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

针对狭窄间隙之间产生的扁平气泡，详细研究了液汽行为与传热之间的关系。在实验中，引入了透明的加热表面，使得加热、从下方观察液汽行为以及获取传热数据的同时成为可能。安装窄间隙测试段以实现重力场中的向上流动，并连接到测试回路以明确规定入口条件。实验采用蒸馏水，压力：P= 0.1 -0.1 8MPa，温度：t_p= 100℃，入口过冷度：DELTAT_<sub, in>= 5 -8K，供给热流平均值q_o=*3.2 chi 10^5W/m^2，入口液体速度：u_<in>= 0.06m/s。改变重力水平以改变气泡上升速度，在狭窄的间隙中，在扁平气泡下方可以看到干燥斑块的延伸和残余微层的溪流。被扁平气泡覆盖并被大量液体流淬灭的周期在加热表面上重复。在气泡上升速度较慢的情况下，由于表面淬火频率的降低，表面温度保持在较高的值。由于表面过热度的增加，热通量得到增强，而传热系数值仅观察到恶化。在极窄的间隙中，扁平气泡的流动变得不稳定，形状复杂的扁平气泡在加热面上向各个方向剧烈运动。结果，干斑延伸和再润湿的重复频率显着降低，并且在一定程度上防止了传热恶化。在本研究中，阐明了核态沸腾中存在两种相互矛盾的趋势：蒸发导致微层厚度减小而强化传热，以及微层中干燥斑块延伸导致传热恶化。较少的

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