액체-기체 2상 유동 오일러 전산해석에서 격자 크기의 영향

Abstract

A two-fluid model using the Eulerian-Eulerian approach is applicable for two-phase flow, which assumes two phases as interpenetration continuum. Each phase shares the same volume, penetrates each other in space, and exchanges mass, momentum, and energy. The volume fraction of a phase is expressed as a continuous function for space and time, and the sum of the volume fraction of the phases is equal to 1. Each phase is governed by conservation law. There are three models for two-fluid flow: a volume of fluid (VOF), a mixture model, and an Eulerian model. In an Eulerian model, the governing equations for all phases must be solved by a coupled numerical scheme and physically reasonable convergence must be achieved. When both liquid and gas exist in a mesh, the size of either bubbles or droplet is required to calculate drag force. But the calculated size of a bubble or a droplet by a model is sometimes larger than the computational cell size. Therefore, in the computational analysis of two-phase flow by Euler model, an appropriate modeling for the size of bubble or droplet is required. Among the liquid-gas two-phase flow, there is a bubble column, which is the most basic phenomenon. In this study, the Eulerian two-phase flow model has been validated with a bubble column analysis data with a focus on the effect of a size model of bubble or droplet. The transient computation of three-dimensional two-phase flow code K-FIX was used for the calculations with a set of additions to the code; material properties and constitutive relations for the interfacial momentum transfer. The model for the size of bubble or droplet was a function of relative velocity between liquid and vapor and it is determined by the critical Weber number. For different drag coefficient model and mesh size, the size of bubble or droplet was examined and in bubbly flow the calculated bubble size was evaluated for the flow variables such as fluid velocity and volume fraction in order to analyze the effect of mesh size in Eulerian model of two-phase flow. A set of calculations has been carried out for three different mesh sizes: base case, a half, and a double of the base case mesh size. At the time of 10 s from the start of air injection, the calculated bubble size was from 0.2 to 1.7 cm in the base case, 0.2 to 4.8 cm in case of half-size mesh, and 0.5 to 1.6 cm in double-size mesh. Considering the bubble size, relative velocity, buoyancy and drag force, the mesh size had a significant effect on the result in case of half-size mesh, but it had an insignificant effect in the case of double-size mesh. In addition, the computation time was significantly reduced by parallelizing the K-FIX code with OpenMP.. The original K-FIX was a serial code.