Comparing the performances of R-134a, R-1234yf and R-600a in a compact spray refrigeration system for electronics cooling

The multi-jet spray cooling unit, integrated with a compact, linear, oil-free R-134a compressor introduced in previous work, is now experimentally evaluated with R-1234yf and R-600a as drop-in replacement alternatives. This unit combines the functions of the evaporator and the expansion device into a single device, allowing the subcooled refrigerant to expand through an array of oblique orifices and form a spray that directly impinges on the heated surface. The experimental analysis quantifies the cooling system thermodynamic performance, including compressor power and coefficient of performance, as well as steady-state heat transfer parameters such as heat transfer coefficient, surface temperature, and critical heat flux. The evaluation considers the influence of refrigerant charge, steady-state applied thermal load (cooling capacity), and refrigerant type. To ensure an unbiased comparison, the refrigerant charge is adjusted so that all refrigerants maintain the same evaporating temperature at the lowest thermal load of 25 W. Experimental tests are conducted across a wide range of evaporation temperatures (4.5 to 20.0 °C). The results indicate a trade-off between heat transfer performance and thermodynamic performance of the refrigeration system when selecting a refrigerant alternative. R-600a required the lowest refrigerant charge to achieve the reference evaporation temperature and exhibited the lowest refrigerant mass flow rate under all tested conditions. However, the heat transfer performance of R-600a is severely penalized compared to R-134a and R-1234yf, with approximately a 40% reduction in the maximum heat transfer coefficient. The maximum values of the heat transfer coefficient for R-134a, R-1234yf, and R-600a are 42.9, 43.4, and 25.8 kW/m${}^2$ K, respectively.