COMBUSTION REGIMES OF PARTICLE-LADEN GASEOUS FLAMES - INFLUENCES OF RADIATION, MOLECULAR TRANSPORTS, KINETIC-QUENCHING, STOICHIOMETRY

We study flat flames propagating steadily in a reactive gaseous premix ture which is seeded with an inert solid suspension. Our main assumpti ons are: (i) the two-reactant, one-step overall reaction we choose as the combustion process has a rate which vanishes at and below a prescr ibed temperature (T-c) and resumes the Arrhenius form at higher temper atures; (ii) both phases are considered as continua and have the same local speed and temperature; (iii) radiation among the particles follo ws the Eddington approximation specialized to a grey medium and the at tenuation length markedly exceeds the conduction-convection length in the gas; (iv) the activation energy is large. The first regimes we con sider comprise a thin flame front (dominated by molecular transports, convection and chemistry) embedded in much thicker radiation-convectio n zones. Jump conditions across the former are derived analytically an d then used as targets in a shooting method to analyse the thickest zo nes and compute the burning speed (U). Such regimes only exist for equ ivalence ratios (phi) above a load-dependent critical value phi(lim) < 1 which corresponds to a turning point of the U(phi) curve. This turn ing point is due to radiative heat losses from the thin flame front to the cooler adjacent zones, which lead to extinction. Over restricted, well defined ranges of composition other regimes may also exist, whic h have monotonic temperature profiles culminating slightly above T-c. When they are too thick to be affected by molecular transports and are thus similar to coal-dust-air flames, their structure, domain of exis tence and speed are investigated analytically and numerically. The cor responding U(phi) curve exhibits an upper limit equivalence ratio phi characterized by an end-point, beyond which such regimes cannot exist . The influence of molecular diffusion is then accounted for and shown to modify the results only slightly.