Conditional moment closure modelling of turbulent methanol jet flames

Conditional moment closure (CMC) predictions for a turbulent piloted jet diffusion flame of methanol in air at velocities of 66.2 and 90.3 m s^?1 are presented. Predictions are compared with the experimental joint Raman-Rayleigh-LIF results of Masri et al and laminar flamelet calculations. Three comprehensive chemical mechanisms (SKELETAL, GRI-Mech and SUBGRI) are used to represent the chemistry of the methanol flame. The SKELETAL mechanism shows the best agreement among the various mechanisms employed. It is found that the SUBGRI mechanism reduces computational cost in terms of memory and CPU time without compromising results where the focus is on the main reactive chemistry.

The k-ε-g turbulence model underpredicts the rate of mixing and the predicted flames are somewhat longer than that reported by experiment. In general, the CMC predictions for conditional mean temperature and species mass fractions are very good and show qualitative agreement with experiment. At downstream locations, the overall trends of predicted temperature and species concentration levels are similar to the upstream ones with the latter showing better agreement with the conditional measured levels. CMC predictions show the same order of agreement at higher velocities.

It is believed that the discrepancies on the fuel-rich side may be due to lack of consideration of the conditional fluctuations. The absence of a radially dependent CMC formulation, excluding differential diffusion effects and the inadequacy of the chemical mechanism may also account, partly, for the degree of discrepancy in the predictions.