Free radicals, determination flames

Chemical Factors. Because knock is caused by chemical reactions in the engine, it is reasonable to assume that chemical stmcture plays an important role in determining the resistance of a particular compound to knock. Reactions that produce knock are generally free-radical chain-type reactions which are different from those that occur in the body of the flame the former occur at lower temperatures and are called cool flame reactions. 

To determine the need for recombination or dissociation processes in a flame, one must first consider the mole number of the final equilibrium composition. A constrained enthalpy and pressure equilibrium calculation will determine the adiabatic flame temperature and the species distribution at that temperature. If the mean molecular weight (IT = Ylk WkXk) is larger than that of the reactants, then recombination must occur. If the W is smaller for the products, then dissociation must take place. Note that the mole number (moles per mass of gas) is the reciprocal of the mean molecular weight. At the adiabatic flame conditions there will be the expected stable products as well as a distribution of other species, including free radicals. 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

An extremely useful intermediate approach, which is capable of handling the whole flame reaction zone, is that employing the quasisteady state (q.s.s.) assumption, referred to in Sect. 5.4.2. In this case a radical pool consisting only of H, OH and O is considered. The growth of the overall pool is now effectively determined by reaction (ii), and its decay by the recombination steps. Its sub-division into the separate components is carried out in rich flames by way of the q.s.s. assumptions on OH and O. In more precise terms, the overall mass flux of free radicals... 

Radicals.—The measurement of emission intensities from electronically excited small free radicals has become an important means of determining radical concentrations in hostile environments such as flames. When combined with laser excitation, the technique is very powerful, offering temporal, spectral, and spatial resolution. Just has reviewed laser techniques for the measurement of both radical concentrations and local temperatures in flames, and has demonstrated the use of laser-induced saturated fluorescence to measure the concentrations of CH and OH radicals in low-pressure acetylene-oxygen flames. Vanderhoff ei al. used a novel Kr " and Ar laser intracavity technique to... 

Spectroscopic methods are required for free radical intermediates. Laser induced fluorescence of hydroxyl radicals has been used successfully to determine elementary rate parameters associated with the isomerization reaction RO2 QOOH [113]. Laser perturbation of hydroxyl radical concentrations in stabilized cool-flames has been used to obtain global kinetic data for chain-branching rates at temperatures of importance to the low-temperature region [79]. These methods appear to be most suited at present to combustion studies in flow systems. There are also several studies of the relative intensity from OH radical fluorescence during oscillatory cool-flames [58,114]. 

We are now equipped to determine what is called the adiabatic reaction temperature. This is the temperature obtained inside the process when (1) the reaction is carried out under adiabatic conditions, that is, there is no heat interchange between the container in which the reaction is taking place and the surroundings and (2) when there are no other effects present, such as electrical effects, work, ionization, free radical formation, and so on. In calculations of flame temperatures for combustion reactions, the adiabatic reaction temperature assumes complete combustion. Equilibrium considerations may dictate less than complete combustion for an actual case. For example, the adiabatic flame temperature for the combustion of CH4 with theoretical air has been calculated to be 2010 C allowing for incomplete combustion, it would be 1920 C. The actual temperature when measured is 1885 C. 

Certain continua have been observed in flames which are due to the direct recombination of radicals and atoms. The best known of these are the alkali metal-hydroxyl recombination continuum, which extends throughout the visible range. This continuum is obscured by the very intense resonance lines of Na and Li. It accounts for the majority of the visible radiation from a K laden flame, where the first and second resonance doublets are at the limits of visibility. James and Sugden showed that the integrated intensity of this continuum was proportional to both the [OH], derived from their measurements of [H] atoms, and to the concentration of free alkali metal, and that the intensity could therefore be used as a relative measure of [OH] concentration. The natural continumn arising from the reaction H + OH -> HjO -1- hv has also been used, as has the H -b Cl continuiun. The direct measurement of the intensity of the OH (306 nm) band can be used to determine the OH profile in a flame however Hollander has pointed out that there is considerable overexcitation and the method is unreliable near the reaction zone. 

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