[Show abstract][Hide abstract] ABSTRACT: Impingement cooling methods, such as spray cooling and jet impingement have demonstrated the capability of cooling high heat flux surfaces while maintaining a low thermal resistance. Most spray cooling and jet impingement experiments attempt to measure the average heat transfer coefficient, even though it is known that heat transfer coefficients are known to change as a function of distance from the impact zone. Secondly, most experiments are done on thick uniformly heated surfaces although most electronic devices are very thin (<0.2 mm) and generate heat very non- uniformly with very large peak heat fluxes (>1000 W/cm<sup>2</sup>) over very small areas (<0.25 mm<sup>2</sup>). In this study an accurate measurement of the uniformity of the spray cooling thermal solution was attained using an Intel supplied thermal test vehicle. The heater block is a thin silicon chip (<0.25 mm thick and 7 cm<sup>2</sup> in surface area) delivering a uniform heat flux to 70 W/cm<sup>2</sup>. The platform also has the ability to power large peak heat fluxes (>1000 W/cm<sup>2</sup>) over small areas (<0.2 5 mm<sup>2</sup>). Experiments using jet impingement with flat spray nozzles angled to the surface were conducted with water, methanol, and HFE-7000. The axial heat transfer coefficient variation was measured under uniform heat loading. Finally, the measurements are compared to modified models from the literature with good agreement.
[Show abstract][Hide abstract] ABSTRACT: Microprocessor performance can be significantly improved by lowering the junction temperature, especially down to the deep subambient levels. This provides the strong motivation for the current study, which focuses on the design and thermohydraulic performance evaluation of high heat flux evaporators suitable for interfacing the microprocessor chip with a cascaded R134 alpha/R508b vapor compression refrigeration system at -80 degrees C. Four compact evaporator designs are examined-a base line slit-flow structure with no microfeatures, straight microchannels, an inline pin fin array, and an alternating pin fin array-all fitting the same size envelope. Pressure drop and heat transfer measurements are reported and discussed to explain the performance of the various evaporator geometries for heat fluxes ranging between 20 W/cm(2) and 100 W/cm2.
[Show abstract][Hide abstract] ABSTRACT: It is well known that microprocessor performance can be improved by lowering the junction temperature. Two stage cascaded vapor compression refrigeration (VCR) is a mature, inexpensive, and reliable cooling technology that can offer chip temperatures down to ?? C. Recent studies have shown that for a power limited computer chip, there is a non-linear scaling effect that offers a 4.3X performance enhancement at ?? C. The heat transfer performance of a compact evaporator is often the bottleneck in sub-ambient heat removal. For this reason, the design of a deep sub-ambient compact evaporator is critical to the cooling system performance and has not been addressed in the literature. Four compact evaporator designs were investigated as feasible designs - a baseline case with no enhancement structures, micro channels, inline pin fin arrays, and alternating pin fin arrays. A parametric experimental investigation of four compact evaporator designs has been performed aiming at enhancing heat transfer. Each evaporator consists of oxygen free copper and has a footprint of 20 mm x 36 mm, with a total thickness of 3.1 mm. The micro channel evaporator contains 13 channels that are 400 um wide by 1.2 mm deep, and the pin fin evaporators contain approximately 80 pin fins that are 400 um wide by 1.2 mm tall with a pitch of 800 um. Two phase convective boiling of R508b refrigerant was investigated in each evaporator at flow rates of 50 - 70 g/min and saturation temperatures of ??to ??C. Pressure drop and local heat transfer measurements are reported and used to explain the performance of the various evaporator geometries. The results are compared to predictions from popular macro- and micro-channel heat transfer and pressure drop correlations. The challenges of implementing a two stage cascade VCR systems for microprocessor refrigeration are also discussed. M.S. Committee Chair: Yogendra Joshi; Committee Co-Chair: Andrei Fedorov; Committee Member: Paul Kohl