Spectral Absorption of Water Vapor and Carbon Dioxide Mixtures in the 2.7 Micron Band

An investigation of the spectral absorption characteristics of water vapor and carbon dioxide mixtures in the 2.7 micron band is described. The absorption of black body radiation by gas mixtures in the central region of a high temperature furnace was measured. The optical path length of 30 cm was bounded by NaCl windows mounted in water-cooled holders. The temperature was essentially uniform in the central one-third of the test cell, and decreased to between 400 800 K at the ends. Representative spectral transmissivity distributions obtained for central temperatures ranging from 1000 to 2200 K and pressures from 0.25 to 3 atm for four different mixture ratios of water vapor and carbon dioxide are presented. Results showed the interaction effect on spectral transmissivity to be greatest at the band center where the maximum change from around 0.40 to 0.48 was observed for approximately equimolar mixtures at a total pressure of 0.5 atm. Since this occurs over a limited wave number range, spectral transmissivities of such mixtures can be acceptably approximated by ignoring the interaction. Spectral distributions predicted from published data for spectral H₂O and CO₂ absorption coefficients and line half-width to spacing ratios are in good agreement with measured distributions.