狭窄加热表面上核态沸腾中汽泡合并特性的实验研究

Experimental Study of Bubble Coalescence Behavior in Nucleate Boiling at a Narrow Heating Surface

狭窄表面的沸腾可以减少沸腾汽泡在观测方向上的干扰,有利于更清晰地研究汽泡生长和合并特性。为探究沸腾过程中汽泡的合并,本文基于常规机械加工的狭窄加热表面,通过高速摄像机完整记录了加热表面上汽泡的生长,合并和脱离过程,并分析了汽化核心间距,汽泡脱离直径对汽泡合并的影响。通过对近壁面处沸腾动态图像的观察,观测到了汽泡合并的不同特性。当沸腾过程中热流密度较低时,近壁面处只发生汽泡水平和倾斜合并,并常常同时存在。而竖直方向上的汽泡合并仅发生在热流密度相对较高时,且常伴随着水平和倾斜方向的合并。相邻汽泡间的合并现象常通过汽泡中心距与合并汽泡脱离直径间的关系来衡量。本实验结果表明,相邻汽化核心上的汽泡发生合并时,汽泡中心距与汽泡脱离直径满足S/D<1.5。同时,相较于粗糙表面,光滑表面的S/D变化范围较小,且平均值有减小趋势。这一结果有助于进一步研究表面结构对汽泡合并的影响。

Narrow surfaces were applied in the current pool boiling experiments as less interference of vapor bubbles in the observation direction can be achieved in the heater.In order to investigate the behaviors of bubble coalescence more clearly,bubble growth,coalescence and departure processes in pool boiling at a narrow machining surface were visualized via a high-speed camera.The effect of bubble departure diameter and the distance between nucleate sites on bubble coalescence were analyzed.Three different coalescence modes have been observed through the observation of the sequential images.It is found that,horizontal and declining colescecces occurs only at lower heat-flux conditions near the heating surface,and the two modes often exist simultaneously.While in higher heat-flux cases,vertical coalescence of bubbles from the same nucleate site may occur and is often accompanied by horizontal of declining coalescence.A dimensionless indicator S/D is introduced as the ratio of distance between bubbles center to the coalesced bubble departure diameter.Results show that,when horizontal coalescence occurs,the criterion S/D <1.5 is generally satisfied in the current experiments,which is consistent with the conclusion based on the boiling experiments at a heating surface with artificial nucleate sites.Moreover,there is a tendency that the average value of S/D decreases with the decrease of the surface roughness and the range of S/D in a smoother surface is not as scattered as that at a rougher surface.Current results will help to investigate the effect of surface structure on bubble coalescence behavior.