Compressor Stator Cascade Research Articles

Binary Repetitive Model Predictive Active Flow Control Applied to an Annular Compressor Stator Cascade With Periodic Disturbances

Abstract Novel pressure gain combustion concepts invoke periodic flow disturbances in a gas turbine's last compressor stator row. This contribution presents studies of mitigation efforts on the effects of these periodic disturbances on an annular compressor stator rig. The passages were equipped with pneumatic active flow control (AFC) influencing the stator blade's suction side, and a rotating throttling disk downstream of the passages inducing periodic disturbances. For steady blowing, it is shown that with increasing actuation amplitudes cμ, the extension of a hub corner vortex deteriorating the suction side flow can be reduced, resulting in an increased static pressure rise coefficient Cp of a passage. The effects of the induced periodic disturbances could not be addressed intrinsically, by using steady blowing actuation, Considering a corrected total pressure loss coefficient ζ*, which includes the actuation effort, the stator row's efficiency decreases with higher cμ due to the increasing costs of the actuation mass flow. Therefore, a closed-loop approach is presented to address the effects of the disturbances more specifically, thus lowering the actuation cost, i.e., mass flow. For this, a repetitive model predictive control (RMPC) was applied, taking advantage of the periodic nature of the induced disturbances. The presented RMPC formulation is restricted to a binary control domain to account for the used solenoid valves' switching character. An efficient implementation of the optimization within the RMPC is presented, which ensures real-time capability. As a result, Cp increases in a similar magnitude but with a lower actuation mass flow of up to 66%, resulting in a much lower ζ* for similar values of cμ.

Corner Separation Control Using a New Combined Slotted Configuration in a High-Turning Compressor Cascade under Different Solidities

In order to comprehensively control the corner separation and the blade trailing edge (TE) separation in a high-turning compressor stator cascade, this research proposes a new combined slotted configuration consisting of one full-span slot and two blade-end slots. Taking into account the effect of the blade solidity, the performance of the original cascade and the combined slotted cascade was calculated and evaluated in a wide incidence angle range at two blade solidities. The results indicated that the blade loading and the corner separation range of the original cascade becomes larger as the blade solidity decreases from 1.66 to 1.36, which leads to higher total pressure loss and lower pressure diffusing capacity under positive incidence angles. The low-momentum fluid in the boundary layer can be significantly re-energized by the high-momentum blade-end and full-span slots jets, hence the combined slotted configuration can eliminate the blade TE separation and reduce the corner separation remarkably in the full incidence angle range at the two blade solidities. By adopting the combined slotted configuration, the total pressure loss, turning angle and static pressure coefficient of the original cascade can be increased by −23.2%, 2.7° and 4.7% on average, respectively, when the blade solidity is 1.66, while they can be increased by −27.7%, 3.3° and 7.6% on average, respectively, when the blade solidity is 1.36. The combined slotted configuration has a significant adaptability to the low blade solidity (or high loading) condition and it shows a certain potential in increasing the aeroengine thrust-to-weight ratio by decreasing the compressor single-stage blade number.

Active flow control by means of endwall synthetic jet on a high-speed compressor stator cascade

The underlying physics of the endwall synthetic jet in improving the aerodynamic performance of a high-speed compressor stator cascade is investigated in this paper. The effects of both injected momentum and actuation frequency are discussed in detail. In the investigations, the injected momentum is controlled by either changing the maximum jet velocity or modifying the tube diameter. Numerical results demonstrate that the streamwise momentum addition and flow mixing enhancement are the key factors of the endwall synthetic jet in improving the cascade performance. The high momentum fluid injected into the flow field can reenergize the passage flow, and the generated streamwise jet vortex contributes to the strengthening of flow mixing. Consequently, the momentum exchange between the low momentum fluid region and the main flow is enhanced and boundary layer separation on the blade suction surface is delayed. The loss characteristic in the corner region is improved as well. The intensified flow mixing will also increase the total pressure loss in the near-endwall region, which as a result will worsen the cascade performance, and hence the total effect of the endwall synthetic jet depends on the sum of its impacts. Moreover, the injected momentum and the actuation frequency have strong influences on the flow control effect. With the momentum coefficient and the reduced frequency being Cµ = 0.131% and F+ = 1.0, the reduction in total pressure loss coefficient and the increment in pressure rise coefficient are 7.3% and 3.3%, respectively.

Flow separation control on a highly loaded compressor cascade using endwall synthetic jets

This paper presents flow separation control conducted on a highly loaded compressor stator cascade using endwall synthetic jets. Numerical methods are employed and mechanisms of endwall synthetic jets in improving the cascade performance are discussed in detail. The influence of several actuation parameters is also investigated. Results show that endwall synthetic jets are able to improve the flows in the blade passage significantly, a maximum loss reduction of 21.63% and a pressure rise increment of 5.60% are obtained at design condition. Apart from energizing the low momentum fluid inside endwall boundary layer by streamwise momentum addition, endwall synthetic jets could induce a streamwise jet vortex and impede the transverse movement of endwall boundary layer through upwash and downwash. Hence, at the expense of slightly degraded near-wall flows, the formation and further evolution of passage vortex would be delayed and flows in the midspan region would be improved notably. The effectiveness of endwall synthetic jets relies on the proper selection of actuation position and jet angle. Flow control turns out to be the most efficient when the actuator is positioned at just upstream of corner separation region with a relatively small jet angle, and a large enough injected momentum is also necessary. Additionally, the adaptability of the actuation at off-design conditions is validated in the present study.

Multivariable Closed-Loop Active Flow Control of a Compressor Stator Cascade

This paper presents a multivariable closed-loop active flow control strategy to control the pressure rise and the wake size of a compressor stator cascade. For flow control, a combination of two actuator concepts, trailing-edge and sidewall blowing, was applied to a linear stator cascade. Detailed five-hole probe measurements of the downstream flowfield at a Reynolds number of were carried out to reveal the effect of the actuation on the cascade flow. An increase in the pressure rise coefficient by 5% and a significant reduction of the wake size could be achieved with the investigated actuation amplitudes of values up to 2.5%. Principal component analysis was used to find coherent structures in the full measurement data set. Based on this analysis, a real-time estimation of the complex downstream flowfield with only three pressure sensors was realized. With this estimation, linear dynamic black-box models for the pressure rise coefficient and the wake size were identified. Variable reference signals with a Strouhal number of 0.03, specified by a later application in a pressure gain combustion system, were successfully tracked, leading to small experimental control errors.

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.

Plasma actuator scaling down to improve its energy conversion efficiency for active flow control in modern turbojet engines compressors

The present work was performed to investigate the employment of micro dielectric barrier discharge plasma actuators for mitigating separation, thereby decreasing wake losses and increasing efficiency, on a highly loaded compressor cascade. To this aim, the experimental characterization of the control device was initially done. A dedicated activity was devoted to microelectronic technology adoption for micro plasma actuator fabrication, together with batch production of electrodes with photolithographic techniques. The actuation effect on quiescent flow was evaluated by measuring the induced wall-jet with particle image velocimetry. The actuator power consumption was estimated by recording the applied voltages and resulting currents.Experimental results were then used to calibrate a multi-physics numerical model, for the prediction of the body forces induced by plasma actuator. Different algebraic models were compared. Numerical modelling was applied to predict the capability of micro plasma actuation to suppress flow separation into a highly-loaded subsonic compressor stator cascade. At first, simulations of the compressor cascade without active flow control were carried out and the results were compared with the literature experimental data. A good agreement was found between the experimental and the numerical results. Active flow control by the micro plasma actuator was then tested under different sinusoidal voltage amplitudes. It was found that the compressor pressure losses were reduced by increasing the applied voltage; actuation brought to a reduction in the pressure loss coefficient up to 14% and to an increase in static pressure up to 3%. When the actuator was on, the isosurface of the Q-criteria showed the reduction of secondary flow structures and the shape factor at the trailing edge of the midspan section was always lower than 2.2, confirming a reattachment of the flow. Furthermore, a conventional macro actuator found in the literature was also modelled and its actuation effect was compared to the one of the micro plasma actuator.In conclusion, the analysis of the actuation cost underlined that the adoption of micro actuation allowed reaching a higher gain when operating at lower voltage and same frequency.

Flow Separation Control on a Compressor-Stator Cascade Using Plasma Actuators and Synthetic and Continuous Jets

AbstractThe effects of active separation control on a highly loaded subsonic compressor stator cascade were numerically studied by comparing the behavior of different devices: continuous jet actuator (CJA), zero-net mass-flux synthetic jet actuator (SJA), and plasma actuator (PA). For the jet actuator modeling, a suction/blowing type boundary condition was used, imposing a time-constant velocity for the CJA case and a prescribed sinusoidal time-varying velocity for the SJA case. For the PA case, the body force, which represents the effect of the plasma actuator on the flow, was added to the momentum equations in the computational fluid dynamics (CFD) code. The PA is slightly more efficient for the reduction of flow separation in the region just downstream of the blade actuators. However, at the same mechanical power delivered by the actuator to the fluid, the SJAs are more advantageous than the CJAs and slightly outperform plasma actuator application from the pressure loss reduction and pressure rise view...

Iterative Learning Active Flow Control Applied to a Compressor Stator Cascade With Periodic Disturbances

This paper presents the capability of iterative learning active flow control to decrease the impact of periodic disturbances in an experimental compressor stator cascade with sidewall actuation. The periodic disturbances of the individual passage flows are generated by a damper flap device that is located downstream of the trailing edges of the blades. The device mimics the throttling effect of periodically closed combustion tubes in a pulsed detonation engine (PDE). For the purpose of rejecting this disturbance, the passage flow is manipulated by fluidic actuators that introduce an adjustable amount of pressurized air through slots in the sidewalls of the cascade. Pressure sensors that are mounted flush to the suction surface of the middle blade provide information on the current flow situation. These data are fed back in real-time to an optimization-based iterative learning controller (ILC). By learning from period to period, the controller modifies the actuation amplitude such that, eventually, a control command trajectory is calculated that reduces the impact of the periodic disturbance on the flow in an optimal manner.

Comparison between synthetic jets and continuous jets for active flow control: Application on a NACA 0015 and a compressor stator cascade

A numerical investigation is performed to analyse the suppression of the boundary layer separation first on a NACA 0015 airfoil and subsequently in a highly loaded subsonic compressor stator cascade using two different active flow control techniques: synthetic jet actuators (SJA) and continuous jet actuators (CJA).In particular, the effort is concentrated towards understanding the physics of the phenomenon, which makes one configuration perform better than the other one as an active flow control (AFC) system. The analysis of the interaction of the jet with the boundary layer was performed, with a description of the vortical structures, which are beneficial for the mixing of the boundary layer and the entrainment of energy from the external flow towards the most inner layers.Regarding airfoils, the comparisons of the SJA with the CJA were performed considering two similarity conditions: identical momentum coefficients and identical amounts of energy fed into the cross flow.Finally, the flow behaviour and the topology structure in a highly loaded compressor cascade with and without AFC are examined. Active flow control using synthetic jet actuators proved to be attractive because it could exploit the unsteady phenomena, inhibit the separation, improve and control the aerodynamics of the flow in both external aerodynamics and turbomachinery components. Furthermore, comparing the CJA and the SJA for active flow control on the compressor at similar momentum coefficients shows that the relative reduction of the total pressure losses for the SJA is approximately twice as large as that for the CJA. It should also be remarked that even if the two AFC configurations present similar effects on the reduction of secondary flow structures, the SJA is more advantageous than the CJA from the regaining energy viewpoint.

Three-Dimensional Flow Field in Highly Loaded Compressor Cascade

Within the present paper, a detailed experimental investigation is presented. The influence of blade loading on the development and interaction of secondary flow structures within an annular compressor stator cascade (CSC) is examined. Experimental results at 3% chord hub clearance were obtained at four different blade loadings. Included are blade and endwall flow visualization, time resolved measurements of the static pressure on the endwall and radial–circumferential hot-wire traverse measurements within the passage as well as five-hole probe traverse measurements at the inlet and the outlet of the passage. The experimentally obtained results give detailed insight on the effect of the incidence on the development and interaction of the clearance vortex, horseshoe vortex, and the passage vortex. Furthermore, it will be shown that a vortex breakdown of the clearance vortex occurs at higher loadings.

Experimental Investigation of Endwall and Suction Side Blowing in a Highly Loaded Compressor Stator Cascade

The following paper describes an experimental investigation of a highly loaded stator cascade with a pitch to chord ratio of t/l=0.6. Experiments without as well as with active flow control by means of endwall and suction side blowing were conducted. Five-hole-probe measurements in pitchwise and spanwise directions as well as endwall oil flow visualizations were carried out in order to determine the performance of the cascade and to analyze the flow phenomena occurring. To quantify the effectivity of the active flow control method, taking the additional energy input into account, corrected losses and an efficiency, which relates the difference of flow power deficit with and without active flow control to the flow power of the blowing jet itself, were evaluated. Even though an increase of static pressure rise could be achieved, a decrease of the total pressure losses was possible for a few operating points only.

Loss Mechanisms of High-Turning Supercritical Compressor Cascades

This paper presents an experimental and numerical study on loss mechanisms of two high-turning compressor cascades at supercritical flow conditions. It is part of a comprehensive advanced compressor blade development program, in which three cascades were designed at a higher supercritical speed (M 1 = 0.87) for a transonic fan stator hub section. The cascades had the same solidity and blade chord but differed significantly in blade profiles. The baseline profile was a conventional controlled diffusion airfoil. The other two were state-of-the-art optimized designs and were experimentally confirmed to outperform the baseline. The first optimized cascade (optimized A) and its performance was published in Song and Ng (Song, B., and Ng, W., Performance and Flow Characteristics of an Optimized Supercritical Compressor Stator Cascade, Journal of Turbomachinery, Vol. 128, July 2006, pp. 435-443). The current paper focuses on comparing the baseline and the second optimized cascade (optimized B) to complement the first publication. Cascade test results are presented to show significant loss reduction (about 30%) from the baseline to the optimized profiles. Experimental and numerical analyses to understand the flow physics underlying the loss reduction are detailed using the following: wake profile, blade surface Mach number, shadowgraph, and blade surface flow visualization. It was found that severe boundary-layer separation occurred in the baseline cascade, whereas the boundary layer of the optimized cascades was well controlled from separation. An understanding of the low-loss flow pattern and design philosophy for high-turning compressor blades at higher supercritical flow conditions, exemplified by the two optimized cascades and relative to the controlled diffusion airfoil, is summarized.

Performance and Flow Characteristics of an Optimized Supercritical Compressor Stator Cascade

An experimental and numerical study was performed on an optimized compressor stator cascade designed to operate efficiently at high inlet Mach numbers (M1) ranging from 0.83 to 0.93 (higher supercritical flow conditions). Linear cascade tests confirmed that low losses and high turning were achieved at normal supercritical flow conditions (0.7<M1<0.8), as well as higher supercritical flow conditions (0.83<M1<0.93), both at design and off-design incidences. The performance of this optimized stator cascade is better than those reported in the literature based on Double Circular Arc (DCA) and Controlled Diffusion Airfoil (CDA) blades, where losses increase rapidly for M1>0.83. A two-dimensional (2D) Navier-Stokes solver was applied to the cascade to characterize the performance and flow behavior. Good agreement was obtained between the CFD and the experiment. Experimental loss characteristics, blade surface Mach numbers, shadowgraphs, along with CFD flowfield simulations, were presented to elucidate the flow physics. It is found that low losses are due to the well-controlled boundary layer, which is attributed to an optimum flow structure associated with the blade profile. The multishock pattern and the advantageous pressure gradient distribution on the blade are the key reasons of keeping the boundary layer from separating, which in turn accounts for the low losses at the higher supercritical flow conditions.