Mechanistic modeling and numerical simulation of in-situ gas void fraction inside ESP impeller

Abstract

When gas is entrained with liquid into an electrical submersible pump (ESP), the in-situ gas void fraction (αG) inside the ESP impeller is closely related to its pressure boosting ability. Due to complex pump geometries, the direct measurement of αG is very difficult to carry out. In this study, a mechanistic model for predicting the in-situ αG inside an ESP impeller is developed and validated by three-dimensional (3D) computational fluid dynamics (CFD) simulations. The pressure increment of ESP obtained from single-phase numerical simulations matches experimental measurement well. With a new bubble size prediction model implemented into multiphase CFD simulations, the calculated ESP pressure increments under gassy flow conditions also agree with experimental pump performance curves. As the inlet gas volumetric fraction (GVF) increases, the ESP boosting pressure deteriorates. The simulated in-situ αG, which is used to verify mechanistic model predictions, increases with bubble size increase and gas density or rotational speed decrease. Based on the radial velocity slippage between gas and liquid phases, the in-situ αG can be determined with GVF, rotational speed, bubble size, and fluid properties etc. Compared with empirical correlations, the proposed mechanistic model can predict in-situ αG better by accounting for the gas-liquid phase interaction using the radial force balance between centrifugal buoyancy and drag forces exerted on a stable bubble.