Increasing Tip Clearance Research Articles

Effects of fusion height of tip winglet on the clearance flow in a compressor cascade

The leakage flow generated by rotor clearance seriously affects efficiency and stability of a compressor. In order to reduce the effect of leakage flow, the tip winglet has been applied at the tip of a transonic compressor cascade. Numerical simulation has been used to investigate the influence of tip winglets with different fusion heights on the flow field structure. Furthermore, the effect of winglets was analyzed under different clearance heights and incidence angles. Results reveal that the winglet can weaken the kinetic energy of the leakage flow. The development of the leakage vortex along the pitchwise is suppressed, and the total pressure loss is decreased near the tip. The increase in the tip separation vortex scale leads to a remarkable decrease in the leakage flow rate. With the increase in fusion height, the scale of tip separation vortex decreases gradually, and the suppression effect on the leakage flow rate is weakened. The optimal scheme can reduce the loss by 3.3% and leakage flow rate by 13.4%. The suppression effect of the winglet on flow loss increases gradually with the increase in incidence angle. Moreover, the suppression effect of the winglet on leakage flow and flow loss increases first and then decreases with the increase in tip clearance.

Prediction of incipient cavitation in micro tubular propeller turbine

Abstract Cavitation is a phenomenon that cavities or bubbles form in a liquid when static pressure falls below the vapor pressure. It can damage the machine, hence it is recommended to be avoided in the design and operation of the machine. In this study, the cavitation characteristics were numerically investigated with a change in the flow rate of a micro tubular propeller turbine. Here, the rotational speed was set to the value corresponding to the best efficiency point for each flow rate. A multiphase flow analysis incorporating water and water vapor was conducted, using the Zwart-Gerber-Belamri (ZGB) model as the cavitation model. Results show that the tip clearance is the first region where the cavitation starts to occur as the flow rate increases. At this time, the Thoma number was found to be 1.178. The second region where the cavitation occurs with further increases in the flow rate is in the vicinity of the leading edge. It turned out that the leading edge cavitation occurs when the Thoma number is below 1.111. As the tip clearance increases, the range of the flow rate that cavitation does not occur increases, but it reduces turbine performance. Therefore, the selection of appropriate tip clearance given the operating flow rate range and performance needs to be considered.

Comparisons of blade tip phantom cooling effectiveness for two tip structures with three tip clearances

The blade tip region of the gas turbine is exposed to an extreme operating condition with a high heat load. The conventional cooling concept that places cooling holes on the tip would consume a lot of cooling air and lead to a reduction in thermal efficiency. Whereas phantom cooling was supposed to help cool the tip region without using additional cooling air. In this paper, computational comparisons were conducted to forecast the tip phantom cooling effects caused by the blade ejections by employing the standard k-ω model. Then, phantom cooling performance was presented under the flat tip (FT) and the squealer tip (ST), with three tip clearances. Results show that the FT shows a better phantom cooling performance compared to the ST. The phantom cooling effects on the FT are distributed at the tip forepart and near the pressure side (PS). Whereas, for the ST, traces of phantom cooling are barely detected on the PS rim. Increasing the tip clearance would weaken the phantom cooling performance on the FT forepart but enhance it on the rear part. For the ST, its phantom cooling effectiveness values decrease as the tip clearance increases. A lower aerodynamic loss is obtained for the ST under any coolant MFR or tip clearance.

Analysis of Tip Clearance Effect on the Transportation Characteristics of a Multiphase Rotodynamic Pump Based on the Non-Uniform Bubble Model

The multiphase rotodynamic pump is widely used in petroleum and gas exploitation, and blade tip clearance may cause flow instability and performance deterioration. In the present work, the influence of tip clearance on the transportation characteristic in a multiphase rotodynamic pump is investigated based on the non-uniform bubble model, in which the bubbles’ coalescence and break-up are considered. The influence mechanism of tip clearance on the energy performance is revealed. The results show that the leakage flow rate increases linearly with the increase in tip clearance, but variation in pump energy performance shows the opposite trend. In addition, a larger tip clearance results in a sharply decreased pressure increment in the impeller, while in the guide vane, the increment is raised slightly. For the 0 mm tip clearance condition, a positive vortex (relative to the impeller rotation direction) is observed in the impeller passage. However, the opposite leakage vortex is also found in the region near the tip clearance when the tip clearance is considered, and the vortex strength increases for a larger tip clearance.

Research on the Influence of Tip Clearance of Axial-Flow Pump on Energy Characteristics under Pump and Turbine Conditions

In order to study the influence of tip clearance on the performance and energy dissipation of the axial-flow pump and the axial-flow pump as a turbine, and find the location of high dissipation rate, this study took an axial-flow pump model as its research object and designed four tip radial clearance schemes (0, 0.2, 1 and 2 mm). The unsteady calculation simulation of each tip clearance scheme was carried out based on CFD technology. The calculated results were compared with the experimental results, and the simulation results were analyzed using entropy production analysis theory. The results showed that, under both an axial-flow pump and axial-flow pump as turbine operating conditions, increasing the blade tip clearance led to a decrease in hydraulic performance. Compared with the 0 mm clearance, the maximum decreases in pump efficiency, head and shaft power under 2 mm tip clearance were 15.3%, 25.7% and 12.3% under the pump condition, and 12.7%, 18.5% and 28.8% under the turbine condition, respectively. Under the axial-flow pump operating condition, the change in blade tip clearance had a great influence on the total dissipation of the impeller, guide vane and outlet passage, and the maximum variation under the flow rate of 1.0Qdes was 53.9%, 32.1% and 54.2%, respectively. Under the axial-flow pump as a turbine operating condition, the change in blade tip clearance had a great influence on the total dissipation of the impeller and outlet passage, the maximum variation under the flow rate of 1.0Qdes was 22.7% and 17.4%, respectively. Under the design flow rate condition, with the increase in tip clearance, the dissipation rate of the blade surface showed an increasing trend under both the axial-flow pump and axial-flow pump as turbine operating conditions, and areas of high dissipation rate were generated at the rim and clearance.

Performance Analysis of Eroded Compressor Stage

Erosion of compressor blades due to operation in dusty environments is a serious problem for the manufacturers and users of gas turbine engines, because of significant degradations in performance. This study has been devoted to estimating the change of performance parameters of an axial compressor stage due to erosive deterioration. The influence of erosion considered as consequences of a reduction of rotor blade airfoils, and an increase of tip clearance. The results of this study obtained using the CFD code “NUMECA Fine/Turbo”. This CFD code is a steady, quasi-three-dimensional Reynolds Averaged Navier-Stokes (RANS) solver. A Spalart-Allmaras turbulence model is used. The compressor stage parameters presented for three rotational speeds. Proposed a new approach to consider the changes of geometric parameters of blades due to erosion. Presented an analysis of the effect of erosion and its individual consequences on the pressure ratio, isentropic efficiency and stability margin of the stage. The obtained results verified using an existing experimental data.

Effect of Tip Vortex Reduction on Air-Cooled Condenser Axial Flow Fan Performance: An Experimental Investigation

AbstractLarge diameter axial flow fans are used in air-cooled condenser (ACC) systems of modern power stations. Efficiency improvements on these fans can significantly reduce the ACC power consumption and increase the net sent-out power to the grid. This study targets fan performance enhancement through blade tip vortex reduction. Experimental investigations are performed on a representative ACC scale fan, where tests consider the effects of tip clearance and two new tip endplate designs on fan performance. Test results confirm the findings of previous studies, showing the negative effect of increasing tip clearance on performance. Despite testing limitations, results from tests incorporating endplates show that fan static pressure coefficient and efficiency increase over large ranges of flow coefficient compared to the datum fan. These outcomes agree with observations from literature and warrants further exploration. Future work is recommended to provide confirmation on the presented trends.

Effect of Tip Clearance on Helico-Axial Flow Pump Performance at Off-Design Case

To reveal the effect of tip clearance on the flow behaviors and pressurization performance of a helico-axial flow pump, the standard k-ε turbulence model is employed to simulate the flow characteristics in the self-developed helico-axial flow pump. The pressure, streamlines and turbulent kinetic energy in a helico-axial flow pump are analyzed. Results show that the tip leakage flow (TLF) forms a tip-separation vortex (TSV) when it enters the tip clearance and forms a tip-leakage vortex (TLV) when it leaves the tip clearance. As the blade tip clearance increases, the TLV moves along the blade from the leading edge (LE) to trailing edge (TE). At the same time, the entrainment between the TLV and the main flow deteriorates the flow pattern in the pump and causes great hydraulic loss. In addition, the existence of tip clearance also increases the possibility of TLV cavitation and has a great effect on the pressurization performance of the helico-axial flow pump. The research results provide the theoretical basis for the structural optimization design of the helico-axial flow pump.

Effects of Tip Leakage Flow on the Aerodynamic Performance and Stability of an Axial-Flow Transonic Compressor Stage

To explain the effect of tip leakage flow on the performance of an axial-flow transonic compressor, the compressors with different rotor tip clearances were studied numerically. The results show that as the rotor tip clearance increases, the leakage flow intensity is increased, the shock wave position is moved backward, and the interaction between the tip leakage vortex and shock wave is intensified, while that between the boundary layer and shock wave is weakened. Most of all, the stall mechanisms of the compressors with varying rotor tip clearances are different. The clearance leakage flow is the main cause of the rotating stall under large rotor tip clearance. However, the stall form for the compressor with half of the designed tip clearance is caused by the joint action of the rotor tip stall caused by the leakage flow spillage at the blade leading edge and the whole blade span stall caused by the separation of the boundary layer of the rotor and the stator passage. Within the investigated varied range, when the rotor tip clearance size is half of the design, the compressor performance is improved best, and the peak efficiency and stall margin are increased by 0.2% and 3.5%, respectively.

Effect of Tip Clearance on Rotating Stall in a Mixed-Flow Pump

Abstract The non-uniform disturbance in the circumferential direction is the essential cause of the occurrence of rotating stall in a turbomachinery. In order to study the effect of tip clearance leakage flow on rotating stall, the mixed-flow pump models with different tip clearances 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 field of the numerical calculation are in good agreement with the experimental results. A saddle region appeared in the energy performance curves of the three tip clearances, and with the decrease of clearance, the head and efficiency of the mixed-flow pump increased to varying degrees, but the critical stall point shifted to the large flowrate, and the stable operating range of the mixed-flow pump decreased, which indicating that the mixed-flow pumps were easier to stall under small tip clearance. Under the deep stall condition, the influence of the leakage flow on the end wall area increases gradually with the decrease of clearance. Under the small clearance, the leakage flow flows away from the suction surface for a certain distance to form a number of tip leakage vortex strips with the mainstream action, overflows the leading edge of the next blade, and then flows downstream into different flow passages, aggravating backflow and secondary flow separation at the blade inlet, which seriously damaged the spatial structure of the inlet flow. This resulted in the earlier occurrence of stall. With the increase of tip clearance, the tip leakage vortex develops along the radial direction to the middle of the flow channel and serious flow separation phenomenon occurs in the downstream channel, which induces the deep stall. Under the 0.8 mm tip clearance, the whole impeller outlet passage is almost blocked by the backflow of the guide vane inlet, and a deep stall was induced.

Flow behaviors in a Kaplan turbine runner with different tip clearances

Tip clearance between the runner blade tip and shroud in a Kaplan turbine is inevitable, and the tip leakage flow (TLF) and tip leakage vortex (TLV) induced by the tip clearance have a considerable effect on the flow behaviors. To reveal the effect of the tip clearance on the flow characteristics, based on the Reynolds time-averaged Navier-Stokes (N-S) equation and the shear stress transfer (SST) k-ω turbulence model, the three-dimensional turbulence flow in a Kaplan turbine is simulated using ANSYS CFX. Meanwhile, the flow laws in the tip clearance are emphatically analyzed and summarized. Results show with the increase of the tip clearance, the negative pressure region in the blade suction side (SS) middle, the SS near the blade tip and the blade tip becomes more and more obvious. In the meantime, the flow behaviors on the blade pressure side (PS) are relatively stable, and the flow separation on the SS near blade tip merges. The larger the tip clearance is, the more obvious the flow separation phenomenon displays. In addition, the TLV is a spatial three-dimensional spiral structure formed by the entrainment effect of the TLF and main flow, and as the tip clearance increases, the TLV becomes more obvious.

Performance assessments and simulations of ROT (radial outflow turbine) for back-pressure turbine generator system

The high pressure steam is regulated by the Pressure-Reducing Valves (PRVs) to suit the various industrial process; however, this, inevitably leads to economic and energy losses. In this study, PRV is replaced by a back-pressure turbine generator comprising a Radial Outflow Turbine (ROT). The structural characteristics of the ROT, such as the easy adjustment of the tip clearance, make it possible to control the steam properties. The steam with selectively reduced pressure (and temperature) can be provided to users. The blade geometry of the ROT is designed using a commercial software. The numerical prediction of the fluid properties is carried out using a real fluid model in ANSYS-CFX. The simulation results are compared with the experimental results obtained under the same off-design conditions. The tip clearance (100, 300 and 500μm) are applied as a parameter to investigate the change of performance and to adjust the properties of the discharged steam. The experimental results show that the maximum electrical power produced at the minimum tip clearance is 12.8 kW, and the temperature and pressure differences tend to decrease with increasing tip clearance. These results demonstrate the potential of the ROT as a PRV and energy recovery device in back-pressure turbines.

Influence of Tip Clearance on the Hydrodynamic Damping Characteristics of a Hydrofoil

AbstractTip clearance in hydraulic machines may complicate the fluid–structure interaction (FSI) effects. In this investigation, a mode-based approach (modal work) is evaluated and employed to quantitatively predict the added mass, added stiffness, and hydrodynamic damping ratio, in relation to the first-order bending mode of a vibrating hydrofoil. The investigated relative tip clearance ranges from 0.067% to 2% of the span length. The predicted vortex shedding frequency, natural frequency, and hydrodynamic damping ratio of the hydrofoil are in good agreement with the previously published experimental results, with relative deviations within 9.92%, 6.97%, and 11.23%, respectively. Simulation results show that the added mass, added stiffness, and hydrodynamic damping ratio increase inversely as the tip clearance increases. In particular, as the relative tip clearance increases from 0.067% to 2%, the added mass in still water, the added stiffness, and hydrodynamic damping ratio at a velocity of 10 m/s decrease by 18.66%, 9.36%, and 27.99%, respectively. As the tip clearance increases, the inversely increased pressure difference between the upper and lower surfaces of the vibrating hydrofoil is the main reason for the inversely increased hydrodynamic damping ratio. This is due to the energy leakages via the tip clearance region increase as the tip clearance increases, which may cause less fluid force to resist the vibration of the hydrofoil, resulting in less negative modal work done by the fluid load on the hydrofoil.

Influence of combined compressor and turbine deterioration on the overall performance of a jet engine using RANS simulation and Pseudo Bond Graph approach

In this study, numerical models are used to analyse the influence of isolated component deterioration as well as the combination of miscellaneous deteriorated components on the transient performance of a high-bypass jet engine. For this purpose, the aerodynamic impact of major degradation effects in a high-pressure compressor (HPC) and turbine (HPT) is modelled and simulated by using 3D CFD (Computational Fluid Dynamics). The impact on overall jet engine performance is then modelled using an 1D Reduced Order Model (ROM). Initially, the HPC performance is investigated with a typical level of roughness on vanes and blades and the HPT performance with an increasing tip clearance. Subsequently, the overall performance of the jet engines with the isolated and combined deteriorated domains is computed by the in-house 1D performance tool ASTOR (AircraftEngine Simulation for Transient Operation Research). Degradations have a significant influence on the system stability and transient effects. In ASTOR, a system of differential equations including the equations of motion and further ordinary differential equations is solved. Compared to common ROMs, this enables a higher degree of accuracy. The results of temperature downstream of the high-pressure compressor and low-pressure turbine as well as the specific fuel composition and the HP rotational speed are used to estimate the degree and type of engine deterioration. However, the consideration of the system stability is necessary to analyse the characterisation in more detail. Finally, a simplified model which merges two engines with individual deteriorated domains into one combined deteriorated engine, is proposed. The simplified model predicts the performance of an engine which has been simulated with combined deteriorated components.

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.

Effect of Tip Clearance on the Aeroelastic Stability of a Wide-Chord Fan Rotor

Abstract This research presents a series of simulations that investigate the effects of tip clearance on the aeroelastic stability of a wide-chord high-speed transonic fan rotor. The results show that the stall margin and the total pressure ratio decreases as the tip clearance increases. The effect of tip clearance on the blade loading can extend to 30% span. The phase of the influence coefficient without tip clearance is different from that with clearance, which causes the most unstable aerodynamic damping to shift in the nodal diameter. As the clearance increases from 0.25 mm to 2 mm, the damping decreases. The nonmonotonic behavior found by other researchers was not observed in this study. We conclude that the tip clearance affects the aeroelastic stability in two ways. The first is to change the blade loading so that the amplitude of the unsteady pressure increases or decreases, while the phase hardly changes, resulting in changes in aerodynamic damping. The second is to change the local flow so that the unsteady pressure amplitude and the phase change locally.

Numerical investigation of the effect of tip clearance on hydrodynamic performance of the linear jet propulsion system and vortex generation behind the rotor

Linear Jet system which has a stator in addition to a rotor combines the best elements of two existing technologies of conventional screw propellers and water jets. In designing this propulsion system, tip clearance plays an essential role, since it causes the appearance of tip vortex that leads to a further loss in efficiency and a probability of cavitation phenomenon. Due to lack of any study in this regard, it is thus necessary to study tip clearance to find the appropriate geometry for linear jet propulsion system. In the current paper, hydrodynamic performance of linear jet propulsion system is numerically investigated. Accordingly, Ansys-CFX software is utilized and RANS unsteady equations are solved using SST turbulent model. Results of the proposed numerical model, in the form of thrust and torque coefficient as well as efficiency, are compared with available experimental data for a ducted propeller. It is concluded that most of the errors at various advance ratios for thrust and torque coefficients are less than 3% and relatively good agreement is observed. Hydrodynamic investigation involves five different sizes of tip clearance (2.5 to 10% of the rotor diameter). Simulation results indicate that thrust and torque coefficients decreases about 10% and 8% respectively, at the same advance coefficient (J) with an increase in tip clearance. Effects of tip clearance on tip-separation vortex and tip leakage vortex are also examined. At about 20% of chord length from the leading edge, separation occurs. As tip clearance size increases, the tip-leakage vortex also increases. At different advance ratios and higher tip clearance, an increase in vortex and a sudden decrease in thrust is generated by the propeller. By changing the time about 0.8 of the rotor periods, the evolution of the vortex generation behind the rotor at the tip of the blade is clearly observed.

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.

Numerical Simulation of the Influence of Tip Clearance on the Performance of Centrifugal Cold Compressor

To improve the efficiency of the Experimental Advanced Superconducting Tokamak (EAST) subcooled helium cryogenic system, a centrifugal cold compressor (CC) was investigated as an alternative to the oil ring pump. A numerical simulation method was applied to analyse variations in CC performance parameters, pressure ratio and polytropic efficiency. Results indicated that the performance curve of the CC and warm compressor (WC) were similar. The tip clearance at the design point was 0.2 mm and increased to 0.231 mm when the CC was operated at 4 K. This increase in tip clearance resulted in a 0.86% drop in the pressure ratio and a 0.54% drop in polytropic efficiency. To verify the simulation model, experiments were done using cold nitrogen.

Thermal design and CFD analysis of the radial inflow turbine for a CO 2 ‐based mixture transcritical Rankine cycle

Transcritical carbon dioxide (CO2) Rankine cycle has exhibited great potential in the field of low-temperature heat utilization. But its application is restricted by the condensing issue and the safety concern due to the relatively low critical temperature and high critical pressure of CO2. Blending CO2 with organic fluids for the transcritical Rankine cycle is regarded as an effective method to solve these problems. And the turbine performance has great influence on the performance of transcritical Rankine cycle. In this paper, the thermal design of the CO2-based mixture turbine is firstly carried out based on the parametric optimization of the system. Then the computational fluid dynamics (CFD) analysis is performed to examine the turbine performance and validate the reliability of thermal design. Furthermore, the effects of blade tip clearance and nozzle-to-rotor clearance on the turbine performance are investigated. Results show that the turbine is well designed with an isentropic efficiency of 84.54%, and the CFD simulation results basically agree with the thermal design results. The influence of leakage flow on mainstream grows significantly as the blade tip clearance increases. When the blade tip clearance is 2 mm, the relative loss of power output could achieve as large as 7.81%. Larger nozzle-to-rotor clearance leads to more uniform distributions of Mach number and pressure, but the flow losses also increase. The effect of trailing edge disturbance on the flow field at the nozzle outlet is almost negligible if the nozzle-to-rotor clearance is 6 mm or more.