A computational investigation of swirl-number and Damkohler-number effects on non-premixed laminar swirling jet flames

Axisymmetric numerical simulations are used to assess the swirl-induced stabilization of low-Mach -number non-premixed jet flames at a moderate Reynolds number (Re = 200). Using a one-step model chemistry describing methane-air partially premixed combustion, we carry out a parametric investigation of the coupling between vortex breakdown and laminar flame liftoff/blowoff in a concentric jet configuration involving a central non-swirling methane jet surrounded by a swirling annular air jet issuing from a pipe with radius R-A' rotating with angular speed Omega'. The analysis considers order-unity values of the two relevant controlling parameters, namely, the Damkohler number D-N, defined as the square of the ratio of the stoichiometric methane-air flame-propagation velocity to the mean air-jet velocity U-A', and the swirl number S = Omega'R-A'/U-A'. As the Damkohler number D-N is decreased the attached edge flame lifts off from the injector rim. The resulting lifted triple flame migrates downstream on further decreasing D-N until a critical blowoff value D-N,D-b is reached. Results for fixed S = 1 exhibit lower values D-N,D-b than the corresponding simulations with fixed S = 0 . For a fixed Damkohler number, it is found that increasing S results in increased entrainment and reduced liftoff heights. At a critical value S-B* of the swirl number, equal to S-B* = 1 . 2 for D-N = 0 . 35 , a recirculation zone abruptly forms upstream of the lifted triple flame, enhancing the mixing and facilitating flame stabilization closer to the injector. (c) 2023 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)