Combustion intermediate species techniques

Computational and experimental methods clearly benefit from a symbiotic relationship in combustion studies. Theoretical calculations can propose important pathways to yield empirically observed intermediates by providing reaction energies and rate coefficients of elementary reactions, thereby guiding experiments. Moreover, theoretical calculations can potentially fill some gaps caused by limitations in experimental approaches the vast majority of analytical techniques fail to distinguish between structural isomers and to identify short-lived intermediate species, both of which are important objectives in delineating overall combustion behavior. Finally, modeling can identify species to look for experimentally. 

Combustion diagnostics today relies on a number of analytical and optical techniques, depending on the species under investigation. For the detection of molecular components with more than two or three atoms, mass spectrometry is usually applied with different ionization techniques. For smaller reactive intermediates, optical techniques are the methods of choice because of their non-invasive natme. One of the most widespread experimental approaches to combustion chemistry is through the measurement of the spatial concentration profiles of the intermediate products of combustion the results from such measurements are then compared with model calculations. 

Developments in computer techniques making it possible to solve complicated fluid motions in a combustion environment that are affected by diffusion and involve complicated chemistry (large numbers of elementary reactions, which individually are not "complex" but quite simple, i.e., most of them involve two reacting species, sometimes three, and the formation or breaking of just one bond), and with a large number of transient intermediates formed in the course of fuel oxidation and pollutant formation. 

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. 

Experiments using the technique of laser-induced fluorescence (LIF) in flames have provided ample demonstration of its selectivity and sensitivity, and hence of its applicability as a probe for the reactive intermediates present in combustion systems. The relationship between the measured fluorescence intensity and the concentration of the molecule probed, however, must take into account the collisional quenching of the electronically excited state pumped by the laser. Because the flame contains a mixture of species, each with different quenching cross sections, it may be difficult to estimate the total quenching rate even if many of these cross sections are known. 

These experiments demonstrate that tunable diode laser absorption spectroscopy is well suited for in situ measurements of species concentrations in combustion flows, when a line-of-sight technique is appropriate, and for accurate measurements of spectroscopic parameters needed to characterize high-temperature absorption lines. The technique is sensitive, species specific and applicable to a large number of important combustion species including reactive intermediates, and hence it should prove to be a useful tool in future studies of combustion chemistry. The potential of tunable laser absorption spectroscopy in particleladen flows should also be noted (12), in that modulation of the laser wavelength on and off an absorption line allows simple discrimination against continuum extinction by particles. 

Although a standard deviation of about 10% is now quite usual for rate constant measurements from fast-flow discharge and pulsed photolysis studies, it is still found that the same reaction studied in different laboratories by the same technique may give results of similar precision, but which differ by far more than would be expected on the basis of that precision. Potential sources of such discrepancies are many but, with experience, likely errors can sometimes be identified in particular cases. For example, reactions between short lived radical intermediates are common in combustion processes. The measurement of the rate constants of such reactions, where the reaction is second order with respect to the transient species, pose particular but well recognized difficulties stemming from the need to determine the absolute concentration of the reacting radical (Chapter 1). This is difficult to achieve and has been a common source of error in this type of determination as exemplified in the series of studies on the rate of the reaction CHO -I- CHO — CH2O -H CO. 

There is a voluminous hterature concerned with the study of flame spectra, but the application of spectroscopy to the study of flame kinetics followed the introduction of flame photometry as a general analytical tool. The chief interest before this was in the spectra of the flames, which could serve to demonstrate the presence of intermediates in the combustion process. These were in general detected by the emission spectra of excited species and therefore were not necessarily indicative of the concentrations of ground state species. The difficulties of constructing burners which were sufficiently large and uniform to allow the study of absorption spectra prohibited a measurement of the species in their ground states, until the development of the multiple pass technique. ... 

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