THE EFFECTS OF FUEL SOOTING TENDENCY AND THE FLOW ON FLAME RADIATION IN LUMINOUS TURBULENT JET FLAMES

In this paper two observations are emphasized with regard to flame radiation in luminous turbulent jet flames: (a) the radiant fraction in turbulent buoyant jet flames is a weak function of the fuel sooting tendency as expressed by the laminar smoke-point heat release rate; and (b) moreover, for very sooty fuels, the radiant fraction saturates to a constant value which is independent of the laminar smoke-point heat release rate, although it depends on the adiabatic flame temperature. These observations can be explained by considering that cooling of the flames is enhanced and the effective radiation temperature drops as soot increases and thus radiant losses increase, until for very sooty fuels flame extinction in the upper part of the flames occurs. The physics of flame radiation phenomena are discussed in the light of a global similarity analysis for flame radiation together with the use of extensive experimental data. In the analysis, complementary to the usual hydrodynamic length scale for combustion, a new radiation length scale is introduced which characterizes the effects of radiant losses on the flow. Scaling relationships of the turbulent radiant fraction in terms of the laminar smoke-point heat release rate are developed first for turbulent buoyant jet flames, wherein flow effects are small, and then extended to momentum-dominated turbulent jet flames. The present results have direct practical significance (a) for predicting radiant fractions in fires, and (b) for the design of enhanced flame radiation burners. The physics presented here for soot concentration and radiant cooling can be incorporated in detailed k-epsilon-g or other turbulence models.