Propagation velocity reaction front

Deflagration A propagating chemical reaction of a substance in which the reaction front advances into the unreacted substance at less than the sonic velocity in the unreacted material. Where a blast wave is produced that has the potential to cause damage, the term explosive deflagration may be used. 

The cooling effect of the channel walls on flame parameters is effective for narrow channels. This influence is illustrated in Figure 6.1.3, in the form of the dead-space curve. When the walls are <4 mm apart, the dead space becomes rapidly wider. This is accompanied by falling laminar burning velocity and probably lowering of the local reaction temperature. For wider charmels, the propagation velocity w is proportional to the effective flame-front area, which can be readily calculated. On analysis of Figures 6.1.2b and 6.1.3, it is evident that the curvature of the flame is a function of... 

OS 90] [R 31] [P 70] At weak electrical field, the propagation velocity of a reaction front in a capillary-flow reactor was increased or decreased depending on the mutual orientation of the electrical field and the reaction zone propagation [68]. The movement of two reaction fronts was given by optical images in [68]. 

The damage effects from an explosion depend highly on whether the explosion results from a detonation or a deflagration. The difference depends on whether the reaction front propagates above or below the speed of sound in the unreacted gases. For ideal gases the speed of sound or sonic velocity is a function of temperature only and has a value of 344 m/s (1129 ft/s) at 20°C. Fundamentally, the sonic velocity is the speed at which information is transmitted through a gas. 

For a deflagration the energy from the reaction is transferred to the unreacted mixture by heat conduction and molecular diffusion. These processes are relatively slow, causing the reaction front to propagate at a speed less than the sonic velocity. 

Detonation - A propagating chemical reaction of a substance in which the reaction front advances into the unreacted substance at or greater than sonic velocity. 

The conditions in the reaction zone determine the release rate of the N-precursors HCN and NH3 [11], Among these conditions are the properties of the fuel (e.g., N-content and particle size), parameters related to the combustion front (temperature and propagation velocity) and the gas composition in and directly above the combustion front. As the prediction of the mass fractions of the N-precursors is important for the final goal of this research, i.e., the prediction of NO formation of the complete furnace, a model is needed in which all these conditions are represented. 

Detailed experimental studies on these gas-solid combustion reactions reveal the dependence of combustion and propagation characteristics, like front propagation velocity, combustion temperature and degree of conversion, on operating parameters like nitrogen pressure, particle size and morphology of the reactant metal and dilution of the gas and solid phases. From these studies the optimum synthesis conditions for a variety of nitrides are determined and information about the mechanisms of several gas-solid combustion reactions is obtained. With the aid of combustion theory, the apparent values of activation energy for several nitridation reactions are calculated from measured combustion characteristics. 

The propagation velocity of the TM does not exceed 85-87% of the theoretical detonation velocity DT. Calculation of DT is carried out under the assumption of a chemical reaction which runs after compression by the shock wave without any thermal or hydrodynamic losses. In the case of the TM, meanwhile, the very possibility of propagation of a fast flame with the velocity of the shock wave depends on a velocity redistribution as a result of braking of the layers adjacent to the wall. In constructing the equations for the motion as a whole, braking plays the role of a loss which reduces the velocity. In fact, the velocity will be even smaller than the value cited besides the losses in the hydrodynamic preparation zone (the zone of velocity redistribution between the shock wave front and the forward point of the flame front, zone I-II in Fig. 19) we must add the losses in the combustion zone (from the forward point of the flame front to the cross-section in which combustion has ended, zone II-III in Fig. 19). 

Indeed, in normal (slow) combustion, which may propagate only due to heat conduction, the heat flux is a quantity of the same order as the combustion heat released in unit time. The width of the front should be of the same order as the product of the chemical reaction time and the flame propagation velocity. 

In the reaction zone, superposition of convective cooling and heat consumption through the endothermic reaction leads to an accelerated cooling of the fixed bed. The ratio of the propagation velocity of the reaction front (uw0) and the thermal front can be estimated as follows ... 

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