An Experimental and Numerical Study of a Laminar Triple Flame

A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded oy a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stahilization region. It is investigated experimentally hy PlY for the velocity field, OH-LlPF imaging, C2H,-LlF imaging. and a lD-Raman techniquc for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented hy numerical simulations ,olving the two-dimensional axisymmetric l\avicr-Stokes equations in the zc:ro Mach numher limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, hut non-unity, Lewis numhers for all species. Chemistry is represented by a ten-step reduced mechanism for methane oxidation, which was derived starting from a 61-step elementary mechanism that includes the C I and C: chains. The agreement between the measured and the predicted flow field is very satisfactory. Owing to gas expansion, the velocity decreases immediately ahead of the flame and increases strongly at the flame front. Further downstream acceleration due to buoyancy is dominant and is predicted accurately. There is a good agreement hetween measurements and computations for flame shape and flame length. The measured OlI-LlPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoich iometric mixture. The results from a newly developed C2H, -LJ F method arc also supported hy calculations. Whil.e these measurements were only l(ualitative, the temperature and mole fractions of the major species could he measured quantitatively with the comhined Raman-!Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the strudule is a triple flame and is influenced essentially h) two external parameters: heat exchange hetween the branches and heat loss at the curved flame front Ilear the triple point.