Abstract
This study experimentally investigated the flow boiling composition variations and heat transfer characteristics of R134a/R245fa zeotropic mixtures with four mass fractions (20/80, 37/63, 52/48 and 60/40 wt%) and pure R245fa in a rectangular channel. The flow boiling waas visualized from the top. The channel had a cross-sectional area of 8 mm × 8 mm and length of 300 mm and was heated on three sides. The composition variations of the R134a/R245fa zeotropic mixtures were measured directly in the channel. The heat transfer coefficients and composition variations were measured for smooth stratified flow and stratified bubbly flow for mass fluxes from 60 to 100 kg/m2 s and heat fluxes from 5 to 70 kW/m2 at the evaporating temperature of 20 °C. The results showed that the liquid composition variations in the smooth stratified flow and stratified bubbly flow varied in opposite directions with the liquid composition changes strongly related to the temperature glide but only slightly affected by the thermophysical properties. The heat transfer coefficients for the zeotropic mixtures were compared with those for pure R245fa to show the effects of the liquid composition differences and the thermophysical properties. A correlation was developed to predict the R134a mass fraction differences in the liquid for stratified bubbly flow as a function of a dimensionless temperature glide parameter, gravity and other key factors. The predictions agree well with the measured R134a mass fraction differences in the liquid for these zeotropic mixtures, with this difference then representing the liquid mass transfer resistance during stratified bubbly flow.
Published Version
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