Flow Field In Cascade Research Articles

Intermittent breakdown of the tip leakage vortex and the resultant flow unsteadiness in the tip-region of a subsonic compressor cascade

In axial-flow compressor rotors, the breakdown of the tip leakage vortex (TLV) occurs in an intermittent manner under some conditions. In order to gain better knowledge of this unsteady phenomenon, a numerical investigation of the tip-region flow field in a subsonic compressor cascade has been conducted using unsteady Reynold-averaged Navier-Stokes simulations. First of all, the reason why TLV breakdown exhibits intermittency has been clarified through the analysis of time-resolved flow fields. It was found that the switch between the breakdown and no-breakdown states is triggered by a feedback mechanism: when the TLV breaks down, either an unsteady induced vortex or a backflow vortex forms and induces low-pressure on the pressure surface of the adjacent blade, which unloads the blade tip and subsequently suppresses the TLV breakdown; as the unsteady induced vortex or backflow vortex migrates downstream, the pressure on the pressure surface increases again so that the loading at the blade tip is increased again and the TLV breakdown is re-established. The amplitude and frequency characteristics of the unsteadiness associated with the intermittent TLV breakdown have been analyzed and presented. It was found that the backflow vortex-dominant breakdown induces much stronger flow fluctuation and has a much lower characteristic frequency than the unsteady induced vortex-dominant breakdown. The underlying flow physics responsible for this difference have been discussed based on the understanding of the feedback mechanism for the intermittent TLV breakdown.

Challenges Associated With Replicating Rotor Blade Deposition in a Non-Rotating Annular Cascade

Abstract A computational analysis is performed to determine if particulate impact events on the external surfaces of gas turbine engine rotor blades can be faithfully replicated in an experimental rotor cascade. The general electric (GE) energy efficient engine (E3) first-stage turbine flow-field at cruise conditions is first solved using a steady-state explicit mixing plane (MP) approach. To model flow in the cascade, a single E3 rotor periodic domain is then constructed with an inlet section matching the relative flow incidence angle from the mixing plane calculation. The mass-averaged relative flow conditions at the inlet and outlet of the mixing plane rotor section are imposed on the cascade boundaries and a steady solution is found. Particles with diameters ranging from 1 to 25 µm are tracked through each domain and the OSU deposition model is implemented to dictate the sticking and rebounding action of particles impacting solid surfaces. It is discovered that both the locations and parameters of the impacts in the cascade vary significantly from the engine environment. For smaller particles, this is credited to a stronger upstream influence of the blade on the cascade flow-field. As size increases, differences in deposition are instead driven by the interaction of the full-stage vane with the particles. The lack of a vane in the cascade causes drastically different particle inlet vectors over the rotor than are seen in the engine setting. The radial differences of particle impact locations are explored, and the role that pressure plays is considered.

Dynamic mode decomposition analysis of plasma aeroelastic control of airfoils in cascade

A dynamic mode decomposition (DMD) is carried out for the flow field in a compressor cascade with plasma actuators employed for aeroelastic control. Numerical assessments performed in previous works have shown that alternate triggering of pressure side/suction side actuators installed at the trailing edge of the blades can effectively reduce vibratory loads and enlarge the flutter boundaries of a linear compressor cascade. With the twofold aim of obtaining an in-depth understanding of the flow physics associated to plasma actuation and ultimately developing an optimized control law for the actuators, the dominant structures of the pressure field are extracted via a dynamic mode decomposition. The decomposition is conducted on the actuated and non-actuated pressure fields at several inter blade phase angles. The fundamental effects of plasma actuations on the flow field, and in turn on the blade loading, are identified and discussed. The procedure allows obtaining an useful picture of the main fluid mechanic phenomena associated to plasma aeroelastic control on turbomachinery blades. Additionally, the DMD spectrum and its coherence are analysed, yielding a wider energetic spread over high-order modes for the plasma-actuated case, in comparison to the clean cascade.

Influences of large fillets on endwall flows in a vane cascade with upstream slot film-cooling

Investigations in cascade employ filleted blades to influence the near endwall secondary flows and total-pressure losses. The secondary flows aggravate the aerodynamic losses and endwall thermal stresses in the gas turbine passages. Investigations of different configurations of the slot-bleed flow from the endwall of cascade upstream show significant influences on the near endwall flows. In the present paper, the near endwall flow-field in a linear cascade employing filleted vanes and bleed flow from the upstream endwall slots is measured experimentally. Two fillet profiles are tested, one is larger than the other. The objectives are to quantify the combined effects of endwall fillet, fillet profiles, and film-cooling flow on the endwall region flow-field. The fillet covers the junction of vane and endwall upstream of the cascade throat region. The discontinuous bleed-slots near the cascade entrance provide the film-cooling flow on endwall and simulate the gaps between combustor/nozzle-vane discs or stator/rotor discs in the gas turbine. The inlet Reynolds number of 2.0E+05 is based on the chord length of vane profile. The density and temperature ratios of the coolant flow to mainstream are both about 1.0. The inlet blowing ratio of the film-cooling flow varies between 1.0 and 2.8. The flow-field is measured through the distributions of flow temperature, yaw angle, axial vorticity, and total-pressure losses along the vane passage. The effects on the flow-field are then presented by comparing the cases of filleted vanes with the cases of un-filleted vanes. The results of flow yaw angle and axial vorticity in the filleted passage are smaller than those in the passage without the fillet. The yaw angles responsible for the endwall secondary flows are the smallest for the smaller fillet (Fillet-2). The temperature field indicates the pitchwise distributions of the coolant on endwall specially near the pressure side are better when the smaller fillet is employed. Also, the weakened passage vortex of the endwall secondary flows in the filleted passage reduces the total-pressure losses. Although the total-pressure losses decrease at very high coolant mass flux with and without the fillet, the losses are always smaller for the filleted passage than for the un-filleted passage. The present investigation is beneficial for improving and optimizing the endwall film-cooling in the gas turbine passage.

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.

Effects of End-Bend and Curved Blades on the Flow Field and Loss of a Compressor Linear Cascade in the Design Condition

Curved blade and end-bend blade are considered as effective passive control methods for improving the aerodynamic performance of a compressor cascade, and their applicability also needs to be further studied. A highly-loaded compressor linear cascade in the design condition was taken as the research objective in this paper. Numerical simulation was used to study the positive and negative effects of these methods on the flow loss of the cascade by redistributing the vortex structures. Results show that: the curved blade could reduce the flow loss to 2.55%, while the end-bend blade and the end-bend+curved blade increase it to 10.58% and 2.19%, respectively. The positive effect of the curved blade weakens the strength and scale of Concentrated Shedding Vortex (CSV), which accounts for 59.2% of the total flow loss. The negative effect of the end-bend blade exacerbates the low-energy fluid clusters from the end wall and wake into CSV. Finally, the end-bend+curved blade can take their own strengths, but it enhances the twisting motion of Passage Vortex (PV) and CSV, which makes it fail to reduce the flow loss. Therefore, the positive and negative effects of these methods on the flow loss of a compressor cascade are much clearer. In addition, we also predict the potential danger of the end-bend blade in the negative incidence conditions and provides some suggestion for future study.

Performance Characteristic Modeling for 2D Compressor Cascades

Abstract The flow field in a compressor cascade is very complex owing to the highly 3D, turbulent, and viscous properties. However, in the through-flow analysis method, the viscosity effects are taken into account using empirical models. These models were based on experimental results for early blades. However, the blade types in modern compressors are quite different from those in older compressors. Therefore, it is not possible to predict the performance of modern compressors using the old empirical models. In this study, several multiple circular-arc (MCA) blades commonly used in modern compressors were simulated. After the simulations, a database of the cascade performance was built. Based on this database, some new models were established to predict the performance of modern cascades using regression analysis methods and an artificial neural network (ANN) method. The accuracy of all these new models is high enough for use in engineering applications.

Visualization of Flow Field in Circular Cascade Having Rotating Stall Suppressed by Tip Injection with PIV

Visualization of Flow Field in Circular Cascade Having Rotating Stall Suppressed by Tip Injection with PIV

REGARDING CERTAIN AERODYNAMIC SOURCES OF NON-STATIONARY FORCE LOADS ON TURBINE PROFILES

The paper deals with selected phenomena present in a blade cascade flow field. The presented research was performed on prismatic blade cascades composed mostly of the tip and root sections of the last stage rotor blade of a large output steam turbine. The unstarted supersonic flow on the cascade inlet, the separation of the flow and the swirl line behind the trailing edge have been identified as the possible sources of the unsteady force effects.

Non-axisymmetric endwall profiling in a highly loaded compressor cascade

Very few studies of non-axisymmetric endwall profiling in compressor have been conducted in published literatures. This study investigates the control mechanism of non-axisymmetric endwall on the flow field of a highly loaded compressor cascade and the guidelines for the design of non-axisymmetric endwall. First, CFD method was validated with existing experimental data. Then, the design method of non-axisymmetric endwall based on trigonometric function and Bezier curve was introduced. The design strategies of non-axisymmetric endwall considering axial position effect, height effect and different patterns of concave/convex were investigated in detail. Results show that non-axisymmetric endwall is an effective way to control the near endwall flow field of the compressor cascade. With the optimal non-axisymmetric endwall in this study, the loss coefficient of the cascade is reduced by 5.5%, and the cascade exhibited suppressed corner separation, reduced passage vortex and more uniformly distributed outflow angles. The optimal axial position of the concave was at 0.22 axial chord, which was near the onset of corner separation, so the best control effect was obtained. There is an optimal height for the concave of the non-axisymmetric endwall. A lower concave will not have sufficient effect on the cascade, whereas a higher concave will be detrimental on the contrary. It is indicated that the influence mechanism of non-axisymmetric endwall on the compressor cascade is basically derived from the superimposed effect of reduced cross-passage pressure gradient and increased spanwise inward pressure gradient. The cross-passage pressure gradient can be reduced by both concave on the suction side and convex on the pressure side of the endwall. However, concave can introduce spanwise inward pressure gradient, whilst convex can introduce spanwise outward pressure gradient. For better improving the performance of compressor cascades, non-axisymmetric endwall is suggested to be a concave on the entire endwall, which introduces both reduced cross-passage pressure gradient and increased spanwise inward pressure gradient.

Flow control mechanisms of a combined approach using blade slot and vortex generator in compressor cascade

Experiments proved the performance gains in a high-load cascade using a new combined flow control approach, but lack of clear explanations on flow interactions between the configurations and the cascade flow. A detailed discussion is conducted here to further reveal the flow control mechanisms based on experimental and numerical results. An overview of experimental studies is firstly presented to conclude the flow control benefits and to put forward the questions for the simulations. Cascade flow fields observed by experiments show that the combined approach works by two aspects: the slot produces high-speed jets to re-energize the suction side separated flows and reattach them to the suction surface; the vortex generator (VG) creates a counter-rotating vortex into cascade passage to further reduce the end-wall cross flows. Thus, both the two main sources of separations in cascade flow are considerably suppressed. The corner separation is suppressed by delaying the passage vortex (PV): The VG counter-balances and deflects the PV while the slot jet further limits its pitch-wise width. Coupling the effects of two devices, the cascade flow structure is improved and main vortices are significantly reduced in size and intensity, result in greater separation control effects than the individuals in the high-load cascade.

Loss Production Mechanism in Stator Cascade Flow Field of a Multi-Stage Transonic Axial Compressor

Loss Production Mechanism in Stator Cascade Flow Field of a Multi-Stage Transonic Axial Compressor

Study of the wet steam flow in the blade tip rotor linear blade cascade

Experimental investigations of non-equilibrium spontaneous condensation in transonic steam flows in a linear blade cascade were carried out. The cascade consists of the rotor blades of the low-pressure (LP) steam turbine last stage. The experimental testing facility is a part of a small-scale steam power plant located at the Silesian University of Technology in Gliwice. The steam parameters at the testing facility inlet correspond to the wet steam conditions in a low-pressure steam turbine. Static pressure measurements on the blade surface as well as Schlieren images were used to assess the flow field in the steam turbine rotor tip blade linear cascade for different flow conditions. The experimental results were used to validate an in-house CFD code.

Numerical simulation of flow through cascade in wind tunnel test section and stand-alone configurations

The paper deals with the numerical simulation of the flow field in a turbine cascade, which corresponds to the tip section of a last low-pressure steam turbine rotor. Considered cascade consists of very thin profiles with high stagger angle. The resulting flow field is complex with interactions of strong shock waves, shear layers and shock reflections. The paper proposes a proper numerical approximation of boundary conditions suitable for cases with supersonic inlet and outlet flow velocities and compares the flow field for two cascade configurations: the first one corresponding to real experiment (cascade with finite number of blades located in the wind tunnel test section) and the second one corresponding to annular cascade. The experimental configuration includes the complicated geometry of wind tunnel. The annular configuration leads to blade to blade periodicity, which is not guaranteed for the experimental configuration. Numerical simulations are based on the Favre-averaged Navier–Stokes equations with SST k–ω turbulence model and the in-house implicit finite volume solver with AUSM-type discretization. This method considers structured multi-block grid. Results are compared with experimental data.

Effects of airfoil-probe tubes on the flow field of a compressor cascade

To explore the effects of airfoil-probe tubes and its installment position on the flow field of the compressor cascade, and find out the mechanism that how the airfoil-probes affect the aerodynamic characteristics of the compressor cascade, this paper performed both numerical and experimental works on the same compressor cascade. The experiment mainly focused on the cases of low Mach number (Ma = 0.1), and cases with different Mach numbers (0.1, 0.3, 0.7) and different incidence angles (-5, 0, 5) are investigated by the numerical method. The case without the airfoil-probe tube was referenced as the baseline, and other three cases with the airfoil-probe tubes installed in different chordwise positions (30%, 50%, 70% of the chord length) were studied. The diameter of the airfoil-probe tube is 3mm, which is configured as 300% amplification of some particular airfoil-probe according to the geometrical similarity principle. The results show that the airfoil-probe tubes have a negative influence on the flow capacity of the cascade at all investigation points. The separations and the large scale streamwise vortices that induced by the airfoil-probe tube on the pressure side cause most the losses at the high Mach number. The influence of the airfoil-probe tube on the flow field in the vicinity of the pressure side surface is local separation at the low Mach number. The airfoil-probe tubes also have a clearly effect on the leakage flow. It decreases the mass flow of the leakage flow and weakens the intensity of the leakage vortex, but enlarges the influence area. The total pressure loss of the case that the tube is installed at the half chordwise position is generally lower than other cases especially at the high Mach number, it can even decrease the losses compared with the basic case.

Effects of non-equilibrium condensation on aerodynamics of the flow field in a steam turbine cascade

In the present paper an in-house CFD code is developed using Roe scheme to simulate condensing two-phase flow in blade to blade passage of a steam turbine. Effects of condensation on the flow field of steam turbine rotor tip section are investigated for different outlet pressures. Firstly, comparison is performed between results of wet and dry cases. Then effects of outlet pressure variations on the flow field are studied. Finally effects of condensation on different specifications of the flow field (total pressure loss coefficient, entropy generation and deviation angle) are investigated. Also the mechanism of flow deviation in the cascade flow field is described. Condensation has a great influence on the behavior of the flow field based on the numerical results of this paper. It changes the outflow direction and consequently the flow entering to the next blade deviates from its on-design condition, thus additional losses are produced. For example, the value of deviation angle reaches to 7.62for wet case and exit Mach number Me=1.45. Also there are stagnation pressure loss and entropy generation due to non-equilibrium condensation that reduce the efficiency of the steam turbine

Assessment of Transition Modeling and Compressibility Effects in a Linear Cascade of Turbine Nozzle Guide Vanes

The flow field in a linear cascade of highly loaded turbine nozzle guide vanes (NGVs) has been numerically investigated at low and high-subsonic regime, i.e., exit isentropic Mach number of M2is = 0.2 and 0.6, respectively. Extensive experimental data are available for an accurate assessment of the numerical procedure. Aerodynamic measurements include not only vane loading and pressure drop in the wake but also local flow features such as boundary layer behavior along both pressure and suction sides of the vane, as well as secondary flow structures downstream of the trailing edge (TE). Simulations were performed by using two computational fluid dynamics (CFD) codes, a commercial one and an open-source based in-house code. Besides computations with the well-established shear-stress transport (SST) k–ω turbulence model assuming fully turbulent flow, transition models were taken into account in the present study. The original version of the γ–Reθ model of Menter was employed. Suluksna–Juntasaro correlations for transition length (Flenght) and transition onset (Fonset) were also tested. The main goal was to establish essential ingredients for reasonable computational predictions of the cascade aerodynamic behavior, under both incompressible and compressible regime. This study showed that transition modeling should be coupled with accurate profiles of inlet velocity and turbulence intensity to get a chance to properly quantify aerodynamic losses via CFD method. However, additional weaknesses of the transition modeling have been put forward when increasing the outlet Mach number.

Stereoscopic PIV measurements of the flow field in a turbine cascade

This paper presents experimental measurements of the flow field in a Low-speed Turbine Cascade using a stereoscopic particle-image velocimetry (SPIV). During the measurements, a pair of frame-straddling-based CCD cameras were configured at different sides of the laser light sheet, and appropriate tracing particles (DEHS) were employed. The measurements were conducted at the incidence angle of 0 degree and exit Reynolds number of 1.7 × 105 with the tip clearance 1.18% of blade chord. The tip flow features, such as the evolution and breakdown of tip leakage vortex, the horseshoe vortex, turbulence characteristics of tip leakage flow, were studied for the flow field analysis. The results showed that the tip leakage flow/vortex mainly dominate flow fields in the tip region. The tip leakage vortex performs as a concentrated vortex before its breaking down and splitting into small vortices. The highest turbulence intensity mainly occurs in the tip region along with the trajectory of tip leakage vortex, and when the vortex breaks down, the turbulence intensity reduces rapidly. Additionally, the SPIV with this configuration also shows an advantage in investigating the flow structures and mechanism inside the turbine cascade.

Closed-loop active flow control of a non-steady flow field in a highly-loaded compressor cascade

The provision of secure compressor operation under circumstances of a pulsed detonation engine is crucial for the success of pressure gaining combustion processes for turbo machinery applications. This paper discusses active flow control as a possible solution to approach this challenge. The presented experiments were conducted on a highly loaded low speed linear compressor stator cascade operated at \(Re=600{,}000\) and \(Ma =0.07\). A choking-device which was located in the wake of the cascade simulated the non-steady outflow condition that is expected under the conditions of pressure gaining combustion. In the discussed experiments, the choking-device generated a periodic disturbance to every passage at a typical Strouhal number of \(Sr =0.03\). The flow structures of the non-steady flow field were strongly correlated to the working-phase of the choking-device. In this paper, an iterative learning controller was used to find an optimized actuation trajectory that was used for closed-loop sidewall-actuation to control the corner separation in the non-steady flow field. The iterative learning controller took advantage of the periodicity of the disturbance to calculate a non-steady actuation trajectory that optimally suppressed the impact of the choking-device on the flow. The active flow control effect was evaluated by means of static pressure rise using five hole probe measurements in the wake of one passage.

Influence of coupled boundary layer suction and bowed blade on flow field and performance of a diffusion cascade

Based on the investigation of mid-span local boundary layer suction and positive bowed cascade, a coupled local tailored boundary layer suction and positive bowed blade method is developed to improve the performance of a highly loaded diffusion cascade with less suction slot. The effectiveness of the coupled method under different inlet boundary layers is also investigated.Results show that mid-span local boundary layer suction can effectively remove trailing edge separation, but deteriorate the flow fields near the endwall. The positive bowed cascade is beneficial for reducing open corner separation, but is detrimental to mid-span flow fields. The coupled method can further improve the performance and flow field of the cascade. The mid-span trailing edge separation and open corner separation are eliminated. Compared with linear cascade with suction, the coupled method reduces overall loss of the cascade by 31.4% at most. The mid-span loss of the cascade decreases as the suction coefficient increases, but increases as bow angle increases. The endwall loss increases as the suction coefficient increases. By contrast, the endwall loss decreases significantly as the bow angle increases. The endwall loss of coupled controlled cascade is higher than that of bowed cascade with the same bow angle because of the spanwise inverse “C” shaped static pressure distribution. Under different inlet boundary layer conditions, the coupled method can also improve the cascade effectively.