| An unsteady three-dimensional numerical model has been built to study ignition, flame decay, and flame growth over a composite solid sample fuel upon non-uniformly distributed radiant heating. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed.;The model is applied to a vertically-oriented sample in a gravitational field similar to the configuration of NASA-STD-6001 Test ;The computed results provide a detailed sequence of the ignition events including the first appearance of the reaction kernel, the spread and decay of the initial premixed flames, and eventually the formation of an anchoring solid diffusion flame. By varying the radiant heating rate, two ignition modes are identified: reaction initiated at the surface at low heating rates, and reaction initiated in the gas phase at high heating rates. Computed ignition boundaries yield the critical heating rate for ignition, as well as the minimum total energy for solid ignition as a function of the heating rate. Parameters varied in the computations include the shape of the radiant heat source, sample thickness, pressure, and gravity. |
