Studies on post-dryout heat transfer in R-134a vertical flow

Abstract

An experimental study of post-dryout heat transfer with the coolant R-134a was performed in a vertical round tube with upward flow. The experiments were conducted in an uniformly heated tube with an inside diameter of 10 mm and a heated length of 3100 mm, at pressures of 11.1 bar, 16 bar and 28 bar, with varying mass flux in the range of 300–1500 kg/m2s.The results show that for higher mass fluxes and higher heat fluxes the wall temperature decreases after reaching a peak close to the critical vapor quality, while at lower mass fluxes the wall temperature keeps increasing after dryout. The phenomenon described last is shifted to even lower mass fluxes with rising pressure. Furthermore, it was found that at increasing pressure as well as at increasing mass flux the dryout location is shifted to lower equilibrium vapor qualities.Based on the experimental results and existing prediction methods, a theoretical model was developed to describe the heat transfer in the post-dryout area. It divides the post-dryout area into a developing and a developed region. A correction factor was introduced to model the droplet distribution in the cross-section of the fully developed region. A comparison with selected post-dryout heat transfer models from the literature for the experimental parameter range in the present study shows a good applicability of the new model.