UC San Diego

In combustion applications such as gas turbines, swirling jets are used to generate a vortex breakdown recirculation region that serves as a non-invasive flame stabilizer. The present work performs direct numerical simulations to study the effects of vortex breakdown on the structure and stabilization of laminar low-Mach-number gaseous diffusion flames.

Vortex breakdown transitions are first studied for heated/cooled variable-density, non-reacting jets in solid body rotation issuing into an unconfined ambient atmosphere. For increasing values of the swirl number $S$, two vortex breakdown modes are observed, the bubble and the cone, and the associated transitions $S^*_B$ and $S^*_C$ are determined for different values of the ambient-to-jet temperature ratio $\Lambda$. Both axisymmetric and three-dimensional simulations show decreasing values of $S^*_B$ with increasing $\Lambda$, while critical swirl numbers for the transition to the cone $S^*_C$, remain relatively constant for a fixed effective Reynolds number.

The first study is then extended to evaluate the same transitions in axisymmetric methane-air flames in the Burke-Schumann limit of infinitely fast chemistry. Transitions $S^*_B$ are relatively unaffected by fuel-feed dilution, and result in jet-like flames. For moderate values of dilution, further increase in $S$ to $S^*_C$ generates a steady conical breakdown, with the flame sheet again passing around the recirculating fuel and products. Extreme dilution, on the other hand, generates an enlarged cone that recirculates the ambient air, stabilizing the flame near the jet inlet.

Effects of bubble vortex breakdown are then explored for finite-rate chemistry flames in an axisymmetric concentric swirling jet configuration, for which a central fuel jet is surrounded by a swirling co-annular stream of air. Liftoff and blow-off are analyzed by systematically varying the two relevant parameters, the swirl number $S$ and the Damk\"{o}hler number, $D_N$. For sufficiently low values of $D_N$, and large values of $S$, flames lift off the injector, and thermal expansion at the base of the triple flame redirects the flow radially inward, promoting the formation of a small recirculation zone. Axisymmetric and three-dimensional simulations of the isothermal flow are used to analyze the mechanism for the onset of the bubble, and identify post-breakdown flow structure for larger values of the Reynolds number.