Liquid hold-up, pressure gradient, and flow patterns in inclined gas-liquid pipe flow

A model is presented for gas-liquid flow and for the stratified-wavy-to-intermittent (slug or semislug) flow-pattern transition in slightly inclined pipes. The main parameter for predicting instabilities in stratified-wavy flow is the liquid hold-up, which attains a maximum, critical value at the flow-pattern transition. The critical liquid hold-up epsilon(L,SW-I) ranged from 0.07 to 0.42 in the experiments and is presented as a function of the dimensionless superficial gas velocity u(GS)/u(G0). It is used successfully to model the results of 1270 flow-pattern-transition experiments, performed in pipes of 26 and 51-mm diameter, at inclination angles beta between 0.1 degrees and 6.0 degrees, using atmospheric air-water and air-tetradecane systems. For calculating the liquid hold-up in the approach to the flow-pattern transition, which is required to predict the flow pattern, the modified apparent rough surface (MARS) model is presented. The MARS model contains separate equations for the interfacial, gas-to-wall, and liquid-to-wall shear stresses and for each of the three corresponding perimeters. Comparison plots show that liquid hold-ups, pressure gradients, flow-pattern boundaries, and dimensionless liquid-to-wall perimeters calculated with the MARS model agree well with their measured counterparts. (C) Elsevier Science Inc., 1997.