Numerical evaluation of boundary layer assumptions for laminar diffusion flames in microgravity

A systematic numerical study of a low Reynolds number laminar diffusion flame is presented. The configuration used is that of a boundary layer flow established over a flat plate burner. The importance of this configuration relies on its potential use for the assessment of the flammability of materials to be used in space vehicles. The study focuses on the validity of boundary layer formulations to the study of these flames. The characteristic velocities are representative of microgravity environments [O(100 mm/s)]. Parietal injection results eventually in flow separation establishing two characteristic regimes: non-separated and separated flows. Non-separated flows show an increased local acceleration but allow the use of two-dimensional assumptions at the plane of symmetry. It was demonstrated that classical boundary layer assumptions can be used if the flow is non-separated. Three-dimensional flow fields at the trailing edge of the injection zone characterize separated flows. Energy release enhances the positive pressure perturbations and leads to flow acceleration that cannot be damped by viscous shear. Acceleration appears at the vicinity of the flame, thus it is dramatically amplified by the decreased density in this region. Significant errors are generated if boundary layer assumptions are used to describe diffusion flames established in separated flows.