Analysis of molecular gas radiation: Real gas property effects

Molecular gas radiation calculations have been performed for CO2 and H2O wi th a line-by-line high-resolution transmission molecular absorption databas e, with and without particle scattering. Both line-by-line (Monte Carlo) an d 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 co mbined with Elsasser narrowband model predictions to assess real gas proper ty effects on radiative heat transfer. It was found that accurate predictio n of the radiative heat flux and divergence requires a more accurate repres entation of the K-distribution than band model data currently provide, It w as also found that the notion of an optically thin molecular gas based on t he Planck mean optical thickness is probably a misleading concept. Analysis of real line-by-line data for typical thermodynamic conditions and path le ngths suggests that in most practical systems at least some portion of a vi bration-rotation band spectrum is optically thick. Hence, Planck mean optic al thickness is an erroneous indicator of the validity of the Planck mean o ptically thin approximation for molecular gas radiation.