Behavior of Thin Liquid Film underneath a Coalesced Bubble in Nucleate Boiling at High Heat Flux

详细研究了窄间隙间产生的扁平气泡的液-气行为与传热的关系。在实验中，引入了一个透明的加热表面，使加热、从下面观察液-气行为和获取换热数据同时成为可能。设置窄间隙试验段，实现重力场下的向上流动，并与试验回路连接，明确规定进口条件。实验采用蒸馏水，压力：P= 0.1 ~ 0.1 8MPa，温度：t_p= 100 C，入口过冷：DELTAT_<sub, in>= 5 ~ 8k，供热通量平均值q_o=*3.2 chi 10^5W/m^2，入口液体流速：u_<in>= 0.06m/s。通过改变重力水平来改变气泡的上升速度，在狭窄的间隙内，在平坦的气泡下可以识别出残留微层的干斑和溪流的延伸。在受热面上迭代了被压扁的气泡覆盖并被大量液体流淬灭的周期。在气泡上升速度较慢的情况下，由于减少了表面淬火的频率，使表面温度保持在较高的水平。表面过热度的增加使热流密度增大，而换热系数的值则减小。在极窄的间隙内，扁平气泡的流动变得不稳定，形状复杂的扁平气泡在受热面上向各个方向剧烈运动。干区扩展和再润湿迭代频率明显降低，在一定程度上防止了换热恶化。本研究阐明了在有核沸腾过程中存在两种相互矛盾的趋势，一种是由于蒸发导致微层厚度减小而增强传热，另一种是由于微层中干燥斑的扩大而使传热恶化。少

The relation between the liquid-vapor behavior and heat transfer is investigated in detail for the flattened bubble generated between narrow gaps. In the experiments, a transparent heating surface was introduced to make the heating, the observation of liquid-vapor behavior from underneath and the acquisition of heat transfer data simultaneously possible. The test section of the narrow gap was installed to realize the upward flow in the gravity field and was connected to test loop to prescribe the inlet condition without ambiguity. The experiments was conducted by the use of distilled water at pressure : P= 0.1 -0.1 8MPa, temperature : t_p= 100ﾟC, inlet subcooling : DELTAT_<sub, in>= 5 -8K, averaged value of heat flux supplied q_o=*3.2 chi 10^5W/m^2, inlet liquid velocity : u_<in>= 0.06m/s. Gravity level was changed to vary the bubble rising velocity, In the narrow gap, extension of dry patches and rivulets of residual microlayers were recognized underneath a flattened bubble. The perio … More ds covered by the flattened bubble and quenched by the bulk liquid flow were iterated on the heating surface. in the case of slow bubble rising velocity, the surface temperature is maintained at high value owing to the reduced frequency of surface quenching. And the heat flux is enhanced due to the increased surface superheat, while only the deterioration is observed in the values of heat transfer coefficient. In the extremely narrow gap, the flow of flattened bubbles becomes unstable and the flattened bubbles with complicated profiles moves vigorously towards the various direction on the heating surface. As a result, the frequency in the iteration of dry patch extension and of re-wetting is markedly decreased and the heat transfer deterioration is prevented to some extent. In the present research, it was clarified in nucleate boiling the existence of two conflicting trends of heat transfer enhancement by the decreased thickness of microlayer due to evaporation and of heat transfer deterioration by the extending dry patches in the microlayer. Less