Transient radiative initiation of the heterogeneous flame in self-propagating high-temperature materials synthesis: Theory and experimental comparisons

Radiative initiation of a combustion wave in the self-propagating high-temperature materials synthesis (SHS) process was analyzed for a mixture compact by use of a heterogeneous theory which accounts for the premixed mode of bulk flame propagation as well as the nonpremixed mode of consumption of the non-metal (or higher-melting-point metal) particles. In the realistic limit of diffusion-controlled combustion, which nevertheless is still highly temperature sensitive because of the Arrhenius nature of mass diffusion, the analysis successfully identified the functional dependence of the various system parameters related to flame initiation on the particle size, a feature of which was not captured in previous theoretical works based on homogeneous flame propagation. Specifically, calculated results showed that the ignition delay increases with decreasing mixture ratio, increasing dilution, decreasing intensity and beam diameter of the heat flux, and increasing particle size. Furthermore, in the presence of heat loss which is inevitable, there also exist absolute ignition limits beyond which the mixture compact is not ignitable. Such nonignition events are manifested by the failure to either establish a combustion wave, for large particles or moderate heat fluxes, or sustain the propagation of an established wave even though the particle size is small and the heat flux is strong. Comparison with experimental data showed satisfactory agreement, both qualitatively and quantitatively, thereby substantiating the viability of the present heterogeneous theory of flame initiation.