Gas-liquid interfacial structure and pressure drop characteristics of churn flow

Oscillatory characteristics of churn flow in the intermediate region between the annular and slug flows were experimentally investigated. To clarify the effects of the oscillatory motion of the liquid flow on the frictional pressure gradient, simultaneous measurements of the pressure drop and the spatio-temporal gas-liquid interface structure were conducted. From a comparison of time-averaged frictional pressure gradients with conventional correlations for steady two-phase flow, it was found that the measured frictional pressure gradient in the oscillatory flow was much greater than that predicted by correlations especially for churn flow under low liquid flow rate conditions. The liquid flow structure of churn flow was characterized by the flooding-type large wave formed on the thin liquid film. As the liquid flow rate decreased, the wave velocity became much higher than the average liquid velocity, which suggested that the frictional pressure gradient was dominantly caused not by the steady liquid film flow, but by the propagation of large wave. A large wave model was proposed, and was confirmed to be valid for predicting the frictional pressure gradient in both the churn and annular flow regions. For a practical calculation, a simple predictive method of the frictional pressure gradient for the churn and annular flows was also presented by means of the correlations of the two-phase friction multiplier and the liquid holdup. (C) 2003 Elsevier Inc. All rights reserved.