Numerical investigation of film thickness and wave statistics in gas-liquid downwards annular flows

Two-phase gas-liquid annular flows are observed in a broad range of industrial processes, such as production and pipeline systems for oil and gas distribution, steam generators, boiling water reactors, and emergency core cooling facilities to protect nuclear reactors. Although the global flow characteristics of annular gas-liquid flows have been studied experimentally for more than 50 years, their numerical modelling is still immature. We present a computational fluid dynamics model based on the volume of fluid method for simulating annular gas-liquid flows, focusing on the regular wave flow regime. We performed transient simulations on a 3-D domain using a commercial code (ANSYS Fluent 2021 R1). The mesh sensitivity analysis indicates that a very fine mesh must be used near the pipe wall to capture the liquid-gas interface correctly (Fig. 1). The code is validated through available experimental data [1] regarding topological flow properties. In particular, we considered mean film thickness, film roughness, base film thickness, and wave film thickness. We studied two operating conditions. The first is characterized by liquid and gas Reynolds numbers of 1 250 and 25 000, respectively. The second has the same liquid Reynolds number as the first, but the gas Reynolds number is increased to 30 000. A post-processing procedure is implemented to obtain the time traces of film thickness at 12 circumferential positions to capture the asymmetries in the flow. The numerical values of the quantities analyzed are in good agreement with the experimental findings, with a maximum error of 21.02% concerning the wave film thickness. The errors regarding the mean film thickness and film roughness are less than 10% for both the case studies. Considering the film thickness of time traces at different circumferential positions, we calculated the cross-correlation coefficients between them. The high values of the cross-correlation coefficients indicate that waves are coherent over the circumference of the pipe, following the experimental findings. Finally, to better understand wave activities, we generated the power spectral density functions for the two cases studied. They are characterized by a quasi-linear power decay, similar to that of the Kolmogorov spectrum for homogeneous and isotropic turbulence, which becomes slightly steeper for the case characterized by a higher gas Reynolds number, in accordance with the experimental data.