利用高速摄像机拍摄顶部浸没竖直向下管口气泡膨胀及脱离演变过程,对比分析管径、平均气速对气泡尺寸、形成时间的影响规律.引入泡龄k表征管口气泡位置特性,用椭球形气泡长短轴比L表征气泡形状特征.研究发现,气泡生成机理有间歇生成气泡、含抽吸间歇生成气泡和连续生成气泡3种;气泡脱离直径随平均气速变化过程明显存在气泡脱离形态转变点;气泡膨胀脱离模式随平均气速变化分为单个气泡形成、气泡聚并形成两种.平均气速为3.1 m·s?1时管口处存在抽吸现象,此时最大泡龄kmax最小;脱离阶段气泡长短轴比L随脱离时间变化过程明显存在时间分界点.在分界点前,L值在2.0附近波动,气泡主要以椭球形存在.分界点后,L值明显增加,气泡形状向扁椭球体发展.%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 whichkmaxis minimized. In the detachment stage, a critical point of detachment time exists in the evolution ofLas a function of time, namely,L allocates around 2.0 with mostly ellipsoidal bubblebefore the critical point, however,L increases significantly with the formation of flat ellipsoidal bubble after the critical point.
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