163 results found
Readshaw T, Jones WP, Rigopoulos S, 2023, On the incorporation of conservation laws in machine learning tabulation of kinetics for reacting flow simulation, Physics of Fluids, Vol: 35, Pages: 1-18, ISSN: 1070-6631
Tabulation of chemical mechanisms with artificial neural networks (ANNs) offers significant speed benefits when computing the real-time integration of reaction source terms in turbulent reacting flow simulations. In such approaches, the ANNs should be physically consistent with the reaction mechanism by conserving mass and chemical elements, as well as obey the bounds of species mass fractions. In the present paper, a method is developed for satisfying these constraints to machine precision. The method can be readily applied to any reacting system and appended to the existing ANN architectures. To satisfy the conservation laws, certain species in a reaction mechanism are selected as residual species and recalculated after ANN predictions of all of the species have been made. Predicted species mass fractions are set to be bounded. While the residual species mass fractions are not guaranteed to be non-negative, it is shown that negative predictions can be avoided in almost all cases and easily rectified if necessary. The ANN method with conservation is applied to one-dimensional laminar premixed flame simulations, and comparisons are made with simulations performed with direct integration (DI) of chemical kinetics. The ANNs with conservation are shown to satisfy the conservation laws for every reacting point to machine precision and, furthermore, to provide results in better agreement with DI than ANNs without conservation. It is, thus, shown that the proposed method reduces accumulation of errors and positively impacts the overall accuracy of the ANN prediction at negligible additional computational cost.
Ren S, Jones WP, Wang X, 2023, Multi-fuel combustion performance analysis and operating characteristics of a vortex-tube combustor, Energy, Vol: 264, Pages: 1-14, ISSN: 0360-5442
In this work, an ultra-steady combustion technique was attempted to achieve fuel adaptability combustion. The combustion performances of five representative gaseous fuels in a stratified vortex-tube combustor were investigated in terms of the stability limit, pressure fluctuation, and flame topology. Results show that the lean stability limit of the global equivalence ratio for the five fuels can always be less than 0.15 with a uniform flame front, whilst the amplitude of pressure fluctuations is always below 2300 Pa, indicating a super-steady combustion process. The non-premixed flame structure guarantees a high mass concentration near the reaction zone, whilst the vortex flow also decreases the local flow velocity, inhibiting flame blow-out, and suggesting good self-adjusting capacity under various global equivalence ratios. The synergistic action of the flow and flame structures transports the interior high-enthalpy burnt gas and exterior unburnt gas to the exterior unburnt gas and reaction zones to promote the ignition and reaction procedures, resulting in an intensified combustion. The large tangential velocity and density gradient result in the large values of Richardson number, which indicates that laminarization of the flow arises and results in good aero-dynamic and thermo-dynamic stabilities. The resultant good self-adjusting capacity and three types of dynamic stabilities are the intrinsic causes of the ultra-steady combustion process in this combustor. Ultimately, the generalized criterion of stabilization can be defined by the combination of Richardson and Rayleigh numbers, for which large Richardson and small Rayleigh numbers are required for a highly steady combustion process.
Tsagkaridis M, Papadakis G, Jones WP, et al., 2023, Large Eddy Simulation of Turbulent Flame Synthesis of Silica Nanoparticles with an Extended Population Balance Model, Flow, Turbulence and Combustion, ISSN: 1386-6184
In the present study, a recently proposed extended population balance equation (PBE) model for aggregation and sintering is incorporated into a large eddy simulation-probability density function (LES-PDF) modelling framework to investigate synthesis of silica nanoparticles in a turbulent diffusion flame. The stochastic field method is employed to solve the LES-PBE-PDF equations, characterising the influence of the unresolved sub-grid scale motions and accounting for the interactions between turbulence, chemistry and particle dynamics. The models for gas-phase chemistry and aerosol dynamics are the same as those recently used by the authors to simulate silica synthesis in a laminar flame (Tsagkaridis et al. in Aerosol Sci Technol 57(4):296–317, 2023). Thus, by retaining the same kinetics without any adjustments in parameters, we focus on the modelling issues arising in silica flame synthesis. The LES results are compared with experimental in-situ small-angle X-ray scattering (SAXS) data from the literature. Good agreement is found between numerical predictions and experimental data for temperature. However, the LES model underestimates the SAXS data for the primary particle diameter by a factor of two. Possible reasons for this discrepancy are discussed in view of the previous laminar flame simulations.
Ren S, Tian B, Jones WP, et al., 2022, Aerodynamic and thermo-acoustic stability analysis in a super-steady reactive flow, FUEL, Vol: 337, ISSN: 0016-2361
Ding T, Rigopoulos S, Jones WP, 2022, Machine learning tabulation of thermochemistry of fuel blends, Applications in Energy and Combustion Science, Vol: 12, Pages: 1-17, ISSN: 2666-352X
The objective of the present work is to develop a machine learning tabulation methodology for thermochemistry that accounts for fuel blends. The approach is based on the hybrid flamelet/random data and multiple multilayer perceptrons (HFRD-MMLP) methodology (Ding et al., 2021), the essence of which is to train a set of artificial neural networks (ANNs) using random data so as to anticipate the composition space encountered in turbulent flame simulations. As such, it is applicable to any combustion modelling approach that involves direct coupling of chemistry and flow, such as transported probability density function (PDF) methods, direct numerical simulation (DNS), conditional moment closure (CMC), unsteady flamelet, multiple mapping closure (MMC), thickened flame model, linear eddy model (LEM), partially stirred reactor (PaSR) as in OpenFOAM and laminar flame computation. In this paper, the HFRD approach is further developed to generate data of varying fuel ratios. Furthermore, radiative heat losses are included and it is shown that the ANN-based simulations are able to account for it. The ANNs generated are first tested on 1-D laminar flame simulations and then applied to two turbulent flames with different fuel compositions: a pure methane flame, Sandia flame D, and Sydney flame HM1, which is a methane/hydrogen flame. The results of species mass fraction and temperature are compared between ANN and direct integration, and excellent agreement are achieved. These results indicate that the methodology has great capacity for generalisation and is applicable to a range of blended fuels. Furthermore, a speed-up ratio of 14 to 17 is attained for the reaction step compared with direct integration, which greatly reduces the computational cost of turbulent combustion simulations.
Fredrich D, Miniero L, Pandey K, et al., 2022, Large Eddy simulation of a reacting kerosene spray in hot vitiated cross-flow, Flow, Turbulence and Combustion, Vol: 109, Pages: 991-1010, ISSN: 1386-6184
The evaporation and combustion characteristics of a kerosene spray injected perpendicularly into a cross-flow of high-temperature vitiated air is investigated. This fundamental flow configuration has wider implications for the future development of ultra-low emission aeronautical combustors, particularly with respect to technologies involving MILD combustion. Large eddy simulations with a Eulerian–Lagrangian framework are performed to investigate the spray evolution and the characteristics of the reaction zone for a range of conditions. For the closure of turbulence-chemistry interactions at the sub-grid scales, a transported probability density function approach solved by the Eulerian stochastic fields method is applied. A configuration based on the use of airblast atomisation is assessed first and compared with experimental observations. The effect of the atomiser air-to-liquid mass flow ratio is studied in greater detail, both in terms of the resulting gas-phase properties and the droplet evaporation process. Then, the effect of ambient pressure on the global spray flame behaviour is examined. For this part of the study, no atomising air is included in the simulation to separate the effects of ambient pressure on the spray from the interaction with the air jet. Analysis of the flame and spray properties at cross-flow operating pressures of 1 atm, 2 bar and 4 bar highlights the strong coupling between the reacting flow and droplet evaporation characteristics, which are highly affected by the penetration of the spray into a flow field characterised by relatively large gradients of temperature. The results reported in this work provide fundamental understanding for the development of novel low-emission combustion technologies and demonstrate the feasibility of applying large eddy simulation with detailed chemistry for the investigation of reacting aviation fuel sprays in hot vitiated cross-flow.
Liu A, Luo KH, Rigopoulos S, et al., 2022, Effects of the electric field on soot formation in combustion: a coupled charged particle PBE-CFD framework, Combustion and Flame, Vol: 239, ISSN: 0010-2180
In this article, a coupled PBE-CFD framework has been proposed to study counterflow non-premixed flames and soot formation under an external electric field. This framework integrates the population balance equation (PBE) for nanoparticle dynamics into an in-house CFD solver for the multicomponent reactive flows. Different electric properties have been considered in this model. An ion mechanism used in both fuel-rich and fuel-lean combustion is combined with a detailed chemistry for neutral gaseous species and small-size aromatics to retain the full chemistry. In order to model soot particles carrying charges and the movement of the reacting fluid medium in the electric field, a second PBE for the production and transport of charges on soot particles is introduced for the first time and incorporated into the original PBE for the number density of particles. Also, the electric force for the gas mixture is included in the momentum equations. The electric drift velocities for ions and soot particles are also considered in the transport equations of ions and the PBE of soot particles, respectively. The simulations have shown that the presence of the electric field modifies the stagnation plane of the counterflow flames and reduces the soot formation in both rich-fuel and lean-fuel conditions in agreement with experimental observations. The application of the soot particle charging model, accompanied by a proper electric correction factor on the nanoparticle processes of nucleation and surface growth, significantly improves the stability of the flame structure. The introduction of the electric correction factor reveals that the suppression of soot formation in an electric field is mainly caused by the inhibited chemical reactions of the PAH nucleation and particle surface growth, which is more important than the electric drift of the charged particles. Reducing the critical size of the particle charging process enhances the electric drift of nascent soot, thus lessening i
Ren S, Jones WP, Wang X, 2022, Stabilization mechanism revelation of a novel vortex-tube combustion technique: LES with sgs-pdf approach, PHYSICS OF FLUIDS, Vol: 34, ISSN: 1070-6631
Gong Y, Jones W, Marquis A, 2021, Numerical analysis of indirect noise generated by compositional inhomogeneities using large eddy simulation, AIP Advances, Vol: 11, Pages: 1-11, ISSN: 2158-3226
In the present work, indirect noises generated by compositional disturbances in a non-isotropic convergent nozzle are studied using Large Eddy Simulations (LESs). An in-house compressible LES code, Boundary Fitted Flow Integrator-LESc, is utilized to simulate the noise generation in the system. A non-reflective outlet boundary condition is used to eliminate numerical reflections and to ensure the reproduction of the operating conditions in the experiments. The experiments are designed to feature two configurations with different injection positions, which enable the separation of direct and indirect noises. Different operating conditions are investigated, including different injection gases and air mass flow rates. This present paper compares computational results with the experimental measurements. The results revealed that the processes of direct and indirect noise generation are successfully reproduced in the LES, with the noise magnitudes in good agreement with those in the measurements. Injection of gases with smaller (He) and larger (CO2) molar masses compared to air is found to generate negative and positive indirect noises, respectively, in the LES, which is consistent with the experimental findings. The effect of different air mass flow rates is also investigated and discussed, and the direct noise and indirect noise amplitudes are both found to be closely related to the air mass flow rate.
Fredrich D, Jones WP, Giusti A, 2021, Numerical investigation of a reacting kerosene spray in hot vitiated cross-flow, 13th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, Publisher: ERCOFTAC, Pages: 1-6
The behaviour of a liquid kerosene spray injectedinto a cross-flow of high temperature vitiated air is in-vestigated. This fundamental flow configuration haswider implications for the design of future aeronauti-cal fuel injectors, particularly with respect to MILDcombustion concepts. Large eddy simulation is ap-plied to numerically study the global flame character-istics, with the main objectives of giving further in-sight into the reacting behaviour of sprays in cross-flow and comparing two different approaches to modelturbulence-chemistry interaction: the conditional mo-ment closure model and the Eulerian stochastic fieldsmethod. Results show that the two approaches givesimilar predictions of the location of the peak meantemperature. Some differences appear in the vicinityof the spray injection location, possibly highlightingthe important role of the modelling of the interactionbetween evaporation and sub-grid mixing. Analysisof the flame and spray behaviour at different pressuresalso demonstrates the strong coupling between the re-acting field and evaporation characteristics, which arehighly affected by the penetration of the spray into aflow field characterised by relatively large gradients oftemperature. Results obtained in this work provide abenchmark for the application of sprays in hot cross-flow for the development of novel combustion tech-nologies.
Ding T, Readshaw T, Rigopoulos S, et al., 2021, Machine learning tabulation of thermochemistry in turbulent combustion: An approach based on hybrid flamelet/random data and multiple multilayer perceptrons, Combustion and Flame, Vol: 231, Pages: 1-23, ISSN: 0010-2180
A new machine learning methodology is proposed for speeding up thermochemistry computations in simulations of turbulent combustion. The approach is suited to a range of methods including Direct Numerical Simulation (DNS), Probability Density Function (PDF) methods, unsteady flamelet, Conditional Moment Closure (CMC), Multiple Mapping Closure (MMC), Linear Eddy Model (LEM), Thickened Flame Model, the Partially Stirred Reactor (PaSR) method (as in OpenFOAM) and the computation of laminar flames. In these methods, the chemical source term must be evaluated at every time step, and is often the most expensive element of a simulation. The proposed methodology has two main objectives: to offer enhanced capacity for generalisation and to improve the accuracy of the ANN prediction. To accomplish the first objective, we propose a hybrid flamelet/random data (HFRD) method for generating the training set. The random element endows the resulting ANNs with increased capacity for generalisation. Regarding the second objective, a multiple multilayer perceptron (MMP) approach is developed where different multilayer perceptrons (MLPs) are trained to predict states that result in smaller or larger composition changes, as these states feature different dynamics. It is shown that the multiple MLP method can greatly reduce the prediction error, especially for states yielding small composition changes. The approach is used to simulate flamelets of varying strain rates, one-dimensional premixed flames with differential diffusion and varying equivalence ratio, and finally the Large Eddy Simulation (LES) of /air piloted flames Sandia D, E and F, which feature different levels of local extinction. The simulation results show very good agreement with those obtained from direct integration, while the range of problems simulated indicates that the approach has great capacity for generalisation. Finally, a speed-up ratio of 12 is attained for the reaction step.
Fredrich D, Jones W, Marquis A, 2021, A combined oscillation cycle involving self-excited thermo-acoustic and hydrodynamic instability mechanisms, Physics of Fluids, Vol: 33, Pages: 1-22, ISSN: 1070-6631
The paper examines the combined effects of several interacting thermo-acoustic and hydrodynamic instability mechanisms that are known to influence self-excited combustion instabilities often encountered in the late design stages of modern low-emission gas turbine combustors. A compressible large eddy simulation approach is presented, comprising the flame burning regime independent, modeled probability density function evolution equation/stochastic fields solution method. The approach is subsequently applied to the PRECCINSTA (PREDiction and Control of Combustion INSTAbilities) model combustor and successfully captures a fully self-excited limit-cycle oscillation without external forcing. The predicted frequency and amplitude of the dominant thermo-acoustic mode and its first harmonic are shown to be in excellent agreement with available experimental data. Analysis of the phase-resolved and phase- averaged fields leads to a detailed description of the superimposed mass flow rate and equivalence ratio fluctuations underlying the governing feedback loop. The prevailing thermo-acoustic cycle features regular flame liftoff and flashback events in combination with a flame angle oscillation, as well as multiple hydrodynamic phenomena, i.e., toroidal vortex shedding and a precessing vortex core. The periodic excitation and suppression of these hydrodynamic phenomena is confirmed via spectral proper orthogonal decomposition and found to be controlled by an oscillation of the instantaneous swirl number. Their local impact on the heat release rate, which is predominantly modulated by flame-vortex roll- up and enhanced mixing of fuel and oxidizer, is further described and investigated. Finally, the temporal relationship between the flame “surface area,” flame-averaged mixture fraction, and global heat release rate is shown to be directly correlated.
Gong Y, Jones WP, Marquis AJ, 2021, Study of a premixed turbulent counter-flow flame with a large eddy simulation method, Flow, Turbulence and Combustion, Vol: 106, Pages: 1379-1398, ISSN: 0003-6994
The turbulent counter-flow flame (TCF) has proven to be a useful benchmark to study turbulence-chemistry interactions, however, the widely observed bulk flow fluctuations and their influence on the flame stability remain unclear. In the present work, premixed TCFs are studied numerically using a Large Eddy Simulation (LES) method. A transported probability density function (pdf) approach is adopted to simulate the sub-grid scale (sgs) turbulence-chemistry interactions. A solution to the joint sgs-pdf evolution equation for each of the relative scalars is obtained by the stochastic fields method. The chemistry is represented using a simplified chemical reaction mechanism containing 15 reaction steps and 19 species. This work compares results with two meshing strategies, with the domain inside nozzles included and excluded respectively. A conditional statistical approach is applied to filter out the large scale motions of the flame. With the use of digital turbulence, the velocity field in the flame region is well reproduced. The processes of local extinction and re-ignition are successfully captured and analysed together with the strain rate field, and local extinctions are found correlated to the turbulent structures in the reactant stream. The predicted probability of localised extinction is in good agreement with the measurements, and the influence of flame stoichiometry are also successfully reproduced. Overall, the current results serve to demonstrate the capability of the LES-pdf method in the study of the premixed opposed jet turbulent flames.
Gallot-Lavallee S, Jones WP, Marquis AJ, 2021, Large eddy simulation of an ethanol spray flame with secondary droplet breakup, Flow, Turbulence and Combustion, Vol: 107, Pages: 709-743, ISSN: 0003-6994
A computational investigation of three configurations of the Delft Spray in Hot-diluted Co-flow (DSHC) is presented. The selected burner comprises a hollow cone pressure swirl atomiser, injecting an ethanol spray, located in the centre of a hot co-flow generator, with the conditions studied corresponding to Moderate or Intense Low-oxygen Dilution (MILD) combustion. The simulations are performed in the context of Large Eddy Simulation (LES) in combination with a transport equation for the joint probability density function (pdf) of the scalars, solved using the Eulerian stochastic field method. The liquid phase is simulated by the use of a Lagrangian point particle approach, where the sub-grid-scale interactions are modelled with a stochastic approach. Droplet breakup is represented by a simple primary breakup model in combination with a stochastic secondary breakup formulation. The approach requires only a minimal knowledge of the fuel injector and avoids the need to specify droplet size and velocity distributions at the injection point. The method produces satisfactory agreement with the experimental data and the velocity fields of the gas and liquid phase both averaged and ‘size-class by size-class’ are well depicted. Two widely accepted evaporation models, utilising a phase equilibrium assumption, are used to investigate the influence of evaporation on the evolution of the liquid phase and the effects on the flame. An analysis on the dynamics of stabilisation sheds light on the importance of droplet size in the three spray flames; different size droplets play different roles in the stabilisation of the flames.
Readshaw T, Ding T, Rigopoulos S, et al., 2021, Modeling of turbulent flames with the large eddy simulation–probability density function (LES–PDF) approach, stochastic fields, and artificial neural networks, Physics of Fluids, Vol: 33, Pages: 1-17, ISSN: 1070-6631
This work proposes a chemical mechanism tabulation method using artificial neural networks (ANNs) for turbulent combustion simulations. The method is employed here in the context of the Large-Eddy Simulation (LES)–Probability Density Function (PDF) approach and the method of stochastic fields for numerical solution, but can also be employed in other methods featuring real-time integration of chemical kinetics. The focus of the paper is on exploring an ANN architecture aiming at improved generalization, which uses a single multilayer perceptron (MLP) for each species over the entire training dataset. This method is shown to outperform previous approaches which take advantage of specialization by clustering the composition space using the Self-Organizing Map (SOM). The ANN training data are generated using the canonical combustion problem of igniting/extinguishing one-dimensional laminar flamelets with a detailed methane combustion mechanism, before being augmented with randomly generated data to produce a hybrid random/flamelet dataset with improved composition space coverage. The ANNs generated in this study are applied to the LES of a turbulent non-premixed CH4/air flame, Sydney flame L. The transported PDF approach is used for reaction source term closure, while numerical solution is obtained using the method of stochastic fields. Very good agreement is observed between direct integration (DI) and the ANNs, meaning that the ANNs can successfully replace the integration of chemical kinetics. The time taken for the reaction source computation is reduced 18-fold, which means that LES–PDF simulations with comprehensive mechanisms can be performed on modest computing resources.
Gong Y, Fredrich D, Jones WP, et al., 2021, Thermoacoustic instabilities of hydrogen-enriched partially premixed flames in a swirl combustor, Turbo Expo 2021 Turbomachinery Technical Conference & Exposition
Fredrich D, Jones WP, Marquis AJ, 2020, Thermo-acoustic Instabilities in the PRECCINSTA combustor investigated using a compressible LES-pdfApproach, Flow, Turbulence and Combustion, Vol: 106, Pages: 1399-1415, ISSN: 0003-6994
This work predicts the evolution of self-excited thermo-acoustic instabilities in a gas turbine model combustor using large eddy simulation. The applied flow solver is fully compressible and comprises a transported sub-grid probability density function approach in conjunction with the Eulerian stochastic fields method. An unstable operating condition in the PRECCINSTA test case—known to exhibit strong flame oscillations driven by thermo-acoustic instabilities—is the chosen target configuration. Good results are obtained in a comparison of time-averaged flow statistics against available measurement data. The flame’s self-excited oscillatory behaviour is successfully captured without any external forcing. Power spectral density analysis of the oscillation reveals a dominant thermo-acoustic mode at a frequency of 300 Hz; providing remarkable agreement with previous experimental observations. Moreover, the predicted limit-cycle amplitude is found to closely match its respective measured value obtained from experiments with rigid metal combustion chamber side walls. Finally, a phase-resolved study of the oscillation cycle is carried out leading to a detailed description of the physical mechanisms that sustain the closed feedback loop.
Fredrich D, Jones W, Marquis A, 2019, The stochastic fields method applied to a partially premixed swirl flame with wall heat transfer, Combustion and Flame, Vol: 205, Pages: 446-456, ISSN: 0010-2180
Large eddy simulations of a partially premixed flame are performed with the purpose of predicting the reacting flow in a swirl-stabilised, low emissions industrial gas turbine combustor. The corresponding sub-grid scale turbulence–chemistry interactions are modelled using a probability density function transport equation, which is solved by the stochastic fields method. A 15-step reduced, but accurate, methane mechanism including 19 species is employed for the description of all chemical reactions. The test case involves a combustor with complex geometry and simulations are carried out for two different combustor operating conditions. Overall, results of the velocity, temperature and species mass fractions (including carbon monoxide) as well as the instantaneous thermochemical properties are shown to be in good agreement with experimental data, demonstrating the capabilities of the applied stochastic fields method. The inclusion of wall heat transfer in the combustion chamber is found to improve temperature and species predictions, especially in the near-wall regions. Comparisons between an oscillating and a ‘stable’ flame case furthermore highlight the influence of experimentally observed thermo-acoustic instabilities on the scalar fluctuations near the combustor centreline. None of the default model parameters were adjusted and the results showcase the accuracy and flexibility of the present large eddy simulation method for an application to complex, partially premixed combustion problems; this being particularly important for the designers of new generation low emission gas turbine combustors.
Fredrich D, Jones WP, Marquis A, 2019, Thermo-acoustic instabilities in the PRECCINSTA combustor investigated using a compressible LES-pdf approach, 11th Mediterranean Combustion Symposium, Publisher: The Combustion Institute, Pages: 1-12
This work predicts the evolution of self-excited thermo-acoustic instabilities in a gas turbine model combustor using large eddy simulation. The developed flow solver is fully compressible and comprises a transported sub-grid probability density function approach in conjunction with the Eulerian stochastic fields method. An unstable operating condition in the PREC-CINSTA test case involving flame oscillation driven by thermo-acoustic instabilities is the chosen target configuration. Good results are obtained in a comparison of time-averaged flow statistics against experimental data. The flame’s self-excited oscillatory behaviour is successfully captured without any external forcing involved. Power spectral density analysis of the oscillation reveals a dominant thermo-acoustic mode at a frequency of 300 Hz providing remarkable agreement with experimental observations. Moreover, the predicted limit-cycle amplitude closely matches the experimental value obtained with rigid metal combustion chamber side walls. Finally, a phase-resolved study of the oscillation cycle is carried out leading to a detailed description of the physical mechanism closing the feedback loop.
Xia Y, Laera D, Jones WP, et al., 2019, Numerical prediction of the Flame Describing Function and thermoacoustic limit cycle for a pressurised gas turbine combustor, Combustion Science and Technology, Vol: 191, Pages: 979-1002, ISSN: 0010-2202
The forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1 large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements.
Jones WP, Marquis AJ, Vogiatzaki K, 2019, Assessing the effect of differential diffusion for stratified lean premixed turbulent flames with the use of LES-PDF framework, Combustion Science and Technology, Vol: 191, Pages: 1003-1018, ISSN: 0010-2202
Lean premixed stratified combustion is rapidly growing in importance for modern engine designs. This paper presents large eddy simulations for a new burner design to assess the predictive capability of the probability density function (pdf) approach to flames that propagate through non-homogeneous mixtures in terms of equivalence ratio. Although various efforts have been made in the past for the simulation of the same test case the novelty of this work lies to the fact that it is the first simulation effort that differential diffusion is accounted for given the relatively low Reynolds numbers (13,800) of the configuration. First mean and root mean square velocity simulations are performed for the isothermal cases to assess the effect of the grid resolution and the overall LES flow field solver. Then instantaneous snapshots of the flame are presented to provide insight to the structure of the flame and the effect of stratification. Finally, results for velocities, temperature and mixture fraction are presented and compared with the experimental data. Overall, the results are in very good agreement with experiments.
Fredrich D, Jones W, Marquis A, 2019, LES-pdf simulation of combustion dynamics in a partially premixed swirl combustor, 9th European Combustion Meeting, Publisher: The Combustion Institute, Pages: 1-6
The dynamic behaviour of the partially premixed, swirl-stabilised PRECCINSTA gas turbine model combustor is studied using a fully compressible large eddy simulation solver with a 15-step and 19 species reduced methane mechanism. The solver applies a transported sub-grid probability density function approach solved by the Eulerian stochastic fields method to allow for a burning regime independent description of turbulent flames. A specific operating condition prone to self-excited thermo-acoustic and hydrodynamic instabilities is targeted to evaluate the predictive capabilities of the computational method. Excellent agreement with experimental data is obtained for the predicted thermo-acoustic mode representing a pronounced limit-cycle oscillation at the correct frequency and amplitude. Moreover, the periodic formation and suppression of a precessing vortex core is identified as well as periodic shedding of toroidal vortices at the burner nozzle rim. Both phenomena are shown to be directly coupled to the dominant thermo-acoustic mode and their respective interaction with the flame is investigated in further detail.
Noh D, Karlis E, Navarro-Martinez S, et al., 2019, Azimuthally-driven subharmonic thermoacoustic instabilities in a swirl-stabilised combustor, Proceedings of the Combustion Institute, Vol: 37, Pages: 5333-5341, ISSN: 0082-0784
A joint experimental and computational study of thermoacoustic instabilities in a model swirl-stabilised combustor is presented. This paper aims to deliver a better characterisation of such instabilities through the examination of measurements in conjunction with the numerical results obtained via Large Eddy Simulation (LES). The experimental configuration features a cylindrical combustion chamber where the lean premixed methane/air flame experiences self-sustained thermoacoustic oscillations. The nonlinear behaviour of the acoustically excited flame is experimentally investigated by broadband chemiluminescence and dynamic pressure measurements. In LES, the Eulerian stochastic field method is employed for the unknown turbulence/chemistry interaction of the gas-phase. Comparisons of the predicted frequency spectrum and phase-resolved flame structure with measurements are found to be in good agreement, confirming the predictive capabilities of the LES methodology. The presence of Period 2 thermoacoustic oscillations, as a consequence of period-doubling bifurcation, is also confirmed in LES. Through the application of nonlinear analysis both to the experimental and numerical acoustic fluctuations, it is highlighted that the nonlinear behaviour of combustion instabilities in the burner under investigation follows a pattern typical of Period 2 oscillations. Furthermore, the current work demonstrates a useful approach, through the use of dynamic mode decomposition (DMD), for the investigation of unstable flame modes at a specific frequency of interest. The experimental and numerical DMD reconstructions suggest that hot combustion products are convected azimuthally at a rate dictated by the subharmonic frequency.
Fredrich D, Jones WP, Marquis AJ, 2019, Application of the Eulerian subgrid Probability Density Function method in the Large Eddy Simulation of a partially premixed swirl flame, Combustion Science and Technology, Vol: 191, Pages: 137-150, ISSN: 0010-2202
A gas turbine model combustor is studied using Large Eddy Simulation with a transported Probability Density Function approach solved by the Eulerian stochastic field method. The chemistry is represented by a reduced methane mechanism containing 15 steps and 19 species while the subgrid scale stresses and scalar fluxes are modelled, respectively, via a dynamic Smagorinsky model and a gradient diffusion approximation. The test case comprises a partially premixed swirl flame in a complex geometry. Four stochastic fields are utilised in the simulations, which are performed for two different combustor operating conditions involving a stable and an unstable flame. Good agreement between the simulation and measurement data is shown in a comparison of mean velocity, temperature and species mass fraction profiles, as well as scatter plots of the instantaneous thermochemical properties. In conclusion, the predictive capabilities of the employed Large Eddy Simulation method are successfully demonstrated in this work.
Fredrich D, Jones WP, Marquis A, 2018, Large eddy simulation of an oscillating flame using the stochastic fields method, 12th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, Pages: 1-6
Large eddy simulation (LES) of a partially pre-mixed, swirl-stabilised flame is performed using atransported Probability Density Function approachsolved by the stochastic fields method to accountfor turbulence-chemistry interaction on the sub-gridscales. The corresponding sub-grid stresses and scalarfluxes are modelled via a dynamic version of theSmagorinsky model and a gradient diffusion approx-imation, respectively.A 15-step reduced methanemechanism including 19 species is employed for thedescription of all chemical reactions. The test case in-volves a widely studied gas turbine model combustorwith complex geometry and the simulation is carriedout for a specific operating condition involving an os-cillating flame. Overall, results of the velocity, temper-ature and major species mass fractions as well as theinstantaneous thermochemical properties are shown tobe in good agreement with experimental data, demon-strating the capabilities of the applied stochastic fieldsmethod. The inclusion of wall heat transfer in the com-bustion chamber is found to improve temperature pre-dictions, especially in the near-wall regions. In sum-mary, this work showcases the LES method’s accuracyand robustness - none of the default model parametersare adjusted - for an application to complex, partiallypremixed combustion problems
Koniavitis P, Rigopoulos S, Jones W, 2018, Reduction of a detailed chemical mechanism for a kerosenesurrogate via RCCE-CSP, Combustion and Flame, Vol: 194, Pages: 85-106, ISSN: 0010-2180
Detailed mechanisms for kerosene surrogate fuels contain hundreds of species and thousands of reactions, indicating a necessity for reduced mechanisms. In this work, we employ a framework that combines Rate-Controlled Constrained Equilibrium (RCCE) with Computational Singular Perturbation (CSP) for systematic reduction based on timescale analysis, to reduce a detailed mechanism for a jet fuel surrogate with n-dodecane, methylcyclohexane and m-xylene. Laminar non-premixed flamelets are utilised for the CSP analysis for different strain rates and therefore different scalar dissipation rate, covering the flammable region of strain rates for the surrogate fuel.Two RCCE-reduced mechanisms are developed via an RCCE-CSP methodology, one with 17 and one with 42 species, and their accuracy is assessed in a range of cases that test the performance of the reduced mechanism under both non-premixed and premixed conditions and its dynamic response. These include non-premixed flamelets with varying strain rate, laminar premixed flames for a range of equivalence ratios and pressures, flamelets ignited by an artificial pilot or by hot air, and unsteady flamelets with time-dependent strain rate.The profiles of both major and minor species, as well as important combustion characteristics such as the ignition strain rate and the laminar flame speed, are investigated. The structure of non-premixed flamelets is very well predicted, while the premixed flames are overall well predicted apart from a few deviations in certain species and an underprediction in the laminar flame speed. Apart from the large reduction in dimensionality, the reduction in computational time is also considerable (up to 19 times). As the detailed mechanism comprises 367 species and 1892 reactions, this paper presents the first application of RCCE to a mechanism of this size, as well as a comprehensive validation in a set of cases that include non-premixed and premixed laminar flames, atmospheric and elevated pressur
Noh D, Gallot-Lavallée S, Jones WP, et al., 2018, Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution, Combustion and Flame, Vol: 194, Pages: 135-151, ISSN: 0010-2180
The current investigation aims to present the numerical results of a turbulent spray jet flame in the framework of Large Eddy Simulation. The injection of n-heptane liquid fuel is achieved through the use of a pressure-swirl atomiser generating a lifted spray flame. The evolution of the sub-grid probability density function of relevant scalars is accounted for by the Eulerian stochastic field method. A non-reactive spray computation is conducted in a preliminary stage. The simulation allows for a stochastic breakup formulation in combination with a stochastic dispersion model to confirm their predictive capabilities. This is achieved as the liquid properties such as droplet diameter and velocity profiles are found to be in excellent agreement between the simulated results and measurements. The main target of the work is to assess the performance of the most widely accepted evaporation models in a turbulent droplet-laden flame. In this paper, a detailed comparison of the evaporation models is conducted thanks to a recent development in measurement techniques which allow to permit the spray temperature profiles. The time-averaged droplet temperature across the spray flame is therefore investigated in order to validate several droplet vaporisation models. All the models under consideration are found to capture the formation of a double reaction zone flame and the measured lift-off height is reproduced within a satisfactory level of accuracy. This good reproduction of the flame morphology additionally confirms the performance of the pdf approach as a closure for the unknown turbulence–chemistry interaction in this type of spray flames. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zones in addition to the profile along the spr ay centreline show a large discrepancy when compared to measurements. This large difference thus requires further investigation both in the modelling of evaporation and in the accuracy of measureme
Xia Y, Laera D, Morgans AS, et al., 2018, Thermoacoustic limit cycle predictions of a pressurised longitudinal industrial gas turbine combustor, ASME Turbo Expo 2018, Publisher: American Society of Mechanical Engineers
Xia Y, Laera D, Morgans AS, et al., 2018, Thermoacoustic limit cycle predictions of a pressurised longitudinal industrial gas turbine combustor
This article presents numerical prediction of a thermoacoustic limit cycle in an industrial gas turbine combustor. The case corresponds to an experimental high pressure test rig equipped with the full-scale Siemens SGT-100 combustor operated at two mean pressure levels of 3 bar and 6 bar. The Flame Transfer Function (FTF) characterising the global unsteady response of the flame to velocity perturbations is obtained for both operating pressures by means of incompressible Large Eddy Simulations (LES). A linear stability analysis is then performed by coupling the FTFs with a wave-based low order thermoacoustic network solver. All the thermoacoustic modes predicted at 3 bar pressure are stable; whereas one of the modes at 6 bar is found to be unstable at a frequency of 231 Hz, which agrees with the experiments. A weakly nonlinear stability analysis is carried out by combining the Flame Describing Function (FDF) predicted by LES with the low order thermoacoustic network solver. The frequency, mode shape and velocity amplitude corresponding to the predicted limit cycle at 209 Hz are used to compute the absolute pressure fluctuation amplitude in the combustor. The numerically reconstructed amplitude is found to be reasonably close to the measured dynamics.
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