Flow boiling heat transfer and dryout characteristics of ammonia in a horizontal smooth mini-tube

Abstract

A boiling two-phase flow heat transfer experimental system for ammonia in a horizontal smooth tube with an inner diameter of 3 mm was established. The purpose is to explore the characteristics of flow boiling and dryout phenomenon. The experimental conditions: heat flux density is 10–30 kW·m−2, the mass flux is 40–200 kg·m−2·s−1, the saturation temperature is −10–10 °C, and the range of vapor quality is 0.1–1. The results show that the dominant heat transfer in different flow patterns is different, intermittent flow with nucleate boiling, annular flow and mist flow with convective boiling; the increase of mass flux can reinforce the convective boiling, the heat flux density can enhance the nucleate boiling, the increase of saturation temperature has no significant effect on the nucleate boiling, but it can weaken the convective boiling. The increase of mass flux and heat flux density will increase the friction pressure drop, and the increase of saturation temperature can decrease the friction pressure drop. The dryout visualization results and attenuation of heat transfer show that the increase of saturation temperature, heat flux density and mass flux can decrease the initial vapor quality of dryout, which is consistent with the results of Mori et al. model. Compared with the experimental data, the existing two-phase flow correlations are not suitable for the post-dryout prediction for ammonia, and the Wojtan et al. model and Mori et al. model can provide a reference for the dryout prediction.