Higher-order conditional moment closure modelling of local extinction and reignition in turbulent combustion

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Abstract
Higher-order, conditional moment closure approaches to modelling local extinction and reignition in turbulent, non-premixed combustion are investigated. This is done by studying closure strategies of the conditional source term itself. Feasibility studies are done using direct numerical simulation (DNS) experiments which exhibit varying degrees of local extinction. The higher-order conditional moments are taken directly from the DNS experiments as opposed to solving modelled transport equations for them. Results show that with moderate levels of extinction, the conditional probability density function (pdf) of the reduced temperature is unimodal, but skewed, and at least third-order terms in a series expansion of the nonlinear chemical source term conditional on the mixture fraction are required to predict the conditional means. With higher levels of local extinction, the conditional pdf shapes can be bimodal and third-order closure breaks-down. The success of a presumed beta pdf shape for conserved scala...