Numerical simulation of premixed chemical reactions with swirl

Direct numerical simulation is used to study the development of exothermic chemical reactions in a dilute, premixed, low speed, inviscid, axisymmetric, swirling flow in a straight, open, cylindrical pipe. Attention is focused on the complex interplay between the swirl and heat release of the chemical reaction and the objective is to determine, as a function of exothermicity, the critical swirl level corresponding to the first appearance of vortex breakdown in the reactive flow. It is found that for a given exothermicity, a large-amplitude structure develops around the pipe axis as the swirl level increases, and a near-stagnant breakdown zone appears when the swirl exceeds a critical level. These features are accompanied by significant changes in the temperature and reactant fields and the appearance of a hot core close to the inlet. As exothermicity is raised from low levels to higher, the critical swirl exhibits a nonlinear change; it first decreases and then, above a certain level of exothemicity, increases. An analysis of the governing equations attributes this behaviour to the nonlinear interaction between the advection of azimuthal vorticity and the baroclinic effects resulting from the coupling between the velocity and temperature gradients.