bluuebunny/crossref_metadata_2025 huggingface dataset:
Calibration of CDM-Based Creep Constitutive Model Using Accelerated Creep Test (ACT) Data
Authors: Hossain Md Abir , Mach Robert , Pellicotte Jacob , Stewart Calvin M.
Published: January 2021

In conventional creep testing (CCT) a specimen is subject to constant load and temperature for a long-duration until creep rupture occurs. Conventional testing can be costly when considering the number of experiments needed to characterize the creep response of a material over a range of stress and temperature. To predict long-term creep-rupture properties, the time-temperature-stress superposition principle (TTSSP) approach has been employed where stress and/or temperature is applied at an elevated level; the result of which are extrapolated down to low stress and/or temperature conditions. These methods have been successful in predicting minimum-creep-strain-rate (MCSR) and stress-rupture (SR) but suffer from an inability to predict the creep deformation curve or account for changes in deformation mechanisms or aging that occurs at long-duration. An accelerated technique, termed the Stepped isostress method (SSM) allows the accelerated testing of materials to determine their creep deformation response. Unlike TTSSP tests, the SSM test employs a single specimen where the stress is periodically step increased until rupture. The SSM creep deformation curve is processed (time and strain shifted) to produce an accelerated creep deformation curve that represent the creep deformation curve at the initial stress level in SSM. A processing procedure for metals has yet to be developed.

The research objective of this study is to develop a processing procedure for SSM test data using a creep-damage constitutive model. Triplicate SSM tests were conducted on Ni-based superalloy Inconel 718 at 650°C with stress being periodically increased until rupture. Triplicate CCT tests were conducted at the initial stress level of the SSM tests. The Sine-hyperbolic (Sinh) creep-damage model was employed in this study. The Sinh creep-damage constitutive model is based on coupled creep strain rate and damage evolution equations; where both rates are dependent on the current state of damage. Calibration is two-step: analytical and numerical optimization. Each stepped creep deformation curve is tackled quasi-analytically to determine MCSR and SR related material constants and accumulated damage. The damage accumulated at the end of each step was then passed onto subsequent steps to calibrate the MCSR, rupture prediction, and damage evolution. Numerical optimization was applied to optimize model constants involved in the creep strain constitutive equations in order to generate best-fitted Sinh creep deformation curves. The Sinh model predictions were compared to the SSM and CCT data. The Sinh model satisfactorily predicts the SSM data and thus the calibrated material constants provides a good estimate of rupture found in the CCT data. Calibration using SSM data reduces the number of tests needed to calibrate a model; significantly reducing costs. A single SSM test replaces numerous creep tests at different stresses.
Effect of Sloped Trench Casing Treatment on Performance and Stability of Axial Compressors
Authors: Jiexuan Hou , Yangwei Liu
Published: January 2021

Numerical simulations are carried out to investigate the effect of the sloped trench casing treatment on the performance and stability of a compressor cascade, an isolated rotor, and a single compressor stage. The research objects alter from a simplified flow model to an actual compressor working environment. Firstly, a detailed study of how the sloped trench casing treatment effects the tip leakage flow structure, especially the tip leakage vortex of a compressor cascade, is presented. Results show that the strength of the tip leakage vortex is weakened as the sloped trench casing treatment transforms the structure of the tip leakage vortex. Then the simulation results of the isolated rotor and the single stage are studied. For both cases, the effect of the sloped trench casing treatment on the tip leakage flow is analogous to that of the cascade case. For the isolated rotor, the improvement on the performance is not obvious. While under the stage environment, different from the traditional casing treatment, both the performance and the stability of the compressor are advanced, by getting the tip leakage vortex under control and letting the downstream stator working under a better inflow condition.
Mid-Span Stall Inception in a Transonic Fan
Authors: Verschueren H. , Hall C. A. , Wilson M. J.
Published: January 2021

In this paper, steady and unsteady CFD have been used to investigate stall inception for a modern low pressure ratio transonic fan. The computational results are validated against measurement data from a high-speed test facility. CFD validation was approached as a blind test case. The results shows good agreement between the experiments and computations.

Stall is triggered by growth of a suction surface separation behind the shock around the mid-span of the rotor blade. As the fan is throttled the separation grows leading to increased blockage in the blade passages. At the point of instability the separation grows further, locally increasing incidence and leading to the formation of a stall cell. It is shown that changes to the tip leakage flow leave the stall inception mechanism unaffected. A computational case with a suction surface slip patch between 25–75% span shows that the reduction in blockage around the mid-span increases the stall margin by 25%. This demonstrates that for cases with mid-span initiated stall it is important to consider the flow away from the tip as well as the flow in the tip region. A redesigned fan is used to illustrate that design changes around the mid-span can be effective to improve flow range. The redesigned fan increases stall margin by 6.7% while maintaining the design point efficiency within 0.1%.
Topology Optimization of a Turbomachinery Casing Structure Using the Level Set Method
Authors: Li Lei , Neofytou Andreas , Paiva Ricardo , Keskin Akin , Kim H. Alicia
Published: January 2021

A turbomachinery casing structure provides the necessary support to many crucial systems and components in an aeroengine such as the combustion chamber, compressors, and turbines. A competitive casing structure should be stiff and durable while minimizing weight. This work investigates the possibilities and potential of using topology optimization to design a turbomachinery casing structure to achieve certain design requirements. As the casing structure is in a high pressure system that undergoes complex mechanical loading conditions, the level set method is employed in contrast to the common density-based topology optimization methods, which could lead to physically uninterpretable gray elements for a complex loading environment. This paper describes the basic concept of the level set method and demonstrates the benefits in terms of boundary definition, speed, and applicability to engineering problems. Topology optimization with a linear-elastic analysis based on an axisymmetric finite element solver is employed in this study. Both minimum compliance and minimum stress problem formulations are considered, and a series of optimum designs are presented and discussed.
A Co-Simulation Approach to Modeling Air-Cooled Condensers in Windy Conditions
Authors: Engelbrecht Ruan , Laubscher Ryno , van der Spuy Johan
Published: January 2021

Air-cooled condensers (ACC) are commonly used for cooling applications in the petro-chemical and power generation industry. Common analysis techniques used include one-dimensional design codes, however, these fail to capture complex flow phenomena associated with ACCs subjected to adverse environmental conditions. Computational Fluid Dynamics (CFD) is seen as an alternative, more accurate analysis tool. The model used in the present study co-simulates the CFD using ANSYS Fluent® to model the air-side with a system-level model in Flownex® to represent the steam-side. A 4 × 3 ACC bank is subjected to adverse cross-wind conditions and a single cell is co-simulated. The volumetric and thermal performance is predicted. Prior to the evaluation a discretization process is undertaken to ascertain an adequate level of detail required to accurately capture the complex flow phenomena which could affect the steam-side performance. A 3 × 3 and 6 × 6 discretization case is compared. A 13% difference in heat transfer rate is predicted between the cases. The 6 × 6 case is used to evaluate the cell performance when the ACC is subjected to cross-wind speeds of 0, 3, 6 and 9 m/s. Results show an increase in outlet steam quality as the cross-wind speed increases due to diminishing ACC performance.
Exploring Topology Optimisation of High Pressure Turbine Blade Tips
Authors: Vincekovic Luka , John Alistair , Qin Ning , Shahpar Shahrokh
Published: January 2021

This work presents the aerodynamic topology optimisation of high pressure turbine rotor blade tips. Before carrying out the topology optimisation on the blade tip, some initial tip design studies were carried out. The winglet shape was optimised using two different design space setups and parameter limits. The optimum winglet design features the largest overhangs and in the case of unconstrained optimisation proved to have 1.40% greater aerodynamic efficiency. Secondly, a radial basis function based parametrisation was set up to allow the creation of single squealer line using the flat tip blade as a baseline geometry. The optimum case proved to increase efficiency 0.46% compared to the flat tip. After that, a combination of winglet and topology free squealer tips was investigated for topology optimisation. The winglet tip was parametrized as in the winglet only optimisation cases and topology free squealer walls were created using mapping of radial basis function surfaces of different complexities. It is shown that by combining both winglet and novel squealer topology optimisation, better designs of different topologies can be produced.
Low DP FlameSheet™ Extended Validation of a Flexible, Low Emissions, Higher Output and Efficiency F-Class Turbine Upgrade
Authors: Rizkalla Hany , Hui Timothy , Hernandez Fred , Yaquinto Matthew , KeshavaBhattu Ramesh
Published: January 2021

Renewables proliferation in the energy market is driving the need for flexibility in gas fired power plants to enable a wider and emissions compliant operability range. The ability for a gas fired plant to peak fire while maintaining emissions compliance, full life interval capability, improved simple and combined cycle heat rate and the ability to achieve extended turndown, positions a gas fired asset to benefit from an improved capacity factor, and overall economic viability in an increasingly renewables’ dependent energy market. The low pressure drop FlameSheet™ combustor variant’s implementation alongside PSM’s Gas Turbine Optimization Package (GTOP3.1) on a commercially operating frame 7FA heavy duty gas turbine in 2018 and as introduced in GT2019-91647, is presented with emphasis on extended validation of operational and emissions/tuning performance at different ambient conditions, higher peak firing and minimum load after one year of continuous commercial operation. The output and heat rate improvement achieved with the FlameSheet™/GTOP3.1 conversion thus enabling improved capacity is also discussed. As shale gas continue to grow as a dominant source of the U.S Natural gas supply, the need for fuel flexible combustion systems enabling tolerance to higher ethane/ethylene concentrations associated with Shale gas is required for improved operability. The adverse impact and means to mitigate such higher ethane/ethylene content on standard F-Class heavy duty combustion systems is also presented as part of said FlameSheet™/GTOP 3.1 conversion.
Flow in a Simulated Turbine Blade Cooling Channel With Spatially Varying Roughness Caused by Additive Manufacturing Orientation
Authors: McClain Stephen T. , Hanson David R. , Cinnamon Emily , Snyder Jacob C. , Kunz Robert F. , Thole Karen A.
Published: January 2021

Because of the effects of gravity acting on the melt region created during the laser sintering process, additively manufactured surfaces that are pointed upward have been shown to exhibit roughness characteristics different from those seen on surfaces that point downward. For this investigation, the Roughness Internal Flow Tunnel (RIFT) and computational fluid dynamics models were used to investigate flow in channels with different roughness on opposing walls of the channel. Three rough surfaces were employed for the investigation. Two of the surfaces were created using scaled, structured-light scans of the upskin and downskin surfaces of an Inconel 718 component which was created at a 45° angle to the printing surface and documented by Snyder et al. [1]. A third rough surface was created for the RIFT investigation using a structured-light scan of a surface similar to the Inconel 718 downskin surface, but a different scaling was used to provide larger roughness elements in the RIFT. The resulting roughness dimensions (Rq/Dh) of the three surfaces used were 0.0064, 0.0156, and 0.0405. The friction coefficients were measured over the range of 10,000 < ReDh < 70,000 for each surface opposed by a smooth wall and opposed by each of the other rough walls. At multiple ReDh values, x-array hot film anemometry was used to characterize the velocity and turbulence profiles for each roughness combination. The friction factor variations for each rough wall opposed by a smooth wall approached complete turbulence. However, when rough surfaces were opposed, the surfaces did not reach complete turbulence over the Reynolds number range investigated. The results of inner variable analysis demonstrate that the roughness function (ΔU+) becomes independent of the roughness condition of the opposing wall providing evidence that Townsend’s Hypothesis holds for the relative roughness values expected for additively manufactured turbine-blade cooling passages.
A Strategy to Tune Acoustic Terminations of Single-Can Test-Rigs to Mimic Thermoacoustic Behavior of a Full Engine
Authors: Haeringer Matthias , Fournier Guillaume J. J. , Meindl Max , Polifke Wolfgang
Published: January 2021

Thermoacoustic properties of can-annular combustors are commonly investigated by means of single-can test-rigs. To obtain representative results, it is crucial to mimic can-can coupling present in the full engine. However, current approaches either lack a solid theoretical foundation or are not practicable for high-pressure rigs. In the present study we employ Bloch-wave theory to derive reflection coefficients that correctly represent can-can coupling. We propose a strategy to impose such reflection coefficients at the acoustic terminations of a single-can test-rig by installing passive acoustic elements, namely straight ducts or Helmholtz resonators. In an iterative process, these elements are adapted to match the reflection coefficients for the dominant frequencies of the full engine. The strategy is demonstrated with a network model of a generic can-annular combustor and a 3D model of a realistic can-annular combustor configuration. For the latter we show that can-can coupling via the compressor exit plenum is negligible for frequencies sufficiently far away from plenum eigenfrequencies. Without utilizing previous knowledge of relevant frequencies or flame dynamics, the test-rig models are adapted within a few iterations and match the full engine with good accuracy. Using Helmholtz resonators for test-rig adaption turns out to be more viable than using straight ducts.
An Approach to Model a Lossless Junction for Fluid Network Calculations in Turbomachinery
Authors: Khandrymailov Andrii , Moroz Leonid , Yevlakhov Viktor , Staple Shanel , Vogel Gregory
Published: January 2021

Turbine secondary flow system calculations are usually performed by utilizing the thermal-fluid network approach. The network consists of branches and nodes. Fluid branches usually describe the flow resistance of the correspondent fluid path section, while fluid nodes are used to connect fluid branches between each other. Fluid flow in branches and nodes is described by the set of conservation equations such as mass and momentum equations. For fluid nodes, usually total or static pressure is used as a variable in these equations. However, such an approach may yield a system of equations that cannot be solved (for theoretical cases where zero resistance branches are present) or may produce significant differences in results compared to the more precise CFD solution (for real cases with resistances). This paper provides an approach on how to create a model of a fluid lossless junction in order to solve the mentioned problems. Such a junction enables the connection of any number of inflow and outflow fluid branches without bringing any flow resistance, which allows fluid branches to handle flow resistance influence by themselves.

The proposed model is based on the idea, that each node should contain not just one pressure (static or total) as a variable, but two variables — both static and total pressures. Such a node can be used to model junctions of different types with flow mixing, as well as separation and sudden cross-sectional area changes. With this approach it is also possible to model chambers by modifying just one equation. Also, this new method can be applied for both compressible and incompressible calculation types and can handle chocked flows as well.
Advanced Automated GT26 Combustion Tuning for Increased Operational Flexibility and Performance
Authors: Singla G. , Delatti B. , Scarpato A. , Barracas J. , Wang W. , Demougeot N.
Published: January 2021

Today’s advanced Gas Turbine (GT) combustion systems are designed to achieve low emission limits over a wide operational range. Regular combustion tuning is usually required to ensure that emissions are kept well within the plant compliance limits and that combustion dynamics levels are low enough to meet the mechanical integrity targets of all hot gas parts. These targets must be met during the full lifetime of the power plant, regardless of ambient conditions, fuel composition or engine degradation. Considering also the recent increase of combustor inspection intervals and the change in the GT loading profile due to the intermittent nature of renewable energies, it became advantageous to develop an automated GT26 combustion tuning control.

Ansaldo Energia self-learning AutoTune system, based on the well proven product developed by Power Systems Manufacturing (PSM), tunes engine operating parameters to maintain emission and combustion dynamics within specified limits under varying ambient conditions, load ranges, gas compositions and increasing engine degradation. It increases the robustness of the operation concept by introducing dynamic set points instead of fixed settings. This results in tuning flexibility, continuous engine optimization and it helps to potentially eliminate manual engine adjustments during a major inspection cycle.

After a general introduction on the operating concept of the GT26, the interaction between AutoTune and the GT controller is described together with the set-up and tuning principles. The universal AutoTune algorithm was adapted to the unique GT26 reheat architecture and flexibility, allowing for: (i) GT26 EV and SEV tuning for emissions and pulsations during stable conditions, (ii) tuning to minimize the minimum environmental load (MEL) with changing fuel gas properties or ambient temperature with the Low Part Load (LPL) concept, (iii) handling of very reactive fuels with high C2+ concentrations, (iv) tuning during transient conditions accounting for the variation of the GT thermal state. Based on robust experience after five successful GT26 installations, tuning attempts and successful tunings are statistically analyzed on a commercial plant over a period of several weeks to demonstrate the enlarged operational flexibility and performance of the GT26 equipped with AutoTune. The data analysis focuses on the Low Part Load operation, where Autotune achieved a robust decrease of MEL of 20MW, corresponding to a reduction of 4% in terms of combined cycle relative load.
Transonic Axial Compressors Loss Correlations: Part II — Loss Models Validation
Authors: Manfredi Marco , Babin Cedric , Fontaneto Fabrizio
Published: January 2021

The quest for greener, more efficient aircraft engines is the main driver for the development of innovative compression system designs. Reduced order design tools rely nevertheless on semi-empirical loss models, whose validity range is often not net or in general not verified. The present work aims at defining a set of loss correlations, which could readily be employed in the analysis and design process of modern transonic axial compressors. In Part I, various loss correlations were deeply described and, in some cases, updated to enhance both their generality and their prediction capability. In Part II, the effectiveness of both original and updated models will be tested for one specific low aspect ratio axial compressor stage. Experimental and numerical data will be used at such extent.
Numerical Investigation of 100% Premixed Hydrogen Combustor at Gas Turbines Conditions Using Detailed Chemistry
Authors: Menon Sachin , Bouten Thijs , Withag Jan , Klein Sikke , Gangoli Rao Arvind
Published: January 2021

The combustion properties of hydrogen make premixed hydrogen-air flames prone to flashback. Several combustor concepts have been proposed and studied in the past few years to tackle the problem of flame flashback in premixed high hydrogen fuel combustors. This study looks at one of the concepts which uses the Aerodynamically Trapped Vortex to stabilize the flame. Burner concepts based on trapped vortex flame stabilization have a higher resistance towards flame blowout than conventional swirl stabilized burners. This work looks at the flow and flame behavior in the proposed Aerodynamically Trapped Vortex Combustor for 100% premixed hydrogen operation. Numerical simulations for the analysis were performed with the commercial CFD simulation package AVL FIRE™. The flow field characterization was focused on the investigation of the influence of both the inlet velocity and inlet turbulence intensity on the mean velocity, wall velocity gradient and turbulence intensity in the combustor. To study the flame stabilization mechanism, reactive simulations were performed at two fuel equivalence ratios. The combustion regime of the flame, turbulent flame speed and temperature distribution in the combustor were quantified from the simulation results.

Combustion is modelled using a detailed chemistry solver with the k–ε turbulence model to resolve turbulence. No additional turbulence-chemistry interaction model is used in the current research. To reduce chemistry computational time, the multi-zone method is employed. To capture the effect of preferential diffusion, two approaches were used to quantify the diffusion coefficient of each species. The diffusion coefficients were calculated using both mixture averaged approach and the multi component diffusion approach. The proposed design for the Aero-dynamically Trapped Vortex combustor was able to stabilize a 100% premixed hydrogen flame without flashback for the simulated conditions.
Uncertainty Quantification and Missing Data for Turbomachinery With Probabilistic Equivalence and Arbitrary Polynomial Chaos, Applied to Scroll Compressors
Authors: Pepper Nick , Montomoli Francesco , Giacomel Francesco , Cavazzini Giovanna , Pinelli Michele , Casari Nicola , Sharma Sanjiv
Published: January 2021

This work presents a framework for predicting unknown input distributions for turbomachinery applications starting from scarce experimental measurements. The problem is relevant to turbomachinery where important parameters are obtained using indirect measurements. In this paper a scroll compressor is used as example but the suggested framework is completely general and can be used to infer missing data on material composition (carbon fiber properties, laser melted specimens for additive manufacturing etc) or input data (such as the turbine inlet temperature).

Scroll compressors are small devices with a very complex geometry that is difficult to measure. Moreover these compressors are highly sensitive to manufacturing errors and clearances. For these reasons we have chosen this example as an ideal candidate to prove the effectiveness of the framework. An input probability distribution for the scroll height is recovered based on a scarce, synthetic data set. The scroll height is used as an example of a missing distribution for a geometric parameter as it has the highest variance and is challenging to measure experimentally.

The framework consists of two main building blocks: an equivalence in a probabilistic sense and a Non-Intrusive Polynomial Chaos formulation able to deal with scarce data. The probabilistic equivalence is defined by a Probability Density Function (PDF) matching approach in which the statistical distance between probability distributions is quantified by either the Kolmogorov-Smirnov (KS) distance or the Kullback-Leibler (KL) divergence. By representing the missing inputs with a generalised Polynomial Chaos Expansion (gPCE) the back-calculation problem can be recast as an optimisation problem in which an arbitrary Polynomial Chaos (aPC) formulation was used to propagate the uncertain input distributions through a computational model of the system and generate a probability distribution for the Quantity of Interest (QoI).

The framework has been tested with multiple non-Askey scheme distributions to prove the generality of the proposed approach.
Fast Model Predictive Control for Aircraft Engine Based on Automatic Differentiation
Authors: Wang Yunlai , Wang Xi
Published: January 2021

Nonlinear model predictive control (NMPC) is a strategy suitable for dealing with highly complex, nonlinear, uncertain, and constrained dynamics involved in aircraft engine control problems. Because of the complexity of the algorithm and the real-time performance of the predictive model, it has thus far been infeasible to implement model predictive control in the realtime control system of aircraft engine. In most nonlinear model predictive control, nonlinear interior point methods (IPM) are used to calculate the optimal solution, which iterate to the optimal solution based on the Jacobian and Hessian matrix. Most nonlinear IPM solver, such as MATLAB fmincon and IPOPT, cannot calculate the Jacobian and Hessian matrix precisely and quickly, instead of using numerical differentiation to calculate the Jacobian matrix and BFGS method to approach to the Hessian matrix. From what has been discussed above, we will 1) improve the real-time performance of predictive model by replacing the time-consuming component level model (CLM) with a neural network model, which is trained based on the data of component level model, 2) precisely calculate the Jacobian and Hessian matrix using automatic differentiation, and propose a group of algorithms to make NMPC strategy quicker, which include making use of the structure of predictive model, and the integrity of weighted sums of Hessian matrix in IPM. Finally, considering input and output constraints, the fast NMPC strategy is compared with normal NMPC. Simulation results with mean time of 19.3% – 27.9% of normal NMPC on different platforms, verify that the fast NMPC proposed can improve the real-time performance during the process of acceleration, deceleration for aircraft engine.
Study on Optimization Design and Flow Control Mechanism of Little Blades in a Compressor Cascade
Authors: Guo Zhengtao , Chu Wuli , Chen Xiangyi
Published: January 2021

In view of the characteristics of flow separation in the compressor cascade corner region, a new flow control method for installing little blades in the front of the cascade passage was proposed, which took into account the flow control advantages of end wall fences and vortex generators. Firstly, the little blades could hinder the cross flow on the end wall and the development of the horseshoe vortex pressure surface branch. Secondly, the little blades could generate induced vortices to take away the low-energy fluid near the end wall and the corner region.

Based on numerical simulations, the effects of different pitchwise positions, stagger angles and heights of the little blades on the aerodynamic performance of the cascade were studied, and the optimal little blades were obtained by NSGA-II using EBF neural network as the agent model. The results show that the little blades have the optimal pitchwise position, stagger angle and height range for improving the aerodynamic performance of the cascade. When the optimized little blades are introduced in the baseline cascade, the stable working range of the cascade is expanded, and the stall type of the cascade changes from the hub-corner stall to the overload of flow separation near the mid-span. At the near stall attack angle of the baseline, the total pressure loss coefficient is reduced by about 10.38% and the static pressure coefficient is increased by about 4.31%. Meanwhile, the loss of the lower span is decreased and the diffuser capacity of the whole span is improved. The passage secondary loss and wake loss are reduced because of the delay of corner separation. Moreover, the strength of the end wall vortex is weakened and the end wall vortex no longer develops as part of the passage vortex. The induced vortex, horseshoe vortex pressure surface branch and initial passage vortex develop into new passage vortex.
Uncertainty Quantification of Aero-Thermal Performance of a Blade Endwall Considering Slot Geometry Deviation and Mainstream Fluctuation
Authors: Tao Zhi , Guo Zhendong , Song Liming , Li Jun
Published: January 2021

With the continuous increase of aerodynamic and thermal load, the endwall of modern gas turbines has become the critical region affected by the uncertainties in the manufacturing and operation process and thus is very likely to suffer performance degradation and thermal failure. Therefore, it is critical to understand and quantify the impacts of uncertainty factors on endwall aero-thermal performance. Based on Kriging surrogate, the frameworks of uncertainty quantification and global sensitivity analysis are constructed for a gas turbine blade endwall. The impacts of slot geometry deviations (slot width, endwall misalignment) and mainstream condition fluctuations (turbulence intensity, inlet flow angle) on endwall aero-thermal performance are quantified and analyzed. Results show that the actual performance of the endwall has a high probability of deviating from its nominal value. With respect to the nominal values, the maximum deviations of aerodynamic losses, averaged film cooling effectiveness and averaged Nusselt number reach up to 0.33%, 45% and 5.0%, respectively. The critical regions which are most sensitive to the input uncertainty parameters are identified. Furthermore, the inlet flow angle is proved to be the most significant parameter affecting the endwall aero-thermal performance through sensitivity analysis. The influence mechanisms of the inlet flow angle on endwall aero-thermal performance are clarified by detailed flow and thermal field analysis. Results show that the inlet flow angle significantly alters the size and strength of the secondary flow structures, resulting in a large variation of endwall aero-thermal performance. Quantitatively, a positive incidence angle of 2 degrees can lead to a 0.1% reduction of total pressure coefficient, a 12% reduction of averaged film cooling effectiveness and a 2% enhancement of averaged Nusselt number.
A Three-Dimensional Unsteady Through-Flow Model for Rotating Stall in Axial Compressors
Authors: Guo Jin , Hu Jun , Wang Xuegao , Xu Rong
Published: January 2021

Rotating stall is a natural limit to the stable operating range of compressors due to the inverse pressure gradient of viscous gas. Effective prediction of compressor stall boundary is an important guarantee for the successful development of aeroengine. In this paper, a three-dimensional unsteady through-flow model based on body force theory is developed to reflect the dynamic stall process of multistage axial compressors with acceptable computational costs. The influence of blade geometric parameters is fully considered in blade force source terms. The source terms are related to the attack angle and Mach number of the blade inlet using the deviation angle and loss model in the through-flow theory. Meanwhile, the temporal lag response of the source terms to the upstream flow conditions is taken into account. Therefore, it can be utilized for predicting the off-design performance and rotating stall characteristics of multistage axial compressors. The developed model is validated on a two-stage low-speed axial compressor. The calculated performance line and stall cell speed are in agreement with the experimental results. The unsteady flow behavior of the compressor during stall is presented by the model. The results indicate that the developed model has the potential to be applied to the preliminary evaluation of compressor stability in design stage.
Effect of Inlet Mach Number on Aerothermodynamics of Sealing Leakage Flow Cooling on Endwall
Authors: Gao Hongyu , Li Xueying , Ren Jing , Jiang Hongde
Published: January 2021

The gas turbine has an assembly gap between the combustion chamber and the first stage vane. The coolant air discharge from the gap can prevent the ingestion of the high temperature gas. This leakage flow also provides a cooling coverage on the vane endwall. Taking the cooling effect of the leakage flow on the endwall into consideration is very important for an efficient cooling design.

In this paper, the cooling effect of leakage flow on endwall is studied by means of experimental and numerical methods. The study included slots at 30°, 45°, and 60° angles, and six blowing ratios of 0.3, 0.6, 1.0, 1.4, 1.7, and 2.0. The experiment and numerical calculation are conducted under the condition that the inlet Mach number is 0.125 and the outlet Mach number is 0.72, which is close to the working Mach number of the real gas turbine.

Under the same slot inclination and blowing ratio, the distribution of endwall adiabatic cooling effectiveness is more nonuniform under the condition of near-real engine Mach number. This is because the passage vortex is weaker under the low Mach number condition, and the leakage flow has a better wall attachment effect. In terms of the spanwise average of endwall adiabatic cooling effectiveness, when the blowing ratio is small, the adiabatic cooling effectiveness is lower under the condition of near-real engine Mach number than that under the condition of low Mach number, but the opposite is true under the condition of large blowing ratio. This is because under the condition of large blowing ratio, the turbulence is stronger under the condition of Mach number of near-real engine. With the reduction of blowing ratio, the turbulent kinetic energy weakens more strongly. In the studied cases, there is a critical blowing ratio of 1.0, and the total endwall cooling adiabatic cooling effectiveness is not significantly affected by the Mach number when it is smaller than M1.0. The average adiabatic cooling effectiveness of the endwall under the condition of near-real engine Mach number is about 7% lower than that under the condition of low Mach number.

It means that the experimental results of leakage flow cooling obtained under the condition of low inlet Mach number need to be corrected by a correction factor, which may be less than 1 to make it engine relevant.
A Degradation Diagnosis Method for Gas Turbine – Fuel Cell Hybrid Systems Using Bayesian Networks
Authors: Mantelli L. , Zaccaria V. , Kyprianidis K. , Ferrari M. L.
Published: January 2021

During the last decades there has been a rise of awareness regarding the necessity to increase energy systems efficiency and reduce carbon emissions. These goals could be partially achieved through a greater use of gas turbine - solid oxide fuel cell hybrid systems to generate both electric power and heat. However, this kind of systems are known to be delicate, especially due to the fragility of the cell, which could be permanently damaged if its temperature and pressure levels exceed their operative limits. This could be caused by degradation of a component in the system (e.g. the turbomachinery), but also by some sensor fault which leads to a wrong control action. To be considered commercially competitive, these systems must guarantee high reliability and their maintenance costs must be minimized. Thus, it is necessary to integrate these plants with an automated diagnosis system capable to detect degradation levels of the many components (e.g. turbomachinery and fuel cell stack) in order to plan properly the maintenance operations, and also to recognize a sensor fault. This task can be very challenging due to the high complexity of the system and the interactions between its components. Another difficulty is related to the lack of sensors, which is common on commercial power plants, and makes harder the identification of faults in the system.

This paper aims to develop and test Bayesian belief network based diagnosis methods, which can be used to predict the most likely degradation levels of turbine, compressor and fuel cell in a hybrid system on the basis of different sensors measurements. The capability of the diagnosis systems to understand if an abnormal measurement is caused by a component degradation or by a sensor fault is also investigated. The data used both to train and to test the networks is generated from a deterministic model and later modified to consider noise or bias in the sensors.

The application of Bayesian belief networks to fuel cell - gas turbine hybrid systems is novel, thus the results obtained from this analysis could be a significant starting point to understand their potential. The diagnosis systems developed for this work provide essential information regarding levels of degradation and presence of faults in gas turbine, fuel cell and sensors in a fuel cell – gas turbine hybrid system. The Bayesian belief networks proved to have a good level of accuracy for all the scenarios considered, regarding both steady state and transient operations. This analysis also suggests that in the future a Bayesian belief network could be integrated with the control system to achieve safer and more efficient operations of these plants.
Study on the Influence of Film Cooling on the Flow Field and Infrared Radiation Characteristics in the Divergent Section of a Nozzle
Authors: Yang Haoqi , Yang Qingzhen , Zhang Saile , Yang Huicheng , He Yubo
Published: January 2021

As the last part of the convergent divergent nozzle, the divergent section is exposed to high temperature and high-speed airflow and thus, it is more easily to be detected by the infrared detector. It is one of the main sources of the infrared radiation in the exhaust system. Film cooling is applied to protect the wall from hot flow and reduce the infrared radiation.

In this paper, the study is conducted on a nozzle with spherical convergence flap in a turbofan engine exhaust system. The effect of film cooling on the internal flow and infrared radiation characteristics of the exhaust system in the divergent section was studied by numerical simulations. The k-ω SST turbulence model was used to simulate the flow field, and the Reverse Monte Carlo Method was employed to calculate the infrared radiation characteristics of the nozzle. Four different kinds of film hole arrangements are involved, they are cylindrical film holes in an in-line pattern, cylindrical film holes in a staggered pattern, converging-expanding film holes in an in-line pattern and converging-expanding film holes in a staggered pattern. The cylindrical film hole and the converging-expanding film hole have a round shape inlet, with an equivalent diameter of d = 5mm on the projection surface perpendicular to the axial direction. Angles between each film hole and the wall surface are 35°. The impact of the heat conduction on the wall was taken into account. The results show that with the given mass flow rate of the coolant, the lengths of the high temperature core zone of the four models with different film cooling structures are slightly shorter than the core zone of the model without cooling structures. However, no significant difference can be found for the length of the core zone of the four models. The average temperature of the wall in the divergent section decreases significantly by using film cooling. No significant difference can be found in the wall temperature distribution for the four models. In the 3∼5μm and 8∼14μm bands, the cooling technique barely affects the infrared radiation of the main exhaust jet flow, while it significantly reduces the infrared radiation of the solid wall in the divergent section, and the decreasing amplitude is from 45% to 51%. Different film hole arrangements result in similar effects on the infrared radiation of the nozzle.

Overall, the usage of film cooling in the divergent section of the nozzle effectively reduces the averaged wall temperature and substantially suppresses the solid infrared radiation on the wall. However, the shape and arrangement of the film holes have no significant influence on the infrared radiation intensity and temperature of the wall in the divergent section.
A Study of Two Variable Cycle Engine Concepts for High Speed Civil Aircraft
Authors: Zhang Jiyuan , Chen Min , Tang Hailong , Liu Xin
Published: January 2021

High speed civil aircraft has become a promising field with the development of globalization. The propulsion system is an indispensable part of the aircraft. Conventional engines have difficulty meeting the performance requirement of the high-speed civil aircraft.

In this article, two variable cycle engines were studied to preliminarily as aircraft propulsion system. Their performance and matching mechanism were analyzed and compared with each other. Firstly, the cycle parameters design was conducted to explain the principle of cycle parameter determination for the high-speed civil aircraft. Secondly, the control law of variable geometry components was studied to optimize engine performance during supersonic cruising. Finally, the throttling process with constant airflow was studied to solve the problem of thrust surplus during subsonic cruising.

According to this study, given same cycle parameters, the engine with variable fan stage can produce equal or slightly higher thrust with slightly less fuel consumption than the engine with core-driven-fan stage. The engine with core-driven-fan stage has advantages in aero-dynamical stabilities. It can also throttle to slightly lower thrust level during subsonic cruising. Considering the advantages in performance and derived development comprehensively, the engine with variable fan stage is a better option for high speed civil aircraft.
Analysis of Friction-Saturated Flutter Vibrations With a Fully-Coupled Frequency Domain Method
Authors: Berthold Christian , Gross Johann , Frey Christian , Krack Malte
Published: January 2021

Flutter stability is a dominant design constraint of modern gas and steam turbines. Thus, flutter-tolerant designs are currently explored, where the resulting vibrations remain within acceptable bounds. In particular, friction damping has the potential to yield Limit Cycle Oscillations (LCOs) in the presence of a flutter instability. To predict such LCOs, it is the current practice to model the aerodynamic forces in terms of aerodynamic influence coefficients, derived for some normal modes of the linearized structural model and fixed oscillation frequency. However, this approach neglects that both the nonlinear contact interactions and the aerodynamic stiffness cause a change in the deflection shape and the frequency of the LCO. This, in turn, may have a substantial effect on the aerodynamic damping. The goal of this paper is to assess the technical importance of these neglected interactions. To this end, a state-of-the-art aero-elastic model of a low pressure turbine blade row is considered, undergoing nonlinear frictional contact interactions in the tip shroud interfaces. The LCOs are computed with a fully-coupled harmonic balance method, which iteratively computes the Fourier coefficients of structural deformation and conservative flow variables, as well as the a priori unknown frequency. The coupled algorithm was tested for various combinations of harmonics in both domains and found to provide excellent computational robustness and efficiency. Moreover, a refinement of the conventional energy method is developed and assessed, which accounts for both the nonlinear contact boundary conditions and the linearized aerodynamic influence. It is found that the conventional energy method may not predict a limit cycle oscillation at all while the novel approach presented here can. Furthermore the refined energy method provides deep understanding of the nonlinear aero-elastic interactions.
Stress Analysis of Fan Blades Subjected to Hail Impact
Authors: Sun Xin , Liu Bo , Han Chang-fu
Published: January 2021

The rapid development of modern aircraft technology is increasingly demanding aero-engines. It not only requires aeroengines to ignite and take off under suitable conditions, but also faces various severe weather and foreign object impact tests during flight. The stress variation caused by the impact of an aero-engine fan blade is studied. Because the compressor guide vane has a small solidity, the hail may not only hit the vane when striking the blade, but also may hit the first-stage rotor blade. The effects of hail impact on the vanes and rotor blades, especially the impact on the blade strength, were studied. Firstly, the UG software is used for geometric modeling based on the existing vane and rotor geometry data. Secondly, the ANSYS/LSDYNA software is used to simulate the process when the hail hits different positions of the blade and rotor at different speeds. To explore the influence of hail impact on the blade. It is found that when the hail hits the guide vane, the maximum effective stress of the vane does not exceed the yield stress of the material. When the hail hits the rotor, the maximum effective stress of the rotor is closely related to the rotor speed, the hail incident velocity, and the relative position of the rotor and the hail. When the impact is severe, the maximum effective stress will exceed the yield stress of the material, and even the blade may be bent and pitted, which has an important impact on the strength and life of the blade.
The Influence of Mach Number on the Boundary Layer Development of High-Lift Low Pressure Turbine in Low Reynolds Number
Authors: Duan Wenhua , Liu Jian , Qiao Weiyang
Published: January 2021

A numerical analysis of the effect of Mach number on the boundary layer development and aerodynamic performance of a high-lift, after loaded low pressure turbine blade is presented in this paper. The turbine blade is designed for the GTF engine and works in a low Reynolds number, high Mach number environment. Three different isentropic exit Mach numbers (0.14, 0.87 and 1.17) are simulated by large eddy simulation method, while the Reynolds number based on the axial chord length of the blade and the exit flow velocity is kept the same (1 × 105). The condition Mais,2 = 0.14 represents the lowspeeed wind tunnel environment which is usually used in the low pressure turbine investigation. The condition Mais,2 = 0.87 represents the design point of the turbine blade. The condition Mais,2 = 1.17 represents the severe environment when the shock wave shows up. A comparison of the boundary layer development is made and the total pressure loss results from the boundary layer is discussed.