Temperature measurements in sooting counterflow diffusion flames using laser-induced fluorescence of flame-produced nitric oxide

Laser-based diagnostic methods are often used for non-intrusive studies of delicate processes of soot formation. When soot particles are heated by the laser pulse, their size distribution can be estimated from the cooling rate, provided that the local gas temperature is known. However, strong light absorption, scattering and fluorescence in sooting environment hinder non-intrusive laser-based temperature measurements. Methods based on fitting of laser-induced fluorescence spectra work well in stationary flames but usually require temperature tracer seeded into the flame. We have shown that in counterflow diffusion flames, often used for soot-formation studies, enough nitric oxide is produced for two-dimensional temperature imaging. Measured temperature profiles agree very well with chemical kinetic calculations for a variety of fuels if laser intensity is reduced to keep NO excitation in the linear regime. Gas composition affects line shapes at temperatures below 600 K and should be taken into account for accurate measurements.