摘要
在混合物流动中,某组元i的质量迁移速度(绝对速度)等于对流速度(牵连速度)与扩散速度(相对速度)之和.扩散速度——以及扩散系数——依对流速度取法之不同而不同.在湍流中,组元i的质量迁移速度是(?)(质量加权的时间平均速度).扩散速度((?)-a)是由湍流扩散速度(?)和分子扩散速度v_1-a所组成((?)是组元i的时平均速度,a是某种取法的对流速度).因此,湍流扩散速度与对流速度的取法无关.组元i的质量守恒方程,其右端的扩散须依其左端对流速度取法之不同而不同.在湍流情况下,它可能没有扩散项,或只有湍流扩散项,或既有湍流扩散项又有分子扩散项,如果我们取(?),或(?),或任何其它速度作为对流速度的话.在层流中会遇到只有分子扩散项而无湍流扩散项的例子.分子扩散总是依赖于对流速度的选取.与混合气体情况不同,在二相流中.分子扩散项的重要性相当于、或甚至超过湍流扩散项.
The mass migration velocity (absolute velocity) of component i in a multicom-pont flow is equal to the convection velocity (frame velocity) plus the diffusion velocity (relative velocity). The diffusion velocity as well as the corresponding diffusion coefficient depends on how the convection velocity is adopted.
In turbulent flow, the mass migration velocity of component i is (mass-weighted time average velocity). The diffusion velocity (- a) consists of turbulent
diffusion velocity ( - ) and molecular diffusion velocity , is the simple time average velocity of component i and a is a certain convection velocity). So, the part of turbulent diffusion velocity is independent of what convection velocity is taken.
In the mass conservation equation for component i, the expression for the diffusion term on its right-hand side will change when the convection velocity on its left-hand side changes. In turbulent flow, there could be no diffusion terms, or a turbulent diffusion term only, or both the turbulent and molecular diffusion terms when vf, or v, or any velocity other than these two is taken as the convection velocity. The case, in which there could be molecular diffusion only without turbulent diffusion, occurs in lamnar flew. The moleculrr diffusion term aways depends on the adoption of convection velocity.
In two-phase flow, the value of the molecular diffusion term is often near or even exceeds that of the turbulent diffusion term, which is quite different from the case in gas mixture flow.
出处
《应用数学和力学》
CSCD
北大核心
1991年第11期1007-1013,共7页
Applied Mathematics and Mechanics
基金
国家自然科学基金
关键词
扩散速度
质量守恒方程
湍流
moecular diffusion, turbulent diffusion, diffusion and convection, diffusion equation
