Experimental study of R134a flow boiling in a horizontal tube for evaporator design under typical Organic Rankine Cycle pressures

The heat transfer characteristics of organic fluids at high pressures for Organic Rankine Cycle (ORC) application has not been well studied and relevant experimental results are scarce. R134a flow boiling was investigated experimentally in a 10.3 mm horizontal tube at high pressures of 2.5-3.3 MPa (0.62-0.81Pc) with mass fluxes of 300-600 kg/m2s, heat fluxes of 20-50 kW/m2. The results showed that dryout more easily occurred at the top surface at higher pressures indicating the predominant flow pattern was stratified flow with partial dryout. With the expansion of dryout, the heat transfer coefficient decreased with increasing vapor quality. The heat flux and pressure affected the heat transfer coefficient at vapor qualities less than around 0.3, while higher mass fluxes improved the heat transfer for the whole range of vapor qualities. An improved correlation was developed for stratified flow conditions by combining the Gungor and Winterton correlation and the Wojtan correlation to overcome their respective defects in predicting cases in this study. The combined correlation has a better prediction accuracy with a mean absolute prediction error of 12.5% for 270 data points for various experimental stratified conditions and the prediction accuracy is less sensitive to the experimental parameters.