Boiling Bubble Dynamics by Numerical Simulation

本文的研究是建立一种基于流体体积（VOF）的热流体流动的数值方法，并将其应用于沸腾现象的气泡动力学。我们提出了一种改进的供体-受体方法，可以更高的精度跟踪液-气界面，并提出了一种新的包括两相间体积变化在内的液-气相变化算法。在第一年，我们模拟了一个氦泡，它最初被放置在一个平面上，并在大气压下加热。采用轴对称二维坐标，计算域在径向和垂直方向分别为100mum和200mum。初始气泡直径为20mum，放置在接触角为50的表面上，过热度为0.2K。计算出的偏离直径约为55mum，该值与实验中通常观察到的值（60-80mum）相当。我们得到了气泡生长和离开过程中的速度矢量、压力和温度分布。计算所得的热边界层与纹影照相所得的热边界层相当接近。第二年，我们尝试模拟一个空腔中的气泡。空腔直径和深度分别为20IOTAm和20IOTAm。在腔内放置初始气泡，并以0.2K的过热度加热。最初的气泡在空腔上生长，最后被浮力抬起。气泡离开后，腔内仍留有小气泡，应为下一个气泡的核。不幸的是，出现了一些不稳定，我们无法继续进一步计算。然而，温度和局部传热系数的分布与通常在实验中观察到的非常相似。本文所建立的数值方法同样适用于固液相变问题。

The present study is to develop a numerical method based on VOF(Volume of Fluid) for thermal and fluid flow with liquid-vapor phase change and to apply its method to the bubble dynamics of boiling phenomena. We developed an improved donor-acceptor method that can trace liquid-vapor interface with higher accuracy and new algorithm for liquid-vapor phase change including volume change between two phases.In the first year, we performed simulation of a helium bubble initially placed on a flat surface and heated under atmospheric pressure. Computation domain is 100mum and 200mum in radial and vertical directions, respectively, using axsymmetric two-dimensional coordinate. Initial bubble of 20mum in diameter is placed on the surface with contact angle of 50 and heated with 0.2K in degree of superheating. The departure diameter calculated is about 55mum and this value is comparable with the values (60-80mum) usually observed in experiments. We obtained velocity vectors, pressure and temperature distributions during the bubble growth and departure. The thermal boundary layer calculated is quite similar to those obtained by schlieren photograph.In the second year, we tried to simulate a bubble from a cavity. The cavity size is 20IOTAm and 20IOTAm in diameter and depth, respectively. Inside the cavity, initial bubble is placed and heated with 0.2K in degree of superheating. The initial bubble grew over the cavity and was finally lifted up by buoyancy. After the departure of bubble there remained small bubble inside the cavity and it should be a nuclei of next bubble. Unfortunately, some instabilities appeared and we could not continue further calculations. However, the distributions of temperature and local heat transfer coefficients are quite similar to those usually observed in experiments.The numerical method developed in the present study is applicable also to solid-liquid phase change problems.