Experimental study on bubble sliding for upward subcooled flow boiling in a narrow rectangular channel

Abstract

In order to analyze the flow boiling heat transfer in narrow channels, visualization experiments were carried out and bubble sliding behaviors were observed for upward subcooled flow boiling in a narrow rectangular channel. The effects of wall heat flux, mass flow rate and inlet subcooling on sliding bubble growth rate and velocity were studied. Based on experimental results, a bubble generating at the nucleation site on the heating surface experiences a rapid growth process in a short time at first and then the growth rate slows down, which continues before the bubble slides out of the observation area. The wall heat flux promotes the initial bubble growth. Additionally, the bubble growth rate increases with the decreasing mass flow rate and decreasing inlet subcooling. Compared with the heat flux, the mainstream temperature influences bubble growth more significantly in the narrow channel. Bubble sliding velocity increases rapidly at the initial stage and tends to stabilize when approaching the mainstream value. Since the heat flux and inlet subcooling affect the bubble size and the forces acting on the bubble, bubble sliding velocity increases as heat flux increases and inlet subcooling decreases. The dependence of bubble velocity on liquid velocity indicates that the drag force caused by velocity difference between bubble and mainstream is the main force driving the bubble sliding motion. New models are established and can well predict the sliding bubble diameter and velocity.