Heat transport characteristics of a flat-evaporator loop heat pipe for use in a thermal management system of electric vehicles

In this work, a flat-evaporator loop heat pipe (FELHP), with a 100 cm2 rectangular thermal interface and an overall length of approximately 2 m was devised for the thermal management of electric vehicles. A simple rectangular-cuboid evaporator was devised using a novel stepped-flat wick structure, and the heat transport characteristics were systematically investigated in terms of the heat load, relative height of the evaporator to the condenser (i.e., position), heat sink temperature, and working fluid. In particular, water-ethanol binary mixtures with different compositions were tested to find an optimum low-freezing-temperature working fluid for use in thermal management system of the electric vehicles driving outdoors. Over the tested heat load range, a significant position dependence of the operating temperature was observed with a thermal resistance change from 0.21 K/W +/- 0.02 K/W to 0.14 K/W +/- 0.01 K/W for a 0.4 m change in the position. In contrast, the operating temperature of the FELHP was insensitive to changes in the heat sink temperature. A binary mixture containing 20% water and 80% ethanol (by volume) resulted in comparable heat transport characteristics to those of pure water but had a much lower freezing temperature (e.g., a thermal resistance of 0.25 K/W +/- 0.03 K/W at 450 W and a freezing temperature of approximately -59 degrees C), demonstrating that this was an optimum composition of the water-ethanol mixture working fluid of the FELHP.