从Rayleigh-Taylor不稳定性视角分析蒸发降膜和自然循环过冷沸腾两相流动不稳定

Rayleigh-Taylor instability is one of classical hydrodynamic instabilities in fluid mechanics. Falling evaporation film and natural subcooled boiling circulation have a very significant role in the safety systems of nuclear power plants due to the passive flow characteristics. But the two processes are subject to instablility in some conditions. It is believed that the RT instability is the driven source of the intability of these two processes. The flow features and the research status quo of the Rayleigh-Taylor instability are thoroughly reviewed and examined. Based on the Lagrangian framework, the new method is proposed for solving the non-linear stage evolution of the RT intability, which is the composite double-parameter pertubation expansion based on the Fourier spectrum and multi-scale method. And it is fully different from the convenstional method based on the Eulerian framework . The mechanism of multi-mode coupling and interaction will be examined and revealed using this method. Based on the built analycal method, the instabilty of falling evaporation film and natural subcooled boiling circualtion will be analyzed in the viewpoint of the RT instability, combined with their respectively specific characteristics.The rich variety of their instability spectrum will be attempted to make exposition. The relation of the critical stable length on the inclined angle and wall superheat will be given out at different pressure on the instabilty of falling evaportion film. On the instabilty of natural subcooled boiling circulation, the effect of the liquid subcool and bubble size will be investigated. The coupling and interaction of whole and local RT instabilities will be analysed.

Rayleigh-Taylor不稳定性是流体力学中经典流动不稳定性。 而蒸发下降液膜和自然循环过冷沸腾由于其非能动的特点，二者在核能电站安全系统中具有非常重要的地位。我们认为RT不稳定性是这两个过程不稳定的内源。本课题在对RT不稳定性研究现状的基础上，有别于其他研究者所采用的方法，提出在Lagrangian框架下采用傅立叶频谱法和多尺度扰动的复合双展开法来研究Rayleigh-Taylor流动不稳定性在非线性阶段的演化，力求揭示出其非线性阶段多模耦合和作用的机理。根据所建立的分析体系，结合蒸发降膜和自然循环的具体特点，从RT视角分析解开这两个过程中不稳定性的丰富性。关于蒸发降膜不稳定性，给出在系统不同压力下临界液膜稳定长度与壁面过热度和倾斜角度的关系。关于自然循环过冷沸腾不稳定性，给出液体过冷度和气泡尺度对RT不稳定性的影响，分析循环过程中微宏观两个RT流动不稳定性的耦合和相互影响。

本项目的背景为研究在核能、热能、化工领域中具有重要应用的下降液膜流动不稳定性，本课题主要探究Rayleigh-Taylor不稳定性、水力驱动、Marangoni效应对加热液膜流动不稳定性的贡献和驱动机理。研究内容包括以下三个方面，其一构建了三维的基于加权余量法液膜流动不稳定性模型，探明了水力驱动、Marangoni效应和Rayleigh-Taylor不稳定性对加热液膜流动不稳定性的贡献和影响，比较了在不同初始扰动条件下液膜表面形成溪流的条件和演化规律。其二采用粒子法进行了冷凝液滴与壁面碰撞的数值模拟，开发了新的表面粒子识别法和基于几何分形的表面张力模型，并成功解决了困扰粒子法的压力振荡难题。其三探究在加热条件下液膜截面速度分布与努塞尔特平板分布的偏离，为加权余量模型速度分布假设提供实验依据，同时测量了液膜厚度与液膜表面温度分布，为计算提供的第一手验证资料。得到了如下的具有重大创新意义的研究成果，其一成功构建了基于加权余量的非等温液膜流动不稳定性模型，指明了Rayleigh-Taylor不稳定性、水力驱动、Marangoni效应对加热液膜流动不稳定性的各自贡献；其二对粒子法的促进和发展做出了较大贡献，提出了新的表面粒子识别法，创立了基于几何分形的表面张力模型，揭示了冷凝液滴与壁面碰撞的机理，解决了困扰粒子法的压力振荡问题。该项目对应用于核电安全系统的包壳安全冷却、核能海水淡化关键工艺----多效蒸馏系统等具有重要的意义。