Two dimensional temperature measurements were performed in a sooting, methane-air flame by a modification of the conventional Rayleigh scattering technique. This new technique, filtered Rayleigh scattering (FRS), uses a molecular absorption filter in the detection path to psectrally spectrally separate part of the gas-phase Rayleigh scattering from the elastic scattering contributions of soot particles. It also suppresses laser-induced glare and background scattered light from walls and windows. Thus, particle and background, unaffected, gas-phase Rayleigh scattering can be detected. This has been done in a weakly sooting flame where the temperature field information was obtained using a calibration of the FRS setup in a nonsooting flame by conventional Rayleigh scattering. This was necessary because the FRS gas scattering intensity is no longer a simple function of temperature and the known scattering cross sections of the individual gas species present. It also depends on the spectral broadening of the Rayleigh line relative to the indicent radiation. Two theoretical models for the calculation of the dependence of the FRS signal on the experimentla conditions, for the kinetic and the hydrodynamic regime, are presented in order to directly calculate gas-phase temperatures from FRS measurements without the need for calibration. For the present flames, the broadening mechanism is in an intermediate regime.
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