Boiling Bubble Dynamics by Numerical Simulation

本研究旨在发展一种基于VOF（Volume of Fluid）的汽液相变传热数值计算方法，并将其应用于沸腾现象的汽泡动力学研究。我们发展了一种改进的施主-受主方法，可以更精确地追踪液-汽界面，并发展了一种新的算法来计算液-汽相变，包括两相之间的体积变化。采用轴对称二维坐标系，计算域为径向100 μ m，垂直方向200 μ m。将直径为20 μ m的初始气泡放置在接触角为50 °的表面上，并以0.2K的过热度加热。计算出的偏离直径约为55 μ m，该值与实验中通常观察到的值（60- 80 μ m）相当。得到了气泡生长和脱离过程中的速度矢量、压力和温度分布。计算得到的热边界层与纹影照片得到的热边界层非常相似。第二年，我们尝试模拟了空腔中的气泡。腔的直径和深度分别为2010 TAm和2010 TAm。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。空泡离开后，腔内仍有小空泡存在，它应该是下一个空泡的核。不幸的是，出现了一些不稳定性，我们无法继续进一步的计算。然而，温度和局部传热系数的分布是非常相似的，通常在实验中观察到的。在本研究中发展的数值方法也适用于固-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.