多相流基础chapter5-driftfluxmodel.ppt

Vg, Vl 为常数, jgl vs. a 为变量 * Remember this is the bubbly flow * * This can be developed from the former plot, the * Why introduce the C0? * * 三副图片分层显示，点击管道分别出现分层流和段塞流动画，在动画下面配有文字说明。 * 三副图片分层显示，点击管道分别出现分层流和段塞流动画，在动画下面配有文字说明。 Chapter five — Drift flux model（第五章－漂移流模型） A type of separated flow model especially aimed at the relative motion of the phases, developed by G.B. Wallis. It is most applicable to bubbly flow and plug flow. It is not particularly relevant to annular flow because it has two characteristic velocities in one phase (liquid film velocity and liquid drop velocity) Introduction（简介） Development of the model slip velocity: then: The definition of drift flux (the gas relative to the liquid): Definition of drift velocities: where: The drift velocities are the difference between the actual velocity and the average velocity. The drift flux is the volumetric flux of a component relative to the surface moving at the average velocity. The drift flux of the gas is: Because we have: We can obtain: Similarly we have: Attentions: From former equations, we can find the two drift flux of gas and liquid are equal and opposite. Commonly only drift flux of gas is used. In upwards flow with upward velocity, the drift flux of gas is positive. For homogeneous flow, because it is a flow with zero slip velocity, the drift flux of gas is equal to zero. For steady-state one-dimensional flow, force balance can be written for the liquid in the absence of wall shear stress: The physical importance of the drift flux For gas: Then we obtain: Tip: In the absence of wall shear, F is a function only of the void fraction, physical properties. It can also be written as: Tip: Both the drift flux jgl and the slip velocity us are functions only of α and of the physical properties of the system. Example: bubbly flow For bubbly flow, Whalley(1987) proposed one equation: Where ub is the rising velocity of a single isolated bubble, because we have,