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NIRE Annual Report
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 1999

Availability of Band Models for Numerical Calculation of Spectral Absorption Coefficient of High Temperature Combustion Gas

Thermal Energy Applications Division
Thermal Energy and Combustion Engineering Department
Objectives
For radiative heat transfer in a furnace filled with high temperature combustion gas medium, it has been widely recognized that the spectral absorption/emission characteristics of CO2 and H2O should be considered instead of the former gray body approximation. For the purpose of engineering calculation of radiative heat transfer in a combustion furnace, a number of band models have been proposed.
In the present study, we adopt the combination of Elsasser's narrow band model and Edwards' wide band model to calculate spectral absorption coefficients and total emissivities of CO2 and H2O for the range of temperature and partial pressure expected in a combustion furnace. And we discussed the availability of the relevant band models by comparing the calculation results with existing experimental measurements.
Results
Fig. 1. compares total emissivities calculated from the combination of the two models and its simplified form with measured values by Hottel et al. at several conditions of optical depth, in the limit of zero partial pressure in a mixture having a total pressure of 1 atm. In both cases of CO2 and H2O the total emissivities calculated from Elsasser model show good agreement with the experiments in the whole temperature range.
Comparison of calculated emissivities from the combined model with those from the simplified form shows discrepancy in relatively lower temperature range and at smaller optical depth conditions. This feature is more significant for the emissivities of H2O than those of CO2. The discrepancy suggests that the discrete line structure in each band can not be neglected at low temperature and small partial pressure conditions.
Adopting the simplified form of the model makes the numerical calculations very convenient. However, its availability is limited and it should be noted that the simplified calculation may lead to serious error.
Fig.1 Comparison of the total emissivities calculated from the band models with experimental measurements.
Fig.1 Comparison of the total emissivities calculated from the band models with experimental measurements.

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