bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Freestream Turbulence Intensity, Turbulence Length Scale, and Exit Reynolds Number on Vane Endwall Secondary Flow and Heat Transfer in a Transonic Turbine Cascade
Authors: Li Zhigang , Bai Bo , Liu Luxuan , Li Jun , Mao Shuo , Ng Wing , Xu Hongzhou , Fox Michael
Published: January 2021

In gas turbine engines, the first-stage vanes usually suffer harsh incoming flow conditions from the combustor with high pressure, high temperature and high turbulence. The combustor-generated high freestream turbulence and strong secondary flows in a gas turbine vane passage have been reported to augment the endwall thermal load significantly.

This paper presents a detailed numerical study on the effects of high freestream turbulence intensity, turbulence length scale, and exit Reynolds number on the endwall secondary flow pattern and heat transfer distribution of a transonic linear turbine vane passage at realistic engine Mach numbers, with a flat endwall no cooling. Numerical simulations were conducted at a range of different operation conditions: six freestream turbulence intensities (Tu = 1%, 5%, 10%, 13%, 16% and 20%), six turbulence length scales (normalized by the vane pitch of Λ/P = 0.01, 0.04, 0.07, 0.12, 0.24, 0.36), and three exit isentropic Mach number (Maex = 0.6, 0.85 and 1.02 corresponding exit Reynolds number Reex = 1.1 × 106, 1.7 × 106 and 2.2 × 106, respectively, based on the vane chord). Detailed comparisons were presented for endwall heat transfer coefficient distribution, endwall secondary flow field at different operation conditions, while paying special attention to the link between endwall thermal load patterns and the secondary flow structures.

Results show that the freestream turbulence intensity and length scale have a significant influence on the endwall secondary flow field, but the influence of the exit Reynolds number is very weak. The Nusselt number patterns for the higher turbulence intensities (Tu = 16%, 20%) appear to be less affected by the endwall secondary flows than the lower turbulence cases. The thermal load distribution in the arc region around the vane leading edge and the banded region along the vane pressure side are influenced most strongly by the freestream turbulence intensity. In general, the higher freestream turbulence intensities make the vane endwall thermal load more uniform. The Nusselt number distribution is only weakly affected by the turbulence length scale when Λ/P is larger than 0.04. The heat transfer level appears to have a significant uniform augmentation over the whole endwall region with the increasing Maex. The endwall thermal load distribution is classified into four typical regions, and the effects of freestream turbulence, exit Reynolds number in each region were discussed in detail.
Numerical Study on Film Effectiveness and Heat Transfer Characteristics of Aero-Engine Nose Cone With Hot Air Film
Authors: Yang Tao , Zhang Li , Zhu Hui-Ren , Wang Xing-Ming
Published: January 2021

When the aircraft works in an environment containing supercooled water droplets, it is easy to cause the engine inlet nose cone on the windward side to freeze, which not only affects the performance of the engine, but also leads to flight accidents. Therefore, it is necessary to research the anti-icing technology of aero-engine nose cone components. At present, the air intake cone of the aircraft mainly forms a thermal anti-icing system by means of hot film anti-icing and heating impingement anti-icing.

In this paper, the effects of blowing ratio, film hole pitch, hole shape and film hole arrangement method on film heating effectiveness and heat transfer characteristics are studied by numerical simulation methods for the hot film anti-icing system of the nose cone. The results show that with the increase of the blowing ratio, the film heating effectiveness in the downstream area quickly decreases first and then increases, and the range of change aggrandizes. Under the given condition, as the pitch between the film holes decreases, the area covered by the film extends, and the heating effectiveness improves significantly. In the case of high blowing ratio, the advantage of film heating effectiveness of waist-shaped film hole is more apparent. The laterally-averaged film heating effectiveness of the staggered film holes is much higher than that of the aligned film holes. For the characteristics of film heat transfer, the hole pitch and hole shape has little effect on the heat transfer characteristics at low blowing ratio. In the case of high blowing ratio, the heat transfer effect will be greatly weakened when the hole pitch is increased, moreover the heat transfer efficiency of the waist-shaped hole is better. In the region near the exit of the secondary row film hole, the heat transfer characteristics of the staggered structure is stronger than that in the aligned structure. In addition, it is found that the high heat transfer region for staggered arrangement shows W–shaped.
Reynolds Number and Roughness Effects on Turbocompressor Performance: Numerical Calculations and Measurement Data Evaluation
Authors: Dietmann Fabian , Casey Michael , Vogt Damian M.
Published: January 2021

Further validation of an analytic method to calculate the influence of changes in Reynolds number, machine size and roughness on the performance of axial and radial turbocompressors is presented. The correlation uses a dissipation coefficient as a basis for scaling the losses with changes in relative roughness and Reynolds number. The original correlation from Dietmann and Casey [6] is based on experimental data and theoretical models. Evaluations of five numerically calculated compressor stages at different flow coefficients are presented to support the trends of the correlation. It is shown that the sensitivity of the compressor performance to Reynolds and roughness effects is highest for low flow coefficient radial stages and steadily decreases as the design flow coefficient of the stage and the hydraulic diameter of the flow channels increases.
Introduction of a Novel Parameter Model to Analyze the Geometric Variation of Airfoil Edges of Ex-In-Service Compressor Airfoils
Authors: Voigt Paul , Voigt Matthias , Mailach Ronald , Abu-Taa Kimon , Rostamian Mirco
Published: January 2021

The leading and trailing edge shapes of high pressure compressor (HPC) airfoils have a high impact on their aerodynamic performance. Typically, the shape of these airfoil edges are designed symmetrically as half circles or half ellipses. Due to manufacturing scatter and operational effects, the edges of real HPC airfoils deviate from the design intent. Especially the change in shape of the airfoil edges due to erosion can lead to a reduction of the aerodynamic efficiency and operability range.

This paper introduces a novel and intuitive parametrization approach to describe the occurring airfoil edge shapes. The introduced parametric model has been applied to a set of postservice airfoils with a variety of airfoil edge shapes. For this purpose, just above 1000 of used HPC airfoils from different engines with different operational conditions and runtimes were digitized by means of a structured light 3D scanner. The high precision of the scanning system enables accurate capturing of the sensitive airfoil edge shapes.

Subsequently, the extracted parameters were used for a statistical evaluation of the leading edge (LE) shape geometry. It turned out that the LEs of the investigated post-service compressor airfoils exhibit a thinner LE shape characteristics compared to the design intent LE shape. Furthermore, in nearly all cases, more material is missing on the suction sided LE of the investigated airfoils. This result is rather contrary compared to the findings of relevant literature, where the LE pressure side is missing more material due to erosion.

Nevertheless, the generated results suggest that the developed method is suitable for a sufficient qualitative and quantitative description of HPC LE shapes. In future work, the introduced parametric model can be used as the basis of a prospective evaluation of the correlation between the geometric variation of airfoil edges and manufacturing scatter as well as engine operational condition.
Unsteady Steam Turbine Optimization Using High Fidelity CFD
Authors: Iranidokht Vahid , Papagiannis Ilias , Kalfas Anestis , Abhari Reza , Senoo Shigeki , Momma Kazuhiro
Published: January 2021

This paper presents the computational methodology, and experimental investigations accomplished to enhance the efficiency of a turbine stage by applying non-axisymmetric profiling on the rotor hub wall. The experimental setup was a two-stage axial turbine, which was tested at “LISA” test facility at ETH Zurich. The 1st stage was considered to create the flow history for the 2nd stage, which was the target of the optimization. The hub cavity of the 2nd stage was designed with large dimensions as a requirement of a steam turbine. The goal was to optimize the interaction of the cavity leakage flow with the rotor passage flow to reduce the losses and increase efficiency.

The computational optimization was completed using a Genetic Algorithm coupled with an Artificial Neural Network on the 2nd stage of the test turbine. Unsteady time-accurate simulations were performed, using in-house developed “MULTI3” solver. Besides implementing all geometrical details (such as hub and tip cavities and fully 3D blade geometries) from the experimental setup into the computational model, it was learned that the unsteady upstream effect could not be neglected. A novel approach was introduced by using unsteady inlet boundary conditions to consider the multistage effect while reducing the computational cost to half. The importance of this implementation was tested by performing a steady simulation on the optimized geometry. The predicted efficiency gain from steady simulations was 4.5 times smaller (and negligible) compared to the unsteady approach. Excluding the cavity geometry was also assessed in a different simulation setup showing 3.9% over-prediction in the absolute efficiency value.

Comprehensive steady and unsteady measurements were performed utilizing pneumatic, Fast Response Aerodynamic (FRAP), and Fast Response Entropy (FENT) probes, on the baseline and profiled test cases. The end-wall profiling was found to be successful in weakening the strength of the hub passage vortex by a 19% reduction in the under-over turning. As a result, the blockage was reduced near the hub region leading to more uniform mass flow distribution along the span. The flow angle deviations at the higher span position were also corrected due to better control of the flow angles. Furthermore, the improvements were confirmed by reductions in entropy, Secondary Kinetic Energy, and pressure unsteadiness. The accurate computational implementations led to an excellent agreement between the predicted and measured efficiency gain.
A Comparison of VOF and Euler-Euler Approaches in CFD Modelling of Two-Phase Flows With a Sharp Interface
Authors: Bertolotti Luc , Jefferson-Loveday Richard , Ambrose Stephen , Korsukova Evgenia
Published: January 2021

In aero-engines, it is important to predict the behavior of shear flows in the different parts such as bearing chambers or gearboxes. In bearing chambers, the thickness distribution of wavy films is well studied as two-phase flows are still very hard to predict depending on the case. Experimental studies remain very expensive to carry out and Computational Fluid Dynamics (CFD) still struggles with two-phase flow prediction especially when a sharp interface between the two phases must be modelled. CFD is used to predict the oil film thickness distribution and interface velocity at different engine operating conditions. Currently Reynold-Averaged Navier-Stokes (RANS) CFD uses a semi-empirical method of turbulence damping, which is inaccurate for wavy films and so impacts the modelling of bearing chambers and gearboxes. With the objective of improving RANS models from Large Eddy Simulation (LES) methods, the Volume of Fluid (VOF) and Euler-Euler methods for two-phase flow modelling are investigated in this study. The Volume of Fluid (VOF) approach assumes a single set of momentum equations for the two phases and volume fractions are 1 or 0 everywhere except in the interface region. An alternative to VOF, is the Euler-Euler method with interface sharpening for shear flows. This approach assumes one set of momentum equations per phase but a shared field of pressure. The VOF and Euler-Euler approaches are compared in this study using LES with the CFD code OpenFOAM v6. The case study is based on experimental work investigating stratified flow in a horizontal channel that will be further detailed in the paper. In this study, a simplified 3D periodic channel filled with two distinct phases: air and water is used. A flow regime is studied in which flows are fully developed and the water phase has a much smaller velocity than the air phase in order to obtain a shear flow. Numerical results are compared with experimental measurements from the literature. With OpenFOAM, the VOF solver used for the study is interFoam and the Euler-Euler solver used is reacting-MultiphaseEulerFoam. Velocity profiles, shear-stress profiles and kinetic energy profiles are compared with experimental measurements for the assessment of the two flow solvers. Maps of vorticity magnitude are also provided to support the comparisons between the Euler-Euler and the VOF approaches as well as an appropriate vortex identification method.
Transonic Axial Compressors Loss Correlations: Part I — Analysis and Update of Loss Models
Authors: Manfredi Marco , 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, the main entropy generation mechanisms are described together with a review of the most commonly employed modelling approaches. Selected loss models are then deeper investigated and updated to increase both their range of validity and the accuracy of their predictions. In Part II, the effectiveness of the investigated models will be tested for one specific low aspect ratio axial compressor stage.
A Multi-Timescale Approach for the Prediction of Temperatures in a Gas Turbine Combustor Liner
Authors: Karalus Megan , Brandt Dustin , Brown Alistair , Lister Vincent
Published: January 2021

The desire for increased engine efficiencies is driving higher firing temperatures. But this increases the risk for hot spots in solid components which can lead to durability issues. These may not be discovered until late in the design process through expensive and time consuming thermal paint tests. Historically, conjugate heat transfer simulations to predict solid temperatures have been done with steady RANS. However, Large Eddy Simulation (LES) is now being used in the early design process for gas turbine engines to account for the multiphysics of reacting flows. Incorporating the solid into these simulations poses a new challenge: the physical response time of the solid components can be orders of magnitude larger than the reacting gas phase. Running a fully coupled unsteady conjugate heat transfer analysis is therefore not tractable, but the high fidelity of the LES reacting solution is still desired.

The objective of this paper is to demonstrate a multi-timescale simulation approach for conjugate heat transfer (CHT) in Simcenter STAR-CCM+ 2019.3. The combustor is solved using LES, including all relevant physics, while steady state conduction is determined in the metal liner and thermal barrier coating. Time averaged boundary conditions are transferred from the combustor to the solids, and temperature is returned through multiple exchanges until the solid temperatures reach a stable solution. A simplified case is used to verify the approach, and then results from a test combustor are compared against data. The investigation compares results obtained with PISO and SIMPLE numerical schemes.
Nonlinear Vibration by Asynchronous Excitation Force in Friction Damper of Turbine Blade
Authors: Umehara Ryuichi , Shiraishi Haruko , Onozato Naoki , Shimmyo Tetsuya
Published: January 2021

Turbine blades are now being used under increasingly severe conditions in order to increase the thermal efficiency of gas turbines. Friction dampers are often used to reduce the vibration of the blade and improve the plant reliability. This is a general study dealing with resonance passing where the natural frequency of the turbine blade coincides with the frequency of specific harmonic excitation forces while increasing the turbine rotation speed. Asynchronous components of excitation forces are also considered in addition to the synchronous components caused by specific harmonic excitation forces. In this study, a new method for predicting the characteristics of nonlinear vibration under excitation force including both synchronous and asynchronous force components is developed. In order to investigate the effect of additional asynchronous loading, time history response analyses considering nonlinear vibration using simulated turbine blades were conducted. Results showed that friction damper slip can be induced by the presence of the additional asynchronous excitation force components even for low values of synchronous excitation force. It is shown that it is possible to use a calibration factor to predict vibration characteristics considering friction slipping by estimating the ratio of the total excitation force to the single harmonic excitation force. To verify the effect of asynchronous excitation force and the validity of the proposed correction method, verification tests were conducted experimentally. The experimental results show that friction slipping occurred under small harmonic excitation force when there was asynchronous excitation force and show good agreement with the numerical results. Moreover, the validity of the proposed method which corrects the dynamic characteristics obtained using of the first order harmonic balance method is confirmed.
Aeroelastic Stability of Axial Compressor Blades Under Different Operating Conditions
Authors: Fang Mingchang , Wang Yanrong
Published: January 2021

Flutter is one of the important issues in turbomachinery analysis. There are four common types of flutter, including sub/transonic stall flutter, choke flutter, supersonic stall flutter, and supersonic non-stall flutter. Flutter may occur under many different operating conditions. Therefore, it is important to study the aeroelastic stability of blades when the compressor operates under different conditions.

Based on the energy method proposed by Carta [1], this paper studied the aeroelastic stability of the second-stage rotor blade of an axial compressor under different operating conditions. It is found that the aerodynamic damping of the blade under the near-stall operating point of the compressor is negative. Three typical operating points are selected to study the differences in flutter mechanism between different operating conditions.

The 90% span section is selected as the reference section to analyze the variation of the aerodynamic work at different operating points. The influence of reduced frequency, modal component, and tip clearance on aerodynamic damping are analyzed under three operating points.
Performance of a Turbine Rim Seal Subject to Rotationally-Driven and Pressure-Driven Ingestion
Authors: Bru Revert Anna , Beard Paul F. , Chew John W. , Bottenheim Sebastiaan
Published: January 2021

This experimental study considered the performance of a chute rim seal downstream of turbine inlet guide vanes (but without rotor blades). The experimental set up reproduced rotationally-driven ingestion without vanes and conditions of pressure-driven ingestion with vanes. The maximum rotor speed was 9000 rpm corresponding to a rotational Reynolds number of 3.3 × 106 with a flow coefficient of 0.45. Measurements of mean pressures in the annulus and the disc rim cavity as well as values of sealing effectiveness deduced from gas concentration data are presented. At high values of flow coefficient (low rotational speeds), the circumferential pressure variation generated by the vanes drove relatively high levels of ingestion into the disc rim cavity. For a given purge flow rate, increasing the disc rotational speed led to a reduction in ingestion, shown by higher values of sealing effectiveness, despite the presence of upstream vanes. At Uax/(Ωb) = 0.45, the sealing effectiveness approached that associated with purely rotationally-driven ingestion. A map of sealing effectiveness against non-dimensional purge flow summarises the results and illustrates the combined rotational and pressure-driven effects on the ingestion mechanism. The results imply that flow coefficient is an important parameter in rim sealing and that rotational effects are important in many applications, especially turbines with low flow coefficient.
Full Annulus Analysis of Inlet Distortion Effects on the Performance of a Compressor
Authors: Cao Dongming , Wang Dingxi , Huang Xiuquan
Published: January 2021

The paper presents a numerical investigation of inlet distortion effects on the performance of a whole compressor. The compressor system consists of three and a half stages: a fan stage, an axial stage, a mixed-flow stage and an outlet guide vane. There are two bypasses after the fan stator. The following two and a half stages are in the core flow. The inlet distortion is generated by blocking part of the upstream inlet duct using a plate. The inlet distortion breaks up the original axisymmetric flow feature, thus a whole annulus computational domain has to be employed throughout the numerical unsteady analyses. The commercial GPU-based software of Turbostream is used to conduct all the analyses on several GPU workstations. The purpose of the numerical analysis is to understand how the inlet distortion propagates within this compressor and how the performance of each stage and the whole compressor is affected by the inlet distortion qualitatively and quantitatively. To achieve this goal, two separate numerical analyses have been performed for the compressor with and without inlet distortion at an interested operation point. For these two simulations, we do DFT analysis for the total pressure distribution at three spans of several different axial positions, such as the inlet and outlet of each stage, to get the spatial harmonic characteristic of each stage. The finding provides the insight needed to redesign the relevant stage to increase the surge margin.
An Experimental and Modelling Strategy for Obtaining Complete Characteristic Maps of Dual-Volute Radial Inflow Turbines
Authors: Serrano José Ramón , Arnau Martínez Francisco José , García-Cuevas Luis Miguel , Samala Vishnu , Guilain Stephane , Batard Samuel
Published: January 2021

Despite the importance of turbocharged engines with radial inflow dual-volute turbines, their characteristic maps and fully predictive modelling using 1D gas dynamic codes are not well established yet. The complexity of the unsteady flow and the unequal admission of these turbines, when operating with pulses of engine exhaust gas, makes them a challenging system. This is mainly due to the unequal flow admission, which generates an additional degree of freedom with respect to well-known single entry vanned or vaneless turbines. This paper has as a main novelty a simple procedure for characterizing experimentally and elaborating characteristic maps of these turbines with unequal flow conditions. This method of analysis allows for easy interpolation within the proposed characteristic maps or conceiving simple models for calculating and extrapolating full performance parameters of dual-volute turbines.

Here, also described are two innovative 0D mean-line models that require a minimum quantity of experimental data for calibrating both: the mass flow parameter model and the isentropic efficiency model. Both models are predictive either in partial or unequal flow admission conditions using as inputs: the mass flow ratio between branches; the total temperature ratio between branches; the blade to jet speed ratio in each branch and the pressure ratio in each branch. These six inputs are generally instantaneously provided by 1D gas-dynamics codes. Therefore, the novelty of the model is its ability to be used in a quasi-steady way for dual-volute turbines performance prediction. This can be done instantaneously when turbines are calculated operating at turbocharged engines under pulsating and unequal flow conditions.
Reynolds Stress Field and Turbulent Kinetic Energy Budget in a Repeating Compressor Stage
Authors: Nawab Ahmad , Wang Feng , di Mare Luca , Adamczyk John J.
Published: January 2021

Turbulence modelling in compressor passages continues to be a challenging problem. In order to better understand the shortcomings of turbulence modelling, a LES and a RANS computation were performed of a repeating compressor stage. The computation was carried out near the aerodynamic design point of the compressor stage, in order to minimise the challenge posed to the turbulence model.

The use of a repeating stage configuration removes the need to specify the statistics of the incoming turbulent field; the statistics become an output of the simulation and not an input. This is a critical fact that greatly increases the credibility of the current LES compressor simulation over many previous simulations. As the computations are performed at mid-span, radial gradients can safely be assumed to be small, thus removing issues associated with capturing flow features attributed to 3D geometry. The flow field is assumed to be incompressible, which is required in order to achieve a true repeating stage environment.

The RANS computation is based on a state-of-the-art turbulence model. At the same flow coefficient, the RANS simulation yielded a total pressure rise very near that of the LES simulation. However, there are nontrivial differences in the flow details. The mean flow and Reynolds shear stress boundary layer profiles are in good agreement in regions of favourable pressure gradient, but significant differences exist in the presence of adverse pressure-gradients. The turbulent kinetic energy profiles however are in poor agreement throughout the flow.

The mean flow production rates predicted by the RANS computation are largely similar to those of the LES simulation forward of mid-chord where the pressure gradient is favourable. A notable exception is the leading-edge region where the LES predicts negative production i.e. a net transfer of energy to the time-mean flow, and the region aft of mid-chord where the pressure gradient is adverse.

Outside of the viscous sub-layer, the dissipation rates are also predicted correctly by the RANS simulation forward of midchord where the pressure gradient is favourable. Aft of mid-chord however, there are significant differences in the dissipation rates.
Evaluation of a Machine Learning Turbulence Model in a Square Transverse Jet in Crossflow
Authors: Costa Fabíola Paula , Díaz Rubén Bruno , Milani Pedro M. , Takachi Tomita Jesuíno , Bringhenti Cleverson
Published: January 2021

Film cooling is an important technique to ensure safe operation and performance fulfillment of turbines. Its ultimate goal is to protect the axial turbine blades from high gas temperatures. An appropriate study is necessary in order to obtain a reliable representation of the flow characteristics involved in such phenomena. Because of the high computational cost of high-fidelity simulations, the low-fidelity simulation method Reynolds Averaged Navier Stokes (RANS) is commonly used in practical configurations. However, the majority of the current turbulent heat flux models fail to accurately predict heat transfer in film cooling flows. Recent work suggests the use of machine learning models to improve turbulent closure in these flows. In the present work, a machine learning model for spatially varying turbulent Prandtl number previously described in the literature is applied to a transverse film cooling flow consisting of a jet square channel. The results obtained in the present work were compared to adiabatic effectiveness experimental data available in the literature to assess the performance of the machine learning model. The results shown that for low blowing ratios (BR = 0.2 and BR = 0.4) the proposed machine learning model has poor performance. However, for the case with the highest blowing ratio (BR = 0.8), the proposed model presented better results. These results are then explained in terms of the resulting turbulent Prandtl number field and suggest that the training set is not appropriate for capturing the turbulent heat flux in fully attached jets in crossflow.
Stress and Modal Analysis of a Rotating Blade and the Effects of Nonlocality
Authors: Tufekci M. , Rendu Q. , Yuan J. , Dear J. P. , Salles L. , Cherednichenko A. V.
Published: January 2021

This study focuses on the quasi-static stress and modal analyses of a rotor blade by using classical and nonlocal elasticity approaches. The finite element method with an additional numerical integration process is used to evaluate the integral equation of nonlocal contionuum mechanics. The blade is assumed to be made of a linear elastic material of weak nonlocal characteristic. Such materials can be composites, metallic foams, nanophased alloys etc. A full-scale fan blade model is chosen as the test case to represent the rotor blade for a modern high bypass ratio turbofan engine. The boundary conditions and loads are chosen based on the steady-state cruising operating conditions of such blades. The nonlocal stresses are calculated by processing the calculated local stresses. To calculate the nonlocal stresses, the integral form of nonlocal elasticity is employed in the discretised domain. The results of the two cases are compared and discussed.
Data-Driven Combustion Modeling for a Turbulent Flame Simulated With a Computationally Efficient Solver
Authors: Talei Mohsen , Ma Man-Ching , Sandberg Richard
Published: January 2021

The use of machine learning (ML) for modeling is on the rise. In the age of big data, this technique has shown great potential to describe complex physical phenomena in the form of models. More recently, ML has frequently been used for turbulence modeling while the use of this technique for combustion modeling is still emerging. Gene expression programming (GEP) is one class of ML that can be used as a tool for symbolic regression and thus improve existing algebraic models using high-fidelity data. Direct numerical simulation (DNS) is a powerful candidate for producing the required data for training GEP models and validation. This paper therefore presents a highly efficient DNS solver known as HiPSTAR, originally developed for simulating non-reacting flows in particular in the context of turbo-machinery. This solver has been extended to simulate reacting flows. DNSs of two turbulent premixed jet flames with different Karlovitz numbers are performed to produce the required data for training. GEP is then used to develop algebraic flame surface density models in the context of large-eddy simulation (LES). The result of this work introduces new models which show excellent performance in prediction of the flame surface density for premixed flames featuring different Karlovitz numbers.
Large Eddy Simulation of Film Cooling for the Real Additive Manufactured Fan-Shaped Holes
Authors: Shi Wei , Li Xueying , Wang Lang , Ren Jing , Jiang Hongde
Published: January 2021

Additive Manufacturing (AM) is a process for making complex parts that were once difficult to machine using traditional manufacturing processes such as forging, casting, and welding. As a new and promising processing technology, AM is being increasingly applied to the manufacturing of high temperature turbine parts. However, before the widespread application of AM can become feasible, the influence of such processes on the performance of turbine hot ends — especially during the film cooling flow heat transfer — requires further study.

This paper focuses a large eddy simulation study done in order to understand the physical phenomena involved in the random roughness caused by the AM of fan-shaped film holes. This paper proposes a set of workflows to connect the AM, CFD simulation, Computed Tomography (CT) and reverse modeling, so that the effect of AM on the flow and heat transfer of film cooling can be studied.

The results of this preliminary workflow reveal several observations. First, that the film cooling effectiveness (η) of AM fan-shaped holes decreases. The area averaged η of the ideal hole is 0.32, while the area averaged cooling effectiveness of the AM hole is 0.29. As such, the η of the AM fan-shaped hole has a significant bifurcation phenomenon. This is because the separation bubble in-tube moves forward, and blocks the flow channel, which bifrucates the flow in-tube. Second, a pressure gradient towards the trailing edge generated at a random rough surface near the leading edge squeezes the fluid. The combined effect of these two mechanisms causes the fluid to flow out of the air film pores mainly from the leading edge with a smaller lateral expansion.
LES Study of the Effects of Oscillations in the Main Flow on Film Cooling
Authors: Baek Seung Il , Ahn Joon
Published: January 2021

The effects of sinusoidal oscillations in the main flow on film cooling in the gas turbine were investigated by Large Eddy Simulation (LES). The film cooling flow fields for the sinusoidal oscillation of 32 Hz in the mainstream from a cylindrical hole inclined by 35° to a flat plate at average blowing ratio of M = 0.5 were numerically simulated. The LES results were compared to the experimental data from Seo, Lee and Ligrani (1998), Jung, Lee and Ligrani (2001) and Reynolds-Averaged Navier-Stokes (RANS) results. The experimental results showed that if the oscillation frequency in the main flow was increased, the adiabatic film cooling effectiveness was decreased. The credibility of the LES results relative to the experimental data was demonstrated by the comparison of time-averaged adiabatic film cooling effectiveness, time and phase-averaged temperature contours, contours of Q-criterion, time-averaged velocity profiles, and time and phase-averaged Urms profiles with the RANS results. The adiabatic film cooling effectiveness by LES model showed a good match to the experimental data, while RANS results highly over-predict the centerline effectiveness. Also, the LES results showed more consistent with the experimental data for the time-averaged and phase-averaged temperature contours, time-averaged velocity profiles and time and phase-averaged Urms profiles than the RANS results. RANS did not predict the peak generated by the jet penetration exactly and Urms profiles obtained by RANS approach was much smaller compared to the experimental results. Paper will discuss these results in detail.
Bayesian Machine Learning for the Prognosis of Combustion Instabilities From Noise
Authors: Sengupta Ushnish , Rasmussen Carl E. , Juniper Matthew P.
Published: January 2021

Experiments are performed on a turbulent swirling flame placed inside a vertical tube whose fundamental acoustic mode becomes unstable at higher powers and equivalence ratios. The power, equivalence ratio, fuel composition and boundary condition of this tube are varied and, at each operating point, the combustion noise is recorded. In addition, short acoustic pulses at the fundamental frequency are supplied to the tube with a loudspeaker and the decay rates of subsequent acoustic oscillations are measured. This quantifies the linear stability of the system at every operating point. Using this data for training, we show that it is possible for a Bayesian ensemble of neural networks to predict the decay rate from a 300 millisecond sample of the (un-pulsed) combustion noise and therefore forecast impending thermoacoustic instabilities. We also show that it is possible to recover the equivalence ratio and power of the flame from these noise snippets, confirming our hypothesis that combustion noise indeed provides a fingerprint of the combustor’s internal state. Furthermore, the Bayesian nature of our algorithm enables principled estimates of uncertainty in our predictions, a reassuring feature that prevents it from making overconfident extrapolations. We use the techniques of permutation importance and integrated gradients to understand which features in the combustion noise spectra are crucial for accurate predictions and how they might influence the prediction. This study serves as a first step towards establishing interpretable and Bayesian machine learning techniques as tools to discover informative relationships in combustor data and thereby build trustworthy, robust and reliable combustion diagnostics.
A Joint Experimental and LES Characterization of the Liquid Fuel Spray in a Swirl Injector
Authors: Vignat Guillaume , Rajendram Soundararajan Preethi , Durox Daniel , Vié Aymeric , Renaud Antoine , Candel Sébastien
Published: January 2021

The quality of liquid fuel spray injection determines to a large extent the steady state performance and dynamics of gas turbine and aero-engine combustors. The present investigation is focused on the detailed characterization of the liquid fuel spray in a single sector test rig targeted at aero-engine applications. The liquid fuel (heptane) is injected in a hollow cone spray pattern by a simplex atomizer and the injector comprises a radial swirler. Two features of the droplet distribution that are less commonly found in the technical literature are identified. First, the distributions of mean droplet diameters exhibit non-axisymmetric patterns, a lack of symmetry that is investigated for three types of swirlers differing by their swirl number and/or head loss. Second, it is found that the size-conditioned velocity distributions feature a single wide peak for small droplets and become bimodal for the largest droplets, with a first peak at low velocities, and a second one at higher velocities. The spray behavior analysis is complemented by making use of Large Eddy Simulations with Lagrangian Particle Tracking. Droplet injection is achieved with a model in which the initial size and velocity distributions are specified from experimental data in the atomizer near field. The initial spray interacts with the lateral injector surface and requires a droplet-wall interaction model accounting for the existence of a liquid film. Simulations do not retrieve the lack of rotational symmetry that is found experimentally indicating that this is not linked to the nature of the swirling flow. This is also consistent with further experiments with a different atomizer confirming that this is due to imperfections in the initial atomizer geometry. Another result is that certain swirler designs appear to be more robust to these atomizer imperfections. Simulations accounting for the liquid film yield a bimodal distribution for the droplets’ axial velocity distribution which would not be obtained without this model indicating that it is important to represent the droplet-wall interaction, a feature that is not commonly found in the literature.
On a New Nonlinear Reduced-Order Model for Capturing Internal Resonances in Intentionally Mistuned Cyclic Structures
Authors: Quaegebeur Samuel , Chouvion Benjamin , Thouverez Fabrice , Berthe Loïc
Published: January 2021

Cyclic structures such as turbomachinery present material and geometrical variations between sectors. These discrepancies are called mistuning and break the cyclic symmetry of the structure. Computing the forced response of mistuned cyclic structures is thus a numerical challenge. The Component Nonlinear Complex Mode Synthesis (CNCMS) is one of the few nonlinear reduced-order model formulations that allow to compute the nonlinear response of tuned and mistuned structures. It has been validated successfully for friction problems. However, in the presence of geometric nonlinearities, internal resonances may arise and they cannot be captured correctly with the CNCMS method. The purpose of this work is therefore to present a new methodology for developing a nonlinear reduced-order model that can successfully capture internal resonances for tuned and mistuned structures. This method, called Component Mode Synthesis with Nonlinear Re-evaluation (CMSNR), is based on a variation of the CNCMS approach. The final modal synthesis uses a multi-harmonic procedure and a re-evaluation of the nonlinear forces on each sector independently. The performance and limitations of the proposed approach are assessed using a simplified example of a blisk subject to polynomial nonlinearities. Different internal resonances are exhibited and studied depending on the type of excitation force and on the level of mistuning.
Nonlinear Vibration Analysis of Turbine Bladed Disks With Mid-Span Dampers
Authors: Ferhatoglu Erhan , Zucca Stefano , Botto Daniele , Auciello Jury , Arcangeli Lorenzo
Published: January 2021

Friction dampers are one of the most common structures used to alleviate excessive vibration amplitudes in turbomachinery applications. There are very well-known types of contact elements exploited efficiently, such as underplatform dampers. However, different design approach is sometimes needed to maximize the effectiveness further. In this paper, computational forced response prediction of bladed disks with a configuration of the secondary structure commonly used by Baker Hughes design, the so-called mid-span dampers, is presented. Mid-span dampers are metal devices positioned at the middle section of the airfoil span and come into contact with the blade by the centrifugal force acting during rotation. Proposed damping mechanism is applied to a realistic steam turbine bladed disk under cyclic symmetric boundary conditions. Friction contact is modeled through a large number of contact nodes between the blade and the damper by using a 2D friction contact element with variable normal load. Harmonic Balance Method and Alternating Frequency/Time approach are utilized to obtain nonlinear algebraic equations in frequency domain and nonlinear forced response is computed by using Newton-Raphson method. The results obtained by numerical simulations show that mid-span dampers are an efficient configuration type of a damping mechanism to be used in the design of the bladed disks for nonlinear vibration analysis.
Micro-, Macromechanical and Aeroelastic Investigation of Glass-Fiber Based, Lightweight Turbomachinery Components
Authors: Iseni Senad , Prasad Mahesh Ramaswamy Guru , Hartmaier Alexander , Holeczek Klaudiusz , Modeler Niels , di Mare Francesca
Published: January 2021

A major technical challenge for modern aero engines is the development of designs which reduce noise and emission whilst increasing aerodynamic efficiency and ensuring aeroelastic stability of low-temperature engine components such as fans and low-pressure compressors. Composites are used in aviation due to their excellent stiffness and strength properties, which also enable additional flexibility in the design process. The weight reduction of the turbomachine components, due to composite materials and lighter engines, is especially relevant for the design and developments of hybrid-electric or distributed propulsion systems [1].

To accomplish this, a representative volume element (RVE) of a glass-fiber reinforced polymer is created, describing the geometrical arrangement of the textile reinforcement structure within the polymer matrix. For both phases, realistic linear elastic properties are assumed. This RVE will be investigated with the finite element method under various loading conditions to assess its anisotropic elastic properties and also its damping behaviour for elastic waves. To study the influence of delamination on the mechanical properties, small defects will be introduced into the model at the interface between reinforcement and matrix. Based on this micromechanical approach, a constitutive model for the composite will be formulated that describes the anisotropic properties as well as the damping behaviour. This constitutive model is then used to describe the material response in a macro-mechanical model, which serves as the basis for an aeroelastic analysis of a 1/3-scaled high-speed fan using a conventional (Ti-6Al-4V) and fiber composite material.
On the Development of a Synchronized Harmonic Balance Method for Multiple Frequencies and its Application to LPT Flows
Authors: Crespo Javier , Contreras Jesús
Published: January 2021

The aim of this paper is to describe the development and application of a multi-frequency harmonic balance solver for GPUs, particularly suitable for the simulation of periodic unsteadiness in nonlinear turbomachinery flows comprised of a few dominant frequencies, with an unsteady multistage coupling that bolsters the flow continuity across the rotor/stator interface.

The formulation is addressed with the time-domain reinterpretation, where several non-equidistant time instants conveniently selected are solved simultaneously. The set of required frequencies in each row is driven into the governing equations with the help of almost-periodic Fourier transforms for time derivatives and time shifted boundary conditions. The spatial repetitiveness inside each row can be exploited to perform single-passage simulations and the relative circumferential positioning of the rotors or stators and the different blade or vane counts is tackled by means of adding fictitious frequencies referring to non-adjacent rows therefore taking into account clocking and indexing effects.

Existing multistage row coupling techniques of harmonic methods rely on the use of non-reflecting boundary conditions, based on linearizations, or time interpolation, which may lead to Runge phenomenon with the resulting numerical instabilities and non-preserving flux exchange. Different sets of time instants might be selected in each row but the interpolation in space and time across their interfaces gives rise to robustness issues due to this phenomenon. The so-called synchronized approach, developed in this work, consist of having the same time instances among the whole ensemble of rows, ensuring that flux transfer at sliding planes is applied more robustly. The combination of a set of shared non-equidistant time instances plus the use of unequal frequencies (real and fictitious) may spoil the Fourier transforms conditioning but this can be dramatically improved with the help of oversampling and instants selection optimization. The resulting multistage coupling naturally addresses typical numerical issues such as flow that might reverse locally across the row interfaces by means of not using boundary conditions but a local flux conservation scheme in the sliding planes.

Some examples will be given to illustrate the ability of this new approach to preserve accuracy and robustness while resolving them. A brief analysis of results for a fan stage and a LPT multi-row case is presented to demonstrate the correctness of the method, assessing the impact in the modeling accuracy of the present approach compared with a time-domain conventional analysis. Regarding the computational performance, the speedup compared to a full annulus time-domain unsteady simulation is a factor of order 30 combining the use of single-passage rows and time spectral accuracy.