1,721,316 research outputs found
Conditional moment closure modelling for spark ignition in a turbulent n-heptane spray
No full textIgnition and flame stabilisation have been simulated in a turbulent, bluff body stabilised spray flame. A complete first order Conditional Moment Closure (CMC) model for spray combustion is presented, as well as CMC modelling for spark ignition. The new elements of the two phase model formalism and the spark ignition models are illustrated using a one dimensional spray ignition example. It is shown that the new spray terms are not significant in the flows considered, however the modelling of the mixture fraction variance equation is critical. Finally, ignition of the experimental spray burner is simulated and compared with the available data, showing reasonable qualitative agreement but over-predicting the speed of flame stabilisation
Numerical investigation of forced ignition in laminar counterflow non-premixed methane-air flames
No full textSimulations of forced ignition of non-premixed laminar counterflow flames are used to study the effect of strain rate on ignition success. A one dimensional calculation is performed, using detailed methane chemical kinetics and treating the spark as an instantaneous heat release in an inert mixing layer. Ignition success depends on the mixture composition at the spark location, resulting in lean and rich ignitability limits for a given spark that can be different from the fuel's static flammability limits. The difference is attributed to the finite spark width and the diffusion of heat from the spark to the flammable mixture. Ignition is prohibited by excessive strain rates, in some cases at levels well below the extinction value. The structure of the evolving flame is examined in terms of temperature, heat release rate and species mass fraction distributions. In the case of successful ignition, the high temperature reached due to the spark energy deposition causes local auto-ignition and subsequently, intense burning rapidly consumes the premixed reactants in the mixing layer and a non-premixed flame survives. In the case of unsuccessful ignition, despite the auto-ignition achieved in the sparked region, the strain rate is sufficiently high or the composition is sufficiently far away from the nominally flammable mixture for the heat and radicals to diffuse without resulting in a flame at long times from the spark event
Numerical investigation of spark-ignition in a laminar methane-air counterflow
No full textSimulations of forced ignition in a non-premixed laminar counterflow are used to study the effect of the strain rate on ignition success. A one dimensional calculation is performed, using detailed methane chemical kinetics, and treating the ignition event as an instantaneous heat release. Ignition success is seen to depend on the mixture composition and spark location, resulting in lean and rich ignitability limits for a given spark that can be different from the nominal flammability limits. Ignition is also prohibited by excessive strain rates, in some cases at levels well below the extinction value. The structure of the evolving ignition region is examined in terms of its temperature, heat release rates and its composition. In the case of successful ignition, the high temperature reached due to the spark energy deposition causes local autoignition. Subsequently, intense burning rapidly consumes the reactants in the remaining region of flammable methane-air mixture. As this intense burning subsides a partially premixed and then a non-premixed diffusion flame are seen to survive
Simulations of edge-flame propagation in turbulent non-premixed jets
No full textIgnition, flame propagation and stabilisation have been simulated and analysed in a turbulent jet of non-premixed methane and air. The first order Conditional Moment Closure (CMC) turbulent combustion model was fully coupled with a Reynolds-Averaged Navier Stokes (RANS) flow simulation. A CMC model was developed to account for spark ignition. The over-prediction of turbulent flame propagation was attributed to the limitations of the first order reaction rate closure, and of the RANS description of the flow in the presence of thermal expansion around the flame front. A new model for the effects of counter gradient turbulent transport in partially premixed flows was implemented and the modification of the flame front was presented. The coupled CMC-CFD model successfully captures the physics necessary to represent unsteady flame evolution and hence may be used for simulation of ignition in practical combustor designs
Micromixing effects in a reacting plume by the Stochastic Fields method
No full textThe Stochastic Fields method for turbulent reacting flows has been used for a reacting plume. The method simulatesmacromixing with an eddy diffusivity and micromixing by a random walk in scalar space with appropriate models for thescalar dissipation. The numerical technique used for the solution of the stochastic partial differential equation that arises from the stochastic fields method is discussed. The predictions are very close to experimental data for a plume of NO in a O3-doped turbulent air flow for a range of Damköhler numbers
Spark ignition of turbulent recirculating non-premixed gas and spray flames: a model for predicting ignition probability
No full textA model that synthesizes previous knowledge from experiments and simulations on spark ignition of gas and liquid-fuelled non-premixed recirculating flames has been developed. Attention is focused on the flame expansion process and the overall filling of the combustor volume with flame. The model is meant to provide a quick assessment of the ignition behaviour of a combustor. It uses information from the flow patterns before ignition and calculates possible trajectories that a flame emanating from a spark may experience. The calculation of these trajectories includes flame extinction to capture the experimentally- observed flame quenching, mixture fraction fluctuations to capture the non-premixed nature of the flame, convection by the mean and the random turbulent flow to capture the probabilistic nature of the flame evolution, and uses recent results on the laminar burning velocity in sprays. The model is applied to gas and spray flames and the calculated ignition probability distributions and the timescale of complete ignition agree reasonably well with experiment. The results of the model provide insights into spark ignition processes in complicated flow patterns
Analysis of Direct Numerical Simulations of ignition fronts in turbulent non-premixed flames in the context of Conditional Moment Closure
Get PDFDirect Numerical Simulations of an igniting non-premixed flame are analysed in order to assess the accuracy of the gradient diffusion model for the conditional turbulent flux term in the context of the CMC equation. This term may become important in realistic combustion situations involving stabilisation, ignition and extinction. The results show that the edge flame probably cannot be treated with first-order CMC and that its structure involves a balance between chemistry, molecular mixing, and spatial transport. The usual gradient diffusion approximation for the conditional turbulent flux seems to be adequate for high turbulence intensity relative to laminar burning velocity of a stoichiometric mixture, but there is strong counter-gradient transport at weak turbulence. An analysis of the major terms in the conditional scalar flux equation shows that the pressure fluctuation is mainly responsible for the counter-gradient transport
Micromixing effects in air pollution modelling
No full textPredicting the dispersion of reacting pollutants close to their source is a topic of importance in Air Quality Modelling.The conventional method of neglecting species concentration fluctuations is not valid for such small-scale problems.Various methods that incorporate segregation are reviewed here and their use for typical atmospheric dispersion problems is illustrated through numerical simulations of a simplified problem.By comparison with experimental data, it is found that micromixing can affect the evolution of the mean reaction rate and that the models presented here are more accurate than if segregation were not included.Further work should focus on the interfacing of these models with practical Air Quality calculations
A practical model for the high-altitude relight of a gas turbine combustor
No full textA model that simulates the possible flame trajectories following spark ignition in a generic recirculating flame has been applied to a realistic aero-engine combustor. The model has been previously validated for gaseous and simple spray flames. It uses a CFD solution of the un-ignited flow and estimates the volume of the combustor that could be ignited given a particular flow field, spray distribution, and spark location, shape and size, and also provides a measure of the variability between independent sparking events. From this information, the ease of igniting the combustor can be assessed, and hence the combustor and injector geometry and spark placement decisions can be informed at a very early stage of the design process. Results for igniting a Rolls-Royce test combustor run with kerosene at high-altitude relight conditions for which experimental data are available and for which a RANS CFD solution has been developed, demonstrate the usefulness of the model’s output. The results are consistent with experiment and also reveal that the spark characteristics and location used in the experiments, developed over a number of years by trial-and-error methods, are indeed close to optimum
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