Comparing spray and microchannel as evaporators of a compact refrigeration system for electronics cooling

This study presents a unique experimental comparison of a multi-jet spray cooling unit and a parallel microchannel evaporator, both integrated into the same compact R-1234yf vapor compression refrigeration system with an oil-free linear compressor. Previous studies have not compared these technologies side-by-side under identical conditions. This work fills that gap by testing both configurations in the same facility, with matched heat transfer surface areas and calibrated flow restrictions to ensure an unbiased comparison. The spray unit merges evaporator and expansion functions, delivering subcooled refrigerant through oblique orifices to form impinging two-phase jets on a heated surface. The microchannel system employs a needle valve and a copper evaporator with 34 channels (260.7μm hydraulic diameter), with the valve configured to match the spray unit's flow coefficient. Tests covered refrigerant charges from 29.9 to 143.6 g and thermal loads from 25 to 250 W. Both systems maintained surface temperatures below 70 °C. The spray unit outperformed at high heat fluxes, achieving higher critical heat flux and lower surface temperatures, while the microchannel unit was more effective at low to moderate loads. Maximum heat transfer coefficients on the order of 43 kW/(m²K) were achieved with spray cooling, versus 37 kW/(m²K) for the microchannel system, though at significantly lower wall heat flux. Thermal performance was benchmarked against classical pool boiling correlations, which serve as reference standards for immersion cooling. Both systems exceeded these baselines, with the spray unit maintaining higher heat transfer coefficients up to critical heat flux levels.