Temperatures in flames

Therefore, the fraction of atoms excited critically depends on the temperature of the flame thereby emphasizing the vital importance of controlling the temperature in Flame Emission Spectroscopy (FES). 

Free radicals, however, are often important intermediate products in chemical reactions. Very reactive free radicals can be produced in electric discharges through gases, by the action of radiation, or at high temperatures in flames. All these methods produce a situation of high energy, sufficient to break a covalent bond. Two radicals are then the temporary results... 

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... 

Sato, A., Hashiba, K., Hasatani, M., Sugiyama, S., and Kimura, J. "A Correctional Calculation Method for Thermocouple Measurements of Temperatures in Flames." Combustion and Flame 24 (1975) 35-41. 

A comparison of the combustion temperatures in flame- and in FLOX -mode should clearly show the reduction of the temperature peaks. Figure 23.13 shows, in the upper diagram, the temperature field of a 400 kW burner in a furnace at 1200°C fired with cold air in flame mode. In the second diagram, the same burner is operated with air at 600°C, but the fuel input is reduced to 245 kW to take into account the preheating and thereby keeping the net heat input constant. The temperature field of a 200 kW FLOX burner using preheated air at 950°C is represented in the bottom diagram. 

At high" temperatures, in flames for example, it is assumed that the radicals Up and p decompose very easily and consequently that the 6 radicals are determining. They are therefore the only ones to be involved in the termination reactions and in the bimolecular reactions. 

Since flame temperatures are very important to the excitation process (refer to Chapter 2), it is important to realize that temperatures in flames are not uniform throughout thus atomic excitation will not be uniform throughout the flame. Figure 9-2 is a typical temperature distribution of the Meeker burner operating with a stoichiometric ratio of natural gas and air. 

Since the intensity of spontaneous Raman lines is proportional to the density N(vi, Ji) of molecules in the initial state (u/, 7/), Raman spectroscopy can provide information on the population distribution A(u/, 7/), its local variation, and on concentrations of molecular constituents in samples. This allows one, for instance, to probe the temperature in flames or hot gases from the rotational Raman spectra [372, 373, 377, 378] and to detect deviations from thermal equilibrium. 

In contrast, absorption spectroscopy allows for more direct and more accurate, absolute concentration and temperature measurements. An increasingly popular approach for the detection of (minor) species in flames is cavity ring-down spectroscopy, CRDS. It combines the advantage of common absorption techniques, i.e. the direct determination of number density, with an effective absorption path of up to a few kilometres. Therefore, the detection of species of very low concentrations is possible. For a description of the principles of CRDS see Section 7.2. A typical experimental setup for the quantitative measurement of species concentration and temperature in flames is shown in Figure 29.7. 

Infrared spectroscopy continues to be a valuable tool for measuring species concentrations and temperatures in flames and combustion gases, especially in field-based studies. The advent of compact, fibre-coupled, tunable diode lasers operating at room temperatures is expanding the use of vibrational spectroscopy beyond the laboratory, and providing a useful complement to broadband methods currently in place. 

On account of their large endothermicity, dissociation reactions of H2,02, or H2O can play a part only in ignition processes at very high temperature in flame propagation they are completely unimportant. Relative collision efficiencies are discussed in Section 1.2. 

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