Chemical kinetics, flame propagation

Studying anew the differential equations of heat conduction, diffusion, gas motion and chemical kinetics under the conditions of a chemical reaction (flame) propagating in a tube, through a narrow slit or under similar conditions, using the methods of the theory of similarity we find the following dimensionless governing criteria ... 

In the previous chapters, the fundamental areas of thermodynamics and chemical kinetics were reviewed. These areas provide the background for the study of very fast reacting systems, termed explosions. In order for flames (deflagrations) or detonations to propagate, the reaction kinetics must be fast—that is, the mixture must be explosive. 

Moreover, the phenomena of combustion themselves prove to be more complicated. For a long period the study of combustion broke away from chemical kinetics and set itself its own specific tasks. These included especially studies of the influence of instrumental parameters on ignition, flame propagation and limits, i.e., the influence of the diameter and length of tubes, the form of vessels, the direction of propagation, etc. 

Over the last 20 years chemical kinetics, and especially the theory of chain reactions, have achieved major successes. A theory of ignition of heated explosive mixtures has been created. However, attempts to directly explain the propagation of flame as the diffusion of active centers, or to explain the limits of propagation by the conditions of chain breaking fail to yield positive results. 

The calculation gave a combustion rate which differed by 15% from the experimental one. Thus, for the first time, the combustion rate was calculated from independent data, and for the first time in this example the practical possibility was demonstrated of reducing the laws of flame propagation to the laws of phenomena which lie at the basis of the process, i.e., to the laws of chemical kinetics and of heat conductivity and diffusion. 

Thus, the study of chemical kinetics in a jet in the MCD sector of the curve in Fig. 4 will be extremely significant for the theory of flame propagation. 

This chapter concerns the structures and propagation velocities of the deflagration waves defined in Chapter 2. Deflagrations, or laminar flames, constitute the central problem of combustion theory in at least two respects. First, the earliest combustion problem to require the simultaneous consideration of transport phenomena and of chemical kinetics was the deflagration problem. Second, knowledge of the concepts developed and results obtained in laminar-flame theory is essential for many other studies in combustion. Attention here is restricted to the steadily propagating, planar laminar flame. Time-dependent and multidimensional effects are considered in Chapter 9. 

L. This chemically reacting flowfield is solved using the SPIN application, while PREMIX is employed to study the freely propagating flame in which the stagnation surface is absent. The GRI 3.0 chemical kinetics mechanism for methane-air combustion is used in both simulations, employing 49 species and 277 elementary reactions steps [18]. 

We summarize a number of simulations aimed at deciphering some of the basic effects which arise from the interaction of chemical kinetics and fluid dynamics in the ignition and propagation of detonations in gas phase materials. The studies presented have used one- and two-dimensional numerical models which couple a description of the fluid dynamics to descriptions of the detailed chemical kinetics and physical diffusion processes. We briefly describe, in order of complexity, a) chemical-acoustic coupling, b) hot spot formation, ignition and the shock-to-detonation transition, c) kinetic factors in detonation cell sizes, and d) flame acceleration and the transition to turbulence. 

In this paper we describe some of the basic effects which arise specifically from the coupling between chemical kinetics and fluid dynamics in combustion systems. Although the particular emphasis here is on the ignition and propagation of detonations, many of the more fundamental interactions described are generally applicable to flames. The selection of topics is by no means meant to be comprehensive rather, it represents a potpourri of ideas which complement each other and those presented in the other papers in this session of the Symposium. Although we mainly use calculations performed at NRL to extract and illustrate the details of the interactions, we have drawn liberally on the results of experiments and analytic theory. 

First we described some of the properties occurring in propagating detonations, for which the structure is highly dependent on the chemical kinetic-fluid dynamic interactions. Finally, in the last section we discussed some processes and mechanisms involved in the transition to turbulence, which is important for flames. 

Pepiot-Desjardins, P. Pitsch, H. (2008). An efficient error-propagation-based reduction method for large chemical kinetic mechanisms. Combustion and Flame Vol (154) pp 67-81... 

---