Numerical simulations and modeling of turbulent combustion

Abstract

Turbulent combustion is the basic physical phenomenon responsiblefor efficient energy release by any internal combustion engine. Howeverit is accompanied by other undesirable phenomena such as noise, pollutantspecies emission or damaging instabilities that may even lead tothe system desctruction. It is then crucial to control this phenomenon,to understand all its mecanisms and to master it in industrial systems.For long time turbulent combustion has been explored only through theoryand experiment. But the rapid increase of computers power during thelast years has allowed an important development of numerical simulation,that has become today an essential tool for research and technicaldesign. Direct numerical simulation has then allowed to rapidlyprogress in the knowledge of turbulent flame structures, leading tonew modelisations for steady averaged simulations. Recently large eddysimulation has made a new step forward by refining the descriptionof complex and unsteady flames. The main problem that arises whenperforming numerical simulation of turbulent combustion is linkedto the description of the flame front. Being very thin, it can nothowever be reduced to a simple interface as it is the location of intensechemical transformation and of strong variations of thermodynamicalquantities. Capturing the internal structure of a zone with a thicknessof the order of 0.1 mm in a computation with a mesh step 10 timeslarger being impossible, it is necessary to model the turbulentflame. Models depend on the chemical structure of the flame, on theambiant turbulence, on the combustion regime (flamelets, distributedcombustion, etc.) and on the reactants injection mode (premixedor not). One finds then a large class of models, from the most simplealgebraic model with a one-step chemical kinetics, to the most complexmodel involving probablity density functions, cross-correlations andmultiple-step or fully complex chemical kinetics.