Double Leakage Research Articles

Numerical Investigation of the Interaction of a Circumferential Groove Casing Treatment and Near-Tip Modifications for a Highly Loaded Low-Speed Rotor Under the Influence of Double Leakage

Abstract Eckel et al. (2023, “Numerical Investigation of Near-Tip Modifications for a Highly Loaded Low-Speed Rotor Under the Influence of Double Leakage,” ASME J. Turbomach., 145(4), p. 041003) proposed using a convex-profiled pressure side region close to the tip, known as belly, as an effective method of extending the operating range of low-speed axial compressor rotors. In the literature, circumferential grooves are another well-described technique for improving the stable working range of a compressor rotor. No research has been conducted to date to determine which modification is more effective and how they interact when used together. This paper numerically investigates the influence of circumferential casing grooves and near-tip modifications on the flow field in the tip region of a highly loaded, low-speed axial compressor rotor. The simulated rotor consists of a hybrid blade configuration with a tandem profile in the mid-span region and single blade profiles near the endwalls. The single blade profile close to the tip features three different convex-profiled elements, which differ in their respective thicknesses. The aim of the numerical analysis is to explain the interaction of the secondary flow phenomena when applying the circumferential grooves and the belly geometries. For this purpose, eight different axial positions of the circumferential groove are investigated for each of the three belly configurations. These are arranged in 10% increments from −7% to 63% along the axial rotor tip chord. The potential of the concept is evaluated by a numerical investigation in the 1.5-stage setup with an inlet guide vane and tandem stator. It is shown that a circumferential groove can further increase the operating range for all belly configurations when positioned axially correctly. In this respect, equalization of the near-casing deceleration in the circumferential direction leads to an extension of the stall margin with both modifications. Concentrated regions of low-momentum fluid with a large extent in the radial direction should be avoided consequently. A tip vortex stability factor is introduced to quantitatively evaluate this effect. The operating range can thus already be estimated in a first approximation at the design point. In general, the groove and belly should be positioned where the tip leakage vortex meets the pressure side of the adjacent blade. If the groove is used together with the belly, the leading edge of the former should be situated at the location of maximum thickness of the near-tip modification. The effects of the circumferential groove and the belly are then superimposed. If using only one modification, the belly appears better suited for ensuring an extension of the operating range while maintaining high efficiencies.

Flow instability control of an ultra-highly loaded transonic compressor rotor using self-excited casing bleed

Flow instability is a common issue encountered by high-speed compressors when they operate outside of their optimal range, especially in highly loaded compressors. This study investigates the potential of an unsteady passive flow control technique, self-excited bleed (SEB), which involves casing modification, to improve the base flow and stability characteristics of an ultra-highly loaded low reaction transonic compressor rotor. Through transient computational fluid dynamics simulations, we demonstrate that SEB can extend the rotor's operating range by up to 14.07%. The physical mechanism underlying this stability enhancement is the suppression of the shock-induced breakdown of the tip leakage vortex (TLV) near the blade leading edge and the attenuation of the double leakage flow by SEB. The unsteady excitation of the bleed effect dominates the tip flow and eliminates the spontaneous closed-loop feedback process based on the dynamic interaction between the TLV breakdown, the tip secondary vortex, and the blade loading. Time-resolved tip-region flow patterns elucidate the self-organization and reconstruction of this feedback mechanism. Frequency spectral analysis further reveals that the self-induced oscillation frequency of the tip leakage flow formed during the feedback process disappears, and the bleed excitation frequency replaces it as the main frequency of the tip flow field. However, increasing the bleed flow rate causes the boundary layer on the suction surface to migrate radially outward, resulting in increased flow blockage at the rear of the tip passage. These two influences of SEB are quantified by a blockage factor, and determining the optimal bleed flow rate requires a trade-off between beneficial and detrimental impacts.

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.

Stall Margin Improvement in a Transonic Compressor With a Casing Treatment: Flow Mechanism

Abstract A compressor casing treatment with circumferential casing grooves (CCGs) is studied in detail. The primary objective of the current paper is to unearth the main driving fluid mechanism that changes the stall margin in a transonic compressor with CCGs. Large eddy simulation (LES) is applied to calculate the transonic compressor flow fields with and without CCGs. The present investigation shows that CCGs reduce the mass flow through the tip gap by about 16% near the stall condition. Calculated flow fields show that most of this reduction of tip leakage flow occurs near the CCGs. Reinjected flow from the CCGs pushes the tip leakage flow radially inward below the casing and changes how the tip leakage flow collides with the incoming main passage flow. However, a detailed examination of the calculated flow in the tip region shows that the reinjected flow does not contribute to the reduction of the overall blockage generation. The primary driver for reducing blockage generation with CCGs is the reduction of overall mass flowrate through the tip gap. In the present investigation, measurements show a very small decrease in efficiency with CCGs at the design flow condition, although the difference in efficiency is within the measurement uncertainty. Results from the LES simulation at the design condition with CCGs show that the tip leakage vortex (TLV) is pulled toward the blade suction side and double leakage flow is eliminated. The result is that the simulated efficiencies with and without CCGs are almost the same.

Numerical Investigation of Near-Tip Modifications for a Highly Loaded Low-Speed Rotor Under the Influence of Double Leakage

Abstract This paper numerically investigates the flow field in the tip region of a highly loaded axial low-speed compressor rotor. The rotor in focus is of a hybrid blade configuration, featuring a tandem profile in the mid-span region and single blade profiles near the endwalls. The simulated 1.5-stage configuration also consists of a single row inlet guide vane (IGV) and a tandem stator. Due to high loading coefficients (Ψ = 0.58) in combination with moderate flow coefficients (ϕ = 0.58), double leakage of the tip leakage vortex plays a major role at all operating conditions. As already shown in the past, double leakage is a stability influencing dominant flow phenomenon in compressor rotors and influencing that can be the key to an efficient working range enhancement on compressor stages. This work introduces a simple and powerful method of enhancing the working range for low-speed rotors under the influence of double leakage. It is shown, that modifications to the pressure side (ps) shape of rotor tip profiles can influence the interaction of the tip leakage vortex with the adjacent blade, comparable to the effects of casing treatments. These unconventional blade shapes result from the transformations from tandem blade configurations to hybrid blades near the endwalls. The resulting single blade profile close to the tip features a convex profile element (an increase in local thickness, called the “belly”), which significantly influences the local static pressure gradients. This unusual pressure-side profiling can change mass flow over the tip and, by correct positioning, may reduce the amount of double leakage and, consequently, increase the rotor’s efficient working range. This work presents the geometry definitions for the hybrid blade single-segments, a detailed analysis of the impact on design and off-design operating conditions together with their underlying flow phenomena, and finally proposes design guidelines for the blade-tip geometries of highly loaded rotors under the influence of double leakage.

Prevention of periprosthetic leakage with double flange voice prosthesis: a systematic review and management protocol proposal

Objective To conduct a systematic review of the use and results of the Provox®VegaTMXtraSealTM in the prevention of periprosthetic leakage and to propose a management protocol for this voice prosthesis. Methods Systematic search based on the PRISMA Statement during February 2020. Keywords were double flange, periprosthetic leakage, voice prosthesis, and laryngectomy. Results Four articles with 315 voice prosthesis (94 XtraSeal and 221 controls) in 55 patients were found. The XtraSeal mean duration was 114.28 ± 73.2 (CI 95%, 98.29–130.26) days compared to 102.98 ± 17.74 (CI 95%, 100.62–105.35) days of the control group. Out of 266 replacements, endoprosthetic leakage was the most frequent cause in both groups (62.41%). Periprosthetic leaks were less frequent in the XtraSeal (9.62%) than in the control group (22.43%). Conclusions The XtraSeal could be effective in preventing periprosthetic leakage and lengthening the time between replacements. Studies with a robust methodology are necessary to confirm these results. Managing voice prosthesis is complex and requires a multidisciplinary and systematic approach by experienced professionals to reduce replacements and complications. Incorrect placement of the XtraSeal could cause a foreign body reaction and consequently inflammation, extrusion, or pressure lesions. The Tower of Hercules protocol: (1) Measurement of the tracheoesophageal fistula using the Provox® Measure, (2) Minimization of XtraSeal slack by avoiding the complete visualization of the prosthesis’ blue ring, (3) Nasofibroscopic examination of the oesophageal wall confirming both flanges are in correct position; could prevent or minimize complications derived from the use of the XtraSeal.

Effect of hub clearance size and shape of a cantilevered stator on the performance of a small-scale transonic axial compressor

To deeply understand the hub leakage flow and its influence on the aerodynamic performance and flow behaviors of a small-scale transonic axial compressor, variations of the performance and the flow field of the compressor with different hub clearance sizes and clearance shapes were numerically analyzed. The results indicate that the hub clearance size has remarkable impacts on the overall performance of the compressor. With the increase of the hub clearance, the intensity of the hub leakage flow increases, resulting in more intense flow blockage near the stator hub, which reduces the compressor efficiency. However, the flow field near the blade mid-span is modified due to the more convergent flow as the reduced effective flow area caused by the passage blockage, and the flow separation range is narrowed, thus the flow stability of the compressor is enhanced. On this basis, two kinds of non-uniform clearance cases of expanding clearance and shrinking clearance with the same circumferential leakage area as the design clearance were investigated. The occurrence position of the double leakage flow which is closely connected with the flow loss and blockage is shifted backward by the expanding clearance, the flow capacity near the stator hub is enhanced, and the unsteady fluctuation intensity of the flow field is attenuated but fluctuation frequency remains. Similarly, the modification of the stator blade root flow field may result in the reduction of stall margin. The effect of the shrinking clearance on compressor performance is opposite to that of the expanding clearance, which reduces the peak efficiency and delays the stall inception.

Modeling Dynamic Pressure of Gas Pipeline With Single and Double Leakage

Pipeline systems are safe and economic infrastructures for transporting gas and oil. However, leakage failure is the most common problem encountered with the use of pipeline and affects operation. The accuracy in locating a leakage point predominantly depends on the flow parameters in the given pipeline. Numerical modeling is a useful tool that allows separate analysis of the different hypotheses used to describe the fluid flow process and parameters. In this study, the distinctive features in the dynamic pressure fluctuation of gas are modeled by using computational fluid dynamics (CFD) software. The model was simulated with a transient operation to reveal the effect of one and two leakage points on the dynamic pressure in the pipeline by varying the leak orifice number and size, after being validated with experimental data. Finally, a consistent response was exhibited: as the leak orifice size or number changed, the leakage rate and dynamic pressure magnitude in the pipeline changed.

Modelling Flow Behavior of Gas Leakage from Buried Pipelineu

Risks in gas transportation are usually comprised of losses of the valuable gas, fire, explosion, and destruction to the environment. The safety of this infrastructure especially flammable gas pipelines is of great importance due to potential associated risks when leakage happens. An accurate understanding of the dispersion characteristics of the leaked gas from the underground pipe is of great importance. A gas leaking model from the buried pipeline was established based on computational fluid dynamics (CFD) technique, to simulate the situation. At the incidence of leakage, gas will propagate out and cause changes in flow behavior, which will prompt the detectors. The leakage position influences significantly much on the strength of leak signals to be detected at the ground surface. Under the simulation process, the double leakage pipeline model was involved. The variation of flow parameters inside the pipeline, outside pipeline, and the effect of leakage position were depicted and analyzed.

Modelling Flow Behavior of Gas Leakage from Buried Pipelineu

Risks in gas transportation are usually comprised of losses of the valuable gas, fire, explosion, and destruction to the environment. The safety of this infrastructure especially flammable gas pipelines is of great importance due to potential associated risks when leakage happens. An accurate understanding of the dispersion characteristics of the leaked gas from the underground pipe is of great importance. A gas leaking model from the buried pipeline was established based on computational fluid dynamics (CFD) technique, to simulate the situation. At the incidence of leakage, gas will propagate out and cause changes in flow behavior, which will prompt the detectors. The leakage position influences significantly much on the strength of leak signals to be detected at the ground surface. Under the simulation process, the double leakage pipeline model was involved. The variation of flow parameters inside the pipeline, outside pipeline, and the effect of leakage position were depicted and analyzed.

Design of a rear-stage subsonic axial compressor with casing treatments

The aerodynamic design of a 3.5-stage high-pressure axial compressor, representing a modern rear stage design with low aspect ratio blades and large relative tip gaps is presented. The subsonic rotor and stator airfoils feature a controlled-diffusion (CDA) type of sectional design, obtained from 2D viscous-inviscid simulations using the MISES flow solver. Improvements of the main gas path, as well as the 3D blade design, are based on 3D steady Reynolds-averaged Navier–Stokes simulations (ANSYS CFX). Sweep and dihedral were applied to improve the design and off-design performance of the compressor. The matured set of blading, with smooth casing, still exhibits large rotor tip vortices and double leakage, limiting the compressors operability towards stall point. Consequently, the application and development of axial slot casing treatments (CT) into the smooth casing is performed, aimed at improving local stage and overall stall margin with particular emphasis on avoiding efficiency penalties within stages. In order to investigate and optimise the geometric properties of the CTs with respect to efficiency a DoE (Design of Experiments) is executed. Based on the DoE a CT-geometry is deduced and applied to the critical rotor. Due to the fact, that the stage operability with casing treatment is limited by the downstream stator, a first adaption of the existing blade design is considered. In a first step, the downstream stator’s incidence is adjusted, resulting in a slightly increased stall margin. The second step incorporates the modification of rotor blade count, following the idea of trading rotor operability for an efficiency increase, due to reduced friction. However, no notable efficiency benefit was achieved, as the reduced friction loss is counterbalanced by increased secondary flow loss resulting from higher blade loadings.

Research on Transmission Performance of Double Leakage Cable Based on LTE-M System under Tunnel Simulation Environment

With the improvement and maturity of LTE technology, the vehicle-ground wireless communication system based on it has become a new technical direction of rail-vehicle-ground communication in recent years. Based on an LTE-M vehicle-to-ground wireless communication system covered by double leaky cables, load simulation service tests were carried out to test the transmission performance of leakage coaxial cables. This study found out that when two leakage cables are HV: one is horizontally polarized and another vertically polarized, the average transmission rate of the vehicle-ground communication system of the same polarization mode increased by about 50%. At the same time, there was no significant decrease with the system transmission rate when the space between them reduced from 1 m to 0.3 m. Thus, an optimized leakage cable deployment scheme was obtained. When the horizontally polarized and vertically polarized leakage cable was used and the space between the two leakage cables was 0.3 m, the optimum transmission rate has been achieved.

Bifurcations in a fractional-order neural network with multiple leakage delays

This paper expatiates the stability and bifurcation for a fractional-order neural network (FONN) with double leakage delays. Firstly, the characteristic equation of the developed FONN is circumspectly researched by employing inequable delays as bifurcation parameters. Simultaneously the bifurcation criteria are correspondingly extrapolated. Then, unequal delays-spurred-bifurcation diagrams are primarily delineated to confirm the precision and correctness for the values of bifurcation points. Furthermore, it lavishly illustrates from the evidence that the stability performance of the proposed FONN can be demolished with the presence of leakage delays in accordance with comparative studies. Eventually, two numerical examples are exploited to underpin the feasibility of the developed theory. The results derived in this paper have perfected the retrievable outcomes on bifurcations of FONNs embodying unique leakage delay, which can nicely serve a benchmark deliberation and provide a comparatively credible guidance for the influence of multiple leakage delays on bifurcations of FONNs.

Numerical investigation into the effects of casing aspiration on the overall performance and flow unsteadiness in a counter-rotating axial flow compressor

The impacts of casing aspiration with circumferential slot over the rear rotor on the overall performance and unsteady flow behaviors have been studied with numerical simulations in a counter-rotating axial flow compressor. The results indicate that casing aspiration is an effective active flow control method to expand the safe operating range of the compressor. The peak efficiency is always decreased to a different extent in the aspiration schemes and the best aspiration scheme is located at near 10% tip axial chord without significant peak efficiency loss. After the casing aspiration, both the relative inlet flow angle and the leading edge blockage are reduced obviously in the tip region. The angle between the exit direction of tip leakage flow (TLF) and axial direction is also reduced over the whole blade chord range, especially below the aspiration slots due to the local blade unloading effect. Additionally, the interface between the TLF and incoming main flow is pushed more downstream in the aspiration schemes and it is more effective to improve the flow stability by controlling the TLF released around mid-chord through casing aspiration. The intensity of both TLF and double leakage flow is attenuated and the overall oscillations inside are alleviated to a different extent in the aspiration schemes. Frequency analysis shows that the fluctuation of TLF with lower frequency component is suppressed by casing aspiration which is thought to be beneficial for the enhancement of stall margin.

Effects of tip clearance size on the unsteady flow behaviors and performance in a counter-rotating axial flow compressor

This paper presents the studies performed to better understand the effects of increased tip clearance size on the unsteady flow behaviors and overall performance under the rotor–rotor interaction environment in a counter-rotating axial flow compressor. The investigation method is based on the three-dimensional unsteady Reynolds-averaged Navier–Stokes simulations. The results show that the intensified tip leakage flow in front rotor (R1) caused by the increased tip clearance size will lead to the growth of incoming incidence angle near the tip of the rear rotor (R2). The increasing of double leakage flow range plays a significant role in the sensitivity of the efficiency to tip clearance size and its extent is enlarged gradually with the increase of tip clearance size. As the tip clearance size is increased to 1.5τ (τ represents the designed tip clearance size) from 0.5τ, the results of the fast Fourier transform for the static pressure near blade tip show that two other new fluctuating frequency components appear due to the happening of tip leakage flow self-unsteadiness in R1 and R2, respectively. Additionally, the fluctuating strength near the tip in R2 is significantly increased. However, both the overall fluctuation in R1 caused by the potential effect from downstream and the oscillation in the hub corner on the pressure side of R2 are decreased obviously. The relative inflow angle tends to increase when the incoming wakes and tip leakage flow from R1 encounter the blade leading edge of R2, which leads to the result that the trajectory of tip leakage flow is shifted more upstream.

An improved model for tip clearance loss in transonic axial compressors

An improved compressible model for estimating tip clearance loss in transonic compressors is presented with the emphasis on the effects of blade tip loading distribution and double leakage flow. Tip clearance flow is treated as three parts along the chord and the progressive relations from upstream to downstream part is revealed to be responsible for the formation of tip clearance flow. Control volume method is applied to simplify the mixing process and calculate the mixed-out loss for the three parts, separately. Computational study shows that mass flow of the incoming flow entering the control volume is consistent with that passing through an equivalent area of about half of tip leakage vortex region. The new model reveals that the second part of tip clearance flow has a larger mixed-out loss capacity than the two other parts. This difference is attributed to two factors: larger injection flow angle and more enrolled incoming flow, and both factors tend to increase the mixed-out loss. The success of the model implies that blade design or flow control strategies turning the tip clearance/main flow interface’s arrival onto blade tip pressure side downstream and the shock’s impingement point onto blade tip suction side upstream may be beneficial in desensitizing compressor performance to tip clearance size, without trading off pressure rise.

Algorithmic localisation of noise sources in the tip region of a low-speed axial flow fan

An objective and algorithmised methodology is proposed to analyse beamform data obtained for axial fans. Its application is demonstrated in a case study regarding the tip region of a low-speed cooling fan. First, beamforming is carried out in a co-rotating frame of reference. Then, a distribution of source strength is extracted along the circumference of the rotor at the blade tip radius in each analysed third-octave band. The circumferential distributions are expanded into Fourier series, which allows for filtering out the effects of perturbations, on the basis of an objective criterion. The remaining Fourier components are then considered as base sources to determine the blade-passage-periodic flow mechanisms responsible for the broadband noise. Based on their frequency and angular location, the base sources are grouped together. This is done using the fuzzy c-means clustering method to allow the overlap of the source mechanisms. The number of clusters is determined in a validity analysis. Finally, the obtained clusters are assigned to source mechanisms based on the literature. Thus, turbulent boundary layer – trailing edge interaction noise, tip leakage flow noise, and double leakage flow noise are identified.

Mechanism study of performance enhancement in a subsonic axial flow compressor with recirculating casing treatment

The improvements in both compressor efficiency and stability have been observed in a subsonic axial flow compressor with recirculating casing treatment. The study aims to understand the underlying flow mechanisms of the beneficial effect on the compressor efficiency of recirculating casing treatment. The recirculating casing treatment was investigated experimentally and numerically. In the experiment, static pressure fluctuations over the rotor tip were measured with fast-response pressure transducers. Whole-passage time-accurate simulations were also implemented to help in understanding the flow details. The unsteadiness of double-leakage flow and its effect on the loss generated at the rotor tip were discussed in detail for the solid casing. The effect of recirculating casing treatment on the double-leakage flow related loss was subsequently investigated. The results indicate that the double leakage flow is a main loss source in the rotor tip region. The double-leakage flow is completely unsteady, which induces a cyclical fluctuation of the amount of loss generated in the rotor tip region for the solid casing. With the recirculating casing treatment installed, the high-energy jet created in recirculating loops changes the unsteady characteristics of double-leakage flow by getting it diffused over the rotor tip gap along the chordwise direction, which results in the loss fluctuation that is dominated by the high-energy jet rather than the unsteadiness of double-leakage flow. The double-leakage flow is also pushed downstream by the jet and the amount is reduced when it passes through the recirculating loops. The effect of recirculating casing treatment on the double-leakage flow is primarily responsible for the improvement in the compressor efficiency.

Modeling of the double leakage and leakage spillage flows in axial flow compressors

A model to predict the double leakage and tip leakage leading edge spillage flows was developed. This model was combined by a TLV trajectory model and a TLV diameter model and formed as a function of compressor one-dimensional design parameters, i.e. the compressor massflow coefficient, ϕ and compressor loading coefficient, Ψ, and some critical blade geometrical parameters, i.e. blade solidity, σ, stagger angle, βS, blade chord length, C, and blade pitch length, S. By using this model, the double leakage and tip leakage leading edge spillage flow could be predicted even at the compressor preliminary design process. Considering the leading edge spillage flow usually indicates the inception of spike-type stall, i.e. the compressor is a tip critical design, this model could also be used as a tool to choose the critical design parameters for designers. At last, some experimental data from literature was used to validate the model and the results proved that the model was reliable.

Inlet Condition Effects on the Tip Clearance Flow With Zonal Detached Eddy Simulation

In the present study, the influence of the inlet condition on the tip clearance flow of an axial compressor is investigated. Two different zonal detached eddy simulations (ZDES) computations are carried out and compared to Reynolds-averaged Navier–Stokes (RANS) and unsteady RANS (URANS) computations as well as to experimental data. A rotating distortion map of the flow cartography is set as inlet condition for the first ZDES computation. An azimuthally averaged inlet condition is used for the second one and uncouples the rotor tip-leakage vortex flutter phenomenon, which stems from the arrival of the inlet guide vane wake from the behavior inherent to the rotor tip-leakage vortex. In the studied configuration, the inlet guide vane tip vortex reveals to lower the effects from double leakage on the rotor. The topology of the rotor tip-leakage vortex is described, and its development is analyzed.