Low Stretch Diffusion Flames Over a Solid Fuel

Abstract

A unique new way to study low gravity flames in normal gravity has been developed. To study flame structure and extinction characteristics in low stretch environments, a normal gravity low-stretch diffusion flame is generated using a cylindrical PMMA sample of varying large radii. Foutch and T'ien used the radiative loss as well as a densimetric Froude number to characterize the blowoff (small Da) and quenching extinction (large Da) boundaries in stagnation-point diffusion flames under various convective conditions. An important conclusion of this study was that the shape and location of the extinction boundary, as well as a number of important flame characteristics, were almost identical for the buoyant, forced, and mixed convective environments they modeled. This theory indicates it should be possible to understand a material's burning characteristics in the low stretch environment of spacecraft (induced by fans and crew movements) by understanding its burning characteristics in an equivalent Earth-based stretch environment (induced by normal gravity buoyancy). Similarly, the material's burning characteristics in Lunar or Martian stretch environments (induced by partial gravity buoyancy) can be assessed. Equivalent stretch rates can be determined as a function of gravity, imposed flow, and geometry. A generalized expression for stretch rate which captures mixed convection includes both buoyant and forced stretch is defined as a = a(sub f) ((1 + (a(sub b))exp 2/(a(sub b))exp 2))exp 1/2. For purely buoyant flow, the equivalent stretch rate is a(sub b) = [(rho(exp e)-rho(exp *)/rho(sub e)][g/R](exp 1/2). For purely forced flow, the equivalent stretch rate is characterized by either a(sub f)= 2U(sub infinity)/R for a cylinder, or a(sub f)=U(sub jet)/d(sub jet) for a jet impinging on a planar surface. In these experiments, the buoyant stretch is varied through R, the radius of curvature, but the buoyant stretch could also be varied through g, the gravity level. In this way the effect of partial gravity, such as those found on the Moon (1/6 g) or Mars (1/3 g) can be captured in the definition of flame stretch.