CORRELATED-KAPPA AND FICTITIOUS GAS METHODS FOR H2O NEAR 2.7 MU-M

The correlated-k distribution (CK) method is extended with a fictitious gas approach in order to account for strong temperature-gradient effects. The correlated-k fictitious gas (CKFG) method involves consideration of the absorbing gas as a mixture of fictitious gases, each characterized by real gas lines with a lower level energy belonging to a fixed range. CK and CKFG parameters have been obtained from a line-by-line (LBL) approach for H2O between 2900 and 4200 cm-1 in the 300-1500 K temperature and 0.2-3 atm pressure ranges. Results obtained from CK, CKFG and narrow-band statistical (NBS) models are compared under various conditions with LBL results. The CK approach yields accurate results for isothermal and slightly nonisothermal media. But for large temperature gradients, only CKFG results agree well with LBL calculations. It is shown that the CK approach leads to an underestimation of more than 50% under some conditions of the radiation emitted by a hot medium and transmitted through a cold medium. For practical use, the variations of the CK and CKFG model parameters with temperature and pressure are approximated by analytical functions with few coefficients fitted from LBL calculations. It is shown that this fitting procedure does not degrade the accuracy of either model. Finally, the utility of each of the models is discussed for use under various radiative transfer situations.