Diffusion flame technique

Singhal J.S. T ien, Flammability Study of Polymer Fuels Using Opposed-Jet Diffusion Flame Technique, Rept No SQUID-TR-CWRU-3-PU, Contract N00014-67-A-0226-0005, Purdue Univ, Lafayette (1975)... 

The chemiluminescent emission spectrum of GeCl2 was obtained by burning GeCl4 in potassium vapor using a diffusion flame technique 11 The spectrum consisted of a series of closely spaced diffuse bands in the region 4900—4100 A with an underlying continuum. The bands resemble those of SnCl2. 

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... 

We have omitted from Table XII.5 the large body of the literature on the reactions of the alkali metal atoms with halogens and alkali halides obtained by the diffusion flame technique of Polanyi. The reason is that the data from these reactions cannot be used to obtain absolute rate constants or activation energies directly, but instead, assumptions must be introduced concerning the kinetic cross reactions and diffusion constants. Reed and Rabi no witch have given an excellent analysis of some of the more troublesome features of the technique. It is of interest, however,... 

This further modification of the diffusion flame technique (see Fig. 5) was used by Polanyi and co-workers [31, 32] in order to prove the formation of free radicals... 

An important development of iiie diffusion flame technique was introduced [33] in 1964 in which a collecting system for the removal of the alkali metal halide products was incorporated. The halide products are the result of the reaction of two separate organic halides with the chosen alkali metal and one of the organic halides is labelled with chlorine-36. This technique, which is of considerable potential, has received relatively little attention [34—37]. It has been extended to the case of two different halides that are not labelled, using a conductimetric analysis of the alkali metal halide mbctures [36]. 

In this method [21], the evaluation of the rate coefficients does not involve the spatial distribution and the rate of diffusion of the alkali metal as in the diffusion flame technique, although the main experimental features are basically similar to that method. Thus... 

Schay [82] has reported on the reactions of both sodium and potassium with the hydrogen halides using the nozzle flame method. The rate coefficients increased for both metals in the order from chloride to bromide to iodide. A similar result was obtained [83] using the diffusion flame technique. The most accurate measurements [84] obtained by the diffusion flame apparatus give rate coefficients for reaction with sodium at 511°K of 4.1 X 10 (HCl) and 3.4 x 10 (DCl) cm mole sec. The mechanism of the reaction has been discussed [53] in terms of the energy surfaces involved and of the zero point energy contribution to the activation energy. 

The chemically inert character of sulfur hexafluoride is responsible for the almost complete lack of exchange of fluorine atoms between SFe and HF (249). It does react with hot alkali metals, however, and a study has been made of the rate of reaction of Na atoms with SF6 gas using the sodium diffusion flame technique. The rate constant at 247° is 2.23 X 10-1 cm mole-1 sec-1 and the energy of activation for the reaction SF6 + Na — SF6 + NaF, is about 37 keal. A film of sodium on a glass wall does not react with SF at room temperature. The reaction sets in at about 200° (57). The fluorides, SF , SF4, and S2F2, have no effect upon the viscosity of liquid sulfur in the range 180-195° (93). Sulfur hexafluoride forms a solid hydrate which has a crystal constant of 17.21 A. It decomposes just above 0° (285). 

Fluoro-olefins have proved popular substrates for kinetic studies during the period of this Report. A study of the relative rates of reaction of active nitrogen, using diffusion flame techniques, has suggested the following order of reactivity ... 

The simultaneous analysis of orthophosphate, glycerol phosphates, and inositol phosphates has been achieved by spectrophotometric analysis of the molybdovanadate complexes. Also, a sensitive and selective chemiluminescent molecular emission method for the estimation of phosphorus and sulphur is described, which is based on passing solutions into a cool, reducing, nitrogen-hydrogen diffusion flame. For organic compounds it was usually necessary to prepare test solutions by an oxygen-flask combustion technique. 

A number of conclusions can be drawn from this first detailed analysis of NO production in methane-air diffusion flames by techniques of RRA. It is found that all production mechanisms have rates dependent on the peak flame temperature T°. The production rates for the thermal and nitrous oxide mechanisms increase sufficiently rapidly with T° that they are calculated by AEA after the peak flame temperature, and superequilibrium radical mole fractions are obtained from the RRA analysis of the primary flame structure. The flame temperature depends on the temperature of the fuel and oxidizer streams and... 

A surprising addition has recently been made to the list of elements which may be usefully determined by vapour generation techniques, namely cadmium.5 Sodium tetraethylborate was used to produce a volatile cadmium species, with citrate being used to mask interference from nickel and copper. Using an argon-diluted hydrogen diffusion flame as atomizer, the detection limit by AFS was 20 ng l-1. 

Because of these difficulties with moment methods for reacting flows, we shall not present them here. A number of reviews are available [22], [25], [27], [32]. There are classes of turbulent combustion problems for which moment methods are reasonably well justified [40]. Since the computational difficulties in use of moment methods tend to be less severe than those for many other techniques (for example, techniques involving evolution equations for probability-density functions), they currently are being applied to turbulent combustion in relatively complex geometrical configurations [22], [31], [32]. Many of the aspects of moment methods play important roles in other approaches, notably in those for turbulent diffusion flames (Section 10.2). We shall develop those aspects later, as they are needed. 

The most important restriction on the method that has been presented is chemical equilibrium, and the second most important is equal dilfusivities. How critical each of these is in diffusion flames is a topic to which research recently has been devoted. In sufficiently fuel-rich portions of hydrocarbon-air diffusion flames, the chemical-equilibrium approximation is not good (see Figure 3.8 and the discussion in Section 3.4.1), but empirical approaches apparently still can be employed to relate nonequilibrium concentrations uniquely to Z with reasonable accuracy for main species [77]. In addition, the extent to which the burning locally proceeds to CO or to CO2 may vary with the fuel, local stoichiometry, and characteristic flow times methods to account for this are being developed [78], [79]. The theoretical methods that have been applied in studying the validity of the two major approximations are expansions for Lewis numbers near unity [80] and expansions in reaction-rate parameters for near-equilibrium flows [27], [28], [81]. The results of the research tend to support a rather broad range of applicability for the predictions obtained by the approach that has been described [27]. However, continuing rsearch is needed on the limitations of the technique. ... 

Measurements have been made of the combustion characteristics of an air blast kerosene spray flame and of droplet sizes within the spray boundary of isothermal sprays. Specific techniques were used to measure velocity, temperature, concentration, and droplet size. Velocities measured by laser anemometer in spray flames in some areas are 400% higher than those in isothermal sprays. Temperature profiles are similar to those of gaseous diffusion flames. Gas analyses indicate the formation of intermediate reactants, e.g., CO and Hg, in the cracking process. Rosin-Rammler mean size and size distribution of droplets in isothermal sprays are related to atomizer efficiency and subsequent secondary atomizer/vaporization effects. 

The diffusion flames of methanol [27], ethanol [27], n- and iso-propanol [27], and the four isomeric butanols [28] have been investigated using a quartz probe technique. The alcohol flames were burned on a pyrex wool wick and samples for analysis were taken from various positions in the flame using a quartz microprobe. Thermocouple measurements showed that the temperature varied from 200 °C at the wick to around 1400 °C at the tip and edges of the flame. 

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