Free radicals augmentation, and large eddy probability-density simulation for high-speed turbulent combusting jets.

Abstract

Theoretical and experimental investigations aimed at altering "nature-prescribed" hydrocarbon combustion are described; the basic intention is to anchor combustion zones in supersonic streams. The well-known free-radicals augmented combustion is studied, but with a novel innovation of donor injection, rather than free-radicals themselves. Standard methane/air combustion is explored in an open jet geometry in the turbulent regime. A new flammability criterion is established in the light of the Kolmogorov microscale mixing. The diagnostics are non-intrusive through infrared thermograms and acoustic emissions. Fifty percent extension of the lean flammability limit is experimentally demonstrated. Unambiguous differences in acoustic power spectra indicate a great increase of the reaction rate. Possible reductions of pollutants observed from thermochemical calculations are further confirmed by infrared imaging processing. Numerical simulations of high-speed turbulent combustion are studied. A new method: Large Eddy Probability-density Simulation (LEPS), has been proposed based on both the large eddy simulation and the probability density function method. In this approach, a mixed finite-spectral method is employed to solve for the velocity field, while the probability density function method is used to solve for the energy and species equations. In the PDF solver, a modified composition joint PDF equation is derived, in which the mean velocity field from the κ-ε model in the traditional PDF equation is replaced by the resolved velocity field from the LES so that the large-scale effect is explicitly represented. The solution algorithms are discussed in full detail. This new method is highly perspective in simulating turbulent reacting flows because both advantages of the LES and PDF approach have been taken, whereas both disadvantages have been offset! Initial simulation results are presented. Future work is outlined.