Stator Vanes Research Articles

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.

Crack cause analysis of a third stage stator vane of the low-pressure compressor in an aeroengine

A cracked third stage stator vane of the low-pressure compressor was found during a major overhaul of an aeroengine. The crack originated from a corrosion pit located on the arc transition between the leading edge and the upper edge plate. The finite element analysis results demonstrated the arc transition was the region with the large von-Mises stress under the wake excitation. The cleavage-like fracture and fatigue striations predominated throughout the fracture surface, indicating it was a pit-related fatigue crack. Meanwhile, Metallographic examination discovered most of corrosion pits beneath the bulging or peeling of the paint layer were distributed on the leading edge and the concave side. Moreover, the composition and phase identification results showed that the corrosion products included sulfate and alumina. Considering the pitting corrosion products and outdoor environment of the aircraft downtime, it was inferred that the cracked stator vane suffered from atmospheric corrosion.

The heat transfer potential of compressor vanes on a hydrogen fueled turbofan engine

Hydrogen is a promising fuel for future aviation due to its CO2-free combustion. In addition, its excellent cooling properties as it is heated from cryogenic conditions to the appropriate combustion temperatures provides a multitude of opportunities. This paper investigates the heat transfer potential of stator surfaces in a modern high-speed low-pressure compressor by incorporating cooling channels within the stator vane surfaces, where hydrogen is allowed to flow and cool the engine core air. Computational Fluid Dynamics simulations were carried out to assess the aerothermal performance of this cooled compressor and were compared to heat transfer correlations. A core air temperature drop of 9.5 K was observed for this cooling channel design while being relatively insensitive to the thermal conductivity of the vane and cooling channel wall thickness. The thermal resistance was dominated by the air-side convective heat transfer, and more surface area on the air-side would therefore be required in order to increase overall heat flow. While good agreement with established heat transfer correlations was found for both turbulent and transitional flow, the correlation for the transitional case yielded decent accuracy only as long as the flow remains attached, and while transition was dominated by the bypass mode. A system level analysis, indicated a limited but favorable impact at engine performance level, amounting to a specific fuel consumption improvement of up to 0.8 % in cruise and an estimated reduction of 3.6 % in cruise NOx. The results clearly show that, although it is possible to achieve high heat transfer rate per unit area in compressor vanes, the impact on cycle performance is constrained by the limited available wetted area in the low-pressure compressor.

Broadband noise characteristics of fans with different blades under identical working conditions

Small axis-flow fans are widely used for air cooling and ventilation systems. A common small axis-flow fan has an unshrouded rotor, structs or stator vanes and a round casing. The fans used in this work are designed based on isolated-airfoil blade design mode and simple radial equilibrium theory supplemented by three-dimensional flow simulations with steady flow. The noise radiated from the fan can be decomposed to discrete tones and broadband noise, which is mainly due to interaction between blades and structs or stators vanes and turbulence, respectively. Under the working conditions of the same pressure rise and volume flow rate, the noise spectra from different fans are analyzed. It is found that with the addition of the fan blade number, the concentration of the broadband sound energy gradually shifts from low frequency to high frequency. And the fewer the number of blades, the less the corresponding total broadband energy. Besides, the calculation results also show the fan with more blades can be absorbed more sound by the same sound absorption materials with the same volume. The findings are useful for improving the acoustic design of small axis-flow fans.

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.

Large Eddy Simulation of Externally Induced Ingress about an Axial Seal by Stator Vanes

Turbine inlet temperatures in advanced gas turbines could be as high as 2000 °C. To prevent ingress of this hot gas into the wheelspace between the stator and rotor disks, whose metals can only handle temperatures up to 850 °C, rim seals and sealing flows are used. This study examines the abilities of large eddy simulation (LES) based on the WALE subgrid model and Reynolds-averaged Navier–Stokes (RANS) based on the SST model in predicting ingress in a rotor–stator configuration with vanes but no blades, a configuration with experimental data for validation. Results were obtained for an operating condition, where the ratio of the external Reynolds number to the rotational Reynolds number is 0.538. At this operating condition, both LES and RANS were found to correctly predict the coefficient of pressure, Cp, located downstream of the vanes and upstream of the seal, but only LES was able to correctly predict the sealing effectiveness. This shows Cp by itself is inadequate in quantifying externally induced ingress. RANS was unable to predict the sealing effectiveness because it significantly under predicted the pressure drop in the hot gas path along the axial direction, especially about the seal region. This affected the pressure difference across the seal in the radial direction, which ultimately drives ingress.

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.

Design Optimization of 1.5-Stage Transonic Compressor Based on BPNN Surrogate Model and NSGA-II

To achieve multi-objective aerodynamics design optimization for a 1.5-stage transonic compressor, a design platform incorporating blade parameterization methods, a BPNN surrogate model, and the NSGA-II optimization method was developed. The stagger angle distribution of three blade rows was selected as the optimization variable, with isentropic efficiency at the new design condition and stall margin set as the goal functions. Results demonstrated that, without altering the blade profile shape and endwall contour, the flow rate at design condition increased by 7.1%, stall margin increased by 1.8%, isentropic efficiency decreased by 0.0087, and total pressure ratio experienced a slight increase. The flow field at different conditions before and after optimization was compared and analyzed. The analysis indicated that the tangential velocity of rotor outlet becomes the determining factor for the compressor’s work capacity. The relative Mach number at the rotor inlet emerged as the key parameter affecting shock wave intensity and shock wave/boundary layer interaction, which directly influenced the efficiency of the rotor passage. At near stall condition, the stator vane root’s stagger angle is crucial for the compressor’s performance.

Large Eddy Simulation of Rotationally Induced Ingress and Egress around an Axial Seal between Rotor and Stator Disks

In gas turbines, the hot gas exiting the combustor can have temperatures as high as 2000 °C, and some of this hot gas enter into the space between the stator and rotor disks (wheelspace). Since the entering hot gas could damage the disks, its ingestion must be minimized. This is carried out by rim seals and by introducing a cooler flow from the compressor (sealing flow) into the wheelspace. Ingress and egress into rim seals are driven by the stator vanes, the rotor and its rotation, and the rotor blades. This study focuses on the ingress and egress driven by the rotor and its rotation. This is carried out by performing wall-resolved large eddy simulation (LES) around an axial seal in a rotor–stator configuration without vanes and blades. Results obtained show the mechanisms by which the rotor and its rotation induce ingress, egress, and flow trajectories. Kelvin–Helmholtz instability was found to create a wavy shear layer and displacement thickness that produces alternating regions of high and low pressures around the rotor side of the seal. Vortex shedding on the backward-facing side of the seal and its impingement on the rotor side of the seal also produces alternating regions of high and low pressures. The locations of the alternating regions of high and low pressures were found to be statistically stationary and to cause ingress to start on the rotor side of the seal. Vortex shedding and recirculating flow in the seal clearance also cause ingress by entrainment. With the effects of the rotor and its rotation on ingress and egress isolated, this study enables the effects of stator vanes and rotor blades to be assessed.

A Study of Preliminary Design Methods for Low Noise Fans

Engineers need theory and computer codes to help them design quiet and efficient fans. A fast method to identify low-noise stator vane counts that might minimize tone noise for axial ducted fans has been developed. The analysis begins with a preliminary aerodynamic fan design. Duct acoustic and rotor-stator interaction theories are combined to predict the set of cut-on circumferential modes that are expected to be generated and propagate through the fan duct. By predicting the number of cut-on circumferential modes for each vane count, a set of low-noise vane counts for a given rotor and duct design can be identified. This method was used to design the stator set for NASA's spacecraft cabin ventilation fan, in order to minimize tone noise generated by rotor-stator interaction at the first three blade passing frequency harmonics. This method was also applied to three other research fans that NASA uses for aeroacoustics research for comparison. This technique can be used to include acoustic design criteria early in the design of an axial fan and provide valuable information to guide the final design of quiet fans.

Forced Response Analysis of an Embedded Compressor Rotor Induced by Stator Disturbances and Rotor–Stator Interactions

Accurate predictions of the blade response in a multi-row compressor is one of the most important tasks within the design process of compressor blades. Some recent studies have shown that the decoupled method considering only the stator disturbances cannot obtain accurate results for cases with strong rotor–stator interactions, especially for the interaction between the rotor and downstream stator, and the coupled method with multi-row configurations is necessary. Factors determine what computational domains to model need to be clarified to find a balance between accuracy requirements and computational costs. To this end, this study conducted full-annulus unsteady calculations with decoupled and coupled configurations to investigate the forced response of an embedded compressor rotor induced by upstream and downstream stator disturbances and rotor–stator interactions, respectively. The results show that the upstream IGV disturbances were dominated by the wake, and the IGV and S1 potential fields had little effect on the R1 response. Meanwhile, the IGV–R1 interactions and S1–R1 interactions were dominated by one cut-on mode, respectively. The comparisons of the blade vibration amplitude and the unsteady pressure field calculated by decoupled and coupled methods revealed the mechanism of the forced response, namely, for the R1 response induced by upstream aerodynamic disturbances, the dominant excitation source was the IGV wake, and the blade vibration amplitude can be predicted by the decoupled method. In terms of the response induced by downstream disturbances, the cut-on S1-R1-interaction mode was dominant and the use of the decoupled method without considering its influence will lead to an inaccurate prediction. This study concluded that the formation process of rotor–stator interactions was the key factor that determines whether the decoupled method or coupled method should be used, and analogized a process independent of the downstream stator disturbance. The results can provide a preliminary configuration for accurate and efficient blade response predictions and explain the reason why including downstream stator vanes is very important.

Numerical Analysis of the Effects of Grooved Stator Vanes in a Radial Turbine Operating at High Pressure Ratios Reaching Choked Flow

The flow through the stator vanes of a variable geometry turbocharger turbine can reach supersonic conditions and generates a shock wave on the stator vanes, which has a potential impact on the flow loss as well as on unsteady aerodynamic interaction. The shock wave causes a sudden increase in pressure and can lead to boundary separation and strong excitation force, besides pressure fluctuation in the rotor blades. Thus, in this study, the flat surface of the vanes of a commercial variable geometry turbocharger turbine has been modified to analyze the effects of two grooved surfaces configuration using CFD simulations. The results reveal that the grooves change the turbine efficiency, especially at higher speed, where the increase in the efficiency is between 2% and 6% points. Additionally, the load fluctuation around the rotor leading edge can be reduced and minimize the factors that compromise the integrity of the turbine. Furthermore, the grooves reduce the supersonic pocket developed on the suction side of the vane and diminish the shock wake intensity. Evaluating the effectiveness of the available energy usage in the turbine, on the one hand, at lower speed, the fraction of energy at the inlet destinated to produce power does not change significantly with a grooved surface on the stator vanes. On the other hand, at higher speed and higher pressure ratio with 5 grooves occurs the most effective approach of the maximum energy.

Unsteady modeling of particle deposition effects on aerodynamics and heat transfer in turbine stator passages with mesh morphing

In order to have a further insight into the dynamic build-up process of particle deposits within the high-pressure turbine stator of an aeroengine and also into their effects on aerodynamic performance and heat transfer characteristics of the entire cascade passages, unsteady simulations coupled with a particle-wall interaction model and a dynamic mesh update technology were conducted in this study. The changing tendencies of flow field structures, aerodynamic losses, and heat transfer coefficients were characterized as the deposits on the stator increased with the exposure time. Results reveal that localized deposits with peak-to-valley patterns are generated with the accumulation of particles. The flow path area of the turbine passage is reduced due to the deposits that are mainly concentrated on the pressure side of the stator vane. Energy dissipation is generated as the mainstream gas flows through the fouled stator, which results in the deterioration of aerodynamic performance. Heat transfer presents a different tendency at specific locations on the vane surface with the dynamic deposition. At the early stage of deposition, slight deposits can reduce the thermal load on the vane to a certain extent, while as particles further build up, the heat transfer coefficient of the entire vane surface increases rapidly.

On the Probabilistic Endurance Prediction Approach for Turbomachinery Blades and Vanes

Abstract The design and aeromechanical assessments of turbomachinery blades and vanes comprise a wide range of complex processes that tend to be based on conventional deterministic methods. These processes often provide a “snapshot” evaluation of the new component designs at the nominal operating conditions. While the deterministic methods can predict the high cycle fatigue (HCF) endurance with reasonable accuracy, they assume that the conservative safety factors applied to cover for the parametric variations, uncertainties, and unknowns will not change during the product life cycle. This approach is intended to be conservative and in some cases may overlook the lack of robustness. The present paper proposes a robust design analysis approach based on probabilistic methodology for the aeromechanical assessment of rotor blades and stator vanes of turbomachinery. The robust design approach can account explicitly for the effects of design and manufacturing variability. This methodology can reduce the levels of conservatism in the deterministic approach and can provide a more thorough risk assessment. This paper offers a generalized aeromechanical analysis formulation based on probabilistic methods to evaluate the HCF capability of turbomachinery components. Herein, this methodology is demonstrated using a typical stator vane of an aero engine compressor and it is based on Monte-Carlo and design of experiment (DOE) simulations. The methodology consists of parametric sensitivity studies and identification of the most influential parameters that control the HCF endurance. Future ideas and roadmap of the aeromechanical probabilistic analysis capability development are also discussed.

Error of Measuring Surface Temperature of Turbine Blades With the Influence of Reflection by Monochromatic Method

A new method is proposed for correcting the error of monochromatic method in measuring the surface temperature of turbine blades with the influence of reflection. The measurement error caused by surrounding components were numerically investigated. The effects of background temperature, adjacent rotor blades, front stator vanes and rear stator vanes on the temperature measurement error are described in detail. Furthermore, the corrected errors of high emissivity and low emissivity are compared and analyzed and the influence of the relative displacement between the turbine blade and the hot-end component on the temperature measurement error is also discussed. Benefiting from the proposed radiation-based error correction method (RECM), we can reduce the monochromatic temperature measurement error to less than 0.3 % at high emissivity and less than 0.8% at low emissivity theoretically. Finally, the RECM method is studied experimentally, and the results show that the temperature measurement error can be corrected to better than 0.6%, which is in good agreement with the Monte Carlo simulation results.

THERMAL BARRIER COATINGS: RECENT DEVELOPMENTS, CHALLENGES, AND PROBABLE SOLUTIONS

The lifespan and performance of the industrial components used in aero engine’s and IC engines can be considerably increased by depositing thermal barrier coatings (TBCs). High-pressure gas turbines’ rotor blades and stator vanes are some of the world’s most heavily loaded engineering parts. In order to safeguard such components, more research is being conducted on TBCs, since the size of deployment and the production techniques used lead to improved life, economics, efficiency, and durability. This paper concentrates on the technological developments in the field of TBCs over the past few years in order to achieve maximum output. Current coating R&D has also placed an emphasis on the factors that contributed to the development of cutting-edge advanced coating-fabrication techniques, such as electron-beam physical vapor deposition (EB-PVD), suspension plasma spray (SPS), additive manufacturing (AM), functionally graded material (FGM) and incorporation of artificial intelligence (AI) in manufacturing of TBCs. This paper examines the current state of TBCs, including the most recent developments in terms of their performance and manufacturing, associated challenges, and suggestions for their potential usage in high temperature sector.

Numerical and experimental investigation of 3D unsteady flow fields of a low head axial flow turbine under different blade number

Low-head axial flow turbines (LHAFT) are important turbomachines suitable for converting low-head water (potential energy) into mechanical (kinetic) or electrical energy. The performance characteristics of this kind of turbine are an essential aspect of its design and operational life cycle. Nevertheless, pressure fluctuations within the turbine affect its operational reliability. Hence its fluctuation characteristics cannot be overlooked due to its influence on vibration, noise, and severity of performance. It is essential to investigate the zones associated with high unsteadiness and turbulence. Furthermore, blade number and variation in flow rate are other important, influential factors on the level of pressure fluctuations. Given the above, numerical and experimental investigation of pressure fluctuation intensities within the flow channels of the LHAFT model under different blade numbers was carried out in this study. The results posit that the pulsating pressure coefficient decreases gradually from the inflow to the guide vane but increases at the runner, then falls as the flow progresses in the outlet pipe. At the runner, the flow experienced the most irregular flow patterns in the turbine. All three runner cases were operated with nine guide vanes, but after one complete revolution, blade numbers z = 2, 3, and 4 revealed regular pressure fluctuations, respectively, in the stationary part (guide vane, inlet and outlet pipes). Regardless of the case study, nine (9) regular peaks and valleys of pressure pulses matching the number of vanes on the static vane were recorded in the runner flow passage. The flow interaction between the static vane and runner is responsible for the pressure fluctuation, which influences the turbine’s vibration and noise. The obtained results would be an essential reference to noise and vibration analytical studies in turbines to optimize them for improved operational reliability.

Evolution of Turbine Cooled Vanes and Blades Applied for Large Industrial Gas Turbines and Its Trend toward Carbon Neutrality

Photovoltaics and wind power are expected to account for a large share of power generation in the carbon-neutral era. A gas turbine combined cycle (GTCC) with an industrial gas turbine as the main engine has the ability to rapidly start up and can follow up to load fluctuations to smooth out fluctuations in power generation from renewable energy sources. Simultaneously, the system must be more efficient than today’s state-of-the-art GTCCs because it will use either Carbon dioxide Capture and Storage (CCS) when burning natural gas or hydrogen/ammonia as fuel, which is more expensive than natural gas. This paper describes the trend of cooled turbine rotor blades used in large industrial gas turbines that are carbon neutral. First, the evolution of cooled turbine stationary vanes and rotor blades is traced. Then, the current status of heat transfer technology, blade material technology, and thermal barrier coating technology that will lead to the realization of future ultra-high-temperature industrial gas turbines is surveyed. Based on these technologies, this paper introduces turbine vane and blade cooling technologies applicable to ultra-high-temperature industrial gas turbines for GTCC in the carbon-neutral era.

Three-dimensional effects of cascade perforations on rotor–stator interaction noise

One novel trend in reducing aero-engine noise is to utilize the silent flight mechanism of owls by applying perforations on fan stator vanes. Consequently, the establishment of relevant theoretical models is of particular interest. The current efforts made in this regard are just targeting the features based on two-dimensional models without including the three-dimensionality. In this paper, we present a three-dimensional solution for acoustic scattering by annular perforated cascades, and the dipole source corresponding to the unsteady pressure loading on the vanes is identified as the dominant sound source. By the singularity method, the acoustic response is obtained with the soft boundary condition applied on the vane surfaces. It is found that considerable noise reduction can be achieved for rotor–stator interaction with a modest uniform porosity, and accordingly two mechanisms are proposed to understand the effect of porosity on propagating sound. The first is that the perforations allowing a normal velocity across the vane reduce the unsteady loading induced by the incident disturbances. The second is that the three-dimensional interactions among the dipole sources at different positions are also dampened by the soft boundaries, thus the distribution of the unsteady pressure loading on the vanes will also change significantly compared to hard-vane cases. Non-uniform distributions of porosity are investigated further, indicating that perforations in the vane upstream area are more effective in reducing propagating noise. Our method is fully three-dimensional and capable of investigating non-uniform porosity, and thus is able to provide useful guidance for future soft vane designs.