New correlations for determining nonuniform liquid film thickness and interfacial friction in two-phase flow within a vertically inclined pipe

Two-phase flow in inclined pipes frequently occurs in industrial applications such as oil and gas production, heat exchangers and nuclear reactors. Understanding liquid film thickness and interfacial friction coefficient is crucial for designing and optimizing these systems. This study investigates the effect of limited inclination angles on liquid film thickness and interfacial friction coefficient in two-phase flow within vertical pipes. An image recording and processing technique analyzes the counter-current flow of water and air, which forms an annular flow pattern in transparent pipes of various sizes. The results are segregated based on superficial velocities, Reynolds numbers and pipe diameters. A multivariate correlation is developed to determine the ratio of maximum to minimum film thickness and an equivalent film thickness correlation for inclined pipes. The interfacial friction coefficient is also evaluated, leading to a tri-variable correlation. The proposed correlations align closely with presented data and prior research (R2=0.96 for film thickness ratio and R2=0.97 for interfacial friction coefficient). The maximum deviation between data and proposed correlations is 2.86% for film thickness and 4.3% for the interfacial friction coefficient. These findings enhance the understanding of two-phase flow behavior in inclined pipes and offer valuable tools for industrial design and optimization.