Mixture Fraction for High-Pressure Turbulent Reactive Flows

A database of Direct Numerical Simulations (DNS) representing the mixing of five species under high-pressure (high-p) conditions, and showing the achievement of turbulent conditions, ignition and combustion, has been created for the purpose of understanding the modeling requirements for the turbulent reaction rate under these conditions. Since in a preliminary study it has been established that the primary flame region is of diffusion type, the focus of the study was on the typical scalar used in advanced turbulent reaction rate models for non-premixed atmospheric-p flames: the mixture fraction. Analysis of the database shows that the mixture fraction is not a conserved scalar, contradicting the basic assumption of the atmospheric-p turbulent reaction-rate models. Moreover, even when using the exact moments extracted from the DNS, it is shown that the probability density function (PDF) of the mixture fraction cannot be represented by a β PDF. Based on these facts, it appears that new strategies are required for the modeling of the turbulent reaction rate in high-p reacting flows