过冷沸腾时气泡表面波触发机制及对气泡动力学行为影响

It was illustrated that surface wave could form on the surface of a vapor bubble in subcooled boiling at certain liquid subcooling, and affected the heat transfer process, while its formation mechanism and its effects on dynamic behaviors of the bubble and boiling heat transfer mechanism need to be further investigated. In this project, transient features and parameters of the surface wave on a vapor bubble during subcooled boiling under different conditions will be obtained firstly with the aid of a high-speed video camera and PIV/PLIF technique, combining with digital image analysis technique developed based on MATLAB environment. It helps to explore the conditions for the occurrence of surface wave as well as its influence on bubble behavior and heat transfer characteristics during subcooled boiling. The parameters of surface wave on a bubble will be realized by applying an acoustic field and adding surfactant to the working fluid. Meanwhile, variations in temperature and flow fields around the bubble surface could be measured with the tracer technique. Thus, detailed experimental data could be presented for illustrating the influence of surface wave. On the basis of experimental studies, CFD method and theory of interface instability analysis will be applied to help reveal the mechanism triggering the wave on a bubble surface and its evolution under subcooled boiling condition. Effects and mechanism of micro convection introduced by surface wave on heat and mass transfer process at the interface will be investigated as well. A new equation based on classical bubble dynamics for predicting bubble motion will be established considering the influence of surface wave, with which an improved method might be presented for calculating nucleate boiling heat transfer. The outcome of this project could give an enrichment and improvement to the current theory of bubble dynamics, and will also play a positive role in future unveiling the mechanism of boiling.

有研究发现，在一定过冷度的沸腾条件下气泡表面上会形成表面波并影响沸腾传热过程，但其形成机制及对气泡动力学和传热机理的影响需进一步探究。本项目拟采用高速摄像和PIV/PLIF技术，结合基于MATLAB环境的数字图像分析技术，获取不同过冷沸腾条件下气泡表面波的波动特征和参数，以探明表面波的发生条件及其对气泡动力学行为和换热过程的影响。试验中通过外加声场和添加表面活性剂调控气泡表面波的特征参数，并结合示踪技术测量气泡周围微观流场和温度场，以获取反映表面波影响的详实试验数据；在试验基础上，结合气泡表面不稳定性分析理论和CFD方法，揭示过冷沸腾时气泡表面波的触发机制和形成过程，并探究表面波引起的微对流对界面传热和传质的影响及机理。在经典气泡动力学理论基础上，建立能反映表面波影响的气泡动力学方程以及核态沸腾传热计算方法。项目的研究成果可丰富和发展现有的气泡动力学理论，对进一步探明沸腾机理具有积极意义。

过冷沸腾广泛应用于能源与动力工程领域，前期研究表明，一定条件的过冷沸腾时气泡表面上将形成表面波，其对沸腾传热有着明显的影响。本项目围绕表面波及其所致界面破碎对过冷沸腾的影响开展实验研究，以期为高效热管理技术的开发提供新的思路和解决方案。主要研究结果如下：（1）在瞬态过冷沸腾实验中发现，一定过冷度下的过渡沸腾阶段，气液界面上会形成剧烈的毛细波，导致蒸汽气膜破碎并引入局部射流过程，促进加热面再湿润，触发微细化沸腾发生，强化沸腾换热。（2）对沸腾声的研究发现，小加热面上过冷沸腾的声信号随热流密度升高呈N型分布的规律，与大尺寸加热面有着明显的差异；在微细化沸腾阶段，表面波引起的界面破碎和体积振荡显著的增强声压强度，可用于微细化沸腾发生的判断依据。（3）表面波的形成将增加气液接触面积，促进冷热流体交换，进而显著加速气泡凝结过程。（4）受限空间核态沸腾时，高振幅超声波促进气泡毛细波，在25K以下过冷度时，表面波的发展破坏气-液-固三相接触线，促进气泡脱离，进而提高临界热流密度；而在25K以上过冷度时，气泡表面波和声流作用加剧蒸汽气泡凝结是临界热流密度提高主要机制。另外，超声作用还可触发微细化沸腾在受限空间内发生，进一步提高换热极限。