The study aimed to assess the flow characteristics of water-air and silicone oil-air in a vertical upward pipe, utilizing computational fluid dynamics (CFD) simulations with the volume of fluid (VOF) model. Structured meshes with various resolutions were employed to ensure mesh independence, and the k-Ɛ realizable model addressed turbulence. Simulations were conducted in a vertical pipe with a diameter of 67 mm, while varying superficial gas velocities. The investigation focused on the impact of superficial gas velocity on flow patterns, radial void fractions, void fraction time series, probability density functions (PDFs), and mean void fractions. Results indicated a transition in flow patterns with increasing superficial gas velocities: water-air shifted from cap bubbly-churn flow, and silicone oil-air transitioned from bubbly-annular flow. Notably, annular flow was observed in silicone oil even at low gas velocity. Substantial alterations were observed in radial void fraction profiles corresponding to changing flow patterns. Void fraction time series showed higher fluctuations for water compared to silicone oil, and PDFs identified regimes. Mean void fraction consistently demonstrated higher values for silicone oil compared to water across all flow conditions. The CFD results were validated against experiments, demonstrating good agreement. Furthermore, the validated model was applied to predict pressure drops and liquid velocities between the two systems. Silicone oil exhibited lower pressure drops compared to water. Significant differences in liquid velocities were observed between the two systems at 0.05 m/s and 5.71 m/s, emphasizing the impact of fluid properties.

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