Highly-loaded Compressor Cascade Research Articles

Influence of Ellipsoidal Dimple Column Number on Performance of Highly-loaded Compressor Cascade

Influence of Ellipsoidal Dimple Column Number on Performance of Highly-loaded Compressor Cascade

Unsteady Flow Structure of Corner Separation in a Highly Loaded Compressor Cascade

Abstract Corner separation is an inherently unsteady flow feature in an axial compressor cascade, and it significantly affects the aerodynamic performance of compressors. The flow field of a highly loaded compressor cascade at the Mach number of 0.59 under the moderate separation condition is simulated based on delayed detached eddy simulation. Comparisons of averaged flow field and transient flow field show that the three-dimensional corner separation flow is highly unsteady and composed of fine-scale vortex structures. The classical recognition of corner separation structures is a consequence of time-averaging. To better understand the contribution of unsteady structures to the averaged flow structures, the evolutions of flow fields in time series and the power spectrums are analyzed. A dominant periodic flow fluctuation is caused by the development of separating vortices with a characteristic frequency around 3500 Hz or at a Strouhal number of 0.75. Further, energy scales and spatiotemporal features of these dominant unsteady behaviors are analyzed using proper orthogonal decomposition and dynamic mode decomposition methods. Results show that the low-frequency behaviors mainly caused by the passage vortex at lower-span regions govern large-scale changes of separation flow in size and intensity and act with certain intermittency. The vortex developing mode around 3500 Hz prevails at higher regions affected by the concentrated shedding vortex. As the separating vortices dissipate approaching the midspan, the effect of the vortex developing mode on axial velocity fluctuation is reduced, although it dominates the pressure fluctuation with good stability in the whole passage.

Effect investigation of single-slotted and double-slotted configurations on the corner separation and aerodynamic performance in a high-load compressor cascade

Controlling the corner separation through blade-end slotting has been widely proven advantageous in compressors. This study proposed a new double-slotted configuration and deeply revealed its benefits and flow control mechanisms by compared with the single-slotted configuration. Based on a high-load compressor cascade, the Slotted_A scheme with a single blade-end slot1 and the Slotted_B scheme with a smaller slot2 behind the slot1 were established and detailedly evaluated. The results showed that, since the slot jet with high momentum has efficacious re-energizing effects for the local low-energy fluid, both the two slotted schemes can prevent the original cascade from entering the corner stall condition. But the slot2 jet can further control the end-wall secondary flow developing and separating behind the slot1 in the Slotted_A cascade. Therefore, the Slotted_B scheme behaves better in controlling the corner separation than the Slotted_A scheme, resulting in larger turning angles, smaller vortex structures, and weaker flow blockage. As a whole, the Slotted_A scheme and Slotted_B scheme can respectively increase the turning angle by an average of 0.93° and 1.53°, improve the static pressure coefficient by 18.2% and 21.7% on average, and achieve an average reduction of 38.3% and 43.7% in the total pressure loss. In contrast, the Slotted_B scheme is more advantageous in improving the pressure diffusion capacity and stability margin for the high-load compressor blade.

Endwall-pulsed blowing of different excitation models to control flow separation on a highly-loaded compressor cascade

Abstract Endwall-pulsed blowing (EPB) is studied for three different excitation waveforms to improve the aerodynamic performance of highly loaded compressors. Some important excitation parameters include the excitation frequency and momentum coefficient, which were analyzed in detail. The results of the EPB are compared with the endwall steady blowing (ESB) case. For EPBs with the three excitation waveforms (Waveforms sine, triangle and trapezoid), excitation frequencies that are equal to an integral multiple of the natural frequency of the vortex shedding are optimal and provide better performances than the ESB with the same time-mean momentum coefficient. Moreover, the EPBs of the three excitation waveforms have significant differences in their aerodynamic performance improvements. The optimal case is achieved by the EPB with Waveform triangle and provides a total pressure loss coefficient with a reduction of 25.64%.

Mechanisms of oscillating suction in controlling the tip leakage flow in a high-load compressor cascade

To control tip leakage flow and reduce related loss in axial-flow compressors, the oscillating suction scheme was introduced in a high-load compressor cascade with a tip clearance, and its flow control mechanisms on the tip leakage flow were studied numerically. It is found that the flow control effect of oscillating suction is the best when the unsteady excitation frequency is equal or harmonic to the feature frequency of the flow field related to the flow separation. The loss of the cascade controlled by the oscillating suction with a reduced frequency of 0.39 and excitation amplitude of 10 kPa was reduced by about 3.4% compared with the steady suction scheme, and about 11.14% less than that of the non-suction scheme. Oscillating suction discretized the concentrated tip leakage vortex into a series of scale-reduced vortex structures by the periodic unsteady excitation, which can be identified by the analysis using proper orthogonal decomposition (POD). The feature frequencies became consistent with the oscillating excitation frequency or its multiple, and the fluctuation generated by the tip leakage flow was enhanced. The energy proportion of high-order POD modes was increased by oscillating excitation, which indicated the vortex structures with weak fluctuation intensity were more active. The mixing and energy exchange among the mainstream, separated flow, endwall secondary flow, and tip leakage flow were enhanced, so that a better flow control effect could be achieved. Furthermore, flow capacity of tip passage was modified, the overall aerodynamic performance of the compressor cascade was improved thereby.

The Investigation of a New End Wall Contouring Method for Axial Compressors

To further control corner separation in high-load axial compressors, this study proposes a new end wall contouring method. It defines multiple standard “surface units” with particular flow control effects and then applies a linear combination, finally forming the geometry of the end wall surface. Based on design experiences, three different end wall contouring cases are generated and calculated on a high-load compressor cascade in the first step. The results show that the new method achieves a clear and intuitive influence on the end wall geometry, with a proper number of design variables, and can effectively combine variables with the development of secondary flow. In the second step, the new method was applied to an axial compressor, with an improvement in the design variables. Although the end wall contouring only improved the efficiency of the compressor stage on the right part of its operating map, the experimental results of the flow field show that the corner separation and end wall loss are suppressed at multiple inflow conditions. The results thus verified the practical effect of the newly developed end wall contouring method.

Flow quality improvement of the wind tunnel testing for a highly-loaded compressor cascade at high incidence

Abstract To obtain reliable and accurate experimental data in cascade testing, the influencing factors and the improving method of the flow quality of a highly-loaded compressor cascade under high incidence were investigated through a series of numerical simulations and experiments. The numerical method was validated by experimental data and agreed well at both incidence angles of 0° and 6°. Under the original upper end wall, both experimental and numerical results indicated an unsatisfactory flow quality of the cascade with an obvious nonuniformity of inlet Mach number, and the incidence of the central blade is 3.6° larger than the theoretical value. Using a small curved upper wall can reduce the severe flow separation on the upper wall and achieve a maximum improvement in flow quality under the critical installation angle, where the incidence deviation of the central blade was reduced to 2.1°. Using the combination of adjustable tailboards and a small curved upper end wall can further improve the cascade flow quality. Under the optimal angle of the tailboards, both the inflow uniformity and the outflow periodicity of the three middle blade passages the test requirements, and the incidence deviation of the central blade is reduced to 0.2°.

Flow quality improvement of the wind tunnel testing for a highly-loaded compressor cascade at high incidence

Abstract To obtain reliable and accurate experimental data in cascade testing, the influencing factors and the improving method of the flow quality of a highly-loaded compressor cascade under high incidence were investigated through a series of numerical simulations and experiments. The numerical method was validated by experimental data and agreed well at both incidence angles of 0° and 6°. Under the original upper end wall, both experimental and numerical results indicated an unsatisfactory flow quality of the cascade with an obvious nonuniformity of inlet Mach number, and the incidence of the central blade is 3.6° larger than the theoretical value. Using a small curved upper wall can reduce the severe flow separation on the upper wall and achieve a maximum improvement in flow quality under the critical installation angle, where the incidence deviation of the central blade was reduced to 2.1°. Using the combination of adjustable tailboards and a small curved upper end wall can further improve the cascade flow quality. Under the optimal angle of the tailboards, both the inflow uniformity and the outflow periodicity of the three middle blade passages the test requirements, and the incidence deviation of the central blade is reduced to 0.2°.

Investigation of flow unsteadiness in a highly-loaded compressor cascade using a dynamic mode decomposition method

Unsteady flow in the hub endwall region has long been a hot topic in the turbomachinery community. However important it is to the performance of the whole engine, the coherent unsteady flow phenomena are still not well understood. In this paper, the complex flow field in the hub endwall of a cantilevered compressor cascade has been investigated through numerical approach. The predicted results were validated by experimental data. To highlight the dominant flow structures among irregular and chaotic motions of various vortices, a Dynamic Mode Decomposition (DMD) method was utilized. The results show that there exist three dominant periodic flow structures: the oscillation of the leakage vortex, a circumferential migration of a Breakdown Induced Vortex (BIV) and the fluctuation of the passage vortex. These three coherent structures all together form a self-sustained closed loop which accounts for the flow unsteadiness of the studied cascade. During this process, the BIV plays a key role in inducing the flow unsteadiness. Only if the BIV is strong enough to affect the passage vortex, the flow unsteadiness occurs. This study expands current knowledge base of flow unsteadiness in a compressor environment, and shows the efficacy of the DMD method for revealing the origin of flow unsteadiness.

Applications of little blades in a high-load compressor cascade with leaned blade

The development of boundary layer affects the compressor cascade performance to a certain extent. Therefore, the compound lean and little blades are selected to redistribute the boundary layer, and the influences of these two flow control technologies on the axial compressor cascade performance are further studied. The calculated results showed that appropriate high pressure region on the blade suction surface near the end-wall is helpful to reduce the total pressure loss of compressor cascade, which can be achieved by positive lean technique. Meanwhile, the maximum stable operation boundary can be expanded by the application of positive leaned blade. On the other hand, the introduction of negative lean angle not only increases the total pressure loss of cascade, but reduces the stable operation range. As the little blades are introduced in the negative lean compressor cascade, the stable operation range is significantly improved by the introduction of little blades. Especially the cascade with −10° lean angle, the maximum stable operation boundary is increased from 1° to 6°. In the positive lean compressor cascade, although more low-energy fluid is accumulated on the blade suction surface near the mid-span, the little blades still show an active role in reducing the total pressure loss and expending the stable operation range, because the influence range of induced vortex reaches 30%span. The results provide a reference for improving the aerodynamic performance of compressor stator, especially when more low-energy fluid is blocked in the range near the mid-span.

Effects of segmented layer suction and micro-vortex generator on a high-load compressor cascade performance

The axial location of full-span boundary layer suction is studied to explore the influences of suction slot on the cascade performance. At the design condition, the slot with 50% axial location shows a superior capability to reduce the total pressure loss. At the near stall condition, the more upstream of the suction slot is moved, the more total pressure loss is reduced, and the suction slot with a location of 0.7 axial chord length cannot effectively reduces the total pressure loss in all conditions. Moreover, a rearranged segmented suction slot according to the distribution characteristics of the flow reversal region is developed and compared with full-span boundary layer suction. The segmented suction slot shows significant advantages in delaying the stall occurrence, and the stall point is delayed from 7.9° to 10.0° compared with the baseline. According to a quantitative analysis method selected to measure the performances of flow control technologies, the wake loss is significantly reduced by the segmented suction slot. Finally, a set of micro-vortex generator is introduced in the cascade with a segmented suction slot, and the conclusion indicates that the portion near the end-wall is very effective to reduce the flow loss.

Reduced order model for unsteady aerodynamic performance of compressor cascade based on recursive RBF

Based on Recursive Radial Basis Function (RRBF) neural network, the Reduced Order Model (ROM) of compressor cascade was established to meet the urgent demand of highly efficient prediction of unsteady aerodynamics performance of turbomachinery. One novel ROM called ASA-RRBF model based on Adaptive Simulated Annealing (ASA) algorithm was developed to enhance the generalization ability of the unsteady ROM. The ROM was verified by predicting the unsteady aerodynamics performance of a highly-loaded compressor cascade. The results show that the RRBF model has higher accuracy in identification of the dimensionless total pressure and dimensionless static pressure of compressor cascade under nonlinear and unsteady conditions, and the model behaves higher stability and computational efficiency. However, for the strong nonlinear characteristics of aerodynamic parameters, the RRBF model presents lower accuracy. Additionally, the RRBF model predicts with a large error in the identification of aerodynamic parameters under linear and unsteady conditions. For ASA-RRBF, by introducing a small-amplitude and high-frequency sinusoidal signal as validation sample, the width of the basis function of the RRBF model is optimized to improve the generalization ability of the ROM under linear unsteady conditions. Besides, this model improves the predicting accuracy of dimensionless static pressure which has strong nonlinear characteristics. The ASA-RRBF model has higher prediction accuracy than RRBF model without significantly increasing the total time consumption. This novel model can predict the linear hysteresis of dimensionless static pressure happened in the harmonic condition, but it cannot accurately predict the beat frequency of dimensionless total pressure.

Comparative Study on Modal Decomposition Methods of Unsteady Separated Flow in Compressor Cascade

The present work investigated the vortex structure and fluctuation frequency characteristics generated by boundary layer separation of a high-load compressor cascade using modal decomposition methods. The dominant modes and dynamic behaviors of unsteady flow in the cascade were obtained, and the differences of three modal decomposition methods (Proper Orthogonal Decomposition, Dynamic Mode Decomposition and Spectral Proper Orthogonal Decomposition) in feature recognition of cascade flow were discussed. The results show that:(1) The POD method can accurately extract the dominant spatial structure of the flow field, but the modal coefficients are multi-frequency coupled, which makes the dominant modal characteristics of cascade flow unclear. (2) The standard DMD method can obtain the spatial-temporal single frequency mode of cascade flow, as well as their growth rates and frequencies. However, this method is likely to capture the suboptimal mode of large amplitude with large attenuation rate, and fails to obtain the high-frequency coherent structure, which makes it impossible to obtain the dominant feature with limited mode number. (3) The SPOD method, based on spectral characteristics, can obtain spatial and temporal single frequency modes, and there is no modal screening problem. The use of spectral estimation method (SPOD) reduces the sensitivity to numerical noise. This method can obtain the low-rank behavior of cascade flow, which is helpful to understand cascade flow mechanisms. Therefore, SPOD method is more advantageous for the modal analysis of unsteady separated flow in high-load compressor cascade.

Control mechanisms of endwall profiling and its comparison with bowed blading on flow field and performance of a highly-loaded compressor cascade

In order to exploit the control mechanism of endwall profiling (EP) on flow field and performance of compressors, this paper carried out investigations on non-axisymmetric endwall profiling (NAEP), axisymmetric endwall profiling (AEP) and the comparison with bowed blading in a highly-loaded compressor cascade. Firstly, the influence mechanism of NAEP on corner separation and its design strategies were investigated. Pattern effect of NAEP, axial effect and height effect of concave/convex of NAEP were studied. Secondly, the influence mechanism of AEP on corner separation and its design strategies were studied. Thirdly, the comparison of flow mechanism between endwall profiling (EP) and bowed blading was carried out. Results show that although all the NAEP patterns can reduce the cross-passage pressure gradient, they exhibit distinct different control effects on corner separation. It is indicated that the radially inward pressure gradient, rather than the cross-passage pressure gradient near the endwall, is the key to control corner separations. The endwall cross-passage pressure gradient remained after AEP. However, the corner separation was also eliminated effectively by AEP with a concave curvature, with loss coefficient reduced by 20.07%. At last, compared with bowed blading, the cascade with EPs exhibits a higher overall performance but a lower control effect on corner separation.

Numerical study on endwall fence with varying geometrical parameters in a highly-loaded compressor cascade

This study investigates the secondary flow control effect of the single diminutive streamwise endwall fence with varying geometrical parameters in a highly-loaded compressor. To determine the effects of the fence parameters upon the flow field structure and aerodynamic performance, numerous fenced cases are numerically simulated at the aerodynamic design point. The results demonstrate that the optimum endwall fence obstructs the migration of the endwall boundary layer from the pressure side (PS) to the suction side (SS), which leads to a measurable reduction in the separation line of corner reverse flow. Moreover, the counter-rotating fence vortex (FV) induced by the endwall fence can suppress the passage vortex (PV) and enhance the momentum exchange between the endwall boundary layer and high momentum freestream fluid. For the negative side, the fence with a larger surface area (the thicker, taller or longer fence) is proved to bring greater additional loss to the cascade, which offsets its positive flow control effect. In this study, the fence device with a width of 0.5 mm, a height of 10% the inflow boundary layer thickness and a length of 75% blade chord provides a 1.55% reduction on the cascade total pressure loss, as well as optimizes secondary flow structure by weakening passage vortex and dissipating corner vortex (CV).

Study of combined flow control strategies based on a quantitative analysis in a high-load compressor cascade

Micro-vortex generator and a jet flow control technique are combined to improve the flow performance of a high-load compressor cascade, and the injection position is further investigated. The calculated results showed that the reverse flow region of baseline grows with the increment of incidence, and an unstable flow phenomenon rapidly occurs near the leading edge when the incidence rises to +7.9°. Meanwhile, two critical points occur on the end-wall, that is considered to be the main reason for occurring stall. When an injection tube is applied to remove the low energy fluid, the total pressure loss appears drastic reduction, and the occurrence of stall shows a slight delay from +7.9° to +8.7° incidence compared with the baseline. With the micro-vortex generator is introduced in the baseline, the occurrence of stall is delayed from +7.9° to +10.0° incidence due to the mixture of low energy fluid and main flow. The combined application shows an excellent effect in delaying the occurrence of stall and reducing the total pressure loss, and the sudden deterioration of the cascade performance occurs at +9.8° incidence. A quantitative analysis of total pressure loss shows that the secondary flow loss and wake loss contribute to the reduction of total pressure loss, the former is reduced by 31.0% and the latter is decreased by 24.1% at the near stall condition. Moreover, the spanwise injection position can affect both the total pressure loss and stall incidence, and the wider operating range is accompanied by the increment of total pressure loss.

A combined application of micro-vortex generator and boundary layer suction in a high-load compressor cascade

In the current study, the effects of a combined application between micro-vortex generator and boundary layer suction on the flow characteristics of a high-load compressor cascade are investigated. The micro-vortex generator with a special configuration and the longitudinal suction slot are adopted. The calculated results show that a reverse flow region, which is considered the main reason for occurring stall at 7.9° incidence, grows and collapses rapidly near the leading edge and leads to two critical points occurring on the end-wall with the increasing incidence in the baseline. As the micro-vortex generator is introduced in the baseline cascade, the corner separation is switched to a trailing edge separation by the thrust from the induced vortex. Meanwhile, the occurrence of failure is delayed due to the mixed low energy fluid and main flow. The synergistic effects between the micro-vortex generator and the boundary layer suction on the performance of the cascade are superior to the baseline at all the incidence conditions before the occurrence of failure, and the sudden deterioration of the cascade occurs at 10.3° incidence. The optimal results show that the farther upstream suction position, the lower total pressure loss of the cascade with vortex generator at the near stall condition. Moreover, the induced vortex with a leg can migrate the accumulated low energy fluid backward to delay the occurrence of stall.

Effects of modified micro-vortex generators on aerodynamic performance in a high-load compressor cascade

In the current study, the effects of micro-vortex generators on the flow characteristics of a high-load compressor cascade are investigated, and four types of micro-vortex generators including “rectangular,” “curved rectangular,” “trapezoidal,” and “curved trapezoidal” are considered and named VGR, VGCR, VGT, and VGCT separately. The calculated results show that a rising reverse flow region, which is considered a main reason for occurring stall at +8° incidence, collapses rapidly from the leading edge in the cascade. Therefore, the micro-vortex generators are all mounted on the end-wall in front of the passage to suppress the development of the secondary flow, and the stall occurrence is delayed from +8° to +11° incidence by applying VGCT. At the design condition, the VGT can make the total pressure loss decrease by 0.54%. The modified micro-vortex generators show an obvious superiority when the range of incidence is between +3° and +8°. At the stall condition, the VGCT can make the total pressure loss decrease by 9.36%. Moreover, the reduction of the secondary flow loss is considered a main goal of the adoption of micro-vortex generators which is an achievement for decreasing the total pressure loss, and the highest reduction of the secondary flow loss reaches 34.6% at the stall condition in the cascade with VGCT.

Effects of slot jet and its improved approach in a high-load compressor cascade

Experimental investigations are conducted in this paper to study a slot jet approach. Measurements inside a high-load compressor cascade were performed to evaluate its gains and reveal its mechanisms. Further, an improved approach is developed on the basis of the individual slot jet approach. Experiments shows that the individual slot jet is highly efficient at reducing trailing-edge separation by directly speeding up separated fluids, but it is not so efficient at reducing the corner stall. To further reduce the corner stall, the improved approach introduces a vortex generator into the flow field. The individual slot jet partly reduces corner stall by restricting the passage vortex and concentrating the separation at the cascade corner, which happens to offer convenient conditions for the vortex generator to further reduce flow loss. The vortex generator, which produces a counter-rotating vortex to counteract the passage vortex, prevents the formation of the end-wall cross flow. Therefore, both the trailing-edge separation and corner stall are significantly suppressed, so that the improved approach achieves more powerful flow control effects.

Advanced Endwall Contouring for Loss Reduction and Outflow Homogenization for an Optimized Compressor Cascade

The following paper deals with the development of an optimized non-axisymmetric endwall contour for reducing the total pressure loss and for homogenizing the outflow of a highly-loaded compressor cascade. In contrast to former studies using a NACA-65 K48 cascade airfoil this study starts with the design of a new high-performance airfoil which is based on the aerodynamic boundary conditions of the NACA-65 K48 cascade. This new airfoil is then used as a basis. Optimizations of the airfoil and of the endwall contour are performed using the German Aerospace Center (DLR) in-house tool AutoOpti and the RANS (Reynolds-averaged Navier-Stokes)-solver TRACE (Turbomachinery Research Aerodynamic Computational Environment). Three operating points at an inflow Mach number of 0.67 with different inflow angles are used to secure a wide operating range. The optimized endwall contour changes the secondary flow in such a way that the corner stall is reduced which, in turn, significantly reduces the total pressure loss. The endwall contour in the outflow region leads to a considerable homogenization of the outflow in the near wall region. Using non-axisymmetric endwall shaping demonstrates a valuable measure to further improve highly-efficient compressor blading on the vane level.