Double Leakage Flow Research Articles

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.

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.

Flow mechanism of self-recirculating casing treatment in a low-reaction transonic compressor rotor

Casing treatment is used to ensure the stability of an aeroengine compressor when it is operated away from its design point. However, currently, there is little published data about the effect of casing treatment on highly loaded axial compressors. In this study, unsteady numerical simulations are applied to explore the flow mechanism responsible for the stability enhancement of a low-reaction (LR) transonic compressor rotor with ultra-high loading owing to self-recirculating casing treatment (SRCT). A special SRCT configuration is designed based on the suction effect, which can simultaneously improve the stall margin and aerodynamic performance. Radial skew in the direction of blade rotation is employed to enhance the control outcomes further. The results show that the best skew angle for stall margin and performance improvement is 45 degrees. Further analysis of the instantaneous flow structures reveals that for the smooth casing, the mechanism by which tip leakage vortex (TLV) breakdown, TSV, and blade loading interact induces periodic oscillations of the tip flow field. As the unsteady suction effect introduced by the SRCT dominates the evolution processes of vortical structures, the original oscillation frequency is converted into the SRCT-induced excitation frequency. Furthermore, a substantial amount of double-leakage flow is inhaled into the recirculation channel without rolling up into the adjacent TLV, which dampens the shock/TLV interaction and the formation of downstream TSVs. The blockage along the pressure side caused by the aforementioned large-scale vortices is effectively eliminated.

Unsteady Effects of Wake on Downstream Rotor at Low Reynolds Numbers

In a compressor, the periodic wake is an inherently unsteady phenomenon that affects the downstream flow conditions and loading distribution. Thus, understanding the physical mechanisms of these unsteady effects is important for eliminating flow losses and improving compressor performance, particularly at low Reynolds numbers. To understand the influence of the upstream wake on the downstream flow field structure, this paper describes numerical simulations of a one-stage high-pressure compressor at altitudes of 10–20 km. The influence of the wake on rotor flow blockage at different Reynolds numbers is analyzed, and the unsteady interaction between the upstream wake and boundary layer or tip leakage flow is discussed. The results indicate that the wake has a beneficial effect on the efficiency of the rotor at high Reynolds numbers, but this weakens and becomes negative as the Reynolds number decreases. The wake can reduce the flow blockage in the mainflow region. Due to the wake, the length of the laminar separation bubble at high Reynolds numbers decreases and that at low Reynolds numbers increases. In addition, the unsteadiness of the wake causes separation bubbles to appear periodically at high Reynolds numbers and induces an open separation bubble at low Reynolds numbers. The Kelvin–Helmholtz instability can dominate the transition process of the boundary layer, which is also affected by the disturbance vortex induced by the wake. Regarding the tip leakage flow, the wake can reduce the flow blockage at high Reynolds numbers but increase the flow blockage at low Reynolds numbers. The interaction at low Reynolds numbers causes a double-leakage flow, which finally leads to the large-scale separation of the suction surface boundary layer. The large-scale separation causes flow blockage in the tip region and prevents the rotor wake from propagating downstream. On the contrary, the unsteady wake can pass through the tip clearance vortex and inhibit the separation of the suction boundary layer at high Reynolds numbers, which is reflected in a larger amplitude of one blade passage frequency. Therefore, the flow loss in the downstream flow field at high Reynolds numbers is significantly reduced at high Reynolds numbers.

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.

Combined acoustic and aerodynamic investigation of the effect of inlet geometry on tip leakage flow noise of free-inlet free-exhaust low-speed axial flow fans

The performance and the emitted noise of a free inlet – free exhaust axial flow fan are influenced by the inlet geometry. In the present paper, the effect of bellmouth entries with various sizes on the tip leakage flow (TLF) noise is discussed. The aerodynamics phenomena are investigated using inlet velocity profile measurements and computational fluid dynamics (CFD) simulations. Noise source maps have been derived from phased array microphone measurements using the Rotating Source Identifier (ROSI) beamforming algorithm. The aerodynamic and acoustic data are evaluated in a combined manner. The results showed that with the use of a bellmouth entry of proper size, the TLF can be moderated, the double-leakage flow (DLF) can be eliminated, and the sound pressure level of the emitted noise can be reduced by 5–6 dB.

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.

Effects of Double-Leakage Tip Clearance Flow on the Performance of a Compressor Stage With a Large Rotor Tip Gap

Effects of a large rotor tip gap on the performance of a one and a half stage axial compressor are investigated in detail with a numerical simulation based on large eddy simulation (LES) and available particle image velocimetry (PIV) data. This paper studies the main flow physics, including why and how the loss generation is increased with the large rotor tip gap. The present study reveals that when the tip gap becomes large, tip clearance fluid goes over the tip clearance core vortex and enters into the next blade's tip gap, which is called double-leakage tip clearance flow. As the tip clearance flow enters into the adjacent blade's tip gap, a vortex rope with a lower pressure core is generated. This vortex rope breaks up the tip clearance core vortex of the adjacent blade, resulting in a large additional mixing. This double-leakage tip clearance flow occurs at all the operating conditions, from design flow to near stall condition, with the large tip gap for the current compressor stage. The double-leakage tip clearance flow, its interaction with the tip clearance core vortex of the adjacent blade, and the resulting large mixing loss are the main flow mechanism of the large rotor tip gap in the compressor. When the tip clearance is smaller, flow near the end wall follows more closely with the main passage flow and this double-leakage tip clearance flow does not happen near the design flow condition for the current compressor stage. When the compressor with a large tip gap operates at near stall operation, a strong vortex rope is generated near the leading edge due to the double-leakage flow. Part of this vortex separates from the path of the tip clearance core vortex and travels from the suction side of the blade toward the pressure side of the blade. This vortex is generated periodically at near stall operation with a large tip gap. As the vortex travels from the suction side to the pressure side of the blade, a large fluctuation of local pressure forces blade vibration. Nonsynchronous blade vibration occurs due to this vortex as the frequency of this vortex generation is not the same as the rotor. The present investigation confirms that this vortex is a part of separated tip clearance vortex, which is caused by the double-leakage tip clearance flow.

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.

Effect of Unsteady Stator Wake—Rotor Double-Leakage Tip Clearance Flow Interaction on Time-Average Compressor Performance

A study has been conducted, using unsteady three-dimensional Reynolds-averaged Navier-Stokes simulations to determine the impact on rotor performance of the interaction between upstream (steady defect and time-varying defect) stator wakes and rotor tip clearance flow. The key effects of the interaction between steady stator wakes and rotor tip clearance flow are: 1) a decrease in loss and blockage associated with tip clearance flow; 2) an increase in passage static pressure rise. Performance benefit is seen in the operability range from near design to high loading. The benefit is modest near design and increases with loading. Significant beneficial changes due to the stator-rotor interaction occur when the phenomenon of tip clearance flow double-leakage is present. Double-leakage occurs when the tip clearance flow passes through the tip gap of the adjacent blade. It is detrimental for compressor performance. The effect of strong stator-rotor interaction is to suppress double-leakage on a time-average basis. Double-leakage typically takes place at high loading but can be present at design condition as well, for modern highly loaded compressor. A benefit due to unsteady interaction is also observed in the operability range of the rotor. A new generic causal mechanism is proposed to explain the observed changes in performance. It identifies the interaction between the tip clearance flow and the pressure pulses, induced on the rotor blade pressure surface by the upstream wakes, as the cause for the observed effects. The direct effect of the interaction is a decrease in the time-average double-leakage flow through the tip clearance gap so that the stream-wise defect of the exiting tip flow is lower with respect to the main flow. A lower defect leads to a decrease in loss and blockage generation and hence an enhanced performance compared to that in the steady situation. The performance benefits increase monotonically with loading and scale linearly with upstream wake velocity defect. With oscillating defect stator wakes, rotor performance shows dependence on oscillation frequency. Changes in the tip region occur at a particular reduced frequency leading to (1) decrease in blockage, and (2) increase in passage loss. The changes in rotor performance at a particular reduced frequency are hypothesized to be associated with the inherent unsteadiness of the tip clearance vortex and its resonance behavior excited by the oscillating wakes.