Single-point thermometry in high-pressure, sooting, premixed combustion environments

We have performed nonintrusive thermometry in the burnt gases of rich, pressurized ethylene/air flames using a frequency measurement based on laser-induced gratings. Light from a continuous-wave probe beam is coherently scattered from a thermal or electrostrictive grating induced by a pair of crossed, pulsed pump beams. The measured Doppler shift of the signal beam is a function of the local speed of sound from which a temperature can be extracted. At equivalence ratios of 1.6, the transient grating temperature agreed with a corrected thermocouple temperature. At higher soot loading, it is necessary to account for the change in local gas composition caused by soot particle vaporization. Soot particles, acting as blackbody absorbers, were observed to generate thermal gratings of diagnostic value.