Boiling and Condensation Heat Transfer of Zeotropic Mixtures in Smooth Tubes

Abstract

Boiling and condensation of mixtures are widely encountered in liquefaction of natural gas process, refrigeration systems, and other chemical and power generation industries. Zeotropic mixtures present a temperature glide between the dew and bubble points. A variation in fluid temperature and shifts in vapor and liquid compositions occur during boiling and condensation processes of zeotropic mixtures. Heat transfer characteristics of mixtures are less investigated, comparing with pure fluids. Problems such as possibly reduced heat transfer performance of mixtures need to be further studied. Better understanding of boiling and condensation heat transfer of mixtures is significant for optimal design and improvement of energy efficiency. A comprehensive investigation of in-tube boiling and condensation of zeotropic binary mixtures was carried out in the present work. For boiling of mixtures, heat transfer and flow characteristics were studied numerically, with non-equilibrium models based on annular and annular-mist flow. It was found that both the fluid physical properties and mass transfer resistance are responsible for the heat transfer degradation during boiling of non-azeotropic mixtures. Moreover, dryout of binary mixtures boiling was investigated by examining effect of various setups for initial entrainment fraction and nucleation-induced entrainment on predicted dryout quality. The initial amount of entrainment was found to be important for the prediction at low critical qualities, while the history effect is unimportant for the prediction at high critical qualities. Condensation heat transfer of mixture was studied both theoretically and experimentally. Equilibrium and non-equilibrium models were presented for predicting condensation heat transfer coefficient of mixtures. For industrial design, a simple equilibrium and empirical model including mixture effects was proposed by studying five experimental datasets covering a wide range of fluids and working conditions. The experimental results were best predicted when applying the additional heat transfer resistance caused by the mixtures on both the annular and stratified heat transfer coefficient. Three models based on film theory were introduced to investigate thermodynamic and hydrodynamic non-equilibrium effects, as well as effect of separated vapor zones. The studies of non-equilibrium models provide references and suggestions for simulation of condensation process, which will further help to understand heat and mass transfer characteristics of binary mixtures. In particular, the thermodynamic non-equilibrium effect was shown to be the most significant one on the condensation heat transfer, while the hydrodynamic non-equilibrium effect and the effect caused by separated vapor are less evident in the present studied cases. For the experimental part, condensation tests were performed for R32/R1234ze(E) mixture at 25/75% mass composition inside an 8mm inner diameter tube, with mass velocity ranging from 100 to 600 kg/m2s at a pressure of 10 bar. Heat transfer coefficient was measured for further validation of the proposed model.