The forced circulation cooling system with rectangular mini-channels for the inverter of electric vehicles

In this study, the flow boiling in an evaporator with narrow rectangular mini-channels has been investigated experimentally to develop a forced circulation cooling system with high performance. The test evaporator has 11 parallel channels (channel width × depth × length: 1.0 mm × 3.5 mm × 34 mm), and new refrigerant, R1234ze(E), is used as working fluid. The experiment is conducted in the following ranges: the input heat flux is controlled from 150 to 2000 kW/m² depending on thermodynamic equilibrium quality at the outlet of the evaporator, the mass velocity of working fluid ranges from 72 to 288 kg/(m²· s), the degree of subcooling of the working fluid are 5, 10 and 15 K, and the pressure of the working fluid is kept 767 kPa, at which the saturation temperature is 40 °C. The pressure drop in the evaporator is calculated using Jige-Koyama correlation, and it agrees with the experimental one with less than 30 % of deviation. Then, the working fluid temperature, thermodynamic equilibrium quality etc. are estimated based on the calculated pressure drop. As the effective heat flux increases, the thermal resistances of boiling and system are dramatically decreased up to 800 kW/m² and then kept constants before the dryout. And the temperature of heating block surface is kept below 90 °C up to about 1600 kW/m² of the effective heat flux in the mass velocity 288 kg/(m²· s). This result indicates the possibility of the forced circulation cooling system applying the flow boiling of R1234ze(E) in narrow rectangular channels.