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

对于窄缝间产生的扁平气泡，详细研究了气液行为与换热的关系。在实验中，引入了一个透明的加热面，使加热、从下面观察汽液行为和同时采集传热数据成为可能。安装窄间隙试验段，实现重力场中的上行流动，并与试验回路相连，明确规定进口条件。实验采用蒸馏水，压力：P=0.1~0.18 Mpa，温度：T_p=100ﾟC，入口过冷度：D_1，In_gt；=5-8K，平均供热流密度Q_o=3.2CHI，10^5W/m~2，入口液体速度U_(？)The Perio…更多的DS被扁平的气泡覆盖，被大体积液体流淬火，在加热面上迭代。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。随着表面过热度的增加，热流密度增大，而换热系数只有恶化。在极窄的缝隙中，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朝着不同的方向剧烈运动。结果表明，干斑扩展迭代和再湿迭代的次数明显减少，在一定程度上防止了换热恶化。在本研究中，阐明了在核态沸腾中，存在两种相互矛盾的趋势，即蒸发导致微层厚度减小而强化换热，以及微层内干斑的延伸导致换热恶化。较少

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