FREE-CONVECTIVE DIFFUSION FLAME SHEET IN POROUS-MEDIA

The nonpremixed burning of a condensed fuel with an oxidizer flow, dri ven by the buoyancy force, adjacent to the wall in porous media is ana lyzed in the reaction-sheet limit. Because of the distinct flow charac teristics in porous media, similarity exists when the problem is scale d by x(1/2), where x is the spatial coordinate along the wall, instead of x(1/4) as for the conventional gaseous natural convective flow. Re sults show that the profiles of the temperature and reactants are qual itatively similar to those of the gaseous combustion situation. Quanti tatively, the flame temperature is reduced as a consequence of the hig h effective thermal conductivity, which results in high Lewis numbers. The standoff distance of the reaction sheet is relatively insensitive to the preferential diffusion of the oxidizer but is decresed with in creasing Lewis number of the fuel vapor. Interestingly, the vertical i nduced flow velocity may have one maximum, which is located at the rea ction sheet, or two maxima, depending upon the temperature dependence of the flow viscosity. The fuel consumption rate, as a result of the c ombined effect of the flame temperature and standoff distance, can be increased or decreased when varying the Lewis numbers.