Experimental analysis of high temperature flow boiling of zeotropic mixture R134a/R245fa in a plate heat exchanger

The potential to improve the performance of organic Rankine cycle systems by using zeotropic mixtures as working fluids has been demonstrated in the open literature. However, there are very few research works in the open literature studying the mixture flow boiling in plate heat exchangers, and none of the existing works address high temperature boiling, which is the prevailing working condition in the evaporator of organic Rankine cycle systems. This paper presents an experimental study of high temperature flow boiling of zeotropic mixtures of R134a/R245fa in a plate heat exchanger. Five compositions were tested at the reduced pressures 0.45, 0.55 and 0.65 and various heat fluxes and mass fluxes. In addition, the predictive performance of several existing prediction methods for the heat transfer coefficient and pressure drop were evaluated, and based on the results, new heat transfer and pressure drop prediction methods were developed. The experimental results suggest that the heat transfer is dominated by the nucleate boiling. The mixture having a 0.427/0.573 mass composition presents the largest heat transfer degradation, up to 42 %, compared with the pure fluids. As far as the pressure drop is concerned, the results suggest that the zeotropic mixtures have the same pressure drop characteristics as the pure fluids, and the five mixtures have almost the same frictional pressure drop due to their similar thermo-physical properties. Existing methods provide a good prediction of the heat transfer data. The new heat transfer correlations enable an even better predictive performance than those available in the literature, however, it remains to evaluate the predictive performance of the new correlation for other working fluids, plate heat exchanger geometries and working conditions than those considered in the paper.