A numerical and experimental study of the structure of laminar triple flames propagating in mixing layers

Abstract

A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded by a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stabilization region. It is investigated experimentally by PIV for the velocity field, OH-LIPF imaging, C2Hx-LIF imaging, and a 1D-Raman technique for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented by numerical simulations solving the two-dimensional axisymmetric Navier-Stokes equations in the zero Mach number limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, but non-unity, Lewis numbers 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 C1 and C2 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 between measurements and computations for flame shape and flame length. The measured OH-LIPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoichiometric mixture. The results from a newly developed C2Hx-LIF method are also supported by calculations. While these measurements were only qualitative, the temperature and mole fractions of the major species could be measured quantitatively with the combined Raman-/Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the structure is a triple flame and is influenced essentially by two external parameters: heat exchange between the branches and heat loss at the curved flame front near the triple point.