Publications
309 results found
Ren S, Tian B, Jones WP, 2024, Assessing the effects of pressure variations on combustion dynamics in vortex-tube flows, Physics of Fluids, Vol: 36, ISSN: 1070-6631
In our investigation, we scrutinize combustion stability in vortex-tube combustion at high operating pressures by examining stability limits, flame configurations, and pressure oscillations. Our examination deeply probes the intricacies of flow and flame behaviors in terms of aerodynamic, thermodynamic, and flamedynamic aspects to identify the fundamental reasons behind stability variances at different pressure levels. The findings indicate that combustion instability escalates with rising operating pressures, marked by increased variability in flame patterns and a monotonic upsurge in pressure oscillation amplitudes. Although aerodynamic and thermodynamic stabilities remain unaffected, the thermoacoustic stability is compromised at elevated pressures. This is evidenced by the strong link between the Rayleigh criterion and the amplitude of pressure fluctuations, with an increased “gain” in the flame transfer function as pressure mounts. The core of the observed thermoacoustic instability is traced back to heightened density variations and mean flow velocities at high pressures, leading to amplified momentum flux oscillations.
Wang F, Wang Y, Wei G, et al., 2024, Flame Structure of Methane and Kerosene Combustion with A Compact Concave Flame-Holder using the LES-pdf Method, Journal of Thermal Science, Vol: 33, Pages: 222-234, ISSN: 1003-2169
Compact flame-holders for afterburners are an increasing requirement for modern aero engines. However, flame-holder design is non-trivial since high inlet temperatures, velocities, and elaborate structures induce complex turbulence, combustion, and spray coupling in modern afterburners. In this work, the LES-pdf and stochastic fields-Lagrangian particle spray methods are used to investigate methane and aviation kerosene combustion structures formed by new-type concave flame-holders. The flow pattern, combustion mode, and flame structure of gaseous and liquid fuel around a concave flame-holder are analyzed, discussed, and compared with experimental results. Results reveal that the flame stability of a concave flame-holder is better than that of the non-concave one. Furthermore, when using liquid fuel, the concave flame-holder forms a stable and compact flame. These results suggest concave flame-holders are a promising design for compact afterburners.
Readshaw T, Franke LLC, Jones WP, et al., 2023, Simulation of turbulent premixed flames with machine learning - tabulated thermochemistry, Combustion and Flame, Vol: 258, ISSN: 0010-2180
The numerical integration of the differential equations describing chemical kinetics consumes the majority of computational time in combustion simulations that involve 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. This step can be accelerated by tabulation, and artificial neural networks (ANNs) have recently emerged as a powerful technique in this domain. To be applicable to a wide family of problems, an ANN tabulation approach must be based on data generated by an abstract process, rather than from the turbulent flame to be simulated. In the present work, the hybrid flamelet/random data and multiple multilayer perceptrons (HFRD-MMLP) method (Ding et al., Combust. Flame 231, 111493, 2021) for non-premixed flames is taken as a basis to develop a thermochemistry tabulation method for premixed flames. In the spirit of maintaining an essentially random data set that still originates in meaningful composition states, a set of one-dimensional premixed flame simulations is employed to generate data that are used as starting points for a random data generation process and subsequently discarded. The approach is applied to large eddy simulations (LES) of the Cambridge/Sandia swirl burner in configurations five and six, with the transported PDF method employed to provide closure for the filtered reaction source terms and the stochastic fields method used for numerical solution. Very good agreement in both major and minor species is observed between the LES-PDF simulations using direct integration of the reaction source term and the ones with the ANNs. Furthermore, the average time taken for reaction source term computations is reduced by fourteen times, while memory requirement
Ren S, Li F, Jones WP, et al., 2023, An advanced vortex-tube technology for pure ammonia combustion with clean and steady peculiarity, PHYSICS OF FLUIDS, Vol: 35, ISSN: 1070-6631
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, Vol: 111, Pages: 1029-1057, ISSN: 0003-6994
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.
Gong Y, Fredrich D, Marquis AJ, et al., 2023, Numerical investigation of combustion instabilities in swirling flames with hydrogen enrichment, Flow, Turbulence and Combustion, Vol: 111, Pages: 953-993, ISSN: 0003-6994
This work presents a numerical study on technically premixed, swirl-stabilised flames in the PRECCINSTA model combustor. The employed method, BOFFIN-LES, comprises a fully compressible formulation to study unsteady combustion with thermo-acoustic instabilities. To allow for this, the iso-thermal flows are first investigated, based on which three reacting cases are established. The investigation delves into various aspects including flame topology, flow characteristics, and the related thermo-acoustic and hydrodynamic instabilities are studied and results are benchmarked against available measurement data. The dominant feedback mechanism of the observed thermo-acoustic fluctuations is identified; the evolution of the helical vortex is discussed together with the related flame stabilisation process. Furthermore, the interplay of the thermo-acoustic oscillations, helical structure, and the flame stabilisation process is summarised in the end, with the potential effect of the wall-heat transfer on them discussed. This work establishes that the Large Eddy Simulation (LES) effectively captures the iso-thermal flow dynamics and the flame topology under various operating conditions, with a good prediction of the thermo-acoustic frequencies in all the cases. The dominant driving mechanism of the observed thermo-acoustic fluctuations was identified as a combined effect of equivalence ratio and velocity fluctuations in all the cases investigated. The effect of Hydrogen enrichment on modifying the flame topology and changing the thermo-acoustic instability features are well predicted by the simulations. Moreover, different modes of the helical vortex are detected, and their periodic excitement, evolution, and effect on flame stabilisation are discussed in great detail. To conclude, this LES-based investigation offers valuable insights into the complex interplay of unsteady combustion, acoustic fluctuations, flow dynamics, and solid boundaries within swirling flames subjected to
Ren S, Tian B, Jones WP, et al., 2023, The response mechanism of pressure fluctuation to the unsteady heat release in a strong rotating environment, Aerospace Science and Technology, Vol: 139, ISSN: 1270-9638
The response mechanism of the pressure fluctuation to the unsteady heat release rate at the ‘unsteady’ combustion state in a strong rotating environment was investigated using a stratified vortex-tube combustor via a large eddy simulation method. Results show that the peak amplitude of heat release fluctuation at the ‘unsteady’ state is just 2.8 × 106 W/m3 and the peak amplitude of pressure fluctuation is always within 3000 Pa based on the vortex-tube configuration, indicating a weak fluctuation degree. The unsteady heat release does not bring about a higher momentum flux fluctuation due to quick momentum damping in the rotating reactive environment. The pressure fluctuation does not relate closely to the momentum flux and its fluctuation; this is due to the effect of the centrifugal force caused by rotating behavior. After splitting the momentum flux into the different components, we found that the tangential component acts in reverse to that of the radial and axial directions in the exterior region. Viz., the pressure increases when the tangential momentum flux increases in the exterior region but decreases when the radial and axial components increase. The laminarization of the fluid suppresses the fluctuations in the radial and axial directions only, therefore the uneven attenuation of the momentum flux fluctuation in different directions is the dominant cause of the enlarged pressure fluctuation in the ‘unsteady’ burning state. Moreover, we find that the pressure fluctuation degree is also closely related to the centrifugal weight coefficient. When this coefficient is far away from zero, the centrifugal effect will become obvious, and vice versa.
Ren S, Jones WPP, Wang X, et al., 2023, Combustion Intensification Mechanism in a Vortex-Tube Reactive Flow, AIAA JOURNAL, ISSN: 0001-1452
Ren S, Tian B, Jones WP, et al., 2023, Aerodynamic and thermo-acoustic stability analysis in a super-steady reactive flow, FUEL, Vol: 337, ISSN: 0016-2361
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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, Hydrogen-enriched methane combustion in a swirl vortex-tube combustor, FUEL, Vol: 334, ISSN: 0016-2361
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.
Fredrich D, Jones WP, Marquis AJ, et al., 2023, Longitudinal and azimuthal thermo-acoustic instabilities in an industrial gas turbine combustor operating at elevated pressure, International Journal of Spray and Combustion Dynamics, ISSN: 1756-8277
This work numerically investigates longitudinal and azimuthal thermo-acoustic instabilities in the swirl-stabilised can-type industrial SGT-100 gas turbine combustor operated at elevated pressures of 3 and 6 bar. Previous experiments have shown that the combustor is susceptible to self-excited flame oscillations sustained by a thermo-acoustic feedback loop at specific operating conditions. In order to gain a better understanding of this feedback loop, a fully compressible large eddy simulation method is applied. The unknown sub-grid scale turbulence-chemistry interactions are modelled via a transported probability density function approach solved by the Eulerian stochastic fields method. First, the reaction zones and global flame topology at both operating pressures are analysed and compared to experimental images providing good qualitative agreement. Radial profiles of time-averaged and root-mean-square quantities furthermore demonstrate good quantitative agreement with the available measurement data. The applied simulation approach is capable of successfully reproducing self-excited thermo-acoustic instabilities in the longitudinal direction. The fundamental frequency of the predicted limit-cycle oscillation matches the experimentally measured frequency with high accuracy. Similar to the experimental observations, the fluctuation amplitudes of the pressure and global heat release rate increase significantly upon increasing the mean operating pressure from 3 to 6 bar. In addition to the dominant longitudinal mode, a high-frequency, low-amplitude azimuthal mode is also identified at both pressures. This azimuthal mode is periodically amplified and attenuated by the superposed longitudinal mode and induces small asymmetric (around the burner circumference) fluctuations of the local fuel and total mixture mass flow rates entering the flame region.
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 <i>sgs</i>-<i>pdf</i> approach, PHYSICS OF FLUIDS, Vol: 34, ISSN: 1070-6631
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Gong Y, Jones W, Marquis A, 2022, Large Eddy Simulation of compositional indirect noises generated in a non-isentropic nozzle
In the present work, noises generated by compositional disturbances in a non-isotropic convergent nozzle is studied using Large Eddy Simulation (LES). The disturbances are created by a cross-flow pulse injection of a secondary gas with a different composition. The experiments are designed to feature two configurations, which enables the separation of direct and indirect noises. Compressible LES code BOFFIN-LES is utilised to account for noise generation and propagation effect. Different injecting positions, main jet mass flow rates and injection gases corresponding to the experiments are studied. 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. The predicted noise amplitudes were found to be closely related to the losses in the system, which was over-predicted in the simulation when the Mach number in the nozzle approaching unity.
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.
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, 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.
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.
Brandon JA, Jones W, Ohman MD, 2019, Multidecadal increase in plastic particles in coastal ocean sediments, SCIENCE ADVANCES, Vol: 5, ISSN: 2375-2548
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Jones WP, 2019, Reconsidering Southern Labor History: Race, Class, and Power, JOURNAL OF AMERICAN HISTORY, Vol: 106, Pages: 425-426, ISSN: 0021-8723
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.
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