Review on flow boiling of refrigerants R236fa and R245fa in mini and micro channels

Flow boiling in mini and micro channels is considered to be one of the most efficient cooling solutions for mobile devices and electronic components. Cooling systems relying on flow boiling should be able to work efficiently in a wide range of operating conditions including saturation temperature. Employing refrigerants characterized by low saturation pressures assure lightweight structure of the cooling system even at high operating temperatures. Therefore, this article focuses on two low-pressure refrigerants R236fa and R245fa (refrigerants which are characterized by low saturation pressures corresponding to high saturation temperatures) with special emphasis on the influence of saturation temperature on flow boiling characteristics. It presents a detailed review on the most recognized heat transfer models during flow boiling and an experimental database on R236fa and R245fa covering channel hydraulic diameters ranging from 0.1 to 3 mm, mass fluxes from 42 to 2500 kg/m2/s, and wall heat fluxes varying between 6.5 and 422.7 kW/m2. The covered vapor qualities range from −0.09 to 1.14 (which translates into subcooled, saturated and superheated flows) and reference saturation temperatures vary between 14 and 120 °C. Subcooling at the heat exchanger inlet changes from 0 to 19 K. The amassed experimental data are compared with the results obtained from 24 theoretical models on flow boiling. The models follow additive, asymptotic, Nusselt-type and phenomenological approaches. The accuracy of the models is assessed on the basis of Mean Absolute Percentage Error MAPE and standard deviation σSD. The additive models of Saitoh et al. with MAPE=27.4% and σSD=43.8%, Chen with MAPE=28.6% and σSD=35.8% and an Nusselt-type correlation of Sun and Mishima with MAPE=28.4% and σSD=43.4% exhibit the best accuracy. The amassed database allowed determination of the range of experimental conditions that need further scientific investigation. Review on heat transfer models together with results of comparison between the models and experimental data allowed identification of the most problematic aspects of modeling flow boiling in mini and micro channels.