BUBBLE SLIDING PROCESS ON SUBCOOLING FLOW BOILING IN VERTICAL RECTANGULAR NARROW CHANNEL

In present work, a visualized investigation was conducted to understand the phenomenon of bubble sliding on vertical heating wall with upward subcooled flow boiling in narrow rectangle channel with gaps of 2.5 mm. Experimental results show that the characteristics of bubble slipping are significantly influenced by mainstream velocity and temperature. With increasing mainstream velocity, bubble sliding distance and speed are all increased. In present working conditions, the bubbles sliding distance is of 3-10 times of the bubble departure diameter. Based on the energy balance on bubble sliding process, the impact of bubble sliding on the characteristics of heat transfer was analyzed as well. Considering the different mechanisms effect on heat transfer, such as micro-layer evaporation, transient conduction and single-phase forced convection between fluid and wall, the ratio of every mechanism was calculated and compared. Analysis results show that the bubble with sliding movement have significant heat transfer enhancement than that of the stationary bubbles. With the proposed concept of disturbance intensity of sliding bubble, the heat flux expressions caused by disturbance of sliding bubble or stationary have been proposed.