Experimental and theoretical study of bubble coalescence and departure behaviors during nucleate pool boiling on uniform smooth and micro-pinfinned surfaces under different subcoolings and heat fluxes

The bubble behavior is significant to the boiling heat transfer process. Among which, the bubble coalescence has been widely concerned by researchers because of the complex flow and heat transfer process, which can promote the bubble departure and enhance the heat transfer coefficient. In this paper, bubble coalescence and departure were investigated in the boiling process. The variation of vapor bridge during the coalescence and the oscillation of bubble radius in the rising period were quantitatively obtained. The departure velocity of the merged bubble under different conditions is also studied, which increases with the increase of the average radius of the single bubble before coalescence. In order to reveal the mechanism of bubble coalescence and departure, the system energy balance is analyzed. Meanwhile, the influence of the net force on the kinetic energy of the merged bubble is also considered. The force-modified energy balance model is proposed and the calculated results are consistent with the experimental data. This paper focuses on the bubble departure behavior after the coalescence and analyzes the factors affecting the bubble departure velocity, which provides a new idea for the surface structure design on heat transfer enhancement from the view of promoting bubble departure.