Flame recombination zone, measurement

One should parenthetically note that the measurement of OH concentration and temperature in the flame recombination zone provides a method of determining the 0 atom concentration as well. The reactions G1 - G3 are usually fast consequently, if one assumes partial equilibrium,... 

The burning velocity, and the temperature and composition profiles in a low temperature, fuel-rich hydrogen—nitrogen—oxygen flame at atmospheric pressure having an unbumt gas composition 2, u = 0.1883, A N2,u 0.7657 and Aqj.u = 0.0460, with = 336 K, were measured by Dixon-Lewis et al. [156] while the burning velocities of a number of flames having compositions not too far from this were also examined by Dixon-Lewis and co-workers [158, 159]. In a number of these flames the main reaction zone extended from approximately 600—1060 K, and the predominantly recombination zone from about 1060—1080 K. The maxi-... 

Species profiles have not been measured directly for dry CO/air or CO/O2 flames in the same way as they have for hydrogen flames. Several investigations, however, have been concerned with the oxidation of carbon monoxide in lean hydrocarbon flames (e.g. refs. 406, 413, 417, 429) or in moist CO flames flames of H2 /CO mixtures in air [167, 406, 414, 418] or O2 [523]. The interest in the oxidation in hydrocarbon flames has arisen since the overall reaction in such flames is a two stage process. In the first rapid stage (the main flame reaction zone) the hydrocarbon is essentially converted to CO and water, with traces of hydrogen also appearing. The second, more extended, stage is devoted to radical recombination and to the slower oxidation of CO, predominantly by reaction (xxiii). 

The interpretation of measured flame profiles by means of the continuity equations may be approached in one of two ways. The direct experimental approach involves the use of the measured profiles to calculate overall fluxes, reaction rates, and hence rate coefficients. Its successful application depends on the ability to measure the relevant profiles, including concentrations of intermediate products. This is not always possible. In addition, the overall fluxes in the early part of the reaction zone may involve large diffusion contributions, and these depend in turn on the slopes of the measured profiles. Thus accuracy may suffer. The lining up on the distance axis of profiles measured by different methods is also a problem, and, in quantitative terms, factor-of-two accuracy is probably about the best that may normally be expected from this approach at the position of maximum rate. Nevertheless, examination of the concentration dependence of reaction rates in flames may still provide useful preliminary information about the nature of the controlling elementary processes [119—121]. Some problems associated with flame profile measurements and their interpretation have been discussed by Dixon-Lewis and Isles [124]. Radical recombination rates in the immediate post-combustion zones of flames are capable of measurement with somewhat h her precision than above. 

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. 

In flames, the bulk of the oxidation occurs in a narrow zone where the rate of reaction is controlled not only by the kinetics but by the flow and by heat transfer. Immediately above this, the concentration of atoms and free radicals is greatly in excess of its equilibrium value. The region of burnt gas, at nearly constant temperature and pressure, is long enough in space and time to enable measurements of recombination rates to be made, and also studies of the reactions of free radicals with various metals added to the flame, and the resultant ionization. In the third chapter. Professor Page describes both the structure of flames and the kinetics of these processes which can be studied in the reaction zone. 

The method of catalytic recombination has been developed for the detection of hydrogen atoms and the measurement of their concentrations in various flames such as H2, CO, C2H2. It is based on the fact that the different catalytic activity of various compounds with respect to surface recombination of atoms and radicals is very specific. The mixed oxide Zn0-Cr203 is such a catalytic compound stimulating preferential recombination of hydrogen atoms. When introduced into the flame zone (as a thin film deposited on the surface of a quartz capillary), this catalyst heats up as a result of the recombination H + H -> H2 [238]. 

---