Flame fronts, resolution

The general shape of a premixed flame stabilized on a pipe approximates to a cone of semi-angle 6. At any point on the flame front, resolution of velocities in the longitudinal directions shows that... 

Attachment of electrons to 02, Reaction 5, was eliminated by Green because 02 - appeared far ahead of the flame front, which indicated that electron attachment occurred in the unburned gas rather than in the flame. However, in low pressure flames (where the spatial resolution is far better) 02- is observed in the flame front (see e.g., Figure 2). 

The development of models for HCSI combustion has been governed by the similarity of flame growth in HCSI engines and premixed turbulent flames. Thin laser-sheets of only 300 pm thickness were used to measure high-resolution cross sections of the temperature and OH radical distribution in flames of a propane-fueled engine. Figure 8.2.3 illustrates the structure where temperature and OH concentration are closely coupled with super equilibrium values for the OH radical close to the flame front [11]. 

Gedanken Flame Experiment. In order to illustrate how the problems caused by the requirements of temporal and spatial resolution and geometric and physical complexity are translated into computational cost, we have chosen to analyze a gedanken flame experiment. Consider a closed tube one meter long which contains a combustible gas mixture. We wish to calculate how the physical properties such as temperature, species densities, and position of the flame front change after the mixture is ignited at one end. The burning gas can be described, we assume, by a chemical kinetics reaction rate scheme which involves some tens of species and hundreds of chemical reactions, some of which are "stiff."... 

Figure 4. An adaptive gridding method, in which the fine resolution is clustered around the flame front, reduces computational time to two days...

A hydrogen/air premixed flame has a burning velocity of the order of 0 5 m s which the linear gas flow must exceed. A typical value would be 1-0 m s . Such a flame would show a slight contraction if burned isothermically. However the burnt gases are actually at a temperature of the order of 2000 K, so there is a sevenfold expansion. Some part of this is taken up by a transverse expansion, but a significant portion produces an acceleration of the burnt gas, with a resultant back pressure across the flame front. The burnt gases pass up the flame with a velocity of the order of 2-0 m s 1 cm height corresponds to 500 fis. Several techniques are available to study flames which have a spatial resolution of 100 fis. The flame photometer in particular can resolve measurements at intervals of 10 /is. 

In this section we consider problems in which there is convective and diffusive transport in one spatial dimension, as well as elementary chemical reaction. The computational solution of such problems requires attention to discretization on a mesh network and solution algorithms. For steady-state situations the computational problem is one of solving a boundary-value problem. In chemically reacting flow problems it is not uncommon to have steep reaction fronts, such as in a flame. In such a case it is important to provide adequate mesh resolution within the front. Adaptive mesh schemes are used to accomplish this objective. 

Diffusion in solids does not ensure the experimentally observed velocity of combustion wave propagation in the systems which are traditionally considered as gasless and burned in the mode of solid flames (gasless solid-state combustion). The phenomenology of indirect interactions, the thermochemistp and dynamics of the gas-phase carriers formation, as well as their participation in the reactants transport are studied in the systems Mo-B and Ta-C. The distributions of the main species in the gas phase of the combustion wave are measured in situ with the use of a dynamic mass-spectrometry (DMS) technique which allows for high temporal and spatial resolution. The detailed chemical pathways of the processes were established. It was shown that the actual mechanism of combustion in the systems under study is neither solid state nor gasless and the reactions are fiilly accomplished in a narrow front. 

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