Combustion reaction zone

If such a process continues to accelerate, the combustion mode may suddenly change drastically. The reactive mixture just in front of the turbulent combustion zone is preconditioned for reaction by a combination of compression and of heating by turbulent mixing with combustion products. If turbulent mixing becomes too intense, the combustion reaction may quench locally. A very local, nonreacting but highly reactive mixture of reactants and hot products is the result. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

A third alternative has been proposed by Anderson and Brown (A6, A9) as an outgrowth of their research on the ignition of composite propellants. Their ignition studies suggest significant contributions to the overall combustion process from the solid phase. Two exothermic reaction zones contributing to combustion are considered, as shown schematically in Fig. 19. 

The most notable theoretical analysis of the instability problem has been presented by McClure and Hart (M5). These investigators postulated a generalized combustion zone that includes a temperature-dependent and pressure-independent solid-phase reaction zone, and a temperature- and pressure-dependent gas-phase reaction zone. From this general model, Hart... 

Williams (W2) has recently modified the analysis of Hart and McClure by considering in more detail the effect of diffusional processes on the gas-phase reaction zone. The results of his study show that the diffusional processes tend to stabilize the gas-phase combustion process, indicating that the postulated solid-phase reactions are probably the underlying cause of the instability. 

Another contributing mechanism is the direct cooling of hot propellant surface by contact with the injected fluid. The fluid should cause the decomposing surface to reduce its pyrolysis rate to a point where combustion cannot be sustained. In addition, the presence of water on the surface would obstruct heat transfer from the gas-phase reaction zones to the solid surface, thus augmenting the cooling of the surface. Proponents of these two approaches have correlated the injection data on the basis of mass of fluid required per unit area of surface, but theoretical justifications for the use of this particular correlating parameter have not been presented. 

On comparing the two flames, it is evident that the flow structure of the lean limit methane flame fundamentally differs from that of the limit propane one. In the flame coordinate system, the velocity field shows a stagnation zone in the central region of the methane flame bubble, just behind the flame front. In this region, the combustion products move upward with the flame and are not replaced by the new ones produced in the reaction zone. For methane, at the lean limit an accumulation of particle image velocimetry (PIV) seeding particles can be seen within the stagnation core, in... 

F. Pintgen, C.A. Eckett, J.M. Austin, and J.E. Shepherd, Direct observations of reaction zone structure in propagating detonations. Combust. Flame, 133, 211-229, 2003. 

Laminar flames Combustion in which the reaction zone is not turbulent. 

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