Measurements of parameters of transient gas flows by a diode laser absorption spectroscopy at elevated pressures and temperatures

Abstract

We have developed a prototype of a diode-laser absorption spectrometer for noncontact determination of the temperature and concentration of water vapor in the combustion zone of mixed supersonic flows. The method is based on fast scanning of the radiation frequencies of two diode lasers in different spectral ranges containing several absorption lines of water molecules, determining the integral intensities of selected lines, and calculating the medium temperature from the intensity ratio of these lines. The described version of the spectrometer allows one to determine the parameters of the gas medium at high pressures (up to 3 atm) and temperatures (up to 2000 K). The criterion of selection of the spectral scanning ranges of the lasers is described, which is based on minimizing the error of determining the temperature at the expected experimental measurement accuracy of the integral intensities of lines. A program for selection of appropriate combinations of lines from spectroscopic databases has been developed. An algorithm for obtaining the temperature by fitting experimental spectra and theoretical spectra simulated using databases has been developed. Errors of determining the temperature for different fitting algorithms have been estimated. We show that, for the temperature estimation to be more accurate, it is necessary to take into account maximal possible number of lines lying in the scanning ranges of the two lasers.