The stabilization mechanism of a diffusion flame over solid combustibles exposed to a convective oxidizing flow is explored with emphasis on radiative heat transfer. The theoretical model includes the two-dimensional Navier-Stokes momentum, energy and species equations with a single step overall chemical reaction and second-order, finite rate Arrhenius kinetics. In order to produce a definitive answer to the question whether radiation is important in determining the flame characteristics, a solution strategy, called the discrete ordinates method, is employed and evolved in detail. Prior to the simulation on the structure and behavior of a flame affected by radiation, the solution strategy is applied to the problem of the natural convection coupled with radiation in a square cavity.
First, the computation is performed over a wide range of Damkohler numbers. For large Damkohler numbers, envelope diffusion flames are found to exist where the computed fuel evaporation rate, flame stand-off distance and the velocity profiles show a similitude. As the Damkohler number is lowered, a transition to open-tip flame takes place where the flame becomes stabilized on the sides of the combustibles. Further decreasing of the Damkohler number pushes the diffusion flame downstream out of the leading edge region of solid fuel.
Secondly, the effects of radiative heat transfer on the behavior of the diffusion flame are scrutinized. Numerical studies are performed over a wide range of mass absorption coefficients characterizing the radiation absorption and emission of fuel vapor gas. When the radiation absorption is significant, radiative heat loss retreats flame downstream over the fuel surface causing the shrinkage of size and diminution of maximum temperature. Results on the effects of wall emissivity and Boltzmann number variation are also presented.
It is inferred that thermal radiation may be the controlling mechanism of diffusion flame stabilization and thus neglecting radiation can cause erroneous prediction on the flame behavior and structure in combustion reaction processes.
