Interfacial friction in gas–liquid annular flow: analogies to full and transition roughness

New film thickness and pressure gradient data were obtained in a 5.08 × 101.6 mm duct for nitrogen and water in cocurrent upward annular flow. Pressures of 3.4 and 17 atm and temperatures of 38 and 93°C were used to vary the gas density between 4 and 20 kg/m3 and liquid viscosity between 0.3 × 10−3 and 0.7 × 10−3 kg/m s. These data are used to compute interfacial shear stresses and interfacial friction factors for comparison with several accepted literature correlations. These comparisons are reasonable for small values of the relative film thickness. However, the new data cover conditions not approached by the data used to construct those correlations. By combining the current data with the results of two other comprehensive modern experimental studies, a new correlation for the interfacial friction factor has been developed. This correlation adds elements of transition roughness to Wallis’ fully-rough analogy to better predict interfacial friction factors over a wide range of gas Reynolds numbers and liquid film thicknesses.