An experimental and theoretical study was investigated on heat transfer and fluid flow characteristics of a photovoltaic pSi cells array. The effect of three different type of collector geometries (Model A, B, and C) on thermal efficiency and pressure drop were considered. The theoretical model using a simple energy balance was well matched with experimental data within 11 %. Electrical generation is modeled by PVT in conjunction with an Organic Rankine Cycle. The results showed that the efficiency of model B boosts of up to 49.6% with the lowest pressure drop.

Over the last decade, rapid development of additive manufacturing techniques has allowed the fabrication of innovative designs which could not have been manufactured using conventional fabrication technologies. One field that can benefit from such technology is heat exchanger fabrication, as heat exchanger design has become more and more complex due to the demand for higher performance systems. One specific heat exchanger design that has shown significant performance enhancement potential over conventional designs and can greatly benefit from additive manufacturing technology is a manifold-microchannel heat exchanger. It is a design. that combines careful fluid distribution through appropriate manifolds with an enhanced heat transfer surface design to achieve specific thermohydraulics performance expectations. Additive manufacturing allows fins as thin as 150 in to be fabricated, which is an important enabler feature for the heat exchanger thermal performance. In addition, additive manufacturing allows the manifold and the microchannel sections to be fabricated as a single piece, which eliminates the need to fuse those sections together through a subsequent bonding process. As part of this work, we fabricated and experimentally tested a high-performance titanium alloy (Ti64) air-water heat exchanger that utilizes manifold-microchannel design. The heat exchanger was fabricated using direct metal laser sintering (DMLS) fabrication technique. The air-side implemented a manifold-microchannel design, while the water side used multiple rectangular channels in parallel. This was because the major thermal resistance occurs on the air side. The pressure drop and heat transfer performance of this heat exchanger were evaluated. The experimental results showed a noticeable performance reduction compared to the ones projected by numerical simulation due to an inaccuracy and low fidelity in printing of thin fin profile. However, despite this manufacturing inaccuracy, compared to a conventional wavy fin surface, 15% - 50% increase in heat transfer coefficient was possible for the same pressure drop value. Compared to a plain plate-fin surface, 95% - 110% increase in heat transfer coefficient was possible for the same pressure drop value. The air-side heat transfer coefficient in the range of 100 - 450 W/m(2)K was achievable using manifold-microchannel technology for air-side pressure drop of 90 - 1800Pa. Since metal based additive manufacturing is still in the developmental stage, it is anticipated that with further refinement of the manufacturing process in future designs, the fabrication accuracy can be improved.

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.

This paper provides analysis of air vs. liquid and two-phase flow cooling for a data center application. A new micro channel-based forced convection evaporation cooling is introduced, and its performance is compared against single phase and conventional phase change cooling systems. The technique offers substantially reduced thermal resistances with associated pumping power requirements significantly below that of conventional systems. It removes the need for compressors in a typical phase change cooling as it relies on a combination of forced convection boiling and thin film evaporation mechanism. Comparison analysis of the three techniques may provide additional incentives for adoption of energy efficient liquid cooling in next generation data centers. Issues remaining with large scale adoption of liquid and phase change cooling in data centers are also addressed in this paper.

This paper describes a multi-objective optimization of single-phase, laminar flow inside a single element of a manifold microchannel flat plate heat exchanger. Approximation assisted optimization was used for the optimization process. The process uses metamodeling in conjunction with Computational Fluid Dynamic (CFD) simulation as a method to minimize the number of function evaluations and thereby obtain substantial reductions in computational time. Two optimization objectives were considered: a) maximizing heat density rate per temperature difference Q/(VΔT) and minimizing pumping power density (P/V), and b) maximizing base heat transfer coefficient (h) and minimizing pumping power per base area (P/Abase). Water and air were used as working fluids to compare the optimum solutions of the two fluids with very distinctive thermo-physical properties. The study shows that both optimization objectives result in similar optimum points. The behaviors of the optimum solutions for water and air are also discussed in detail. Additionally, as a case study using the optimization results, it was demonstrated that for an array of microchannels with volume as low as 4,250 mm3 on one side, pumping power of 138 W and heat transfer rate of 56.7 kW can be achieved using water.

A model was developed to simulate the performance of a microgrooved surface undergoing steady thin film evaporation subject to a specified superheat on the groove wall. A theoretical thin film model was coupled with a meniscus curve model to accurately model the complete system. A numerical routine was successfully implemented to solve the governing non-linear differential equations of an evaporating thin film subject to a specified set of groove wall superheat and fluid/interface properties. The resulting thin film profile was used to correlate the heat transfer characteristics as a function of radius of curvature of the intrinsic meniscus. These correlations were then used by another numerical routine to solve for the meniscus curve profile as a function of groove geometry and fluid properties. The total heat, wetted length, heat transfer coefficient, and if desired, 3-D surface plot of the liquid bulk in the microgroove were then extracted from the results. The model results were then compared to the available experimental results. Results of the preliminary comparison with the experiments, as well as future planned tasks, are discussed in this paper.

An experimental investigation on characterization of copper-finned micro-grooved surfaces for effective evaporation heat transfer with applications to cooling of high flux electronics was conducted in the present study. Performance of the copper-finned microstructures were studied as a function of operating parametric values of fin density, fin height, fin length, and channel width over a surface which was rosin soldered to a 10 mm × 10 mm heating block (typical size of an electronic chip). The performance of the copper-finned microstructures versus a flat/smooth nichrome plate in HFE-7100 was significantly higher. Two experimental conditions were investigated. In the first set of experiments pool boiling over the groves was examined, where as in the second set of experiments the fluid was forced-fed into the grooves in a forced convection mode. It is shown that the forced fed mode yields higher heat transfer coefficients than the submerged/pool boiling mode. In general the micro-grooved surfaces performed at least three times better than the flat/smooth surface and preliminary results with the forced-fed evaporation experiments suggest that an order of magnitude heat transfer coefficients are possible when compared with a smooth surface.

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 heal-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.

The capillary pumped loop (CPL) is a state-of-the-art device for efficient cooling of electronic components used in spacecraft and telecommunications. CPL functions on a two-phase heat transport process in which a working fluid is driven by the pumping effect of the capillary action of a wick material imbedded in the evaporator, The wick structure is imbedded only in the CPL evaporator, and the rest of the loop is a simple wickless smooth tuber Therefore, compared to the widely used heat pipe, CPL provides a substantially higher heat transport capacity, more flexibility of installation, and a much greater distance of heat transport. The major disadvantages of the CPL, however, are the long and complicated start-up procedure, and the possibility of depriming at the high heat input and load variation. The main focus of this paper is on selection of potential working fluid candidates for use in Electrohydrodynamic (EHD)-enhanced CPLs. In this connection, a series of feasibility studies on various CPL working fluids have been performed and will be discussed. Among the working fluid candidates, propane and propylene are found to be the promising candidates, For appropriate electrode configuration, experimental results showed that the EHD pumping head of propane and propylene was high. Therefore, the two natural refrigerants, propane and propylene, are expected to improve the performance of EHD-enhanced CPLs.

This paper will present results of an experimental study in which the Electrohydrodynamic (EHD) technique was utilized to provide compound enhancement of pool boiling heat transfer on three commercially available tubes. The tubes tested were: a low-fin 19 fins per inch, a modified high-fin 52 fins per inch, and a mechanically formed re-entrant cavity type. All tubes tested had a length of 63.5 mm (2.5 ") and an outer diameter of 19 mm (3/4 "). Five different electrode designs were tested, each over a predetermined heat flux and saturation temperature. The enhanced boiling mechanism in each case and a comparison of results for the three tubes are described. It was found that for the electrodes and heat fluxes tested, the specially designed (modified) high-fin tube yielded the highest enhancement amongst the three, with a nearly three-fold increase in the heat transfer coefficient. In the worst case, the EHD power consumption was less than 0.4% of the total heat transfer rate in the test section.

The capillary pumped loop (CPL) is a state-of-the-art technique for cooling of spacecraft and telecommunication devices. It provides substantially higher cooling capacity than most heat pipes, more flexibility of installation, and much greater distance of heat transport because of the small diameter of wickless transport lines. Major disadvantages of the CPL are long and complicated startup procedures and the possibility of deprime at high heat input or load variation. The present work was an experimental study to characterize the start-up process for an electrohydrodynamically (EHD) assisted CPL system. Startup is achieved by establishing stable differential pressure and average temperature at the evaporator wall. When an electric field is applied to the evaporator wick, the liquid-vapor separation, the EHD pumping, and the instability-induced Maxwell stresses collectively contribute to reduce the startup time, as well as provide substantial improvement in CPL thermal performance. The experimental data in the present study show that at a power level of 10 W, the EHD can reduce the startup time by as much as 50% at an applied voltage of 10 kV. A similar trend is observed at power levels of 20 and 50 W.

In this paper, results of an experimental study on the EHD-assisted external condensation of R-134 over a plain tube will be presented and discussed. The experiments performed can be divided into two groups: (1) EHD-assisted external condensation on a vertical smooth tube and (2) EHD-assisted external condensation experiments on a horizontal smooth tube. In each case, experiments were conducted as a function of parametric values such as heat flux, electrode gaps and applied electric field potential. All of the experiments were conducted using a specially designed helical electrode and with R-134a as the working fluid.Experimental results demonstrate a remarkable potential in utilizing EHD to enhance external condensation heat transfer. It is concluded that the enhancement is driven by the effective removal of the condensate through EHD-induced liquid extraction and dispersion phenomena.

The two widely used experimental methods for pool boiling experiments are evaluated. Boiling heat transfer data employing either the water heating or the electric heating method obtained by various researchers are compared and discussed. The electric heating method is a constant heat pur process, while the water heating method represents a pseudo-exponential temperature profile for the heating water along the tube. It is concluded that the temperature profile along the tube is the major cause of the difference between results from the two methods. The parameters that shape the temperature profile and cause differences are discussed.

Heat transfer enhancement of falling-film evaporation on commercially available horizontal tubes using an applied electric field was studied experimentally. The tube surfaces tested included: smooth, 19 fins per inch (19 fpi) low-fin type, and Turbo BIII which is a state-of-the-art commercially available boiling tube. The nominal outside diameters of all the tubes were 19 mm. Experiments were performed with R-134a at a saturation pressure of 550 kPa. Effects of heat flux, film flow rate, applied electric field potential, and heat transfer surface on the heat transfer coefficient were investigated. In addition, the effect of Poloyl-ester oil on the heat transfer coefficients was also investigated. Experiments were conducted for oil concentrations ranging from 0.5 percent to 5 percent on a mass basis. Small concentrations of a poloyl-ester lubricant were found to improve the heat transfer performance, while large concentrations reduced the heat transfer coefficient. [S0022-1481(00)00702-7]

The capillary pump loop (CPL) is the current state-of-the-art space cooling system, it provides higher cooling capacity than most heat pipes, more installation flexibility, and much greater distance of heat transport due to the small diameter of wickless transport lines. Major disadvantages of the CPL include long and complicated startup procedures and the possibility of depriming at high heat input and load variation. The presented work was an experimental study to characterize the startup process for an EHD-assisted CPL system. Startup is achieved by an almost stable differential pressure and average temperature at the evaporator wall. When the electric field is applied, it interacts with the vapor/liquid distribution inside the core and the wick. It also provides an additional pumping effect of liquid to the evaporator surface. As a result, less time is needed to build up the meniscus. Furthermore, the instability-induced EHD pumping at liquid-vapor interface pushes the liquid-vapor interface near the evaporator wall to enhance the phase-change. These EHD-enhanced mechanisms collaborate to reduce the required duration at different regimes and hence realize the EHD-reducing startup time for a CPL system. Experimental data showed that about 50% startup time, reduction was attainable.

The capillary pump loop (CPL) is a state-of-the-art technique for cooling spacecraft and telecommunication devices. It is a two-phase heat transport device in which the pumping action is provided by the capillary action of the wick material in the evaporator. Compared to the widely used heat pipes, it provides a substantially higher cooling capacity, more flexibility of installation, and a much greater distance of heat transport due to the small diameter of wickless transport lines. The major disadvantages of the CPL are long and complicated start-up procedures and the possibility of deprime at high heat input and load variation. This paper investigates the liquid-vapor separation and thermal management with the EHD technique for the porous material inside an R-134a CPL system. An experimental investigation along with a mechanism analysis was employed to evaluate the potential of the EHD technique on CPL thermal performance improvement. Experimental results showed that up to three times heat transfer coefficient enhancement can be obtained by applying an electric field at different power levels.

In-tube evaporation heat transfer coefficients of ammonia and CO2 were determined experimentally in a smooth tube. The test section was a smooth tube with inner diameter of 5.44mm (0.21 in), a wall thickness of 1.27 mm (1/20 in) and length of 1.78 m (70 in). AN experiments were conducted at an evaporative temperature of 15 degreesC. The corresponding saturation pressure for ammonia was 737.7 kPa and for CO2 was 5000 kPa The experiments were conducted as a junction of mass flow rare (which ranged from 130 to 190 kg/(m(2)s) for ammonia and 140 to 440 kg/(m(2)s) for CO2), and heat flux (which ranged from 1000 w/m(2) to 50000 w/m(3) for ammonia and 2250 w/m(2) to 40000 w/m(2) for CO2). It was found that at the same mass flow rate and heat flux, the hear transfer coefficient of ammonia was 10 to 15% higher than that of CO2, and higher than R22 and R134 alpha. However, in practice for a given system capacity, one can expect that the heat transfer coefficient of CO2 will be comparable to that of ammonia due to higher mass flux of CO2.

Earlier works on frost formation characteristics and selected predictive models are reviewed The selected analytical models and empirical correlations for predicting frost density and thickness represent the simplified version of the rather extensive research in this area. In the light of recent studies demonstrating that an electric field can substantially influence the frost structure, a comprehensive review of recent work on the effect of electrohydrodynamics (EHD) on frost formation is also discussed in this paper.

The capillary pumped loop (CPL) is a passively pumped two-phase heat transport device that has demonstrated performance capabilities up to an order of magnitude greater than heat pipes, which are the current state-of-art. CPL technology has been developed to a near ready state for use as a thermal control device for advanced spacecraft systems. To further improve CPL performance, on a system level, the pumping head generated within the wick material must be enhanced. Utilizing the effect of a phenomenon known as liquid extraction (or EHD pumping), the Electrohydrodynamic (EHD) technique can effectively improve the liquid pumping capacity in a CPL system. EHD uses an electric field that can collect, guide, and pump liquid to the evaporating surface. This paper presents an experimental investigation of the feasibility of using EHD technology for improving CPL performances. The experimental study included EHD-enhanced pumping across a felt material that simulated a CPL wick. The results show more than 80% increase in heat transfer coefficients at the evaporator wick, with R-123 as the working fluid. Calculations, using the experimental data, demonstrate substantial increase in the corresponding additional pressure head developed across the wick through EHD effects.