The timing and location of autoignition can be highly sensitive to turbulent fluctuations of composition.Second-order Conditional Moment Closure (CMC) provides transport equations for conditional(co)variances in turbulent reacting flows. CMC equations accounting for compressibility and differentialdiffusion are analyzed using data from direct numerical simulation of an autoignitive lifted turbulenthydrogen jet flame [C.S. Yoo, R. Sankaran, J.H. Chen, Three-dimensional direct numerical simulationof turbulent lifted hydrogen/air jet flame in a heated coflow. Part 1. J. Fluid. Mech., (2008)]. At theflame base, second-order moments were required to accurately model the conditional reaction rates.However, over 80% of the second-order reaction rate component was obtainable with a small subset(16%) of the species-temperature covariances. The balance of the second-order CMC equation showedthat turbulent transport across spatial composition gradients initiates generation of conditionalvariances.
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