INFLUENCES OF SAMPLE THICKNESS ON THE EARLY TRANSIENT STAGES OF CONCURRENT FLAME SPREAD AND SOLID BURNING

The effects of solid thickness on the initial stages of concurrent fla me spread are studied through numerical simulation. The two-dimensiona l mathematical formulation of the problem is based on the fully ellipt ic, reactive Navier-Stokes equations coupled to energy and mass conser vation equations for a charring solid. For all fuel thicknesses, unifo rm burn-out, pyrolysis and flame propagation rates are approached afte r an accelerative stage. As with the opposed-flow problem, three main regimes of spread rate are established based on the dependence on fuel thickness. The first (kinetic) regime, where spread rates increase wi th the thickness, is established for samples below 0.008 x 10(-2) m. B oth frame and pyrolysis lengths are very short. In the second (thermal ly thin) regime, the spread rates decrease as the solid thickness is i ncreased while the flame and the pyrolysis regions become successively larger. Finally, as the fuel thickness is increased above 0.5 x 10(-2 ) m, the thermally thick regime, signified by constant spread rates, i s simulated. As no experimental measurements of spread rate dependency on the thickness of charring materials are available, numerical predi ctions are compared with a thermal theory to assess its validity limit s. Copyright (C) 1996 Elsevier Science Ltd.