Impeller Shroud Research Articles

Study on the pulsation and fluid-structure coupling characteristics of rotating parts of vane type mixed-flow pump

Abstract Vane type mixed flow pumps (VTMFP) are widely used in water conservancy engineering, with a wide range of head applications. The impeller is the core component of the pump unit, and its internal flow and hydraulic excitation force affect the safe and stable operation of the unit. Aiming the pulsation characteristics inside the impeller, the VTMFP under different conditions is studied to examine the deformation and stress distribution by CFD, and the simulated results agree with the test data. The simulated results indicate that the impeller blade passing frequency consistently dominates the impeller inlet under various operating conditions. Furthermore, it is observed that the impeller rotating parts experience the highest levels of stress and deformation at low flow rate. The distribution of the equivalent force decreases from the hub to the shroud, while the deformation displacement decreases from the impeller shroud to the hub. In light of these findings, it is recommended that the design process takes into consideration the stress concentration near the blade root and the deformation at the blade edge.

A multi-constituent model for assessing the effect of impeller shroud on the thrombosis potential of a centrifugal blood pump.

Centrifugal blood pumps can be used for treating heart failure patients. However, pump thrombosis has remained one of the complications that trouble clinical treatment. This study analyzed the effect of impeller shroud on the thrombosis risk of the blood pump, and predicted areas prone to thrombosis. Multi-constituent transport equations were presented, considering mechanical activation and biochemical activation. It was found that activated platelets concentration can increase with shear stress and adenosine diphosphate(ADP) concentration increasing, and the highest risk of thrombosis inside the blood pump was under extracorporeal membrane oxygenation (ECMO) mode. Under the same condition, ADP concentration and thrombosis index of semi-shroud impeller can increase by 7.3% and 7.2% compared to the closed-shroud impeller. The main reason for the increase in thrombosis risk was owing to elevated scalar shear stress and more coagulation promoting factor-ADP released. The regions with higher thrombosis potential were in the center hole, top and bottom clearance. As a novelty, the findings revealed that impeller shroud can influence mechanical and biochemical activation factors. It is useful for identifying potential risk regions of thrombus formation based on relative comparisons.

Flow instability of an axial flow pump-as-turbine using relative streamline coordinates

When axial flow pumps-as-turbines (PATs) operate under off-design conditions, unstable and unsteady flow structures appear in the internal flow field, resulting in suboptimal functioning. These operating conditions not only decrease the efficiency of the hydraulic machines but also affect their mechanical reliability. This study establishes relative streamline coordinates, based on the blade's mean camber line, to investigate flow instabilities in axial flow PATs from a new perspective. Numerical simulations on an axial flow PAT were performed and validated using experimental data. The results show that flow separation is more likely to occur due to the more curved profile at the blade's suction surface, leading to considerable fluctuations in velocity along the flow direction and enstrophy amplitude near both the hub and impeller shroud. Moreover, the poor matching of the relative inflow angle of the impeller with the blade inlet angle leads to impingement losses near their leading edge, generating unstable flows and significant pressure pulsations, which induces hydraulic instability within the impeller. In addition to rotor-stator interference effects, the curvature of the blade suction surface profile and the bend structure of inlet conduit are significant factors that influence the pressure pulsation distribution of the PAT. An analysis of the enstrophy transport equation indicates that the relative vortex generation and the Reynolds stress dissipation terms play a key role in both vortex generation and dissipation, whereas the viscous term has a lower influence. These findings can serve as a reference for the optimization and efficient design of axial flow PATs.

Passive flow control at impeller radial bend for stall delay in centrifugal compressors with fishtail pipe diffusers

This paper proposes a new strategy for extending centrifugal compressor stall margin through shroud passive flow control placed at the impeller radial bend. Relative to the usual leading-edge location, the radial bend presents fewer geometrical constraints for flow control devices and allows them to have potentially more impact on low-momentum flow structures that lead to stall. Two centrifugal compressors with fishtail pipe diffusers are chosen for study: a high-speed transonic design exhibiting diffuser stall and a low-subsonic design with exducer stall. Four passive flow control techniques placed on the impeller shroud at the radial bend are evaluated for each compressor, in terms of stall margin improvement and efficiency penalty at design mass flow, using unsteady RANS CFD simulations. These are circumferential groove, slots, impeller recirculation pipe and diffuser-impeller recirculation pipe. The results indicate that passive flow control at the impeller shroud radial bend can be significantly more effective than at the impeller leading edge. For each compressor, the flow mechanisms associated with stall initiation and its delay by the most effective studied flow control technique as well as the associated efficiency penalty are elucidated. The findings indicate that techniques having flow injection with high spanwise penetration and low relative streamwise momentum are most effective for delaying diffuser stall, while the opposite applies to exducer stall. The design point efficiency penalty is due to mixing/shear losses from the jet-in-cross flow phenomenon in the impeller and shear losses from flow redistribution in the diffuser.

Particle Image Velocimetry in a Centrifugal Pump: Influence of Walls on the Flow at Different Axial Positions

Abstract 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.

Effect of Trimmed Rear Shroud on Performance and Axial Thrust of Multi-Stage Centrifugal Pump With Emphasis on Visualizing Flow Losses

Abstract Multi-stage centrifugal pumps are frequently used in high-lift applications and consume considerable energy, but suffer from poor performance and large axial force. The rear shroud of impeller is trimmed for reducing axial thrust, but this degrades performance. This study analyzes performance degradation and optimizes performance and axial force. Experiments and simulations are conducted on different ratios of rear shroud to front shroud (λ). Total pressure losses are calculated, and flow losses are visualized using the entropy generation method. Both measured and simulated performances decrease as the rear shroud is trimmed. Designs with different λ meet the head coefficient requirement of 1.1. However, λ of 0.86 has the best efficiency of 42.7%, λ of 0.83 reaches 42.5%, λ of 0.8 shows the lowest efficiency of 39.9%. Efficiency in the middle channel improves as the rear shroud is trimmed, but this cannot offset increased losses in the impeller and rear side chamber. Entropy production is exacerbated in the axial passage between impeller and rear side chamber due to the collision between impeller-driven flow and pressure-driven backflow. When λ is reduced by 0.03, axial thrust drops by 7%. To compromise between performance and axial thrust, λ should be designed at 0.83.

Influence of thermodynamic effects on rotor–stator cavity flow in liquid oxygen turbopump

Thermodynamic effects of the cryogenic medium have not been researched adequately for the accurate solution of the turbopump axial thrust, which is a key technique for the reusable rocket engine. In this paper, a liquid oxygen turbopump was chosen to reveal the influence of thermodynamic effects. Experimental tests using liquid nitrogen were carried out to verify the numerical model, and the numerical results under liquid oxygen were discussed to reveal the thermodynamic effects. The results show that the head coefficients and the efficiencies decrease under all operating conditions due to the alterations of the physical properties caused by the thermodynamic effects of the cryogenic medium. The total axial thrusts decrease in the range of 1.63% to 3.22%, and the maximum variations of the axial thrust acting on the impeller shroud and hub are 2.96% and 2.69%, separately, owing to the divergences of the cavity structure. The entropy generation rate was chosen to analyze the power loss, and the minimum deviation caused by the thermodynamic effects is 5.01% at the normal condition; the distributions of the entropy generation rate in the rotor–stator cavities are obviously changed due to the addition of the thermodynamic effects. The new omega method was selected to compare the vortex distribution. The vortex strength changes slightly, owing to the reduction of the medium viscosity caused by the temperature rise. It is critical to consider the thermodynamic effects of cryogenic media for accurately calculating the axial thrust of a high power-density turbopump.

150USRT Class R-513A Refrigerant Two-Stage Centrifugal Compressor Design Point and Separation Point Flow Field Simulation Analysis

This study used computational fluid dynamics for low greenhouse effect refrigerant (R-513A) simulation analysis in the two-stage 150 USRT class oil-free centrifugal refrigerant compressor using integrated part load value (IPLV) and internal flow field. The compressor rotor speed and mass flow rate for the IPLV working conditions with various loads were planned using Stage 1 and Stage 2 simulations, respectively. The performance and flow field numerical simulation analyses for the two-stage centrifugal compressor is discussed. This study used Ansys-CFX software for numerical simulation analysis and the conservation form of a 3D steady-state Navier–Stokes equation set with the finite volume discretization method for computation. The computing mode produced better computing efficiency and flexible mesh setting using the k-omega (k-ω) model, which has better computational performance in the near wall boundary layer and low Reynolds number flow field (used as the turbulence model) for simulation. The R-513A refrigerant property setting was calculated using the Soave–Redlich–Kwong gas equation. This study discusses the shaft work, pressure ratio, and isentropic efficiency and also describes the main research findings with the meridional pressure, isentropic efficiency contour maps, and flow field velocity vector diagrams. According to the numerical simulation results, in Stage 1 and Stage 2 simulations, the isentropic efficiency produced the highest numerical results in the 75% load case, which are 88.19% and 89.06%, respectively. The isentropic efficiency decreased to 75.93% and 82.26%, respectively, in the 25% load case. The flow field velocity vector diagram shows that in the 25% load case, there was back-flow field distribution near the impeller shroud. The compressor performance was also analyzed.

Onset of Flow Separation Phenomenon in a Low-Specific Speed Centrifugal Pump Impeller

AbstractThe efficiency of centrifugal pumps drops sharply when the flowrate is reduced below a threshold value. This is due to a profound change in the flow structure, characterized by a large of portion flow separation near the impeller blades and the formation of energy-intensive recirculation zones. So far, it is not clear how such flow separation may initiate and develop. This study combines state-of-the-art experiments and numerical simulations to explore the onset of flow separation in centrifugal impellers. In particular, a high-frequency particle image velocimetry (PIV) system is used to visualize the velocity field in impeller channels. The continuous relative velocity value and deviation angle relative to the blade surface are displayed before the stall inception conditions. Meanwhile, the validated numerical simulation method is used to compute the flow at similar experimental conditions. The results clearly show a cylindrical vortex band exists near the impeller shroud. As the flowrate decreases, the vortex grows gradually stronger, while moving to the junction between the impeller shroud and blade suction side, and then toward impeller hub along the blade suction side. This growing and moving vortex is the main cause of the flow separation near blade suction side observed in our experiments. Interestingly, the impeller head remains insensitive to this vortex until it causes the flowrate in the adjacent impeller channels to be redistributed. This led us to believe that stalled flow can be detected before it affects the hydrodynamic performances.

Energy loss mechanism due to tip leakage flow of axial flow pump as turbine under various operating conditions

Owing to its ability to handle large flows, an axial flow pump as turbine (PAT) can generate considerable amounts of electricity in small-scale hydropower plants. However, a PAT's efficiency can be hindered by tip leakage flow (TLF), namely, flow through the clearance between the impeller blade tip and shroud. Accordingly, this study investigates the influences of TLF on the PAT's energy performance through numerical simulations in which the entropy production method has been adopted. TLF and the associated tip leakage vortex (TLV) are found to both decrease the hydraulic efficiency and increase the flow rate; the shaft power output is also affected, especially near the machine's best efficiency point. The effect of TLF on the pressure distribution along the blade depends on the flow conditions, and the form of the TLV directly generated by TLF is affected by the flow incidence angle. The vorticity transport equation reveals that the vortex stretching term plays a dominant role in the spatial evolution of the TLV and has the greatest impact on the pressure distribution. Finally, different operating conditions lead to different energy loss mechanisms: turbulent dissipation is the main cause of energy loss, and high flow conditions are marked by an increase in TLF-dependent wall shear stress dissipation.

Multi-objective optimization of a two-stage liquefied natural gas cryogenic submerged pump-turbine in pump mode to reduce flow loss and cavitation

To simultaneously minimize the energy loss and optimize the suction performance in a two-stage LNG (liquefied natural gas) cryogenic submerged pump-turbine in pump mode (PTP), a multi-objective optimization work containing numerical models of cryogenic cavitation flow, novel suction performance prediction, entropy production diagnosis and multi-criteria decision-making method is conducted to obtain the optimal geometric variables, as well as the corresponding performances under 0.7 Q0 with typical growth trend in energy loss after cavitation. Results show that the inlet angle β1 at the shroud and the arc section radius of the shroud r1 of the first-stage impeller and the wrap angle Φ at the shroud of the second-stage impeller are the significant parameters affecting the energy loss and suction performance of the PTP. Then, the ideal compromise solution, β1 = 14.34°， r1 = 26.28 mm and Φ = 127.44°, is obtained by multi-objective optimization and decision-making. The active suppression of the impinging flow, flow separation and rotating stall in the two-stage impellers after optimization yields a 6.18 % drop in the total entropy production rate with sacrificing a 1.25 % drop in pump head. In addition, the PTP is more likely to maintain a better suction performance for the optimal design over a wide range of flow rates. This research contributes to the search for the tradeoff between the conflicting hydraulic performance of the multistage pump-fed unit.

Modeling of the Blade Leading-Edge Pressure Drop of Centrifugal Impeller Based on Machine-Learning

Blade leading-edge pressure drop is an important parameter that strongly influences the safe and stable operation of centrifugal pump. It is sensitive to blade leading-edge geometry. To understand the inlet flow state of centrifugal pump, it is necessary to evaluate the location and the minimum value of blade leading-edge pressure drop. However, the operation condition is complex. A large amount of experiments and numerical simulations should be conducted for a fully understanding of the local pressure field. With the development of artificial intelligence, machine learning can be used for accelerating the evaluations with potential laws of fluid mechanism. Therefore, neural network is used in this case to fit the solution space of condition, leading-edge geometry and local pressure drop. The influence of different factors can be well analyzed. The conditions which is not directly tested will be predicted by well-trained neural network. With the increasing of the leading-edge elliptic ratio of the blade inlet, the minimum pressure position of the blade moves from the impeller hub to the impeller shroud. This study provides a preliminary study and a good reference of the evaluation of blade inlet leading edge flow state in the future researches.

Investigation of the Flow Field and Aerodynamic Load on Impellers under Guide Vanes with a Self-Induced Slot in Compressor Radial Inlet

The flow field and the efficiency of the compressor can be improved and increased by the guide vane in the radial inlet chamber. However, the guide vane generates the wake and results in the rotor–stator interaction, which threatens the safety of the impeller. This paper investigated the guide vane with a self-induced slot (SIS) in a radial inlet, and the self-induced slot was a passive flow control method. Through computational fluid dynamics (CFD) simulation, the radial inlet containing unevenly distributed guide vanes (UGVs) in the hydrogen compressor was studied to clarify the flow phenomenon in the radial inlet and the aerodynamic load on the impellers. The simulation results showed that the self-induced slot did not affect the compressor performance but improved the pure wake style to the weak wake near the shroud region. The aerodynamic load on the impeller leading edge was obtained under different radial inlets through unsteady simulation. The dominant frequency and the pulse amplitude of aerodynamic load were obtained by fast Fourier transforms (FFTs). The SIS model had lower amplitude values at the impeller passing frequency, and the reduction in amplitude was about 18% compared to the UGV model near the impeller shroud region.

Sensitivity analysis on the influence factors of clearance axial force of a varying-speed centrifugal pump

Abstract Hydraulic axial force is an important parameter of the performance of centrifugal pump. It strongly affects the operation stability and the life-time of thrust bearing. Under the influence of flow pressure distribution, the hydraulic axial force of centrifugal pump has mainly two parts. One is the force inside impeller while the other is the force in clearance. In this case, the situation is more complex because the pump operates with varying-speed. Under the influence of rotation speed, pump head and minimum clearance width, the variation law of clearance axial force is still not fully understood. Therefore, the orthogonal experiment is conducted based on Large Eddy Simulation (LES). The local flow and hydraulic axial force in impeller shroud clearance is mainly studied. Results show that the pressure difference between impeller inlet and outlet will strongly impact the pressure distribution in clearance. Then, it affects the clearance axial force. The rotation speed and pump head are proved influential. However, range analysis shows that the minimum clearance width is the most dominant factor. It can somehow adjust the pressure distribution in clearance. The geometric design of impeller clearance will be important in reducing hydraulic axial force especially for varying-speed centrifugal pumps.

Effect of blade squealer tips on aerodynamic performance and stall margin in a transonic centrifugal compressor with vaned diffuser

In order to enhance the aerodynamic performance of a centrifugal compressor with a vaned diffuser while also improving its stall margin, squealer tip cavities were applied to the tips of the impeller blades. Computational analysis was carried out by solving the compressible three-dimensional Reynolds-averaged Navier-Stokes equations. The numerical approach was validated by comparing the results from the flat tip reference case with experimental measurements. Various squealer tips with different cavity depths were applied to both the main and splitter blades and were numerically tested to investigate the influence of cavity depth on the performance and stall margin of the compressor. The results show that squealer tips with a proper cavity depth were effective in enhancing compressor efficiency by 0.32% at the design point, in addition, they were able to improve the stall margin by 1.02%. It was found that the squealer tips reduce tip leakage flow and circumferential velocity near the impeller shroud compared to using flat tips due to the generation of vortical structure within the cavity, which blocks leakage flow in a similar way to labyrinth seals. However, it was observed that by increasing the cavity depth, the size of this vortex becomes larger and the entropy production is increased inside the cavity. Moreover, it was found that the proper cavity depth decreases the mixing tip loss by about 4% compared to flat tips, this plays an important role in the stall margin improvement.

Effect of impeller shroud trimming on the hydraulic performance of centrifugal pumps with low and medium specific speeds

The effect of pump impeller modification by a new approach, pump impeller shroud trimming (PIST), on the flow field and pump performance is examined both numerically and experimentally. The primary purpose of this study is to investigate the hydraulic performance of single-phase centrifugal pumps, which are refined by the PIST method, with low and medium specific speeds. In this type of shroud trimming, different trimming sizes are applied only to the shroud plate of the closed impeller, while the geometries (diameters) of the hub and blades remain unchanged. This modification, which increases the clearance between the impeller and casing from the shroud side, causes desirable conditions for pumping fluid containing undissolved gas. Computational fluid dynamics software (ANSYS-CFX) is used to predict the hydraulic performance of centrifugal pumps. Two well-known turbulence models, the renormalization group (RNG) k-ϵ developed model and shear stress transport (SST) k-ω, are used to predict the flow patterns. The computational results are verified through the comparison of experimental and unsteady numerical simulations with steady numerical data. After ensuring the accuracy of the flow-field simulation approach, further numerical analysis is performed for both pumps by changing the impeller shroud diameter. The effects of the geometric changes on the performance curves, efficiency, flow field, pressure distribution inside the pump components and the radial force acting on both pump types are investigated comprehensively. The results show that the shroud trimming reduces the produced head and efficiency at the design points. Examination of the radial force, which is applied on the rotating parts, shows that shroud-trimmed impellers experience higher radial forces than closed impellers in both pump types owing to the lack of a uniform pressure distribution around the impeller outlet. Accordingly, the relevant information obtained is used to modify the existing coefficients to predict the radial forces.

Flow inside impeller sidewall gaps and disk friction of radial centrifugal pump

Disk friction losses can significantly influence the overall pump effciency. It is most pronounced in the case of machines with low or moderate specific speed, which has to be kept in mind during a design process of a pump. However, analyses using computational fluid dynamics (CFD) often completely neglect the sidewall gaps geometry. Correctly solved flow in the sidewall gaps can have a great impact on the whole flow pattern inside the pump and therefore it can considerably influence the calculated effciency. The paper is focused on the precise simulation of the flow in the spaces between impeller shrouds and casing. CFD study using embedded Large Eddy Simulation (LES) approach in order to achieve accurate insight into the flow in sidewall gaps and reasonable computational time was performed. Obtained results are compared with data from experimental measurements.

Численное моделирование турбулентного течения в высоконапорном осерадиальном рабочем колесе центробежного компрессора холодильных машин

The paper presents the results of viscous flow modeling using different turbulence models in the Ansys CFX program for the D. Eckardt and H. Krain stage. The impellers are with an outlet angle of 90 and 60 degrees. Isentropic efficiency and the total pressure ratio are compared with the experimental data. The evaluation of the static pressure contours structure at the impeller shroud is made. Recommendations for qualitative modeling of high-head stages are given.

Roles of recirculating bubble on the performance of centrifugal compressors

Research works over the years have identified a recirculating bubble locating at impeller shroud in centrifugal compressor operating under off-design conditions. However, its roles on compressor performance have been traditionally oversimplified. In this paper, the characteristics of the recirculating bubble and the full extent of its impact on compressor performance are analyzed through a well-validated three-dimensional numerical simulation. The simulation results show that the bubble is initiated within the impeller passage and stretches gradually in both streamwise and spanwise directions with decreasing flowrates. During this process, it continues to reshape the effective area and flow conditions at the impeller inlet through its effects of blockage, preheating, and preswirl. The evolution of the recirculating bubble can be divided into two phases depending on whether the center of the recirculating bubble passes beyond the impeller inlet. In the initial phase, the blockage rapidly increases and helps to alleviate the impeller incidence and the relevant losses of the active flow. Meanwhile, the negative effects of preheating and preswirl remain relatively low. In the secondary phase, the upstream propagation of the recirculating bubble speeds up and reinforces its negative effects of preheating and preswirl. The fact that the positive and negative effects are not synchronous implies different roles that the bubble plays in the two phases. A model is established to characterize the effects of the recirculating bubble. A combination of the model and a single-zone meanline method is then employed to quantitatively assess the roles of the recirculating bubble by comparing the compressor performance with and without the bubble. The results show that in the initial phase, the recirculating bubble acted as a positive role in suppressing the incidence loss, the tip clearance loss, and the blade loading loss, leading to higher compressor efficiency. However, in the secondary phase, the negative effects overwhelmed the positive effects, which deteriorated the efficiency. The implications of the two-phase evolution of the recirculating bubble on centrifugal compressor design and flow control are also discussed.

Efficiency-House Optimization to Widen the Operation Range of the Double-Suction Centrifugal Pump

Most pumping machineries have a problem of obtaining a higher efficiency over a wide range of operating conditions. To solve that problem, an optimization strategy has been designed to widen the high-efficiency range of the double-suction centrifugal pump at design (Qd) and nondesign flow conditions. An orthogonal experimental scheme is therefore designed with the impeller hub and shroud angles as the decision variables. Then, the “efficiency-house” theory is introduced to convert the multiple objectives into a single optimization target. A two-layer feedforward artificial neural network (ANN) and the Kriging model were combine based on a hybrid approximate model and solved with swarm intelligence for global best parameters that would maximize the pump efficiency. The pump performance is predicted using three-dimensional Reynolds-averaged Navier–Stokes equations which is validated by the experimental test. With ANN, Kriging, and a hybrid approximate model, an optimization strategy is built to widen the high-efficiency range of the double-suction centrifugal pump at overload conditions by 1.63%, 1.95%, and 4.94% for flow conditions 0.8Qd, 1.0Qd, and 1.2Qd, respectively. A higher fitting accuracy is achieved for the hybrid approximation model compared with the single approximation model. A complete optimization platform based on efficiency-house and the hybrid approximation model is built to optimize the model double-suction centrifugal pump, and the results are satisfactory.