Impeller Tip Clearance Research Articles

Effect of different blade number and thickness on performance of micro-turbine

At present, due to its special operating environment and operating conditions, there are few relevant theoretical studies and numerical simulation studies on micro-turbines. Therefore, the use of simulation technology to explore the internal flow law of micro-turbines is of great significance for the design optimization guidance of micro-turbines. The research method of this paper is through CFD software, using SST k-ω algorithm model. This paper studies the efficiency and output of micro-turbine with 8 blade numbers of 13,15,17,19,21,23,25 and 28, as well as the efficiency and output of micro-turbine with thickness of 12mm, 12.5mm, 13mm, 13.5mm and 14mm. The internal flow field characteristics, the pressure pulsation characteristics of impeller region and tip clearance region and the power loss distribution diagram are studied and analyzed. The results show that the maximum efficiency is 22.42% when the number of blades is 15, and 23.92% when the thickness is 10.5mm.

Effect of Flow on the Energy Conversion Characteristics of Multiphase Pumps Based on Energy Transport Theory

Multiphase pumps operate under different flow conditions with different work performance. In order to reveal the energy conversion regulations in multiphase pumps under different flows, this paper presents an analysis of the effects of different flows on the pressure propulsion power, Lamb vector dispersion, and vortex enstrophy dissipation in the pressurization unit of a multiphase pump based on energy transport theory. It is found that at different flows, the pressure propulsion power near the impeller inlet decreases sharply, the pressure propulsion power is mainly located in the first half of the impeller near the suction side of the blade, and with the increase in the flow, the pressure propulsion power in the pressurization unit increases gradually, as well as its energy loss, while the Lamb vector dispersion gradually increases and the area of scattering region tends to be narrow under the small impeller tip clearance, while the Lamb vector dispersion region area slowly decreases with the flow rate when the impeller tip clearance is larger. The effect of flow on the vortex enstrophy dissipation in the multiphase pump is mainly located in the middle of the impeller near the blade pressure surface, and as the flow increases, the value of the vortex enstrophy dissipation in the impeller pressurization unit increases accordingly, and the vortex enstrophy dissipation in the first half of the impeller is even more chaotic. The investigation results have significant theoretical meaning for the deep mastery of the energy conversion characteristics in multiphase pumps.

The Effect of Impeller Tip Clearance on the Performance of a MGT Mixed Flow Compressor Stage Fitted with a Crossover Diffuser

Mixed flow compressor stage designs are becoming increasingly popular for use in Micro Gas Turbine (MGT) engines. Due to manufacturing constraints and structural considerations, such compressors typically feature unshrouded impeller designs. A numeric investigation of the effect of impeller-shroud tip clearance on the performance of a MGT mixed flow compressor stage fitted with a crossover diffuser is presented. Three baseline test compressors are designed using an in-house-developed application based on one-dimensional mean line theory. These cover a wide range of design mass flow rates, design speeds, and impeller meridional exit (mixed flow) angles. The performance of each of the test compressors are evaluated at an impeller-shroud tip clearance of 0.1 mm, 0.3 mm, and 0.6 mm. Commercial CFD software is used to verify the performance results of the various test compressor configurations. It is found that an increase in impeller tip clearance negatively impacts both compressor performance (total-to-total pressure ratio and efficiency) and choke margin. Conversely, stall margin increases with an increase in impeller tip clearance.

Leakage Flow Characteristics in Blade Tip of Shaft Tubular Pump

The shaft tubular pump device is widely used in various water diversion projects because of its ultra-low head and large flow characteristics. Due to the tip clearance between the blade and the shroud, it is easy to cause hydraulic mechanical performance changes, induced vibration, and noise, which seriously affects the safe and stable operation of the pump. Steady and unsteady three-dimensional flow field numerical simulations of a shaft tubular pump device were carried out using computational fluid dynamics to investigate the impeller flow properties of the device under various flow conditions, including the tip clearance leakage flow (TCLF) and change rule of pressure pulsation. The TCLF, vortex morphology evolution, and pressure pulsation properties of the impeller tip clearance were analyzed. The results show that with an increase in the flow rate, the influence of the tip clearance size on the tip clearance flow decreases, the TCLF decreases, and the axial velocity of the water flow at the tip clearance increases. When the flow rate increases, the swirling strength of the tip leakage vortex decreases, and the distance between the tip leakage vortex and the suction surface of the blade increases. With the increase in flow rate, the pressure pulsation amplitude at the tip clearance increases first and then decreases. The focus of this study is to analyze the variation of tip clearance flow field and pressure pulsation under multiple working conditions, aiming to provide some help for improving the performance of the pump device and ensuring its safe operation.

Design, computational and experimental investigation of a small-scale turbopump for organic Rankine cycle systems

The hydraulic design, computational analysis, and experimental investigations of a high-speed small-scale turbopump for mobile waste heat recovery applications based on organic Rankine cycle systems are presented in this paper. Such applications demand high-pressure rise, lightweight, and compact pumping systems with simple construction. The investigated turbopump features an unshrouded 37.75 mm tip diameter single-stage centrifugal pump equipped with eight radial blades, eight splitters blades, and a rectangular axisymmetric volute. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar at a designed rotational speed of 25,000 rpm. Due to uncertainties observed in employing state-of-the-art 1-d methods that are valid for much larger machines, computational fluid dynamics is utilized to obtain a design meeting the specifications. The pump’s performance is evaluated experimentally at different rotational speeds, mass flow rates, and impeller tip clearances. The excellent agreement between experimental data and predictions from computational fluid dynamics validates the design methodology and computational results. The turbopump’s characteristics are then utilized to estimate the possible performance improvement of a target organic Rankine cycle using the novel turbopump instead of a commercial multi-stage centrifugal pump. The comparison suggests that the novel turbopump increases the efficiency of the target organic Rankine cycle by 0.3 % points and decreases its back-work ratio by nearly 50 %. The novel turbopump is approximately ten times more compact compared to commercial systems.

Research on the Influence of Impeller Tip Clearance on the Internal Flow Loss of Axial Circulating Pump Under Unpowered Driven Condition

Abstract An axial-flow pump (AFP) a key hydraulic component in the circulating water system of large ships. When the speed of a large ship meets certain requirements, the AFP operates in the unpowered driven condition and has the characteristics of low speed, positive rotation and driven rotation, thereby achieving energy recovery efficiency without investing in the driving device. The unsteady internal flow characteristics of the AFP under the unpowered driven condition differ from those of conventional conditions. The blade tip clearance (TC) is an essential basis for the comprehensive technical indicators of the lift pump and the safe and stable operation of the circulating water system. In this study, the performance of the AFP with different TCs under the unpowered driven condition is investigated based on experimental tests and numerical simulations. Based on the entropy production theory, the energy loss characteristics of the AFP with different TCs are studied, and it is concluded that increasing the TC increases the strength of tip leakage flow. Particularly at large TCs, the energy loss of the pump increases significantly, causing flow separation on the blade surface and the formation of large-area vortex structures on the blade. In addition, under the large spacing TC, the instantaneous entropy production of the pump is unstable, and the entropy production frequency and amplitude are much larger than those of the conventional TC.

Concurrent theoretical, experimental and numerical analyses of mixed-flow turbopump design

The article expands on the ongoing assessment of the reduced-order model proposed by some of the authors for the geometric definition and noncavitating performance evaluation of mixed-flow centrifugal turbopumps. The most significant predictions of the model are compared with URANS (Unsteady Reynolds-Averaged Navier-Stokes) simulations of the non-cavitating flow through a typical six-bladed, unshrouded, mixed-flow turbopump for liquid propellant rocket engines, operating at design and off-design conditions with different values of the impeller tip clearance. Both methods are preliminarily and successfully validated against the experimental results of the reference turbopump. The comparison of the analytical and numerical approaches highlights the detailed features of the flow phenomena not explicitly accounted for by the theoretical model, but also indicates that the model predictions of the most relevant dynamic aspects of the flow through the machine are in satisfactory agreement with the simulations over a broad range of operating conditions. Therefore, the results confirm the capability of the proposed model to generate useful engineering solutions for the turbopump preliminary design problem at a negligible fraction of the computational cost required by 3D numerical simulations. This feature can be expected to prove useful in efficiently addressing the preliminary parametric optimization of centrifugal turbopump design.

Experimental and Numerical Investigation of Clearance Effects in a Transonic Centrifugal Compressor

Extensive research studying tip clearance effects on compressor performance has been conducted during the past few decades with significant progress achieved. However, most of the previous investigations focus on stage-level analysis at the design condition, with very limited studies at the component level or part speed conditions. To fill this gap, this paper investigates the effects of impeller tip clearance on the performance of a high-speed centrifugal compressor, at both stage and component levels, covering a variety of operating speeds resulting in subsonic to transonic inlet conditions. Experiments were conducted on the Single Stage Centrifugal Compressor facility at two different running tip clearances. With increased clearance, the stage and impeller pressure ratio, isentropic efficiency, and work factor drop, but the static pressure recovery coefficient in the diffuser increases. As the compressor transitions from subsonic to transonic operation from 60 to 85% corrected speed, the sensitivities of stage and impeller pressure ratio and efficiency reach their peak values. Lastly, numerical simulations at design speed showed that the computational fluid dynamics (CFD) model sufficiently predicts the sensitivities due to tip clearance for most of the parameters at the stage and component levels, indicating the potential of using CFD to investigate the flow physics.

Effects of geometric and operational uncertainties on aerodynamic performance of centrifugal compressor stage

No real centrifugal compressor can exactly conform to its design geometry and expected operating conditions due to the uncertainties existing in the manufacturing and operational processes. Such uncertainties have been increasingly recognised to be detrimental to compressor performance. However, quite few studies have focused on the combined effects of geometric and operational uncertainties on compressor performance, and the underlying flow mechanism even remains unclear. In this context, we here present an uncertainty analysis of a centrifugal compressor stage, with both geometric and operational uncertainties taken into account. With the combination of CFD simulation and nonintrusive sparse grid based stochastic collocation methods, the combined and individual effects of total inlet temperature, total inlet pressure, outlet mass flow, impeller tip clearance and hub fillet radius on the stage/impeller performance are quantified and analysed. Particular attention is paid to elucidating the compressor performance variations through flow field and energy decomposition analyses. Results show that the considered uncertainties exert more influence on the compressor stage performance rather than on the impeller performance. Amongst the examined uncertainties, the impeller tip clearance contributes the most to the stage performance. The underlying mechanism lies in that the wake of impeller tip clearance produces distorted flow downstream towards the diffuser, which causes complicated vortex structures and less conversion of kinetic energy to pressure rise in the diffuser passage. The present study lays a theoretical foundation for the further uncertainty quantification and robust design of centrifugal compressors against various sources of uncertainties.

Analysis of the impeller's tip clearance size on the operating characteristics of the underwater vehicle with a new high-speed water jet propulsion pump

To make up for the lack of study on high-speed water jet propulsion pump tip clearances, a newly developed water jet pump system close to the critical specific speed nsc has been investigated. Based on the body of the underwater vehicle and the non-uniform flow, the pump system with different tip clearance sizes (0.1, 0.2, 0.4, 0.8 mm) is evaluated and validated via numerical simulations. Preliminary steady simulating results demonstrate that increasing the tip clearance size can reduce the overall performance of the head, the efficiency, pump thrust, and the consuming power. Following further unsteady simulations, we reveal the unsteady dynamic performance induced by the tip clearances, related to the similar distribution characteristics of the pressure intensities and the pressure pulsations around the underwater vehicle. Furthermore, the pressure intensity and pressure pulsations in the pump increase with the tip clearance. Quantitative and comparative analysis of the distribution characteristics indicates that the blade loading and scrapping effect of the shroud are key factors influencing the leakage flow. At last, the leakage flow's dynamic distributing law under different tip clearances have been clarified.

Transient characteristics of internal flow fields of mixed-flow pump with different tip clearances under stall condition

This paper investigates the influence of different tip clearances on the transient characteristics of mixed-flow pump under stall condition. The instantaneous internal flow fields of mixed-flow pump with four tip clearances (0.2 mm, 0.5 mm, 0.8 mm and 1.1 mm) are explored by conducting unsteady time accurate simulations. Reynolds-averaged Navier-Stokes (RANS) equations are employed in the simulations and the results of computations are compared with experimental data. The results show that the pump head decreases by 22.1% and the pump efficiency drops by 13.9% at design flow condition when the impeller tip clearance increases from 0.2 mm to 1.1 mm. The swirling flow occurs in the inlet pipe of the mixed-flow pump with different tip clearances under stall condition, and the initial starting point of the swirling flow gets further away from the impeller inlet with increase in tip clearance because of increase in circumferential velocity and change in momentum of the tip leakage flow (TLF). The high turbulent eddy dissipation (TED) regions in the flow are attributed to the TLF, swirling flow, back flow and stall vortex, and their intensity are affected by the change in tip clearance. The oscillating trend of time domain distribution of TED enhances first and then decreases with increase in tip clearance and it exhibits a propagation feature under the effect of stall vortex, while most of the energy in the frequency domain remains concentrated in the low frequency part under stall condition.

Experimental Study of Pressure Pulsation in a Mixed-Flow Pump with Different Tip Clearances Based on Wavelet Analysis

In order to investigate the effect of various impeller tip clearances on the pressure pulsation in a mixed-flow pump, the energy performance test and pressure pulsation experiment of a mixed-flow pump with different tip clearances are studied simultaneously. The pressure pulsation signals at impeller inlet, middle, and oulet are processed and analyzed by the wavelet transform. The results show that the change of tip clearance can affect the head and efficiency of mixed-flow pump. Under designed flow rate, when the tip clearance increases from 0.2 mm to 1.1 mm, the head and efficiency decrease by 18.1% and 11.6%, respectively. Due to the strong influence of blade passing frequency (BPF) at impeller middle, the period of pressure pulsation curve is about 1/4 of impeller rotation period. At impeller inlet, the low-frequency pulsation with energy concentration is the main disturbance frequency, and, with the increase of tip clearance, not only does the high-value region of wavelet spectrum expand to low-frequency direction but also it is easy to form second-order peaks in the time-averaged wavelet curve. At impeller outlet, affected by BPF and rotor-stator interaction (RSI), the high-frequency disturbance in RSI area decreases first and then increases. The wavelet coherence demonstrates the stable low-frequency disturbances in comparison to others and it will affect the flow field at impeller middle.

Energy characteristics of mixed-flow pump under different tip clearances based on entropy production analysis

In order to investigate the effect of impeller tip clearance on the internal flow fields and the hydraulic losses in mixed-flow pump, the entropy production method with computational fluid dynamics (CFD) is employed to analyze the energy losses in a low specified number mixed-flow pump with guide vane. The results show that the size of tip clearance is closely related to the external characteristic performance of mixed-flow pump, and the effect of tip clearance on the flow fields of mixed-flow pump is obvious at design flow rate condition. When the tip clearance raises from 0.2 mm to 1.1 mm, the head drop loss coefficient increases 1.62 times in the impeller. As the tip clearance augments from 0.2 mm to the 1.1 mm, the total entropy production in impeller increases by 142%. Whereas, the total entropy production in guide vane descends by 21.8% slightly. It indicates that the increase of tip leakage flow (TLF) may increase the energy losses in impeller but the hydraulic losses in guide vane is suppressed to some extent as a result of an existence of TLF. Therefore, for the sake of improving the energy performance of mixed-flow pump, it is necessary to take the scale of blade tip clearance into account and consider optimizing the hydraulic design structure of guide vanes comprehensively to match the tip clearance.

Theoretical Analyses of the Number of Backflow Vortices on an Axial Pump or Compressor

Abstract Backflow vortices occasionally occur in the annular mixing zone between the main and axially reverse whirling flows from the impeller tip clearance on an axial pump or compressor. A number (N) of tornado-like backflow vortices rotate around themselves and revolve around the casing axis with a diameter (d) and a revolving angular velocity (ω). To investigate the factors determining N and the movement of the backflow vortices, theoretical analyses are performed. Each backflow vortex is generated in the mixing zone; the core region of each backflow vortex is considered to be a forced vortex, while the outer region is considered to be a free vortex. The ratio (f) of the forced vortex to the distance between the backflow-vortex center and the casing is defined. Each backflow vortex has a circulation and induces movements of all the other backflow vortices depending on the distance between the vortices. The casing restricts the movements of all the backflow vortices, and imaginary image vortices are considered on the other side of the casing. Consequently, for d, ω, N, and f, any parameter can be determined if the other three parameters are specified. As an application of the present theory to an inducer representing an axial pump or compressor, the number (Ncav) of “backflow-vortex cavitations,” which occur around the backflow-vortex center, is predicted. Cavitation is visible; therefore, Ncav is quantitatively measurable. In the parameter ranges studied for the tested inducer, the predicted value of N accurately agrees with the experimentally measured value of Ncav.

Influence of tip leakage flow and inlet distortion flow on a mixed-flow pump with different tip clearances within the stall condition

In order to investigate the effect of impeller tip clearance on internal flow fields and the rotating stall inception impacted by tip leakage vortex and inlet unsteady flow in a mixed-flow pump, mixed-flow pump models with tip clearances of 0.5 mm, 0.8 mm, and 1.1 mm were numerically calculated, and then the energy performance curves and internal flow structures were obtained and compared. The results show that the pump efficiency and the internal flow fields of numerical calculation are in good agreement with experimental results at design flow rate and near-stall condition. A portion of the positive slope segment appears in the energy performance curves under different tip clearances. The lowest head of the mixed-flow pump in the positive slope region decreases with the increase of the tip clearance while the highest head shows an opposite situation indicating that mixed-flow pumps are easier to stall under small tip clearance. At the design flow rate condition, the tip leakage vortex is relatively stable under different tip clearances and appears as a “snail shell” shape, whereas in rotating stall conditions, the “snail shell” shape disappear and the tip leakage flow on blade front forms a “flat” vortex structure. The inlet swirl flow not only affects the tip leakage flow in rotating stall conditions under different tip clearances, but also blocks the fluid from the inlet pipe. Under the circumstance of the same tip clearance, the main frequency amplitude of pressure pulsation coefficient gradually shifts away from blade passing frequency (96.67 Hz) to the axial frequency (24.17 Hz) when the pump operates in the stall condition.

Design and Analysis of S-CO2 Cycle and Radial Turbine for SOFC Vehicle Waste-Heat Recovery

Solid oxide fuel cell (SOFC) vehicles are considered to have broad prospects for development, and the high operating temperature of SOFC results in great potential for waste-heat recovery. Many concepts for utilizing waste heat of SOFC have been suggested and studied, and most of them directly couple an SOFC to a gas turbine, which require the SOFC to operate at an elevated pressure and make the system less flexible and thus harder to operate. In recent years, with the development of turbine and heat exchanger technology, the supercritical carbon dioxide (S-CO2) power cycle has raised widespread attractions for the waste recovery. This study explores the potential of S-CO2 Brayton cycle to realize waste-heat recovery for an SOFC vehicle. The SOFC can operate at atmospheric pressure, and the hybrid system is easier to operate than the directly coupled systems. In this paper, a simple recuperated S-CO2 Brayton cycle is proposed and the key component, radial inflow turbine is designed and focused. The flow state of the designed turbine is analyzed in detail based on computational fluid dynamics (CFD) numerical simulation. Five cases with different impeller tip clearances are numerically simulated to study its influence on the turbine performance. In addition, off-design performance analysis of the radial inflow turbine is conducted considering the temperature fluctuation of SOFC in practical applications.

Numerical Study of a Left Ventricular Assist Device (LVAD) With Different Blade Heights and Tip Clearances

One treatment modality for heart failure is to employ a mechanical heart assist device to increase blood flow to peripheral organs. There are various kinds of axial and centrifugal type mechanical pumps available for implantation depending on patient condition. Axial pumps are smaller in size comparatively, although centrifugal pumps have the advantages of lower rotational speed as well as better maintaining any native blood flow pulsatility. This work presents the results of the numerical study of the centrifugal blood pump configured as a Left Ventricular Assist Device (LVAD). The pump design utilized standard industrial centrifugal pump design principles but applied to smaller sized blood pumps. Flow characteristics are modelled using 3-dimensional steady state models operating at design speed of 2000 rpm using Newtonian blood properties for the fluid. Two design parameters of the pump are studied, the impeller blade height and tip clearance resulting in nine model variants. Analysis includes the hydrodynamic performance of the pump and the flow characteristics in the pump. A haemolysis prediction model quantifying red blood cell stress from exposure time and shear stress was used for quantitative predictions of haemolysis within the blood pump. Blood damage estimation was calculated along each path-line and averaged to a single value. By using a ranked selection method, the model with the 15 mm blade height and 800 µm tip clearance was selected as the preferred configuration with Haemolysis Index of 0.01 mmHg, efficiency of 58% at 104 mmHg outlet pressure.

Influence of tip clearance on pressure fluctuations in an axial flow pump

Rotor-stator interaction in axial pumps can produce pressure fluctuations and further vibrations even damage to the pump system in some extreme case. In this paper, the influence of tip clearance on pressure fluctuations in an axial flow water pump has been investigated by numerical method. Three-dimensional unsteady flow in the axial flow water pump has been simulated with different tip clearances between the impeller blade tip and the casing wall. In addition to monitoring pressure fluctuations at some typical points, a new method based on pressure statistics was proposed to determine pressure fluctuations at all grid nodes inside the whole pump. The comparison shows that the existence of impeller tip clearance magnifies the pressure fluctuations in the impeller region, from the hub to shroud. However, the effect on pressure fluctuation in the diffuser region is not evident. Furthermore, the tip clearance vortex has also been examined under different tip clearances.

Effects of Blade Deformation on the Performance of a High Flow Coefficient Mixed Flow Impeller

The effects of blade deformation under running conditions on the performance of a highly loaded transonic mixed flow impeller were investigated. Two impellers were manufactured, one using the “running” blade profiles as designed and one using the converted “unrunning” or “cold” geometry. Both impellers were tested experimentally and investigated numerically. The test data taken with smooth casing showed that at maximum speed, the isentropic efficiency and pressure ratio of the running geometry was higher than the unrunning geometry by about 0.4% and 1.4%, respectively. However, the difference in performance diminished in the presence of recirculating casing treatment. Numerical calculations suggested that the differences at high speeds were mainly due to the variation in the impeller tip clearance. The calculations using deformed blade profiles under centrifugal load only, predicted performance differences which were about twice as high as the measured values. However, closer predictions were obtained when the effects of pressure loads on blade deformation were included using closely coupled fluid-structural analyses.

1615 遠心圧縮機の羽根車翼端隙間が内部流れに及ぼす影響

The CFD analysis was conducted to investigate the effect of impeller tip clearance on internal flow in a centrifugal compressor with vaned diffuser. In previous studies, the leading-edge vortex (LEV) was produced near the diffuser leading-edge and the growth of the LEV deteriorated compressor performance. In addition, the LEV was enlarged by the tip-leakage vortex. In case of using impeller without clearance, corner vortex produced on pressure side of the impeller plays a similar role to tip-leakage vortex. Furthermore, the LEV develops and forms a huge flow blockage within diffuser passages at off-design operation, considering impeller tip clearance. However, the flow fields in diffuser passages were stable because the LEV was stationary on the suction side of the diffuser leading-edge in analysis of using impeller without tip clearance. Therefore, the tip-leakage vortex was considered to be the cause of diffuser stall at off-design operation.