Boiling heat transfer and pressure drop of R-134a and R-1234yf in minichannels for low mass fluxes

Analysis of various existing correlations of predicting local heat transfer and pressure drop in minichannels is performed. Heat transfer coefficient is compared to experimental results. Tests are conducted in a 6-channel tube. Channels are rectangular with 1.1 mm hydraulic diameter. Test parameters were varied in the following ranges: mass flux G = 20–100 kg m−2 s−1, heat flux q” = 2–15 kW m−2, and vapor quality 0–1.Two working fluids are used R-134a and R-1234yf.Contrary to previous studies, in this study convective boiling was identified as the dominant boiling regime. The effect of the critical heat flux on the heat transfer coefficient for low mass flux is also studied. The realized tests are compared to several correlations. Four different intervals of mass and heat fluxes are detected. A correlation of the heat transfer coefficient is proposed for these different intervals.

► Boiling in minichannels is experimentally studied for low mass fluxes (G < 100 kg m−2 s−1). ► None of the correlations in the literature can predict the heat transfer coefficient. ► Convective boiling is dominant for low mass fluxes. Heat transfer coefficient is dependant of mass flux and vapor quality. ► Critical heat flux and boiling curves are showed. Local dry-out is identified. ► A new correlation is proposed for convective boiling for low mass fluxes.