60005 - Vortex Breakdown and Recirculation Bubble Formation in Counter Swirl Flows 

For achieving a high degree of fuel-air mixing within a short distance, or for enhancing atomization of liquid fuels, gas turbine engine combustor premixers often employ counter-rotating swirlers. Counter-swirl flows create strong shear layers in the tangential direction, that produce high turbulence levels resulting in enhanced atomization and mixing. However, the counter-swirl results in rapid decay of tangential velocities due to strong counteraction of the oppositely rotating flows. Since swirl flows create low pressure at the center, the decay in swirl results in corresponding pressure rise at the center, i.e, adverse pressure gradient. The adverse pressure gradient induces premature vortex breakdown and recirculation bubble formation. The recirculation bubbles could be formed inside the mixing passages of the premixers instead of the combustor region.  The bubbles having low velocities could hold a flame resulting in mixer metal burnout. In addition, the bubbles increase flow residence time that leads to auto-ignition problems. The gas turbine mixer design requires an optimal level of counter rotation that achieves sufficient mixing to minimize NOx, while not large enough to result in premature vortex breakdown.

Though the vortex breakdown phenomenon had been investigated in a large number of prior studies, nearly all of them were in unidirectional swirl flows. There haven’t been any studies on breakdown in counter swirl flows. In a unidirectional swirl flow, swirl number mainly affects breakdown occurrence. On the other hand, in a counter swirl flow in addition to swirl number, degree of counter-swirl (DCS) defined as the ratio of tangential momentum fluxes of the two flows could play a significant role. In this study the effect of DCS on breakdown is investigated in a premixer with two sets of axial swirler vanes having opposite rotational directions. The premixer consists of a converging mixing passage downstream of the swirler vanes. The premixer design is similar to some of the practical gas turbine engine premixers. RANS and LES CFD simulations are performed using ANSYS Fluent to simulate the swirler flow field. The DCS is varied by varying swirler vane angles and swirler diameters. In addition to DCS, the effect of external pressure gradient on breakdown is investigated by varying the mixer passage convergence angle. Results from this study would significantly improve our understanding of breakdown in counter swirl flows and may lead to improved premixer designs