Mechanical Engineering

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    Experimental Study of Hybrid Cooled Heat Exchanger
    (2011) Tsao, Han-Chuan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A test system for a hybrid cooled heat exchanger was designed, and the test facility was constructed based on ASHRAE Standard 41.2-1987. A conventional air-cooled tube-fin heat exchanger was tested with and without application of wetting water. The baseline tests were conducted to investigate the heat exchanger performance improvement by applying evaporative cooling technology. The heat exchanger capacity and air side pressure drop were measured while varying operating conditions and heat exchanger inclination angles. The results show the heat exchanger capacity increased by 170% with application of the hybrid cooling technology, but the air side pressure drop increased by 130%. Additional research investigating air fan power was also conducted, which increased 120% from the dry condition to the hybrid cooled condition. In summary, the potential for improving the heat exchanger performance by applying hybrid cooling is shown in this research.
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    PERFORMANCE OF A MICROCHANNEL-THERMOELECTRIC POWER GENERATOR WITH ALUMINA-IN-WATER NANOFLUIDS AS COOLANTS
    (2010) Ahuja, Herwin Singh; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In the past two decades, the rapid advancement of military aircraft in terms of performance and power consumption in order to accomplish evermore demanding missions has introduced new challenges, namely, having to conserve of non-renewable petroleum, minimize carbon emissions, and accomplish more mission per unit energy. This thesis describes the work done to evaluate the performance of a renewable-energy device termed the microchannel-thermoelectric power generator (MC-TEPG), which uses alumina-in-water nanofluids as coolants, that is intended to replace or supplement current non-renewable power supplies such as battery packs in order to contribute to overcoming the abovementioned challenges. The MC-TEPG recovers waste heat internally generated by motors of military aircraft and converts it to usable electric power via the Seebeck effect. This thesis studies nanofluid flow and heat transfer in the MC-TEPG microchannels, and thermoelectric power generation under varying conditions. Current results show MC-TEPG feasibility and suggest future promise.
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    On-Chip Thermoelectric Cooling of Semiconductor Hot Spot
    (2007-08-28) Wang, Peng; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Moore's Law progression in semiconductor technology, including shrinking feature size, increasing transistor density, and faster circuit speeds, is leading to increasing total power dissipations and heat fluxes on silicon chip. Moreover, in recent years, increasing performance has resulted in greater non-uniformity of on-chip power dissipation, creating microscale hot spots that can significantly degrade the processor performance and reliability. Application of conventional thermal packaging technology, developed to provide uniform chip cooling, to such chip designs results in lower allowable chip power dissipation or overcooling of large areas of the chip. Consequently, novel thermoelectric cooler (TEC) has been proposed recently for on-chip hot spot cooling because of its unique ability to selectively cool down the localized microscale hot spot. In this dissertation the potential application of thermoelectric coolers to suppress on-chip hotspots is explored using analytical modeling, numerical simulation, and experimental techniques. Single-crystal silicon is proposed as a potential thermoelectric material due to its high Seebeck coefficient and its thermoelectric cooling performance is investigated using device-level analytical modeling. Integrated on silicon chip as an integral, on-chip thermoelectric cooler, silicon microcooler can effectively reduce the hotspot temperature and its effectiveness is investigated using analytical modeling and numerical simulation, and found to be dependent of doping concentration in silicon, electric contact resistance, hotspot size, hotspot heat flux, die thickness and microcooler size. The other novel on-chip hotspot cooling solution developed in this dissertation is to use a mini-contact enhanced TEC, where the mini-contact pad connects the silicon chip and the TEC to concentrate the thermoelectric cooling power onto a spot of top surface of the silicon chip and therefore significantly improve the hotspot cooling performance. Numerical simulation shows hotspot cooling is determined by thermal contact resistance, thermoelectric element thickness, chip thickness, etc. Package-level experiment demonstrates that spot cooling performance of such mini-contact enhanced TEC can be improved by about 100%.