International Journal of Heat and Mass Transfer, Vol.118, 1276-1283, 2018
High power density air-cooled microchannel heat exchanger
We present single-phase heat transfer in a compact cross-flow microchannel heat exchanger, with air flowing through the heat exchanger to remove heat from a closed-loop flow of refrigerant R245fa. The 1 cm(3) heat exchanger was monolithically fabricated from a block of copper alloy using micro electrical-discharge machining. Air carrying channels of diameter 520 gm were oriented in cross-flow to the refrigerant-carrying channels of size 2.0 x 0.5 mm(2). High-speed air flowed with Reynolds number between 1.2 x 10(4) and 2.05 x 10(4), which corresponded to air speeds between 20 and 100 m/s, while refrigerant flowed at Reynolds number between 1000 and 2300. Using an equivalent fin model and finite element simulations, we predicted the heat exchanger performance and used the simulations to interpret the measured behavior. Temperature, pressure, and flow rates were measured over a variety of operating conditions to determine heat transfer rate, j-factor, and friction factor. We observed a maximum power density of 60 W/cm(3) when the air inlet temperature was 27 degrees C and the refrigerant inlet temperature was 80 degrees C. The high speed of air flow caused large friction on the air side, resulting in goodness factor j/f near 0.5. This work demonstrates that high power density can be achieved in miniature heat exchangers, and that micromachined metal devices can enable this performance. The results could be broadly applied to other types of microchannel devices. (C) 2017 Elsevier Ltd. All rights reserved.