Laser combustion probes

Figure 1. Schematic energy-level diagrams for the three most widely used spectroscopic laser combustion probes (-------), virtual states (---), real states. Thick...

The statistical nature of the turbulent flame required the analysis of many temperature and density data points from separate pulses for accurate results. Thus, an overall computer system was used to control the various components of the combustion probe apparatus, and to collect and interpret the resultant data in an accurate and timely fashion. This system produced a block of data for each laser shot that included information about the Raman signals, LV readings, and ancillary data such as an identifying shot number and corresponding dye laser pulse energy. Typical current operation permits about twenty experimental run conditions daily, with up to several hundred shots per run. 

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

D. R. Crosley, Laser Probes for Combustion Chemistry, American Chemical Society, ACS Symposium Series, 134, Washington, DC, 1980. 

Schoenung, S. M., and R. K. Hanson. 1981. CO and temperature measurements in a flat flame by laser absorption spectroscopy and probe techniques. Combustion Science Technology 24 227-37. 

Crosley DR. Laser probes for Combustion Chemistry (ACS S5mposium Series 134). Washington, DC American Chemical Society 1980. 

Some molecules in this group (HONO, NC j 0, HONC ) have been extensively studied because the photofragments OH and NO can be probed by tunable lasers. These molecules are important minor constituents in the earth atmosphere and their photochemistry plays a major role in air pollution. Atmospheric pollutants N0X (NO, NO2, NO3) are formed from combustion of fuel and subsequent chemical reactions in the atmosphere. Photolysis of alkyl oxides produces NO and NO2 that can be probed by LIF the internal energy distribution provides an important clue to the mechanism of photodissociation. 

Laser-induced fluorescence (LIF) depends on the absorption of a photon to a real molecular state, and is therefore a much more sensitive technique, capable of detection of sub-part-per-billion concentrations. Thus, this is the most suitable for measurement of those minor species which are the transient intermediates in the reaction network. Here a tunable laser is required, as well as an electronic absorption system falling in an appropriate wavelength region serendipitously, many of the important transient species have band systems which are suitably located for application of LIF probing. The ability to sensitively detect transitions originating from electronically as well as vibrationally excited levels of a number of molecules offers the possibility of inquiring into the participation of non-equilibrium chemistry in combustion processes. 

The second is concerned with the need to have a complete and sensible chemical mechanism, valid over a wide range of temperature. Even a relatively simple combustion system will involve dozens of reactions, so that a well established reaction rate data base is essential. It is equivalently essential that the results be verified by comparison with detailed experimental data--such as that provided by laser probes. For example, in a study of the ozone decomposition flame (20). it was found that certain alternative but wrong choices of key input parameters were not discernible if flame speed were used as the sole predicted result for verification however, these choices did produce considerable differences in the profiles of the transient oxygen atom concentration and the temperature. 

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

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