Henrique Stel

CFD Investigation of the Effect of Viscosity on a Three-Stage Electric Submersible Pump

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.Copyright

Numerical Study of the Influence of Viscosity on the Performance of an Electrical Submersible Pump

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.Copyright

Numerical Simulation of the Flow in a Centrifugal Pump With a Vaned Diffuser

This work presents a numerical investigation of the water flow in the first stage of a two-stage centrifugal pump with a vaned diffuser. The geometry consists of a pipe intake, a 8-blade impeller, a 12-blade diffuser and an outlet extension chamber. The numerical modeling comprises a transient rotorstator interface connection between the impeller and the other static domains, and it was implemented in the commercial code ANSYS-CFX. The numerical runs were carried out for four impeller speeds and a wide range of volumetric flow rates. The standard k-e turbulence model was used. An experimental loop is also used to measure pump head values to validate the numerical approach. Comparison of numerical head values with the experimental data showed a good agreement. Similarity relations used for the numerical head values for different impeller speeds also shows good agreement within the range studied. A transient analysis of the pressure values at the impeller-diffuser interface showed that, using a steady state frozen rotor approximation as the initial condition for the transient calculation, generally no more than half of a complete impeller revolution is needed for the pump to achieve temporal periodicity. This numerical procedure saves significantly the computational time. Moreover, the numerical results confirm that, once the periodic regime is achieved, an azimuthal periodicity at each 90° interval is also achieved, just as expected from the 8-to-12 blades ratio between impeller and diffuser. Comparison of the numerical efficiencies of the single-stage pump with the experimental counterpart showed significant discrepancies. These must be related to the geometric simplifications of the numerical model and volumetric pressure losses of the real pump not included in the numerical model. Consequently, the Best Efficiency Point of the single-stage pump was found to be different from the two-stage assembly, and the flow field analysis apparently confirms this feature.Copyright

Numerical investigation of the effect of viscosity in a multistage electric submersible pump

ABSTRACT Electric 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.

Numerical Study of the Free Surface Flow in a Centrifugal Gas-Liquid Separator

This work presents a numerical investigation of the free surface flow in a gas-liquid separator with a cylindrical expansion chamber using Computational Fluid Dynamics. The centrifugal flow is set through a tangentially oriented nozzle at the chamber wall and is modeled using an inhomogeneous Eulerian-Eulerian multiphase flow model with the free surface approach to capture the phases interface. Fluid dynamics is examined for a range of fluid viscosities and flow rates for a single-phase liquid flow at the inlet. Liquid-gas bubbly-flow at the inlet is also considered to investigate some aspects of the phases separation inside the cylindrical chamber. Analysis of the flow field reveals that the impact of the fluid at the chamber wall combined with the centrifugal movement push part of the fluid upwards, stabilizing a liquid level above the nozzle. Near the inlet, the flow dynamics is characterized by a strong centrifugal motion, which decreases continuously below the entrance position due to gravity and viscous effects. The liquid level over the chamber wall and the centrifugal intensity increase with the flow rate, but decrease with viscosity. Viscosity also tends to enlarge the liquid layer thickness over the chamber wall and diminish the residence time of the liquid from the inlet down to the chamber’s bottom exit. Investigation of liquid-gas bubbly-flows in this equipment shows that separation occurs mainly near the chamber entrance due to the sudden expansion and the formation of a thin fluid layer over the chamber wall. In a percentage basis, phases tend to be more effectively separated for higher inlet gas volume fractions, lower liquid viscosities and bigger gas bubbles. These conclusions give technically interesting information for dimensioning hydrocyclones for gas-liquid flow separation.Copyright

Numerical Study of the Fluid Flow in a Cylindrical Hydrocyclone Separator

This work presents a numerical study on the flow inside a gas-liquid cylindrical hydrocyclone separator. This equipment operates with a free-surface liquid film flow, which is a combination of a centrifugal and a gravitational movement originated by a tangential nozzle. The computational package ANSYS-CFX was employed to simulate the flow using an inhomogeneous Eulerian-Eulerian multiphase flow model with the free surface approach to capture the phases interface. Fluid dynamics is examined for a range of fluid viscosities and flow rates for a single-phase liquid flow at the inlet. The results of the simulations provided basis for the development of a compact mechanistic model for calculating velocity components, film thickness and other variables. This model was derived by analyzing the motion of a fluid element and then by including additional terms that represent the sudden expansion of the flow at the inlet of the cylindrical chamber. Then, the terms included and some model coefficients were calibrated using the numerical results. The outcomes of this work can be used to predict the flow dynamics in a hydrocyclone, which is a fundamental step for more complex evaluations such as estimating the separation efficiency and developing new constructive concepts for the equipment.Copyright