Entropy production in a radiation affected flame.

Abstract

The effect of radiation on the entropy production in a flame on a flat flame burner was studied. The flame was modelled as an infinitesimally thin sheet at which the energy due to combustion is released. Because of the complex nature of flame radiation, the flame was modelled as either of two optical extremes, transparent or opaque. The burned and unburned gases were assumed to be optically thin. Scattering effects were neglected. A qualitative underst and ing of the effect of radiation on entropy production was determined by solving the linearized equations. The quantitative effect was obtained by numerically integrating the nonlinear equations using available integration techniques. Results showed that the major contribution to entropy production in the flame occurs in the unburned gases, between the burner surface and the flame. The temperature gradients in the unburned gas decrease with increasing radiation due to decreased flame temperatures and increased flame st and -off distances. As a result, the entropy production due to heat transfer in a flame on a flat flame burner decreases with increasing radiation. The entropy production due to heat transfer in a radiating propane and air flame was calculated for both the transparent and opaque flame assumptions. Flame emissivity was based upon gas only, "nonluminous", or soot only, "luminous", optical properties. The entropy production for the nonluminous transparent flame was 6.4% less than the entropy production calculated when radiation was neglected. For the luminous transparent flame this reduction increased to 53.6%. The entropy production in the nonluminous opaque flame was 12% less than the entropy production calculated neglecting radiation. The luminous opaque flame model did not predict the correct propane and air flame temperature. Because the luminous transparent flame result showed a significant decrease in entropy production, it is important to include the effect of radiation on the entropy production in a flame on a flat flame burner. The results suggest a detailed model of radiation due to soot in a flame is required to accurately determine the entropy production in the flame. Neglecting radiation can result in large errors in the predicted entropy production from a flame.