Flame-vortex dynamics in an inverse partially premixed combustor: The Froude number effects

In this paper we report on a computational and experimental investigation of the transient combustion characteristics of an inverse partially premixed flame established by injecting a fuel-rich (CH4-air) annular jet sandwiched between a central air jet on the inside and coflowing air on the outside. A time-dependent, axisymmetric, reacting flow model is used to simulate the flame dynamics. A global 1-step and a relatively detailed 52-step mechanism are used to model the CH4-air chemistry. Results focus on the dynamic flame structure and flame-vortex interactions at different Froude numbers (Fr), the scaling of the flame flicker frequency, and the global comparison of experimental and computational results. At high Froude numbers (nonbuoyant limit), the computed flame exhibits a steady-state structure, which is markedly different from that of a jet diffusion flame. The flame structure reveals two distinct reaction zones consisting of an inner premixed region followed by two nonpremixed flames at the wings. Methane is converted to CO and H-2 in the premixed reaction zone and these intermediate species provide fuel for the outer nonpremixed flames. Main reaction pathways associated with the double-flame structure are identified. For intermediate Fr, the buoyant acceleration becomes significant, causing a periodic rollup of toroidal vortices. While the rollup process is highly periodic, the flame exhibits steady-state behavior, since vortices are relevant only in the plume region. For Fr < 1.0, the rollup occurs closer to the burner port, resulting in flame-vortex interactions and a dynamic flame. A distinguished characteristic of this flame is the rollup of two simultaneous vortices corresponding to inner and outer diffusion flames, which convect downstream at the same velocity, and interact with the twin flame surfaces, causing flame flicker and stretch. Both numerical and laboratory experiments are employed to obtain a correlation between the Strouhal number (S), associated with the vortex rollup or flame flicker frequency, and the Froude number. Simulations yield a correlation S = 0.56 Fr-0.38, while measurements yield S = 0.43 Fr (-0.38), indicating an excellent agreement, considering that the flow conditions in the numerical and laboratory experiments are only globally matched in terms of overall stoichiometry, Fr, and Reynolds number, and not with respect to burner size and jet velocity. Finally, the effects of chemical kinetics on the computed flame structure are examined. Both the time-averaged and the dynamic flame structure, including flame height, peak temperature, and flicker frequency, are found to be influenced by chemical kinetics. However, the scaling of the dominant frequency or Strouhal number with Fr is essentially the same for the two mechanics. In addition, the frequency is found to be independent of the chemical kinetic parameters used in the global mechanism. (C) 1997 by The Combustion Institute.