Forced-Convection Extinction of a Diffusion Flame Sustained by a Charring Body

摘要

The subsonic isobaric forced-convective flow of air past a pyrolyzing, char-forming body outgassing combustible fuel vapor is examined; the goal is to seek a generalized approach toward establishing approximate criteria for extinction of burning. A Shvab-Zeldovich formulation (incorporating, inter alia, a one-step irreversible exothermic finite-rate chemical reaction) is adopted to describe the gas-phase burning of initially unmixed reactants. For specificity, a sublimation model is adopted to describe the endothermic gasification at the surface of the (isothermal) pristine body; a nonisothermal porous char layer (through which combustible vapor, from pyrolysis of the pristine-body surface, can diffuse to the surrounding air) envelopes the pristine body. Important features of the steadily burning configuration can be obtained for any simple bluff body, and sequentially the question of flame stability can be addressed via exploitation of the well-studied counterflow-diffusion-flame two-point-boundary-value problem for describing extinction of unpremixed burning. Thus, known results for the minimum Damkohler number (ratio of forced-convective-flow time to chemical-reaction time) compatible with vigorous burning of large-activation-energy, large-heat-of-combustion, counterflow diffusion flames are applicable.