A numerical investigation of the flame structure of an unsteady inverse partially premixed flame

Abstract

The flame structure of unsteady flickering partially premixed flames is numerically investigated, and detailed results are provided for a flame established at Fr = 0.5, Re = 500, and overall Φ = 1. A numerical study is conducted in an inverse configuration in which a fuel-rich (CH 4-air) annular jet is sandwiched between an axisymmetric air jet (on the inside) and coflowing air (on the outside). The computations involve a time-dependent, axisymmetric model based on a direct numerical simulation methodology, and a detailed 52-step mechanism to model the CH 4-air chemistry. The calculations show that the flame structure of the partially premixed flames differs from that of a typical nonpremixed laminar jet flame. The fuel-rich annular ring close to the nozzle exit undergoes premixed combustion, but once oxygen is depleted inside the annular ring, diffusion flames are established on both sides of it due to excess fuel emerging from the premixed zone. The two high temperature nonpremixed regions later merge into a single surface in the plume. Subsequently, buoyant acceleration of hot gases outside the diffusion flame surface causes shear-layer rollup, leading to the formation of toroidal vortex rings, which then interact with the plume surface. While the premixed flame exhibits relatively steep temperature gradients, and is thin, the two diffusion flames display broader gradients, and are thicker. The carbon monoxide and molecular hydrogen formed due to premixed combustion provide “fuel” for two nonpremised flames connected to either wing of the premixed flame. The oxygen present in the fuel-rich annulus is completely consumed in the premixed zone, primarily by atomic hydrogen through the reaction H + O 2 ⇔ OH + O. Considering the dominant reactions, the premixed region can be characterized by the overall reaction CH 4 + O 2 - (H) ⇔ (CO + 2H 2 + OH). Oxygen consumption in the nonpremixed region is due to the OH chemistry. Adding the significant reactions in the nonpremixed region and eliminating the steady-state intermediate species, the overall reaction (CO + 2H 2 + OH) + O 2 ⇔ CO 2 + 2H 2O + (H) describes combustion in that zone. The effect of unsteadiness on flame structure is investigated by comparing the scalar profiles at different times with respect to a conserved scalar. The predictions do not reveal significant effects of unsteadiness on the flame structure, although buoyant convection may influence the heat transfer to the unreacted flow. This is because the rollup process essentially affects the postflame, or plume, region and not the actual “flame.” The vortex rollup process can be delayed by increasing either the Froude (Fr) or Reynolds (Re) numbers. The shortest rollup length is for the flame corresponding to the lowest values of Fr and Re. The “flame” flicker frequency increases as the Froude number is decreased, but the qualitative nature of the flame (plume)-vortex dynamics remains essentially the same. Detailed numerical simulations of analogous counterflow flames were performed using a more detailed chemical mechanism for the sake of comparison. There is remarkable similarity between the steady counterflow profiles and those of the unsteady co-annular flame.