Analysis of molecular gas radiation: Real gas property effects

Molecular gas radiation calculations have been performed for CO2 and H2O with a line-by-line high-resolution transmission molecular absorption database, with and without particle scattering. Both line-by-line (Monte Carlo) and K-distribution calculations were conducted The results confirmed that, if the absorption-coefficient distribution function is sufficiently resolved, essentially line-by-line accuracy can be achieved. In addition, comparison was made between line-by-line results and an exponential wideband model combined with Elsasser narrowband model predictions to assess real gas property effects on radiative heat transfer. It was found that accurate prediction of the radiative heat flux and divergence requires a more accurate representation of the K-distribution than band model data currently provide, It was also found that the notion of an optically thin molecular gas based on the Planck mean optical thickness is probably a misleading concept. Analysis of real line-by-line 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.