Piloted diffusion flames of diluted methane near extinction: Mean structure from raman/rayleigh fluorescence measurements

Time- and space resolved measurements have been made, for the first time using simultaneous Raman, Rayleigh and LIF techniques in a turbulent jet diffusion flame of CH₄, diluted alternatively with N₂ and air. The spontaneous Raman/Rayleigh data are processed to yield instantaneous species mass fractions of CH₄,O_2,N₂, H₂0, H₂, CO and CO₂ as well as temperature, density and mixture fraction. The LIF data is used to obtain the OH radical mass fraction. The effects of dilution on flame shape, soot interference, temperature and species mass fractions are reported for two cases each of N₂ and air dilution. In all cases the flames are close to extinction so that finite. chemistry effects are important. The results show that dilution shortens the flame and that air dilution markedly increases the blowoff jet velocity whilst N₂ dilution reduces it. All visible (yellow) soot radiation disappears, and the interference by soot precursors with the Raman signals, a serious problem in undiluted CH₄ flames, is greatly reduced, in particular with N₂ dilution. Due to the shift in the stoichiometric profile towards the middle of the shear layer with increasing dilution, as well as wider reaction limits, a larger fraction of the sampled data is simultaneously reactive and in a region of high strain rate. Results for the major species and temperature lie between the limits of fully reacted and frozen chemistry, whilst values in excess of the fully reacted limit are found for H₂ and OH.