Specie concentration measurements utilizing Raman scattering of a laser beam

Abstract

Abstract : The feasibility of utilizing Raman scattering as a diagnostic technique to measure individual specie concentrations in typical gas mixtures found in gas dynamic applications has been investigated and demonstrated. Utilizing this technique, either the local density of a pure gas or the concentration of individual diatomic (or polyatomic) species in a gas mixture can be uniquely determined. The range and limitations of this technique were investigated and evaluated under controlled static conditions. A Q-switched ruby laser, which has a pulse duration of approximately 10 nanoseconds, was used as a radiation source. The scattered radiation was monitored utilizing a high gain, wide spectral range photomultiplier tube in conjunction with a spectrograph. Measurements were also made utilizing narrow bandpass filters in place of the spectrograph. The species which were investigated include O2, N2, CO2, and CH4. Quantitative results are given for these gases in their pure state as well as in various mixture proportions. The relative and absolute intensity of the scattered radiation from the species investigated were compared with that predicted by theory. Measurements were also made of the vibrational temperature of O2. A description of the pertinent theory and concepts of Raman scattering is also included as well as a discussion of the limitations of the technique.