Influence of gas radiation on combustion on surface burners

The effect of gas radiation on the temp. decrease in the post-flame zone of flat flames is investigated. Flat flames are stabilized on surface burners and provide the framework for the laminar flamelet concept used in turbulent combustion. Flat flames can be modelled using a one-dimensional model with complex chem. to obtain accurate predictions for the temp. and species profiles in the preheating zone and the flame front. However, the temp. profile in the post-flame zone, which governs the relatively slow CO conversion and NO formation, is mainly detd. by gas radiation, which is essentially three-dimensional of nature. The temp. decrease due to gas radiation depends on the difference between emission and absorption, which on its turn depends on the dimensions of the hot gas vol. and thus on the burner dimensions. The limiting cases of the burner dimensions are therefore investigated first. A very small burner is modelled using the optical thin limit where self absorption is neglected. An infinite 1D burner is modelled using the Edwards exponential wide band model. The results of these calcns. show that a large difference exists in the temp. decrease between the two limiting cases. The relation between the burner size and the heat loss due to gas radiation is further investigated by simulating the post-flame zone and the environment using a 2D axi-sym. model. The temp. decrease is subsequently correlated to the results of the one-dimensional model in which the radiative heat loss is calcd. using the optical thin model scaled by a geometry factor. A relation for this geometry factor is proposed based on these results and the limiting situations. Using these relations the radiative heat loss can be calcd. in a one-dimensional model or, possibly, in laminar flamelet models for turbulent flames while incorporating the effect of self absorption of radiation by the hot gases