Low Specific Speed Centrifugal Pump Research Articles

Near-Wall Flow Characteristics of a Centrifugal Impeller with Low Specific Speed

In order to study the near-wall region flow characteristics in a low-specific-speed centrifugal impeller, based on ANSYS-CFX 15.0 software, Reynolds averaged Navier-Stokes (RANS) methods and renormalization group (RNG) k-ɛ turbulence model were used to simulate the whole flow field of a low specific speed centrifugal pump with five blades under different flow rates. Simulation results of external characteristics of the pump were in good agreement with experimental results. Profiles were set on the pressure side and suction side of impeller blades at the distances of 0.5 mm and 2 mm, respectively, to study the distributions of flow characteristics near the wall region of five groups of blades. The results show that the near-wall region flow characteristics of five groups of blades were similar, but the static pressure, relative velocity, cross flow velocity, and turbulent kinetic energy of profiles on the pressure side were quite different to those on the suction sides, and these characteristics also changed with the alternation of flow rate. As the flow rate was 13 m3/h or 20 m3/h, within the radius range of 40 to 50 mm, there was an extent of negative relative velocity of the profiles on the pressure side, and a counter-current happened not on the suction side, but on the pressure side in the low specific speed centrifugal impeller. The flow characteristics of profiles at the distances of 0.5 mm and 2 mm also showed a small difference.

The effect of tongue geometry on pump performance in reverse mode: An experimental study

Low specific speed centrifugal pumps can operate as turbine in micro hydropower plants with relatively high head and low flowrate. Improving the hydraulic and mechanical characteristics of reverse pumps in such applications is highly appreciated. In the present study, the effect of tongue geometry on the performance and radial force of a low specific speed centrifugal reverse pump (10.3 [rpm, m3/s, m]) was studied experimentally. By measuring the flowrate, torque, inlet and outlet pressure as well as pressure distribution around the impeller periphery, hydraulic and mechanical characteristics of reverse pump for tongues of different stretchings and angles were obtained. Results showed that medium stretching can be used for reducing the radial force while further stretching of tongue is not beneficial. However, the longest tongue can boost efficiency the most. At nominal and high flowrates, tongue with medium stretching of angle −5° can rise the efficiency and output power at the highest level comparing with other angles, while that of −10° angle gives better condition at low capacities. The longest tongue of −10° could rise efficiency and output power up to 8% and 32% respectively in off-design conditions which makes it the best geometry for the reverse pump.

Design optimization of a low specific speed centrifugal pump with an unshrouded impeller for cryogenic liquid flow

Rocket engines require centrifugal pu mps used in its propellant feeding system to be smaller and lighter to increase the final speed of payloads, and be produced with lo wer costs. Reducing the number of stages and removing shroud rotating wall make pu mps lighter and easier to manufacture. Therefore, these pumps should be designed with higher head to reduce the number o f stages and have an unshrouded impeller. In this study, various shapes of shrouded and unshrouded impellers with the same meridional plane, that have various blade angle distributions, and splitter b lade shapes, have been analysed by a co mputational flu id dynamics(CFD) approach. Based on this survey, the impeller blade shape which demonstrates the highest head and efficiency has been designed considering the internal flow of the pump. As a result, it was observed that unshrouded impellers had the loss generation structure due to tip leakage flow wh ich was different fro m shrouded impellers had. Based on the above, a design method considering the tip leakage flow of unshrouded impeller was suggested.

Numerical investigation of tongue effects on pump performance in direct and reverse modes

This paper investigates the effects of tongue geometry on the performance and radial force of a low specific speed centrifugal pump in direct and reverse modes numerically. For the evaluation of unknown Reynolds stresses, the k-ω turbulence model was employed. The nonlinear convective terms in all transport equations were approximated by the second-order upwind scheme. The unstructured computation grid with 1.75×106 cells was used for the computation. Numerical results indicated that as expected, the tongue geometry significantly alters the performance and radial force in both direct and reverse modes. In the next step, the effects of two geometrical parameters of tongue, namely; tongue stretching and tongue angle were studied. The results obtained by CFD simulations in pump mode proved that the tongue with the most stretching yields the lowest head value in high flowrates and decreases efficiency. Moreover, it is found that the tongue with medium stretching has no sensible effect on the radial force, while one with the most stretching can increase the radial force significantly. In addition, this study proved that changing the tongue angle increases the radial force exerted on impeller and can lead to lower efficiencies in the pump. The numerical study of fluid flow in PAT mode revealed that by increasing the tongue stretching of a tongue while keeping the angle constant, both head and power increases. For the tongues with medium stretching, it is found there is no direct relation between tongue angle and head but by increasing the tongue angle both power and efficiency increase. In the tongue with the most stretching, the tongue with angle of -10° produced the highest power while that of -5° leads to the highest efficiencies in all flowrates.

A novel volute design for reducing radial force in pump and PAT

In the current paper a novel volute design is proposed to reduce the radial force of a low specific speed centrifugal pump in both direct and reverse modes. First, the fluid flow in the pump in both modes was simulated by utilizing commercial CFD software. For evaluation of unknown Reynolds-Stresses, the k-ω turbulence model was employed. The nonlinear convective terms in all transport equations were approximated by the second-order upwind scheme. The unstructured computation grid with 1.75×106 cells was used for the computation. The numerical results were verified against the measured data and acceptable agreement between them was found. Numerical results showed that changing the operating mode of a pump from direct to reverse increases the flowrate, head and power in the BEP but deteriorates the radial force. Therefore, a novel volute based on BEP flowrate of PAT was proposed. In pump mode, it was found that although this novel design decreases the efficiency in low flowrates slightly, it increases the head and efficiency in high flowrates considerably. Interestingly, the radial force is decreased significantly and the point of minimum radial force is shifted to 125% of BEP flowrate. The comparison of PAT performance with the original volute and the novel one revealed that the new design leads to lower head and power in all flowrates. However, this design provides more uniform pressure distribution around the impeller periphery and thus reduces the radial force.

Investigation of flow instability characteristics in a low specific speed centrifugal pump using a modified partially averaged Navier–Stokes model

In this study, a modified partially averaged Navier–Stokes (MPANS) model is applied to investigate the flow instability characteristics in a low specific centrifugal pump. In MPANS model, the unresolved-to-total ratio of kinetic energy fk is determined according to the local grid size and turbulence length scale. The numerical results by MPANS model are compared with that simulated by SST k-ω model and the available experimental data. It is noted that MPANS model shows better performance for investigating the unstable flow in the current pump under part-load operation conditions. The time-averaged internal flow and flow incidence in the pump impeller depicts that with the decreasing flow coefficient, flow separation develops in the impeller. Owing to the strong separation flow as well as vortex evolution, incidence angle is large and varies remarkably at the entrance of blade-to-blade passage in the pump impeller. The evolution dynamics of rotating stall is further discussed in detail based on vorticity transport equation. During the evolution of rotating stall, the vortex stretching term has an important effect on vorticity transport under the part-load conditions. The analysis of the pressure fluctuations excited by periodic evolution of rotating stall shows that the rotating stall cell propagates along the rotational direction, and identifies the rotating stall frequency ( fstall), which is much lower than the rotational frequency of the impeller, fn ( fstall = 8.76% fn). Finally, two-dimensional Lagrangian coherent structure (LCS) is used to reveal the separation flow in blade-to-blade passages of the pump by monitoring the trajectory of the particles. Both LCS and vortex structure by λ2 can clearly demonstrates the passage blockage and flow separation under the part-load operation conditions, depicting that the separation flow occurs at blade suction side and develops from the leading edge to the main passage in the impeller.

Effects of modifying the blade trailing edge profile on unsteady pressure pulsations and flow structures in a centrifugal pump

In the present work, the effects of modifying the blade pressure side (EPS profile) on unsteady pressure pulsations and flow structures in a low specific speed centrifugal pump are carried out by experimental and numerical methods. Results are compared to the original trailing edge (OTE profile). Unsteady pressure signals are captured at twenty measuring points at flow rate of 0–1.6Qd. It is observed that the pump head of the EPS profile is improved for all the concerned working conditions. Pressure amplitudes at the blade passing frequency are compared and discussed in detail. It is found that the EPS profile contributes to pressure pulsation reduction obviously. For all the measured flow rates, pressure amplitudes are attenuated evidently at major measuring positions, especially at high flow rates. As for the mean pressure amplitude of twenty measuring points, pressure amplitude is reduced more than 20% at the nominal flow rate using the EPS profile. From relative velocity distribution, it is found that the uniformity of flow field at the blade outlet region would be improved significantly by the EPS profile. Besides, the corresponding vorticity magnitude at the blade outlet would be reduced compared to the OTE profile. The combined effects contribute to the reduction of pressure amplitude using the EPS profile.

Influence of an Inlet Rotating Axial Device on the Cavitation Processes in a Low Specific Speed Centrifugal Pump

Influence of an Inlet Rotating Axial Device on the Cavitation Processes in a Low Specific Speed Centrifugal Pump

Cavitating flow-induced unsteady pressure pulsations in a low specific speed centrifugal pump.

With the development of cavitation, the high-energy pressure wave from a cavitation bubble collapsing is detrimental to the stable operation of centrifugal pumps. The present paper concentrates on pressure pulsations under cavitation conditions, and pressure amplitudes at the blade-passing frequency (fBPF) and RMS values in the 0–500 Hz frequency band are combined to investigate cavitation-induced pressure pulsations. The results show that components at fBPF always dominate the pressure spectrum even at the full cavitation stage. For points P1–P7 on the volute side wall, with a decreasing cavitation number, the pressure energy first remains unchanged and then it rises rapidly after the critical point. For point In1 in a volute suction pipe located close to the cavitation region, the pressure energy changes slightly at high cavitation numbers; then for a particular cavitation number range, the pressure energy decreases, and finally increases again. For different flow rates, the pressure energy at the critical point is much lower than the initial amplitude at the non-cavitation condition for In1. This demonstrates that the cavitation cloud in the typical stage is partially compressible, and the emitted pressure wave from a collapsing cavitation bubble is absorbed and attenuated significantly. Finally, this leads to the pressure energy decreasing rapidly for the measuring point In1 near the cavitation region.

Investigation of cavitation instabilities in a centrifugal pump based on one-element theory

In order to investigate the cavitation instabilities in a centrifugal pump, the unsteady cavitating flow in a low-specific speed centrifugal pump was simulated by using a modified SST k-ω turbulence model combined with Kubota cavitation model. The intrinsic and system instabilities of cavitating flow were comprehensively analyzed and interpreted. For the cavity patterns formation process in a centrifugal pump, the separated flow is dominant instead of re-entrant jet flow. The transient incidence angle, cavitation number, cavitation compliance and mass flow gain factor of the middle streamline were addressed in the centrifugal pump at design point based on classical one-element theory in order to characterize the system instability. The results show that the bubbles are static stability at cavitation inception stage, which can reduce pressure fluctuations by varied cavity volume and hard to develop to rotating cavitation. It performs static instability at the stage of development because of the positive-feedback between cavity volume and flow rates, the decreasing of bubble volume would lead to the increasing of flow rate, which can induce undesirable cavitation instabilities.

Numerical prediction of rotating stallin a low-specific speed centrifugal pump

Rotating stall is a common unstable flow phenomenon in centrifugal pumps, which usuallyoccurs in part-load conditions. Rotating stall will cause performance instability when thepump is running, and even excite the resonance of the whole pump system in someextreme cases. In this paper, the stall phenomena at part-load conditions have beeninvestigated by CFDsimulation for a centrifugal pump. The CFD results have beencompared with the experiments, including characteristic curves, internal flow structures andcorresponding frequency spectrum analyses. Furthermore, a parameter named theblockage coefficient for the pump impeller passage is put forward to quantitativelyevaluate the magnitude of stall.The large-scale vortex in the stalled channel is defined as astall cell.At the operation condition Q=0.35Q0, five rotating stall cells can be observedin the impeller, which propagate among different impeller channels with a rotatingfrequency lower than that of the impeller. From the frequency spectrum analysis, the mainfrequency of the stall cells is about 23.4% of the impeller rotational frequency, resulting in4.68% for each stall cells. For the operation condition of Q=0.38Q0, a “stationary stall”phenomenon has been observed in the impeller passages.

On the flow field and performance of a centrifugal pump under operational and geometrical uncertainties

• Effects of uncertainties on the flow field and performance of a centrifugal pump are studied. • Combined geometrical and operational uncertainties are considered. • Geometrical uncertainties are modeled using two Karhunen–Loéve (KL) expansions. • The uncertainties have significant influences on the flow field and performance of the pump. • Variation of the pump head is significant, while the efficiency shows a more robust behavior. Uncertainties are present in most engineering applications such as turbomachines . The performance of turbomachinery can be highly affected by the presence of uncertainties and these effects should be considered in the design procedure. The present study is the first work to quantify the impact of various uncertainties in a hydraulic machine. The paper is concerned with the combined effects of geometrical and operational uncertainties on the flow field and performance of a low specific-speed centrifugal pump. The uncertainty analysis is performed for three different flow rates, i.e., Q / Q d = 0.825 , 1.0 and 1.1175. The volumetric flow rate , rotational speed and blade geometry of the pump are assumed to be stochastic with uniform Probability Distribution Functions (PDFs). The randomness in the blade geometry, due to the manufacturing tolerances, is imposed through two Karhunen–Loève (KL) expansions. The eigenvalues of covariance kernel of KL expansions rapidly decay due to the large correlation lengths and thus a few terms are used in the truncated KL expansion to describe the blade geometrical uncertainties. The uncertainties are propagated in the flow field and performance of the pump using the Non-Intrusive Polynomial Chaos (NIPC) method. The respective effects of assumed uncertain variables on the quantities of interest are assessed using Sobol’ indices. The uncertain operational and geometrical conditions have significant influences on the flow field and performance of the pump. It is observed that while variation of the pump head coefficient under operational and geometrical uncertainties is significant, the pump efficiency shows a robust behavior under assumed uncertain conditions.

Unsteady flow structure and its evolution in a low specific speed centrifugal pump measured by PIV

The intense fluid-dynamic interaction associated with the complex flow structure in the centrifugal pump is detrimental to the stable operation of the pump. Based on the particle image velocimetry (PIV) measuring technique, flow structures in a low specific speed centrifugal pump are measured and presented in detail. Emphasis is laid upon the complex flow structures around the volute tongue region, where intense rotor–stator interaction appears. Results show that the typical jet-wake flow pattern at the blade outlet is observed for flow rates lower than the nominal working condition, and the inflection point of the jet-wake region is found to be at the center of the blade channel. At high flow rate, the jet-wake flow pattern is not apparent due to the high momentum fluid concentrating on the blade suction side. Within the pump, several typical vorticity sheets (both positive and negative) are captured on the blade pressure and suction sides at high flow rates, while at low flow rates, the negative vorticity sheets on the blade suction side disappear due to the separated and reverse flow structures occurring. When the vorticity sheet sheds from the blade trailing edge, it would impinge on the volute tongue with subsequent cutting and distortion. Three typical interacting modes are observed at different flow rates, and the cutting effect of the volute tongue on the vorticity sheet is dependent strongly on the local absolute velocity distribution. At high flow rates, the cutting effect is more significant, and the entire vorticity sheets (positive and negative) would be cut and torn by the volute tongue.

Prediction of the Effect of Impeller Trimming on the Hydraulic Performance of Low Specific-Speed Centrifugal Pumps

The effect of trimming of radial impellers on the hydraulic performance of low specific-speed centrifugal pumps is studied. Prediction methods from literature, together with a new prediction method that is based on the simplified description of the flow field in the impeller, are used to quantify the effect of trimming on the hydraulic performance. The predictions by these methods are compared to measured effects of trimming on the hydraulic performance for an extensive set of pumps for flow rates in the range of 80% to 110% of the best efficiency point. Of the considered methods, the new prediction method is more accurate (even for a large impeller trim of 12%) than the considered methods from literature. The new method generally overestimates the reduction in the pump head after trimming, and hence results less often in impeller trims that are too large when the method is used to determine the amount of trimming that is necessary in order to attain a specified head.

Effect of the blade loading distribution on hydrodynamic performance of a centrifugal pump with cylindrical blades

The effect of the blade loading distribution on head, radial force and pressure pulsation of a low specific-speed centrifugal pump with cylindrical impeller blades were investigated in the present study. Blade shapes were obtained by adopting the 1D inverse design method, impellers with different blade loading curves were obtained while the distribution of the blade loading was carefully tailored. Threedimensional URANS simulation method based on the Shear stress transport (SST) k-ω turbulence model was employed for the analyzation of flow patterns. Numerical results including the pressure distribution and velocity profile were validated by comparing with the available experimental data, and an acceptable agreement was obtained. Three typical parameters of the blade loading curve, including the location of the fore-loading point (mpre), location of the aft-loading point (mpost) and slope of the rectilinear segment (K), were analyzed. Results showed that the well-designed blade loading curve, such as the fore-loading impeller, can effectively reduce the pressure pulsation amplitude and the radial force. The significant effect of the variation of the aft-loading point on pump hydrodynamic performance was also investigated. Meanwhile, pressure and velocity distributions at different slopes of the blade loading curves show that the fore-loading impeller produces more uniform flow issuing from the impeller than that of the pump with aft-loading impeller, thus reduces the radial force and pressure pulsation of the pump.

Establishing a Relationship between Hydraulic Efficiency and Temperature Rise in Centrifugal Pumps: Experimental Study

AbstractHydraulic losses of a low specific speed centrifugal pump originate in the friction and turbulent dissipations in all components between the suction and discharge nozzles as well as fluid r...

Numerical investigation of pressure distribution in a low specific speed centrifugal pump

To study the pressure distribution of the volute casing, front casing and back casing in a prototype centrifugal pump, the pressure experiments and numerical simulations are carried out at six working conditions in this paper. The experimental results shows that the asymmetry of static pressure distribution on volute casing and front cavity is caused by the tongue of the volute and it may result in high radial and axial resultant force which can cause vibration and noise in the centrifugal pump. With the increasing of flow rate, the asymmetry of static pressure distribution and the magnitude of static pressure values reduce. The numerical results indicate that the pressure fluctuation near the tongue is strongest and it becomes slighter at point away from the tongue. With the increasing of flow rate, the local high-pressure region in impeller passage reduces and the flow becomes smoother accordingly, whereas the fluid speed becomes much higher which may cause further flow losses. The results predicted by numerical simulation are in coincident with the experimental ones. It shows that the turbulence model for simulating the flow field in centrifugal pumps is feasible.

The effect of wear ring clearance on flow field in the impeller sidewall gap and efficiency of a low specific speed centrifugal pump

In this paper, the flow in the impeller sidewall gap of a low specific speed centrifugal pump is analyzed to study the effect of wear ring clearance and the resultant through-flow on flow field in this cavity and investigate the overall efficiency of the pump. Centrifugal pumps are commonly subject to a reduction in the flow rate and volumetric efficiency due to abrasive liquids or working conditions, since the wear rings are progressively worn, the internal leakage flow is increased. In the new operating point, the overall efficiency of the pump cannot be predicted simply by using the pump characteristic curves. The flow field is simulated with the use of computational fluid dynamics and the three-dimensional full Navier–Stokes equations are solved using CFX software. In order to verify the numerical simulations, static pressure field in volute casing and pump performance curves are compared with the experimental measurements. The results show that, for the pump with minimum wear ring clearance, the disk friction efficiency is the strongest factor that impairs the overall efficiency. Therefore, when the ring clearance is enlarged more than three times, although volumetric efficiency decreases effectively but the reduction in overall efficiency is remarkably smaller due to improvement in the disk friction losses.

Numerical and Experimental Analysis of Cavitating Flow in a Low Specific Speed Centrifugal Pump With Different Surface Roughness

Three-dimensional (3D) simulations with ansys cfx 16.1 as well as measurements of the cavitating flow in a low specific speed centrifugal pump (nq = 12 min−1) are performed for different operation conditions and varying surface roughness. Surface roughness is considered by wall functions in the flow simulations. Good agreement between measured and calculated head is achieved for noncavitating flow. Net positive suction head (NPSH3%) rises toward overload due to incidence, flow separation, and vapor zones at the volute tongue. The NPSH3% rise is slightly higher for rough walls according to measurements and significantly overestimated by the wall function approach, irrespective of the roughness level in the simulation. A low-Reynolds number approach at the volute tongue leads to a more accurate prediction of NPSH3% than wall functions, at the cost of high computational effort.

Investigation of rotating stall for a centrifugal pump impeller using various SGS models

The accurate modeling and prediction of the rotating stall in a centrifugal pump is a significant challenge. One of the modeling techniques that can improve the accuracy of the flow predictions is the large eddy simulation (LES). The quality of the LES predictions depends on the sub-grid-scale (SGS) model implemented in the LES. This paper assesses the influence of various SGS models that are suitable for predicting rotating stall in a low-specific speed centrifugal pump impeller. The SGS models considered in the present work include the Smagorinsky model (SM), the dynamic Smagorinsky model (DSM), the dynamic non-linear model (DNM), the dynamic mixed model (DMM) and the dynamic mixed non-linear model (DMNM). The results obtained from these models are compared with the PIV and LDV experimental data. The analysis of the results shows that the SGS models have significant influences on the flow field. Among the models, the DSM, the DMM and the DMNM can successfully predict the “two-channel” stall phenomenon, but not the SM and the DNM. According to the simulations, the DMNM gives the best prediction on the mean velocity flow field and also indicates improvements for the simulation of the turbulent flow. Moreover, the high turbulent kinetic energy predicted by the DMNM is in the best agreement with the experiment data.