Flow boiling of hydrocarbons and their zeotropic binary mixtures under pre- and post-dryout conditions

Abstract

This paper concerns an experimental evaluation of the flow boiling heat transfer coefficient of R290, R600a and R1270 focusing on the replacement of R134a in commercial, and residential air-conditioning and refrigeration applications. Moreover, new zeotropic binary mixtures (R600a/R290, 70/30% molar fraction, and R600a/R1270, 75/25% molar fraction) with similar range of temperature applications than R134a are proposed and evaluated. Experiments are performed inside a conduit with a diameter of 9.43 mm for mass velocity ranging from 50 to 300 kg/(m2 s), local heat flux from 10 to 30 kW/m2, vapor quality from 0 to 1, and saturation temperature of 5 °C. In general, higher heat transfer coefficients are obtained for pure hydrocarbons, followed by the mixtures, therefore, showing a better heat transfer performance than R134a. Similar heat transfer coefficients are noticed among pure hydrocarbons, with R600a presenting slightly higher values under certain conditions. A behavior typical of heat transfer dominated by convective effects is noticed for the hydrocarbons with increasing quality at mass velocities higher than 100 kg/(m2 s), while for R134a such a behavior is verified only for mass velocities higher than 200 kg/(m2 s). Nucleate boiling dominated heat transfer is noticed only for mass velocities lower than 100 kg/(m2 s) and 200 kg/(m2 s) for hydrocarbons and R134a, respectively. In general, the heat transfer coefficient increases with mass velocity and heat flux. The heat transfer coefficient and dryout incipience vapor quality data are compared against prediction methods from literature, however, none predicting the current data. Generally, lower dryout incipience vapor qualities are noticed for the hydrocarbons compared to R134a, with the R600a presenting the lowest values. The dryout is delayed in terms of vapor quality for the mixtures compared to the pure hydrocarbons.