Numerical simulation of the convective instability in a dump combustor

The mechanism of the convective instability in a channel with a sudden expansion, known in combustion literature as a dump, followed by a sudden contraction is investigated. The problem is analyzed using vortex simulations at a high Reynolds number. Of particular interest is the effect of the length of the cavity between the expansion and contraction sections on the flow structure and dynamics. We show that two instabilities may be encountered, depending on the ratio of the cavity length to its depth. For short cavities, the roll-up of the separating shear layer is the dominant mode of oscillation, producing small eddies that propagate along the top of the cavity. As the length-to-depth ratio increases, a lower frequency mode is observed to coexist with the separating shear-layer mode. For long cavities, the recirculation zone behind the step sheds large-scale, low-frequency eddies that dominate the dynamics, mixing, and oscillations within the cavity.