Vortex Breakdown and Recirculation Bubble Formation in Counter Swirl Flows

For achieving better fuel-air mixing within a short distance or for improved atomization of liquid fuels counter rotating swirler designs are preferred in gas turbine engine combustors. In this study, vortex breakdown phenomenon is investigated in co and counter rotating swirlers using CFD. The swirler assembly consists of two axial swirlers, an inner and an outer swirler both with straight vanes. Swirler vane angles are varied from 30° to 60° in steps of 10° while keeping inner and outer swirler vane angles equal. CFD simulations are performed with air at ambient conditions as the working fluid at a constant mass flow rate. It is observed that strong shear layers are created in counter swirl flows due to the opposite flow rotation. The shear layers result in rapid decay of inner swirler tangential velocities for the counter swirlers compared to the co-swirlers. The tangential velocity decay is characterized with a parameter named tangential velocity integral (TVI). TVI was observed to decay faster for the counter swirl flows compared to the co-swirl flows. The faster decay in TVI for the counter swirlers is found to result in a stronger adverse pressure gradient in the axial direction at the center. The strong adverse pressure gradient resulted in higher pressure excess ratios (PER) for the counter swirlers. The higher PERs are observed to induce vortex breakdown in counter swirlers even at low vane angles whereas in co-swirlers vortex breakdown is not observed except for the highest vane angle. It is demonstrated that vortex breakdown could be suppressed in counter swirlers using a converging mixer passage. The converging mixer passage creates a favorable pressure gradient that counters the adverse pressure gradient due to swirl decay, resulting in breakdown suppression.