84 results found
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.
Gallot-Lavallee S, Jones WP, Marquis AJ, 2021, Large eddy simulation of an ethanol spray flame with secondary droplet breakup, Flow, Turbulence and Combustion, 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.
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.
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.
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.
Gong Y, Jones W, Marquis A, 2019, Study of an opposed jet turbulent flame using the sub-grid PDF method, 9th European Combustion Meeting
Turbulent premixed flames generated by a counter-flow burner are studied numerically using Large EddySimulation (LES). A transported probability density function (pdf ) approach is adopted to simulate sub-gridscale (sgs) turbulence-chemistry interaction. A solution to the joint sgs-pdf evolution equation of the scalarsis obtained by the Eulerian stochastic field method. The chemistry is represented by means of a simplifiedchemical reaction mechanism containing 15 reaction steps and 19 species, and the subgrid scale stressesand scalar fluxes are modelled by a dynamic Smagorinsky model and a gradient type model respectively.This work investigates the the effect of the interaction of turbulent flames and combustion product on thepremixed counter-flow flame in terms of the probability of localised extinction. The simulations show goodagreement with the experimental measurements in terms of velocity fields in both absolute and relativeframes, and the local progress variable which reflects the probability of finding the fresh combustion productare reasonably reproduced using 8 stochastic fields. Overall, the results serve to demonstrate the capabilityof the LES-pdf method in the study of the premixed opposed jet turbulent flame.
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.
Bartholomew PT, Denner F, Azis H, et al., 2018, Unified formulation of the momentum-weighted interpolation for collocated variable arrangements, Journal of Computational Physics, Vol: 375, Pages: 177-208, ISSN: 0021-9991
Momentum-weighted interpolation (MWI) is a widely used discretisation method to prevent pressure–velocity decoupling in simulations of incompressible and low Mach number flows on meshes with a collocated variable arrangement. Despite its popularity, a unified and consistent formulation of the MWI is not available at present. In this work, a discretisation procedure is devised following an in-depth analysis of the individual terms of the MWI, derived from physically consistent arguments, based on which a unified formulation of the MWI for flows on structured and unstructured meshes is proposed, including extensions for discontinuous source terms in the momentum equations as well as discontinuous changes of density. As shown by the presented analysis and numerical results, the MWI enforces a low-pass filter on the pressure field, which suppresses oscillatory solutions. Furthermore, the numerical dissipation of kinetic energy introduced by the MWI is shown to converge with third order in space and is independent of the time-step, if the MWI is derived consistently from the momentum equations. In the presence of source terms, the low-pass filter on the pressure field can be shaped by a careful choice of the interpolation coefficients to ensure the filter only acts on the driving pressure gradient that is associated with the fluid motion, which is shown to be vitally important for the accuracy of the numerical solution. To this end, a force-balanced discretisation of the source terms is proposed, that precisely matches the discretisation of the pressure gradients and preserves the force applied to the flow. Using representative test cases of incompressible and low Mach number flows, including flows with discontinuous source terms and two-phase flows with large density ratios, the newly proposed formulation of the MWI is favourably compared against existing formulations and is shown to significantly reduce, or even eliminate, solution errors, with an increased stabi
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
Jones WP, marquis A, Noh D, 2017, An investigation of a turbulent spray flame using Large Eddy Simulation with a stochastic breakup model, Combustion and Flame, Vol: 186, Pages: 277-298, ISSN: 0010-2180
A computational investigation of a turbulent methanol/air spray flame in which a poly-dispersed droplet distribution is achieved through the use of a pressure-swirl atomiser (also known as a simplex atomiser) is presented. A previously formulated stochastic approach towards the modelling of the breakup of droplets in the context of Large Eddy Simulation (LES) is extended to simulate methanol/air flames arising from simplex atomisers. Such atomisers are frequently used to deliver fine droplet distributions in both industrial and laboratory configurations where they often operate under low-pressure drop conditions. The paper describes improvements to the breakup model that are necessary to correctly represent spray formation from simplex atomisers operated under low-pressure drop conditions. The revised breakup model, when used together with the existing stochastic models for droplet dispersion and evaporation, is shown to yield simulated results for a non-reacting spray that agree well with the experimentally measured droplet distribution, spray dynamics and size-velocity correlation. The sub-grid scale (sgs) probability density function (pdf) approach in conjunction with the Eulerian stochastic field method are employed to represent the unknown interaction between turbulence and chemistry at the sub-filter level while a comprehensive kinetics model for methanol oxidation with 18 chemical species and 84 elementary steps is used to account for the gas-phase reaction. A qualitative comparison of the simulated OH images to those obtained from planar laser-induced fluorescence (PLIF) confirms that the essential features of this turbulent spray flame are well captured using the pdf approach. They include the location of the leading-edge combustion (or lift-off height) and the formation of a double reaction zone due to the polydisperse spray. In addition, the influence of the spray flame on the structure of the reacting spray in respect of the mean droplet diameters and sp
Fredrich D, Jones WP, Marquis A, 2017, Application of the Eulerian sub-grid probability density function method in the large eddy simulation of a partially premixed swirl flame, 10th Mediterranean Combustion Symposium, Publisher: The Combustion Institute, Pages: 1-12
Large Eddy Simulations (LES) of a partially premixed, swirl-stabilised flame are performedusing a transported Probability Density Function (PDF) approach in combination with the Eu-lerian stochastic field method to account for turbulence-chemistry interaction. The correspond-ing subgrid scale (sgs) stresses and scalar fluxes are modelled using a dynamic version of theSmagorinsky model and a gradient diffusion approximation, respectively. A 15-step reducedmethane (CH4) mechanism containing 19 species is employed for the description of all chem-ical reactions. The test case involves a widely studied gas turbine model combustor with com-plex geometry. Simulations are carried out for two combustor operating conditions utilisingboth one and four stochastic fields. The overall velocity, temperature and major species massfraction results as well as the instantaneous thermochemical properties are shown to be in goodagreement with experimental measurement data demonstrating the capabilities of the employedLES method
Fredrich D, Gallot-Lavallée S, Jones WP, et al., 2017, Large eddy simulation of a gas turbine model combustor using the Eulerian subgrid probability density function method, 8th European Combustion Meeting, Publisher: The Combustion Institute, Pages: 1803-1808
The Eulerian stochastic field method is applied to solve the subgrid Probability Density Function (PDF) accounting for turbulence-chemistry interaction in the Large Eddy Simulation (LES) of a swirl stabilised gas turbine model combustor. A dynamic version of the Smagorinsky model for the subgrid scale stresses and a gradient diffusion approximation for the scalar fluxes complete the formulation. The chemical reaction is described by a 15-step reduced CH4 mechanism involving 19 species. Velocity, temperature and species results as well as basic thermochemical properties are shown to be in good agreement with experimental data validating the capabilities of the employed LES method.
Wu B, Parkes MP, de Benedetti L, et al., 2016, Real-time monitoring of proton exchange membrane fuel cell stack failure, Journal of Applied Electrochemistry, Vol: 46, Pages: 1157-1162, ISSN: 1572-8838
Uneven pressure drops in a 75-cell 9.5-kWe protonexchange membrane fuel cell stack with a U-shaped flowconfiguration have been shown to cause localised flooding.Condensed water then leads to localised cell heating, resultingin reduced membrane durability. Upon purging of the anodemanifold, the resulting mechanical strain on the membranecan lead to the formation of a pin-hole/membrane crack and arapid decrease in open circuit voltage due to gas crossover.This failure has the potential to cascade to neighbouring cellsdue to the bipolar plate coupling and the current densityheterogeneities arising from the pin-hole/membrane crack.Reintroduction of hydrogen after failure results in cell voltageloss propagating from the pin-hole/membrane crack locationdue to reactant crossover from the anode to the cathode, giventhat the anode pressure is higher than the cathode pressure.Through these observations, it is recommended that purging isavoided when the onset of flooding is observed to preventirreparable damage to the stack.
Gallot-Lavallee S, Jones WP, Marquis AJ, 2016, Large Eddy Simulation of an ethanol spray flame under MILD combustion with the stochastic fields method, Proceedings of the Combustion Institute, Vol: 36, Pages: 2577-2584, ISSN: 1540-7489
A combustion device operating in Moderate or Intense Low-oxygen Dilution (MILD) condition is numerically simulated. The burner consists of a cylindrical hot co-flow generator with an injector installed on the central axis. The hot co-flow is obtained by the lean combustion of Dutch Natural Gas (DNG). The spray is injected through an industrial pressure swirl atomiser using liquid ethanol as fuel. MILD combustion consists in burning the fuel in a high temperature environment so that the temperature gradients are limited and the production of pollutants such as NOx reduced. MILD combustion has been investigated for furnace applications and for various gaseous fuels, recently it has also been explored for spray combustion. The simulation is performed in the context of Large Eddy Simulation (LES) and the transported pdf equation for the scalars is solved using the stochastic fields approach. The validation of the results is based on the comparison with experimental data. The characteristics of the injector are obtained a posteriori by the use of the measurements at downstream location. The simulation correctly reproduces the velocity profiles of the particles within their size classes and the integral particle size distribution which is represented by the Sauter Mean Diameter (SMD). The gas phase temperature and velocity are also in good agreement with the measurements, however, some discrepancies are observed, presumably because of the lack of modelling of primary atomisation. The model employed appears to reliably reproduce the behaviour of the spray combustion system under MILD conditions.
Jones WP, Marquis AJ, Noh D, 2016, A stochastic breakup model for Large Eddy Simulation of a turbulent two-phase reactive flow, Proceedings of the Combustion Institute, Vol: 36, Pages: 2559-2566, ISSN: 1873-2704
The current work presents numerical investigations of model burner in which a non-swirling air-assisted methanol spray is injected using a pressure-swirl atomiser. A stochastic breakup model is formulated in the context of Large Eddy Simulation (LES) and validated by detailed comparisons of the results with measurements. An excellent agreement is achieved for the non-reactive case in terms of the dispersion of the spray, the mean droplet distributions and the time-averaged spray velocities. The transported probability density function (pdf) equation/Eulerian stochastic field method are used to represent the interaction of turbulence and chemistry while the gas phase reaction of the methanol/air spray flame is described by a reduced reaction mechanism involving 18 chemical species and 84 elementary steps. The sub-grid scale (sgs) chemistry model in conjunction with the formulated breakup model are found to capture the influence of the flame on droplet dynamics together with the formation of a double reaction zone typically resulting from a polydisperse spray to a good accuracy.
Brauner T, Jones WP, Marquis AJ, 2016, LES of the Cambridge Stratified Swirl Burner using a Sub-grid pdf Approach, 9th Mediterranean Combustion Symposium, Publisher: Springer Verlag, Pages: 965-985, ISSN: 1573-1987
The sub-grid scale probability density function equation is rearranged in order to separate the resolved and sub-grid-scale (sgs) contributions to the sgs mixing term. This allows modelling that is consistent with the limiting case of negligible sub-grid scale variations, a property required for applications to laboratory premixed flames. The new method is applied to the Cambridge Stratified Swirl Burner for 6 operating conditions, 2 isothermal and 4 burning, with varying degrees of swirl and mixture stratification. The simulations are performed with the Large Eddy Simulation (LES) code BOFFIN in which the modelled pdf transport equation is solved using the Eulerian stochastic field method. Eight stochastic fields are used to account for the influence of the sub-grid fluctuations and the chemistry is modelled with a reduced version of the GRI 3.0 mechanism for methane involving 19 species and 15 reaction steps. The simulated velocities for both the isothermal and burning cases show good agreement with the experimental data. The measured temperature and major species profiles are also reproduced to a good accuracy.
Jones WP, Jurisch M, Marquis AJ, 2015, Examination of an oscillating flame in the turbulent flow around a bluff body with large eddy simulation based on the probability density function method, Flow, Turbulence and Combustion, Vol: 95, Pages: 519-538, ISSN: 1386-6184
The present paper describes a Large Eddy Simulation modelling framework for the simulation of oscillating flames in practical flow configurations. The unresolved sub-grid scale motion is modelled using the dynamic Smagorinsky model in combination with the Probability Density Function method. It is shown that the Large Eddy Simulation method is capable of reproducing the characteristic shape of the reaction zone as well as the non-linear evolution of the total heat release rate in a bluff-body stabilised combustor. Commonly used measures for quantifying the variation of the total heat release rate are evaluated and examined in the present flow configuration of a lean-premixed ethylene-air flame. It was found that formaldehyde-based measures do not appropriately reproduce the amplitude and phase of the total heat release rate. A significantly improved correlation was achieved by employing the product of the mass fractions of molecular oxygen (O2) and the ketenyl radical (HCCO) as a means of characterising the variation of the total heat release rate.
Bluck MJ, Wolfendale MJ, Marquis AJ, 2015, An analytical solution to the heat transfer problem in Shercliff flow, International Journal of Heat and Mass Transfer, Vol: 86, Pages: 542-549, ISSN: 0017-9310
The study of flow in a rectangular duct, subject to a strong transverse magnetic field is of interest in a number of applications. An important application of such flows is in the context of coolants, where the principle issue of interest is convective heat transfer. For fully developed laminar flows, the problem can be characterised in terms of two coupled partial differential equations. In the case of perfectly electrically insulating boundaries, there is a well known analytical solution due to Shercliff, which provides the velocity and induced magnetic field profiles. In this paper, we demonstrate analytical solutions to and heat transfer problems for the Shercliff case in rectangular ducts and obtain temperature profiles and corresponding Nusselt numbers as functions of aspect ratio and Hartmann number.
Jones WP, Marquis AJ, Wang F, 2015, Large eddy simulation of a premixed propane turbulent bluff body flame using the Eulerian stochastic field method, Fuel, Vol: 140, Pages: 514-525, ISSN: 0016-2361
A premixed propane/air flame stabilized on a triangular bluff body was studied by numerical simulation as a simplified model of many practical propulsion and power generation systems. The sub-grid scale (sgs) probability density function (pdf) approach in conjunction with the stochastic fields solution method is used to account for sgs turbulence–chemistry interactions; a skeleton chemistry mechanism with 4 reaction steps and 7 species was used to describe the propane–air reaction. Three cases, one non-reacting and two reacting were studied. The instantaneous flow pattern and CO concentration are discussed and the averaged velocity and temperature, the RMS velocity and temperature fluctuations and averaged CO mass fraction profiles are compared with the experimental data. The simulations show very good agreement with the experimental data demonstrating the capability of the LES method coupled with the sgs-pdf method in representing premixed combustion in complex flame configurations.
Jones WP, Marquis AJ, Noh D, 2015, LES of a methanol spray flame with a stochastic sub-grid model, Proceedings of the Combustion Institute, Vol: 35, Pages: 1685-1691, ISSN: 1873-2704
This paper describes the Large Eddy Simulation (LES) of a methanol/air turbulent nonpremixed spray flame. An Eulerian stochastic field method is employed for the turbulence-chemistry interaction of the gas phase while a Lagrangian formulation is used for the liquid phase. A reduced reaction mechanism (18 species and 14 reactions) is adopted and stochastic models are used to account for the influence of sub-grid scale (sgs) motions on droplet dispersion and evaporation. Comparisons of the predicted gas phase and droplet statistics with measurements show a good agreement confirming that the droplet dispersion and evaporation models used in this work are adequate. The general features of the spray flame such as the occurrence of external group combustion and its development into separate combusting islands are well captured.
Bulat G, Jones WP, Marquis AJ, 2014, NO and CO formation in an industrial gas-turbine combustion chamber using LES with the Eulerian sub-grid PDF method, Combustion and Flame, Vol: 161, Pages: 1804-1825, ISSN: 0010-2180
The advances in computing power and numerical schemes allow Large Eddy Simulation (LES) to use more detailed turbulent combustion models as well as to be applied to real gas turbine combustors. In this work, we investigate the emissions formation in an industrial gas-turbine combustion chamber using LES with an Eulerian stochastic sub-grid pdf model with reduced chemistry. Sub-grid stresses are represented by a dynamic version of the Smagorinsky model and sub-grid species fluctuations are characterised by eight stochastic fields. The chemistry was represented by an ARM reduced GRI 3.0 mechanism with 15 reaction steps and 19 species. All calculations were carried out using a detailed block-structured mesh capturing all geometrical features of the Siemens SGT-100 burner operating at a pressure of 3 bar. The influence of the radiation heat losses was investigated and the impact of an alternative 4-step chemical mechanism was discussed. The results show good agreement with the experimental data. The NO formation rates were quantified with prompt NO dominating the thermal and formation paths.
Wu B, Parkes MP, Yufit V, et al., 2014, Design and testing of a 9.5 kWe proton exchange membrane fuel cell-supercapacitor passive hybrid system, International Journal of Hydrogen Energy, Vol: 39, Pages: 7885-7896, ISSN: 0360-3199
The design and test of a 9.5 kWe proton exchange membrane fuel cell passively coupled with a 33 × 1500 F supercapacitor pack is presented. Experimental results showed that the system reduced dynamic loads on the fuel cell without the need for additional DC/DC converters. Fuel efficiency gains of approximately 5% were achieved by passive hybridisation in addition to addressing two main operational degradation mechanisms: no-load idling and rapid load cycling.Electrochemical Impedance Spectroscopy measurements indicated that the supercapacitor capacitance dropped with decreasing cell voltage and suggested that operation below 1.3 V is not recommended. Knee-frequency measurements suggested little benefit was gained in using passive systems with load cycles that have frequency components above 0.19 Hz. Analysis of system sizing suggested using the minimum number of supercapacitors to match the open circuit voltage of the fuel cell to maximise load buffering.
Jones WP, Marquis AJ, Vogiatzaki K, 2014, Large-eddy simulation of spray combustion in a gas turbine combustor, Combustion and Flame, Vol: 161, Pages: 222-239, ISSN: 0010-2180
The paper describes the results of a comprehensive study of turbulent mixing, fuel spray dispersion and evaporation and combustion in a gas-turbine combustor geometry (the DLR Generic Single Sector Combustor) with the aid of Large Eddy Simulation (LES). An Eulerian description of the continuous phase is adopted and is coupled with a Lagrangian formulation of the dispersed phase. The sub-grid scale (sgs) probability density function approach in conjunction with the stochastic fields solution method is used to account for sgs turbulence-chemistry interactions. Stochastic models are used to represent the influence of sgs fluctuations on droplet dispersion and evaporation. Two different test cases are simulated involving reacting and non-reacting conditions. The simulations of the underlying flow field are satisfying in terms of mean statistics and the structure of the flame is captured accurately. Detailed spray simulations are also presented and compared with measurements where the fuel spray model is shown to reproduce the measured Sauter Mean Diameter (SMD) and the velocity of the droplets accurately.
Cumpsty NA, Marquis AJ, 2014, AN APPROXIMATE METHOD TO OBTAIN THERMODYNAMIC GAS PROPERTIES FOR USE IN GAS TURBINES, ASME Turbo Expo: Turbine Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS
Cooper SJ, Kishimoto M, Tariq F, et al., 2013, Microstructural Analysis of an LSCF Cathode Using In Situ Tomography and Simulation, ECS Transactions, Vol: 57, Pages: 2671-2678, ISSN: 1938-6737
Electrode tortuosity factor is a key input parameter in many fuel cell simulations. Three-dimensional microstructural data obtained from in-situ synchrotron X-ray nano-computed tomography is used as the basis for comparing five approaches to quantify the tortuosity factor. Three of these techniques are based on diffusivity simulations and showed strong correlation, but had consistently different absolute values. A random walk method showed a good degree of correlation to the diffusive approaches, but had the largest values overall. Lastly, a calculation that used a mean pore centroid approach showed little correlation to any of the other three methods, but compared well with the conventional Bruggeman correlation. Due to the diffusive nature of the ionic transport in electrodes, the authors would recommend calculating tortuosity factors using a diffusive approach based on the voxels rather than a remeshed volume.
Cooper SJ, Eastwood DS, Gelb J, et al., 2013, Image Based Modelling of of Microstructural Heterogeneity in LiFePO4 Electrodes for Li-ion Batteries, Journal of Power Sources, Vol: 247, Pages: 1033-1039, ISSN: 0378-7753
Battery and fuel cell simulations commonly assume that electrodes are macro-homogeneous and isotropic. These simulations have been used to successfully model performance, but give little insight into predicting failure. In Li-ion battery electrodes, it is understood that local tortuosity impacts charging rates, which may cause increased degradation. This report describes a novel approach to quantifying tortuosity based on a heat transfer analogy applied to X-ray microscopy data of a commercially available LiFePO4 electrode. This combination of X-ray imaging and image-based simulation reveals the microscopic performance of the electrode; notably, the tortuosity was observed to vary significantly depending on the direction considered, which suggests that tortuosity might best be quantified using vectors rather than scalars.
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