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Yu Xia

ANSYS, Applications Team, Faculty Member

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I am a Senior R&D Engineer at Ansys UK Ltd. I am also a Chartered Engineer (CEng) and full Member of Institute of Mechanical Engineers (MIMechE). I received my PhD degree at the Department of Mechanical Engineering, Imperial College London. Before this I did my Master of Research (MRes) with Distinction on Fluid Dynamics (2014-15), and Master of Science (MSc) with Distinction on Advanced Computational Methods (2013-14), both at Imperial College London, and Bachelor of Power Engineering (Hon) at Nanjing University of Aeronautics and Astronautics (NUAA), from 2009-13. For more details please visit my Personal Website: https://sites.google.com/view/steven-xia
Supervisors: Aimee S Morgans, William P Jones, and Patrick Sharkey

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Zubairu Abubakar

Edwin Van Der Weide

Technische Universität Darmstadt

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Videos by Yu Xia

[Video Presentation] GT2021-59100 Dynamic Mesh Adaption For Scale-Resolving Reacting Flow Simulations

A dynamic adaptive mesh refinement method is used in conjunction with a hybrid scale-resolving tu... more

4 views

[Video Presentation] GT2022-80733 Numerical Simulations of a Lifted hydrogen Jet Flame Using Flamelet Generated Manifold Approach

https://www.researchgate.net/publication/360065122_Video_Presentation_GT2022-80733_Numerical_Simu... more

2 views

[Video Presentation] Numerical Prediction of Pressurized Hydrogen Leakage and Spontaneous Ignition

The current work focuses on numerically predicting the auto-ignition patterns of suddenly release... more The current work focuses on numerically predicting the auto-ignition patterns of suddenly released hydrogen from a pressurized tank into the atmosphere. The latest density-based CFD solver is utilized to simulate the 3D turbulent structures during the mixing of the fast-propagating hydrogen with the ambient air, using Large Eddy Simulation for turbulence and Finite Rate Chemistry for combustion. The mesh is automatically refined to capture the turbulent eddies, shock waves and thin reaction zones. The numerical calculations are performed at three tank pressures (250 bar, 100 bar and 50 bar) within two sector domains (1 degree and 30 degree).
The turbulent diffusion and auto-ignition patterns of the leaked hydrogen are correctly predicted across different operating conditions.

15 views

[Video Presentation] GT2020-14717 SBES/FGM Simulation of Film-Cooled Surface Heat Transfer and Near-Wall Reaction

Accurate numerical prediction of surface heat transfer in the presence of film cooling within aer... more Accurate numerical prediction of surface heat transfer in the presence of film cooling within aero-engine sub-components, such as blade effusion holes and combustor liners, has long been a goal of the aero-engine industry. It requires accurate simulation of the turbulent mixing and reaction processes between freestream and the cooling flow. In this study, the Stress Blended Eddy Simulation (SBES) turbulence model is used together with the Flamelet Generated Manifold (FGM) combustion model to calculate the surface heat flux upstream and downstream of an effusion cooling hole.

59 views

[Video Presentation] GT2022-78296 Prediction of Thermoacoustic Instability and Fluid-Structure Interactions for Gas Turbine Combustor

https://www.researchgate.net/publication/357186379_GT2022-78296_Prediction_of_Thermoacoustic_Inst... more

2 views

Papers by Yu Xia

Analysis of Entropy Production Mechanisms in Turbulent Diffusion Flames

AIAA SciTech Forum and Exposition 2025, 2025

Exergy based analysis for thermodynamic systems has become increasingly important in understandin... more Exergy based analysis for thermodynamic systems has become increasingly important in understanding the efficiency of energy-work conversion. Entropy generation, associated with the irreversibility in practical processes, decreases the available useful work, and the mechanisms responsible for the production differ significantly between premixed and nonpremixed combustion. In addition, unsteady combustion generates temperature fluctuations and when accelerated produces entropy waves – referred to as indirect noise. Prior research by the authors have considered the dynamics of entropy generation purely due to heat release for both premixed and non-premixed flames. However, for non-premixed flames where the diffusion-controlled processes are important, transport terms must be considered for consistency. In this work, the entropy production mechanisms due to different sources are analyzed, firstly, by doing a mathematical study on the entropy transport equation and deriving the key forms of the source terms. Then reacting flow simulations are performed for the Sandia D flame, a well-documented configuration, for the numerical analysis of the transport equation. A range of operating conditions have been considered to illustrate the relative importance of different entropy source terms. Overall entropy production rate budget is calculated, and trends are observed for different operating conditions. In addition, across different sections of the flame, local entropy production is analyzed. Subsequently, reacting flow simulations are performed for the classical Burke-Schumann flame, a laminar nonpremixed flame, and the same analysis is carried out to completely gauge the trends of entropy production in non-premixed flames.

[GTP-24-1550] Large Eddy Simulation of Ammonia-Hydrogen Non-premixed Flames Stabilized on a Bluff Body Burner

Journal of Engineering for Gas Turbines and Power, 2024

The demand for carbon-free fuels such as NH3 and H2 has been growing due to stricter environmenta... more The demand for carbon-free fuels such as NH3 and H2 has been growing due to stricter environmental policies on carbon emissions. Since ammonia is easier to store and transport, it is considered an alternative fuel for marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emissions of NOx are important challenges that need to be overcome if ammonia is to be used as a practical fuel for industrial use. Recent works have provided experimental data on the characteristics of NH3 flames under premixed and non-premixed combustion, focusing on flame speed, turbulence-chemistry interaction, and especially the dissociation/cracking of NH3 into H2 and N2. These measurements have encouraged the use of CFD for the simulation and scaling of practical ammonia systems, evaluating the performance of different numerical models for NH3 combustion. In this study, LES is utilized to predict a series of NH3/H2/N2 non-premixed flames stabilized on a bluff body burner. The accuracy of the FGM combustion model combined with LES has been examined, using a commercial CFD solver. The different cracking levels of NH3 into H2 and N2 lead to simulation cases ranging from an H2/N2 flame (i.e., “fully cracked”), to a flame with 50% NH3 (i.e., “half cracked”). The different flame shapes, temperature distributions, and NOx emissions have been simulated, generally matching well experimental data. The developed numerical settings and workflows may be applied to simulating more complex combustors which use NH3 as a fuel.

[GTP-24-1228] Adjoint-Based Optimization for the Venturi Mixer of A Burner

Journal of Engineering for Gas Turbines and Power, 2024

The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and... more The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and minimizing emissions. In this study, we utilize the adjoint method to analyze and refine the design of a Venturi mixer. Our numerical simulations integrate the species transport equation with the eddy dissipation model (EDM) for reacting flow and the generalized k-x (GEKO) model to simulate turbulence. By solving adjoint equations, we effectively compute the shape sensitivity for various observables, including pressure drop, outlet fuel variance/ uniformity deviation index, air and fuel mass flow rates, and outlet CO mass fraction. The shape sensitivity analysis uncovers the interplay between the observables and the appropriate weights for multiple objective optimizations. Subsequently, we perform gradient-based optimizations to enhance the mixer's performance, employing both shape sensitivity and mesh morphing techniques. We conduct a series of case studies focusing on both cold and reacting flows. The optimization of cold flow provides an in-depth exploration of various optimization strategies, encompassing single-objective and multi-objective optimization with diverse weight combinations. Following this, the optimization under reacting flow conditions improves the fuel/air mixing, leading to the increase of combustion efficiency and hence the reduction of CO emissions. Our findings showcase the potential of an adjoint-based optimization framework in designing Venturi mixers that are efficient and emit lower levels of pollutants.

CFD Modeling of High-Enthalpy Hypersonic Flows for FIRE-II Re-Entry Vehicle

AIAA Aviation Forum, 2024

Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. W... more Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. While accurate physical modeling and numerical prediction of hypersonic flows are crucial for the design and analysis of this class of vehicles, the extreme conditions make this class of numerical simulations challenging to converge to a stable and steady solution. The authors' previous study utilized a "two-temperature model" to simulate the hypersonic flow and thermal features of the FIRE-II re-entry vehicle. This previous work demonstrated basic trends for surface heat flux, temperature field, ionization of air, etc., at five operating points along the vehicle's re-entry trajectory. The present work is a higher-fidelity investigation and validation performed on the FIRE II vehicle. In particular, the work aims at comparing the effects on the thermal environment of (1) two different chemical reaction mechanisms, and of (2) two wall catalysis models, full and partial-catalytic wall (PCW). Both chemical mechanisms, the Park and the Gupta models, account for the recombination of the ionized species and have been proven to predict reasonably well the heating trends on the surface of the capsule. The results show that the use of the catalytic wall model improves the prediction of surface heat flux on the vehicle across all the operating points as compared to previous studies. While trends in heat flux are captured well for different altitudes, the absolute values and the match with the experiment still present some discrepancies. These discrepancies are in large part attributable to the presence of a large uncertainty in the instantaneous flight conditions, in the assumption of steady-state conditions at each altitude as well as unknown catalytic conditions of the capsule's thermal protection system (TPS). The study demonstrates

GT2024-125957 LARGE EDDY SIMULATION OF AMMONIA-HYDROGEN NON-PREMIXED FLAMES STABILIZED ON A BLUFF BODY BURNER

Turbo Expo 2024 Turbomachinery Technical Conference & Exposition, 2024

The demand for carbon-free fuels such as ammonia (NH3) and hydrogen (H2) has been growing rapidly... more The demand for carbon-free fuels such as ammonia (NH3) and hydrogen (H2) has been growing rapidly in the past decade due to stricter environmental policies being applied. The storage and transport of NH3 in the liquid form, at a relatively low tank pressure, are comparatively easier than those of H2. As a result, NH3 is considered as an alternative H2-carrier fuel. Since NH3 is a more economical and eco-friendly fuel (compared with hydrocarbon fuels such as natural gas), it has become a potential energy provider for the marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emission of nitrogen oxides (NOx) during its combustion have prevented NH3 from being a practical fuel for industrial combustors. Recent experiments have investigated the fundamental characteristics of NH3 flames under premixed and non-premixed combustion modes, focusing on flame speed, turbulence-chemistry interaction, and the impact of dissociation/cracking of NH3 into H2 and N2. Various computational fluid dynamics (CFD) simulations have also been performed, where a series of CFD combustion models have been evaluated and validated against experimental data. The validated numerical models and workflows can then be used to decrease the number of experiments needed and eventually to reduce the total costs of designing NH3-fueled burners. To predict the highly turbulent flame behavior, scale-resolving models such as Large Eddy Simulation (LES) have been proven to be more accurate than the Reynolds-Averaged Navier-Stokes (RANS) approach. In this work, the performance of the Flamelet Generated Manifold (FGM) combustion model in combination with the LES turbulence model has been evaluated using commercial CFD software to predict bluff-body stabilized NH3/H2 non-premixed jet flames. The experimental data were obtained by Alfazazi et al. at King Abdullah University of Science and Technology (KAUST) (Combustion and Flame, Volume 258, Part 2, 2023, 113066. The different cracking levels of NH3 into H2 and N2 result in four different NH3/H2 blending scenarios, ranging from a pure H2 flame (i.e., "fully cracked") to a flame with 72% volume of NH3. The flame shapes, temperature distribution, and NO and unburnt NH3 emissions have all been simulated, which generally match well the available experimental data and are found to be more accurate than previous RANS predictions. The numerical settings and workflow developed in this study may be applied to more complex configurations and contribute to a wider deployment of NH3 as a practical fuel for multiple industries.

GT2024-124337 Adjoint Solver Based Shape Optimization for a Venturi Mixer

Turbo Expo 2024 Turbomachinery Technical Conference & Exposition, 2024

The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and... more The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and minimizing emissions. In this study, we utilize the adjoint method to analyze and refine the design of a Venturi mixer. Our numerical simulations integrate the species transport equation with the Eddy Dissipation Model (EDM) for reacting flow and the generalized k-ω (GEKO) model to simulate turbulence. By solving adjoint equations, we effectively compute the shape sensitivity for various observables, including pressure drop, outlet fuel variance/uniformity deviation index, air and fuel mass flow rates, and outlet CO mass fraction. The shape sensitivity analysis uncovers the interplay between the observables and the appropriate weights for multiple objective optimizations. Subsequently, we perform gradient-based optimizations to enhance the mixer's performance, employing both shape sensitivity and mesh morphing techniques. We conduct a series of case studies focusing on both cold and reacting flows. The optimization of cold flow provides an in-depth exploration of various optimization strategies, encompassing single-objective and multi-objective optimization with diverse weight combinations. Following this, the optimization under reacting flow conditions improves the fuel/air mixing, leading to the increase of combustion efficiency and hence the reduction of CO emissions. Our findings showcase the potential of an adjoint-based optimization framework in designing Venturi mixers that are efficient and emit lower levels of pollutants.

Numerical Simulation of Liquid Film Evaporation and Condensation for Water Reactors

The 9th World Congress on Momentum, Heat and Mass Transfer (MHMT 2024), 2024

A detailed analysis of liquid film evaporation and condensation has become increasingly important... more A detailed analysis of liquid film evaporation and condensation has become increasingly important, especially in water reactor designs. For example, the prediction of liquid film evaporation is critical to the design of Passive Containment Cooling Systems (PCCS), which are important passive safety facilities in modern Pressurized Water Reactors (PWR). To keep the maximum design values of pressure and temperature under control, a water/air counter-current flow is often employed within the external channel of the PCCS. It is thus essential to analyse the evaporation mass flow rate and heat transfer of the water film for the overall performance of PCCS. On the other hand, hydrogen might be released into the reactor containment during an incident due to cladding oxidation at elevated temperatures. Given the high flammability of gaseous hydrogen, it is paramount to know its local distribution in the containment building while considering wall condensation, which will increase the non-condensable gas concentration on the walls. Another example is evaluating the containment loads in the Light Water Reactor (LWR) plants and predicting their condensation behaviour in the presence of non-condensable gases. With the advantages of Computational Fluid Dynamics (CFD) and recent model developments, the numerical analysis of evaporation and condensation of liquid films has become more convenient than expensive experiments for real-world applications. In this paper, the numerical model of Eulerian Wall Film (EWF) implemented within Ansys Fluent® has been validated based on two canonical cases. The first one refers to the experiments performed by Hu et al. [1], where the heat transfer during the water film evaporation on a vertical plate has been studied. The average heat flux and water film evaporative ratio match the available experimental data well. The second case relates to the experiments done by Ambrosini et al. [2], also known as the "CONAN" case, where the wall film condensation in the presence of non-condensable substances has been studied under different steam mass fraction and velocity conditions. The surface heat flux and condensate mass flow rate also match the experimental measurements. The present numerical workflow using EWF could be applied to more complex devices for liquid film evaporation and condensation simulations.

Entropy wave transport in an industry alike model geometry using a linear framework

Symposium on Thermoacoustics in Combustion: Industry meets Academia (SoTiC 2023), 2023

This study investigates the transport of entropy waves in a industrial complex geometry using Lar... more This study investigates the transport of entropy waves in a industrial complex geometry using Large Eddy Simulation and linear analysis. The LES include a transport equation for a passive scalar, which models entropy. The fluctuations in the passive scalar (or entropy waves) are introduced at the temporally averaged flame location via a harmonically fluctuating source term. The advection and diffusion of the fluctuations in passive scalar are investigated by time stepping the LES. Subsequently, the linear analysis of the entropy wave convection is conducted. The transport equation of the passive scalar is linearized around the temporal mean state, which is obtained by the LES, and transferred into frequency domain, which leads to a transport equation of passive scalar fluctuations (entropy waves). The diffusion of entropy waves is modeled using a turbulent diffusivity, which in this study is determined by a k-epsilon model. The linearized transport equation of the passive scalar is solved for the same frequencies, which are investigated in the LES, using the finite element approach. The results obtained by the linearized approach is in good agreement with the LES, which demonstrates that it is well suited to address the transport of entropy waves in real world combustors.

GT2023-102039 Numerical Simulation of Turbulent Premixed Flames Using Thickened Flame Model

ASME Turbo Expo 2023, 2023

The numerical simulation of turbulent premixed flames in a complex industrial combustor has been ... more The numerical simulation of turbulent premixed flames in a complex industrial combustor has been challenging, due to the very fine mesh required to resolve the thin flame structure using Large Eddy Simulation (LES). The Thickened Flame Model (TFM) has therefore been developed to enable the simulation of such flames on relatively coarser meshes with respect to the laminar flame thickness, by artificially increasing the flame thickness without modifying the flame speed. However, the thickening of a thin flame implies the turbulent eddies at intermediate length scales (between the original and the modified flame thicknesses), which are no longer able to wrinkle the flame. This leads to a smaller flame surface area and correspondingly a reduced heat release rate, which may cause incorrect predictions of the flame shape, emission level, and thermoacoustic stability of the combustion devices, etc. The above reduction in the flame heat release rate may be compensated for by introducing an efficiency function. The efficiency functions proposed by Colin et al. (Physics of Fluids, 2000) and Charlette et al. (Combustion and Flame, 2002) have been widely used, both of which require an estimate of the velocity fluctuation intensity. Multiple implementations of the velocity fluctuation have been proposed to compute the efficiency function, such as (i) the conventional Smagorinsky sub-grid velocity model computed at the mesh filter scale, and (ii) a more recent one developed by Durand and Polifke (Turbo Expo 2007), computed at the thickened flame thickness filter scale. An appropriate combination of the velocity fluctuation model and the efficiency function is vital to the correct representation of the thickened flame behavior. This work primarily examines the above different modeling options for TFM, and evaluates their impact on the numerical solutions. The numerical model applied has been initially validated using the detailed measurement data obtained for a turbulent premixed Bunsen flame (Chen et al. Combustion and Flame, 1996). The present study utilizes the Finite Rate Chemistry (FRC) combustion model with the native TFM model available in Ansys Fluent®, and investigates the effects of different TFM velocity fluctuation and efficiency functions on the accuracy of the simulated flames. Two velocity fluctuation models (Smagorinsky vs. Durand & Polifke) and two efficiency functions (Colin et al. vs. Charlette et al.) are mainly compared. The numerical solutions of flow field, flame shape, turbulent kinetic energy, and multiple major and minor species mass fractions, etc., generally match the experimental data across all the measurement locations. Another key aspect of this work is to use more detailed chemical mechanisms. Most of the recent work on TFM utilized global mechanisms. This study examines a computationally efficient way of applying more detailed mechanisms for TFM, which is made possible using the Dynamic Mechanism Reduction (DMR). This DMR approach utilizes reduced mechanisms that are tailored for local cell compositions for performing reaction integration. This work shows that DMR coupled with TFM leads to a non-trivial speedup without compromising the accuracy of the simulation. The results from this study have been used to develop optimal TFM settings in simulating thin, premixed flames, including the velocity fluctuation model, efficiency function, and reaction integration method, etc. The best practices proposed by this paper may be relevant for industrial configurations which are using the same LES/FRC numerical workflow.

Numerical Prediction of Pressurized Hydrogen Leakage and Spontaneous Ignition

by Yu Xia, Didier Bessette, Abhijit Patil, and Sourabh Shrivastava

AIAA SCITECH 2023 Forum, 23-27 January 2023, National Harbor, MD and Online, 2023

As hydrogen is gaining popularity as an alternative source of energy, there is a strong focus on ... more As hydrogen is gaining popularity as an alternative source of energy, there is a strong focus on understanding the hazards related to its storage and transport. Nowadays there are two means of transporting and storing hydrogen: in liquid state, or in gaseous state at a very high pressure. For the latter, the hydrogen is often stored in highly pressurized tanks, which could be subject to accidental damages and subsequent leakages. As hydrogen is highly flammable, the auto-ignition of leaked hydrogen may further cause a potentially serious safety hazard. The current work focuses on numerically predicting the auto-ignition patterns of suddenly released hydrogen from a pressurized tank into the atmosphere. The latest density-based CFD solver is utilized to simulate the 3D turbulent structures during the mixing of the fast-propagating hydrogen with the ambient air, using Large Eddy Simulation for turbulence and Finite Rate Chemistry for combustion. The mesh is automatically refined to capture the turbulent eddies, shock waves and thin reaction zones. The numerical calculations are performed at three tank pressures (250 bar, 100 bar and 50 bar) within two sector domains (1 degree and 30 degree).
The turbulent diffusion and auto-ignition patterns of the leaked hydrogen are correctly predicted across different operating conditions.

Numerical Simulations of a Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

Journal of Engineering for Gas Turbines and Power, 2022

A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated... more A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated, using the Flamelet Generated Manifold (FGM) combustion model with Large Eddy Simulations (LES). Due to the hot vitiated H2/air co-flow, the primary stabilization mechanism is the autoignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the autoignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at autoignition location but premixed in the post flame. A series of LES/FGM simulations are completed in this work by reducing the co-flow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the co-flow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high co-flow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. This work provides some useful guidelines in simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.

Prediction of Thermoacoustic Instability and Fluid-Structure Interactions for Gas Turbine Combustor

by Yu Xia, Patrick Sharkey, and Alok Khaware

Journal of Engineering for Gas Turbines and Power , 2022

This work simulates a laboratory-scale three-dimensional methane/air burner, which features a blu... more This work simulates a laboratory-scale three-dimensional methane/air burner, which features a bluff body stabilized, lean partially premixed flame experiencing strong limit cycle oscillations. A thin steel liner is installed around the combustion chamber, which heavily interacts with the flow field and produces large amplitude structural deformation via fluid-structure interaction (FSI). An unsteady Reynolds averaged Navier-Stokes (URANS) approach uses the shear stress transport (SST) turbulence model and a flamelet generated manifold (FGM) combustion model to predict the thermoacoustic oscillations in the turbulent reacting flow. The solver also has a built-in finite element structure model, which solves the structural governing equations simultaneously with the computational fluid dynamics (CFD)-computed, finite volume flow equations. This way, a fully coupled, two-way FSI simulation can be performed to predict the thermoacoustic instabilities and the associated solid deformations in the burner. Overall, the predicted strongest pressure oscillation and wall displacement modes (frequency and amplitude) are all in good agreement with the experimental data across different operating conditions. The established workflow may support realistic gas turbine combustor design and prognosis.

GT2022-80733 Numerical Simulations Of A Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

ASME Turbo Expo 2022, 2022

A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated... more A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated, using the Flamelet Generated Manifold (FGM) combustion model with Large Eddy Simulations (LES). Due to the hot vitiated H2/air co-flow, the primary stabilization mechanism is the autoignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the autoignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at autoignition location but premixed in the post flame. A series of LES/FGM simulations are completed in this work by reducing the co-flow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the co-flow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction

GT2022-78296 Prediction Of Thermoacoustic Instability And Fluid-structure Interactions For Gas Turbine Combustor

ASME Turbo Expo 2022, 2022

This work simulates a laboratory-scale 3D methane/air burner, which features a bluff body stabili... more This work simulates a laboratory-scale 3D methane/air burner, which features a bluff body stabilized, lean partially premixed flame experiencing strong limit cycle oscillations. A thin steel liner is installed around the combustion chamber, which heavily interacts with the flow field and produces large amplitude structural deformation via Fluid-Structure Interactions (FSI). An unsteady RANS approach uses the Shear Stress Transport turbulence model and a Flamelet Generated Manifold combustion model to predict the thermoacoustic oscillations in the turbulent reacting flow. The solver also has a built-in finite element Structure Model, which solves the structural governing equations simultaneously with the CFD-computed, finite volume flow equations. This way, a fully coupled, two-way FSI simulation can be performed to predict the thermoacoustic instabilities and the associated solid deformations in the burner. Overall, the predicted strongest pressure oscillation and wall displacement modes (frequency and amplitude) are all in good agreement with the experimental data across different operating conditions. The established workflow may support realistic gas turbine combustor design and prognosis.

Stress-Blended Eddy Simulation/Flamelet Generated Manifold Simulation of Film-Cooled Surface Heat Transfer and Near-Wall Reaction

by Yu Xia and Florian Menter

Journal of Turbomachinery, 2021

Accurate numerical prediction of surface heat transfer in the presence of film cooling within aer... more Accurate numerical prediction of surface heat transfer in the presence of film cooling within aero-engine sub-components, such as blade effusion holes and combustor liners, has long been a goal of the aero-engine industry. It requires accurate simulation of the turbulent mixing and reaction processes between freestream and the cooling flow. In this study, the Stress Blended Eddy Simulation (SBES) turbulence model is used together with the Flamelet Generated Manifold (FGM) combustion model to calculate the surface heat flux upstream and downstream of an effusion cooling hole. The SBES model employs a blending function to automatically switch between RANS and LES based on the local flow features, and thus significantly reduces the computational cost compared to a full LES simulation. All simulations are run using ANSYS Fluent, a commercial finite-volume CFD solver. The test case corresponds to an experimental rig run at MIT, which is essentially a flat plate brushed by a uniform freestream of argon with ethylene seeded inside, and is cooled by either a reacting air or non-reacting nitrogen jet inclined at 35 degrees to the freestream. Calculations are performed for both reacting and non-reacting jet cooling cases across a range of jet-to-stream blowing ratios, and compared with the experimental data. The effects of mesh resolution are also investigated. Calculations are also performed across a range of Damkohler number (i.e. flow to chemical time ratio) from zero to 30, with unity blowing ratio, and the differences in the maximum surface heat flux magnitude in the reacting and non-reacting cases at a specific location downstream of the hole are investigated. Results from these analyses show good correlation with the experimental heat flux data upstream and downstream of the cooling hole, including the heat flux augmentation due to local reaction. Results from the Damkohler number sweep also show a good match with the experimental data across the range investigated.

[Video Presentation] GT2021-59100 Dynamic Mesh Adaption For Scale-Resolving Reacting Flow Simulations

A dynamic adaptive mesh refinement method is used in conjunction with a hybrid scale-resolving tu... more

4 views

[Video Presentation] GT2022-80733 Numerical Simulations of a Lifted hydrogen Jet Flame Using Flamelet Generated Manifold Approach

https://www.researchgate.net/publication/360065122_Video_Presentation_GT2022-80733_Numerical_Simu... more

2 views

[Video Presentation] Numerical Prediction of Pressurized Hydrogen Leakage and Spontaneous Ignition

The current work focuses on numerically predicting the auto-ignition patterns of suddenly release... more The current work focuses on numerically predicting the auto-ignition patterns of suddenly released hydrogen from a pressurized tank into the atmosphere. The latest density-based CFD solver is utilized to simulate the 3D turbulent structures during the mixing of the fast-propagating hydrogen with the ambient air, using Large Eddy Simulation for turbulence and Finite Rate Chemistry for combustion. The mesh is automatically refined to capture the turbulent eddies, shock waves and thin reaction zones. The numerical calculations are performed at three tank pressures (250 bar, 100 bar and 50 bar) within two sector domains (1 degree and 30 degree).
The turbulent diffusion and auto-ignition patterns of the leaked hydrogen are correctly predicted across different operating conditions.

15 views

[Video Presentation] GT2020-14717 SBES/FGM Simulation of Film-Cooled Surface Heat Transfer and Near-Wall Reaction

Accurate numerical prediction of surface heat transfer in the presence of film cooling within aer... more Accurate numerical prediction of surface heat transfer in the presence of film cooling within aero-engine sub-components, such as blade effusion holes and combustor liners, has long been a goal of the aero-engine industry. It requires accurate simulation of the turbulent mixing and reaction processes between freestream and the cooling flow. In this study, the Stress Blended Eddy Simulation (SBES) turbulence model is used together with the Flamelet Generated Manifold (FGM) combustion model to calculate the surface heat flux upstream and downstream of an effusion cooling hole.

59 views

[Video Presentation] GT2022-78296 Prediction of Thermoacoustic Instability and Fluid-Structure Interactions for Gas Turbine Combustor

https://www.researchgate.net/publication/357186379_GT2022-78296_Prediction_of_Thermoacoustic_Inst... more

2 views

Analysis of Entropy Production Mechanisms in Turbulent Diffusion Flames

AIAA SciTech Forum and Exposition 2025, 2025

Exergy based analysis for thermodynamic systems has become increasingly important in understandin... more Exergy based analysis for thermodynamic systems has become increasingly important in understanding the efficiency of energy-work conversion. Entropy generation, associated with the irreversibility in practical processes, decreases the available useful work, and the mechanisms responsible for the production differ significantly between premixed and nonpremixed combustion. In addition, unsteady combustion generates temperature fluctuations and when accelerated produces entropy waves – referred to as indirect noise. Prior research by the authors have considered the dynamics of entropy generation purely due to heat release for both premixed and non-premixed flames. However, for non-premixed flames where the diffusion-controlled processes are important, transport terms must be considered for consistency. In this work, the entropy production mechanisms due to different sources are analyzed, firstly, by doing a mathematical study on the entropy transport equation and deriving the key forms of the source terms. Then reacting flow simulations are performed for the Sandia D flame, a well-documented configuration, for the numerical analysis of the transport equation. A range of operating conditions have been considered to illustrate the relative importance of different entropy source terms. Overall entropy production rate budget is calculated, and trends are observed for different operating conditions. In addition, across different sections of the flame, local entropy production is analyzed. Subsequently, reacting flow simulations are performed for the classical Burke-Schumann flame, a laminar nonpremixed flame, and the same analysis is carried out to completely gauge the trends of entropy production in non-premixed flames.

[GTP-24-1550] Large Eddy Simulation of Ammonia-Hydrogen Non-premixed Flames Stabilized on a Bluff Body Burner

Journal of Engineering for Gas Turbines and Power, 2024

The demand for carbon-free fuels such as NH3 and H2 has been growing due to stricter environmenta... more The demand for carbon-free fuels such as NH3 and H2 has been growing due to stricter environmental policies on carbon emissions. Since ammonia is easier to store and transport, it is considered an alternative fuel for marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emissions of NOx are important challenges that need to be overcome if ammonia is to be used as a practical fuel for industrial use. Recent works have provided experimental data on the characteristics of NH3 flames under premixed and non-premixed combustion, focusing on flame speed, turbulence-chemistry interaction, and especially the dissociation/cracking of NH3 into H2 and N2. These measurements have encouraged the use of CFD for the simulation and scaling of practical ammonia systems, evaluating the performance of different numerical models for NH3 combustion. In this study, LES is utilized to predict a series of NH3/H2/N2 non-premixed flames stabilized on a bluff body burner. The accuracy of the FGM combustion model combined with LES has been examined, using a commercial CFD solver. The different cracking levels of NH3 into H2 and N2 lead to simulation cases ranging from an H2/N2 flame (i.e., “fully cracked”), to a flame with 50% NH3 (i.e., “half cracked”). The different flame shapes, temperature distributions, and NOx emissions have been simulated, generally matching well experimental data. The developed numerical settings and workflows may be applied to simulating more complex combustors which use NH3 as a fuel.

[GTP-24-1228] Adjoint-Based Optimization for the Venturi Mixer of A Burner

Journal of Engineering for Gas Turbines and Power, 2024

The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and... more The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and minimizing emissions. In this study, we utilize the adjoint method to analyze and refine the design of a Venturi mixer. Our numerical simulations integrate the species transport equation with the eddy dissipation model (EDM) for reacting flow and the generalized k-x (GEKO) model to simulate turbulence. By solving adjoint equations, we effectively compute the shape sensitivity for various observables, including pressure drop, outlet fuel variance/ uniformity deviation index, air and fuel mass flow rates, and outlet CO mass fraction. The shape sensitivity analysis uncovers the interplay between the observables and the appropriate weights for multiple objective optimizations. Subsequently, we perform gradient-based optimizations to enhance the mixer's performance, employing both shape sensitivity and mesh morphing techniques. We conduct a series of case studies focusing on both cold and reacting flows. The optimization of cold flow provides an in-depth exploration of various optimization strategies, encompassing single-objective and multi-objective optimization with diverse weight combinations. Following this, the optimization under reacting flow conditions improves the fuel/air mixing, leading to the increase of combustion efficiency and hence the reduction of CO emissions. Our findings showcase the potential of an adjoint-based optimization framework in designing Venturi mixers that are efficient and emit lower levels of pollutants.

CFD Modeling of High-Enthalpy Hypersonic Flows for FIRE-II Re-Entry Vehicle

AIAA Aviation Forum, 2024

Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. W... more Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. While accurate physical modeling and numerical prediction of hypersonic flows are crucial for the design and analysis of this class of vehicles, the extreme conditions make this class of numerical simulations challenging to converge to a stable and steady solution. The authors' previous study utilized a "two-temperature model" to simulate the hypersonic flow and thermal features of the FIRE-II re-entry vehicle. This previous work demonstrated basic trends for surface heat flux, temperature field, ionization of air, etc., at five operating points along the vehicle's re-entry trajectory. The present work is a higher-fidelity investigation and validation performed on the FIRE II vehicle. In particular, the work aims at comparing the effects on the thermal environment of (1) two different chemical reaction mechanisms, and of (2) two wall catalysis models, full and partial-catalytic wall (PCW). Both chemical mechanisms, the Park and the Gupta models, account for the recombination of the ionized species and have been proven to predict reasonably well the heating trends on the surface of the capsule. The results show that the use of the catalytic wall model improves the prediction of surface heat flux on the vehicle across all the operating points as compared to previous studies. While trends in heat flux are captured well for different altitudes, the absolute values and the match with the experiment still present some discrepancies. These discrepancies are in large part attributable to the presence of a large uncertainty in the instantaneous flight conditions, in the assumption of steady-state conditions at each altitude as well as unknown catalytic conditions of the capsule's thermal protection system (TPS). The study demonstrates

GT2024-125957 LARGE EDDY SIMULATION OF AMMONIA-HYDROGEN NON-PREMIXED FLAMES STABILIZED ON A BLUFF BODY BURNER

Turbo Expo 2024 Turbomachinery Technical Conference & Exposition, 2024

The demand for carbon-free fuels such as ammonia (NH3) and hydrogen (H2) has been growing rapidly... more The demand for carbon-free fuels such as ammonia (NH3) and hydrogen (H2) has been growing rapidly in the past decade due to stricter environmental policies being applied. The storage and transport of NH3 in the liquid form, at a relatively low tank pressure, are comparatively easier than those of H2. As a result, NH3 is considered as an alternative H2-carrier fuel. Since NH3 is a more economical and eco-friendly fuel (compared with hydrocarbon fuels such as natural gas), it has become a potential energy provider for the marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emission of nitrogen oxides (NOx) during its combustion have prevented NH3 from being a practical fuel for industrial combustors. Recent experiments have investigated the fundamental characteristics of NH3 flames under premixed and non-premixed combustion modes, focusing on flame speed, turbulence-chemistry interaction, and the impact of dissociation/cracking of NH3 into H2 and N2. Various computational fluid dynamics (CFD) simulations have also been performed, where a series of CFD combustion models have been evaluated and validated against experimental data. The validated numerical models and workflows can then be used to decrease the number of experiments needed and eventually to reduce the total costs of designing NH3-fueled burners. To predict the highly turbulent flame behavior, scale-resolving models such as Large Eddy Simulation (LES) have been proven to be more accurate than the Reynolds-Averaged Navier-Stokes (RANS) approach. In this work, the performance of the Flamelet Generated Manifold (FGM) combustion model in combination with the LES turbulence model has been evaluated using commercial CFD software to predict bluff-body stabilized NH3/H2 non-premixed jet flames. The experimental data were obtained by Alfazazi et al. at King Abdullah University of Science and Technology (KAUST) (Combustion and Flame, Volume 258, Part 2, 2023, 113066. The different cracking levels of NH3 into H2 and N2 result in four different NH3/H2 blending scenarios, ranging from a pure H2 flame (i.e., "fully cracked") to a flame with 72% volume of NH3. The flame shapes, temperature distribution, and NO and unburnt NH3 emissions have all been simulated, which generally match well the available experimental data and are found to be more accurate than previous RANS predictions. The numerical settings and workflow developed in this study may be applied to more complex configurations and contribute to a wider deployment of NH3 as a practical fuel for multiple industries.

GT2024-124337 Adjoint Solver Based Shape Optimization for a Venturi Mixer

Turbo Expo 2024 Turbomachinery Technical Conference & Exposition, 2024

The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and... more The optimization of Venturi mixers in burners is critical for enhancing combustion efficiency and minimizing emissions. In this study, we utilize the adjoint method to analyze and refine the design of a Venturi mixer. Our numerical simulations integrate the species transport equation with the Eddy Dissipation Model (EDM) for reacting flow and the generalized k-ω (GEKO) model to simulate turbulence. By solving adjoint equations, we effectively compute the shape sensitivity for various observables, including pressure drop, outlet fuel variance/uniformity deviation index, air and fuel mass flow rates, and outlet CO mass fraction. The shape sensitivity analysis uncovers the interplay between the observables and the appropriate weights for multiple objective optimizations. Subsequently, we perform gradient-based optimizations to enhance the mixer's performance, employing both shape sensitivity and mesh morphing techniques. We conduct a series of case studies focusing on both cold and reacting flows. The optimization of cold flow provides an in-depth exploration of various optimization strategies, encompassing single-objective and multi-objective optimization with diverse weight combinations. Following this, the optimization under reacting flow conditions improves the fuel/air mixing, leading to the increase of combustion efficiency and hence the reduction of CO emissions. Our findings showcase the potential of an adjoint-based optimization framework in designing Venturi mixers that are efficient and emit lower levels of pollutants.

Numerical Simulation of Liquid Film Evaporation and Condensation for Water Reactors

The 9th World Congress on Momentum, Heat and Mass Transfer (MHMT 2024), 2024

A detailed analysis of liquid film evaporation and condensation has become increasingly important... more A detailed analysis of liquid film evaporation and condensation has become increasingly important, especially in water reactor designs. For example, the prediction of liquid film evaporation is critical to the design of Passive Containment Cooling Systems (PCCS), which are important passive safety facilities in modern Pressurized Water Reactors (PWR). To keep the maximum design values of pressure and temperature under control, a water/air counter-current flow is often employed within the external channel of the PCCS. It is thus essential to analyse the evaporation mass flow rate and heat transfer of the water film for the overall performance of PCCS. On the other hand, hydrogen might be released into the reactor containment during an incident due to cladding oxidation at elevated temperatures. Given the high flammability of gaseous hydrogen, it is paramount to know its local distribution in the containment building while considering wall condensation, which will increase the non-condensable gas concentration on the walls. Another example is evaluating the containment loads in the Light Water Reactor (LWR) plants and predicting their condensation behaviour in the presence of non-condensable gases. With the advantages of Computational Fluid Dynamics (CFD) and recent model developments, the numerical analysis of evaporation and condensation of liquid films has become more convenient than expensive experiments for real-world applications. In this paper, the numerical model of Eulerian Wall Film (EWF) implemented within Ansys Fluent® has been validated based on two canonical cases. The first one refers to the experiments performed by Hu et al. [1], where the heat transfer during the water film evaporation on a vertical plate has been studied. The average heat flux and water film evaporative ratio match the available experimental data well. The second case relates to the experiments done by Ambrosini et al. [2], also known as the "CONAN" case, where the wall film condensation in the presence of non-condensable substances has been studied under different steam mass fraction and velocity conditions. The surface heat flux and condensate mass flow rate also match the experimental measurements. The present numerical workflow using EWF could be applied to more complex devices for liquid film evaporation and condensation simulations.

Entropy wave transport in an industry alike model geometry using a linear framework

Symposium on Thermoacoustics in Combustion: Industry meets Academia (SoTiC 2023), 2023

This study investigates the transport of entropy waves in a industrial complex geometry using Lar... more This study investigates the transport of entropy waves in a industrial complex geometry using Large Eddy Simulation and linear analysis. The LES include a transport equation for a passive scalar, which models entropy. The fluctuations in the passive scalar (or entropy waves) are introduced at the temporally averaged flame location via a harmonically fluctuating source term. The advection and diffusion of the fluctuations in passive scalar are investigated by time stepping the LES. Subsequently, the linear analysis of the entropy wave convection is conducted. The transport equation of the passive scalar is linearized around the temporal mean state, which is obtained by the LES, and transferred into frequency domain, which leads to a transport equation of passive scalar fluctuations (entropy waves). The diffusion of entropy waves is modeled using a turbulent diffusivity, which in this study is determined by a k-epsilon model. The linearized transport equation of the passive scalar is solved for the same frequencies, which are investigated in the LES, using the finite element approach. The results obtained by the linearized approach is in good agreement with the LES, which demonstrates that it is well suited to address the transport of entropy waves in real world combustors.

GT2023-102039 Numerical Simulation of Turbulent Premixed Flames Using Thickened Flame Model

ASME Turbo Expo 2023, 2023

The numerical simulation of turbulent premixed flames in a complex industrial combustor has been ... more The numerical simulation of turbulent premixed flames in a complex industrial combustor has been challenging, due to the very fine mesh required to resolve the thin flame structure using Large Eddy Simulation (LES). The Thickened Flame Model (TFM) has therefore been developed to enable the simulation of such flames on relatively coarser meshes with respect to the laminar flame thickness, by artificially increasing the flame thickness without modifying the flame speed. However, the thickening of a thin flame implies the turbulent eddies at intermediate length scales (between the original and the modified flame thicknesses), which are no longer able to wrinkle the flame. This leads to a smaller flame surface area and correspondingly a reduced heat release rate, which may cause incorrect predictions of the flame shape, emission level, and thermoacoustic stability of the combustion devices, etc. The above reduction in the flame heat release rate may be compensated for by introducing an efficiency function. The efficiency functions proposed by Colin et al. (Physics of Fluids, 2000) and Charlette et al. (Combustion and Flame, 2002) have been widely used, both of which require an estimate of the velocity fluctuation intensity. Multiple implementations of the velocity fluctuation have been proposed to compute the efficiency function, such as (i) the conventional Smagorinsky sub-grid velocity model computed at the mesh filter scale, and (ii) a more recent one developed by Durand and Polifke (Turbo Expo 2007), computed at the thickened flame thickness filter scale. An appropriate combination of the velocity fluctuation model and the efficiency function is vital to the correct representation of the thickened flame behavior. This work primarily examines the above different modeling options for TFM, and evaluates their impact on the numerical solutions. The numerical model applied has been initially validated using the detailed measurement data obtained for a turbulent premixed Bunsen flame (Chen et al. Combustion and Flame, 1996). The present study utilizes the Finite Rate Chemistry (FRC) combustion model with the native TFM model available in Ansys Fluent®, and investigates the effects of different TFM velocity fluctuation and efficiency functions on the accuracy of the simulated flames. Two velocity fluctuation models (Smagorinsky vs. Durand & Polifke) and two efficiency functions (Colin et al. vs. Charlette et al.) are mainly compared. The numerical solutions of flow field, flame shape, turbulent kinetic energy, and multiple major and minor species mass fractions, etc., generally match the experimental data across all the measurement locations. Another key aspect of this work is to use more detailed chemical mechanisms. Most of the recent work on TFM utilized global mechanisms. This study examines a computationally efficient way of applying more detailed mechanisms for TFM, which is made possible using the Dynamic Mechanism Reduction (DMR). This DMR approach utilizes reduced mechanisms that are tailored for local cell compositions for performing reaction integration. This work shows that DMR coupled with TFM leads to a non-trivial speedup without compromising the accuracy of the simulation. The results from this study have been used to develop optimal TFM settings in simulating thin, premixed flames, including the velocity fluctuation model, efficiency function, and reaction integration method, etc. The best practices proposed by this paper may be relevant for industrial configurations which are using the same LES/FRC numerical workflow.

Numerical Prediction of Pressurized Hydrogen Leakage and Spontaneous Ignition

by Yu Xia, Didier Bessette, Abhijit Patil, and Sourabh Shrivastava

AIAA SCITECH 2023 Forum, 23-27 January 2023, National Harbor, MD and Online, 2023

As hydrogen is gaining popularity as an alternative source of energy, there is a strong focus on ... more As hydrogen is gaining popularity as an alternative source of energy, there is a strong focus on understanding the hazards related to its storage and transport. Nowadays there are two means of transporting and storing hydrogen: in liquid state, or in gaseous state at a very high pressure. For the latter, the hydrogen is often stored in highly pressurized tanks, which could be subject to accidental damages and subsequent leakages. As hydrogen is highly flammable, the auto-ignition of leaked hydrogen may further cause a potentially serious safety hazard. The current work focuses on numerically predicting the auto-ignition patterns of suddenly released hydrogen from a pressurized tank into the atmosphere. The latest density-based CFD solver is utilized to simulate the 3D turbulent structures during the mixing of the fast-propagating hydrogen with the ambient air, using Large Eddy Simulation for turbulence and Finite Rate Chemistry for combustion. The mesh is automatically refined to capture the turbulent eddies, shock waves and thin reaction zones. The numerical calculations are performed at three tank pressures (250 bar, 100 bar and 50 bar) within two sector domains (1 degree and 30 degree).
The turbulent diffusion and auto-ignition patterns of the leaked hydrogen are correctly predicted across different operating conditions.

Numerical Simulations of a Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

Journal of Engineering for Gas Turbines and Power, 2022

A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated... more A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated, using the Flamelet Generated Manifold (FGM) combustion model with Large Eddy Simulations (LES). Due to the hot vitiated H2/air co-flow, the primary stabilization mechanism is the autoignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the autoignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at autoignition location but premixed in the post flame. A series of LES/FGM simulations are completed in this work by reducing the co-flow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the co-flow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high co-flow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. This work provides some useful guidelines in simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.

Prediction of Thermoacoustic Instability and Fluid-Structure Interactions for Gas Turbine Combustor

by Yu Xia, Patrick Sharkey, and Alok Khaware

Journal of Engineering for Gas Turbines and Power , 2022

This work simulates a laboratory-scale three-dimensional methane/air burner, which features a blu... more This work simulates a laboratory-scale three-dimensional methane/air burner, which features a bluff body stabilized, lean partially premixed flame experiencing strong limit cycle oscillations. A thin steel liner is installed around the combustion chamber, which heavily interacts with the flow field and produces large amplitude structural deformation via fluid-structure interaction (FSI). An unsteady Reynolds averaged Navier-Stokes (URANS) approach uses the shear stress transport (SST) turbulence model and a flamelet generated manifold (FGM) combustion model to predict the thermoacoustic oscillations in the turbulent reacting flow. The solver also has a built-in finite element structure model, which solves the structural governing equations simultaneously with the computational fluid dynamics (CFD)-computed, finite volume flow equations. This way, a fully coupled, two-way FSI simulation can be performed to predict the thermoacoustic instabilities and the associated solid deformations in the burner. Overall, the predicted strongest pressure oscillation and wall displacement modes (frequency and amplitude) are all in good agreement with the experimental data across different operating conditions. The established workflow may support realistic gas turbine combustor design and prognosis.

GT2022-80733 Numerical Simulations Of A Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

ASME Turbo Expo 2022, 2022

A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated... more A turbulent lifted H2/N2 jet flame in a vitiating co-flow environment is numerically investigated, using the Flamelet Generated Manifold (FGM) combustion model with Large Eddy Simulations (LES). Due to the hot vitiated H2/air co-flow, the primary stabilization mechanism is the autoignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the autoignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at autoignition location but premixed in the post flame. A series of LES/FGM simulations are completed in this work by reducing the co-flow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the co-flow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction

GT2022-78296 Prediction Of Thermoacoustic Instability And Fluid-structure Interactions For Gas Turbine Combustor

ASME Turbo Expo 2022, 2022

This work simulates a laboratory-scale 3D methane/air burner, which features a bluff body stabili... more This work simulates a laboratory-scale 3D methane/air burner, which features a bluff body stabilized, lean partially premixed flame experiencing strong limit cycle oscillations. A thin steel liner is installed around the combustion chamber, which heavily interacts with the flow field and produces large amplitude structural deformation via Fluid-Structure Interactions (FSI). An unsteady RANS approach uses the Shear Stress Transport turbulence model and a Flamelet Generated Manifold combustion model to predict the thermoacoustic oscillations in the turbulent reacting flow. The solver also has a built-in finite element Structure Model, which solves the structural governing equations simultaneously with the CFD-computed, finite volume flow equations. This way, a fully coupled, two-way FSI simulation can be performed to predict the thermoacoustic instabilities and the associated solid deformations in the burner. Overall, the predicted strongest pressure oscillation and wall displacement modes (frequency and amplitude) are all in good agreement with the experimental data across different operating conditions. The established workflow may support realistic gas turbine combustor design and prognosis.

Stress-Blended Eddy Simulation/Flamelet Generated Manifold Simulation of Film-Cooled Surface Heat Transfer and Near-Wall Reaction

by Yu Xia and Florian Menter

Journal of Turbomachinery, 2021

Accurate numerical prediction of surface heat transfer in the presence of film cooling within aer... more Accurate numerical prediction of surface heat transfer in the presence of film cooling within aero-engine sub-components, such as blade effusion holes and combustor liners, has long been a goal of the aero-engine industry. It requires accurate simulation of the turbulent mixing and reaction processes between freestream and the cooling flow. In this study, the Stress Blended Eddy Simulation (SBES) turbulence model is used together with the Flamelet Generated Manifold (FGM) combustion model to calculate the surface heat flux upstream and downstream of an effusion cooling hole. The SBES model employs a blending function to automatically switch between RANS and LES based on the local flow features, and thus significantly reduces the computational cost compared to a full LES simulation. All simulations are run using ANSYS Fluent, a commercial finite-volume CFD solver. The test case corresponds to an experimental rig run at MIT, which is essentially a flat plate brushed by a uniform freestream of argon with ethylene seeded inside, and is cooled by either a reacting air or non-reacting nitrogen jet inclined at 35 degrees to the freestream. Calculations are performed for both reacting and non-reacting jet cooling cases across a range of jet-to-stream blowing ratios, and compared with the experimental data. The effects of mesh resolution are also investigated. Calculations are also performed across a range of Damkohler number (i.e. flow to chemical time ratio) from zero to 30, with unity blowing ratio, and the differences in the maximum surface heat flux magnitude in the reacting and non-reacting cases at a specific location downstream of the hole are investigated. Results from these analyses show good correlation with the experimental heat flux data upstream and downstream of the cooling hole, including the heat flux augmentation due to local reaction. Results from the Damkohler number sweep also show a good match with the experimental data across the range investigated.

GT2021-59100 Dynamic Mesh Adaption For Scale-resolving Reacting Flow Simulations

by Yu Xia and P. Stopford

ASME Turbo Expo 2021, 2021

In this paper, a dynamic adaptive mesh refinement method is used in conjunction with a hybrid sca... more In this paper, a dynamic adaptive mesh refinement method is used in conjunction with a hybrid scale-resolving turbulence model to solve industrial combustion problems. The objective of the adaption method is to track and resolve characteristic turbulent structures arising from swirlers, pilot injectors and flame propagation in industrial burner configurations. By employing Polyhedral Unstructured Mesh Adaption (PUMA) within Ansys Fluent solver, local regions of mesh are refined to capture gradients in temperature, velocity and other key variables. For Scale-Resolving Simulations (SRS), highly refined meshes are required to resolve a sufficient range of turbulent scales. In this work, a strategy is proposed to evaluate the scale-resolving quality of the mesh and to refine it dynamically in a transient simulation. The condition used for adapting the mesh is based on the gradients of key variables such as temperature and velocity, whilst the large-scale eddies are resolved using an approach based on the LES mesh resolution index. This strategy is then applied to a series of test cases (a piloted dilution flame, a bluff body premixed flame and a swirl stabilized flame), using the hybrid Stress-Blended Eddy Simulation (SBES) turbulence model and a Flamelet Generated Manifold (FGM) combustion model. The numerical results are compared with available experimental data, and the accuracy of the solutions is discussed.

Thermoacoustic analysis of the combustor consisting long flames using low order network model approach

International Symposium: Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry Meets Academia, 2016

A low-order network modelling accounting for the local flame transfer function is proposed in thi... more A low-order network modelling accounting for the local flame transfer function is proposed in this work. The long flame is axially split to NOS elements with an equal width. In each segment, the acoustic network is coupled with the local flame transfer function via the acoustic jump conditions. The heat release response to acoustic forcing is determined using the Large Eddy Simulation (LES). The LES utilises the open source Computational Fluid Dynamics (CFD) toolbox, OpenFOAM, with a low Mach number approximation for the flow-field and combustion modelled using the PaSR (Partially Stirred Reactor) model with a one-step chemical reaction mechanism for propane/air. The effect of number of segment (NOS) on the predicted thermoacoustic eigenvalues and modeshapes of the dominant unstable mode is investigated in this work. The predicted results are compared to the reference results determined by a linearised Euler equation (LEE) method. With increasing NOS, the predicted eigenvalues and modeshapes approach the “real” values. A moderate value of NOS = 8 is sufficient for the thermoacoustic prediction for the target flame while keeping the signal to noise ratio (SNR) enough large for extracting the local FTFs from the LES results.

Dispersion of entropy waves advecting through combustor chambers

The 23rd International Congress on Sound & Vibration (ICSV23), 2016

In gas turbines, unsteady combustion generates temperature fluctuations, which are often termed e... more In gas turbines, unsteady combustion generates temperature fluctuations, which are often termed entropy waves. When these are accelerated through the turbine stages, they represent a significant source of noise. This noise mechanism is known as entropy noise, or indirect combustion noise. It generates both upstream-travelling acoustic waves, which can contribute to the feedback loop of combustion instabilities, and downstream-travelling acoustic waves, which can appear as exhaust noise. Prediction of entropy noise relies on (i) knowledge of how entropy waves are generated by unsteady combustion, (ii) how they advect with the turbulent flow through the combustion chamber , and (iii) how they generate acoustic waves upon reaching the turbine stages, where they are accelerated. As the strength of the two acoustic waves is directly proportional to the amplitude of the entropy wave reaching the inlet of the first stator blade, low order models need accurate knowledge of this wave strength. Many such models currently neglect the advection of the en-tropy wave through the turbulent combustion chamber, assuming that they advect undisturbed, except for a time delay. The aim of this work is to study the advection of entropy waves through a real combustion chamber flowfield, exhibiting realistic flow features such as swirl and recircula-tion zones. The combustor flowfield is found to weaken the strength of the entropy wave exiting the combustor chamber, but significant wave strength still remains at the inlet of turbine stages.

Computation of Local Flame Describing Functions for Thermoacoustic Oscillations in a Combustor with a Long Flame

The 8th European Combustion Meeting (ECM8), 2017

This work performs a numerical study of the ORACLES combustor which contains a 1-meter-long turbu... more This work performs a numerical study of the ORACLES combustor which contains a 1-meter-long turbulent premixed flame and sustains low frequency thermoacoustic instabilities. The flame heat release rate response to acoustic disturbances is characterized, but because the flame is long, the axial variation of the response is accounted for using an axial sequence of local flame describing functions (FDF). Low-Mach number large eddy simulations (LES) are performed using the OpenFOAM toolbox, with the turbulence-combustion interaction modelled by the Partially Stirred Reactor (PaSR) model with a global one-step propane/air reaction mechanism. LES results are validated through comparison with experimental data. The computational zone in the combustion chamber is equally split to N s = 40 axial segments, and the flame heat release rate perturbations in each flame segment extracted and used to construct a sequence of local FDF. The dependence of this on the numbers of flame segments used is investigated. 1. Introduction A common way to numerically analyse thermoacous-tic instabilities is to decouple the calculations of acoustic wave perturbation and the flame response. The unsteady flame heat release rate that results from acoustic excita-tion can be described by a flame transfer function (FTF) for linear analysis [1, 2] or a flame describing function (FDF) for weakly nonlinear analysis [3, 4]. The acoustic perturbations are often captured by a low-order acoustic network model [2, 5] or a Helmholtz solver [6], both of which exploit the fact that the acoustic wave behaves linearly in gas turbine combustors [7]. These acoustic models always assume that the flame is "acoustically com-pact" with its thickness much smaller than the dominant acoustic wavelength [8]. This assumption enables the coupling of the acoustic and flame response models for thermoacoustic analysis [2, 3, 8]. However, when the flame length is of the order of the dominant acoustic wavelengths, this assumption is no longer valid. The prediction of thermoacoustic oscillations then needs to account for the spatial variation in the heat release rate along the flame under acoustic excita-tions. This can be achieved using the concept of a "local" and continuously varying flame model, characterising the axially-varying flame responses to normalised velocity perturbations at a fixed reference location. This concept is practically implemented using a discrete series of local FTFs or FDFs extracted from numerical or experimental data, applied at different downstream zones within the long flame [9]. The spatial precision of such local FTFs or FDFs relies on the number and hence the spatial extent of the selected flame zones, and there is a trade-off between the increasing number of flame zones which captures the non-compactness more accurately, and the deteriorated signal to noise ratio (SNR) of the local flame models as each zone now contains less flame heat release.

GT2017-63247 Predicting Thermoacoustic Instability in an Industrial Gas Turbine Combustor: Combining a Low Order Network Model With Flame LES

ASME Turbo Expo 2017, 2017

The thermoacoustic modes of a full scale industrial gas turbine combustor have been predicted num... more The thermoacoustic modes of a full scale industrial gas turbine combustor have been predicted numerically. The predictive approach combines low order network modelling of the acoustic waves in a simplified geometry, with a weakly nonlinear flame describing function, obtained from incompressible large eddy simulations of the flame region under upstream forced velocity perturbations, incorporating reduced chemistry mechanisms. Two incompressible solvers, each employing different numbers of reduced chemistry mechanism steps, are used to simulate the turbulent reacting flowfield to predict the flame describing functions. The predictions differ slightly between reduced chemistry approximations, indicating the need for more involved chemistry. These are then incorporated into a low order thermoacoustic solver to predict thermoacoustic modes. For the combustor operating at two different pressures, most thermoacoustic modes are predicted to be stable, in agreement with the experiments. The predicted modal frequencies are in good agreement with the measurements, although some mismatches in the predicted modal growth rates and hence modal stabilities are observed. Overall, these findings lend confidence in this coupled approach for real industrial gas turbine combustors.

[Presentation] Analysis of Entropy Production Mechanisms in Turbulent Diffusion Flames

AIAA SciTech Forum and Exposition 2025, 2025

Exergy based analysis for thermodynamic systems has become increasingly important in understandin... more Exergy based analysis for thermodynamic systems has become increasingly important in understanding the efficiency of energy-work conversion. Entropy generation, associated with the irreversibility in practical processes, decreases the available useful work, and the mechanisms responsible for the production differ significantly between premixed and nonpremixed combustion. In addition, unsteady combustion generates temperature fluctuations and when accelerated produces entropy waves – referred to as indirect noise. Prior research by the authors have considered the dynamics of entropy generation purely due to heat release for both premixed and non-premixed flames. However, for non-premixed flames where the diffusion-controlled processes are important, transport terms must be considered for consistency. In this work, the entropy production mechanisms due to different sources are analyzed, firstly, by doing a mathematical study on the entropy transport equation and deriving the key forms of the source terms. Then reacting flow simulations are performed for the Sandia D flame, a well-documented configuration, for the numerical analysis of the transport equation. A range of operating conditions have been considered to illustrate the relative importance of different entropy source terms. Overall entropy production rate budget is calculated, and trends are observed for different operating conditions. In addition, across different sections of the flame, local entropy production is analyzed. Subsequently, reacting flow simulations are performed for the classical Burke-Schumann flame, a laminar nonpremixed flame, and the same analysis is carried out to completely gauge the trends of entropy production in non-premixed flames.

[Presentation] CFD Modeling of High-Enthalpy Hypersonic Flows for FIRE-II Re-Entry Vehicle

AIAA Aviation Forum 2024, 2024

Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. W... more Hypersonic flows pose significant challenges in aerospace engineering and atmospheric sciences. While accurate physical modeling and numerical prediction of hypersonic flows are crucial for the design and analysis of this class of vehicles, the extreme conditions make this class of numerical simulations very difficult to converge to a stable and steady solution. The authors' recent study utilized a "two-temperature model" to simulate the hypersonic flow and thermal features of the FIRE-II re-entry vehicle. This previous work helped in understanding important trends for surface heat flux, temperature field, and ionization of air, etc., at five operating points along the vehicle's re-entry trajectory. In the present work, further investigations have been performed on this vehicle by (1) comparing two different chemical reaction mechanisms; (2) applying full and partial catalytic wall (PCW) conditions for surface heat flux predictions. Both chemical mechanisms, the Park and the Gupta models, account for the recombination of the ionized species and have proven to better predict the heating trends on the surface of the capsule. The use of catalytic wall model also improves the prediction of surface heat flux on the vehicle across all the operating points compared to previous studies. While trends in heat flux are captured well for different altitudes, the absolute values and the match with the experiment still present some discrepancies. These discrepancies are in large part attributable to the presence of a large uncertainty in the exact, instantaneous flight conditions, in the assumption of steady-state conditions at each altitude as well as unknown catalytic conditions of the capsule's thermal protection system (TPS). The developed workflow and best practice settings from this study can be applied to more complex hypersonic applications.