Spray Cooling With HFC-134a and HFO-1234yf for Thermal Management of Automotive Power Electronics

This study aimed to experimentally investigate the spray cooling characteristics for active two-phase cooling of automotive power electronics. Tests were conducted on a small-scale, closed loop spray cooling system featuring a pressure atomized spray nozzle. Two types of refrigerants, HFC-134a (R-134a) and HFO-1234yf, were selected as the working fluids. The test section (heater), made out of oxygen-free copper, had a 1-cm² plain, smooth surface prepared following a consistent procedure, and served as a baseline case. Matching size thick film resistors, attached onto the copper heaters, generated heat and simulated high heat flux power electronics devices. The experiments were performed with saturated working fluids at room temperature level (22°C) by controlling the heat flux in increasing steps, and recording the corresponding steady-state temperatures to obtain cooling curves. Performance comparisons were made based on heat transfer coefficient (HTC) and critical heat flux (CHF) values. Effects of spray characteristics and liquid flow rates on the cooling performance were determined with three types of commercially available nozzles that generate full-cone sprays with fine droplets, and with varying flow rates between 1.6 to 5.4 ml/cm².s. The experimental data showed that HFC-134a provided better performance compared to HFO-1234yf, in terms of HTC and CHF, which is believed to be dictated by the thermophysical properties that affect both the spray characteristics and heat acquisition ability. Overall, this study provided a framework for spray cooling performance with the current and next-generation refrigerants aimed for advanced thermal management of automotive power electronics.