7

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Martinus A. Arie, Amir H. Shooshtari, Ratnesh Tiwari, Serguei V. Dessiatoun, Michael M. Ohadi, Joshua M. Pearce

Experimental characterization of heat transfer in an Additively manufactured polymer Heat Exchanger

Applied Thermal Engineering, 113 (2017) 575-584.

In addition to their low cost and weight, polymer heat exchangers offer good anticorrosion and antifouling properties. In this work, a cost effective air-water polymer heat exchanger made of thin polymer sheets using layer-by-layer line welding with a laser through an additive manufacturing process was fabricate and experimentally tested. The flow channels were made of 150 lm-thick high density polyethylene sheets, which were 15.5 cm wide and 29 cm long. The experimental results show that the overall heat transfer coefficient of 35–120 W/m2 K is achievable for an air-water fluid combination for air-side flow rate of 3–24 L/s and water-side flow rate of 12.5 mL/s. In addition, by fabricating a very thin wall heat exchanger (150 lm), the wall thermal resistance, which usually becomes the limiting factor on polymer heat exchangers, was calculated to account for only 3% of the total thermal resistance. A comparison of the air-side heat transfer coefficient of the present polymer heat exchanger with some of the commercially available plain plate fin heat exchanger surfaces suggests that its performance in general is superior to that of common plain plate fin surfaces.

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David M. Hymas, Martinus A. Arie, Farah Singer, Amir H. Shooshtari, and Michael M. Ohadi

Enhanced Air-Side Heat Transfer In An Additively Manufactured Polymer Composite Heat Exchanger

The present study builds upon our prior work in integrating additive manufacturing into next-generation heat/mass exchanger devices. In this paper, we will report an analysis of the fabrication, testing, and performance of an additively manufactured polymer composite heat exchanger. This heat exchanger utilizes a novel approach to achieve enhanced airside heat transfer coefficients and overall mass reduction. This device relies on the Cross-Media Fiber concept where two fluid flows are thermally linked by high-conductivity fins, passing through a low-conductivity channel wall. Through this, the authors have met the required pressure containment, coefficient of performance, and heat flow rate targets, which were 28 psig, 100 and 150 W respectively. The advances that are discussed throughout this paper have allowed this novel polymer composite heat exchanger to be produced through a newly developed form of additive manufacturing that can potentially lead to the economical production of large scale Cross-Media Fiber heat exchangers.

5

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Martinus Adrian Arie, Amir Shooshtari, Michael Ohadi

Air Side Enhancement of Heat Transfer in an Additively Manufactured 1 kW Heat Exchanger for Dry Cooling Applications

Additive manufacturing is a fast-growing technique due to its ability to fabricate complex objects layer by layer from a preprogrammed digital model. Additive manufacturing can greatly enhance the heat exchanger manufacturing field, as it makes possible the fabrication of complex heat exchanger designs that are challenging to fabricate using conventional methods. In the present work, an air-to-water manifold-microchannel heat exchanger made of titanium alloy (Ti64) with size of 15 cm x 15 cm x 3.2 cm was fabricated using direct metal laser sintering (DMLS) additive manufacturing technique. The manifoldmicrochannel feeds the fluid flow into an array of parallel microchannels for better flow distribution as well as short flow travel length, thus yielding significantly enhanced heat transfer performance with low pressure drop penalty. Upon successful fabrication, the heat exchanger was experimentally tested, and the results were analyzed against conventional heat transfer surfaces. Based on the experimental results, for the case where the heat exchanger heat flow rate is 900 W, air-side Reynolds number is less than 100 and the temperature difference between the inlet air and water temperature is 27.5 oC, heat transfer coefficient of 180 W/m2K and pressure drop of 100 Pa are observed. Compared to the conventional surfaces like wavy fin, louvered fin, and plain plate fins, up to 80%, 120%, and 190% improvement in air-side heat transfer coefficients were recorded, respectively, with an air-side pressure drop of less than 100 Pa. The results strongly suggest that additive manufacturing could be implemented for materials and complex designs that are otherwise difficult to fabricate with conventional technologies.

4

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Martinus A. Arie, Amir H. Shooshtari, Michael M. Ohadi

Experimental characterization of an additively manufactured Heat Exchanger for dry cooling of power plants

Applied Thermal Engineering, 2018, 129 187-198.

Air-cooled heat exchangers for power plant cooling are receiving much attention lately, as they require little or no water for cooling when compared to water-cooled systems. This paper focuses on the design, fabrication, and experimental characterization of a novel additively manufactured air–water heat exchanger for dry cooling of power plants. The heat exchanger consists of manifold-microchannels on the air side and rectangular channels on the water side in a cross-flow configuration. By using additive manufacturing, the manifold-microchannel heat exchanger can be fabricated as a single component, which eliminates the assembly process. Three prototype heat exchangers were fabricated using direct metal laser sintering (DMLS) out of stainless-steel (SS17-4), titanium alloy (Ti64), and aluminum alloy (AlSi10Mg). Air-side heat transfer coefficients in the range of 100–450 W/m2 K at pressure drops of 50–2000 Pa were recorded for the titanium alloy heat exchanger for air flow rate ranging from 1.89 L/s to 18.9 L/s. Based on our analysis and compared to conventional heat exchangers, the performance of this manifold-microchannel heat exchanger was superior. Compared to wavy fin and plain plate fin heat exchangers, up to 30% and 40% improvement, respectively, in gravimetric heat transfer density was recorded for the entire range of experimental data. Compared to state-of-the-art dry cooling, nearly 27% improvement in gravimetric heat transfer density was noted at air-side coefficient of performance (COPair) of 172.

3

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Xiang Zhang, Ratnesh Tiwari, Amir H. Shooshtari, Michael M. Ohadi

An additively manufactured metallic manifold-microchannel heat exchanger for high temperature applications

Applied Thermal Engineering, 2018.

This work presents an additively manufactured manifold-microchannel heat exchanger made of Inconel 718 and experimentally tested for high temperature aerospace applications. The heat exchanger core with a size of 66mm×74mm×27mm was fabricated as a single piece through the direct metal laser sintering process. A minimum fin thickness of 180 μm was achieved. Successful welding of additively manufactured headers with the heat exchanger core and conventionally manufactured flanges was demonstrated through the fabrication of the unit. The heat exchanger was tested using nitrogen (N2) on the hot-side and air on the cold-side as the working fluids. The experimental tests were conducted at 600 °C on the hot-side and 38 °C on the cold-side. A maximum heat duty of 2.78 kW and a maximum overall heat transfer coefficient of 1000 W/m2K were achieved during the experiments. The decent agreement between the experimental and the numerical results demonstrates the validity of the numerical analysis model used for heat transfer and pressure drop prediction of the additively manufactured manifold-microchannel heat exchanger. Compared to conventional plate fin heat exchangers, nearly 25% improvement in heat transfer density—the ratio between heat duty and mass (Q/m)—was noted at a coefficient of performance (COP) of 62.

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Hadi Keramati, Fabio Battaglia, Martinus A. Arie, Farah Singer, and Michael M. Ohadi

Additive Manufacturing of Compact Manifold-Microchannel Heat Exchangers utilizing Direct Metal Laser Sintering

Direct Metal Laser Sintering is a metal additive manufacturing technique which uses a laser to fuse metal powders, layer by layer to form a 3D object. In this study, Direct Metal Laser Sintering is used to fabricate compact high temperature manifold-microchannel heat exchangers. Compared to the state of the art heat exchangers, manifoldmicrochannel heat exchangers have been proven to yield superior performances. However, fabrication of manifoldmicrochannel heat exchangers using conventional fabrication methods is a challenge due to their complex geometry. Additive manufacturing processes, like Direct Metal Laser Sintering, allow fabricating the manifold-microchannel heat exchanger as a single component, which significantly simplify its production process. In order to fully utilize the performance potential of manifold-microchannel heat exchangers, small fins (0.1-0.2 mm) and channels (0.2-0.3 mm) are required. In this study, three different machines were used to study the effect of geometries and printing parameters, such as laser power, powder size, and layer thickness, on the fins and channel size of the fabricated microchannel heat exchangers. A comprehensive study has been performed to achieve fin thickness as small as 0.110 mm. Pressure containment tests were also performed to evaluate the minimum base thickness that can hold the designed pressure. A 3”􀵈3”􀀃􀵈3” size microchannel heat exchangers was then successfully fabricated with straight fins of 0.133 mm out of Maraging Steel, and a 4”􀵈4”􀀃􀵈4” size microchannel heat exchangers was successfully fabricated with fin size of 0.22mm out of Inconel 718. The results thus pave the way for more advancements in optimized additive manufacturing of next-generation heat exchangers.

1

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J. Darabi, M. M. Ohadi, S. V. Desiatoun

Falling Film and Spray Evaporation Enhancement Using an Applied Electric Field

The effect of an electric field on the falling-film evaporation of refrigerant R-134a on a vertical plate and three commercially available tubes was investigated experimentally. The plate test section was 25.4 mm wide and 76.2 mm long, and each tube test section was 19 mm in diameter and 140 mm long. Experiments were conducted in both falling film and spray evaporation modes. The effects of various parameters such as heat flux, refrigerant flow rate, electrode gap, and applied voltage were investigated. It was found that in the presence of an applied electric field, the maximum enhancement in the heat transfer coefficient for both falling film and spray evaporation modes on a plate was nearly the same. A maximum enhancement of fourfold in the heat transfer coefficient with the plate, 90 percent with the smooth tube, 110 percent with the Turbo BIII, and 30 percent with 19 fpi tube were obtained. The electrohydrodynamic power consumption in all cases was less than 0.12 percent of the total energy exchange rate in the test section.

                                       2

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Zhang, X., Keramati. H., Arie, M. A., Singer, F., Tiwari, R., Shooshtari, A., and Ohadi, M.

“Recent Development in High Temperature Heat Exchangers: A Review”

Frontiers in Heat and Mass Transfer(FHMT), 11(18), pp. 1-14.

Embedded two-phase microchannel cooling has demonstrated potential for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. Further thermal performance improvements can be made by utilizing different substrates, e.g. single crystalline SiC, and applying geometrical enhancements, as found in FEEDS (thin-Film Evaporation and Enhanced Delivery System), a manifold-microchannel type cooler. Here, we report the performance of FEEDS cooler with SiC capable of cooling 1-kW/cm2 while exceeding 85% outlet vapor quality with R245fa. However, similar to other bulk cooling methods, FEEDS method alone has difficulty in addressing remediation of local hotspots. Thermoelectric coolers (TEC), on the other hand, are scalable and suited for localized cooling. Thus, we report our work on the integration of a micro-contact enhanced TEC with FEEDS. Combining these two thermal management schemes provides effective heat removal over the entire electronic chip surface. Integration of the two methods, however, poses several challenges including manufacturing, hermetic sealing, wiring of the TEC, and thermal/electrical short-circuits. This study proposes a novel design that resolves such challenges and shows capability of cooling 5-kW/cm2 hotspot and 1-kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Lastly, various integration components are optimized, and numerical results demonstrate that with 30°C temperature rise at the SiC chip’s background surface, less than 35°C hotspot temperature rise with respect to the coolant fluid temperature is achieved.

1

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Arie, M. A., Shooshtari, A. H., and Ohadi, M. M.

“Experimental Characterization of An Additively Manufactured Heat Exchanger for Dry Cooling of Power Plants”

Applied Thermal Engineering, 129, pp. 187–198, 2018.

Embedded two-phase microchannel cooling has demonstrated potential for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. Further thermal performance improvements can be made by utilizing different substrates, e.g. single crystalline SiC, and applying geometrical enhancements, as found in FEEDS (thin-Film Evaporation and Enhanced Delivery System), a manifold-microchannel type cooler. Here, we report the performance of FEEDS cooler with SiC capable of cooling 1-kW/cm2 while exceeding 85% outlet vapor quality with R245fa. However, similar to other bulk cooling methods, FEEDS method alone has difficulty in addressing remediation of local hotspots. Thermoelectric coolers (TEC), on the other hand, are scalable and suited for localized cooling. Thus, we report our work on the integration of a micro-contact enhanced TEC with FEEDS. Combining these two thermal management schemes provides effective heat removal over the entire electronic chip surface. Integration of the two methods, however, poses several challenges including manufacturing, hermetic sealing, wiring of the TEC, and thermal/electrical short-circuits. This study proposes a novel design that resolves such challenges and shows capability of cooling 5-kW/cm2 hotspot and 1-kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Lastly, various integration components are optimized, and numerical results demonstrate that with 30°C temperature rise at the SiC chip’s background surface, less than 35°C hotspot temperature rise with respect to the coolant fluid temperature is achieved.

                                       7

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Arie, M. A., Shooshtari, A. H., Tiwari, R., Dessiatoun, S. V., and Ohadi, M. M.

“Experimental Characterization of Heat Transfer in An Additively Manufactured Polymer Heat Exchanger”

Applied Thermal Engineering, 113, pp. 575–584, 2017.
Embedded two-phase microchannel cooling has demonstrated potential for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. Further thermal performance improvements can be made by utilizing different substrates, e.g. single crystalline SiC, and applying geometrical enhancements, as found in FEEDS (thin-Film Evaporation and Enhanced Delivery System), a manifold-microchannel type cooler. Here, we report the performance of FEEDS cooler with SiC capable of cooling 1-kW/cm2 while exceeding 85% outlet vapor quality with R245fa. However, similar to other bulk cooling methods, FEEDS method alone has difficulty in addressing remediation of local hotspots. Thermoelectric coolers (TEC), on the other hand, are scalable and suited for localized cooling. Thus, we report our work on the integration of a micro-contact enhanced TEC with FEEDS. Combining these two thermal management schemes provides effective heat removal over the entire electronic chip surface. Integration of the two methods, however, poses several challenges including manufacturing, hermetic sealing, wiring of the TEC, and thermal/electrical short-circuits. This study proposes a novel design that resolves such challenges and shows capability of cooling 5-kW/cm2 hotspot and 1-kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Lastly, various integration components are optimized, and numerical results demonstrate that with 30°C temperature rise at the SiC chip’s background surface, less than 35°C hotspot temperature rise with respect to the coolant fluid temperature is achieved.

6

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Arie, M. A., Shooshtari, A. H., Rao, V. V., Dessiatoun, S. V., and Ohadi, M. M.

“Air Side Heat Transfer Enhancement Utilizing Design Optimization and An Additive Manufacturing Technique”

Journal of Heat Transfer, 139(3), pp. 031901, 2017.
Embedded two-phase microchannel cooling has demonstrated potential for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. Further thermal performance improvements can be made by utilizing different substrates, e.g. single crystalline SiC, and applying geometrical enhancements, as found in FEEDS (thin-Film Evaporation and Enhanced Delivery System), a manifold-microchannel type cooler. Here, we report the performance of FEEDS cooler with SiC capable of cooling 1-kW/cm2 while exceeding 85% outlet vapor quality with R245fa. However, similar to other bulk cooling methods, FEEDS method alone has difficulty in addressing remediation of local hotspots. Thermoelectric coolers (TEC), on the other hand, are scalable and suited for localized cooling. Thus, we report our work on the integration of a micro-contact enhanced TEC with FEEDS. Combining these two thermal management schemes provides effective heat removal over the entire electronic chip surface. Integration of the two methods, however, poses several challenges including manufacturing, hermetic sealing, wiring of the TEC, and thermal/electrical short-circuits. This study proposes a novel design that resolves such challenges and shows capability of cooling 5-kW/cm2 hotspot and 1-kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Lastly, various integration components are optimized, and numerical results demonstrate that with 30°C temperature rise at the SiC chip’s background surface, less than 35°C hotspot temperature rise with respect to the coolant fluid temperature is achieved.

5

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Bae, D., Yuruker, S., Mandel, R. K., Barletta, P., Bar-Cohen, A., Bao, Y., McKluskey, P., and Ohadi, M.

“Integration, fabrication, and characterization of micro-contact enhanced TFTEC within a FEEDS cooler”

CPMT 2017, 2017.

Embedded two-phase microchannel cooling has demonstrated potential for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. Further thermal performance improvements can be made by utilizing different substrates, e.g. single crystalline SiC, and applying geometrical enhancements, as found in FEEDS (thin-Film Evaporation and Enhanced Delivery System), a manifold-microchannel type cooler. Here, we report the performance of FEEDS cooler with SiC capable of cooling 1-kW/cm2 while exceeding 85% outlet vapor quality with R245fa. However, similar to other bulk cooling methods, FEEDS method alone has difficulty in addressing remediation of local hotspots. Thermoelectric coolers (TEC), on the other hand, are scalable and suited for localized cooling. Thus, we report our work on the integration of a micro-contact enhanced TEC with FEEDS. Combining these two thermal management schemes provides effective heat removal over the entire electronic chip surface. Integration of the two methods, however, poses several challenges including manufacturing, hermetic sealing, wiring of the TEC, and thermal/electrical short-circuits. This study proposes a novel design that resolves such challenges and shows capability of cooling 5-kW/cm2 hotspot and 1-kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Lastly, various integration components are optimized, and numerical results demonstrate that with 30°C temperature rise at the SiC chip’s background surface, less than 35°C hotspot temperature rise with respect to the coolant fluid temperature is achieved.

  4

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Bae, D., Mandel, R., and Ohadi, M.

“Effect of bonding structure and heater design on performance enhancement of FEEDS Embedded manifold-microchannel cooling”

InterPACK2017, 2017.

This work presents the design and characterization of a two-phase, embedded manifold-microchannel (MMC) system for cooling of high heat flux electronics. The study uses a thin-Film Evaporation and Enhanced fluid Delivery System (FEEDS) MMC cooler for high heat flux cooling of electronics. The work builds upon our group’s earlier work in this area with a particular focus on the use of an improved bonding structure and implementation of uniform heat flux heaters that collectively contribute to enhanced performance of the system. In many MMC systems targeted for high heat flux applications microchannels and manifolds are fabricated separately due to different dimensions and tolerances required for each. However, assembly of the system often leaves a gap between the channels and the manifold, thus causing the working fluid to leak through the top of the microfins leading to decreased cooler performance. The effect of this gap is parametrized and analyzed with ANSYS Fluent CFD simulations and discussed in this paper. The findings show that even a few microns wide gap can cause a noticeable degradation of the MMC system performance. Imperfect assembly and the deformation of a microchannel chip due to working fluid pressure can cause gaps, indicating the necessity of uniform and hermetic bonding between the manifold and the tips of the microfins. Furthermore, this work presents the need for better heater designs to enable uniform and high heat flux to the heat transfer surface. Serpentine heaters are often used to mimic electronics in a laboratory environment, but there is a lack of study on the performance characterization of the heaters themselves. In the current work, the performance of a conventional serpentine heater is characterized using ANSYS thermo-electric modeling software. The results show that conventional serpentine heaters are insufficient at providing uniform heat flux in applications where there is a lack of heat spreading—such as in the current embedded cooler— showing deviations ranging over 200 % of the nominal value. The deviations are caused by the many bends present in a serpentine pattern where current density concentrations vary significantly. Two alternate designs are proposed, and numerical simulations show that these new heater designs are capable of providing uniform heat flux, not deviating more than 20% from the nominal heat flux value. The conventional and newly proposed heaters are fabricated, tested, and analyzed with a working FEEDS system.

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Bangerth, S., Ohadi, M. M., and Jenkins, C. A.

Energy Analysis of a LEED Silver Certified Dining Hall on an Academic Campus—A Revisit Three Years after its Initial Certification

ASHRAE Trans. LV-17-011, 123(1), 2017.

Many organizations take pride in improving the energyefficiency of their buildings through certification programs. However, because of limited accountability for energy consumption at the individual building level, certification does not always guarantee efficient building operation in the long run. The purpose of this study was to perform an energy analysis and a building energy simulation of a Leadership in Energy and Environmental Design® (LEED®) Silver dining hall that appeared among a facilities management list of buildings that needed prompt attention on the University of Maryland’s campus in College Park, MD. The focus of this paper is on energy analysis and discussion of operational issues that led to a substantial fall in the energy-efficiency performance for this building. Simulations of energy-efficiency measures predict saving opportunities of $231,632 per year or nearly 60% of the current total building energy consumption. The site energy utilization index (EUI) is estimated to fall from current 349 kBtu/ft2 · yr (1100 kWh/m2 ·yr) to 143 kBtu/ft2·yr (452 kWh/m2 ·yr). This work contributes to a better understanding of the causes of poor energy performance of what may be typical of some dining halls on some academic campuses. It proposes that calculation of exact saving potentials at the building level can improve accountability and thus serve as a proxy for direct financial liability.

2

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Deisenroth, D. C., Moradi, R., Shooshtari, A. H., Singer, F., Bar-Cohen, A., and Ohadi, M.

Review of heat exchangers enabled by polymer and polymer composite additive manufacturing

Heat Transfer Engineering, 2017.

This paper presents an overview of the most common polymer additive manufacturing processes, including vat photopolymerization, material jetting, sheet lamination, powder bed fusion, and fused filament fabrication. Next, the general strengths and challenges of the common methods are discussed and are followed by discussions on efforts to increase thermal performance of polymers used with the various manufacturing methods. Finally, heat exchangers enabled by polymer additive manufacturing are reviewed to show that novel designs in metal, ceramic, and polymer heat exchangers can be made possible by the unique properties of polymers and the advantages offered by additive manufacturing. By reporting what has been proven possible—and the associated challenges—we hope to stimulate the community and further development in a burgeoning field enabled by polymer additive manufacturing.

1

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Arie, M. A., Shooshtari, A. H., Tiwari, R., Dessiatoun, S. V., and Ohadi, M. M.

Experimental Characterization of Heat Transfer in An Additively Manufactured Polymer Heat Exchanger, (Journal Article)

Applied Thermal Engineering, Vol. 113 pp. 575-584, 2017.

In addition to their low cost and weight, polymer heat exchangers offer good anticorrosion and antifouling properties. In this work, a cost effective air-water polymer heat exchanger made of thin polymer sheets using layer-by-layer line welding with a laser through an additive manufacturing process was fabricated and experimentally tested. The flow channels were made of 150 μm-thick high density polyethylene sheets, which were 15.5 cm wide and 29 cm long. The experimental results show that the overall heat transfer coefficient of 35–120 W/m2 K is achievable for an air-water fluid combination for air-side flow rate of 3–24 L/s and water-side flow rate of 12.5 mL/s. In addition, by fabricating a very thin wall heat exchanger (150 μm), the wall thermal resistance, which usually becomes the limiting factor on polymer heat exchangers, was calculated to account for only 3% of the total thermal resistance. A comparison of the air-side heat transfer coefficient of the present polymer heat exchanger with some of the commercially available plain plate fin heat exchanger surfaces suggests that its performance in general is superior to that of common plain plate fin surfaces.

3

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Arie, M. A., Shooshtari, A. H., Dessiatoun, S. V., and Ohadi, M. M.

Air-Side Heat Transfer Enhancement Utilizing Design Optimization and an Additive Manufacturing Technique, (Journal Article)

Journal of Heat Transfer Vol. 139(2)pp. 031901, 2016.

This paper focuses on the study of an innovative manifold microchannel design for air-side heat transfer enhancement that uses additive manufacturing (AM) technology. A numerical-based multi-objective optimization was performed to maximize the coefficient of performance and gravimetric heat transfer density (Q/MΔTQ/MΔT) of air–water heat exchanger designs that incorporate either manifold-microchannel or conventional surfaces for air-side heat transfer enhancement. Performance comparisons between the manifold-microchannel and conventional heat exchangers studied under the current work show that the design based on the manifold-microchannel in conjunction with additive manufacturing promises to push the performance substantially beyond that of conventional technologies. Different scenarios based on manufacturing constraints were considered to study the effect of such constraints on the heat exchanger performance. The results clearly demonstrate that the AM-enabled complex design of the fins and manifolds can significantly improve the overall performance, based on the criteria described in this paper. Based on the current manufacturing limit, up to nearly 60% increase in gravimetric heat transfer density is possible for the manifold-microchannel heat exchanger compared to a wavy-fin heat exchanger. If the manufacturing limit (fin thickness and manifold width) can be reduced even further, an even larger improvement is possible.

2

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Arie, M. A., Shooshtari, A. H., Dessiatoun, S. V., and Ohadi, M. M.

Thermal performance of a polymer composite webbed-tube Heat Exchanger, (Journal Article)

International Journal of Heat and Mass Transfer Vol. 98 pp. 845-856, 2016.

This paper presents an in-depth study of the thermal performance of a gas-to-liquid ‘‘webbed tube” poly-mer heat exchanger. The heat exchanger was fabricated from a thermally enhanced polymer composite,consisting of a Nylon 12 matrix filled with carbon fibers. A laboratory-scale prototype heat exchanger wasbuilt using injection molding and tested on a cross-flow air-to-water heat exchange apparatus. The ther-mal performance of this laboratory webbed-tube polymer composite heat exchanger is studied in depththrough an extended set of experiments, application of existing empirical correlations, and detailed com-putational fluid dynamic (CFD) simulations. The laboratory webbed-tube heat exchanger prototype pro-vided a maximum UA value of 1.8 W/K and a volume-specific heat transfer coefficient of 14 kW/m3K. Theexperimental results, in conjunction with numerical simulations, were used to determine an ‘‘effective”thermal conductivity of 1.8 W/m K for the injection-molded Nylon-carbon composite material in thewebbed-tube heat exchanger configuration.

1

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Arie, M. A., Shooshtari, A. H., Dessiatoun, S. V., and Ohadi, M. M.

Thermal performance of a polymer composite webbed-tube Heat Exchanger, (Journal Article)

International Journal of Heat and Mass Transfer Vol. 98 pp. 845-856, 2016.

This paper focuses on the study of an innovative manifold microchannel design for airside heat transfer enhancement that uses additive manufacturing (AM) technology. A numerical-based multi-objective optimization was performed to maximize the coefficient of performance and gravimetric heat transfer density (Q=MDT) of air–water heat exchanger designs that incorporate either manifold-microchannel or conventional surfaces for air-side heat transfer enhancement. Performance comparisons between the manifold-microchannel and conventional heat exchangers studied under the current work show that the design based on the manifold-microchannel in conjunction with additive manufacturing promises to push the performance substantially beyond that of conventional technologies. Different scenarios based on manufacturing constraints were considered to study the effect of such constraints on the heat exchanger performance. The results clearly demonstrate that the AM-enabled complex design of the fins and manifolds can significantly improve the overall performance, based on the criteria described in this paper. Based on the current manufacturing limit, up to nearly 60% increase in gravimetric heat transfer density is possible for the manifold-microchannel heat exchanger compared to a wavy-fin heat exchanger. If the manufacturing limit (fin thickness and manifold width) can be reduced even further, an even larger improvement is possible.

1

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Arie, M. A., Shooshtari, A. H., Dessiatoun, S. V., Al-Hajri, E., Ohadi, M. M.

Numerical modeling and thermal optimization of a single-phase flow manifold-microchannel plate heat exchanger(Journal Article)

International Journal of Heat and Mass Transfer, 81 pp. 478–489, 2015.

Manifold-microchannel technology has demonstrated substantial promise for superior performance over state of the art heat exchangers, with potential to reduce pressure drop considerably while maintaining the same or higher heat transfer capacity compared to conventional microchannel designs. However, optimum design of heat exchangers based on this technology requires careful selection of several critical geometrical and flow parameters. The present research focuses on the numerical modeling and optimization of a manifold-microchannel plate heat exchanger to determine the design parameters that yield the optimum performance for the heat exchanger. A hybrid method that requires significantly shorter computational time than the full Computational Fluid Dynamic (CFD) model was developed to calculate the coefficient of performance and heat transfer rates of the heat exchanger. The results from the hybrid method were successfully verified with the results obtained from a full CFD simulation and experimental work. A corresponding multi-objective optimization of the heat exchanger was conducted utilizing an approximation-based optimization technique. The optimized manifold-microchannel plate heat exchanger showed superior heat transfer performance over chevron plate heat exchanger designs.

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Vibhash Jha, Sergeui V. Dessiatoun, Amir H. Shooshtari, Ebrahim S. Al-Hajri, Michael M. Ohadi

Experimental Characterization of a Nickel-Alloy Based Manifold-Microgroove Evaporator (Journal Article)

Heat Transfer Engineering, pp. 00-00, 2014ISSN: 0145-7632 1521-0537.
Effective heat and mass exchangers are vital for further improvement of absorption cooling systems. In the current study, a novel manifold-microchannel evaporator was developed and tested. This paper reports heat transfer coefficients and pressure drop for a nickel alloy-based tubular microgrooved evaporator consisting of a novel manifold guided flow. The evaporator was designed for refrigerant-to-liquid heat exchange, and the heat transfer surface consisted of fine high-aspect-ratio microchannels having 100μm channel width and 600μm channel height. The refrigerant-side flow was guided through square manifold feeds with sides of 2 mm in length. A tube insert providing an annular gap of 2.5 mm was used on the water side. Experiments were conducted with R134a as the refrigerant for a flow rate range of 5 30 g/s and water-side flow rate range of 100–600 ml/s. An overall heat transfer coefficient of more than 10,000 W/m2-K was measured with a modestmaximum pressure drop of 120 mbars and 100 mbars on the refrigerant and water sides, respectively.

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Harish Ganapathy, Amir H. Shooshtari, Sergeui V. Dessiatoun, M. Alshehhi, Michael M. Ohadi

Fluid Flow and Mass Transfer Characteristics of Enhanced Co2 Capture in a Minichannel Reactor (Journal Article)

Applied Energy, 119 pp. 43-56, 2014.

CO2 absorption using amine solvents can be significantly enhanced by the use of microscale reactors having high surface area to volume ratio. The present paper reports an experimental investigation of the fluid flow and mass transfer characteristics during reactive gas-liquid absorption in a minichannel reactor. Absorption of CO2 mixed with N2 into aqueous diethanolamine was studied in a channel having hydraulic diameter of 762μm and a circular cross-sectional geometry. High-speed imaging of the two-phase flow was conducted to visualize the flow regimes. Image-processing analysis of the acquired flow patterns was performed to determine the interfacial area. The performance of the reactor was studied with respect to the absorption efficiency, pressure drop, mass transfer coefficient, interfacial area, enhancement factor, and Sherwood number. Parametric studies investigating the effects of phase superficial velocity, liquid reactant concentration, and CO2 concentration in the gas phase were performed and are discussed. High levels of absorption efficiency, close to 100%, were observed under certain operating conditions. An empirical model for the Sherwood number was developed and compared against experimental data. The mass transfer coefficient was found to be higher at reduced channel lengths, which was attributed to improved utilization of the absorption capacity of the amine solution for a given reactor volume. The presently achieved values of mass transfer coefficient and specific interfacial area are between 1 and 4 orders of magnitude higher than those reported for most conventional absorption systems.

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K. Choo, R. M. Galante, Michael M. Ohadi

Energy Consumption Analysis of a Medium-Size Primary Data Center in an Academic Campus (Journal Article)

Energy and Buildings, 76 pp. 414-421, 2014.

Experimental, numerical, and simulation studies were performed to evaluate the energy efficiency performance, develop energy conservation measures (ECMs), and conduct overall energy analysis of a medium-size data center at the campus of the University of Maryland, College Park. Based on the analysis, the PUE (power usage effectiveness) of the data center was found to be 2.73, suggesting ample opportunity for energy saving measures. The IT, cooling, and electrical loads consume 36.6%, 32.9%, and 21.7% of the total data center energy consumption, respectively. Four ECMs are suggested to reduce energy consumption by optimizing the thermo-fluid flow in the data center: (1) eliminate unnecessary CRACs (computer room air conditioning units); (2) increase the return air temperature at the CRACs; (3) add cold aisle containment; (4) implement fresh air cooling. In addition, a transient analysis was performed under a total power failure scenario of all cooling systems, as well as failure of individual CRACs as a separate analysis, to predict the corresponding temperature increase with time in the data center and electronics. © 2014 Elsevier B.V.

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E. Al-Hajri, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi

Performance Characterization of R134a and R245fa in a High Aspect Ratio Microchannel Condenser (Journal Article)

International Journal of Refrigeration, 36 (2), pp. 588-600, 2013ISSN: 0140-7007.

An experimental study on parametric characterization of two-phase condensing flows of refrigerants R134a and R245fa in a single microchannel was carried out utilizing a microchannel with a cross-section of 0.4 mm × 2.8 mm (7:1 aspect ratio) and length of 190 mm. The study investigated parametric effects of variations in saturation temperatures between 30 °C and 70 °C, mass flux between 50 and 500 kg m-2 s-1, and inlet superheats between 0 °C and 20 °C on the average heat transfer coefficient and overall pressure drops in the microchannel. The results of the study suggest that while the saturation temperature and mass flux have a significant effect on both the heat transfer and overall pressure drop coefficients, the inlet superheat has little or no effect. In addition, the applicability of the Dobson-Chato correlation for heat transfer coefficient and Lockhart-Martinelli correlation for pressure drop for the range of parameters was investigated. 

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Raphael Mandel, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi

Streamline Modeling of Manifold Microchannels in Thin Film Evaporation (Inproceeding)

pp. V002T07A024–V002T07A024, American Society of Mechanical Engineers, 2013ISBN: 9780791855485.

Manifold microchannels utilize a system of manifolds to divide long microchannels into an array of parallel ones, resulting in reduced flow length and more localized liquid feeding. Reducing flow length is desirable because it enables the simultaneous enhancement of heat transfer rate and reduction of pressure drop. Furthermore, localized feeding reduces potential for localized dryout, increasing the operational heat flux. Because of the failure of the available conventional heat transfer correlations to predict the thermal performance of manifold microchannels operating in two phase mode, a “streamline” model was created. The heat transfer surface area was divided into parallel, non-interacting streamlines, and the quality, void fraction, film thickness, heat transfer coefficient, heat flux, and pressure drop was calculated sequentially along the streamline. The mass flow rate through each streamline was adjusted in order to obtain the specified pressure drop, and the value of this pressure drop was adjusted in order to obtain the desired microchannel mass flux. Finally, the average wall heat transfer coefficient was calculated, and temperature profile in the fin was adjusted to correspond with the analytical 1-D temperature distribution of a thin fin with an average wall heat transfer coefficient and specified base superheat. The average wall heat transfer coefficients predicted by the model was then compared to the available experimental data with sufficiently good agreement with a wide variety of geometries and working fluids at low mass fluxes.

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Harish Ganapathy, Sascha Steinmayer, Amir H. Shooshtari, Sergeui V. Dessiatoun, Mohamed Alshehhi, Michael M. Ohadi

Enhanced Carbon Capture in a Multiport Microscale Absorber (Inproceeding)

pp. V06BT07A006-V06BT07A006, American Society of Mechanical Engineers, 2013ISBN: 978-0-7918-5629-1.

Increasing concerns on the effects of global warming leading to climate change has necessitated the development of efficient technologies to separate acid gas components, such as carbon dioxide and hydrogen sulfide, from gaseous mixtures. Microscale technologies have the potential to substantially enhance gas-liquid absorption processes on account of their inherent high surface area to volume ratio. The present work reports the mass transfer characteristics during gas-liquid absorption in a multiport microscale absorber. The reactor was designed to comprise of 15 straight, parallel channels having a hydraulic diameter of 456 micrometer and square cross-sectional geometry. The absorption of CO2 mixed with N2 into aqueous diethanolamine was investigated. The performance of the absorber was characterized with respect to the absorption efficiency and mass transfer coefficient. Parametric studies investigating the effects of the gas and liquid phase superficial velocity were performed and discussed. Additionally, the effect of varying the liquid reactant concentration was investigated and discussed.

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Harish Ganapathy, Amir H. Shooshtari, Sergeui V. Dessiatoun, M. Alshehhi, Michael M. Ohadi

High Efficiency Carbon Capture Technology Utilizing Advanced Micro-Structured Surfaces (Inproceeding)

AiChE, Alexandria, VA, 2013.

Manifold microchannels utilize a system of manifolds to divide long microchannels into an array of parallel ones, resulting in reduced flow length and more localized liquid feeding. Reducing flow length is desirable because it enables the simultaneous enhancement of heat transfer rate and reduction of pressure drop. Furthermore, localized feeding reduces potential for localized dryout, increasing the operational heat flux. Because of the failure of the available conventional heat transfer correlations to predict the thermal performance of manifold microchannels operating in two phase mode, a “streamline” model was created. The heat transfer surface area was divided into parallel, non-interacting streamlines, and the quality, void fraction, film thickness, heat transfer coefficient, heat flux, and pressure drop was calculated sequentially along the streamline. The mass flow rate through each streamline was adjusted in order to obtain the specified pressure drop, and the value of this pressure drop was adjusted in order to obtain the desired microchannel mass flux. Finally, the average wall heat transfer coefficient was calculated, and temperature profile in the fin was adjusted to correspond with the analytical 1-D temperature distribution of a thin fin with an average wall heat transfer coefficient and specified base superheat. The average wall heat transfer coefficients predicted by the model was then compared to the available experimental data with sufficiently good agreement with a wide variety of geometries and working fluids at low mass fluxes.

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Harish Ganapathy, Amir H. Shooshtari, K. Choo, Sergeui V. Dessiatoun, M. Alshehhi, Michael M. Ohadi

Volume of Fluid-Based Numerical Modeling of Condensation Heat Transfer and Fluid Flow Characteristics in Microchannels (Journal Article)

International Journal of Heat and Mass Transfer, 65 pp. 62-72, 2013.

The present work proposes a numerical model for the simulation of condensation heat transfer and fluid flow characteristics in a single microchannel. The model was based on the volume of fluid approach, which governed the hydrodynamics of the two-phase flow. The condensation characteristics were governed by the physics of the phenomena and did not include any empirical expressions in the formulation. The conventional governing equations for conservation of volume fraction and energy were modified to include source terms that accounted for the mass transfer at the liquid-vapor interface and the associated release of latent heat, respectively. A microchannel having characteristic dimension of 100 μm was modeled using a two-dimensional computational domain. The working fluid was R134a and the vapor mass flux at the channel inlet ranged from 245 to 615 kg/m2 s. The channel wall was maintained at a constant heat flux ranging from 200 to 800 kW/m2. The predictive accuracy of the numerical model was assessed by comparing the two-phase frictional pressure drop and Nusselt number with available empirical correlations in the literature. A reasonably good agreement was obtained for both parameters with a mean absolute error of 8.1% for two-phase frictional pressure drop against a recent universal predictive approach, and 16.6% for Nusselt number against an available correlation. Further, a qualitative comparison of various flow patterns against experimental visualization data also indicated a favorable agreement. © 2013 Elsevier Ltd. All rights reserved.

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Harish Ganapathy, E. Al-Hajri, Michael M. Ohadi

Phase Field Modeling of Taylor Flow in Mini/Microchannels, Part II: Hydrodynamics of Taylor Flow (Journal Article)

Chemical Engineering Science, 94 pp. 156-165, 2013ISSN: 0009-2509.
Multiphase heat and mass transfer in microscale devices is a growing field of research due to thepotential of these devices for use in various engineering applications. Before the heat and masstransport phenomena in such systems can be modeled, the hydrodynamics of adiabatic multiphaseflow, in the absence of specie transport across interfaces, must be accurately predicted. In the presentpaper, a finite element implementation of the phase field method is applied to simulate Taylor flow in mini/microchannels. Channels with characteristic dimensions ranging from 100 to 500mm are modeledand criteria present in the literature for domain discretization are assessed. The effects of phase fieldparameters, namely mobility and interface thickness, on the predicted flow features are discussed. Thepredicted Taylor bubble lengths are compared against empirical correlations as well as available experimental data in the literature. The predicted gas void fraction data for different channeldimensions are compared with numerous experimental studies. The present results indicate a linear variation of gas void fraction with respect to volumetric flow ratio for all channel sizes

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Harish Ganapathy, E. Al-Hajri, Michael M. Ohadi

Mass Transfer Characteristics of Gas-Liquid Absorption During Taylor Flow in Mini/Microchannel Reactors (Journal Article)

Chemical Engineering Science, 101 pp. 69-80, 2013ISSN: 0009-2509.
This paper reports a numerical study of the mass transfer characteristics during Taylor flow in mini/microchannel reactors. A finite-element implementation of the phase field method was used to predict the hydrodynamics of the two-phase flow. The phase distribution thus obtained was used to define the computational domain to model the reactive mass transfer. The reaction system of the absorption of CO2 into aqueous NaOH solution was considered. Channels with characteristic dimensions ranging from 100μm to 750μm were modeled with cross-flow and flow-focusing inlet configurations. The effect of channel length was studied by varying the residence time in the transient simulation. The results indicated that channels having a small characteristic dimension could yield reductions in the residence time, and therefore the reactor size, by as much as 85%. This reduction was further enhanced by higher concentration levels of the liquid reactant and increased temperatures. The inlet mixing region was found to have a significant influence on the total mass transfer. The channel wall wettability was found to affect the mass transfer characteristics negligibly. The predictions from the currently proposed model were compared with available experimental data, as well as with predictions of an earlier unit cell-based model, and a good agreement was obtained

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David Boyea, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi

Heat Transfer and Pressure Drop Characteristics of a Liquid Cooled Manifold-Microgroove Condenser (Inproceeding)

pp. V003T23A003–V003T23A003, American Society of Mechanical Engineers, 2013
High performance condensers are essential components in energy conversion, electronics cooling and process systems. Increased capacity and functionality with less and less available space has been a main driving force for development of next generation of condensers in energy systems. Our previous work in this area has demonstrated that manifold-microgroove heat exchangers operating in single-phase or two-phase modes offer substantially higher heat transfer performance with a greatly reduced pumping power when compared to state-of-art microchannel heat exchangers. The goal is to enhance heat transfer while minimizing the pumping power, volume and weight. A compact lightweight manifold microgroove condenser, with 60 × 600 micron microgrooves and cooling capacity of 4kW, was fabricated, assembled and tested using different manifolds. Experiments using R236fa and R134a as a working fluids were performed measuring inlet and outlet temperatures, flow rates and pressure drops for the refrigerant and water side. Overall heat transfer coefficient and pressure drop across condenser were determined and refrigerant side heat transfer coefficient were calculated based on water side heat transfer coefficient. Experimental results indicate significant effect of manifold geometry on condenser performances. Refrigerant side heat transfer coefficient of 60 kW/m2K with pressure drop of just 7 kPa has been demonstrated using R-134-a.

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M. Alshehhi, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi

Parametric Study on the Separation of Fine Immiscible Oil Droplets from a Gaseous Stream Using an Electrostatic Force(Journal Article)

HVAC&R Research, 19 (5), pp. 471-480, 2013ISSN: 1078-9669.
The interest in the development of efficient techniques for the separation of fine aerosols from moving gaseous media has grown considerably in recent years, both within the HVAC and refrigeration community as well as other similar industries. A major challenge faced by refrigeration systems is the removal of micron- and submicron-sized oil droplets from refrigerants. Electrostatic separators, when optimally designed, could potentially result in a highly efficient separation technology suitable for the above application. This experimental study provides insights into the physics of electrostatic separation in support of better understanding of the phenomenon and examining its potential for implementation in applications of practical significance. Specifically, the feasibility of using electrostatic forces to separate oil droplets from airflow was conducted. Parametric study analyzing the effects of the applied potential, emitter polarity, fluid velocity, and working fluid temperature on the performance of electrostatic separation were examined and discussed. The results of this study demonstrate that a separation efficiency as high as 99.99% can be achieved with very low power consumption. Collective findings may indicate a high potential for this technology for separation of fine aerosols from a gas flow.

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Amir H. Shooshtari, R. Kuzmicki, Sergeui V. Dessiatoun, M. Alshehhi, E. Al-Hajri, Michael M. Ohadi

Enhancement of Co2 Absorption in Aqueous Diethanolamine Amine Using Microchannel Contactors (Inproceeding)

pp. 1057-1065, 2012ISBN: 9781618396112

Carbon dioxide (CO2) is the largest volume contributor and the fastest growing component of greenhouse gases. Based on current technology the only commercially available process that can absorb a reasonable amount of CO2 from flue gases is chemical absorption. The other techniques are generally less energy efficient and more expensive. Microchannel technology can be used to enhance the mass transfer rate by increasing surface-to-volume ratio and improving the thermal controllability of the absorption process. In the current study we investigated the performance of microchannel contactors for absorption of CO2 in aqueous diethanolamine (DEA). A series of experiments was performed to measure CO2 absorption rate and removal efficiency for various gas-to-amine flow rate ratios. The rate of absorption was determined based on the variation of electrical conductivity of the aqueous DEA due to the CO2 absorption process. The effect of contactor length was studied for 200, 500, and 800 mm long microchannels. The pressure drops of two-phase flow for various flow rate ratios and microchannel length were measured. The results demonstrated high potential of the microchannel contactors for enhancement of the absorption process.

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Vibhash Jha, Sergeui V. Dessiatoun, Michael M. Ohadi, Ebrahim Al Hajri

Experimental Characterization of Heat Transfer and Pressure Drop inside a Tubular Evaporator Utilizing Advanced Microgrooved Surfaces (Journal Article)

Journal of Thermal Science and Engineering Applications, (4), pp. 41009, 2012ISSN: 19485085

Performance enhancement of heat exchangers with a focus in optimum weight/volume and the amount of working fluid in circulation is of significance to a diverse range of industries. This paper presents heat transfer and pressure drop characteristics of a compact tubular evaporator which utilizes a manifold force-fed microchannel design. A microgrooved structure with an aspect ratio of 3:1 (channel width of 100 μm and channel height of 300 μm) forms the channels used on the refrigerant side and minichannels of 1 mm depth were used on the water side. The system was tested using R134a as the refrigerant with a refrigerant flow rate of 6 to 22 g/s and water flow rate of 150 to 640 ml/s. Overall heat transfer coefficients of more than 10,000 W/m2 K were obtained with modest values of pressure drop. The present results indicate a significant enhancement in thermal performance when compared to the state-of-the-art technologies in the same application area.

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M. D. Islam, A. A. Alili, I. Kubo, Michael M. Ohadi

Measurement of Solar-Energy (Direct Beam Radiation) in Abu Dhabi, Uae (Journal Article)

Renewable Energy, 35 (2), pp. 515-519, 2010ISSN: 0960-1481.

This paper presents actual measurements of direct solar radiation in Abu Dhabi (24.43°N, 54.45°E) with the existing meteorological conditions encountered during the measurement throughout the year. High resolution, real-time solar radiation and other meteorological data were collected and processed. Daily and monthly statistics of direct solar radiation were calculated from the one-minute average recorded by a Middleton Solar DN5-E Pyroheliometer. The highest daily and monthly mean solar radiation values were recorded as 730 and 493.5 W/m 2, respectively. The highest one-minute average daily solar radiation was recorded as 937 W/m 2. In addition to direct beam radiation, the daily average clearness indexes, surface temperature variations, wind speeds and relative humidity variations are discussed. When possible, direct beam radiation and some meteorological data are compared with corresponding data of the 22-year average of NASA’s surface meteorology and solar-energy model. The measured data (direct beam radiation and meteorological) are in close agreement with the NASA SSE model with some discrepancy. © 2009 Elsevier Ltd. All rights reserved.

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P. Foroughi, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi

Experimental Characterization of an Electrohydrodynamic Micropump for Cryogenic Spot Cooling Applications (Journal Article)

Heat Transfer Engineering, 31 (2), pp. 119-126, 2010ISSN: 0145-7632.
This article presents a study on the characterization of a planar, multistage, electrohydrodynamic (EHD) ion-drag micropump for pumping of liquid nitrogen. Two designs of the pump, consisting of different emitter configurations (flat and saw-tooth), similar emitter-collector spacing (50 microns), and similar gaps between successive electrode pairs (100 microns), were tested at DC voltages ranging from 0 to 2.5 kV. The electric currents they generated and the corresponding static pressure heads were measured to characterize the pumping performance. Pressure and current onset voltages as well as pressure-voltage (P-V) and pressure-current (P-I) relationships were investigated. The highest pressure head (30 Pa at 1700 V) was generated with the saw-tooth design. After collecting and processing the data for various prototypes, it was evident that incorporating saw-tooth electrodes can significantly improve the performance of the micropump.

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M. Alshehhi, Amir H. Shooshtari, Sergeui V. Dessiatoun, Michael M. Ohadi, A. Goharzadeh

Parametric Performance Analysis of an Electrostatic Wire-Cylinder Aerosol Separator in Laminar Flow Using a Numerical Modeling Approach (Journal Article)

Separation Science and Technology, 45 (3), pp. 299-309, 2010ISSN: 0149-6395.
A numerical methodology based on the Lagrangian approach is outlined to study the performance of a select class of electrostatic aerosol separators. This modeling method is used to perform a parametric study on the efficiency of a wire-cylinder separator in separation of water aerosols from air. The geometry consists of an 80 mu m diameter wire placed in the centerline of a 20mm diameter cylinder. The work focuses on the effect of applied voltage (in the range of 4 to 8kV), flow velocity (in the range of 0.3 to 1.5m/s), flow temperature (in the range of 280K to 320K), and separator length (in the range of 0.05 to 0.15m) on charging of water aerosols and on separator collection efficiency in laminar flow. The aerosols size ranges between 0.01-10 mu m. The results of the study show that applied voltage, flow rate, and separator length affect the separation efficiency significantly, while the effect of flow temperature seems negligible.

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M. D. Islam, I. Kubo, Michael M. Ohadi, A. A. Alili

Measurement of Solar Energy Radiation in Abu Dhabi, Uae (Journal Article)

Applied Energy, 86 (4), pp. 511-515, 2009.

This paper presents data on measurement of actual solar radiation in Abu Dhabi (24.43°N, 54.45°E). Global solar radiation and surface temperatures were measured and analyzed for one complete year. High resolution, real-time solar radiation and other meteorological data were collected and processed. Daily and monthly average solar radiation values were calculated from the one-minute average recorded values. The highest daily and monthly mean solar radiation values were 369 and 290 W/m2, respectively. The highest one-minute average daily solar radiation was 1041 W/m2. Yearly average daily energy input was 18.48 MJ/m2/day. Besides the global solar radiation, the daily and monthly average clearness indexes along with temperature variations are discussed. When possible, global solar energy radiation and some meteorological data are compared with corresponding data in other Arab state capitals. The data collected indicate that Abu Dhabi has a strong potential for solar energy capture. © 2008 Elsevier Ltd. All rights reserved.

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M. Rada, Amir H. Shooshtari, Michael M. Ohadi

Experimental and Numeral Simulation of Meso-Pumping of Liquid Nitrogen – Application to Cryogenic Spot Cooling of Sensors and Detectors (Journal Article)

Sensors and Actuators, A: Physical, 148 (1), pp. 271-279, 2008ISSN: 0924-4247

Superconducting electronics for sensors and detectors require cryogenic conditions and certain conventional electronic applications exhibit better performance at low temperatures. Most of these applications require modest power dissipation capabilities while having strict requirements on the spatial and temporal temperature variations. Controlled surface cooling techniques ensure more stable and uniform temporal and spatial temperature distributions that allow better signal-to-noise ratios for sensors and elimination of hot spots for processors. Small-scale or meso-pumping is a promising technique that could provide pumping and mass flow rate control along the cooled surface. In the present work, an ion-drag electrohydrodynamics (EHD) meso-pump is utilized for the first time, to achieve pumping of liquid nitrogen for spot cooling applications. Successful implementation of these cooling techniques could provide on-demand and on-location pumping power which would allow tight cryogenic temperature control on the cooling surface of sensors, detectors and other cold electronics. © 2008 Elsevier B.V. All rights reserved.

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G. H. Kuang, Michael M. Ohadi, Sergeui V. Dessiatoun

Semi-Empirical Correlation of Gas Cooling Heat Transfer of Supercritical Carbon Dioxide in Microchannels (Journal Article)

HVAC&R Research, 14 (6), pp. 861-870, 2008ISSN: 1078-9669.
This paper provides a comprehensive review of existing correlations for supercritical heat transfer of CO2 in microchannels, as well as a comparison of these correlations with experimentally measured data. Based on the experimental data, a new semi-empirical correlation is developed to predict the gas cooling heat transfer coefficient of supercritical CO2 in microchannels, within an error of 15% for most (91%) of the presented experimental data that were obtained in an 11-port microchannel tube with an internal diameter of 0.79 mm and with a pressure range of 8 to 10 MPa and mass flux range of 300 to 1200 kg/m2s.

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P. Foroughi, Sergeui V. Dessiatoun, Amir H. Shooshtari, Michael M. Ohadi

Experimental Characterization of an Ehd Ion-Drag Micropump for Cryogenic Micro-Pumping Applications (Inproceeding)

pp. 1779-1786, 2008ISBN: 0791843025; 9780791843024.
This paper presents a study on the characterization of a planar multi-stage electrohydrodynamic (EHD) ion-drag micropump for pumping of liquid nitrogen. Four designs of the pump – consisting of different emitter configurations (planar and saw-tooth), emitter-collector spacings (20 and 50 microns), and gaps between successive electrode pairs (80, 100 and 200 microns) – were tested at DC voltages ranging from 0 to 2.5 kV. The generated electric currents and static pressure heads were measured to characterize the pumping performance. After collecting and processing the data for the various designs, it was evident that the purify of the liquid plays a vital role in the repeatability of the pumping results. In all cases high-purity liquid nitrogen was used. The complex interaction between the liquid and the electrodes along with the probabilistic nature of the ion-generation process sometimes prohibited achieving the same pumping performance under identical voltage levels, thus purity of the nitrogen used was very important. The highest pressure head (30 Pa at 1700 V) was generated with a (50,100,s) design. The (50,100,s) stands for saw-tooth emitters and planar collectors, 50 μm inter-electrode spacing, and 100 μm electrode-pair spacing. Copyright © 2007 by ASME.

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Thomas B. Baummer, E. Cetegen, Michael M. Ohadi, Sergeui V. Dessiatoun

Force-Fed Evaporation and Condensation Utilizing Advanced Micro-Structured Surfaces and Micro-Channels (Journal Article)

Microelectronics Journal, 39 (7), pp. 975-980, 2008.
This paper presents results of an experimental study on phase change cooling of high flux electronics utilizing an innovative phase-change technique involving force-fed evaporation and condensation. The technique utilizes high-performance micro-structured surfaces consisting of alternating fins and channels, coupled with a force-fed mechanism in the evaporator and condenser. The force-fed mechanism provides a highly vigorous micro-channel convective heat transfer environment with the net effect of substantially higher heat transfer coefficients without the high-pressure drop penalties that are normally associated with such flows. Our recent results demonstrate dissipation heat flux levels well above 300 W/cm2 with corresponding heat transfer coefficients of close to 90,000-100,000 W/m2 K, using HFE-7100 as the working fluid. For the condensation mode, the force-fed method produces heat fluxes up to 58 W/cm2 with a heat transfer coefficient of 32,000 W/m2 K using HFE-7100. © 2007 Elsevier Ltd. All rights reserved.

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V. Tudor, Michael M. Ohadi

The Effect of Stationary and Sweeping Frequency AC Electric Fields on Frost Crystal Removal on a Cold Plate

International Journal of Refrigeration, 29(4). pp. 669-677, 2007, ISSN: 0140-7007.
The effect of stationary and sweeping frequency AC electric fields on frost crystals growth and frost control/removal on a cold plate was studied for the first time in this paper. The main results of this study showed that the presence of AC electric fields can greatly affect both the frost crystals growth pattern and mass accumulation on cold surfaces. The ice surface electrical properties and basic electrostatics were used to explain the main findings in this paper. Up to 46% frost reduction was obtained when the electric field frequency spanned 370 Hz to 7.5 kHz while the applied voltage was 14.5 kV. Two different sets of environmental conditions were tested, which showed that the plate temperature placed an important effect on frost crystals growth under electric fields. An optimum application time of the AC electric fields was found based on least frost mass accumulation on the cold plate. © 2005 Elsevier Ltd and IIR.

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S. Moghaddam, K. T. Kiger, Michael M. Ohadi

Measurement of Corona Wind Velocity and Calculation of Energy Conversion Efficiency for Air-Side Heat Transfer Enhancement in Compact Heat Exchangers (Journal Article)

HVAC&R Research, 12 (1), pp. 57-68, 2006.
Corona discharge utilizes the effect of electrically induced secondary motions to enhance the air-side heat transfer coefficients between high-density fins in a fin structure. The objective of this study was to measure the velocity associated with the air jet generated by corona discharge for parametric ranges of interest to air-side heat transfer enhancement in compact heat exchangers. This is the first study of its kind where the nonintrusive laser Doppler velocimetry (LDV) technique was used to measure the velocity, with careful attention paid to correction for the electric field effects on the particles and the resulting slip velocity. Experiments were conducted with a prototypical wire-to-plate geometry (positively charged wire and grounded plate). In two series of experiments, the flow was seeded with 2.5 μm glass microspheres and 0.5 μm polystyrene nanospheres. The corrected velocity measurements were used to calculate the kinetic energy flux to the fluid and the resulting efficiency of the electric-to-kinetic energy conversion.

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A. Bar-Cohen, M. Arik, Michael M. Ohadi

Direct Liquid Cooling of High Flux Micro and Nano Electronic Components (Journal Article)

Proceedings of the IEEE, 94 (8), pp. 1549-1570, 2006.

The inexorable rise in chip power dissipation and emergence of on-chip hot spots with heat fluxes approaching 1 kW/cm2 has turned renewed attention to direct cooling with dielectric liquids. Use of dielectric liquids in intimate contact with the heat dissipating surfaces eliminates the deleterious effects of solid-solid interface resistances and harnesses the highly efficient phase-change processes to the critical thermal management of advanced IC chips. In the interest of defining the state-of-the-art in direct liquid cooling, this paper begins with a discussion of the thermophysics of phase-change processes and a description of the available dielectric liquid cooling techniques and their history. It then describes the phenomenology of pool boiling, sprayjet impingement, gas assisted evaporation and synthetic jet impingement with dielectric with dielectric liquids. Available correlations for predicting the on chip hot spot cooling, are also provided and compared.

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V. Tudor, Michael M. Ohadi, M. A. Salehi, J. V. Lawler

Advances in Control of Frost on Evaporator Coils with an Applied Electric Field (Journal Article)

International Journal of Heat and Mass Transfer, 48 (21-22), pp. 4428-4434, 2005ISSN: 0017-9310.

This paper introduces an innovative technique on use of an applied electric field for control of frost over evaporator coils with fin density and geometric configuration of interest to freezer/refrigerator applications. The technique discussed in this paper, referred to as the “dielectric barrier discharge” (DBD) method, may be particularly suitable for application in evaporator coils with high fin density. Experiments conducted with a small-scale laboratory test-module, as well as a full-scale supermarket evaporator are presented. The DBD technique is based on generating localized non-resistive heating within fins of an evaporator coil via application of a high-voltage, alternating current through electrodes. Our experiments demonstrate that the defrosting time using DBD is substantially shorter than conventional techniques, while the energy consumption associated with the process is less than one half of the corresponding energy of the electrical resistance heating methods. Basic operational principles of the technique, its advantages and limitations when compared to conventional electrical defrosting techniques are discussed and presented for the first time in this paper. © 2005 Elsevier Ltd. All rights reserved.

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S. Moghaddam, Michael M. Ohadi

Effect of Electrode Geometry on Performance of an Ehd Thin-Film Evaporator (Journal Article)

Journal of Microelectromechanical Systems, 14 (5), pp. 978-986, 2005ISSN: 1057-7157.

This paper presents details of an optimization process of electrode geometry for an electrohydrodynamically (EHD) driven thin-film evaporator. The operation principle of the device is based on the action of the EHD force on the molecules of a dielectric liquid in a highly convergent electric field. The force starts at the end of a pair of electrodes, where the electric field changes from zero far from the electrodes to a finite value in between the electrodes. This force drives the liquid up into the spacing between the electrodes. The electrodes in this study were deposited thinly on a SiO2/Si wafer, so the liquid could be held within micrometers of thickness over the surface. Since the performance of the device in removing heat from the surface is a function of its pumping head and consequently its electrode geometry, the performances of different electrodes were evaluated by testing twelve sets of electrode pairs with different geometries. Then the optimum electrode design was incorporated into the design of a large size (32 × 32 mm2) EHD thin-film evaporator. The device was fabricated, and its pumping and heat transfer performances were tested. A pumping head equal to the full height of the electrodes and a heat transfer coefficient of 1.9 W/cm2.°C was achieved using HFE-7100 liquid. © 2005 IEEE.

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Y. Zhao, Michael M. Ohadi

Experimental Study of Supercritical Co 2 Gas Cooling in a Microchannel Gas Cooler (Inproceeding)

ASHRAE Transactions, pp. 291-300, 2004ISSN: 12505.

An experimental study was conducted to investigate the heat transfer characteristics of supercritical CO 2 gas cooling down in a microchannel gas cooler over a range of operating conditions encountered in typical residential heat pumps. The microchannels used in the present study had a hydraulic diameter of approximately 1 mm. The experiments were conducted to evaluate the heat transfer performance of the microchannel gas cooler at different test conditions by varying airflow rates, air temperatures, refrigerant inlet temperatures, and mass flow rates. All experimental results are tabulated in the present paper. It was found that the refrigerant mass flow rate is the dominant factor for the capacity of a CO 2 gas cooler, and a significant portion of the heat transfer in a CO 2 gas cooler was carried out in the heat exchanger module on the refrigerant inlet side. The temperature and pressure of CO 2 significantly affect the heat transfer and fluid flow characteristics due to the fact that some important thermal physical properties of CO 2 (such as specific heat, density, viscosity) are strongly dependent on its temperature and pressure. All experiments were successfully conducted with an energy balance of ±3%.

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A. Bologa, Michael M. Ohadi

External Electric Field as the Factor of Heat Transfer Enhancement in the System with Refrigerator Agent R-134a (Journal Article)

Surface Engineering and Applied Electrochemistry (Elektronnaya Obrabotka Materialov), (6), pp. 49-52, 2004.

An experimental study was conducted to investigate the heat transfer characteristics of supercritical CO 2 gas cooling down in a microchannel gas cooler over a range of operating conditions encountered in typical residential heat pumps. The microchannels used in the present study had a hydraulic diameter of approximately 1 mm. The experiments were conducted to evaluate the heat transfer performance of the microchannel gas cooler at different test conditions by varying airflow rates, air temperatures, refrigerant inlet temperatures, and mass flow rates. All experimental results are tabulated in the present paper. It was found that the refrigerant mass flow rate is the dominant factor for the capacity of a CO 2 gas cooler, and a significant portion of the heat transfer in a CO 2 gas cooler was carried out in the heat exchanger module on the refrigerant inlet side. The temperature and pressure of CO 2 significantly affect the heat transfer and fluid flow characteristics due to the fact that some important thermal physical properties of CO 2 (such as specific heat, density, viscosity) are strongly dependent on its temperature and pressure. All experiments were successfully conducted with an energy balance of ±3%.

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V. Tudor, Michael M. Ohadi, F. H. R. Franca

An Experimental Investigation on Frost Control Using Dc and Ac Electric Fields on a Horizontal, Downward-Facing Plate (Journal Article)

Hvac&R Research, (2), pp. 203-213, 2003ISSN: 1078-9669.

The effects of DC and AC electric fields on frost formation on a horizontal downward facing flat plate was investigated in the present experimental study. Frost growth was influenced and controlled by electric fields generated by an insulated wire electrode. In order to quantify the effects of DC and AC electric fields on frost growth, both mass transfer and frost height were measured. Experiments were carried out in a test section where environmental conditions including air temperature, humidity, and flow rate were controlled. When an AC electric field (with frequency ranging from 135 to 1000 Hz) was applied at predefined time intervals, the frost mass reduction ranged from 12% to 36%, while the frost height reduction was between 22% and 33%. On the other hand, continuous application of high voltage and high frequency (above 100 Hz) electric fields was found to increase the mass of frost by as much as 44% while strongly reducing the frost height (up to 65%). An intermittent DC electric field showed interesting effects on frost formation, with a mass frost reduction of up to 10%.

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A. P. Rupani, M. Molki, Michael M. Ohadi, F. H. R. França

Enhanced Flow Boiling of R-134a in a Minichannel Plate Evaporator (Journal Article)

Journal of Enhanced Heat Transfer, 10 (1), pp. 1-7, 2003.

In this article, we report on heat transfer and pressure drop coefficients for flow boiling of R-134a in a minichannel formed by two parallel aluminum plates. The channel walls are augmented by means of round beads. The heat transfer coefficients and pressure drops were evaluated for mass fluxes ranging from 40 to 190 kg/m 2-s and inlet-exit quality bands of 0.25-0.50,0.50-0.75, and 0.75-0.90. The minichannel evaporator design was evaluated by comparing its performance with a similar plate with a different number of round beads. These results were also compared with available correlations for a better understanding of the heat transfer mechanism in the minichannel. Over the range of parameters tested, an enhancement of two- to sixfold in the heat transfer coefficient was obtained, compared to smooth tubes with the same nominal area. From the comparison and study of correlations, a simple correlation was proposed that agrees with the experimental data within 20%. © 2003 by Begell House, Inc.

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S. Moghaddam, M. Rada, Amir H. Shooshtari, Michael M. Ohadi, Y. Joshi

Evaluation of Analytical Models for Thermal Analysis and Design of Electronic Packages (Journal Article)

Microelectronics Journal, 34 (3), pp. 223-230, 2003.

The objective of this study is to evaluate the use of several analytical compact heat transfer models for thermal design, optimization, and performance evaluation in electronic packaging. A model for heat spreading in orthotropic materials is developed. The developed model is used in conjunction with the other available heat transfer models in a resistance network for calculation of heat transfer rate and junction temperatures in a multi-chip module (MCM). Refrigeration cooled MCM of an IBM server is used to illustrate the methodology. Results of the analytical model and resistance network analysis are compared with a numerical solution. Capability of the analytical model in predicting the thermal field is discussed and effectiveness of using the analytical models in thermal design and optimization of electronic packages is demonstrated. © 2003 Elsevier Science Ltd. All rights reserved.

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A. Gidwani, M. Molki, Michael M. Ohadi

Ehd-Enhanced Condensation of Alternative Refrigerants in Smooth and Corrugated Tubes (Journal Article)

HVAC and R Research, (3), pp. 219-237, 2002ISSN: 1078-9669.

An experimental study of EHD-enhanced in-tube condensation of alternative refrigerants is presented. The refrigerants tested were the single-component refrigerant R-134a, the zeotropic mixture R-407c, and the near-azeotrope R-404a. Tests for R-404a and R-407c were performed in smooth and corrugated tubes, whereas R-134a tests were performed only in corrugated tubes, with smooth tube data extracted from Singh (1995). The tests were performed with internally mounted cylindrical electrodes. It was found that, in general, all three refrigerants respond remarkably well to the EHD enhancement. The heat transfer performance of near-azeotrope R-404a is enhanced 18.8-fold in the smooth tube at the highest applied voltage of 18 kV, with a corresponding pressure drop penalty of 11.8-fold, and the maximum enhancement inside corrugated tube is 5.8-fold for the range of conditions tested in this study. R-134a has a maximum heat transfer enhancement of 8.3-fold, with a corresponding pressure drop penalty of 20.8-fold at an applied voltage of 18 kV inside the corrugated tube. Among the three refrigerants tested, R-407c shows the lowest heat transfer enhancement, with a 3.9-fold maximum enhancement at an applied EHD voltage of 18 kV inside the smooth tube, and a maximum enhancement of 2.9-fold inside the corrugated tube.

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J. Darabi, M. Rada, Michael M. Ohadi, J. Lawler

Design, Fabrication, and Testing of an Electrohydrodynamic Ion-Drag Micropump (Journal Article)

Journal of Microelectromechanical Systems, 11 (6), pp. 684-690, 2002.

This paper presents the design, fabrication, and testing of a novel electrohydrodynamic (EHD) ion-drag micropump. In order to maximize the electrical field gradients that are responsible for EHD pumping, we incorporated three-dimensional (3-D) triangular bumps of solder as part of the EHD electrodes. To form these humps, Niobium was sputter-deposited onto a ceramic substrate, coated with photoresist, optically exposed and etched using a reactive ion etcher to define the electrode pattern. The substrate was then “dipped” into a molten solder pool. Since the solder adheres only to the metallic film, bumps of solder form on the electrodes, giving the electrodes a significant 3-D character. The overall dimensions of the micropump are 19 mm × 32 mm × 1.05 mm. Four different designs were fabricated and tested. Static pressure tests were performed with a 3M Thermal Fluid (HFE-7100) as the working fluid and the optimum design was identified. The results with the thermal fluid were highly promising and indicated a pumping head of up to 700 Pa at an applied voltage of 300 V. The experimental results for the four different designs show that the presence of the 3-D bump structures significantly improves the pumping performance. Also, a much better pumping performance was obtained with the micropump in which the emitter had a saw-tooth shape.

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Michael M. Ohadi, S. G. Buckley

High Temperature Heat Exchangers and Microscale Combustion Systems: Applications to Thermal System Miniaturization (Journal Article)

Experimental Thermal and Fluid Science, 25 (5), pp. 207-217, 2001ISSN: 0894-1777.

The objective of traditional research and development on heat exchangers (HEs) has been to improve the performance and/or reduce the size and cost of the HE. Traditional research in power conversion has focused primarily on efficiency issues. However, rapidly developing applications in high temperature power and propulsion, pollution control/heat recovery, and high density power electronics has introduced new opportunities and challenges in developing cost-effective high performance, high temperature heat exchangers (HTHE) and microscale power systems. In this article the focus is placed on HTHEs for power/propulsion and thermal incineration/heat recovery applications, and on enabling technologies for microscale combustion systems. First a brief review of the growing need for HTHEs and microscale combustors and the state-of-the-art materials and fabrication technologies is presented. Next, various heat transfer augmentation techniques and their potential applicability to performance enhancement of HTHEs are discussed. Selected results of a case study involving a carbon fiber HE enhanced with an active heat transfer augmentation technique are presented. Issues associated with microscale combustion systems are presented, and technology enabling their development, namely, catalytic combustion and electrohydrodynamic (EHD) reaction rate augmentation techniques are discussed. © 2001 Elsevier Science Inc. All rights reserved.

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J. Darabi, Michael M. Ohadi, D. DeVoe

An Electrohydrodynamic Polarization Micropump for Electronic Cooling (Journal Article)

Journal of Microelectromechanical Systems, 10 (1), pp. 98-106, 2001ISSN: 1057-7157.

This paper presents the design, fabrication, and characterization of an innovative microcooling device for microelectronics applications. The device incorporates an active evaporative cooling surface, a polarization micropump, and temperature sensors into a single chip. The micropump provides the required pumping action to bring the working fluid to the evaporating surface, allowing the effective heat transfer coefficient through a thin-film evaporation/boiling process. The device is based on VLSI microfabrication technology, allowing the electrohydrodynamic (EHD) electrodes to be integrated directly onto the cooling surface. Since the EHD electrodes are fabricated using the same technology as the electronic systems themselves, the proposed microelectronic cooling system in the form of an integrated microchip is very suitable for mass production. The prototype devices demonstrated a maximum cooling capacity of 65 W/cm2 with a corresponding pumping head of 250 Pa. The results of this investigation will assist in the development of future microcooling devices capable of operating at high power levels.

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L. Tang, Michael M. Ohadi, A. T. Johnson

Flow Condensation in Smooth and Micro-Fin Tubes with HCFC-22, HFC-134a and HFC-410 Refrigerants. Part II: Design Equations (Journal Article)

Journal of Enhanced Heat Transfer, (5), pp. 311-325, 2000.

An empirical study of single-phase convection and flow condensation heat transfer in horizontal tubes was conducted. Three refrigerants (HCFC-22, HFC-134a and HFC-410A), a smooth tube, and three micro-fin tubes (axial, helical and crosshatch enhancement) were examined. Commonly cited correlations were evaluated, utilizing the experimental data obtained in Part I of this study. Although these correlations had fairly good agreement with HCFC-22 and HFC-134 results, all of them failed to predict HFC-410A performance. A modified Shah equation was developed for smooth tube annular flow condensation, which overcomes the shortcomings of the existing correlations. Furthermore, design equations were developed for single-phase heat transfer and two-phase condensation with all three micro-fin tubes investigated, and covering all three refrigerants tested.

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M. Salehi, M. Molki, Michael M. Ohadi

Effect of Electric Field on Flow Pattern in Flow Boiling of R-134a – a Visualization Study (Journal Article)

American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD, 365 pp. 127-136, 2000.

An experimental study was performed to qualitatively investigate the effect of an applied electric field on two-phase flow patterns in convective flow boiling of R-134a inside a channel. The experiments were performed in a horizontal channel with 3 mm hydraulic diameter. Inlet vapor qualities of 5%, 40%, and 80% at a flow Reynolds number of 1000 and heat flux of 20 kW/m 2 are reported here. The flow regimes were analyzed by a high-speed digital camera. The camera could provide a frames speed of up to 3025 frames/s. The electric field was maintained between the upper and lower walls of the channel. The current visualization study suggests some definitive characteristics of the flow regime and the mechanism behind the EHD-enhanced convective boiling in a narrowly-spaced channel. It is found that at the lower range of applied potential, the flow patterns change from stratified wavy flow to a plug flow type with large slugs spanning the height of the channel. At the higher range of applied potential, a new flow regime took place. This flow pattern was associated with the cross motion of the liquid inside the channel. One reason for heat transfer enhancement can be attributed to the strong heat/momentum transfer between the vapor slugs and liquid cross motions.

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D. A. Nelson, S. Zia, R. L. Whipple, Michael M. Ohadi

Corona Discharge Effects on Heat Transfer and Pressure Drop in Tube Flows (Journal Article)

Journal of Enhanced Heat Transfer, (2), pp. 81-95, 2000ISSN: 1065-5131.

This work presents and discusses the results of a series of experiments investigating effects from corona discharge in air on the heat transfer rate and on the pressure drop in tube flows. Two electrode geometries were studied: a single wire electrode, concentric with the grounded tube wall and dual equipotential wire electrodes which were offset 0.4 cm from center in the horizontal plane. Both positive and negative discharge were examined for the single-wire geometry, at Reynolds numbers in the range 1,000≤Re D≤20,000. The dual-wire geometry was studied using positive polarity discharge only, over the range Re D = 1,000 to Re D = 10,000. Heat transfer rates were determined at electrode potentials from 6.00 kV (DC) to 7.75 kV (DC), depending on polarity and electrode configuration. Baseline data were also obtained with the electrode(s) at ground potential. Results demonstrate increases in the Nusselt number of more than two hundred per cent over the values obtained in the absence of discharge. Relative increases in the friction coefficients were generally comparable to the corresponding Nusselt number enhancement. The extent of the increase in either quantity was highly dependent on discharge current and on the Reynolds number. The relative enhancements of both Nusselt number and friction loss coefficient were generally reduced at higher Reynolds numbers (Re D≥5000). However, the fall-off of enhancement with Reynolds number was less pronounced in the offset, dual-electrode geometry. Results suggest the enhancement mechanism may significantly depend on the electrode geometry, independent of the geometry effects on discharge current. The observed trends are discussed in the context of current theory.

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M. Molki, Michael M. Ohadi, B. Baumgarten, M. Hasegawa, A. Yabe

Heat Transfer Enhancement of Airflow in a Channel Using Corona Discharge (Journal Article)

Journal of Enhanced Heat Transfer, (6), pp. 411-425, 2000ISSN: 1065-5131.

A numerical and experimental investigation was performed to study the effect of corona discharge on the flow field and heat transfer enhancement of airflow in a channel. The electric field was applied via a charged electrode situated at the centerline along the channel axis. The numerical approach was based on the Large-Eddy Simulation turbulence model to investigate the potential turbulence generated by the electric field and was applied to the fully-developed region. The experiments were performed in an earlier study and represent the data available to compare with the present computations. Thermal boundary condition was the uniform wall heat flux. In both numerical and experimental approaches, Reynolds number ranged from 500 to 2000, corona current 0.059 to 2.420 mA/m, and applied voltage -5.655 to – 6.900 kV. The numerical results revealed the secondary flows in the cross-section of the channel. This corona-induced secondary flow was the main mechanism behind the enhanced heat transfer coefficients in both fully-developed and developing regions of the channel. The maximum heat transfer enhancement in the fully-developed flow was Nu/Nu0 = 3.4 for Re = 500 -2000, while that in the developing flow was much smaller and ranged from 1.56 to 2.01. © 2000 OPA (Overseas Publishers Association) N.V.

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B. Mo, Michael M. Ohadi, Sergeui V. Dessiatoun, K. R. Wrenn

Capillary Pumped-Loop Thermal Performance Improvement with Electrohydrodynamic Technique (Journal Article)

Journal of Thermophysics and Heat Transfer, 14 (1), pp. 103-108, 2000ISSN: 0887-8722.

The capillary pumped loop (CPL) is a state-of-the-art technology for cooling spacecraft and telecommunication devices. It is a two-phase heat-transport device in which the driving force is provided by the capillary action of the wick material in the evaporator. Compared to the widely used heat pipes, it provides a higher heat-transport capacity, more flexibility of installation, and greater heat-transport distance because of wickless transport lines and the absence of liquid and vapor counterflowing. The major disadvantages of the CPL are long and complicated startup procedures and the possibility of deprime at high heat input and large load variations. This paper investigates the liquid-vapor separation and thermal management with the electrohydrodynamic (EHD) technique for an EHD-assisted CPL using R-134a as the working fluid. An experimental investigation, along with a mechanism analysis, was employed to evaluate the potential of the EHD technique for thermal performance improvement of CPL systems. Experimental results showed that enhancements, up to three times, could be obtained in heat-transfer coefficients by applying an electric field at different heat load levels. The depriming conditions of a capillary pump can also be prevented with the EHD technique.

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J. Darabi, Michael M. Ohadi, Sergeui V. Dessiatoun

Compound Augmentation of Pool Boiling on Three Selected Commercial Tubes (Journal Article)

Journal of Enhanced Heat Transfer, (5), pp. 347-360, 2000.

The capillary pumped loop (CPL) is a state-of-the-art technology for cooling spacecraft and telecommunication devices. It is a two-phase heat-transport device in which the driving force is provided by the capillary action of the wick material in the evaporator. Compared to the widely used heat pipes, it provides a higher heat-transport capacity, more flexibility of installation, and greater heat-transport distance because of wickless transport lines and the absence of liquid and vapor counterflowing. The major disadvantages of the CPL are long and complicated startup procedures and the possibility of deprime at high heat input and large load variations. This paper investigates the liquid-vapor separation and thermal management with the electrohydrodynamic (EHD) technique for an EHD-assisted CPL using R-134a as the working fluid. An experimental investigation, along with a mechanism analysis, was employed to evaluate the potential of the EHD technique for thermal performance improvement of CPL systems. Experimental results showed that enhancements, up to three times, could be obtained in heat-transfer coefficients by applying an electric field at different heat load levels. The depriming conditions of a capillary pump can also be prevented with the EHD technique.

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