A new drag force model for the wake acceleration effect and its application to simulation of bubbly flow

• Proposal of drag force model with consideration of the wake acceleration effect on bubbly flow.
• Proposal of correlations which describe turbulent wake velocity profile based on systematic CFD simulation.
• Validation of the proposed drag force model with experimental data of bubbly flow.

Prediction accuracy of dispersed gas–liquid multiphase flow is strongly dependent on modeling of drag force. In this paper, a new drag force model was proposed to predict the relative velocity of bubbly flow. The proposed model considers the wake acceleration effect under large bubble diameter and high volume fraction conditions. Key relation of the new drag force model is the modeling of the relationship between the increased bubble rising velocity and the velocity profile of unstable wake, for which the concept of defining a proper reference fluid velocity is applied. It is found that there is a strong dependency of the increased bubble rising velocity on bubble aspect ratio and wake velocity. A systematic CFD simulation covering a wide range of bubble diameter was carried out, with which the velocity profiles of unstable wake in both radial and vertical directions were investigated in detail. Based on turbulent boundary layer theory, correlations were proposed to describe the velocity profile of unstable wake in terms of bubble terminal velocity and bubble diameter. Finally, the new drag force model was employed to predict the relative velocity in bubbly flow for validation calculation with selected test cases conducted in pipes of different sizes.