Analysis of molecular gas radiation: Real gas property effects

Detailed molecular gas radiation calculations have been performed for CO₂ and H₂O using the line-by-line HITRAN data base, with and without particle scattering. Both exact, line-by-line (Monte Carlo) and K-distribution calculations were conducted. The results confirmed what had been earlier demonstrated with synthetic property data-that if the absorption coefficient distribution function (K-distribution) is sufficiently resolved, essentially exact line-by-line accuracy can be achieved with a significant reduction in computation time. In addition, comparison was made between exact line-by-line results and exponential wide band model predictions to assess real gas property effects on radiative heat transfer. It was found that accurate prediction of the radiative heat flux distribution in the gas (i.e., flux divergence field) requires line-by-line (LBL) or pseudo-LBL (e.g., K-distribution) treatment (or a more accurate representation of the K-distribution than band model data currently offers) while current band model data appears to be adequate for predicting heat flux at system boundaries when the temperature field is known. It was also found that the notion of an optically thin molecular gas is probably a misleading concept. Analysis of real LBL data for typical thermodynamic conditions and path lengths suggests that in most practical systems, at least some portion of a vibration-rotation band spectrum is optically thick. Hence, Planck mean optical thickness is an erroneous indicator of the validity of the Planck mean optically thin approximation for molecular gas radiation.