Combustion wave velocity

Within the region of optimal experimental parameters, the combustion wave velocity remains constant and the temperature profile T(t) has the same form at each point of the reaction medium. This regime is called steady propagation of the combustion synthesis wave, or steady SHS process. As the reaction conditions move away from the optimum, where the heat evolution decreases and/or heat losses increase, different types of unsteady propagation regimes have been observed. These include the appearance of an oscillating combustion synthesis... 

The time of production by CS methods is much shorter than that for conventional methods of powder metallurgy. Also, materials and articles can be produced directly in the combustion wave. Furthermore, a continuous mode of production may be realized, where reactants are fed at a rate equal to the combustion wave velocity. 

The theory discussed here applies mainly to kinetic-controlled reactions. However, combustion synthesis reactions involve many processes, including diffusion control, phase transitions, and multistage reactions, which result in a complex heat release function, Z4> (r/,7). An analysis of the system of combustion equations for various types of heat sources was compared with experimental data, which led to the conclusion that in general, the combustion wave velocity can be represented as follows (Merzhanov, 1990a) ... 

The second model (Fig. 20c) assumes that upon melting of reactant A, a layer of initial product forms on the solid reactant surface. The reaction proceeds by diffusion of reactant B through this layer, whose thickness is assumed to remain constant during the reaction (Aleksandrov et al., 1987 Aleksandrov and Korchagin, 1988). The final product crystallizes (C) in the volume of the melt after saturation. Based on this model, Kanury (1992) has developed a kinetic expression for the diffusion-controlled rate. Using this rate equation, an analytical expression for the combustion wave velocity has been reported (Cao and Varma, 1994)... 

In an early work by Kottke and Niiler (1988), a cellular model was used to simulate the combustion wave initiation and propagation for the TH-C model system. The interactions between neighboring cells were described by the electrical circuit analogy to heat conduction. At the reaction initiation temperature (i.e., melting point of titanium), the cell is instantly converted to the product, TiC, at the adiabatic combustion temperature. The cell size was chosen to be twice as large as the Ti particles (44 /xm). Experimentally determined values for the green mixture thermal conductivity as a function of density were used in the simulations. As a result, the effects of thermal conductivity of the reactant mixture on combustion wave velocity were determined (see Fig. 21). Advani et al. (1991) used the same model, and also computed the effects of adding TiC as a diluent on the combustion velocity. 

Figure 14. Modeling results of the influence of the mixture ratio of nonmetai to the metal, p, on the combustion wave velocity in the (a) Ti-B system, (b) Zr-B system [19]. Tq is the initial temperature data from [20-25].

Figure 20. The effect of the particle size of carbon on the combustion wave velocity in the synthesis of TiC [26],...

Figure 25. The effect of relative density of the metal compact on the combustion wave velocity (a) for the synthesis of TiN, (b) for the synthesis of NbN [33,35].

Figure 51. Combustion wave velocity against A1 particle size for the combustion synthesis of TiC-AI2O3 composites [100],...

Fig. 5.11 Relative pressure amplitude P IPq and combustion wave velocity of H2 + O2 mixtures in the obstructed tube versus the equivalence ratio (j)...

Retention interval in heated tank Maximum blast pressure time Combustion velocity in obstructed tube Combustion wave velocity... 

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