LES AND DNS OF IGNITION PROCESSES AND COMPLEX-STRUCTURE FLAMES WITH LOCAL EXTINCTION Proceedings of the International COST Workshop
Prediction of the Ignition Phases in Aeronautical and Laboratory Burners using Large Eddy Simulations1190(2009); http://dx.doi.org/10.1063/1.3290168View Description Hide Description
Being able to ignite or reignite a gas turbine engine in a cold and rarefied atmosphere is a critical issue for many aeronautical gas turbine manufacturers. From a fundamental point of view, the ignition of the first burner and the flame propagation from one burner to another are two phenomena that are usually not studied. The present work presents on‐going and past Large Eddy Simulations (LES) on this specific subject and as investigated at CERFACS (European Centre for Research and Advanced Training in Scientific Computation) located in Toulouse, France. Validation steps and potential difficulties are underlined to ensure reliability of LES for such problems. Preliminary LES results on simple burners are then presented, followed by simulations of a complete ignition sequence in an annular helicopter chamber. For all cases and when possible, two‐phase or purely gaseous LES have been applied to the experimentally simplified or the full geometries. For the latter, massively parallel computing (700 processors on a Cray XT3 machine) was essential to perform the computation. Results show that liquid fuel injection has a strong influence on the ignition times and the rate at which the flame progresses from burner to burner. The propagation speed characteristic of these phenomena is much higher than the turbulent flame speed. Based on an in‐depth analysis of the computational data, the difference in speed is mainly identified as being due to thermal expansion and the flame speed is strongly modified by the main burner aerodynamics issued by the swirled injection.
1190(2009); http://dx.doi.org/10.1063/1.3290166View Description Hide Description
Flame stabilization and the mechanisms that govern the dynamics at the flame base have been subject to numerous studies in recent years. Recent results using a combined Large Eddy Simulation‐Conditional Moment Closure (LES‐CMC) approach to model the turbulent flow field and the turbulence‐chemistry interactions has been successful in predicting flame ignition and stabilization by auto‐ignition, but LES‐CMCs capability of the accurate modelling of the competition between turbulent quenching and laminar and turbulent flame propagation at the anchor point has not been resolved. This paper will consolidate LES‐CMC results by analysing a wide range of lifted flame geometries with different prevailing stabilization mechanisms. The simulations allow a clear distinction of the prevailing stabilization mechanisms for the different flames, LES‐CMC accurately predicts the competition between turbulence and chemistry during the auto‐ignition process, however, the dynamics of the extinction process and turbulent flame propagation are not well captured. The averaging process inherent in the CMC methods does not allow for an instant response of the transported conditionally averaged reactive species to the changes in the flow conditions and any response of the scalars will therefore be delayed. Stationary or quasi‐stationary conditions, however, can be well predicted for all flame configurations.
1190(2009); http://dx.doi.org/10.1063/1.3290167View Description Hide Description
Recently there is has been increased interest in modelling combustion processes with high‐levels of extinction and re‐ignition. Such system often lie beyond the scope of conventional single scalar‐based models. Large Eddy Simulation (LES) has shown a large potential for describing turbulent reactive systems, though combustion occurs at the smallest unresolved scales of the flow and must be modelled. In the sub‐grid Probability Density Function (pdf) method approximations are devised to close the evolution equation for the joint‐pdf which is then solved directly. The paper describes such an approach and concerns, in particular, the Eulerian stochastic field method of solving the pdf equation. The paper examines the capabilities of the LES‐pdf method in capturing auto‐ignition and extinction events in different partially premixed configurations with different fuels (hydrogen, methane and n‐heptane). The results show that the LES‐pdf formulation can capture different regimes without any parameter adjustments, independent of Reynolds numbers and fuel type.
1190(2009); http://dx.doi.org/10.1063/1.3290169View Description Hide Description
Spark ignition of turbulent non-premixed gaseous and spray flames is of fundamental and technological importance, but has been very little studied compared to spark ignition of premixed combustion. In addition to the spark parameters, the value of the mixture fraction and its gradient at the spark location and instant and the local fluid velocities can affect the success of generating a kernel. These phenomena are briefly discussed through recent laminar and turbulent flame simulations and experiments and pointers to the recent literature are given.
1190(2009); http://dx.doi.org/10.1063/1.3290170View Description Hide Description
Numerical simulation of forced ignition is performed in the framework of Large‐Eddy Simulation (LES) combined with a tabulated detailed chemistry approach. The objective is to reproduce the flame properties observed in a recent experimental work reporting probability of ignition in a laboratory‐scale burner operating with Methane/air non premixed mixture . The smallest scales of chemical phenomena, which are unresolved by the LES grid, are approximated with a flamelet model combined with presumed probability density functions, to account for the unresolved part of turbulent fluctuations of species and temperature. Mono‐dimensional flamelets are simulated using GRI‐3.0  and tabulated under a set of parameters describing the local mixing and progress of reaction. A non reacting case was simulated at first, to study the unsteady velocity and mixture fields. The time averaged velocity and mixture fraction, and their respective turbulent fluctuations, are compared against the experimental measurements, in order to estimate the prediction capabilities of LES. The time history of axial and radial components of velocity and mixture fraction is cumulated and analysed for different burner regimes. Based on this information, spark ignition is mimicked on selected ignition spots and the dynamics of kernel development analyzed to be compared against the experimental observations. The possible link between the success or failure of the ignition and the flow conditions (in terms of velocity and composition) at the sparking time are then explored.
1190(2009); http://dx.doi.org/10.1063/1.3290171View Description Hide Description
The simulation of a piloted target flame of the TNF workshop namely Sandia D is presented. A Large Eddy Simulation approach with a constant Smagorinky subgrid scale model is used in this study. First order conditional moment closure is used to model the turbulence‐chemistry interaction. To calculate the conditional scalar dissipation and conditional velocity, conditional volume averaging with smoothing is used in this work. For the sake of simplicity a one‐step methane chemistry is used . The governing equations are solved with an incompressible Navier‐Stokes LES solver based on a fractional step approach.
1190(2009); http://dx.doi.org/10.1063/1.3290172View Description Hide Description
The paper describes mixing of injected CNG with air and combustion process in spark ignition internal combustion engine. Because of higher ignition temperature of CNG the SI engines have more effective ignition system than conventional engines. The gas motion, turbulence, charge temperature and obviously electrical energy of the ignition coil have a big influence on the ignition and burning process in the combustion chamber. The paper includes theoretical and experimental investigations of ignition process in the high charged SI engines with direct CNG injection by using LES technique in KIVA program. Simulation of CNG combustion in the caloric chamber was carried in the environment of OpenFOAM program with LES model and also the experimental test was carried out for comparison of results in the chamber with the same geometry. The influence of the “tumble” and “swirl” on the sparking is shown by modelling of this process in premixed charge by using LES technique. The charge motion and also considerably turbulence effect influence strongly on the ignition process.
1190(2009); http://dx.doi.org/10.1063/1.3290164View Description Hide Description
The basic idea of the flamelet model is to represent turbulent flames as an ensemble of laminar flamelets, which are described by one‐dimensional transport equation. The key parameter appearing in the flamelet equation is the conditional scalar dissipation rate, which is usually modeled as a function of the mixture fraction variable in various forms. In this study, the accuracy and validity of the existing models are assessed by using direct numerical simulation data for turbulent counterflow flames. Post‐processing and filtering of the data allowed a comparison of four models for the scalar dissipation rate. It was found that neither model gives satisfactory accuracy in predicting the mean scalar dissipation rate. Once the analysis was limited to points close to stoichiometric compositions, the correlation remained qualitatively the same with significant scatter. We can assume therefore, that the errors are not coupled with the flame. On the other hand, the turbulence parameters used in the models seem to be the main source of discrepancies. Further analysis of the results showed that the best correlation can be found for mean scalar dissipation rate and for gradient of mean mixture fraction Z. It is especially apparent for smaller filter sizes. One can expect such behavior, because downsizing the filter leads to DNS‐type model, when filter size is smaller that the smallest turbulence scale. The presented study provides a good background for building sub‐model of the mean scalar dissipation rate for LES schemes.
1190(2009); http://dx.doi.org/10.1063/1.3290165View Description Hide Description
The paper presents preliminary results of pulverized coal combustion process modeling using Large Eddy Simulation. First the methodology for the testing of mesh resolution is presented. The combustion process was carried out using equilibrium model with single mixture fraction approach.