Effects of initial bubble size on flow pattern transition in a 28.9 mm diameter column

The experimental results described in this paper were carried out in a 28.9 mm diameter column at a constant water velocity of 0.356 m/s for four different bubble sizes. The void fraction waves were measured with impedance void fraction meters. It has been found that the initial bubble size has strong effects on the flow pattern transition and the instabilities of void fraction waves. The critical void fraction at which the flow pattern transition happens decreases with increasing bubble size. At a constant liquid velocity with increasing gas flow rate, the point at which the system gain factor becomes larger than 1, and the point where the wave velocity gradient first becomes negative, also decrease with increasing bubble size. This study has confirmed that the instability of the void fraction wave is not the factor that causes the bubble-to-slug flow pattern transition in gas–liquid verticle flow.