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.
Wall laws for viscous fluids near rough surfaces
