Part-load Flow Research Articles

Effects of tip clearance on energy performance of three-stage electrical submersible pump

In order to investigate the influence of tip clearance on the energy performance of electrical submersible pump (ESP), numerical simulation and experimental study were carried out. The three-dimensional transient Reynolds-Averaged Navier–Stokes (RANS) equation is solved by using the ANSYS CFX software. The experimental results were used to verify the reliability of the numerical method. The results shown that the increase of tip clearance leads to the decrease of energy performance. The increase of tip clearance leads to a proportional drop in energy performance under part-load flow rate conditions, while the increase of tip clearance leads to a significant loss under large flow rate conditions due to increased leakage. The rotor-stator interaction between the impeller and the diffuser is the main factor for the enhanced turbulence at part-load flow rate conditions, resulting in energy losses. The maximum turbulence intensity along the axial direction in the tip clearance is 4.4 times and 4.7 times of the minimum turbulence intensity at rated flow rate 1.0Qdes and large flow rate 1.4 Qdes operating conditions, respectively. The results could be referred to predict the internal flow loss and optimize the design of ESPs.

Entropy production analysis for energy dissipation of a vertical mixed-flow pump device under asymmetric inflow conditions

To investigate the influence of the inflow angle on the hydraulic loss and internal flow fields of a vertical mixed-flow pump device, unsteady numerical simulations were performed under different inflow angles ( θ = 0°, 10°, 20°, and 30°). The calculation results were validated using an external characteristic test at an inflow angle of 0°. The entropy production theory was applied to analyse the energy loss and dissipation rate distribution in a vertical mixed-flow pump device under different inflow angles. Influenced by asymmetric inflow, the design point of the external characteristics deviated towards the overload flow rate, and the maximum deviations were obtained under 1.2 Q des, whereas the influence of the inflow angle on the internal flow fields was the most significant under the part-load flow rate. When the inflow angle increased from 0° to 30°, the total entropy production in the pump device increased by 18.35% under 0.6 Q des, and indirect dissipation accounted for the largest proportion. The internal flow patterns in the impeller and guide vane were also distorted by asymmetric inflow. The indirect dissipation rate on the suction side of the impeller blade, interface between the impeller and diffuser, and pressure side of the guide vane increased significantly with an increase in the inflow angle. To restrain the influence of asymmetric inflow, it is of great significance to optimise the hydraulic design structure of the inflow runner. In this study, a middle spacer pier was used to improve the operational efficiency of the pump device.

Influence of asymmetric inflow on the transient pressure fluctuation characteristics of a vertical mixed-flow pump

In order to investigate the influence of asymmetric inflow on the internal flow fields of a vertical mixed-flow pump, numerical simulation of unsteady flow was carried out under four inflow angles ( θ = 0°, 10°, 20°, and 30°). The reliability of the calculation results was validated through an external characteristic experiment. The fluctuation value of the pressure was defined to obtain the transient pressure fluctuation intensity distribution. Three monitoring points were arranged to compare pressure fluctuation characteristics under different inflow angles. The results suggested that the increase in the inflow angle caused the design operation points to deviate to the over-load flow rate. When the inflow angle increased from 0° to 30°, the axial velocity distribution coefficient and weighted average angle at the inflow runner outlet both decreased. The overall transient pressure fluctuation intensity inside the impeller increased, especially on the pressure side of the blade. Additionally, the transient pressure fluctuation intensity on the pressure side of the guide vane also increased significantly. As a result, the amplitude of pressure fluctuation at the dominant frequency of each monitoring point increased. Relatively speaking, the negative influence of asymmetric inflow on the operational stability of the pumping station was further extended under the part-load flow rate.

Effect of Rotation Speed and Flow Rate on Slip Factor in a Centrifugal Pump

This work analyzes the causes of the slip phenomenon in the impeller on the basis of the internal flow mechanism. Detailed optical measurements of the flow inside the rotation passages of a five-bladed centrifugal pump impeller are obtained through particle image velocimetry (PIV). On the basis of experimental data, the deviation coefficient of slip velocity is proposed and then revised according to the slip factor calculation formula of Stechkin. Results show that, at the same rotation speed, the slip factor increases with the flow rate and reaches the maximum value at 1.0 QBEP flow rate. At different rotation speeds, the slip factor increases with the rotation speed and shows a relatively large variation range. Moreover, a revised slip factor formula is proposed. The modified model is suitable for the correction of slip factor at part-load flow rates and serves as a guide for the hydraulic performance design and prediction of centrifugal pumps.

Numerical Study of Rotor–Stator Interaction of a Centrifugal Pump at Part Load With Special Emphasis on Unsteady Blade Load

Abstract Three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes (URANS) flow simulations are conducted to investigate the highly unsteady flow field at part load operation of a centrifugal pump. By the availability of unsteady flow field measurement data in the impeller wake region, a thorough validation of the simulation method is performed. Grid independence of the results is ensured. Unsteady characteristics in terms of head and shaft power as well as transient blade loads are evaluated to assess the unsteady pump performance. Significant mis-loading of the blading is revealed when one blade passes the volute tongue and associated with the strong unsteady and 3D flow field in the impeller-volute tongue region. Negative radial velocity in the tongue region is the origin of a vortex at the blade pressure side and a subsequent pressure drop that leads to even temporally negative blade loading. The results provide a detailed insight in the complex part load flow field that might be utilized for an improved pump design. As a valuable secondary outcome, a comparison of results obtained by two widely used computational fluid dynamics (CFD) codes for pump flow simulation is provided, i.e., the commercial code ansyscfx and the branch foam-extend of the open source software openfoam. It is found that the results of both methods in terms of unsteady characteristics as well as local ensemble-averaged velocity field are consistent.

Modelling and Evaluation of Waste Heat Recovery Systems in the Case of a Heavy-Duty Diesel Engine

In the present study, the effects of Organic Rankine Cycle (ORC) and turbo-compound (T/C) system integration on a heavy-duty diesel engine (HDDE) is investigated. An inline six-cylinder turbocharged 11.5 liter compression ignition (CI) engine employing two waste heat recovery (WHR) strategies is modelled, simulated, and analyzed through a 1-D engine code called GT-Power. The WHR systems are evaluated by their ability to utilize the exhaust excess energy at the downstream of the primary turbocharger turbine, resulting in brake specific fuel consumption (BSFC) reduction. This excess energy is dependent on the mass flow rate and the temperature of engine exhaust gas. However, this energy varies with engine operational conditions, such as speed, load, etc. Therefore, the investigation is carried out at six engine major operating conditions consisting engine idling, minimum BFSC, part load, maximum torque, maximum power, and maximum exhaust flow rate. The results for the ORC and T/C systems indicated a 4.8% and 2.3% total average reduction in BSFC and also maximum thermal efficiencies of 8% and 10%, respectively. Unlike the ORC system, the T/C system was modelled as a secondary turbine arrangement, instead of an independent unit. This in turn deteriorated BSFC by 5.5%, mostly during low speed operation, due to the increased exhaust backpressure. It was further concluded that the T/C system performed superiorly to the ORC counterpart during top end engine speeds, however. The ORC presented a balanced and consistent operation across the engines speed and load range.

Influence of Guide Ring on Energy Loss in a Multistage Centrifugal Pump

Multistage centrifugal pumps are highly efficient and compact in structure. Pump efficiency can be improved by an effective understanding of hydraulic behavior and energy loss, however, the traditional hydraulic loss evaluation method does not readily reveal the specific locations of energy loss in the pump. In this study, a guide ring was imposed in multistage pumps, and an entropy production theory was applied to investigate irreversible energy loss of a multistage pump with and without guide ring. Detailed distributions of energy losses in the pumps were calculated to determine the respective entropy production rates (EPRs). The EPR values as calculated are in close accordance with actual hydraulic loss values in the pumps. EPR values were higher in the multistage pump with the guide ring than the pump without a guide ring under part-load flow conditions (0.2Qd). However, the vortex flow in the pump was weakened (or eliminated) by the guide ring as flow rate increased; this reduced energy loss in the chambers. Flow passing the chamber was stabilized by the guide ring, which decreased shock and vortex loss in the chamber and guide vane. Under both designed flow condition and overload conditions, the EPR values of the guide ring-equipped multistage pump were lower than those without the guide ring. Furthermore, minimum efficiency index (MEI) values were also calculated for the two chamber structures; it was found that overall efficiency of pump with guide ring is better than that without.

Effect of tip clearance gap and inducer on the transport of two-phase air-water flows by centrifugal pumps

The transport of two-phase (air/water) was experimentally investigated in a semi-open pump impeller of radial type. The influence of increasing the tip clearance gap and installing an upstream inducer on the pump performance were thoroughly investigated and compared, helping to select the optimal operation based on the considered flow conditions. Three experimental procedures were used to set the desired flow conditions to check a possible hysteresis due to gas accumulation. The head degradation, pump surging and flow instabilities were investigated for all cases. The whole pump was made of transparent acrylic glass, providing high-quality flow visualization. The two-phase flow regimes were recorded and identified using a high-speed camera. For single-phase flow, the results show a considerable performance reduction for the increased gap, while the inducer has only insignificant effects. For gas volume fractions within 1%⩽ε⩽3%, the performance deterioration of the semi-open impeller with standard gap is very gradual. However, the performance is strongly reduced for ε⩾4% due to the onset of big gas accumulations in the impeller. The abrupt performance drop could be delayed to ε=7% for the increased gap. Similarly, installing the inducer resulted in more robust performance up to ε=5% and ε=7% for overload and part-load flow conditions, respectively. For ε⩾8%, the performance is very low in all cases due to the occurrence of gas-locking phenomenon. Comparing different experimental procedures, obvious performance hysteresis could be seen within 4%⩽ε⩽6% when the standard gap is used. However, installing the inducer could strongly reduce the performance hysteresis, while increasing the gap could completely eliminate it. Flow instabilities and pump surging occur mainly in overload conditions for 4%⩽ε⩽5%. Having the inducer installed could positively damp the instabilities and reduce the surging region. The obtained two-phase maps show the strongly increased resistance to gas accumulation for the larger gap and the moderately improved resistance when installing the inducer. All the obtained experimental results are used for checking and validating CFD numerical models in a companion study.

Computational analysis of part-load flow control for crossflow hydro-turbines

Crossflow turbines are commonly used in low-head small hydro systems, typically less than 50 m, and produce power up to a few hundred kWs. A crossflow turbine is a unique type of hydro-turbine in that the flow passes twice through the runner. Therefore, the power is extracted in two stages. These turbines often employ a guide vane in the nozzle for controlling the flow entering the runner. However, a guide vane significantly reduces the quality of the entry flow by splitting it into two jets and producing non-uniform entry flow angles which can cause a serious loss in turbine efficiency. Adhikari and Wood (2017) and Adhikari and Wood (2018) showed that well designed crossflow turbines without a guide vane can achieve full-load efficiencies of over 90%. This leaves open the issue of maintaining high efficiency at part-load, defined as reduced flow at constant head. As for other hydro-turbine types, efficient part-load operation cannot be achieved solely by employing power electronics to reduce the shaft speed as the flow decreases. This paper analyzes a slider or “Cink” control device at the entry to the runner to reduce the entry arc, i.e. the angular extent of the runner entry, as the flow rate decreases, (Sinagra et al., 2014). We show that a properly operated slider will maintain the radial and azimuthal entry velocities at their full-load values and therefore will not change the optimum shaft speed as the flow rate changes. This simplifies the power electronics required for the turbine generator. Three-dimensional Reynolds-Averaged Navier-Stokes simulations of a 0.53 kW turbine with 88% full-load efficiency showed that the slider maintains high efficiency at part-flow conditions. We conclude that the slider shows significant advantages over a guide vane as the primary means of crossflow turbine control at part-load.

Deep Part Load Flow Analysis in a Francis Model turbine by means of two-phase unsteady flow simulations

Hydropower plants are indispensable to stabilize the grid by reacting quickly to changes of the energy demand. However, an extension of the operating range towards high and deep part load conditions without fatigue of the hydraulic components is desirable to increase their flexibility. In this paper a model sized Francis turbine at low discharge operating conditions (Q/QBEP = 0.27) is analyzed by means of computational fluid dynamics (CFD). Unsteady two-phase simulations for two Thoma-number conditions are conducted. Stochastic pressure oscillations, observed on the test rig at low discharge, require sophisticated numerical models together with small time steps, large grid sizes and long simulation times to cope with these fluctuations. In this paper the BSL-EARSM model (Explicit Algebraic Reynolds Stress) was applied as a compromise between scale resolving and two-equation turbulence models with respect to computational effort and accuracy. Simulation results are compared to pressure measurements showing reasonable agreement in resolving the frequency spectra and amplitude. Inner blade vortices were predicted successfully in shape and size. Surface streamlines in blade-to-blade view are presented, giving insights to the formation of the inner blade vortices. The acquired time dependent pressure fields can be used for quasi-static structural analysis (FEA) for fatigue calculations in the future.

Statistical characteristics of suction pressure signals for a centrifugal pump under cavitating conditions

Centrifugal pumps are often used in operating conditions where they can be susceptible to premature failure. The cavitation phenomenon is a common fault in centrifugal pumps and is associated with undesired effects. Among the numerous cavitation detection methods, the measurement of suction pressure fluctuation is one of the most used methods to detect or diagnose the degree of cavitation in a centrifugal pump. In this paper, a closed loop was established to investigate the pump cavitation phenomenon, the statistical parameters for PDF (Probability Density Function), Variance and RMS (Root Mean Square) were used to analyze the relationship between the cavitation performance and the suction pressure signals during the development of cavitation. It is found that the statistical parameters used in this research are able to capture critical cavitation condition and cavitation breakdown condition, whereas difficult for the detection of incipient cavitation in the pump. At part-load conditions, the pressure fluctuations at the impeller inlet show more complexity than the best efficiency point (BEP). Amplitude of PDF values of suction pressure increased steeply when the flow rate dropped to 40 m3/h (the design flow rate was 60 m3/h). One possible reason is that the flow structure in the impeller channel promotes an increase of the cavitation intensity when the flow rate is reduced to a certain degree. This shows that it is necessary to find the relationship between the cavitation instabilities and flow instabilities when centrifugal pumps operate under part-load flow rates.

Experimental analysis of the flow pattern of a pump turbine model in pump mode

Reversible pump turbines are the only means to store primary energy in an highly efficient way. Within a short time their operation can be switched between the different operational regimes thus enhancing the stabilization of the electric grid. These qualities in combination with the operation even at off-design conditions offer a high flexibility to the energy market. However, pump turbines pass through operational regimes where their behaviour becomes unstable. One of these effects occurs when the flowrate is decreased continuously down to a minimum. This point is the physical limitation of the pump operation and is very difficult to predict properly by numerical design without a model test. The purpose of the present study is to identify the fluid mechanical phenomena leading to the occurrence of instabilities of pump turbines in pump mode. A reduced scale model of a ANDRITZ pump turbine was installed on a 4-quadrant test rig for the experimental investigation of unstable conditions in pump mode. The performed measurements are based on the IEC60193-standard. Characteristic measurements at a single guide vane opening were carried out to get a detailed insight into the instabilities in pump mode. The interaction between runner and guide vane was analysed by Particle Image Velocimetry. Furthermore, high-speed visualizations of the suction side part load flow and the suction recirculation were performed. Like never before the flow pattern in the draft tube cone became visible with the help of a high-speed camera by intentionally caused cavitation effects which allow a qualitative view on the flow pattern in the draft tube cone. Suction recirculation is observed in form of single vortices separating from each runner blade and stretching into the draft tube against the main flow direction. To find an explanation for the flow phenomena responsible for the appearance of the unstable head curve also characteristic velocity distributions on the pressure side were combined with high-speed visualizations on the suction side of the pump turbine model. The results enhance the comprehension of the physical background leading to the instability and improve the numerical predictability of the instability in pump mode.

Numerical analysis of unsteady tip leakage vortex cavitation cloud and unstable suction-side-perpendicular cavitating vortices in an axial flow pump

The objective of this work is to simulate and analyze the formations of three-dimensional tip leakage vortex (TLV) cavitation cloud and the periodic collapse of TLV-induced suction-side-perpendicular cavitating vortice (SSPCV). Firstly, the improved SST k–ω turbulence model and the homogeneous cavitation model were validated by comparing the simulation result with the experiment of unsteady cavitation shedding flow around the NACA66-mod hydrofoil, and then the unsteady TLV cloud cavitation and unstable SSPCV in an axial flow pump were predicted using the improved numerical method. The predicted three-dimensional cavitation structures of TLV and SSPCV as well as the collapsing features show a good qualitative agreement with the high speed photography results. Numerical results show that the TLV cavitation cloud in the axial flow pump mainly includes tip clearance cavitation, shear layer cavitation, and TLV cavitation. The unsteady TLV cavitation cloud occurs near the blade trailing edge (TE) where the shapes of sheet cavitation and TLV cavitation fluctuate. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, and the entrainment affect of the TLV and the influence of the tip leakage flow at the tailing edge contribute to the orientation and development of the SSPCV. The existence of SSPCV was evidently approved to be a universal phenomenon in axial flow pumps. At the part-load flow rate condition, the SSPCV may trigger cavitation instability and suppress the tip cavitation in the neighboring blade. The cavitation cloud on the SS surface of the neighboring blade grows massively, accompanying with a new SSPCV in the neighboring flow passage, and this SSPCV collapses in a relatively short time.

Numerical and experimental investigation of tip leakage vortex trajectory and dynamics in an axial flow pump

Tip leakage vortex (TLV) in an axial flow pump was simulated by using the shear-stress transport (SST) k–ω turbulence model with a refined high-quality structured grid at different flow rate conditions. The TLV trajectories were obtained by using the swirling strength method corresponding to the cross-sections of streamlines of the TLV. High-speed photography experiments were conducted to observe the TLV trajectory based on cavitation tracing bubbles in an axial flow pump with a transparent casing. The TLV trajectories predicted by the SST k–ω turbulence model agreed well with the visualization results. The numerical and experimental results show that the starting point of the TLV is near the leading edge at part-load flow rate condition (Q/QBEP=0.85), and it moves towards the trailing edge to approximately 20% blade chord at the design flow rate condition (Q/QBEP=1.0). At large flow rate conditions (Q/QBEP=1.2), the starting point of the TLV shifts to about 40% blade chord, and the relative angle between the TLV trajectory and the blade chord is gradually reduced with the increased flow rate. Detailed statistics of the fluid dynamics of the end-wall shear layer and the TLV at design and off-design conditions were discussed based on the numerical results. The shear layer and jetting flow in the tip gap are highly affected by the pressure difference between the pressure side (PS) and suction side (SS). It was also found that the distributions of static pressure, turbulent kinetic energy (TKE) and vorticity inside the TLV core are associated with the TLV structure which is affected by blade loading and operation conditions of the axial flow pump.

CFD investigation and PIV validation of flow field in a compact return diffuser under strong part-load conditions

The internal flow fields in a compact return diffuser under strong part-load conditions are investigated both numerically and experimentally. For numerical simulation, three-dimensional unsteady Reynolds-Averaged Navier-Stokes equations are solved on high-quality structured grids in conjunction with the shear stress transport k-ω turbulence model by employing the computational fluid dynamics (CFD) software ANSYS-Fluent 14.5. For flow field measurements, a special test rig is designed and the two-dimensional particle image velocimetry (PIV) measurements are conducted in the diffuser midplane to capture the complex flow field and for validation of the CFD results. The analysis of the results has been focused on the flow structure in the diffuser, especially under part-load conditions. The detailed comparison between CFD and PIV results is performed. Vortical flow and recirculation flow patterns in the diffuser are captured and analyzed. Large flow separation and backflow appear under the part-load flow conditions. This paper provides a good data set for developing as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a compact return diffuser used with multistage pumps.

Numerical analysis of rotating stall instabilities of a pump- turbine in pump mode

Rotating stall may occur at part load flow of a pump-turbine in pump mode. Unstable flow structures developing under stall condition can lead to a sudden drop of efficiency, high dynamic load and even cavitation. CFD simulations on a pump-turbine model in pump mode were carried out to reveal the onset and developed mechanisms of these unstable flow phenomena at part load. The simulation results of energy-discharge and efficiency characteristics are in good agreement with those obtained by experiments. The more deviate from design conditions with decreasing flow rate, the more flow separations within the vanes. Under specific conditions, four stationary separation zones begin to progress on the circumference, rotating at a fraction of the impeller rotation rate. Rotating stalls lead to the flow in the vane diffuser channels alternating between outward jet flow and blockage. Strong jets impact the spiral casing wall causing high pressure pulsations. Severe separations of the stall cells disturb the flow inducing periodical large amplitude pressure fluctuations, of which the intensity at different span wise of the guide vanes is different. The enforced rotating nonuniform pressure distributions on the circumference lead to dynamic uniform forces on the impeller and guide vanes. The results show that the CFD simulations are capable to gain the complicated flow structure information for analysing the unstable characteristics of the pump mode at part load.

PIV measurements in a compact return diffuser under multi-conditions

Due to the complex three-dimensional geometries of impellers and diffusers, their design is a delicate and difficult task. Slight change could lead to significant changes in hydraulic performance and internal flow structure. Conversely, the grasp of the pump's internal flow pattern could benefit from pump design improvement. The internal flow fields in a compact return diffuser have been investigated experimentally under multi-conditions. A special Particle Image Velocimetry (PIV) test rig is designed, and the two-dimensional PIV measurements are successfully conducted in the diffuser mid-plane to capture the complex flow patterns. The analysis of the obtained results has been focused on the flow structure in diffuser, especially under part-load conditions. The vortex and recirculation flow patterns in diffuser are captured and analysed accordingly. Strong flow separation and back flow appeared at the part-load flow rates. Under the design and over-load conditions, the flow fields in diffuser are uniform, and the flow separation and back flow appear at the part-load flow rates, strong back flow is captured at one diffuser passage under 0.2Qdes.

Rotating stall mechanism and stability control in the pump flows

Based on experimental measurements and flow dynamics analysis, the mechanism of flow separations in the pump flow has been investigated and revealed. The possible flow separation including the rotating stall in the vaned diffuser under the normal flow condition has been confirmed to be merely a diffuser effect and caused by the oversized expansion angle of the diffuser. The dynamics of the part load flow separation of both the stationary and the non-stationary character has been identified to be related to the induced flow circulation at the impeller inlet. The associated flow separation mechanism has been revealed with respect to the build-up of a flow dynamical barrier on the shroud side of the impeller. It exactly explains the coincidence of the flow separation at the impeller inlet and that at the impeller outlet. To quantify the onset of the flow separation, a hypothesis is established based on the revealed mechanism. This has been satisfactorily validated by tests with flow visualizations, pressure measurements and laser doppler anemometry applications. The method for stability control of the pump flows with effective suppression of the flow separation has also been presented.

Rotating stall mechanism and stability control in the pump flows

Investigations based on experimental measurements and flow dynamics analysis have been completed to reveal the mechanism of flow separations in the pump flow. The possible flow separation including the rotating stall in the vaned diffuser under the normal flow condition has been confirmed to be merely a diffuser effect and caused by the oversized expansion angle of the diffuser. The part load flow separation of both the stationary and the non-stationary character has been identified to have its occurrence origin in the flow at the impeller inlet. The mechanism of the flow separation at the impeller inlet has been revealed with respect to the built-up of a flow dynamical barrier on the shroud side. It exactly explains the coincidence of the flow separation at the impeller inlet and that at the impeller outlet. To quantify the onset of the flow separation, a hypothesis is established based on the revealed mechanism. This has been satisfactorily validated by the test based on the flow visualization, pressure measurements and LDA applications. The method for stability control of the pump flows with effective suppression of the flow separation has also been presented.