Abstract
This paper investigates the stabilization mechanism of turbulent jet flames with highly inhomogeneous inlet conditions. A modification of the standard piloted burner is employed here with the addition of a central tube (inner) carrying fuel that can slide within the existing (outer) tube carrying air. Both tubes are located within the pilot annulus and inhomogeneity is varied by translating the inner tube upstream of the jet exit plane. Two flames with identical overall air/fuel ratios, bulk jet velocities, and pilot conditions but different levels of homogeneity in the fuel/air mixture are selected for detailed investigations. Measurements are performed using Sandia's Raman-Rayleigh-LIF line facility, and Large Eddy Simulation (LES) using the stochastic fields approach are also conducted for the same flames. Results reported here focus on the early stabilization region.
The flame with inhomogeneous inlet conditions is more stable being at 57% of blow-off compared to the homogeneous counterpart, which is at 78% of blow-off. It is found that, very close to the jet exit plane, premixed combustion dominates the flame with an inhomogeneous profile. This is in contrast to the homogeneous case, which behaves like a diffusion flame. Further downstream, but still within the pilot region, partial mixing starts to occur between richer samples and hot combustion products. A comparison of the relative conditional scalar dissipation rates, chi(r) shows that in the upstream region, and within the reactive limits, the homogeneous case has higher values of chi(r). Premixed combustion with higher rates of heat release and lower scalar dissipation rates in the near field are therefore key reasons for the improved stability of the flames with inhomogeneous inlets. These findings are corroborated by results from LES. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.