Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determi ned using a test section composed by a 13x3 staggered bundle of smooth copp er tubes, measuring cooling water temperature in the inlet and the outlet o f each tube, and measuring the vapour temperature along the bundle. All dat a were taken at the inlet vapour temperature of 40 degreesC with a wall sub cooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m( 2) and the cooling water flow rate from 120 to 300 l/h for each tube. The v isualisation of the HFC134a condensate flow by means of transparent glass t ubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer d ata with the binary mixtures HFC23-HFC134a show the important effects of te mperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the differ ent zeotropic mixtures were compared with the data calculated with the clas sical condensation model based on the equilibrium model. An improvement of this model is proposed. (C) 2001 Elsevier Science Ltd and IIR. All rights r eserved.