Secondary Flow Control Research Articles

Experiment investigation on secondary flow loss and flow control mechanisms in a variable compressor cascade with penny cavities

Variable Stator Vanes (VSVs) with penny cavities are an important method to improve the efficiency of compressors. However, the annular and radial gaps in VSVs introduce complex flow structures, posing challenges in enhancing compressor efficiency. This article employs low-speed wind tunnel experiments to study the impact mechanisms of VSVs' adjustment angle and radial gap size on secondary flow losses and implements single-hole suction as a means of flow control to reduce total pressure loss. The results indicate that radial gap leakage has a certain inhibitory effect on annular gap leakage at general adjustment angles. Total pressure loss decreases initially as the radial gap increases but once the radial gap further enlarges and dominates the loss components, the total pressure loss increases proportionally with the radial gap. At extreme adjustment angles, where the lateral pressure gradient is significant, radial gap leakage intensity is high and comprises the major part of the loss, leading to an increase in total pressure loss in proportion to radial gap enlargement. Additionally, single-hole suction is effective under various conditions, with the potential to reduce total pressure loss by up to 19.4 %. These findings provide guidance for the application of VSVs in further improving compressor efficiency.

Experimental and numerical investigation on turbine flow control through integrating endwall contouring with section profiling at different Mach numbers

The efficiency of turbomachinery is notably impacted by losses occurring within the turbine cascade, which are intricately linked to the configuration and intensity of secondary flows. Non-axisymmetric endwall contouring, as well as three-dimensional blade designs, are effective methodologies for regulating secondary flows within the cascade passage. These passive control strategies are garnering increased attention due to their adaptability under off-design conditions. However, previous investigations have primarily scrutinized the flow intricacies in linear cascades. This study provides an in-depth examination of the adaptation of endwall contouring, both independently and in conjunction with section profiling, within annular cascades and a specific emphasis on Mach numbers and their influence on secondary flow control. Employing a blend of experimental and numerical techniques, the research elucidates how radial pressure gradients impact pressure distributions near the throat, leading to notable discrepancies in secondary structures between the lower and upper passages. Furthermore, the study highlights how endwall contouring significantly inhibits the progression of secondary vortices. The integration of endwall contouring with section profiling demonstrates enhanced performance by modulating flow angles compared to endwall contouring in isolation. Notably, the effects of these methodologies on pressure variations and secondary flow control exhibit an escalating trend with increasing Mach numbers, attributed to flow compressibility effects. Additionally, the study observes a consistent trend in the ability to mitigate losses as Mach numbers vary.

Experimental and numerical research on blended blade and end wall technique in a compressor cascade

In this paper, to explore the effects and mechanisms of the blended blade and end wall (BBEW) technique on corner separation control, two different BBEW configurations, including the side blended blade and end wall (S-BBEW) and full blended blade and end wall (F-BBEW), are studied and compared through experimental and numerical analyses. First, for a linear cascade, the S-BBEW and F-BBEW designs are carefully built. Then, both numerical simulations and experiments are conducted for each design, demonstrating a significant reduction in the loss coefficient. The results indicate that the S-BBEW is effective in the control of boundary layer development and delays the initial point of separation, especially at the design working condition. Moreover, the F-BBEW design nearly eliminates the corner separation by the control of both boundary layer development and secondary flow, which provides a higher performance improvement, especially at a large incidence angle.

Analysis of Brush Seal Interaction with Steam Turbine Rotor-Shafts

High-speed turbines are a major source for power production. They utilize high pressure and temperature fluid flow. Sealing of these machines to decrease the flow losses has been a major engineering challenge since the inception of steam turbines. From an engineering viewpoint, seals are used to introduce friction in the fluid flow path. This reduces the flow leakage. Improved seal performance offers substantial opportunities for turbine performance. Reduced leakages in steam turbines can lead to greater efficiency and power output. They can also allow tighter control of turbine secondary flows. However, sealing these machines is a major challenge. There are several seal locations on a steam turbine that have significant performance derivatives. These include the interstage shaft packing, the end packing, and the bucket tip seals. Brush seals are ideal for these locations because they can reduce the flow losses. This study analysed the efficiency of the steam turbine by using ANSYS APDL. It assessed the leakage performance of the brush seal for different cases and geometries. It also used porous medium modelling of the bristle pack to provide insight for designers. The outputs of interest were the rotor-shaft temperature and the efficiency of the entire turbine with the brush seal.

Evolution and Action Mechanism of Relative Negative Pressure Zone in Spiral Groove Gas Face Seal for Aero-Engine

Abstract As an advanced sealing technology, the application of gas face seal in aero-engine is few; if possible, lower specific fuel consumption, higher thrust-weight ratio, and effective secondary flow control at minimal cost would be brought. A relative negative pressure zone (RNPZ) was found in the spiral groove gas face seal, and the evolution and action mechanism of RNPZ was investigated in detail, which may promote the above application. Three film thicknesses of spiral groove gas face seals at different rotational speeds and inlet pressures were numerically compared to obtain the pressure field in the groove area and the formation of RNPZ. Then, the radial and circumferential velocities in the groove were calculated to quantify the impact of the obstruction effect, viscous pumping, and shear effect, which revealed the evolution mechanism of the RNPZ stage by stage. At last, the action mechanism of RNPZ was clarified through the hydrodynamic performance analysis. It is found that the pressure field evolution in the gas face seal is in stage Ⅲ under the high rotational speed and low inlet pressure conditions in an aero-engine. Under the same film thickness, RNPZ can suppress leakage to a certain extent in stage Ⅱ, while in stage Ⅲ, it increases the opening force and stiffness-leakage ratio. This work can provide theory and data to help with the subsequent optimization design of gas face seals for aero-engine.

Flow Control in Serpentine Inlet Duct Using Vortex Generator Jets

The objective of the present research is to study the effectiveness of steady and pulsed vortex generator jets in reducing inlet flow distortion and in improving pressure recovery by effective secondary flow control in uniform inflow serpentine duct diffuser. The measurements were carried out at a test Reynolds number of 6.5 x 105 based on the diffuser inlet width. Serpentine duct diffuser consisted of two main portions, namely, a square to circular constant area transition duct followed by a circular diffusing duct. Investigations show that the flow in the serpentine diffusing duct suffers from stall on the inner wall and consequently the outflow at AIP has considerable flow distortion due to the combined effect of secondary flow and the inner wall stall. It is observed that the use of vortex generator jets, both in steady and pulsed modes, improve the performance substantially. The number of jets, location of jets, velocity ratios and the pulse frequency are some of the variables that are studied. The results obtained so far suggest that use of pulsed jet not only gives better performance but also reduces the amount of air that is needed to be injected through the jets in comparison to steady jets for similar velocity ratios.

Nonuniform height endwall fence optimization of a low-pressure turbine cascade

As the lift of turbine increases, the secondary flow in the endwall region seriously restricts improvement of the aerodynamic performance of low-pressure turbine. The particle swarm optimization is improved to pursue stronger optimization ability. An optimization platform is built that combines Bayesian optimization (BO), the eXtreme Gradient Boosting (XGBoost) algorithm and improved chaos particle swarm optimization. The test results show that the surrogate model optimization method proposed in this paper has more prominent application potential in the field of aerodynamic optimization. The above optimization method is used for the optimization design of the nonuniform height endwall fence (NHEF) of a high-lift low-pressure turbine cascade to achieve an efficient secondary flow control effect. The NURBS curve is used to realize the parameterization of the NHEF. The secondary flow control mechanism of traditional fences and NHEF has been compared and analysed in detail. The numerical results show that the NHEF can effectively prevent the cross migration of the boundary layer, and the fence vortex induced by the fence can suppress the development of the passage vortex. The total pressure loss at the outlet of the cascade installed NHEF is reduced by 14.82%. The secondary flow control effect of the NHEF is better than that of a traditional fence. The feature importance analysis results of input variables show that the pitchwise position of the fence has the most obvious influence on the loss of the cascade. In addition, based on the results of feature importance, a 53% length reduction fence is designed to reduce the extra weight brought by the fence. The design achieves the effect of comprehensively considering the weight and aerodynamic performance.

Integrated optimization of a high-lift low-pressure turbine cascade based on dynamic support vector regression

With the increase of the lift of turbine components, the performance of the turbine is significantly affected by the secondary flow. The combined optimization of the root modification and the profiled endwall (PEW) is investigated for more efficient secondary flow control in the endwall region of a high-lift low-pressure turbine cascade. The NURBS surface is used in this paper to perform parametric modelling. Support vector machines (SVM) and particle swarm optimization (PSO) are used to complete the optimal design of the combination of cascade root and endwall. To ensure the accuracy of the regression model, dynamic parameter adjustment and dynamic point addition strategies are introduced in the optimization process to update the regression model. The study found that when the separate profiled endwall was optimized for the cascade, the total pressure loss of the cascade decreased by 10.39%; when the cascade was optimized by the combined shape of the cascade root and the endwall, the total pressure loss of the cascade decreased by 12.88%. The detailed flow field analysis shows that both the profiled endwall and integrated optimization can effectively weaken the cross-passage pressure gradient in the endwall region of the cascade passage. Compared with only profiled endwall, the integrated optimization restrains the passage vortex and the suction side leg of horseshoe vortex from developing towards the middle of the cascade, and the secondary flow loss in the endwall region is obviously reduced.

Investigation on gas-solids hydrodynamics and matching enhancement in feedstock injection zone of double-level nozzle riser

With the development of current energy economy, it is necessary to improve the product distribution of fluid catalytic cracking process, which is achieved by a riser reactor with double-level of nozzles. The new riser is constructed by adding a level of secondary nozzle 0.5 m below the main nozzle of traditional riser. This paper investigates the gas-solids flow and oil-catalyst matching feature based on the optical fiber and tracer technologies. According to the distribution of solids holdup, particle velocity and dimensionless jet concentration, the feedstock injection zone can be divided into the upstream flow control region, the main flow control region, and the secondary flow control region in the radial direction. The size of the regions is changed by the jet gas velocity and axial height. There is a poor match of secondary nozzle jet to particles below the main nozzle. The jet gas from secondary nozzles can improve the matching effect of oil-catalyst near the wall and reduce the probability of coking above the main nozzle.

Wake stabilization behind a cylinder by secondary flow over the leeward surface

Porous coating and blowing jets are both effective flow control methods for a bluff body. In the present study, we conducted wind tunnel experiments to investigate the combined control effects on a circular cylinder. The flow control was achieved by active steady blowing flows through the structured porous surface on the leeward side of the cylinder. The Reynolds number Re in the experiments, based on the cylinder outer diameter, was 1.0×104. The control effects were evaluated by a non-dimensional blowing momentum coefficient Cμ, which was determined by various blowing mass flow rates, incoming wind speed, and the geometry of the porous surface. Reduced-order models, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were employed to analyze the wake stabilization effects of the secondary jet flows. We found that, under the control of secondary flows ejected from the porous region of the cylinder, POD modal characteristics in the global flow wake were changed; temporal and spatial properties of DMD transformed; frequency and mode of the vortex shedding process shifted; statistical turbulent flow characteristics ameliorated; and the estimated drag coefficients restrained. Experimental results in the present study demonstrated that the secondary flow ejected from the structured porous surface and the resultant small-scale vortices could stabilize the cylinder wake with proper Cμ values.

Numerical investigation on the endwall fences with varying geometrical parameters on secondary flow control

This study investigates the effect of the fences with varying height and chord length on the secondary flow reduction. Three-dimensional flow structure on the endwall and streamline on the blade surface are analyzed to determine the effect of fence geometry on the secondary flow reduction. The results demonstrate that the pressure leg of the horseshoe vortex induced by the fences can prevent the passage vortex of the main blade. In this work, a fence geometry is found to reduce the secondary flow loss and decrease the total pressure loss coefficient of the main blade by about 4%. However, the fences with large chord length are found to bring a greater additional flow loss, which is induced by the flow separation of the fences. Thus, the fences could be applied only when the fences induced additional loss is lower than the loss due to the secondary flow that they prevent.

The flow mechanism of the shock structure and secondary flow control in transonic centrifugal compressors with leading edge sweep

Limitations still exist in the research on the effects of leading edge sweep on transonic centrifugal compressors. The lack of understanding of this mechanism also leads to confusion regarding the magnitudes of the performance changes that occur due to the use of leading edge sweep. According to a detailed numerical simulation verified by experiments on two different compressors, the effect and flow mechanism of leading edge sweep on transonic centrifugal compressors were investigated in detail to alleviate the confusion as clearly as possible. The conclusions show that leading edge sweep has different impacts on shock waves at different positions; a forward sweep leading edge can weaken the strength of the shock wave in front of the leading edge, while a backward sweep leading edge can weaken the strength of the passage shock wave that is swallowed in the blade passage. The improved compressor performance attributed to leading edge sweep is due to the weakening of the shock waves and the interaction between the shock waves and the secondary flow of the inducer tip. Furthermore, the intensity of the inducer tip secondary flow influences the potential for improved performance induced by using sweep leading edges in transonic centrifugal compressors. This study provides a valuable reference for applying leading edge sweep to existing transonic centrifugal compressors or future advanced centrifugal compressor designs.

A simultaneous use of a leading-edge fillet and a non-axisymmetrically contoured endwall in a turbine stage

Secondary flow minimization is a crucial problem in a turbine passage. In the present paper, three different strategies are investigated to reduce the secondary-flow-related total pressure loss. These are leading-edge fillet, non-axisymmetric endwall contouring, and combining these two approaches. An experimental investigation in a single-stage turbine facility and RANS-based computations are performed to evaluate the designs. The experiments are carried out in the large-scale Axial Flow Turbine Research Facility AFTRF. A reference Flat Insert is installed on the nozzle guide vane passage's hub surface to allow future endwall design implementation during the experimental phase. The stereolithography manufactured reference insert has a constant thickness with a cylindrical shape. This investigation also uses four different leading-edge fillets, and they are attached to cylindrical Flat Insert initially. The same fillets are also attached to a contoured endwall. Simultaneous use of a leading-edge fillet and a non-axisymmetric contoured endwall for secondary flow control is the main objective of this research. Total pressure measurements are taken at the rotor inlet plane with a Kiel probe. The probe traversing is completed along one vane pitch and from 8% to 38% span. The leading-edge fillets effectively managed to weaken the horseshoe vortex occurring in front of the leading-edge. In one of the fillet cases sitting on the Flat Insert, the computational results at the NGV exit showed that the mass-averaged loss value was reduced by 1.31%. Three fillet designs decreased the area-averaged loss with a maximum reduction of 15.06%.

Studies on Secondary Flow Control in a Low Pressure Turbine Cascade for Aero-Engines by a Composite-Type Device

Studies on Secondary Flow Control in a Low Pressure Turbine Cascade for Aero-Engines by a Composite-Type Device

Numerical Investigation on the Secondary Flow Control by Using Splitters at Different Positions with Respect to the Main Blade

In turbomachinery, strong secondary flow can produce significant losses of total pressure near the endwall and reduce the efficiency of the considered turbomachine. In this study, splitters located at different positions with respect to the main blade have been used to reduce such losses and improve the efficiency of the outlet guide vane (OGV). Three different relative positions have been considered assuming a NACA 65-010 profile for both the main blade and the splitter. The numerical results indicate that splitters can effectively reduce the total pressure loss by suppressing the secondary flow around the main blade, but the splitters themselves also produce flow losses, which are caused by flow separation effects.

Numerical investigation and loss estimation of high-pressure turbine cascade flow with contoured endwall and incoming wakes

With the aim of suppressing the irreversible losses, the secondary flow at endwall and incoming vortices interactions are assumed to be the most concerned physics in gas turbine for modern aero-engine and military vehicles. However, the combining effect of contoured endwall and incoming periodic wakes were seldom discussed previously. In the present work, the research subject is the high-pressure turbine cascade with non-axisymmetric endwall and incoming wakes induced by moving bar fulfilled with periodic boundary conditions in pitchwise direction. Firstly, the aerodynamics and thermodynamics in the turbine cascade with secondary flow control technique, which denotes to non-axisymmetric endwall, were experimentally and numerically investigated. For what concerns the profile aerodynamic loadings, total pressure loss coefficients and vortices coefficient were surveyed to quantify the influence of interaction between incoming wakes and secondary flow at contoured endwall. The cumulative difference of entropy production rate associated with dissipation is then employed to evaluate the loss under different inflow condition. Combined with dynamic mode decomposition (DMD) method, the energy loss features associated with incoming wakes at different passing frequencies was captured. The results indicated that the flow separation at the suction wall with the axial position of 70%∼80% Cax upstream trailing edge was suppressed in the cases with incoming wakes for high pressure turbine cascade. And stronger incoming wakes tends show better suppression effect on the flow separation. However, when combined with contoured endwall, the negative influence of non-axisymmetric endwall at the middle passage was enlarged. And the general aerodynamics of the cascade flow may not be necessarily improved in term of energy loss characteristics with the increase of passing frequency of incoming wakes. The present work preserves guiding significance for the design and flow control of high pressure turbine cascade.

Numerical study of the influence of splitter geometry on secondary flow control

In turbmachinery, splitters are used to control secondary flow and to improve the aerodynamic performance of outlet guide vane (OGV) of axial fan. However, there is few information in the open literature focusing on the effect of the splitter geometry on the secondary flow control. In this work, the numerical investigations were performed for NACA 65-010 profile with different splitters. The spacing ratio of the main blade was 1, the spacing between the splitters and the main blade was 30mm. Three different splitters were compared to investigate the effect of the splitter geometry on secondary flow control. The flow structures near the end-wall, the streamlines on the suction surface of blade and the distribution of the flow losses on the trailing edge of blade was shown and discussed. The results showed the splitters can control the secondary flow and reduce the area of the high flow losses at the junction of end-wall and blade. However, the flow separation of the splitters also causes flow losses in the wake behind the splitters. Therefore, only the specific configuration of splitters can reduce the flow losses of the blade.

Effect of Camber on Non-axisymmetric End-wall Contouring Performance in Compressor Linear Cascades

In order to explore non-axisymmetric end-wall’s mechanism of secondary flow control in compressor cascade, based on three-dimensional numerical calculation, this paper analysed effects of camber on non-axisymmetric end-wall contouring performance in compressor linear cascades. Results showed that non-axisymmetric end-wall reduces the corner separation loss mainly by suppressing the interaction between the crossflow and the suction surface boundary layer and decreasing the adverse pressure gradient on suction surface. Meanwhile, non-axisymmetric end-wall partly enhances the cross flow, increasing the secondary flow loss. At the incidence of 0°, the increase of the secondary flow loss caused by the non-axisymmetric end-wall plays the leading role when the camber angle is 20°; with the increase of camber angle, the reduction of corner separation loss becomes the leading role. At the incidence of 7°, the hub-corner stall is removed by the non-axisymmetric end-wall when the camber angle increases to 40°, decreasing the loss remarkably.

Leakage and Dynamic Force Coefficients of a Pocket Damper Seal Operating Under a Wet Gas Condition: Tests Versus Predictions

AbstractHigh-performance centrifugal compressors presently favor pocket damper seals (PDSs) as a choice of secondary flow control element offering a large effective damping coefficient to mitigate rotor subsynchronous whirl motions. Current and upcoming multiple-phase compression systems in subsea production facilities must demonstrate long-term operation and continuous availability, free of harmful rotor instabilities. Plain annular seals and labyrinth (LABY) seals are notoriously bad choices, whereas a PDS, by stopping the circulation of trapped liquid, operates stably. This paper presents experimental and computational fluid dynamics (CFD) results for the leakage and dynamic force coefficients obtained in a dedicated test facility hosting a fully partitioned PDS (FPPDS), four ribbed and with eight pockets per cavity. The test PDS, operating at a rotor speed 5250 rpm (surface speed 35 m/s) and under a supply pressure/discharge pressure ratio up to 3.2, is supplied with a mixture of air and ISO VG 10 oil whose maximum liquid volume fraction (LVF) is 2.2%, equivalent to a liquid mass fraction of 84%. When supplied with just air (dry condition), the measured leakage increases nonlinearly with supply pressure. Under a wet gas condition, the recorded mass flow increases on account of the large difference in density between the liquid and the gas. CFD-derived mass flow rates for both dry and wet gas conditions agree with the measured ones. The test dry gas PDS produces a direct dynamic stiffness (HR) increasing with frequency, whereas the direct damping (C) and cross-coupled dynamic stiffness (hR) coefficients remain relatively constant. The CFD-predicted damping agrees best with the test C albeit overpredicting HR at low excitation frequencies and hR at all frequencies (<175 Hz ∼ twice rotor speed). Under a wet gas condition with LVF = 0.4%, the test force coefficients show great variability over the excitation frequency range; in particular, HR < 0, though growing with frequency due to the large liquid mass fraction. The CFD predictions, on the other hand, produce a dynamic direct stiffness HR > 0 for all frequencies. Both experimental hR and C for the wet gas PDS are larger than their counterparts for the dry gas seal. The CFD-predicted C and hR, wet versus dry, show a modest growth, yet remaining lower than the test data. The CFD-derived flow field for a wet gas condition shows that the seal radial partition walls (ridges) reduce the circumferential flow velocity and liquid accumulation within a pocket. Both the test data and the CFD prediction show that the magnitude of the flexibility function for the PDS test system reduces when the two-component mixture flows through the seal, hence revealing the additional effective damping, more pronounced for the test data rather than that from the predictions. Further work, experimental and CFD based, will continue to advance the technology of wet gas seals while bridging the gap between test data and computational physics model simulations.

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).