Two-phase flow in a vertical pipe and the phenomenon of choking: Homogeneous diffusion model. Part I. Homogeneous flow models

Abstract

The paper examines the topologic structure of all possible solutions which can exist in flows through adiabatic constant-area ducts for which the homogeneous diffusion model has been assumed. The conservation equations are one-dimensional with the single space variable z, but gravity effects are included. The conservation equations are coupled with three equations of state: a pure substance, a perfect gas with constant specific heats, and a homogeneous two-phase system in thermodynamic equilibrium. The preferred state variables are pressure P, enthalpy h, and mass flux G{sup 2}. The three conservation equations are first-order but nonlinear. They induce a family of solutions which are interpreted as curves in a four-dimensional phase space conceived as a union of three-dimensional spaces (h, G{sup 2};z) with G{sup 2} = const treated as a parameter. It is shown that all points in these spaces are regular, so that no singular solutions need to be considered. The existence and uniqueness theorem leads to the conclusion that through every point in phase space there passes one and only one solution-curve. The set of differential equations, treated as a system of algebraic equations at each point of the phase space, determines the components of a rate-of-change vector which are obtained explicitly by Cramer's rule. This vector is tangent to the solution curve. Each solution curve turns downward in z at some specific elevation z*, and this determines the condition for choking. Choking occurs always when the exit flow velocity at L=z* is equal to the local velocity of propagation of small plane disturbances of sufficiently large wavelength, that is when the flow rate G becomes equal to a specified, critical flow rate, G*. A criterion, analogous to the Mach number, which indicates the presence or absence of choking in a cross-section is the ratio K=G/G** of the mass-flow rate G to the local critical mass flow rate, G**, K=1 denoting choking. The critical parameters depend only on the thermodynamic properties of the fluid and are independent of the gravitational acceleration and shearing stress at the wall. The topological characteristics of the solutions allow us to study all flow patterns which can, and which cannot, occur in a pipe of given length L into which fluid is discharged through a rounded entrance from a stagnation reservoir and whose back-pressure is slowly lowered. The set of flow patterns is analogous to that which occurs with a perfect gas, except that the characteristic numerical values are different. They must be obtained by numerical integration and the influence of gravity must be allowed for. The preceding conclusions are valid for all assumptions concerning the shearing stress at the wall which make it dependent on the state parameters only, but not on their derivatives with respect to z. However, the study is limited to upward flows for which the shearing stress at the wall and the gravitational acceleration are codirectional.