Lagrangian simulation of the early stages of a reacting jet

Vortex simulation, using the transport element method, is used to study shear flow-combustion interactions in a reacting jet at high, Reynolds number. We use an unsteady, low-Mach number model of combustion in which exothermic energy deposition produces volumetric expansion and baroclinic vorticity, while shear flow instability induces entrainment and a strong strain field. The numerical scheme is a Lagrangian, grid-free, field method in which computations are confined to the vorticity-reaction zone. Solutions are obtained for a two-dimensional flow with a single-step, Arrhenius kinetics. Results show that, as the Damkohler number increases, the reaction zone structure changes from a distributed zone within the cores of the large eddies, which form due to the rollup of the jet vorticity, to a thin zone surrounding their outer edges. The structure of the product distribution, however, remains almost the same, being highest within the cores and falling sharply within the braids, and exhibiting strong similarity to the vorticity distribution. The mechanism leading to this similarity is entrainment. At low Damkohler number, reaction occurs, following the mixing of entrained reactants, inside the eddy core. At high Damkoler number, reaction takes place on the outer edges of the eddies, followed by the entrainment of products towards the eddy center. In both cases, the cores act as exothermic centers which support combustion. With finite-rate kinetics, faster reactions show more resistance to extinction within zones of high strain rates. Instability suppression by heat release is weak at finite-rate kinetics, and can be overcome by forcing the jet at high amplitudes, especially when ignition delay is longer than the time of eddy formation.