窄流道润湿特性对汽泡附壁区域演化及泡底热质传输的影响机制

The evolution process of bubble contact areas can significantly affect the heat transfer capacity of facility, and the corresponding geometric parameters are also the key input parameters of many boiling models. In recent years, the enhancement of boiling heat transfer by changing the surface wettability has attracted wide attentions, and the bubble behaviors can present different characteristics. Moreover, the bubbles are confined by the channel wall, which can promote the coalescence phenomena between bubbles and further complicate the evolution processes of bubble contact areas. In this project, the applicant plans to select the rectangular narrow channel with four-side visibility, and the transparent glass coated with ITO film is selected for the heating wall. Meanwhile, the evolution characteristics of bubble contact areas and the temperature distribution of heating surface are simultaneously recorded from the bottom of transparent heating wall by means of the high-speed camera and the infrared thermal imager. By means of image recognition, the spatial-temporal distribution characteristics of the geometric parameters of bubble contact areas and the high-precision temperature field of heating surface are analyzed. According to the dynamic force balance analyses of bubbles, the influence mechanisms of surface wettability and channel confinement on the evolution of bubble contact areas are clarified. Then, the corresponding statistical analysis model is established. The effect mechanism of bubble contact area on the boiling heat transfer capacity of heating surface is revealed, and a theoretical model is constructed to describe the heat and mass transfer mechanism underneath bubble. According to this project, it is possible to develop a new mechanism method for calculating the boiling heat transfer.

加热表面汽泡附壁区域的演化过程显著影响了设备的传热能力，其几何参量也是众多沸腾模型的关键输入参数。近年来，改变表面润湿特性强化沸腾传热受到了广泛的关注；受润湿性的影响，汽泡行为将呈现出不同的特征。汽泡在窄流道内受到壁面的限制，导致更易于聚合，进一步复杂了汽泡附壁区域的演化过程。本项目拟基于四面可视的矩形窄流道，选择具有不同润湿特性的透明ITO玻璃为加热本体，通过高速摄像机与红外热像仪同步从加热本体底部记录汽泡附壁区域演化与加热表面的温度分布。通过图像识别，分析汽泡附壁区域几何参量与加热表面高精度温度场的时空分布特征。基于汽泡动态力平衡分析，阐明窄流道润湿特性与限制作用对汽泡附壁区域演化的影响机制，建立相应的统计学分析模型。揭示汽泡附壁区域演化对加热表面沸腾传热能力的影响机制，构建理论模型描述泡底传热传质机理。该项目将有可能开发出新的机理性沸腾传热计算方法。

伴随着汽泡生长、聚合与脱离的沸腾传热现象广泛存在于核工程与航空航天等众多领域，而具有大比表面积与大高宽比的矩形窄流道也被越来越多的紧凑式换热设备所采用。其中，汽泡附壁区域的演化过程不仅显著影响设备换热性能，其几何特征参量也是众多沸腾模型的关键输入参数。近年来，改变表面润湿性强化沸腾传热受到了广泛关注；当加热面润湿性发生改变，汽泡行为也将呈现出不同特征。归因于窄流道的壁面限制作用，当汽泡未能及时流向下游，上游或周围汽泡更易与其发生聚合，形成占据整个流道的大汽泡或汽团，进一步复杂了汽泡附壁区域的演化过程。本项目设计了一种加热面润湿性可改变的矩形窄流道试验方案，通过高分子纳米陶瓷、氮化钛、聚四氟乙烯、以及高分子陶瓷聚合物涂层，分别实现超亲水性、亲水性、疏水性、和超疏水性，并通过高速摄像仪开展精细化的汽泡动力学行为研究。通过开展流动沸腾试验发现，在相同热流密度和质量流速下，随着加热面润湿性增加，即从超疏水变化至超亲水，汽泡尺寸会逐渐变大。这是因为亲水表面的壁面过热度高于疏水表面，当汽泡成核后，汽泡会迅速长大，生长曲线更为陡峭。而在汽泡聚合过程中，汽泡之间的部分液体会被捕获到聚合后所形成的大汽泡底部，形成“厚液膜”，其演化过程主要包括：捕获过程、破裂过程以及蒸发过程。不同尺寸的汽泡通过连续不断地聚合会形成了受限大汽弹，即使加热壁面被受限汽泡或汽弹所覆盖，泡底厚液膜内仍存在明显的二次核态沸腾现象。最后，本项目从窄流道壁面限制效应和加热面润湿性影响汽-液相界面的角度，建立了尺度效应和润湿性效应耦合下的沸腾传热机理模型。并在此基础上，开发了包括随机碰撞和尾翼夹带引起的汽泡聚合模型，湍动涡旋引起的汽泡破碎模型，核化相变、界面传热冷凝和惯性湮灭引起的界面面积变化模型。