Size and shape of solid fuel diffusion flames in very slow speed flows

The effect of very low speed forced flows on the size and shape of a diffusion flame adjacent to a solid fuel slab are investigated experimentally and theoretically. Flows due to natural convection are eliminated by performing the experiment in low gravity. The range of velocities tested is 1.5 om/s to 6.3 om/s and the mole fraction of oxygen in the O₂/N₂ atmosphere ranges from .15 to .19. Although the flames did not reach the final steady state in the 5.2 seconds to which the experiment was limited, trends in many flame characteristics, such as flame size, shape and color can be deduced. As the flow “elocity is reduced, the flames move further from the fuel surface, and its for suggest a lower flame temperature. As the oxygen concentration is lowered the flames move closer to the fuel sample and the flame temperature is lowered. With stand off distances up to 6.5 mm and thicknesses around 1 or 2 millimeters, these flames are much weaker than flames observed at normal gravity. A corresponding theoretical model is solved using two-dimensional Navier-Stokes system with a one-step finite-rate chemical reaction and surface radiative loss. Computed flame shape and size compare favorably with experimental results, but the computed trend of maximum temperature does not appear in agreement. The implications of these comparisons are discussed.