MECHANISMS OF COMBUSTION WAVE-PROPAGATION IN HETEROGENEOUS REACTION SYSTEMS

For many condensed heterogeneous systems. thr propagation of the combu stion front occurs in a planar. steady manner, when observed using typ ical imaging rates (30 frames/s) and magnifications (Ix). However, usi ng much higher magnification (> 400x) and imaging rates (1000 frames/s ). two different mechanisms (quasiho-mogeneous and relay-race) oi comb ustion propagation have been observed recently. For the former, the mi crostructure of the combustion wave resembles what is viewed macroscop ically (i.e., at typical magnification and imaging rates). Under these conditions. steady, planar propagation occurs. In the relay-race mech anism, while planar at the macroscopic level, the combustion front pro files are irregularly shaped, with are-shaped convexities and concavit ies at the microscopic level. Moreover. the reaction front propagation consists of a series of rapid jumps and hesitations. In this paper, b ased on the local conditions during combustion (i.e., the microstructu re of the combustion wave), we develop and evaluate several new criter ia for determining the mechanisms of combustion wave propagation. Usin g these criteria, the boundaries between quasihomogeneous and relay-ra ce mechanisms are determined experimentally as a function of the initi al organization of the reaction medium (i.e., particle size and porosi ty). Further, the characteristics of the two mechanisms in terms of te mperature gradients and ignition delays are also studied. (C) 1998 by The Combustion Institute.