Assessment of conditional moment closure models of turbulent autoignition using DNS data

• Published in: Proceedings of the Combustion Institute 2002, Vol.29 (2), p.2069-2077
• Main Authors: Sreedhara, S., Lakshmisha, K.N.
• Format: Article
• Language: English
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/S1540-7489(02)80252-8

A systematic assessment of the submodels of conditional moment closure (CMC) formalism for the autoignition problem is carried out using direct numerical simulation (DNS) data. An initially non-premixed, n-heptane/air system, subjected to a three-dimensional, homogeneous, isotropic and decaying turbulence, is considered. Two kinetic schemes, (1) a one-step and (2) a reduced four-step reaction mechanism, are considered for chemistry. An alternative formulation is developed for closure of the mean chemical source term ( w/ν), based on the condition that the instantaneous fluctuation of excess, temperature is small. With this model, it is shown that the CMC equations describe the autoignition process all the way up to near the equilibrium limit. The effect of second-order terms (namely, conditional variance of temperature excess δ 2 and conditional correlations of species q ij ) in modeling ( w/η) is examined. Comparison with DNS data shows that δ 2 has little effect on the predicted conditional mean temperature evolution, if the average conditional scalar dissipation rate ( N/η) is properly modeled. Using DNS data, a correction factor is introduced in the modeling of nonlinear terms ( Y iY j/η ) to include the effect of species fluctuations. Computations including such a correction factor show that the species conditional correlations q ij have little effect on model predictions with a one-step reaction, but those q ij involving in termediate species are found to be crucial when four-step reduced kinetics is considered. The “most reactive mixture fraction” is found to vary with time when a four-step kinetics is considered. First-order CMC results are found to be qualitatively wrong if the conditional mean scalar dissipation rate is not modeled properly. The autoignition delay time predicted by the CMC model compares excellently with DNS results and shows a trend similar to experimental data over a range of initial temperatures.