Gas bubbles in liquid influence efficiency of mass and heat transfer. Much efforts on the observation of expansion and detachment of bubble at upward nozzle have been conducted

while that of downward nozzle is rarely known. In this article

the expansion and detachment of bubble at the downward nozzle of vertical flat pipe was observed by a high speed camera. Effect of the nozzle diameter and average gas velocity on bubble size and time of formation was investigated. The bubble age

k and the diameter ratio of long axis to short axis

L describe bubble position and bubble shape

respectively. It showed that three kinds of mechanism of intermittent formation

intermittent formation with suction and continued formation exist in bubble formation. With the variation of detachment diameter of bubble as a function of average gas velocity

a critical point representing the evolution of bubble detachment exists. The expansion and detachment of bubble along with average gas velocity

follow two types of single bubble formation and coalescence formation. The suction phenomenon occurred at 3.1 m·s-1 of average gas velocity

of which kmax is minimized. In the detachment stage

a critical point of detachment time exists in the evolution of L as a function of time

namely

L allocates around 2.0 with mostly ellipsoidal bubble before the critical point

however

L increases significantly with the formation of flat ellipsoidal bubble after the critical point.