Diagnostics of Combustion Processes

In the previous chapter we have seen how lasers can be used in a multitude of ways to gain basic information on atomic and molecular systems. Thus, the laser has had a considerable impact on basic research, and its utility within the field of applied spectroscopy is just as great. We shall discuss here some applications of considerable interest. Previously, we have mainly chosen examples of atomic rather than molecular spectroscopy, but in this chapter we shall mainly discuss applied molecular spectroscopy. First we will describe the diagnostics of combustion processes and then discuss atmospheric monitoring by laser techniques. Different aspects oi laser-induced fluorescence in liquids and sofids will be considered with examples from the environmental, industrial and medical fields. We will also describe laser-induced chemical processes and isotope separation with lasers. Finally, spectroscopic aspects of lasers in medicine will be discussed. Appfied aspects of laser spectroscopy have been covered in [10.1, 10.2]. 

A detailed understanding of combustion must start with simple processes such as hydrogen, methane or acetylene combustion in oxygen or air. Normal hquid hydrocarbons are considerably more complex and wood or coal combustion can hardly be attacked at a inolecular level. Below we give some effective chemical reactions leading to a transformation of fuel and oxidant 

Flame Effective reactions Temperature [K] Energy release im 

Reactive molecular fragments or radicals, such as OH, H and O, are very important in combustion. The combustion zone of a stoichiometric CH4/O2 flame contains about 10% OH, and 5% each of H and O. In the second of the two reactions given above the number of radicals is doubled. A fast increase in radical formation frequently leads to explosive combustion. Because of the high reactivity of radicals they cannot be measured by probe (extraction tube) techniques, since wall reactions immediately ehminate them. Thus, laser techniques are particularly valuable for radical monitoring. Pollution formation in flames should also be considered. Nitrogen and sulphur oxides, incompletely burnt hydrocarbons and soot particles form important pollutants. It is of the utmost importance to understand which elementary reactions form and eliminate pollutants. The formation of nitric oxide is reasonably well understood. At temperatures above 2000 K the nitrogen in the air is attacked  

The two reactions constitute the so-called Zeldovich mechanism. NO is then oxidized to the toxic NO2 by the oxygen in the air. NO can be elimated from the post flame gases by the addition of NH3. The following reactions occur [10.5] 

Combustion occurs with a large number of intermediate steps and even simple processes, such as the ones listed in Table 10.1, occur through dozens of coupled elementary reactions. With computer simulations it is possible to describe the interaction between the reactions, and concentration profiles can be calculated. In order to perform the computer calculations it is necessary to know the rate constants for the individual elementary reactions. Comparisons between theory and experiments are best made for a flat, premixed flame, which in its central part can be considered to have only onedimensional (vertical) variation, allowing computer calculations to be performed comparatively easily. The most important reactions are included in the computer description. In Fig. 10.1 experimental and theoretically calculated concentration curves are given for the case of low-pressure ethane/ oxygen combustion. As examples of important elementary processes we give the reactions 

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Ebert V, Schulz C, Voipp H R, Wolfrum J and Monkhouse P 1999 Laser diagnostics of combustion processes from chemical dynamics to technical devices Israel J. Chem. 39 1-24... 

The method is not restricted to neutral molecules but can also be applied to ionic species. This is of importance when LIF is used for diagnostics of combustion processes [194, 195] or plasmas [196]. 

Thermometric measurements are of primary importance in the analysis and the control of combustion processes. As a matter of fact, the chemical routes, established by specific chemical reactions, depend very much on the temperature at which they take place. For this reason, the temperature is one of the most important parameters that dramatically influence the whole combustion efficiency as well as heat release and formation of the pollutants. It is then not surprising that many laser measurements, designed for diagnostic purposes, are meant to obtain an experimental evaluation of the high temperatures developed during the combustion. Various techniques can offer a solution to the problem. The most known are Rayleigh, LIP, SpRS, and CARS. They are compared in the following Table 12.2. 

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. 

The purpose of this paper is to review the use of laser induced fluorescence spectroscopy (LIFS) for studying combustion processes. The study of such processes imposes severe constraints on diagnostic instrumentation. High velocities and temperatures are common, as well as turbulent inhomogeneities, and there is a need to make space and time resolved species concentration and temperature measurements. The development of LIFS has reached the point where it is capable of making significant contributions to experimental combustion studies. 

There are many reasons that one might want to record, assign, and interpret the electronic spectrum of a diatomic molecule. These include qualitative (which molecular species are present) and quantitative (what is the number density of a known quantum state of a known molecule) analysis, detection of trace constituents (wanted, as in analysis of ore samples for a precious metal, or unwanted, as in process diagnostics where specific impurities are known to corrupt an industrial process), detection of atmospheric pollutants, monitoring of transient species to optimize a combustion process by enhancing efficiency or minimizing unwanted byproducts, laboratory determinations of transition frequencies and linestrengths of interstellar molecules, and last but certainly not least, fundamental studies of molecular structure and dynamics. 

The various techniques of Raman scattering that enable laser-based diagnostics of technical combustion processes as well as species identification on the micrometer scale or remote sensing of molecular species and pollutant in the atmosphere. 

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