Variable Stator Vanes Research Articles

A new contact force model for revolute joints considering elastic layer characteristics effects

The contact forces generated during the motion of revolute joints with clearances significantly impact the dynamic performance of mechanisms, making it essential to develop an accurate contact force model. Traditional contact models often neglect the influence of elastic layers or assume that the bushing thickness is equal to its radius, which limits their applicability. To address the dynamic contact problem in revolute joints with small clearances, this study employs an improved Winkler elastic foundation model and introduces an elastic layer correction coefficient to refine the static contact model. The model's validity is verified using the finite element method. Building on this, a new contact force model is proposed, incorporating the optimal damping term from multiple continuous contact models. Finally, experimental validation is conducted using the crank-slider mechanism and typical applications in the Variable Stator Vane (VSV) mechanism. The results demonstrate that the proposed contact force model effectively predicts the dynamic characteristics of mechanisms with clearances.

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 study on pulsed suction under different momentum coefficients in variable compressor cascade with annular gaps

Variable Stator Vanes (VSVs) with penny cavities are crucial in the compressors of variable cycle engines, where complex interactions between annular and radial gaps can adversely affect aerodynamic performance. Unsteady active flow control techniques have significant potential for reducing secondary flow losses in compressors. This research investigates the internal flow characteristics of a variable compressor cascade with penny cavities using low-speed wind tunnel experiments and flow visualization techniques. This study is the first to experimentally explore the relationship between loss reduction and momentum coefficient using pulsed suction (PS) compared to steady continuous suction (SCS), evaluating the effects of various excitation frequencies. The findings show that at a momentum coefficient of 0.11 and an excitation frequency of 100 Hz, PS achieves optimal aerodynamic performance, reducing total pressure losses by 11.59 %, marking a 92.59 % improvement over SCS. As the momentum coefficient increases, the effectiveness of PS control significantly improves, whereas the changes in SCS effectiveness are less pronounced. The control effects of PS under varying excitation frequencies are complex and influenced by multiple factors. At higher momentum coefficients, the loss reduction effectiveness of PS is strongly correlated with excitation frequency, while at lower coefficients, performance changes due to frequency variations are minimal. This study provides valuable guidance for the design of modern multistage variable compressors.

A simplified model for the wear prediction of plain bearings in the variable stator vane system

The wear of plain bearings in the variable stator vane (VSV) systems in a turbojet engine is a significant problem for compressor efficiency and stability requiring adapted models. The VSV bearings are located in a complex system and are exposed to combined and oscillating loads. This study aims to predict the wear evolution of the VSV bearings, considering the complexity of the system’s geometry and the loads with adapted computational time. This paper presents a three-dimensional formulation of this highly nonlinear wear problem under simplifying hypotheses in the objective to obtain a compromise between the quality of the prediction and the CPU time.

A wear simulation approach based on the energy consumption model and its application to the VSV bushing wear analysis

Wear is the primary cause of damage to the bushing area of the Variable Stator Vane (VSV) mechanism. This paper presents an approach for predicting the wear-damage behavior of the bushing region to enhance the long-term effectiveness and safety of the VSV mechanism. The approach is based on an energy dissipation model and innovatively combines the FRIC COEF and UMESHMTION subroutines embedded in finite element software to perform wear simulations. The new approach takes into account the variation of the friction coefficient with normal load and sliding velocity and applies it to the wear simulation. The friction and wear models used in the simulations are based on the fitting of real experimental data. The simulation results are then compared with the wear results based on the Archard model. The results demonstrate that the new approach is more effective in simulating the wear of the bushing material. The study revealed that the contact stress between the upper and lower specimens decreases as wear behavior develops. Additionally, the wear on the surface disperses the distribution of the contact stress.

Performance Simulation of Thermodynamic Design for a New Turboshaft Engine

For a search and rescue (SAR) helicopter, the requirements of long-range and twin-engine are becoming increasingly important. In this paper, the overall performance of a new turboshaft engine for SAR helicopters is simulated. Firstly, the main performance parameters of competitor engines are collected and compared, and the thermodynamic cycle parameters of the new engine are proposed subsequently. Then, an engine model is created in Turbomatch and the overall performance of this model at the design point is calculated. Finally, the off-design performances at different conditions of output power, ambient temperature, altitudes and flight Mach numbers (Ma) are investigated. A strategy of variable stator vane (VSV) for the anti-surge control of the axial compressor is also introduced. The results of the performance simulation indicate that this engine is quite competitive with lower specific fuel consumption (SFC) and higher specific power (SP) compared to the current SAR helicopter engines.

Design of Aerodynamically Balanced Transonic Compressor Rotors

Abstract This paper describes a simple and efficient physics-based method for designing optimal transonic multistage compressor rotors. The key to this novel method is that the spanwise variation of the parameter which controls the three-dimensional shock structure, the area ratio between the throat and the inlet, “Athroat /Ainlet”, is extracted directly from the 3D computational fluid dynamics (CFD). The spanwise distribution of the area ratio is then adjusted iteratively to balance the shock structure across the blade span. Because of this, the blade design will be called “aerodynamically balanced.” The new design method converges in a few iterations and is physically intuitive because it accounts for the real changes in the 3D area ratio that directly controls the shock structure. Specifically, changes in both the spanwise 3D flow and the rotor’s operating condition; thus aiding designer understanding. To demonstrate this, two example design cases are shown in the paper: a transonic rotor within a multistage civil compressor with variable upstream stator vanes, and an embedded rotor within a multistage military fan. The method is shown to (1) improve both the operating range and the design efficiency while retaining the compressor’s overall matching, and (2) allow a balanced design to be simultaneously achieved at multiple shaft speeds. The result is a method which simplifies the multistage transonic compressor rotor design process and therefore has great practical utility.

Tribo-Informatics Approach to Investigate the Friction and Wear of Bushings in the Variable Stator Vane System

Abstract Determining the friction and wear behaviors of aero-engine key components under realistic conditions is important to improve their long-term reliability and service life. In this paper, the friction and wear behaviors of different bushing materials in the variable stator vane (VSV) system were investigated through the basic pin-on-disc test and actual shaft-bushing test. Different machine learning (ML) models were established based on the experimental information to predict the coefficient of friction (COF) and wear-rate. The results indicated that there is a significant temperature warning line for the wear amount of the polyimide material, while the high-temperature alloy material exhibited stable tribological performance under experimental load and temperature conditions. ML analysis indicated that the extreme gradient boosting (XGB) outperformed other ML algorithms in predicting the COF (R2 value = 0.956), while the kernel ridge regression (KRR) produced the best performance for predicting the wear-rate (R2 value = 0.997). The tribo-informatics research for bushings in the VSV system can accelerate the structural optimization and material selection and support the evaluation of new structures and materials.

Characterization of joint contact model considering friction of variable stator vane mechanism and application to principle-level mechanism

For the characterization of joint contact model of the variable stator vane (VSV) mechanism, the dry friction properties of joint materials must be taken into account. In this paper, the important parameters characterizing the joint contact model are experimentally tested, and then the characterized contact model containing the parameters is utilized for the dynamic analysis of a principle-level VSV mechanism test rig, including the dry friction rotary pair. Firstly, the friction tests and the vibration tests induced by the friction process are carried out. Then an ideal comprehensive dynamic model of the scaled system of VSV mechanism’s single kinematic chain is established through the Lagrange multiplier method. The friction coefficient values obtained from the test are embedded into the contact force model considering friction factors and compared with other macro models. The accuracy of the hybrid contact model is verified through a one-shot contact-impact simulation and the corresponding displacement is measured for evaluation with accuracy of established ideal dynamic model. Finally, the influence of different driving methods on the dynamic characteristics of the system is analyzed.

Secondary flow and loss mechanisms of variable stator vanes in an annular cascade

Axial compressors with variable stator vanes require annular gaps and radial gaps from the endwalls for smooth adjustment, which induces complex secondary flows such as the penny leakage vortex and tip leakage vortex, leading to a negative impact on the aerodynamic performance. To better understand these mechanisms, numerical investigations were conducted on four different clearance configurations. The results show that the penny leakage vortex moves toward the suction side under the transverse pressure gradient and mixes with the hub corner stall vortex. This causes the corner separation to be further developed, leading to an increase in total pressure loss by 13.6%. However, the tip clearance leakage flow could reduce the transverse pressure gradient, which prevents penny leakage vortex from mixing with low-energy fluid in the corner region. Moreover, the hub corner stall vortex is also replaced by the tip leakage vortex, which effectively suppresses the range of corner separation. Under the comprehensive effects of the penny leakage vortex and the tip leakage vortex, the total pressure loss coefficient is increased only by 7.6%. Therefore, the mixing effect between the penny leakage vortex and low-energy fluid in the corner separation is the main reason for higher loss production of the cascade, and these findings provide theoretical support for the future application of flow control technology to reduce secondary flow loss.

Investigation of spatial plane joint characteristic for dynamic analysis of VSV mechanism based on similarity scaling technique

The Variable Stator Vane (VSV) mechanism is utilized to adjust the stator vane angles so as to delay the compressor stall. Establishing the dynamic model of the system, which is the basis to obtain different joint characteristics more in line with the real case, is worth analyzing. In this work, a comprehensive dynamic model of the scaled system of VSV mechanism’s single kinematic chain by the Lagrange multiplier method is built, thus the spatial plane joint characteristic’s effects on the behavior of the scaled system are investigated. Since a full-scale facility is unavailable for testing sometimes, the technique of similarity scaling is resorted to establishing the scaled principle-level model which has a certain relationship with the prototype in structure and material and so on. Then an example of the dynamic modeling procedure of the spatial RSSR mechanism included in the scaled model is presented with the aim of establishing a guide to model the whole system. Therefore, the equation of motion of the scale system is expediently established using some systematic method. Finally, inertial navigation sensors are used to measure the corresponding displacement for evaluation with accuracy of the established dynamic model.

Axial Compressor Map Generation Leveraging Autonomous Self-Training Artificial Intelligence

Abstract The paper describes the study performed by SoftInWay in the scope of the Phase I SBIR project funded by the National Aeronautics and Space Administration (NASA). The project was dedicated to a study of optimization of the variable geometry reset angle schedules with the use of innovative autonomous artificial intelligence (AI) technology. In the scope of the project, an automated compressor performance data generation workflow was developed. Three highly loaded multistage axial compressors were designed. The developed workflow was used to generate the training, validation, and test data sets for all three compressors. Multiple different architectures of artificial neural networks were studied, and parametric models for the representation of performance speedlines were developed. Utilizing the developed approaches, artificial neural networks were trained for all three compressors to predict their performance with a relative error below 3%. The trained neural networks were successfully used in the optimization of the variable inlet guide vanes and variable stator vanes reset angle schedules with a relative error of total-to-total pressure ratio prediction below 2% for most of the points and relative error of total-to-total efficiency prediction below 1% for all the points of the operational line. The capability of the developed AI models to accurately predict the optimal combination of reset angles and efficiency of the axial compressor with multiple vanes controlled independently allowed doing quick evaluations of efficiency and stability margins. The availability of such information enables the opportunity to make technical-economical decisions about the reasonability of implementation of independent variable vanes and their number during engine system analysis.

Numerical investigation of a variable stator vane with nonuniform partial radial gaps in an annular compressor cascade

Abstract A variable stator vane (VSV) with nonuniform partial radial gaps is numerically investigated in an annular compressor cascade. Five adjusting angles (α = −5.5°, −5°, 0°, 5° and 10°) are chosen to simulate different working conditions. The VSV is adjusted in negative direction means that the stagger angle increases and the VSV is more closed. Since the VSV is installed in an annular cascade, the heights of the partial gaps are nonuniform and reaches 3.8% of the span. Results show that as the VSV is adjusted negatively, the total pressure loss rises by 20.3% and a huge area of corner separation appears. The outflow angle is also more distorted along radial direction. Comparisons with a fixed-configuration stator show that although the VSV would cause higher loss, it indeed plays an important role in adjusting the outflow angle.

Mean-line model development for off-design performance prediction of transonic axial compressor of an industrial gas turbine based on computational fluid dynamics database

In this work, a mean-line model with the row-by-row stacking scheme is developed to simulate the off-design operation of a multi-stage transonic axial compressor of an industrial gas turbine. A database of computational fluid dynamics (CFD) simulation results validated with the field data is generated at different rotating speeds and pressure ratios by using CFD simulation tools and available detail geometry of the studied compressor. The compressor characteristic parameters including inlet/exit blockage factor, deviation angle parameter, rotor temperature and pressure coefficients, and the pressure loss of the stator for all stages, plus variable inlet guide vane, are extracted from CFD simulation results. Based on analytical/empirical relations in the literature, a generic formulation for the characteristic parameters is defined as a function of airflow parameters such as inlet Mach number, inlet flow angle, and velocity ratio. Since the accuracy of this type of model depends on the formulated characteristic parameters, and due to the error propagation potential of the sequential stacking solutions, a multi-objective optimization program is developed to specify the coefficients and exponent values of the generic formulations. The functionality of characteristic curves versus airflow parameters for different compressor rows is comprehensively investigated. The comparison between the results of the mean-line model and CFD simulations shows the high accuracy of the model. The developed model can be used in applications such as condition monitoring of in-service gas turbines, studying the effects of rescheduling the variable stator vanes, and investigating the wet compression process.

Rough surface damping contact model and its space mechanism application

For the dry conformal friction pairs, the contact damping effect between rough surfaces should be considered. In this study, a new rough surface damping contact (RSDC) model is firstly proposed and then applied to the spatial tribo-dynamics analysis of the variable stator vane (VSV) mechanism, which consists of dry conformal friction joints. The results show that the contact damping is very significant and cannot be ignored, otherwise the calculation process will not converge, which is quite different from the case when lubricants exist. In the tribo-dynamics simulation of the VSV, the damping effect play a role in ensuring stability, and the clearance size affects the distribution of damping forces and rigid forces.

Parametric Analysis of Variable Stator Vane System in Gas Turbines Based on Cosimulation of its Refined Model and System Dynamic Performance Model

AbstractThe variable stator vane (VSV) system is a key component of gas turbine supercharged rotor components for anti‐surge, and its performance directly affects the working stability and economy of the gas turbine. The effect of the VSV system on the dynamic characteristics and control performance of the gas turbine cannot be known. Then a refined model of the VSV system refined to design parameters established and studied to explore the effect of gas turbine dynamic performance based on co‐simulation. The effects of key parameters on the performance of the gas turbine system are analyzed by embedding the refined VSV model into the engine model. The simulation results prove that servo amplifier gain, viscous friction coefficient and dead band are sensitive structural parameters that affect the dynamic performance of the gas turbines. In the proposed appropriate interval, the optimal sensitive parameters can reduce the overshoot and adjustment time by about 51% and 7%. The gain of the servo amplifier is positively correlated with the maximum opening of the VSV. When the dynamic performance of the control system has been exerted to its extreme, some nonlinear parameters of the actuator can greatly improve the overall performance of the complex system.

Effect of an Axially Tilted Variable Stator Vane Platform on Penny Cavity and Main Flow

Abstract This paper presents the impact of an axially tilted variable stator vane (VSV) platform on penny cavity flow and passage flow, with the aid of both optical and pneumatic measurements in an annular cascade wind tunnel as well as steady computational fluid dynamics (CFD) analyses. Variable stator vanes in axial compressors require a clearance from the endwalls. This means that penny cavities around the vane platform are inevitable. Production and assembly deviations can result in a vane platform which is tilted about the circumferential axis. Penny cavity and main flow in geometries with and without platform tilting were compared in an annular cascade wind tunnel. Detailed particle image velocimetry (PIV) measurements were conducted inside the penny cavity and in the vane passage. Steady pressure and velocity data was obtained by two-dimensional multi-hole pressure probe traverses in the inflow and the outflow. Furthermore, pneumatic measurements were carried out using pressure taps inside the penny cavity. Additionally, oil flow visualization was conducted on the airfoil, hub, and penny cavity surfaces. Steady CFD simulations have been benchmarked against experimental data. The results show that tilting the vane platform reduces the penny cavity leakage mass flow. By decreasing penny cavity leakage, platform tilting also affects the passage flow where it leads to a reduced turbulence level and total pressure loss in the leakage flow region. In summary, the paper demonstrates the influence of penny platform tilting on cavity flow and passage flow and provides new insights into the mechanisms of penny cavity-associated losses.

Investigating the friction, wear and damage behaviour of plain bearing bushes of the variable stator vane system

Determining the friction, wear and damage behaviour of aero engine components under realistic operation conditions is important to improve the long-term reliability and efficiency. A special test rig for single plain bearing bushes of the variable stator vane (VSV) system was designed to investigate these components under engine-like conditions. It is found that four main wear and failure phenomena occur during the testing of VSV bushes. These include heavy abrasive wear on the inner contact area, wear on the flange contact area, plastic deformation and material failure as well as heavy wear and debris on the spindle. Results from polyimide-based plain bearing bushes show that the operational torque and wear rate remain almost constant, as long as no failure of the bush occurs.

Aerodynamic Loading Considerations of Three-Shaft Engine Compression System During Surge

Abstract Compressor surge imposes a limit on aero-engine operability and can compromise integrity because of significant aerodynamic loads imparted on the engine components. The aim of this article is to use 3D unsteady computational fluid dynamics (CFD) to predict the surge loadings on a compression system of a modern three-spool engine. The compression system is matched at a high power condition and computations are performed using a whole-assembly approach. In this study, the effect of two types of surge initiation on the maximum loading recorded during surge are studied: throttling of the high pressure compressor (HPC) and turning of the intermediate pressure compressor (IPC) variable stator vanes (VSV). An explanation of the main physical phenomena that contribute to those loadings is offered. It was found that in an aero-engine surge event, the maximum overpressure is driven by a combined effect of the surge shock wave passing and high pressure gas blown toward the front of the engine during depressurization. The amplitude of maximum overpressure is dictated by the compression system exit pressure at the moment of surge inception. The surge initiation via HPC throttling produces larger overpressure and therefore should be considered for design intent.

Stator re-stagger optimization in multistage axial compressor

As a widely applied technique in multistage axial compressors, variable stator vanes (VSV) can flexibly rematch the blade rows to fulfil a variety of aerodynamic performance requirements, such as high efficiency and wide surge margin. The purpose of this paper is to develop an optimization method to quickly determine VSV settings during the preliminary design phase. A mean-line method with a model calibration procedure is adopted to evaluate compressor performance, and the NSGA-II algorithm is employed for automatic optimization. The developed optimization system is then employed to determined re-stagger arrangement in a multistage compressor. A single-speed optimization with performance constraints of specific operating point and a multi-speed optimization with different control laws are conducted. Results are compared with available experimental re-stagger scheme, which verifies the effectiveness of the re-stagger optimization method. Moreover, method is proposed to determine operating parameters of a working point with a user-defined pressure ratio or mass flow rate after variable geometry.