Flame systems

The total-consumption burner can be adjusted to produce a fuel-rich or oxidant-rich environment very easily since danger of an explosion is very low. All the nebulized sample enters the flame however, droplet size is quite variable and some droplets pass through the flame without complete evaporation and dissociation into atoms. The flame, because of its turbulence and high velocity, will entrain air surrounding the flame, which may react with sample elements and other constituents in the flame. Some use of sheathed turbulent flow burners has been made. The flame is surrounded by a sheath of inert gas to prevent entrainment of air into the flame. Such flames are said to provide greater flame stability and higher flame temperatures than unsheathed flames. 

FIGURE 10-14. Perkin-Elmer premixed laminar flow burner. [Courtesy the Perkin-Elmer 

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Flame Throwers and Projectors. One advance ia flame throwers siace World War II was a mechanized flame thrower kit for a variety of armored vehicles other than the main battle tank. The multishot, lightweight, shoulder-fired, four-tube flame system capable of firing one to four flame rounds semiautomaticaHy is replacing the portable flame thrower. Indeed the mechanized flame thrower is expected to become obsolete because of the family of large-cahber flame rounds. 

A minimum sample volume of between 0.5 and 1.0 mL is needed to give a reliable reading by aspiration into a flame system. 

Macek [27] examined the flammability limits for premixed fuel-air systems and small diffusion flames under natural convection conditions, and computed the equilibrium flame temperature for these flame systems. Data were considered for the alkanes and alcohols at their measured premixed lower flammability limits, and at their measured... 

From the above definition it is quite evident that the sensitivity takes no cognizance of the noise-level of the base-line, therefore, it is more or less of no use as a definite guide to the least quantity of an element which may be estimated. However, the sensitivity of a 1% absorption-is a pure theoretical number only that would undergo a change solely depending on the efficiency of the lamp (hollow-cathode-lamp), atomizer, flame-system employed, monochromator (prism, grating used), and finally the photomultiplier used. 

The recombination zone falls into the burned gas or post-flame zone. Although recombination reactions are very exothermic, the radicals recombining have such low concentrations that the temperature profile does not reflect this phase of the overall flame system. Specific descriptions of hydrocarbon-air flames are shown later in this chapter. 

FIGURE 4.14 Velocity and temperature variations through non-one-dimensional flame systems. 

In order to calculate the thermal NO formation rate from the preceding expression, it is necessary to know the concentrations of 02, N2, O, and OH. But the characteristic time for the forward reaction (8.49) always exceeds the characteristic times for the reaction systems that make up the processes in fuel-oxidizer flame systems thus, it would appear possible to decouple the thermal NO process from the flame process. Using such an assumption, the NO formation can be calculated from Eq. (8.52) using local equilibrium values of temperature and concentrations of 02, N2, O, and OH. 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

If indeed S02 and S03 are effective in reducing the superequilibrium concentration of radicals in flames, sulfur compounds must play a role in NO formation from atmospheric nitrogen in flame systems. Since S02 and S03 form no matter what type of sulfur compound is added to combustion systems, these species should reduce the oxygen atom concentration and hence should inhibit NO formation. Wendt and Ekmann [46] have reported flame data that appear to substantiate this conclusion. 

The limit of detection is a useful figure which takes into account the stability of the total instrumental system. It may vary from instrument to instrument and even from day to day as, for example, mains-borne noise varies. Thus, for atomic absorption techniques, spectroscopists often also talk about the characteristic concentration (often erroneously referred to as the sensitivity—erroneously as it is the reciprocal of the sensitivity) for 1% absorption, i.e. that concentration of the element which gives rise to 0.0044 absorbance nnits. This can easily be read off the calibration curve. The characteristic concentration is dependent on such factors as the atomization efficiency and flame system, and is independent of noise. Both this figure and the limit of detection give different, but useful, information about instrumental performance. 

The methane flame may be used as an example of the present state of knowledge of a flame system. In the reaction zone of this flame, the attack of hydroxyl radical on methane is followed rapidly by the further decomposition of the methyl radical into carbon monoxide and active species. CO is oxidized slowly in an equilibration zone by the reaction... 

Not illustrated is the use of multiphoton excitation of fluorescence (12.15). thus far demonstrated in flame systems only for excitation of atoms. It affords the means to excite otherwise inaccessible states and offers other potential advantages in spatial resolution and for optically thick flames, in spite of inherently low signal levels. 

Dirty Flames. At this point one could well ask so what happens in real combustors which are turbulent, soot and particle laden and are highly luminous By the end of this morning s session you should be convinced that CARS can be applied to these systems. I don t want to steal all of Alan Eckbreth s slides so I will show only two more. Figure 13 shows the BOXCARS spectrum of N- with a computer fit to a temperature of 2000°K in a laminar sooting propane diffusion flame (12). Figure 14 shows the vertical temperature profile for this same flame system. It should be pointed out that care must be taken under these conditions to account for the laser interaction with carbon in the flame which can generate laser induced Swan Band emission from C2-... 

Historical Development. Saturation spectroscopy in flame systems dates back only to about 1972. The important early work... 

Coherent Antistokes Roman Spectroscopy has been used to make both concentration and terrperature measurements in flame systems. The accuracy and detection limits vary depending upon the type of... 

Of course, this is unacceptable. Ideally such a simple calculation should take about 100 seconds (See Figure 3). What are needed are numerical algorithms which have the resolution in time and space only where it is required. Furthermore, these algorithms should be optimized to take advantage of what is known about the physics and chemistry of the problem. This will be discussed further below where it is shown how the application of various numerical algorithms can be used to reduce this flame system to a tractable computational problem. 

Flame atomisation systems have some disadvantages, however, which limit their potential and convenience in use. These drawbacks have led many workers to devise techniques for the atomisation of samples for analysis that are not based entirely on nebuliser/flame systems. Some of the possible drawbacks of flames for analytical work are ... 

Recommended conditions for flame and approximate values for ETA (graphite rod, etc.) atomizers are given in Table 2 for a number of elements important with regard to air pollution studies. Conditions are included in the table for the flame system used when hydrides of arsenic, antimony and selenium are generated and passed through the flame. Burrel [16] discusses generation of metal hydrides and cold-vapor mercury evolution techniques in great detail. 

Tantalum-carbon, tantalum-boron, and niobium-boron mixtures may be classified as so-called solid flame systems, where T c is lower than the melting points of both reactants and products. Consequently, the sizes of reactant particles are not expected to change in the combustion wave. The dependence of U on the average metal particle size for these systems is presented in Fig. 47. 

Fig. 47. Effect of metal particle size on combustion velocity in solid flame systems.

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