Extinction thickness in the SHS flame propagation in two-layered composite medium

Extinction of the self-propagating high-temperature synthesis (SHS) flame in a condensed composite medium, consisting of adjacent layers of reactants and inert, has been analyzed for the synthesis of functionally graded materials (FGMs), which are anticipated to be useful in future space applications. The essential heterogeneous nature of the SHS flame propagation is appropriately described and the effects of the particle size on the combustion and/or extinction behavior are captured. The analysis, based on the temperature sensitivity of the mass diffusion process in the condensed phase, yields the extinction thickness due to heat transfer to the neighboring inert layer. The study further identifies the extinction thickness as function of the system parameters, which include the mixture ratio, the degree of dilution, the particle size, the representative length of the specimen cross section, and the initial temperature. It is demonstrated that the extinction thickness strongly depends on the particle radius, increasing with increasing particle size, due to the reduced heat generation rate at the particle surfaces, It is also shown that the thickness increases with decreasing mixture ratio and/or increasing degree of dilution, due to the reduced amount of the heat of combustion. Furthermore, it is found that preheating can be quite effective in reducing the extinction thickness when there exist restrictions in reducing the particle size for the production of composite materials.