This work presents a numerical investigation of the fluid flow in the first stage of a two-stage centrifugal pump with a vaned diffuser. A computational fluid dynamics (CFD) analysis is performed by using the ANSYS-CFX software for a wide range of volumetric flow rates and also for different rotor speeds. The numerical results are validated against measured values of pressure rise through the impeller and diffuser of the first stage with an overall good agreement. Nevertheless, not only the best efficiency point evaluated numerically is overestimated in comparison with the measured two-stage pump values but also the computed hydraulic efficiency of the first stage. Investigation of the flow pattern for different flow rates reveals that the flow becomes badly oriented for part-load conditions. In such cases, significant levels of turbulence and blade orientation effects are observed, mainly in the diffuser. In spite of different flow rates or rotor speeds, the flow pattern is quite similar if the flow dimensionless coefficient is kept constant, showing that classical similarity rules can be applied in this case. By using such rules, it was also possible to derive a single equation for the pump head to represent the whole operational range of the pump.
Turbulent flow in d-type corrugated pipes of various aspect ratios has been numerically investigated in terms of flow pattern and friction factor, for Reynolds numbers ranging from 5000 to 100,000. The present numerical model was verified by comparing the friction factor with experimental and numerical results from the literature. The numerical analysis suggested that d-type behavior exists for groove aspect ratios up to w/k = (groove width/rib height) = 2 independent of the pitch. However, for a ratio of w/k = 3 an important change in the flow pattern occurs so that the pressure drag exerted by the groove walls becomes important. It is shown that the friction factor is independent of the groove height as long as the flow is similar to a flow in a d-type corrugated pipe. Moreover, the friction factor curve for d-type pipes shows a logarithmic behavior as function of the Reynolds number, so that a simple method can be used to derive an expression for the friction factor as a function of the Reynolds number and the relative groove width only. The results may be useful to engineering projects that require a better prediction of the friction factor in d-type corrugated pipes.
(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
Centrifugal pumps operate below their nominal capacity when handling gas–liquid flows. This problem is sensitive to many variables, such as the impeller speed and the liquid flow rate. Several works evaluate the effect of operating conditions in the pump performance, but few bring information about the associated gas–liquid flow dynamics. Studying the gas phase behavior, however, can help understanding why the pump performance is degraded depending on the operating condition. In this context, this paper presents a numerical and experimental study of the motion of bubbles in a centrifugal pump impeller. The casing and the impeller of a commercial pump were replaced by transparent components to allow evaluating the bubbles' trajectories through high-speed photography. The bubble motion was also evaluated with a numerical particle-tracking method. A good agreement between both approaches was found. The numerical model is explored to evaluate how the bubble trajectories are affected by variables such as the bubble diameter and the liquid flow rate. Results show that the displacement of bubbles in the impeller is hindered by an increase of their diameter and impeller speed but facilitated by an increase of the liquid flow rate. A force analysis to support understanding the pattern of the bubble trajectories was provided. This analysis should enlighten the readers about the dynamics leading to bubble coalescence inside an impeller channel, which is the main reason behind the performance degradation that pumps experience when operating with gas–liquid flows.
This work presents the characterization of the flow in a centrifugal separator. The study is focused on the behavior of the liquid phase in this kind of equipment, so that only single-phase liquid flow at the inlet of the separator is considered. The parameters investigated are the film thickness, the flow velocity components, the flow streamlines angles and the liquid level height above the inlet. These quantities were assessed for different flow rates using numerical simulations and experimental measurements. Results show that an increase on the inlet flow rate causes an increase on the liquid film thickness and the liquid level height. Also, the centrifugal movement is intensified when the flow rate increases. The numerical results proved to describe the overall flow behavior satisfactorily when compared to the experiments. From the outcomes of this study, information can be extracted to understand the separation process in this type of separator.
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.
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