Tip Clearance Size Research Articles

Investigation of Tip Leakage Vortex Breakdown in a High-Speed Multistage Axial Compressor

AbstractIn this article, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near-surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in the static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge were observed. It was found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship among this unsteady flow phenomenon, tip clearance size, and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference was proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the aforementioned results, a high-speed multistage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted, and an increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade, which suppressed the unsteady tip leakage flow instability, was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

Effect of Tip Clearance Size on the Cavitation Flow and Noise Characteristics of the NACA0009 Hydrofoil

The noise characteristics of NACA0009 hydrofoil with tip clearance are discussed. Select SST k- ω Turbulence model and Schnerr Sauer cavitation model simulate tip leakage cavitation flow. The results show that the simulation results are in good agreement with the original records, and the tip clearance reduces the energy characteristics of the hydrofoil. With the expansion of tip clearance model, TLV and cavitation compressive strength gradually decrease. In addition, the study of far-field noise shows that the nonspecific curve of the total noise coefficient of far-field noise shows a disc shaped symmetric structure. Compared with the original hydrofoil, when the tip clearance is 20 mm, the far-field dipole noise value at 170 ° angle is reduced by 17.68%. Tip clearance reduces offline and far-field noise. Far field noise decreases with the increase of tip clearance model.

The effect of wake vortex on the unsteady characteristics of tip leakage flow in a high-loading counter-rotating compressor

The effect of upstream wake vortex on the unsteady characteristics of downstream tip leakage flow (TLF) in a high-loading counter-rotating compressor, have been revealed. For the strength of the tip leakage vortex (TLV) is stronger than that of the upstream wake vortex and the TLF spills from the leading edge (LE) of the neighbor blade under near stall (NS) condition, the wake vortex can only transport bypass the interface, formed by the TLF and main flow, just below the tip. The TLV is separated into several parts by the wake vortex such that it weakened rapidly and the dominant frequency of the static pressure fluctuation near the trailing edge (TE) is about 1 relative blade passing frequency (RBPF) under the NS condition. When the extent of TLV breakdown is enough, the TLV is responsible for the dominant frequency near the LE. Therefore, there are two dominant frequencies (one is located near LE, the other is located near TE) in the tip region of the downstream supersonic rotor under NS condition. It’s harmful for the stable operation of compressors due to lower frequency resulting from TLV. At small tip clearance size, affected by the upstream wake vortex, when the static pressure fluctuation caused by the TLV at the LE of the downstream rotor just appeared, the momentum ratio increases. And it locates above the limit line (applied to judge the static pressure fluctuation caused by the self-excitation of the TLF), this means the wake vortex is beneficial to suppress the self-excitation of the TLF.

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.

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.

Large Eddy Simulation on the Cavitation Flow and Noise Characteristics of a NACA0009 Hydrofoil With Different Tip Clearance Sizes

Abstract All hydraulic machinery has a tip clearance, which not only produces tip-leakage vortexes (TLVs), but also reduces the energy performance of the machinery. In addition, tip clearance leads to cavitation and attendant vibration and noise. Therefore, investigating tip-leakage cavitating flow and noise characteristics is of great practical importance. In this paper, the energy performance and noise characteristics of NACA0009 hydrofoils with different tip clearance sizes are studied. A large eddy simulation model and Schnerr–Sauer cavitation model are employed to simulate tip-leakage cavitating flow. Additionally, a broadband noise source model and the Ffowcs Williams–Hawkings (FW–H) equation are used to calculate the noise source and far-field radiated noise characteristics, respectively. Results show that the numerical simulation of cavitation vortex and velocity field is in good agreement with the experimental data, illuminating the characteristics of energy performance, flow pattern, cavitation flow, broadband noise source, and near-field and far-field radiated noise. Compared with the original NACA0009 hydrofoil, the tip clearance reduces the noise of the Curle dipole on the hydrofoil surface and Proudman noise around the hydrofoil. Moreover, study of the far-field noise shows that the directivity curve of the overall sound pressure level (SPL) is distributed in a butterfly shape, symmetrically. Evidently, the tip clearance size has a large impact on the energy performance of the hydrofoil, the intensity of the TLV, and the cavitation. This paper lays a solid foundation for further research on cavitation flow in large-scale hydraulic machinery.

Extending Highly Loaded Axial Fan Operability Range Through Novel Blade Design

Abstract The tip clearance size has historically been considered to be the main factor affecting stability range in axial fan and compressors. This paper reveals that the stall characteristics are defined by the axial momentum flux of the tip leakage flow and that tip clearance is primarily a strong driver for this metric. A bespoke methodology for carefully tailoring the axial momentum via three-dimensional design is presented, which enables a higher degree of control over the stability range for cases where the tip clearance responds to other considerations and cannot be defined for this purpose. The effect of the axial momentum on efficiency is also addressed and the trade-off between operability range and design point performance is derived. The results show that the conditions for optimal stability differ from those for optimal efficiency and that control over the axial momentum enables tuning the design for a desired exchange. Numerical simulations have been employed to drive the analysis through a high-fidelity computational model whose behavior is supported by rich set of experimental data. Contrary to current belief, results further indicate that an accurate characterization of stall, including onset mechanism, can be achieved through steady-state simulations, minimizing the need for expensive time-accurate computations during the design phase.

Method of variable-depth groove on vortex and cavitation suppression for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag performance of the hydrofoil.

Influence of tip leakage flow on the aerodynamic damping of compressor blade torsional vibration

Based on harmonic balance calculation technology with a single-passage phase-lag periodic boundary, the influence of tip gap size on the aerodynamic damping characteristics of rotor for first-order torsional vibration is analyzed with energy method. This research shows that the aerodynamic damping coefficient of the blade has a parabolic distribution law of first decreasing and then increasing with the increase of the tip clearance size. There is a most dangerous clearance value of torsional flutter characteristics in the blade, and the aerodynamic damping under this clearance is 12.67 % lower than the maximum damping value. The influence of the leakage flow on the time-averaged flow field near the blade tip leads to significant differences in the local aerodynamic work. In addition, the effect of leakage flow on the amplitude and phase of the local first-order unsteady pressure also bring about a significant influence on local aerodynamic work near blade tip.

Tip Clearance Effect on The Tip Leakage Vortex Evolution and Wake Instability of a Ducted Propeller

The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TLV became thicker, indicating damage to the energy recycling process. The fast Fourier transform (FFT) of several wake points showed pressure pulsations of the wake ranging from the blade-passing frequency to the shaft frequency, and the evolution process accelerated with the increase in the GSR. The power spectral density (PSD) analysis showed that the energy of the wake enhanced with the increase in the GSR. In particular, the vortex interactions could cause pulses in low-GSR conditions, which could intensify the excitation force of the propeller and also have a certain impact on the noise level.

Research on Cavitation Flow Dynamics and Entropy Generation Analysis in an Axial Flow Pump

The entropy generation theory is introduced to investigate the effects of different NPSH and tip clearance size on the cavitation flow dynamics and mechanical energy dissipation intuitively and quantitatively within an axial flow pump through numerical simulations. The results indicate that main mechanical energy dissipation of the pump gathers in part impeller and diffuser, and most are turbulent dissipation. Meanwhile, the impeller is the largest place of mechanical energy dissipation of the pump under cavitation conditions, accounting for more than 50%. NPSH has significant effects on the cavitation pattern, which reflects on the field that the areas of attached sheet cavitation and tip leakage vortex cavitation around blades increase obviously with NPSH reducing under the tip clearance of 0.1% span. With NPSH decreasing, high regions of turbulent dissipation in the impeller mainly expands along blades and move downstream, with span S0.98 near the shroud having larger turbulent dissipation. Besides, high regions of turbulent dissipation are mainly distributed at the rear part of the cavity for every corresponding span of the impeller, which indicates that the turbulent dissipation has a strong relation with the cavitation pattern. In the impeller, the unstable flows cause cavity shedding at the rear of the cavity and wake flows near the blade trail induce higher turbulent kinetic energy, finally resulting in higher turbulent dissipation there. Under the same NPSH, areas of tip leakage vortex cavitation and areas of tip clearance cavitation around the tip both expand with the tip clearance increasing from 0.1% span to 0.8% span. And high areas of turbulent dissipation also are distributed at the rear of the cavity and moving downstream along the blade suction side, especially at span S0.98. Therefore, the tip clearance width mainly affects the cavitation development and turbulent dissipation distribution near the impeller’s shroud under same NPSH.

Study on the effect of rotor tip fillet edge in axial compressor

The compressor flowfield is sensitive to the small geometry in the tip region. The blade tip edge, which was usually idealized as “sharp” edge in the numerical simulation, had been actually filleted during the process of machining, coating, and wearing. However, the fillet edge was completely neglected in the compressor performance estimation. Thus, in this paper, the effect of the fillet edge is evaluated by establishing several models with tip fillet edges as well as tip sharp edge in a transonic axial compressor. By the comparison of the flowfield between models with fillet edge and sharp edge, it was found that the tip fillet edge can eliminate the separation bubble at the pressure side of the tip clearance. Also, it can affect the tip clearance outflow by changing the reversed flow. As a result, the leakage flow is increased and the compressor performance is decreased in efficiency and pressure ratio. The local leakage mass flow rate along the camber line can be increased up to 40% when the fillet radius is as much as 20% of the tip clearance size. The flowfield in the downstream is also affected by the fillet edge. By the comparing among the models with different working conditions, it is found that the sensitivity of the performance to the fillet edge is different between design and off-design conditions. Furthermore, the effect of the tip fillet edge on the leakage flow is qualified and compared with the effect of the tip clearance.

Numerical and experimental investigations of tip clearance effects in a high-flow-coefficient centrifugal compressor

Abstract This paper focuses on the investigation of the blade deformation and tip clearance effects of a high-flow-coefficient centrifugal compressor. Blade deformation caused by the pressure load and centrifugal load under operating condition is considered by fluid/solid interaction method to simulate the stage performance accurately. Hence the numerical results are in good agreement with the experimental data. The blade deformation is then analyzed in detail. Also, the effect of the size of tip clearance is studied by evaluating the performance with five various gaps. The result shows that the increment of the tip clearance from 0 to 2.0 mm leads to a 5.14% peak efficiency decline. However, increasing tip gap contributes to enlarge the stable operating range in terms of stall margin and choke margin.

Numerical Investigation of Tip Clearance Effects on the Performance and Flow Pattern Within a Sewage Pump

Abstract Sewage pumps are one of the most popular types of fluid machinery in municipal, industrial, and environmental engineering and many other fields. The influence of tip clearance size on the pump performance and internal flow field distribution within a sewage pump was investigated in this study based on numerical calculation and test verification. Three models with different tip clearances were obtained by adjusting the assembly structure and simulated by the computational fluid dynamics software to solve the Navier–Stokes equations for three-dimensional steady flow. The accuracy of numerical methods was verified by comparison with the experimental results. The sewage pump performance under different tip clearances was then compared and analyzed. The structure and propagation of the vortices inside the impeller caused by the leakage flow with different tip gaps were also analyzed. The leakage flow in the impeller couples with the cover reflection and secondary flows, forming different vortex structures in various locations of the impeller channel. These vortex structures cause large hydraulic losses near the blade suction surfaces. This finding could provide useful insights for the improvement of the hydraulic performance and operational stability of semi-open sewage pumps.

Comparative Study of Tip Injection in a Transonic and Subsonic Compressor

Abstract Discrete tip injection is an effective method to enhance stability of compressors. This study compares the effects of injection parameters on compressor performance and underlying mechanisms in two different compressors. The transonic compressor is studied using unsteady simulations, and the subsonic compressor is mainly investigated with experiments. Results show that tip injection improves stable operating range by 35.6% and 77.9% for the transonic compressor and subsonic compressor, respectively, without decreasing compressor efficiency. The effects of circumferential coverage percentage and injector throat height on compressor stability are similar in the two compressors when the injection velocity is double the velocity of the main flow. The optimal injector throat height which is normalized by the tip clearance size is the same for the two compressors, and the best circumferential coverage percentage for the subsonic compressor is lower than that in the transonic compressor. For the two compressors, the adaption of the main flow to the discrete tip injection is unsteady, and the hysteresis effect that the recovery of tip blockage lags behind the recovery of tip leakage vortex accounts for the improved stability using partial coverage of injection. The injection efficiency, which is defined to quantify the improved quality of the flow field in the injection domain, is proven to determine the stall limits by studying the effects of different injection parameters. The guidelines built in the subsonic compressor can be used in the transonic compressor to design tip injection, but the optimal values of some injection parameters should be reconfirmed.

Some effects of non-axisymmetric tip clearance layout on axial compressor aerodynamics

A steady and unsteady numerical research is carried out to explore some effects of a specific non-axisymmetric tip clearance layout on the overall performance and stability of an axial compressor stage. For a 4-stage low-speed research compressor (LSRC) in Shanghai Jiao Tong University (SJTU), one-eighth annulus of the inlet guide vane and the first stage rotor was modeled for this study. After the validation for the uniform tip clearance case, a specific non-axisymmetric tip clearance layout is chosen from several random cases generated by the Gaussian Probabilistic Density Function method. Unsteady time-averaged results at the near stall condition show that the chosen non-axisymmetric layout can improve the isentropic efficiency by 1.3% and extend the stall margin by 4%. Detailed analyses on flow fields are carried out to interpret the performance improvement. Due to the circumferential layout of clearance sizes, the inlet mass flow and incidence are redistributed in both the radial and circumferential directions. It leads to blade loading and tip leakage flow varying with the tip clearance size. The quantification of blockage manifests that the blockage arising from the tip leakage flow is significantly alleviated in the non-axisymmetric layout, which leads to improvements in overall performance and stall margin. Transient flow fields at the rotor tip are also analyzed at the near stall condition. For the non-axisymmetric layout, low-momentum regions originating from larger clearance sizes oscillate and develop downstream in one blade passage period.

Comparison of Turbulence Modeling for a Compressor Rotor at Different Tip Clearances

Due to the high Reynolds numbers together with the complex turbulent motions in turbomachinery, selecting a proper turbulence-modeling method is of vital significance in interpreting the real flow physics. In this paper, shear stress transport (SST) as a Reynolds-averaged Navier–Stokes model and scale-adaptive simulation (SAS) and zonal large eddy simulation (ZLES) as two scale-resolving simulation approaches were chosen to simulate the flow in a low-speed axial compressor rotor at three different blade tip clearances (1.3, 2.6, and 4.3% of chord length). Results of unsteady simulations at the design point were compared to the experimental data. The results prove that the ZLES model performs best in characterizing the experimental data regardless of the blade tip clearance size. The performance of the SST model in capturing the tip flow is close to the measurement data when the blade tip clearance is small, while the deviation from the experimental results increases with the clearance size enlarged. In contrast, the use of the SAS model does not result in any benefits over the SST model for the small tip clearance but demonstrates similar performance with the ZLES model for the configurations with middle and large tip clearance sizes.

Effect of Tip Clearance Size on Tubular Turbine Leakage Characteristics

To study the effect of tip clearance on unsteady flow in a tubular turbine, a full-channel numerical calculation was carried out based on the SST k–ω turbulence model using a power-plant prototype as the research object. Tip leakage flow characteristics of three clearance δ schemes were compared. The results show that the clearance value is directly proportional to the axial velocity, momentum, and flow sum of the leakage flow but inversely proportional to turbulent kinetic energy. At approximately 35–50% of the flow direction, velocity and turbulent kinetic energy of the leakage flow show the trough and peak variation law, respectively. The leakage vortex includes a primary tip leakage vortex (PTLV) and a secondary tip leakage vortex (STLV). Increasing clearance increases the vortex strength of both parts, as the STLV vortex core overlaps Core A of PTLV, and Core B of PTLV becomes the main part of the tip leakage vortex. A “right angle effect” causes flow separation on the pressure side of the tip, and a local low-pressure area subsequently generates a separation vortex. Increasing the gap strengthens the separation vortex, intensifying the flow instability. Tip clearance should therefore be maximally reduced in tubular turbines, barring other considerations.

Effects of tip clearance size on vortical structures and turbulence statistics in tip-leakage flows: A direct numerical simulation study

Direct numerical simulations of the tip-leakage flow, generated by a gap between a straight National Advisory Committee for Aeronautics 0012 hydrofoil and the end wall of a channel, have been performed to investigate the effects of tip clearance size on vortical structures and turbulence statistics. The tip-leakage vortex, tip separation vortex, and induced vortex are the predominant vortical structures in tip-leakage flow for a relatively large gap (3.33%Ca), while the reverse flow vortex dominates the tip clearance region for smaller gaps (1.67%Ca), where Ca is the truncated chord length of the hydrofoil. Detailed analysis of turbulence statistics reveals that the tip-leakage vortex is caused by the rollup of the tip-leakage jet, while the spanwise inflow interacting with the sidewall of the hydrofoil leads to the formation of a reverse flow vortex. The turbulent kinetic energy contours show an arc-shaped distribution on the suction side of the hydrofoil, but their locations are significantly affected by the tip clearance size. In addition, the investigation of tip-leakage loss indicates that increasing the size of the tip clearance can reduce the tip-leakage loss across the hydrofoil. This can be attributed to the dominant vortical structures in the tip-leakage flow.

Numerical study on the effect of blade tip clearance change on stall margin of the transonic axial flow compressor rotor

The change of the blade tip clearance size has an important impact on the performance of the compressor. Considering that the performance curve of the compressor is often limited by surge and stall boundaries, this paper used the numerical simulation method to investigate the influence mechanism of the blade tip clearance size change on the stall margin of transonic axial flow compressor rotor. By mathematically decomposing the calculation formula of the stall margin of rotor, the approximate calculation formula of the change of rotor’s stall margin was obtained. Then, the detailed quantitative analysis of the factors that affect the rotor’s stall margin was carried out, the influence weights of various factors on the rotor’s stall margin was also obtained. Finally, the physical mechanism of the change of the rotor’s performance parameters was obtained by the analysis of rotor tip flow field after the blade tip clearance size change.