Clearance Leakage Research Articles

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.

Investigation on effect of anti-cavitation edge on performance and leakage vortex characteristics of tubular turbine for ocean power generation

Anti-cavitation edge directly affects turbine runner performance and clearance leakage flow. This paper takes a tubular turbine as the object of study and uses the numerical simulation method of SST-CC turbulence model based on rotation and curvature correction to study the effect of anti-cavitation edge thickness on the turbine internal flow patterns and the flow characteristics of tip leakage vortex under different conditions. The results show that the high-altitude bubble concentrated area on the blade suction surface is reduced after installing the anti-cavitation edges. Meanwhile, the leakage volume decreases by 13.28% under the optimal condition, and the maximum leakage decreases by 37.72% under the off-design condition. The raised structure of the anti-cavitation edge can effectively push the tip leakage vortex (TLV) away from the blade suction surface and destroy the original vortex structure; the strength of tip separation vortex (TSV) is obviously reduced. The energy loss increasingly reduces with the increase of anti-cavitation edge thickness, and the loss reduction value is greater in the off-design condition. The small negative helicity vortex at the draft tube wall surface disappears. The above research results provide a certain reference for the performance optimization of tubular turbine.

Prediction model for energy conversion characteristics during transient processes in a mixed-flow pump

The global energy shortage is gradually intensifying. A pump is a versatile device with significant power consumption in energy conversion equipment. Of the world's total energy generated, pumps consume about 20%. With the continuous expansion of the application field, the pump's transient working characteristics are increasingly getting important for energy saving and consumption reduction. Due to ignoring the influence of impeller rotational acceleration and fluid inertia, traditional methods have significant errors in predicting transient energy conversion characteristics, which leads to improper design and unreasonable lectotype, resulting in low pump operation efficiency. Improving the prediction accuracy of transient energy characteristics and ultimately improving the pump operation efficiency is considered to be the key solution to reducing energy consumption. Therefore, this research paper proposes a prediction model of transient energy characteristics in a mixed-flow pump. Firstly, the dynamic hydraulic torque, an important parameter of energy growth during the start-up process was calculated according to the momentum moment equation. Secondly, based on the torque energy balance equation and the relative Bernoulli equation, the transient head expression during the start-up process of the mixed-flow pump is derived, and the saddle area correction is carried out. Finally, considering the additional shaft power loss caused by the pump's backflow and clearance leakage flow, a calculation method for the instantaneous energy conversion efficiency of the mixed-flow pump is established. The accuracy of the model is compared with traditional quasi-steady-state calculation results. The results show that the predicted head of the model is in good agreement with the test head, the head estimation error is reduced from 15% to 5%, and the modified head in the saddle area is closer to the test value. In addition, this study also shows the ability of the model to simulate different start-up modes and start-up times. The models presented in this paper help guide the transient hydraulic design and prediction of transient energy characteristics of pumps.

Effects of the Shock Wave Structure on the Tip Clearance Leakage Flow in Transonic Compressor Rotors

The interaction between shock waves and the tip clearance flow in a transonic compressor rotor has important effects on the tip clearance flow and the rotor aerodynamic performance. In this paper, two transonic rotors with a high pressure ratio are selected to study the tip flow, one of which has one normal shock wave at the blade tip and the other has two shock waves (an oblique shock wave and a normal shock wave) at the tip. The two rotors have the same meridional flow channel, design point flow rate, pressure ratio, and rotation speed to focus on the influence caused by the effect of the shock wave structure. The numerical results for the flow fields show the following conclusions. The strength of the two shock waves at the blade tip is weaker than that of one normal shock, and the former two shock waves are less stable than the latter. Therefore, with increasing tip clearance, the efficiency, pressure ratio, and stall margin of the rotor with the two shock waves decrease more rapidly. The static pressure difference between the pressure and suction sides of the tip clearance is the only driving factor of the tip clearance leakage flow, and the leakage flow depends on the local pressure difference and the secondary leakage caused by adjacent blades. The movement speed of the annular wall is less than that of the leakage flow, which has a minor blocking effect on the tip clearance leakage flow. The change in tip clearance has little effect on the chordal distribution of the static pressure difference and leakage flow rate per unit area, so the total leakage flow varies linearly with the tip clearance size.

Study on the blade squealer tip affecting tip leakage flow and performance of a multiphase pump

Tip clearance is the distance required between the blade tip and the pump body wall of an impeller in a helicon-axial multiphase pump, which tends to induce tip leakage flow. The tip leakage vortex formed by the interaction of tip leakage flow with the mainstream can seriously affect the performance of the multiphase pump. To minimize the adverse effects of tip leakage flow in the multiphase pump, a method to design a squealer tip on the impeller blade is proposed in this paper. The effect of the squealer tip on external characteristics, tip clearance flow characteristics, and energy dissipation of the multiphase pump is analyzed. Research results indicate that the blade squealer tip can effectively improve hydraulic efficiency of the multiphase pump. At the optimal efficiency point, the head and hydraulic efficiency of the multiphase pump with a squealer tip increased by 3.62% and 4.15%, respectively, compared with the original model. The influence of tip leakage flow in the axial rear half passage of the multiphase pump impeller is far greater than that in the axial forward half passage, especially on the back position in the middle of the impeller passage. The squealer tip can restrain the reverse of leakage flow from the pressure side to the suction side of the impeller blade, and the clearance leakage flow of the model with a squealer tip is smaller than that of the original model. The squealer tip on blade will reduce the energy dissipation caused by unsteady flow in the mainstream. The research results in this paper can provide theoretical support for effectively restraining the influence of the tip leakage vortex on the mainstream of the helicon-axial multiphase pump and contribute to engineering practice value of improving the performance of the multiphase pump.

NACA0009 Hydrofoil Groove Design for Suppressing TLV and Noise

In hydraulic equipment, tip clearance may result in a leakage vortex known as a tip clearance leakage vortex (TLV), which reduces the machinery’s energy efficiency. In order to lessen the damage brought on by tip clearance, this research sets the angle, location, spacing, and depth of the grooves as significant criteria. The orthogonal approach is used to design the hydrofoil’s grooves. Numerous simulations are carried out using the SST k-turbulence model and the wideband noise source model. The results demonstrate how closely the numerical simulation matches what was seen in the experiments. The results demonstrate that grooves are a successful method for lowering TLV when compared to the original hydrofoil. The vortex area of the hydrofoil is reduced by 4.15% under the M5 design compared to the original hydrofoil. The groove significantly alters the dipole noise distribution of the hydrofoil’s leading edge in comparison to the original hydrofoil, resulting in a reduced decibel noise level at the hydrofoil’s tip clearance region. In comparison to the original hydrofoil, the average Curle APL of M5 and M8 decreased by 1.78 and 1.65%, respectively. Due to the way the groove blocks the TLV and muffles the noise it generates, M5 is in the optimum operational condition.

A novel approach for suppressing leakage flow through turbine blade tip gaps

Tip clearance leakage flow of the turbine bade is an important factor limiting the augment of the high pressure turbine efficiency, which should be suppressed utilizing certain methods. However, the passive control method with the traditional structure is more and more difficult to satisfy the suppressing ability of the advanced turbine demand. In the present paper, a synergetic suppressing method by combining the approach of blade shape modification and spontaneous injection is adopted, to construct a novel tip structure. The aerodynamic characteristics of the tip leakage flow (TLF) with different blade tip configurations, such as the squealer, squealer-winglet (SW) and squealer-winglet-spontaneous injection holes (SWS) composite configurations, are numerically investigated. The impacts of several key geometric parameters, such as the winglet width and the space ng of spontaneous injection holes, are also discussed. Due to the adjustment of the winglet, the SW tip configuration can get better suppressing effect on TLF than the squealer tip. The SWS synergetic suppression tip decrease the leakage flow rate and the leakage mixing loss on the basis of the SW tip due to the blocking effect of the spontaneous injection flow. The key geometric parameters study shows that the suppressing effect of the TLF can be improved by reasonably increasing the winglet width and reducing the spacing between spontaneous injection holes.

Experimental study on the aerodynamic performance of variable geometry low-pressure turbine adjustable guide vanes

An annular cascade testing rig of a variable-geometry low-pressure turbine with a large expansion angle was designed and built, combining a five-hole probe, a surface static pressure test system, and a PIV(Particle Image Velocimetry) measurement system. The effects of exit Mach numbers (0.2, 0.3, 0.4), turning angles (−1.5°, 0°, 3°), and adjustable device diameters (30 mm, 40 mm) on turbine cascade aerodynamic performance were experimentally investigated. Numerical simulations were also carried out to gain more insight into the flow fundamentals. Results show that the endwall clearance and the adjustable device significantly influence the flow field, with leakage flows before, after, and around the disc. The leakage vortex and passage vortex formation resulted in two high-loss regions in the shroud and hub endwall regions. The aerodynamic loss structures at the exit section were similar at different Mach numbers and varied considerably at different turning angles. Besides, a larger area of the adjustable device (d = 40 mm) was found to reduce the leakage loss caused by upper and lower clearance leakage flows and inhibit the development of secondary flows, resulting in a more straightforward secondary flow structure and a smaller influence range. Quantitatively, the adjustable device with a diameter of 40 mm could decrease the average total pressure loss coefficients by 11.5%, 16.6%, and 6.3% at three turning angles (−1.5°, 0°, 3°), respectively, compare to the adjustable device with a diameter of 30 mm.

Pulsed suction towards unsteady active flow control in an axial compressor cascade including clearance leakage effects

Tip leakage flow (TLF) induced by the gap between the stationary and rotating components brings complex flow structures, and this leads to a considerable loss in the compressor. Boundary layer suction is considered an efficient method to compensate the disadvantages brought by the TLF. In this study, the effect of hole-type pulsed suction (PS) on the TLF in a compressor cascade is investigated through a series of wind-tunnel experiment. PS is a more advanced technology over the constant suction (CS), and it considers improving of the compressor flow field from both spatial and frequency domain. The results indicate that the optimal aerodynamic performance is obtained when using PS at 25 % cx with a mass flow rate of 0.66 % and an excitation frequency of 100 Hz. First, flow control mechanisms are analyzed in terms of flow structures based on oil visualization and dynamic pressure results. The PS technique can induce significant periodic features and increase local intermittently momentum exchange. A connection between the local intensity of the excitation frequency and loss reduction at the cascade outlet is observed. Subsequently, the influence of the PS hole axial position is investigated, and the PS is found to contribute further control to the vortex structures than the CS. Finally, the effectiveness of the PS technique at different tip clearance sizes and inlet incidences is assessed to confirm its superiority over the CS technique in controlling TLF.

Experimental and numerical investigation on suppressing end-clearance leakage flow for variable nozzle turbine guide vane

Variable geometry turbine (VGT) exists in turbocharged gas engines to solve the problems of insufficient low-speed torque and poor transient response. However, there is a performance penalty associated with the end-wall clearance required for the movement of the guide vanes. Aiming at suppressing the end-wall clearance leakage flow loss to improve turbine performance, a novel scheme of end-clearance sealed guide vane (ESGV) is proposed and the effect assessment of this configuration is studied numerically and experimentally in a real variable geometry turbine. The numerical predictions reveal the three-dimensional leakage flow characteristics associated with the sealed end-wall clearance that optimizes the mixing process between the clearance leakage flow and the main flow and further improve the design and off-design performance of the turbine. Results exhibit that the relative leakage flow in the ESGV turbine is reduced by 26.81%, 7.33%, and 0.99% for 10%, 50%, and 90% nozzle openings, respectively, compared to that of the original turbine. Moreover, most of the leakage loss is concentrated in the groove clearance of the disk. The efficiency advantage of 20% and 4.4% points for the ESGV turbine at the peak efficiency point under 10% and 50% opening conditions, respectively. Besides, an experimental investigation is carried out to further verify the ESGV turbine performance predictions and analytical assumptions. Overall, the ESGV is found to be the best choice for the variable turbine guide vanes. The present results can provide valuable references for the vane-end clearance design of variable geometry turbines.

An Investigation of the Performance and Internal Flow of Variable Nozzle Turbines with Split Sliding Guide Vanes

In order to effectively weaken the leakage flow and shock intensity of traditional “swing” type guide vanes in a variable nozzle turbine, a new flow control device named the “split sliding guide vane” (SSGV) is studied in the present work. Steady and unsteady calculations were carried out on both the SSGV and base model at 10%, 40%, and 100% open positions. The shock test was performed to verify the accuracy of the numerical method. The results showed that at 10%, 40%, and 100% open positions, the leakage flow of the SSGV was 43%, 51%, and 40% of that of the base model, respectively. When 10% open, the turbine efficiency increased by 12%, compared with the base model, since the SSGV could effectively inhibit the clearance leakage flow. Due to the increased distance between the rotor and guide vane, the shock intensity of the SSGV was only 52% of that of the base model when 40% was open. The SSGV could reduce the static pressure loss on the guide vane pressure surface, but for the guide vane suction surface, the static pressure distribution appeared in a “W” shape due to the influence of the vane profile. Finally, the flow in the rotor was studied, which showed that the weakening of the shock and reduction of the clearance leakage flow in the guide vane were also beneficial for the strength of downstream rotor blades.

Prediction for the Influence of Guide Vane Opening on the Radial Clearance Sediment Erosion of Runner in a Francis Turbine

In this paper, the Eulerian–Lagrangian method and Tabakoff erosion model are used to study the solid–liquid two-phase flow in a Francis turbine. Through the analysis of the overall flow pattern, particle flow, particle concentration, and wear in the bladeless area of the unit under different guide vane openings, the influence of runner radial gap flow on the surrounding flow field characteristics and wear under different guide vane openings is revealed. The results show that the smaller the opening of the guide vane, the greater the influence on the vortices and flow pattern and the particle distribution in the runner. The overall wear in the hydraulic turbine unit with the optimal opening is the smallest. The long-term wear of the runner inlet and guide vane outlet will cause the loss of local structures, an increase in the radial clearance of the runner, an increase in the clearance leakage, an increase in the vibration of the unit, and a reduction in efficiency. The research results provide a basis for the structural and hydraulic optimization of the Francis turbine.

Influence of tip clearance inclination on the performance of axial flow impeller in storage mode

Tidal energy is one of the most mature energy sources used in all kinds of ocean energy. Axial flow impeller plays a key role in converting potential energy and kinetic energy of seawater into electric energy. In this study, aiming at the problem of three different clearance inclination in axial flow impeller, the axial flow impeller is studied by numerical simulation and experimental verification. The results show that the efficiency first decreases rapidly, then fluctuates within a certain range, and then decreases rapidly as the inclination changes from negative to positive. When the positive inclination is larger, the axial force and radial force are smaller. Meanwhile, the length of the clearance leakage vortex and the included angle with the axial direction are smaller. This paper provides a reference for improving the utilization efficiency of tidal energy. It also has guiding significance for the design of axial flow impeller.

Study on the Effect of the Guide Vane Opening on the Band Clearance Sediment Erosion in a Francis Turbine

Sediment erosion is a negative phenomenon for the water turbine. The purpose of this paper is to study the effect of the guide vane opening on the particle motion and sediment erosion in the band chamber using the Euler–Lagrangian approach. The software Ansys CFX and Tabakoff erosion model are used to simulate the sediment laden flow in the full flow passage of the hydraulic turbine. The results are in good agreement with the actual erosion character on site. Results show that the guide vane opening has a positive correlation with the flow in the non-clearance channel. The increase of the guide vane opening will increase the erosion of the runner blade head, but the friction wear on the outlet side of the blade surface will decrease. The rotating action of the runner makes the sediment particles in the band chamber rotate rapidly around the center of the runner and constantly collide with the band chamber wall. Under the small opening, the smaller the opening, the easier the leakage of the band clearance occurs. The unsteady flow in the band chamber will disturb the motion trajectory of particles, change the impact angle of particles, and affect the wear in the band chamber. Under different openings, the change law of the erosion in the band clearance is closely related to the change law of clearance leakage.

Fluid-solid coupled simulation on thermal management of hydrodynamic gas foil bearing with an axial cooling throughflow

The axial throughflow cooling is a realistic solution for the purpose of thermal management in the hydrodynamic gas foil bearings operating at ultra-high rotational speed and mini bearing clearance. In order to illustrate the effects of axial throughflow cooling within the fluid-solid coupled heat transfer mode on the aerodynamic and thermal behaviors of hydrodynamic gas foil journal bearing, a steady three-dimensional fluid-solid coupled numerical simulation is performed in the present study by assuming that the journal-bearing works in a steady-state condition with a fixed eccentricity ratio of 0.9. The results show that the shearing-induced circumferential flow is significantly dominant in the hydrodynamic air film layer. At both edges of the foil bearing section, local suction flow into the bearing clearance and local leakage flow out the bearing clearance happen simultaneously at different circumferential positions. The increase of axial throughflow mass flowrate plays an effective role on improving the cooling capacity. However, it should be noteworthy that the bleeding air pressure is prompted in a nearly quadratic relationship along with the increase of axial throughflow mass flowrate. Due to the non-uniformity nature of viscous-shearing heat generation inside the hydrodynamic gas foil bearing with a big eccentricity, the temperature on the shaft and top foil surfaces takes on a non-uniform distribution. The peak temperature generally has a 20 K–30 K difference with respect to the mean temperature.

Influence of uneven blade tip clearances on aeroacoustic characteristics of centrifugal compressors

The blade tip clearance is a crucial factor in the compressor performance and the aeroacoustics. There are usually main blades and splitter blades, two different types of blades in centrifugal compressors. Keeping almost the same work input, three different tip clearance configurations were designed in which the tip clearance gaps of main blades and splitter blades were uneven. Then experimental investigations were carried out to study the influence of uneven tip clearances on the aeroacoustic characteristics of centrifugal compressors. The results showed that larger main blade tip clearances or larger splitter blade tip clearances would induce rotating instabilities (RI) near flow instability boundary at low rotating speeds. However, the broadband noises below the blade passing frequency (BPF) in the case with uneven blade tip clearances were suppressed significantly which might be due to the incongruous interaction between the tip clearance leakage flows of main blades and splitter blades. In addition, the blade nonsynchronous vibrations (NSV) were observed in the analysis of the frequency spectrum of far-field noise in the case which had smaller main blade tip clearances and larger splitter blade tip clearances. Based on the jet core feedback theory, a physical explanation was given for NSV occurred in this case.

COVID-19 and renal involvement: a prospective cohort study assessing the impact of mild SARS-CoV-2 infection on the kidney function of young healthy males

PurposeCOVID-19 frequently affects the kidneys with symptoms ranging from mild proteinuria to progressive acute kidney injury. This prospective study aimed to assess the short- and long-term impact of asymptomatic and mild COVID-19 on the renal function of healthy young adults, and to determine the correlation between viral load and kidney function among these patients.MethodsThis was a prospective cohort study conducted over a period of 6 months. Patients were followed-up at baseline, and then after 3 and 6 months, respectively. Real-time PCR cycle threshold (CT) was used to determine the viral load and disease activity. Patients were classified into two groups with either asymptomatic COVID-19 or mild pneumonia. The assessment parameters were variables that could directly or indirectly relate to the renal function.ResultsA total of 48 patients were included and evaluated. The majority of patients (62.5%) had asymptomatic COVID-19 disease. Patients with mild pneumonia had significantly higher serum creatinine (SCr) at the time of COVID-19 diagnosis (beta = 12.836, 95% CI = 2.405–23.268, P = 0.019), after 3 months (beta = 14.345, 95% CI = 1.149–27.542, P = 0.035), and after 6 months (beta = 14.100, 95% CI = 0.730–27.470, P = 0.040) compared to asymptomatic patients. Mild pneumonia was also significantly associated with lower serum albumin level at the time of COVID-19 diagnosis (beta = – 6.317, 95% CI = – 9.448–− 3.185, P < 0.001).ConclusionMild COVID-19 is associated with mild renal involvement without AKI. Changes in the renal function appear to be related to reduced creatinine clearance and possible albumin leakage in the acute phase of the disease. The reduction in creatinine clearance is not predicted by viral load, and it appears to be a long-term effect of the disease that can last for at least 6 months.

CFD and Energy Loss Model Analysis of High-Speed Centrifugal Pump with Low Specific Speed

High-speed centrifugal pumps with low specific speed have the characteristics of a small flowrate, a high head, and being compact and light weight, making them promising candidates for applications in the thermal management of aerospace and electronic devices. The energy loss in the low specific speed pump is critical and complex due to the large impeller diameter, the narrow and long flow channel, and the small outlet width. In this paper, an analysis method based on an energy loss model and computational fluid dynamics simulations (ELM/CFD) is proposed to analyze the performance of the low specific speed pump with a fully sealed structure. Experiments were carried out under variable water flowrates. The results show that the empirical correlation method failed to accurately predict the performance of high-speed centrifugal pumps, because the bearing clearance leakage and motor channel leakage are ignored. Moreover, the volume loss and hydraulic loss are calculated based on the empirical parameters of commonly used pumps that are different from the high-speed pump with the low specific speed in the complex flow channel structure. The ELM/CFD method calculates various loss power based on the simulation results and can predict the head and efficiency with deviations less than 2% and 5%, respectively. ELM/CFD can accurately analyze the optimization direction of the pump. The hydraulic loss and the volume loss of the impeller are the dominant factors that restrict the pump efficiency under the lower flowrates, while the hydraulic loss of subsequent flow channels becomes important under the larger flowrates.

Unsteady film cooling characteristics of the high-pressure turbine shroud with blade rotation in an aero-engine

Abstract Three-dimensional unsteady numerical simulations were conducted to investigate the detailed film cooling mechanism of the high-pressure turbine shroud with the first-stage turbine blade and guide vane for an aero-engine under the high-speed rotation of blades and rotor-stator interaction. The slip mesh was used to realize the relative motion between the rotating blade and the stationary turbine shroud. It is found that the coolant jet is alternately influenced by the hot mainstream, tip clearance leakage flow and leakage vortex due to the high rotational speed of blades. The film cooling characteristics of the turbine shroud significantly present an unsteady and periodic flow and heat transfer phenomenon. The insufficient cooling margin for film holes at the upstream of the blade leading edge can occur not only under high blowing ratios due to the coolant jet liftoff, but also at low blowing ratios due to the insufficient coolant flow rate as a result of the high exit pressure. A novel shroud cooling structure with coolant supply by the added throttle chamber is put forward, and expected to provide better thermal protection for the high-pressure turbine shroud near the leading edge of blades with no extra increase in the total mass flow rate of coolant.

Performance Optimization of the High-Pressure Compressor in Series Two-Stage Turbocharging System Based on Low-Speed Performance Requirements of Diesel Engine

Abstract The performance of the high-pressure (HP) compressor is very important for the two-stage turbocharging system. However, the performance of HP compressor on the engine most of the time is poor at low speed and low mass flow conditions. These will lead to poor engine performance at low engine speed. The purpose of this paper is to improve the performance of the HP compressor at low speed and low mass flow conditions. The Latin hypercube design of the experiment method is used to establish the Kriging model and global optimization by multi-objective genetic algorithm NSGA-II to optimal HP compressor. The simulation results indicated that the flow field within the compressor was improved and the high entropy generation area was reduced. The new design delayed the mixing between the tip clearance leakage vortex flow and main flow. The low-speed performance of the HP compressor was improved. The turbocharger gas stand tests and engine bench tests were carried out. The results showed that the efficiency and pressure ratio of the optimized design is increased by 2.1% and 3%, respectively. The engine achieved better performance in low-speed conditions. The pumping means effective pressure (PMEP) and intake airflow increased by 7% and 4.98%, respectively, while brake specific fuel consumption (BSFC) and soot emissions decreased by 0.56% and 32.8%, respectively.