Diagnostics of water-containing gas mixtures using thermal laser-induced gratings

Abstract

Laser-induced gratings (LIGs) experiments, employing vibrational-overtone excitation of H 2O molecules through rovibrational transitions of the (2 1 1) ← (0 0 0) band around 820 nm to produce gratings, have been performed in humid ambient air, in a room-temperature cell with neat water vapor or that diluted in buffer gas at pressures of 0.02–4 bar, and in the vicinity of premixed atmospheric pressure CH 4-air flame, with the aim to investigate the feasibilities of the techniques for accomplishing gas diagnostics or tracing H 2O at various temperatures and pressures. In highly vibrationally-excited H 2O rapid multi-stage collisional thermalization processes occur, and as a result both strong oscillatory (due to standing acoustic waves) and stationary (due to stationary density modulations) contributions to LIGs have been observed in gas mixtures. Temporal profiles of LIG signals recorded at various conditions of gas mixtures employed have been analysed in frame of a model of a two- or three-stage R– T, V– V′, and V– T collisional relaxation of H 2O and buffer gas molecules. The informative parameters, like adiabatic sound velocity, gas temperature, H 2O mole fraction have been derived. The results of the analysis are presented, demonstrating possibilities and challenges of the technique in application to gas diagnostics.