experimental study of CHF and post-CHF phenomena on flow boiling of R-134a with grid effects in annular geometry

Abstract

This study deals with experimental works performed in the dryout and the post-dryout condition using R-134a to investigate the CHF and post-CHF characteristics and the effects of spacer grids in a vertical annular geometry. The experiments were conducted under the outlet pressure of 11 - 20 bar, the mass fluxes of 100 - 400 kg/ $m^2$ s, and the inlet temperatures of 25 - 51 $\circ$ C. The critical heat flux (CHF) experiments in the forced convection boiling were performed with one heated rod and four spacer grids using R-134a as a working fluid. The parametric trend of the CHF data was well consistent with the previous understanding in water. The comparison between the present results and water data in similar geometry showed R-134a can be a modeling fluid for simulating water CHF in high pressure and high temperature condition and indicate the effect of the flow obstacle enhancing the CHF. The visualization works were performed in the condition of dryout and post-dryout heat transfer to give an insight to the interfacial characteristics, flow regimes, and heat transfer mechanisms. Dryout patches on inner liquid film playing the important role in CHF occurrence were observed. The direct observation of flowing bubble behaviors were achieved, which enhances the understanding of the effect of flow obstacles in the flow boiling heat transfer. The post-CHF experiments were also performed to investigate the post-CHF heat transfer characteristics in an annular geometry and about 700 of post-CHF data were obtained in the annular geometry. These post dryout data were compared with the existing thermal equilibrium and thermal non-equilibrium models. The results showed a large prediction uncertainty. 301 data points were extracted from the post-dryout experiments for the film boiling region, and the empirical correlation was developed with the present experimental results. The heat transfer coefficient was calculated based on the saturated vapor temperature an...