Study on Dry-out and Flow Boiling Heat Transfer for R717 in a Horizontal Smooth Mini-channel

The paper explores the characteristics of boiling two-phase flow heat transfer and the annular flow dry-out for ammonia (R717) in a horizontal smooth tube with an inner diameter of 3 mm. The purpose is to explore the characteristics of dry-out for the annular flow. The experimental conditions are: saturation temperature -10∼10℃, heat flux density 10∼30 kW/m2, the mass flow rate 40∼200 kg/(m2•s), and the vapor quality 0.1∼1. The experimental results show that the increase of mass flow rate, saturation temperature and heat flux density will enhance boiling heat transfer. Increasing the mass flow rate will strengthen the convective boiling in annular flow; increasing the saturation temperature and heat flux density will mainly strengthen the nucleate boiling in intermittent flow. In the boiling two-phase flow, the dry-out phenomenon is often accompanied by the deterioration of heat transfer. The heat transfer coefficients before and after dry-out are fitted respectively. The intersection of fitted curves is defined as the initial vapor quality of dry-out. The fitting results show that the increase of saturation temperature, heat flux density and mass flow rate will reduce the initial dryness of drying and accelerate the drying process, and the critical vapor quality of dry-out is equal to 0.9±0.03, which is consistent with the visualization results. © 2022, Science Press. All right reserved.