Laser-induced breakdown spectroscopy measurements in turbulent methane flames

Laser Induced Breakdown Spectroscopy (LIBS) in uniform flowing methane-air mixtures of compositions ranging from only air to only fuel was investigated in order to explore calibration schemes for further measurements in turbulent non-premixed and premixed flames. A portable low-cost spectrometer was used to record plasma emission spectra. The differences among the various spectra are discussed and the intensity ratios Hα(656.3nm)/O(777.3nm) and C2(516.5nm)/CN(388.3nm) were found to depend monotonically to the mole fraction of methane in the ranges 0-0.3 and of 0.3-1.0 respectively, therefore providing a scheme for measurement in non-premixed systems spanning a wide range of equivalence ratios. Based on this and using laser excitation conditions such that the escaping plasma emission contains spectral signatures from reactants, flame intermediates, and combustion products, the measurement can be thought to give an estimate of the local and instantaneous mixture fraction. The mean estimated mixture fraction in turbulent axisymmetric non-reacting and reacting jets was in reasonable agreement with empirical correlations. The technique was also applied to a swirling recirculating premixed flame and it is shown that the equivalence ratio is successfully measured