Centrifugal pumps are essential for many human activities, accounting for a considerable portion of the global electricity consumption. However, despite decades of study, the flow within the pump’s impeller and its effects on the performance are far from being fully understood, particularly when the flow involves fluids more viscous than water. In this context, this paper reports experiments using time-resolved particle image velocimetry (TR-PIV) for investigating the flow of a 14-cP-viscosity mineral oil in a transparent pump with radial impeller. We found that: (i) at low flow rates, the positions of vortices depend on the fluid properties; (ii) at higher flow rates, the oil flows aligned in the radial direction, while the water flows following closely the blade curvature; (iii) the velocity profiles for the oil are approximately parabolic, whereas those for water are flatter; (iv) the average deflection angle of the velocity vectors relative to the blade curvature changes significantly with viscosity; (v) contrary to common expectation, the turbulent kinetic energy is up to four times higher for oil than for water; (vi) vortices are periodically formed and dissipated with a frequency proportional to the rotational speed. Our results provide new insights into the flow of viscous fluids in pumps, with valuable information for their design and installation.
Tag: Particle image velocimetry
Analysis of energy losses and head produced by a radial impeller using particle image velocimetry
Centrifugal pumps play a crucial role in industrial operations involving fluid transport. The quest for optimizing efficiency and reducing energy usage is a driving force behind research into their performance. The literature continues to offer opportunities for the creation of models that accurately depict the head generated by pumps, with a particular focus on impellers. The current pumps, however, are still far from being completely optimized. The idea of this paper is to conduct an analysis of energy losses and propose a mathematical expression to represent the head produced by a radial impeller, P23 model, working with water flow, considering that head is influenced by losses due to recirculation, shock/incidence, internal friction. The head losses are quantitatively evaluated from experimental data acquired via particle image velocimetry, which provides information on velocity vector direction and wall shear stress, both useful for the analysis. Our results reveal that the loss due to friction is the most significant, accounting for 40–90% of the total head loss, while shock and recirculation losses are restricted to 35% and 25%, respectively. Friction factors vary from 1.0 to 26 depending on the flow rate, as a result of wall shear stresses reaching up to 430 N/m2, mainly influenced by pressure and pseudoforces. The head calculated through the new proposed expression is finally compared with the actual head generated by the impeller, measured via experiments dedicated to assess the pump performance. According to our results, the relative deviations between the calculated and measured heads are limited to 5%. Although our results have been validated for a single P23 impeller geometry, the methodology developed here can be extended to other impellers in the future. The results may thus represent a step forward for designing more efficient and power-saving pumps.
Particle image velocimetry in a centrifugal pump: Influence of walls on the flow at different axial positions
For almost a century, humans have relied on centrifugal pumps for the transport of low-viscous fluids in commercial, agricultural, and industrial activities. Details of the fluid flow in impellers often influence the overall performance of the centrifugal pump and may explain unstable and inefficient operations taking place sometimes. However, most studies in the literature were devoted to understanding the flow in the midaxial position of the impeller, only with a few focusing their analysis on regions closer to solid walls. This paper aims to study the water flow in the vicinity of the front and rear covers (shroud and hub) of a radial impeller to address the influence of these walls on the fluid dynamics. For that, experiments using particle image velocimetry (PIV) were conducted in a transparent pump at three different axial planes, and the PIV images were processed to obtain the average velocity fields and profiles, as well as turbulence levels. Our results suggest that: (i) significant angular deviations are observed when the velocity vectors on the peripheral planes are compared with those on the central plane; (ii) the velocity profiles close to the border are similar to those in the middle, but the magnitudes are lower close to the hub than to the shroud; (iii) the turbulent kinetic energy on the periphery is up to eight times greater than that measured at the center. Our results bring new insights that can help propose mathematical models and improve the design of new impellers. A database and technical drawings of the centrifugal pump are also available in this paper so that other researchers can perform numerical simulations and validate them against experimental data.
Particle image velocimetry in a centrifugal pump: Details of the fluid flow at different operation conditions
Centrifugal pumps are present in the daily life of human beings. They are essential to several industrial processes that transport single- and multi-phase flows with the presence of water, gases, and emulsions, for example. When pumping low-viscous liquids, the flow behavior in impellers and diffusers may affect the centrifugal pump performance. For these flows, complex structures promote instabilities and inefficiencies that may represent a waste of energetic and financial resources. In this context, this paper aims at characterizing single-phase water flows in one complete stage of a centrifugal pump to improve our understanding of the relationship between flow behavior and pump performance. For that, a transparent pump prototype was designed, manufactured and installed in a test facility, and experiments using particle image velocimetry (PIV) were conducted at different conditions. The acquired images were then processed to obtain instantaneous flow fields, from which the flow characteristics were determined. Our results indicate that the flow morphology depends on the rotational speed of the impeller and water flow rate: (i) the flow is uniform when the pump works at the best efficiency point (BEP), with streamlines aligned with the blades, and low vorticity and turbulence in the impeller; (ii) the velocity field becomes complex as the pump begins to operate at off-design conditions, away from BEP. In this case, velocity fluctuations and energy losses due to turbulence increase to higher numbers. Those results bring new insights into the problem, helping validate numerical simulations, propose mathematical models, and improve the design of new impellers.
Flow Visualization In The Impeller And Diffuser Of A Centrifugal Pump Using Time-Resolved Particle Image Velocimetry
The present paper describes an experimental study on the flow dynamics within a centrifugal pump impeller. A transparent pump prototype made of acrylic parts was firstly developed for flow visualization purposes. Then, single-phase flow experiments were conducted in different impeller rotational speeds and water flow rates. A time-resolved particle image velocimetry (TR-PIV) system was used as the flow visualization method. As a result, velocity fields were obtained in the whole impeller. They reveal that the flow behaviour is dependent on the pump operational condition. When the pump works at the best efficiency point (BEP), the flow is uniform and the streamlines follow the blade curvature. However, when the machine works at off-design conditions, the flow becomes complex, with the presence of turbulent structures which cause a reduction in the pump performance. This type of result may be useful to validate numerical simulations and support the proposition of new mathematical models, new impeller geometries, among other applications.
Flow visualization in centrifugal pumps: A review of methods and experimental studies
Methods for flow visualization have been decisive for the historical development of fluid mechanics. In recent years, technological advances in cameras, lasers, and other devices improved the accuracy and reliability of methods such as High-Speed Imaging (HSI) and Particle Image Velocimetry (PIV), which have become more efficient in visualizing complex transient flows. Thus, the study of centrifugal pumps now relies on experimental techniques that enable a quantitative characterization of single- and two-phase flows within impellers and diffusers. This is particularly important for oil production, which massively employs the so-called Electrical Submersible Pump (ESP), whose performance depends on the behavior of bubbles and drops inside its impellers. Visualization methods are frequently used to study gas-liquid flows in pumps; however, the visualization of liquid-liquid dispersions is complex and less common, with few publications available. Methods to characterize the motion of gas bubbles are often unsuitable for liquid drops, especially when these drops are arranged as emulsions. In this context, there is room to expand the use of visualization techniques to study liquid-liquid mixtures in pumps, in order to improve the comprehension of phenomena such as effective viscosity and phase inversion with focus on the proposition of mathematical models, for example. This is a main motivation for this paper, which presents a review of researches available in the literature on flow visualization in centrifugal pumps. A broad set of studies are reported to provide the reader with a complete summary of the main practices adopted and results achieved by scientists worldwide. The paper compares the methods, investigates their advantages and limitations, and suggests future studies that may complement the knowledge and fill the current gaps on the visualization of single-phase flows, gas-liquid, and liquid-liquid mixtures.