Variable Stator Vanes Research Articles

Vibration Analyses of an Axial Turbine Wheel With Intentional Mistuning

Abstract The last-stage bladed disk of a steam turbine is analyzed with respect to both flutter susceptibility and limitation of forced response. Due to the lack of variable stator vanes, unfavorable flow conditions may occur, which can lead to flow separation in some circumstances. Consequently, there is the risk of flutter in principle, particularly at nominal speed under part load conditions. For this reason, intentional mistuning is employed by the manufacturer with the objective to prevent any self-excited vibrations. A first step in this direction is done by choosing alternate mistuning, which keeps the manufactural efforts in limits since only two different blade designs are allowed. In this sense, two different series of blades have been made. However, it is well known that small deviations from the design intention are unavoidable due to the manufacturing procedure, which could be proved by bonk tests carried out earlier. The influence of these additional but unwanted deviations is considered in numerical simulations. Moreover, the strong dependence of blade frequencies on the speed is taken into account since centrifugal stiffening effects significantly attenuate the blade-to-blade frequency difference in this particular case. Focusing on the first flap mode, it could be shown that a mitigation of flutter susceptibility is achieved by prescribing alternate mistuning, which indeed evokes an increase of originally small aerodynamic damping ratios. Nevertheless, the occurrence of negative damping ratios could not be completely precluded at part load conditions. That is why optimization studies are conducted based on genetic algorithms with the objective function of maximizing the lowest aerodynamic damping ratios. Again, only two different blade designs are admitted. Finally, mistuning patterns could be identified causing a tremendous increase of aerodynamic damping ratios. The robustness of the solutions found could be proved by superimposing additional random mistuning.

Gas Turbine Engine Condition Monitoring Using Gaussian Mixture and Hidden Markov Models

This paper investigates the problem of condition monitoring of complex dynamic systems, specifically the detection, localisation and quantification of transient faults. A data driven approach is developed for fault detection where the multidimensional data sequence is viewed as a stochastic process whose behaviour can be described by a hidden Markov model with two hidden states — i.e. ‘healthy / nominal’ and ‘unhealthy / faulty’. The fault detection is performed by first clustering in a multidimensional data space to define normal operating behaviour using a Gaussian-Uniform mixture model. The health status of the system at each data point is then determined by evaluating the posterior probabilities of the hidden states of a hidden Markov model. This allows the temporal relationship between sequential data points to be incorporated into the fault detection scheme. The proposed scheme is robust to noise and requires minimal tuning. A real-world case study is performed based on the detection of transient faults in the variable stator vane actuator of a gas turbine engine to demonstrate the successful application of the scheme. The results are used to demonstrate the generation of simple and easily interpretable analytics that can be used to monitor the evolution of the fault across time.

Effectiveness of Variable Rotor Blades Technique&Variable Stator Vanes Technique in AdjustingAxial-flow Compressor

Multi-stage Axial-flow compressor is the main driver of continuous transonic wind tunnel and it usually has ultra-wide operating range. High performance wind tunnel generally requires its multi-stage axial-flow compressor has high economic performance and stability in its ultra-wide operating range. The combination of variable compressor rotational speed and variable geometry technique, variable rotor blades technique (VRBT) and variable stator vanes technique (VSVT), has been widely adopted in extending the stable working range with high efficiency of multi-stage axial-flow wind tunnel compressor. Adjusting Effectiveness was defined in this paper to quantify the effectiveness of VRBT and VSVT in extending stable and high aerodynamic efficiency operating range of wind tunnel axial-flow compressor under various operating conditions. The results show that the adjusting effectiveness of VRBT and VSVTis a function of reaction degree, flow coefficient, loading coefficient and the amount of loading coefficient change.

Off-design performance of CAES systems with low-temperature thermal storage under optimized operation strategy

Compressed air energy storage (CAES) systems usually operate under off-design conditions due to load fluctuations, environmental factors, and performance characteristics of the system. However, the current research on off-design performance of CAES systems, especially in the aspects of advanced compressed air energy storage systems and operation strategies, is deeply insufficient. The complete off-design model of a compressed air energy storage system with thermal storage (TS-CAES) and optimal regulations by adjusting variable inlet guide vane (VIGV) and variable stator vane (VSV) is established for the first time. Using this model, we uncover the off-design characteristics common in TS-CAES systems caused by changes in load, ambient temperature, and operation type (constant pressure operation and sliding-pressure operation), and which is conducted under the optimal regulation strategy for the compression and expansion sections. It is found that system parameters fluctuate around a balance position with load fluctuation. But under the optimal operation, the fluctuation of the system efficiency and energy density is relatively small. System efficiency and energy density increase marginally with the decrease of ambient temperature, and thermal storage temperature increases linearly and markedly with rise in ambient temperature. We also found that system efficiency under constant pressure operation is decreased by 1.74 percent compared to that under sliding-pressure. This study will provide theoretical support for the design, operation, and control of TS-CAES systems.

A novel tribometer for investigating bushing wear

Bushing wear is regarded as the main cause of Variable Stator Vanes (VSV) mechanism failure in compressor airflow control systems. In order to examine the bushing health status, a new tribometer was designed to evaluate the bushing frictional performance under various working conditions. These include high temperature, changing axial load, and reciprocating rotation. The prototype apparatus was validated in real-world operation conditions. Results indicate that key test environment parameters were well controlled during the operation. Bushing made of SP2515, SCP5050 and SCP50094 were tested at 364 °C. It is found that transfer films were formed because of polyimide adhesion on the spindle contacting surface. Furthermore, the wear coefficient of the special contact configuration was derived. Assessment based on frictional torque and wear resistance, SP2515 demonstrated the best tribological performance though polyimide is not applicable in the testing temperature.

Analysis of mistuned forced response in an axial high-pressure compressor rig with focus on Tyler–Sofrin modes

ABSTRACTThis paper aims at contributing to a better understanding of the effect of Tyler–Sofrin Modes (TSMs) on forced vibration responses by analysing a 4.5-stage research axial compressor rig. The first part starts with a brief review of the involved physical mechanisms and necessary prerequisites for the generation of TSMs in multistage engines. This review is supported by unsteady CFD simulations of a quasi 2D section of the studied engine. It is shown that the amplitude increasing effect due to mistuning can be further amplified by the presence of TSMs. Furthermore, the sensitivity with respect to the structural coupling of the blades and the damping as well as the shape of the expected envelope is analysed.The second part deals with the Rotor 2 blisk of the research compressor rig. The resonance of a higher blade mode with the engine order of the upstream stator is studied in two different flow conditions realised by different variable stator vane (VSV) schedules which allows to separate the influence of TSMs from the impact of mistuning. A subset of nominal system modes representation of the rotor is used to describe its mistuned vibration behaviour, and unsteady CFD simulations are used to characterise the present strength of the TSMs in the particular operating conditions. Measured maximum amplitude vs blade pattern and frequency response functions are compared against the predictions of the aeromechanical models in order to assess the strength of the TSMs as well as its influence on vibration levels.

Off‐design performance and operation strategy of expansion process in compressed air energy systems

Compressed air energy storage (CAES) systems usually operate under off-design conditions due to load fluctuations, environmental factors, and performance characteristics of the system. In order to optimize design and operation of CAES systems, it is significant to study off-design performance. The expansion process plays an important role in the whole system. The main objective of this study is to explore the off-design characteristics and optimization strategy of the multistage expansion process of CAES systems with thermal storage. Two kinds of off-design operating modes, which are equal-power-ratio (EPR) operation and optimizing variable stator vane rotation angle (OVRA) operation, are proposed and compared for the first time. Correlation between key parameters such as total output power ratio, exergy efficiency, outlet air/water temperature versus mass flow rate ratio, and inlet pressure are revealed. Furthermore, optimal operation principles are obtained. The primary optimizing operation principle is to optimize the isentropic efficiencies of low-pressure stages to improve the whole efficiency of expansion process. Lastly, the optimized regulating law for variable stator vane is expressed in a polynomial form.

A simplified model of a fueldraulic actuation system with application to load estimation

In this work, a simplified model of the compressor variable stator vane fueldraulic actuation system of a jet engine is presented. The actuation system is a sub-assembly of the engine’s hydro-mechanical unit. A unique characteristic of the actuator is an internal cooling flow which prevents the overheating of fuel. It is shown that the effect of the cooling flow is well represented by a static input nonlinearity. The resulting model is of the Hammerstein structure. It is then shown that the model can be used for the estimation of the actuator’s external load. The results are validated using an accurate real system simulator.

The Off-Design Performance Simulation of Marine Gas Turbine Based on Optimum Scheduling of Variable Stator Vanes

As one of the antisurge techniques, the adjusting scheme of VSV under off-design conditions has a significant impact on the performance of gas turbines. In this paper, the one-dimensional characteristic of the compressor calculation program is embedded into the zero-dimensional overall gas turbine model, which replaces the original compressor characteristic module. Based on the assembling relationship of the actual components of the marine gas turbine, the architecture of the modular model library is designed, and an integrated simulation platform of marine gas turbine is developed by using MATLAB/GUI software. The influence of the first 3 rows of variable stator vanes of the 9-stage axial compressor working alone on the performance of the compressor at different speeds and different angles was analyzed by the HARIKA compressor characteristic calculation program. Taking the economics and stability of the gas turbine as the optimization objective, the optimization of the first three-stage stator vanes regulation schemes under different working conditions was carried out. The steady-state performance parameters under each working condition of gas turbine of power generation with or without variable stator vane mode were calculated. The study results can provide references for the adjusting scheme of VSV under gas turbine off-design conditions operating process.

Compressor map computation based on 3D CFD analysis

The focus of the paper is on procedures and strategies to compute high-fidelity compressor maps for aero engines based on 3D CFD. The developed automatic process starts with an operation point analysis where a convergence checker terminates the running 3D flow analysis as soon as physical quantities such as mass flow or aerodynamic blade row loss have converged. Subsequently, the corresponding compressor speed line is determined, where operation limits like surge and choke are detected by solving optimization and root search problems, respectively. Such speed lines also have to be calculated for various other shaft speeds to obtain the whole performance map. This is achieved by adjusting shaft speed and boundary conditions, where the mesh for variable stator vanes and the amount of bleed mass flow are adapted automatically according to given schedules. Finally, the developed process is applied to a 4.5-stage axial compressor to demonstrate feasibility of the proposed strategies.

Rotating Stall Observations in a High Speed Compressor—Part II: Numerical Study

In this two-part paper the phenomenon of part span rotating stall is studied. The objective is to improve understanding of the physics by which stable and persistent rotating stall occurs within high speed axial flow compressors. This phenomenon is studied both experimentally (Part I) and numerically (Part II). The experimental observations reported in Part I are now explored through the use of 3D unsteady Reynolds-averaged Navier–Stokes (RANS) simulation. The objective is to both validate the computational model and, where possible, explore some physical aspects of the phenomena. Unsteady simulations are presented, performed at a fixed speed with the three rows of variable stator vanes adjusted to deliberately mismatch the front stages and provoke stall. Two families of rotating stall are identified by the model, consistent with experimental observations from Part I. The first family of rotating stall originates from hub corner separations developing on the stage 1 stator vanes. These gradually coalesce into a multicell rotating stall pattern confined to the hub region of the stator and its downstream rotor. The second family originates from regions of blockage associated with tip clearance flow over the stage 1 rotor blade. These also coalesce into a multicell rotating stall pattern of shorter length scale confined to the leading edge tip region. Some features of each of these two patterns are then explored as the variable stator vanes (VSVs) are mismatched further, pushing each region deeper into stall. The numerical predictions show a credible match with the experimental findings of Part I. This suggests that a RANS modeling approach is sufficient to capture some important aspects of part span rotating stall behavior.

INFLUENCE OF THE INCORRECT SETTINGS OF AXIAL COMPRESSOR INLET VARIABLE STATOR VANES ON GAS TURBINE ENGINE WORK PARAMETERS

The paper deals with the problem of influence of changes variable stator vanes axial compressor settings of gas turbine engine on work parameters of compressor and engine. Incorrect operation of change setting system of variable vanes could make unstable work of compressor and engine. This paper presents theoretical analysis of situation described above and results of own research done on real engine. When in the compressor construction there is assembled system of setting change of variable stator vanes its task is to make optimal cooperation engine units during the permanent improvement of compressor characteristic. Perturbations in the operation of this system could cause changes in work of compressor and engine similarly as in the case of changes caused by changes of rotational speed or polluted interblades ducts of compressor. The purpose of investigations, which was carried out on real engine was determination influence of incorrect operation of axial compressor inlet guide variable stator vanes control system of gas turbine engine on compressor and engine work parameters. The object of research is type DR 77 marine gas turbine engine, which is a part of power transmission system of war ship. In compressor construction configuration of this engine there are used inlet guide stator vanes which make possibilities to change the setting angle incidance (change of compressor flow duct geometry) in depend on engine load. On the base of results of experiment there were determined the mathematical equations modelling the changes of particular engine work parameters in the function of variable inlet guide stator vanes setting angle.

Rotating Stall Observations in a High Speed Compressor—Part I: Experimental Study

In this two-part paper, the phenomenon of part span rotating stall is studied. The objective is to improve understanding of the physics by which stable and persistent rotating stall occurs within high speed axial flow compressors. This phenomenon is studied both experimentally (Part I) and through the use of unsteady RANS simulations (Part II). In this paper, the behavior of an eight stage high speed compressor is studied during slow acceleration maneuvres along a fixed working line. Casing mounted pressure transducers and rotor mounted strain gages are used to examine the spectral content of any unsteadiness in the flow and its behavior across the operating range. By deliberate aerodynamic mismatching of the front stages through adjustment of three rows of variable stator vanes (VSVs), stable rotating stall is initiated. The observed behavior falls into two “families” of high and low frequency when tracked on the instrumentation. Further analysis based on the Doppler shift between the static and rotating measurements confirms that these respective phenomena are due to rotating stall of high and low cell count. Acoustic modes resulting from stall/rotor interaction are also identified. Strong correlation of the stall intensity with simple 1D meanline predicted loading parameters suggests that these families of behavior are independently linked to the stalling of different regions within the compressor.

Robust Passive Fault Tolerant Control Applied to Jet Engine Equipment

In order to minimize the occurrence of unexpected costly flight failures modern aircraft engines industry focuses especially on increasing product’s availability. In this work, we propose to monitor the health of a Variable Stator Vane (VSV), subsystem controlling the amount of airflow through the High Pressure Compressor (HPC), allowing optimum compressor performance. This control of airflow prevents the engine from stalling. The proposed methodology is based on an original approach for real time on-board Passive Fault Tolerant Control (PFTC). The objective of the proposed PFTC is to provide acceptable performance and preserve stability when faults occur. The method relies on the design of a specific Robust Virtual Sensor in a Linear Parameter Variable (LPV) polytopic framework. The robustness to model uncertainties is ensured by a Neural Extended Kalman Filter (NEKF) accommodating, in real time, the model prediction. In the proposed methodology, an off-line closed-loop identification scheme is first used to elaborate a multi local linear state space models, after that a multi-model observer based on Linear Matrix Inequalities (LMI) optimization is used to build the virtual sensor. The NEKF is added to circumvent online model accuracy problems. The efficiency and limit of the approach are shown and discussed through simulations on a complete numerical engine test bench.

Method of calculating GTE gas-thermodynamic parameters with blade row description of an axial multistage compressor

A method of calculating the operating performance of a turboshaft gas turbine engine for steady- state conditions has been developed based on a mathematical model with a row- after- row presentation of a multistage axial compressor. This method as opposed to the existing ones makes it possible to determine gas turbine engine gas-thermodynamic parameters at different ratings taking into account such factors as air bleeding and blow off directly from the air-gas channel, the variation of geometrical parameters of compressor blade rows and meridian bypasses and changes in the programme of regulating variable stator vanes. The results of investigating the thermodynamic parameters of the main units, their elements and performance characteristics of a turboshaft gas turbine engine are presented for a wide range of rotor speeds (throttle performance). Comparisons between the computational results and the experimental data are made, their agreement is shown to be satisfactory.

Vortex development induced by part gap geometry and endwall configurations for variable stator vanes in a compressor cascade

The article describes investigations on the three-dimensional (3D) flowfield development near the endwall of a linear compressor cascade which is caused by specific part gap and endwall concepts of adjustable stator vanes. Their beneficial or harmful characteristics, previously measured with outlet loss and flow turning distributions, are investigated inside the passage at different stagger angles to analyse the origin and the interaction of the geometry-induced vortex system. Qualitative blade-to-blade measurements were conducted with particle image velocimetry in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Improved three-dimensional (3D) numerical Reynolds-averaged Navier–Stokes (RANS) calculations with Reynolds-stress turbulence models were carried out and enhance the experimental findings. Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vortex development and its impact to the blade boundary layers downstream the passage is visualised and enhances the understanding of the geometry effect. Also, the impact of the endwall concepts without radial clearances to the endwall boundary layer is shown and explains their beneficial characteristics compared to a reference geometry.

The effect of different clearance geometry configurations on aerodynamic performance in a high-load compressor cascade

The clearance flow of compressor variable stator vanes has a significant impact on compressor performance. Most clearance flow experimental research work has been carried out in compressor cascades with lower turning angle (lower load). The clearance flow in a higher load compressor cascade is relatively more complicated due to three-dimensional (3D) flow separation. In the present work, the effects of different heights and locations of clearances in a compressor cascade on aerodynamic performance and separation flow were investigated experimentally in a low-speed plane cascade wind tunnel. The objective of this investigation is to study the characteristics of different clearance configurations depending on the clearance height and the clearance axial position, and make an attempt to verify the possibility of a local clearance flow that improves the performance. The cascade outlet section aerodynamic parameters were measured by “L” five-hole probe. The ink-trace flow visualizations on the cascade surface and lower end wall were performed. The results show that with the increase in clearance size, the core area of high loss is moving to the end wall and the morphology of limiting streamlines on the suction surface changes gradually from separation lines to reattachment lines. The separation lines on end wall gradually moved to the middle of the cascade and the separation area increased gradually. The above reasons caused high loss zone gradually extended to the end wall. Thus, the loss at mid-span declined slightly while the loss on the end wall was increasing. Otherwise, cascade load reduced significantly when the clearance height became 3 mm, and the change magnitude of limiting streamline morphology in the flow field which was influenced by clearance flow at the rear of the blade reached to a minimum. So the loss and load came to a minimum, respectively. The impact of the clearance in the rear of a blade on loss and flow is weaker than the other schemes, with a slightly larger loss and separation with respect to the original scheme without a clearance. It may be expected to improve the cascade performance further by an appropriate design of clearance near the trailing.

Constrained Sparse Estimation for Improved Fault Isolation

Least-squares-based methods are very popular in the jet engine community for health monitoring purpose. Their isolation capability can be improved by using a prior knowledge on the health parameters that better matches the expected pattern of the solution i.e., a sparse one as accidental faults impact at most one or two component(s) simultaneously. On the other hand, complimentary information about the feasible values of the health parameters can be derived in the form of constraints. The present contribution investigates the effect of the addition of such constraints on the performance of the sparse estimation tool. Due to its quadratic programming formulation, the constraints are integrated in a straightforward manner. Results obtained on a variety of fault conditions simulated with a commercial turbofan model show that the inclusion of constraints further enhance the isolation capability of the sparse estimator. In particular, the constraints help resolve a confusion issue between high pressure compressor and variable stator vanes faults.

Methodology for gas turbine performance improvement using variable-geometry compressors and turbines

Poor part-load performance is a well-known undesirable characteristic of gas turbines. Running off-design, both compressor and turbine lose performance. Flow misalignment at the various rows causes losses to increase sharply, thereby decreasing net output faster than decreasing fuel consumption. To bring the flow to alignment with the blade passages, it is required to restagger the blades both at the compressor and at the turbine. To avoid mechanical complexities, it is generally accepted to restagger only the stators. This work deals with a numerical approach to the simulation of a gas turbine equipped with variable stators at the compressor and at the turbine, enabling the search for better-performance operation. A computer program has been developed to simulate virtually any gas turbine having variable stators at the compressor stages and turbine nozzle guide vanes. Variable-inlet guide vanes (VIGVs), variable-stator vanes (VSVs), variable-nozzle guide vanes (VNGVs), variable-geometry compressors (VGCs) and variable-geometry turbines (VGTs) are the focus in this work, which analyses a one-shaft free power turbine for power generation in the search for performance improvement at part load.

Damped torque shaft assembly

A torque shaft assembly includes a hollow tube with a central axis disposed between and fixedly connected to first and second crankshafts at first and second distal ends, respectively, of the tube for movement of an array of connecting members such as push rods. A hollow interior of tube between the first and second crankshafts is filled with a sufficient quantity of flowable inertia material or damping media to absorb vibratory energy by friction during operation of the engine. The hollow interior is preferably filled with a quantity of damping media to a level by volume of about 98% and a preferred flowable inertia material is round steel shot. A plurality of spaced-apart clevises are fixedly attached to a tube wall on an outer surface of the tube wall surrounding the hollow interior. Each clevis includes connection means disposed away from the wall outer surface for connection with an actuator for movement of the array of adjustable devices such as variable stator vanes.