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(2017) Numerical investigation of the effect of viscosity in a multistage electric submersible pump, Engineering Applications of Computational Fluid Mechanics, 11:1, 258-272, DOI: 10.1080/19942060.2017 ABSTRACTElectric submersible pump (ESP) systems are commonly used as an artificial lift technique by the petroleum industry. Operations of ESPs in oil wells are subjected to performance degradation due to the effect of oil viscosity. To understand this effect a numerical study to simulate the flow in three stages of a multistage mixed-flow type ESP operating with a wide range of fluid viscosities, flow rates, and rotational speeds was conducted. The problem was solved by using a commercial computational fluid dynamics (CFD) software. The numerical model was validated with experimental head curves from the literature at different viscosities and rotational speeds available for the same ESP model used in this study, and good agreement was found. Performance degradation was evaluated by analyzing the effect of viscosity on head and flow rate. In addition, a flow field analysis to compare the flow behavior when the pump operates at different viscosities was carried out. The interaction between stages was also analyzed, and the influence of a previous stage on the upstream flow was evidenced. The flow field was analyzed at a curved surface that follows the complex mixed-flow geometry of the stages. CFD proved to be useful for exploring this kind of feature, a task whose accomplishment by means of experimental methods is not trivial. Such analysis helps to understand the flow pattern behind head and flow rate degradation when the Reynolds number is decreased. The results from this work are helpful as they provide a basis to estimate performance degradation for general scenarios. ARTICLE HISTORY

This work presents a numerical analysis on the influence of viscosity on the performance of a semi-axial electrical submersible pump (ESP) such as the ones used in offshore petroleum production. A single stage composed of an impeller with seven blades and a diffuser with seven vanes is considered. Flow simulations for water and other fluids with viscosities ranging from 60 to 1020 cP were performed with the aid of Computational Fluid Dynamics, and both design and off-design flow rates and impeller speeds were investigated. The numerical model was validated with experimental measurements of the static pressure difference on a given stage of a three-stage ESP system. Results showed good agreement between the computed and the measured pressure difference values. Analyzes of the water flow inside the pump revealed that the flow is blade-oriented at the best efficiency point as expected, while large separation zones are found in the impeller and diffuser channels for part-load conditions. However, flow is not strictly blade-oriented at the best efficiency point for fluids other than the water. Examination of performance for water and fluids with higher viscosities shows that similarity laws are restricted for water, and that the best efficiency point is shifted when considering viscous fluids. Also, head values for viscous fluids are degraded not just due to viscosity and high flow rates, but also with rotor speed. The flow pattern analysis and the results found may provide useful information for engineers concerned with highly viscous fluid pumping and, possibly, shed some light on the understanding of more complex phenomena associated with actual offshore oil production operations such as multiphase pumping of viscous fluids.

Electric Submersible Pumps (ESP’s) are multistage pump arrangements used in offshore petroleum production. Most of their applications are subject to viscous oil pumping, which causes performance degradation with respect to the regular service with water and changes some characteristics related to the flow dynamics inside the pump. The purpose of this work is to use CFD to investigate numerically the flow in a semi-axial type ESP with three stages operating with fluids of different viscosities. Both design and off-design flow rates are simulated, as well as different impeller rotation speeds. Head curves of the ESP for these cases are compared with experimental data and show good agreement. The importance of considering more than a single stage when studying ESP’s is discussed. The flow fields inside the pump channels for different operating conditions are compared, showing for instance that the flow is not always blade-oriented at the best efficiency point for service with fluids more viscous than water. The effect of the fluid viscosity and the rotation speed on the performance degradation is also explored. In addition, dimensional analysis is used in favor of a better understanding on how the pump performance degrades when working out of the design figure.

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