A 3D Transient Model for the Multiphase Flow in a Progressing-Cavity Pump

Abstract

Summary This paper presents the development of a computational-fluid-dynamics (CFD) model for the 3D transient two-phase flow within a progressing-cavity pump (PCP). The model implementation was only possible because of the meticulous mesh-generation and mesh-motion algorithm, previously published by the authors, which is briefly described herein. In this algorithm, a structured mesh was generated by defining all nodes’ positions and connectivities, for each rotor position by means of FORTRAN subroutines, which were embodied into ANSYS CFX software. The model is capable of predicting accurately the volumetric efficiency and the viscous losses, and it provides detailed information of pressure and velocity fields and void distribution along the pump. Such information could be of fundamental importance for product development and/or optimization for field operation. In field applications, the common situation is that in which the oil comes into the pump accompanied with free gas, which characterizes a multiphase flow. Simplified models on the basis of the calculation of the backflow or “slippage,” which is subtracted from the displaced flow rate, fail to characterize the PCP performance under multiphase conditions because the slip is variable along the pump. In this model, the governing equations were solved with an element-based finite-volume method in a moving mesh. The Eulerian-Eulerian approach, considering the homogeneous model, is used to model the flow of the gas/liquid mixture. The compressibility of the gas is taken into account, which is one of the main shortcomings in positive/constant displacement pumps. The effects of the different gas-volume fractions (GVFs) in pump volumetric efficiency, pressure distribution, power, slippage flow rate, and volumetric flow rate were analyzed, and some new insights are presented about the slippage in PCPs operating in multiphase conditions. The results show that the developed model is capable of reproducing pump dynamic behavior under multiphase-flow conditions performed early in experimental works.