Stator Vanes Research Articles

Nozzle Passage Endwall Effectiveness Values With Various Combustor Coolant Flowrates—Part 2: Endwall and Vicinity Surface Effectiveness Measurements

AbstractEffective coolant schemes are required for providing cooling to the first-stage stator vanes of gas turbines. To correctly predict coolant performance on the endwall and vane surfaces, these coolant schemes should also consider the effects of coolant streams introduced upstream in the combustor section of a gas turbine engine. This two-part paper presents measurements taken on a first-stage nozzle guide vane cascade that includes combustor coolant injection. The first part of this paper explains how coolant transport and coolant-mainstream interaction in the vane passage is affected by changing the combustor coolant and endwall film coolant flowrates. This paper explains how those flows affect the coolant effectiveness on the endwall and vane surfaces. Part one showed that a significant amount of coolant injected upstream of the endwall is present along the pressure surface of the vanes as well as over the endwall. Part two shows effectiveness measurement results taken in this study on the endwall and pressure and suction surfaces of the vanes. Sustained endwall coolant effectiveness is observed along the whole passage for all cases. It is uniform in the pitch-wise direction. Combustor coolant flow significantly affects cooling performance even near the trailing edge. The modified flowfield results in the pressure surface being cooled more effectively than the suction surface. While the effectiveness distribution on the pressure surface varies with combustor and film coolant flowrates, the distribution along the suction surface remains largely unchanged.

Nozzle Passage Endwall Effectiveness Values With Various Combustor Coolant Flowrates—Part 1: Flowfield Velocity and Coolant Concentration Measurements

Abstract The stators of the first stage of a gas turbine are exposed to severe temperatures. The coolant streams introduced to prevent the stators from thermal damage further complicate the highly three-dimensional vane passage flow. Recent results have shown that the coolant streams injected for cooling the combustor also influence the flow physics and the cooling effectiveness in the first-stage stator vanes passage. However, the effects of changing the mass flowrate of these combustor coolant streams on the passage flowfield have not been studied. As understanding the coolant transport is necessary for analyzing changes in cooling effectiveness in the vane passage, detailed aerodynamic and thermal measurements along the whole vane passage are required. This two-part paper presents such measurements taken for a variety of combustor coolant and endwall film coolant flowrates. The experiments were conducted in a low-Mach number facility with engine-representative Reynolds numbers and large-scale high-level turbulence. The objective of the first part is to describe the flow that influences endwall and vane surface cooling effectiveness distributions, which are presented in the second part. The measurements show changes in the passage flowfield due to changes in both combustor coolant and endwall film coolant flowrates. Overall, the flow physics remains largely unaffected by changes in coolant flowrates except in the endwall-vane surfaces region where the combustor coolant flowrate dominates changes in coolant transport. This is shown to have a high impact on endwall and vane surface cooling.

Failure analysis of blades and vanes of a compressor for a gas turbine engine

The flame-out occurred during the ground slide for take-off. As a result of immediate disassembly and inspection after failure, damages to blades and vanes were observed from the stage IV of a compressor. In particular, all stator vanes of stages IV and V, five rotor blades of stage V, and a rotor blade of stage VI were failed. In a fractured stator vane of stage IV, a fatigue-fracture surface, which was originated from the tip, appeared. However, the origin of fatigue cracking was not observed due to severe oxidation and wear. In the case of rotor blades at stages V and VI, the fatigue cracking was initiated from the nick damage part. The same nick damage was also observed in the stator vane of stage V, but fatigue cracking started from the convex surface. Comparing the microstructures of sound and fractured products showed that the convex surface of the fractured product had coarser grains compared to that of the sound product. This indicates that the coarse grains led to premature failure by fatigue cracking.

Conjugate Thermal Optimization With Unsupervised Machine Learning

Abstract With advances in additive manufacturing of metal components, commercial production of complex turbine components is becoming feasible. Thus, designers are not constrained to the limitations of conventional manufacturing methods. A new conjugate optimization technique is proposed, which is not computationally demanding and can be used when several heat transfer modes are working simultaneously. For this study, film cooling holes in the leading edge of a gas turbine airfoil are optimized without trial and error simulations. Since the machine learning technique is not dependent on thermal analysis, the optimization technique can be applied to any nonlinear problem. Film hole sizes are optimized to minimize coolant flow rate while reducing the temperature variations in the stationary vane. The technique used a transfer function based iterative optimization process with unsupervised machine learning that has been termed nonlinear optimization with replacement strategy (NORS). It uses a grading metric to replace the worst performing hole combinations with one that has been optimized with a given objective and several constraints. Optimized results show significant reductions in vertical temperature variations along the leading edge while minimizing coolant flow rate. Reduced temperature variation results in reduced thermal stresses. The finite element (FE) model and the associated correlations are not part of the unsupervised machine learning technique; therefore, the proposed optimization model can be generalized for any engineering design with multiple inputs for learning and multiple outputs for grading.

Dust Ingestion in a Rotorcraft Engine Compressor: Experimental and Numerical Study of the Fouling Rate

Helicopters often operate in dusty sites, ingesting huge amounts of contaminants during landing, take-off, hover-taxi, and ground operations. In specific locations, the downwash of the rotor may spread soil particles from the ground into the environment and, once ingested by the engine, may stick to the compressor airfoils. In the present work, the Allison 250 C18 engine’s multistage axial-flow compressor is employed to study the fouling rate on rotor blades and stator vanes from both numerical and experimental standpoints. The compressor is operated in a typical ground-idle operation, in terms of the rotational regime and contaminant concentration, in laboratory-controlled conditions. The mass of deposits is collected from the airfoil surfaces at the end of the test and compared to that estimated through the numerical model. The experimental test shows that the airfoils collect almost 1.6% of the engine’s total mass ingested during a ground-idle operation. The capability of numerical methods to predict the fouling rate on the rotating and stationary airfoils of a multistage compressor is tested through the implementation of literature based deposition models. Sticking models show a good agreement in terms of the relative results; nevertheless, an overestimation of the deposited mass predicted is observed.

Performance Degradation Due to Fouling and Recovery After Washing in a Multistage Test Compressor

Abstract Land-based power units have to fulfill even more high levels of production and reliability. In harsh environments (desert and tropical installations, typically), the power unit ingests high amounts of dust that might deposit inside the compressor. In this paper, the analysis of a multistage compressor performance that operates under sandy and humid conditions has been assessed. The compressor units, which equips the Allison 250 C18 compressor, has been subjected to multiple runs under severe conditions of soil dust ingestion. The compressor has been operated according to subsequent runs, and at the end of each run, the performance curve was recorded; the performance losses, in terms of pressure ratio, have been measured during the operations. The characteristic curve of each run is representative of the level of contamination of the unit. Finally, the compressor has been washed, and the performance curve, in the recovered conditions, has been recorded. The results show the modification and the downward shift of the characteristic curves which lead to a gradual loss of the compressor performance. The curves realized after dust ingestion have been compared with the recovered curve after online washing. The measurement shows a promising recovery of the performances, even if the compressor flow path appears affected by localized deposits able to resist to the droplet removal action. Detailed photographic reports of the inlet guide vane (IGV) and the first compressor stages have been included in this analysis. After each run, the IGV, the rotor blade and stator vane of the first stage, and the hub and the shroud surfaces have been photographed. The pictures show the deposition patterns on the blades and the compressor surfaces. The comparison of the pictures of the internal surfaces, before and after the washing, highlights the parts that are more critical to clean and needy of attention during offline washing and overhaul.

Embedded Temperature Sensor Evaluations for Turbomachinery Component Health Monitoring

Current rotorcraft gas turbine engines typically use titanium alloys and steel for the compressor section and single-crystal nickel superalloys for the hot-section turbine stator vanes and rotor blades. However, these material selections are rapidly changing due to increased requirements of power-density and efficiency. Future U.S. Army gas turbine engines will be using ceramic matrix composites for many high temperature engine components due to their low density and improved durability in high temperature environments. The gas turbine industry is also actively developing adaptive concept technologies for production and assembly of modular gas turbine engine components with integrated sensing. In order to actively monitor engine components for extended seamless operation and improved reliability, it is essential to have intelligent embedded sensing to monitor the health of critical components in engines. Under this U.S. Army Foreign Technology Assessment Support (FTAS) program funded research project, embedded fiber-optic temperature sensors from U.K.-based company, Epsilon Optics Ltd (Fordingbridge, UK)., were experimentally evaluated to measure temperature responses on typical turbomachinery component material coupons. The temperature responses from this foreign technology sensor were assessed using a thermomechanical fatigue tester with a built-in furnace to conduct thermal cycling durability experiments. The experimental results obtained from the durability performance of this embedded fiber Bragg sensor are reported in this paper. This sensor technology, upon maturation to higher TRL (technology readiness level), can greatly reduce the lifecycle cost of future U.S. Army gas turbine engines.

Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions

Abstract As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.

Evolution of Emission Species in an Aero-Engine Turbine Stator

Future energy and transport scenarios will still rely on gas turbines for energy conversion and propulsion. Gas turbines will play a major role in energy transition and therefore gas turbine performance should be improved, and their pollutant emissions decreased. Consequently, designers must have accurate performance and emission prediction tools. Usually, pollutant emission prediction is limited to the combustion chamber as the composition at its outlet is considered to be “chemically frozen”. However, this assumption is not necessarily valid, especially with the increasing turbine inlet temperatures and operating pressures that benefit engine performance. In this work, Computational Fluid Dynamics (CFD) and Chemical Reactor Network (CRN) simulations were performed to analyse the progress of NOx and CO species through the high-pressure turbine stator. Simulations considering turbulence-chemistry interaction were performed and compared with the finite-rate chemistry approach. The results show that progression of some relevant reactions continues to take place within the turbine stator. For an estimated cruise condition, both NO and CO concentrations are predicted to increase along the stator, while for the take-off condition, NO increases and CO decreases within the stator vanes. Reaction rates and concentrations are correlated with the flow structure for the cruise condition, especially in the near-wall flow field and the blade wakes. However, at the higher operating pressure and temperature encountered during take-off, reactions seem to be dependent on the residence time rather than on the flow structures. The inclusion of turbulence-chemistry interaction significantly changes the results, while heat transfer on the blade walls is shown to have minor effects.

A Multi-Stage Axial Compressor Characteristic Calibration Method based on One-Dimensional Average Streamline Method

Abstract As the core component of modern gas turbines, multistage axial compressors have a decisive influence on the performance of gas turbines. This paper proposes a multi-stage axial compressor characteristic calibration method based on one-dimensional average streamline method. Firstly, according to the physical laws followed by the multi-stage axial compressor, the aerodynamic analysis method of axial compressors is established which based on the one-dimensional average streamline method. Secondly, develop an automatic calibration method for the cascade performance model, use genetic algorithms to establish a complete blade wake Momentum thickness coefficient scalar database to realize the automatic calibration of compressor characteristics. This method has fast calculation speed and versatility. It can predict, encrypt and extrapolate compressor characteristics. It can be used to predict the characteristics of inlet guide vanes and stationary vanes when they are adjustable, or to calculate the overall performance without experimental data or CFD data.

Analysis of Unsteady Flow Field in Rotating Detonation Turbine Engine

The rotating detonation turbine engine is a research hotspot in recent years, but the research is mainly at the phase of theoretical analysis. In this paper, the influence of the circumferentially rotating pulsating flow field on the performance of the turbine is studied by numerical simulation. The results show that the uneven turbine inlet flow will increase the working load of the stator vane, but will produce greater flow loss. At the same time, the strength of the channel vortex and the tip leakage vortex inside the rotor is also increasing. The mass flow rate and work efficiency of the turbine decrease with the increase of the unevenness of the flow field.

Generation of 3D Turbine Blades for Automotive Organic Rankine Cycles: Mathematical and Computational Perspectives

Organic Rankine cycle technology is gaining increasing interest as one of potent future waste heat recovery potential from internal combustion engines. The turbine is the component where power production takes place. Therefore, careful attention to the turbine design through mathematical and numerical simulations is required. As the rotor is the main component of the turbine, the generation of the 3D shape of the rotor blades and stator vanes is of great importance. Although several types of commercial software have been developed, such types are still expensive and time-consuming. In this study, detailed mathematical modelling was presented. To account for real gas properties, REFPROP software was used. Moreover, a detailed 3D CFD numerical analysis was presented to examine the nature of the flow after generating the 3D shapes of the turbine. Moreover, finite element analysis was performed using various types of materials to obtain best-fit material for the current application. As the turbine is part of a larger system (i.e., ORC system), the effects of its performance on the whole ORC system were discussed. The results showed that the flow was smooth with no recirculation at the design point except at the last part of the suction surface where strong vortices were noticed. Despite the strong vortices, the mathematical model proved to be an effective and fast tool for the generation of the 3D shapes of turbine blades and vanes. The deviations between the 1D mean-line and 3D CFD in turbine efficiency and power output were 2.28% and 5.10%, respectively.

Supersonic separation technology for carbon dioxide and hydrogen sulfide removal from natural gas

Abstract A cleaner approach was proposed for removing CO2 and H2S from highly acid natural gas by supersonic separation technology. For this purpose, different structures of Laval nozzles and supersonic swirling separation devices were designed. The mathematical models of gas flow in the nozzle were established, and the supersonic flow characteristics of CH4–H2S–CO2 ternary mixture were accurately predicted by adding the real gas state equation and the transport equation to describe the rotational and irrotational flow field. The effects of nozzle structure, swirl angle of static vanes and inlet parameters on the refrigeration performance of the nozzle and the separation characteristics of the swirl device were systematically investigated. The results show that the average temperature at the nozzle outlet can be as low as 135.2 K under the condition of inlet temperature 293.15 K, inlet pressure 4 MPa and 20 mol % acid gas. It indicates that the refrigeration effect produced by high speed expansion of acid natural gas in the supersonic nozzle can provide a favorable liquefying environment for the acid components, which confirms the feasibility of capturing acid components by supersonic separation device. Better refrigeration performance can be achieved in the nozzle with higher outlet Mach number, but the pressure recovery capacity of the device will be suppressed accordingly. As swirl angle increases from 73.90° to 81.79°, the maximum tangential velocity in the nozzle rises from 119.6 m/s to 212.5 m/s, the separation performance of the swirl device increases, while the pressure and temperature at the same location decrease slightly. However, the increase of swirl intensity will restrict the flow capacity of the separator. For the given supersonic swirling separation device, it shows a good adaptability in separation performance when changing the inlet parameters.

Heat Transfer Analysis in a Single Spool Gas Turbine by Using Calculated-Estimated Coefficients with the Finite Element Method

In this paper, a calculation procedure is presented to estimate the heat transfer coefficients of a single spool gas turbine designed to generate 5 kN of thrust. These heat transfer coefficients are the boundary conditions which govern the heat interaction between the solid parts and the working fluid in the gas turbine. However, the calculation of these heat transfer coefficients is not a trivial task, since it depends on complex fluid flow conditions. Empirical correlations and assumptions have been used to find convective heat transfer coefficients over most components, including stator vanes, rotor blades, disc faces, and disc platforms. After defining the heat transfer coefficients, the finite element method was used to determine the temperature distribution in one eighth section of the gas turbine making use of the problem cyclic symmetry. Both static and rotating assemblies have been modeled. The results allowed the prediction of the thermal expansion behavior of the whole single spool gas turbine with special attention to the safety margin of clearances. Furthermore, having the temperature distribution defined, it is possible to calculate the thermal stresses in any mechanical component. Additionally, it is possible to specify suitable metallic alloys for achieving appropriate performance in every case. The structural integrity of all components was then assured with the temperature distribution and thermal expansion behavior under knowledge. Thus, the mechanical drawings could be released to manufacturing.

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.

Evaluating the Use of Leaned Stator Vanes to Produce a Non-Uniform Flow Distribution Across the Inlet Span of a Mixed Flow Turbine Rotor

Abstract Current trends in the automotive industry have placed an increased emphasis on downsized turbocharged engines for passenger vehicles. The turbocharger is increasingly relied upon to improve power output across a wide range of engine operating conditions, placing a greater emphasis on turbocharger off-design performance. An off-design condition of significant importance is performance at low turbine velocity ratios, since it is relevant to engine transient response and also to efficient energy extraction from pressure pulses in the unsteady exhaust flow. An increased focus has been placed on equipping turbochargers with mixed flow turbine rotors instead of conventional radial flow turbine rotors to improve off-design performance and to reduce rotor inertia. A recognized feature of a mixed flow turbine is the spanwise variation of flow conditions across the blade leading edge. This is a consequence of the reduction in leading edge radius from shroud to hub, coupled with the increasing tangential velocity of the flow due to conserved angular momentum as the radius decreases. The result is increasingly positive incidence toward the hub side of the leading edge. The resulting region of highly positive incidence at the hub produces separation from the suction surface and generates significant loss within the rotor passage. The aim of this study was to determine if the losses in a mixed flow turbine (MFT) could be reduced by the use of leaned stator vanes, which deliberately created a significant spanwise variation of flow angle between hub and shroud at rotor inlet, to reduce the positive incidence at the hub. The turbine performance with a series of leaned vanes was compared against that of a straight vane using a validated computational fluid dynamics (CFD) model. It was found that increasing vane lean improved turbine performance at all operating points considered. An increase of 3.2 percentage points in stage total-to-static efficiency was achieved at a key off-design operating point. Experimental testing of a set of leaned vanes and the baseline vanes confirmed the advantage of the leaned vanes at all operating points, with an increase in measured efficiency of 2.6 percentage points at the key off-design condition. Unsteady CFD models confirmed the same level of improvement at this operating point. The CFD and experimental results confirmed that the losses in an MFT can be reduced by the use of leaned stator vanes to shape the flow at rotor inlet.

Design of a two spool contra-rotating turbine for a turbo-fan engine

Contra-rotating turbines offer enhanced performance over their conventional co-rotating configurations. In addition, vaneless contra-rotating turbine stages offer lesser stage length along with improved performance. Contra-rotating turbines with a vaned LP stages offer a controlled work-split between the stages over a wider range of operating conditions by maintaining inlet swirl to the second rotor. The objective of the present work is to design an equivalent vaned contra-rotating turbine for an existing co-rotating configuration of a two-spool turbo fan engine. The contra-rotating turbine is designed by retaining the existing flow path and HP turbine, and redesigning the LP turbine for fixed radial distributions of inlet total temperature, pressure and swirl. A comparative study between performance of the co-rotating and contra-rotating turbines is carried out for different speeds. Cascade testing of the LP stator and rotor mean sections was carried out to validate the analysis. The LP stage of contra-rotating turbine exhibits a performance improvement by 2% points at design point, as per flow predictions. The reduced flow deflection in stator row is the primary reason for significant reduction in profile and secondary losses in contra-rotating turbine, which contributed to the performance improvement. A significant reduction of 23% in blade weight and 45% in LP stator vane count is obtained.

High-Fidelity Finite Element Modeling and Analysis of Adaptive Gas Turbine Stator-Rotor Flow Interaction at Off-Design Conditions

ABSTRACTThe objective of this work is to computationally investigate the impact of an incidence-tolerant rotor blade concept on gas turbine engine performance under off-design conditions. When a gas turbine operates at an off-design condition such as hover flight or takeoff, large-scale flow separation can occur around turbine blades, which causes performance degradation, excessive noise, and critical loss of operability. To alleviate this shortcoming, a novel concept which articulates the rotating turbine blades simultaneous with the stator vanes is explored. We use a finite-element-based moving-domain computational fluid dynamics (CFD) framework to model a single high-pressure turbine stage. The rotor speeds investigated range from 100% down to 50% of the designed condition of 44,700 rpm. This study explores the limits of rotor blade articulation angles and determines the maximal performance benefits in terms of turbine output power and adiabatic efficiency. The results show articulating rotor blades can achieve an efficiency gain of 10% at off-design conditions thereby providing critical leap-ahead design capabilities for the U.S. Army Future Vertical Lift (FVL) program.

Failure analysis of turbine stator vanes of aero-engine

Failed stator vanes of various aero-engines were subjected to investigations. Stator segment designated as SV-1 failed due to impact by large iron containing object generated possibly inside the turbine. Damage on the vanes of stator segment termed as SV-2 has taken place by stress rupture at low stress and high temperature. Stator segment of another aero-engine termed as SV-3 is degraded by impact abrasion of iron containing particles. Investigation reveals that thermo-mechanical fatigue is responsible for large crack generation of stator vanes obtained from another aero-engine referred as SV-4. In contrast, multiple small cracks are formed because of creep fatigue interaction also known as thermal fatigue for impairment of another segment of the same aero-engine denoted as SV-5. Finally, measures to be taken for improving service life of these stator segments are discussed.

Noise attenuation and performance study of a small-sized contra-rotating fan with microperforated casing treatments

This study investigates the benefits of noise control for a small-sized contra-rotating (CR) fan using microperforated-panel (MPP) casing treatments over the rotor. Several configurations of this type of casing treatment were proposed and then simulated with the finite element method (FEM) to determine their acoustic performance. In general, the FEM simulations indicated that the configurations of placing the treatment over the front rotor are more acoustically effective than those over the rear rotor. Experimentally, it was found that each axial row of holes of the MPP casing behaves like a stator vane, interacting with the fan rotor and thus generating a noisy interaction tone. However, such a tone can be sufficiently suppressed by optimizing the row numbers at the design stage of the MPP casing to increase the interaction mode and frequency. Experimental results further showed that for the treatment over the front rotor, the fan performance was not significantly influenced, especially in the flow rate region above stall point, while for the treatment over the rear rotor, a pressure loss of no more than 10 Pa was perceived, particularly in the flow rate region around the fan design point. Acoustically, the former was able to reduce overall noise throughout the flow rate region, with a maximum noise reduction of 3.5 dBA around the design point, whereas the latter increased overall noise slightly. Moreover, an enlarged volume lowering cavity stiffness for the treatment over the front rotor was found to improve the reduction of overall noise by 0.7 dBA (from 2.8 dBA to 3.5 dBA) as well as to attenuate rotor-alone tones effectively, of which the front treatment with the regular cavity failed in noise reduction. In contrast, no substantial enhancements of noise reduction were noticed for the rear treatment with an enlarged cavity. It is anticipated that the current investigation can provide a reference for controlling CR fan noise with MPP casing treatments.