Experimental evaluation of the in-tube condensation heat transfer of pure n-pentane/R245fa and their non-azeotropic mixture as an ORC working fluid

Abstract

The existence of temperature gliding in non-azeotropic mixture working fluids provides a promising opportunity for greatly improved resource utilization in organic Rankine cycle (ORC) systems for geothermal applications. The condensation heat transfer degradation associated with working fluid mixtures is an unavoidable penalty that may reduce the utilization and system benefits by necessitating larger heat exchangers. The objective of the current study is an experimental investigation of condensation heat transfer for R245fa, n-pentane, and a composition of their mixture. Quasi-local condensation heat transfer coefficients are measured in a straight horizontal smooth tube with flow conditions at mass flux G = 100 and 150 kg (m2 s)-1, local equilibrium quality x = 0.05-0.80, and reduced pressures Pr = 0.05-0.15. The reduced condensation heat transfer coefficients from the test are compared with the predictions from already established mixture correction methods in the literature. Pure components R245fa and n-pentane condensation heat transfer data are well predicted by Shah (1979) correlations for all test conditions. However, their non-azeotropic mixture condensation heat transfer data require additional correction parameter to account for the heat transfer degradation. This correction parameter is correlated by the temperature gliding gradient of mixture fluids. Test data with mixture working fluids from current study are reasonably well predicted by the corrected in-tube correlation.