Regimes in turbulent non-premixed combustion

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In many practical applications of non-premixed combustion the flamelet model is used. There remains, however, the question, to which extent the underlying assumptions of this model are valid for intensive and small scale turbulence. In this proposal this question shall be assessed on the basis of Direct Numerical Simulations (DNS). For this the numerical set-up of a temporarily evolving planar jet flame will be used. The jet consists of a methane-nitrogen mixture which has the same nitrogen mass fraction as the surrounding oxidizer stream. As oxidizer air or air diluted with nitrogen will be used. Because of the large dilution and the resulting relatively low temperatures these are situations where the flamelet assumptions could be particularly critical. Two cases with variable Lewis numbers and two cases with unity Lewis numbers will be calculated. In addition to the equations for the temperature and the mass fractions of the chemically reacting species an equation for the mixture fraction will be solved. The mixture fraction field will be decomposed space fillingly into so-called dissipation elements (DE) which contain a smooth region between a local minimum and a local maximum of the mixture fraction. In addition to the scalar difference of the mixture fraction delta Z at the two extremal points, the linear distance between the points and the resulting mean mixture fraction gradient across the elements will be calculated, such that the elements are characterized by the two parameters delta Z and g. The reactive structures which exist within the individual elements can be analyzed in detail on the basis of the DNS results. The reactive structures: burning flamelets, edge flames, fine scale mixing zones and broken reaction zones shall be identified in a regime diagram which is formed by the two parameters delta Z and g. By integrating over the joint probability density function of these two parameters, which is obtained from the DNS, the probability of occurrence of the four different reactive structures shall be calculated for difference regions in the jet flame. This analysis will be extended to flames of practical interest which have different turbulence properties and different dilutions.