Influence of saturation temperature and heat flux on pool boiling of R245fa

Low-pressure refrigerants such as R245fa allow compact design of IT cooling systems intended for harsh environments. Nucleate boiling of R245fa on an electrically heated flat horizontal surface was experimentally studied. Heat fluxes qw were 30–65 kW/m2 and saturation temperatures 40–70 ∘C. Results show that the heat-transfer coefficient HTC increases with both heat flux and temperature, and that higher heat fluxes/temperatures lessen the HTC dependence on temperature/heat flux. For instance, increasing temperature from 40 to 70 ∘C translates into change of the exponent n in the HTC∝qwn from 0.60 to 0.50. Therefore, pool-boiling correlations employing n=const do not accurately reflect the HTC∝qwn dependence. Comparing the experimental data with nucleate boiling correlations showed that the models of Kruzhilin and Mostinski were the most accurate with the Mean Absolute Percentage Error MAPE equal to 16.6% and 32.9%, respectively. The data were then used to optimize the model of Rohsenow and increase its accuracy. This model was chosen for optimization as it is the only model among the analyzed in this paper which incorporates a description of both surface roughness and contact angle. These properties are addressed using an empirical parameter Csf. Optimizing this parameter allowed the characterization of the interface between R245fa and heating surface used in this study. The resulting MAPE is 5.9%.