Three-dimensional interface structures and characteristics in a stratified gas–liquid pipe flow

Abstract

In this study, the air–water flow in a pipe with a diameter of 100 mm was tested to investigate the three-dimensional (3D) interfacial characteristics of gas–liquid stratified flow in horizontal and slightly inclined pipes. The gas–liquid stratified flow interface exhibited a nonflat phenomenon (concave or convex). In particular, this phenomenon was verified by the transformation of the interface shape when the gas–liquid ratio exceeded 200 and 5 in the horizontal and inclined pipes, respectively. Compared with the flow in the horizontal pipe, the liquid holdup gradually decreased with an increase in the inclination angle in the downward stratified flow, and liquid height changes at the center was slight; however, the convex interface shape was obvious. In addition, the interface exhibited regular fluctuations in the flow direction under fixed operating conditions. Therefore, the hypothesis of a nonflat interface for stratified flow was proposed based on the experimental phenomena, and the interface shape function and flow equations were constructed to predict the degree of interface bending and critical conditions for gas–liquid stratified flow by analyzing the theoretical and influencing factors. The acceleration pressure drop was introduced to quantitatively characterize interfacial fluctuations in the flow direction; therefore, a modified model for nonflat stratified flow was developed. The proposed model could characterize the interface features in both the cross-sectional and flow directions of the pipe and provided a solution for the 3D interface of stratified flow. A comparison with the experimental results revealed that the proposed model performed the prediction satisfactorily. By solving the proposed model, the interfacial information in the pipe cross section and flow direction can be effectively predicted.