Flame chemistry

Lamprecht, A., Atakan, B., and Kohse-Hoinghaus, K., Fuel-rich propene and acetylene flames A comparison of their flame chemistries. Combust. Flame, 122, 483, 2000. 

Cool, T.A. et al., Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry,. Chem. Phys., 119,8356,2003. 

Cool, T.A. et al., Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source. Rev. Sci. lustrum., 76,094102,2005. 

Figure 7.2.5 provides a visualization of a localized extinction event in a turbulent jet flame, using a temporal sequence of OH planar LIF measurements. The OH-LIF measurements, combined with particle image velocimetry (PIV) reveal that a distinct vortex within the turbulent flow distorts and consequently breaks the OH front. These localized extinction events occur intermittently as the strength of the coupling between the turbulent flow and the flame chemistry fluctuates. The characteristics of the turbulent flame can be significantly altered as the frequency of these events increases. 

The ability to resolve the dissipation structures allows a more detailed understanding of the interactions between turbulent flows and flame chemistry. This information on spectra, length scales, and the structure of small-scale turbulence in flames is also relevant to computational combustion models. For example, information on the locally measured values of the Batchelor scale and the dissipation-layer thickness can be used to design grids for large-eddy simulation (LES) or evaluate the relative resolution of LES resulfs. There is also the potential to use high-resolution dissipation measurements to evaluate subgrid-scale models for LES. 

Flame retardants currently in use which operate by inhibiting vapor phase flame chemistry may be far from optimum. Those flame retardant systems which evolve hydrogen chloride, and perhaps even those which evolve hydrogen bromide, may be acting by little more than a physical effect (1). Some of our own work on tris(dichloroisopropyl) phosphate in polyurethane foams also suggests a physical mode of action (2). 

The cool-flame phenomenon [15] is generally a result of the type of experiment performed to determine the explosion limits and the negative temperature coefficient feature of the explosion limits. The chemical mechanisms used to explain these phenomena are now usually referred to as cool-flame chemistry. 

TABLE 8.4 Major Flame Chemistry Reactions of Sulfur under Rich Conditions... 

Using laser fluorescence measurements on fuel-rich H2/02/N2 flames seeded with H2S, Muller et al. [43] determined the concentrations of SH, S2, SO, S02, and OH in the post-flame gases. From their results and an evaluation of rate constants, they postulated that the flame chemistry of sulfur under rich conditions could be described by the eight fast bimolecular reactions and the two three-body recombination reactions given in Table 8.4. 

The starting point for any study of this kind is a set of elementary reactions and their associated reaction-rate parameters. Although literally hundreds of elementary steps are potentially relevant, calculations with full detailed mechanisms show that most of them are unimportant. A starting chemical-kinetic mechanism needs to be selected that includes all of the important elementary steps. Since the nitrogen chemistry is a small perturbation on the chemistry of the main flame, it is convenient to separate the flame chemistry from the nitrogen chemistry in the starting mechanism. The starting chemistry, which... 

In combustion experiments, there are two key considerations first, generating a flame and second, detecting the species of interest. Gaseous flows in a flame can be classified as laminar (streamlined layers) or turbulent. While these flames can be analyzed directly, it is less confounding to study flame chemistry through controlled generation of reactive species in one of a wide variety of experimental apparata. 

Many of these vapours will break down spontaneously to atoms in the flame. Others, particularly diatomic species such as metal monoxides (e g. alkaline earth and rare earth oxides), are more refractory. Monohydroxides which can form in the flame can also give problems. The high temperature and enthalpy of the flame aid dissociation thermodynamically, as does a reducing environment. The role of flame chemistry is also important. Atoms, both ground state and excited, may be produced by radical reactions in the primary reaction zone. If we take the simplest flame (a hydrogen-oxygen flame), some possible reactions are the following ... 

The mode of operation of homogeneous or heterogeneously initiated reactions cannot be excluded in all processes under discussion. The reaction temperatures are typically between 623 K and 873 K, well within the range of pyrolysis temperatures [12]. There is ample evidence of complex and selective homogeneous reactions of unfunctionalized hydrocarbon molecules in flame chemistry [13-15]. 

Laser-Induced Fluorescence A Powerful Tool for the Study of Flame Chemistry... 

In the following we present an application of laser induced fluorescence to a study of the chemistry of sulfur in rich hydrogen/oxygen/nitrogen (H2/O2/N2) flames and demonstrate a simple rationale for taking quench effects into account. Fluorescence measurements for S2, SH, S02, SO, and OH along with measurements of flame temperature and H-atom (in sulfur free flames) have been employed to develop a kinetic model for the highly coupled flame chemistry of sulfur. The kinetic aspects of the study already have been presented in considerable detail (6). 

Experiments with lithium also show a change in behavior. This is not as directly obvious due to the flame distribution of lithium between Li and LiOH which occurs via the controlling reaction Li + H20 = LiOH + H and which relates Li to H-atom concentrations under normal non-radiated conditions. However, it is apparent that a similar behavior to sodium is exhibited but is disguised to a large extent by the normal lithium flame chemistry. 

These instantaneous temperature and velocity values can be related to values of the average fluctuating mass flux

for our experimental conditions, utilizing assumptions of the ideal gas law and fast flame chemistry. Here p and u are fluctuation values of density and velocity, respectively, Knowledge of flame properties such as p u > provides key data needed for developing improved combustion models. 

The technical synthesis of graphite, diamond and a variety of other forms of sp2 carbons (Fig. 3) is described in a review [39] and is not covered here. As the unintended formation of carbon in deactivation processes and the modification of primary carbon surfaces during chemical treatment (in catalytic service and during oxidative reactivation) and their chemical properties arc frequent problems encountered in catalytic carbon chemistry, it seems appropriate to discuss some general mechanistic ideas which mostly stem from the analysis of homogeneous combustion processes (flame chemistry) and from controlled-atmosphcre electron microscopy. 

In used lubricating oils the wear metals are normally present in an extremely finely divided form and thus flame chemistry, similar to that for metals in true solution, is normally found. However, in some oils, such as those from heavy duty equipment or where excessive dust has been accumulated, larger particles may be present. In these cases more accurate results may be obtained by employing an ashing procedure to ensure proper dissolution of the metals (see Section II.D). 

Ballschmiter K, Braunmiller I, Niemczyk R, Swerev M (1988), Chemosphere 17 995-1005.. .Reaction pathways for the formation of polychlorodibenzodioxins (PCDD) and -furans (PCDF) in combustion processes II Chlorobenzenes and chlorophenols as precursors in the formation of polychlorodibenzodioxins and -dibenzofurans in flame chemistry"... 

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