Concentration in flames

Unfortunately, OH and O concentrations in flames are determined by detailed chemical kinetics and cannot be accurately predicted from simple equilibrium at the local temperature and stoichiometry. This is particularly true when active soot oxidation is occurring and the local temperature is decreasing with flame residence time [59], As a consequence, most attempts to model soot oxidation in flames have by necessity used a relation based on oxidation by 02 and then applied a correction factor to augment the rate to approximate the effect of oxidation by radicals. The two most commonly applied rate equations for soot oxidation by 02 are those developed by Lee el al. [61] and Nagle and Strickland-Constable [62],... 

A plasma is an appreciably ionized gas(about 1% or more) having no net charge, and may have a wide range of densities. Plasmas are of particular interest because of the possibility of initiating nuclear fusion in them, but they also appear in such phenomena as a neon sign, a lightning stroke, the ionosphere about the earth, shock waves, and the compressed layer of hot gas about an object entering the earth s atmosphere They appear in flames and detonation waves. It seems well established that free radicals and ions are present at well over equilibrium concentrations in flames (Ref 1). The ions appear to be produced not by thermal processes but by chemical factors which cause abnormal electronic excitation... 

Later Kaskan7,8 also determined hydroxyl concentrations in flames by the linear absorption method. 

Two very distinguishing features of flames are the emission of visible radiation and the presence of an abnormal number of ions in the reaction zone (Cl, Gl). Both of these features appear to be kinetic by-products, which are completely unnecessary for flame propagation.5 The radical CH is responsible for much radiation and now also appears to be the precursor of many of the ions observed in flames (C2). Enhancement of CH or other radical or ion concentration in flames could result in any number of applications if an understanding of their formation made such control possible. 

The results in Figure 1 imply that the abundance of chemiions initially present in the exhaust is 109/cm3. As noted by Yu and Turco [75], this number is consistent with the measurement of charge concentrations in flames, and with known ion-ion recombination coefficients, when the time scale of the exhaust emission into the atmosphere is taken into account. More recently, direct sampling of massive ion clusters (greater than 9500 amu) in fresh jet exhaust confirms that the chemiion concentration near the exit plane is of the order of 109/cm3 [90-92]. 

Concentration Measurements. The use of LIFS to measure atomic species concentrations in flames has been demonstrated repeatedly in analytical applications and the field is well reviewed by Winefordner and Elser (23) and Winefordner (24). 

For atomic species the saturation approach appears to be most fruitful, although care must be taken to avoid chemical effects. Daily and Chan (7) have measured sodium concentrations in flames using saturated LIFS with a pulsed laser source and compared the results with absorption measurements. Smith, et al. 

PCAH have been observed in a flame using laser induced fluorescence spectroscopy by injecting individual species into the post-reaction zone. While the spectra are broadened by the elevated temperature, the spectra are qualitatively similar to low temperature (100 C) spectra and are indicative of the particular species injected. Thus, the injection procedure appears to be a feasible method of determining PCAH spectra at flame temperatures. These spectra will be used as a data base to determine individual PCAH concentrations in flames from their LIF spectra. 

The determination of carbon dioxide concentrations in flames by Raman spectroscopy has been described by Blint et al. (1979). The mole fraction of CO2 is obtained by calculating the ratio of the intensity of the Raman lines to the intensity of the N2 band. It is pointed out that the calculation for each spectrum must include the respective partition function. The theoretical fitting of computer processed laser Raman spectra of methane and propane air flames has been discussed by Stephenson and Blint (1979). 

Unstable species fall into two categories free radicals, including reactive atoms, and ions. The latter are present in such small concentrations that their measurement is not normally required. Sampling coupled with mass spectrometry may be used for free radicals as for molecules. Absorption and ESR spectrometry have also found limited application, although they are potentially very useful. The technique which has been most widely used depends on the addition to the flame of an indicator which reacts at a known rate or to a known extent with one of the radicals or atoms present. The rate of this reaction is followed either by sampling or spectroscopically. The details of this type of technique depend to a large extent on the nature of the species concerned, and the methods which have been developed naturally involve those radicals, such as H, OH and O, which tend to be present in high concentration in flames . ... 

Figure 1. Estimated median ion masses based on Homann and coworkers heavy ion masses (see text) in sooting flames. Solid line used to convert Langmuir probe currents to ion concentrations in flames with 0 >2.5 dashed line is geometric mean of solid line and mass 39 and was used in 0 = 2.5 flame, o, charged particle mass (it) and density = 1.5 g cm . ...

The approach used assumes that H2O rapidly assumes an equilibrium concentration in flames. With H2O fixed in this way, one can explicitly verify whether or not Reactions 1 and 2 are equilibrated by computing the ratios ... 

Our initial experiments have centered on the C2 radical, which is known to exist in high concentrations in flame and other combustion environments. The presence of a low-lying excited electronic state (a H ) within 610 cm of the ground state... 

The measurement of the positive ion concentration in flames is most effectively carried out by diffusion-limited negatively biased electrostatic probes, frequently known as Langmuir probes. ... 

Fig. 7. Relative decrease of H (solid lines) and OH (dashed lines) maximal concentration in flames doped with 0.04% TMP with different D versus equivalence ratio modeling data using the updated mechanism...

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