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Show The quality of predictions from a combustion chemistry model is a strong function of the accuracy of the reaction rate coefficients for the elementary reaction steps and the thennochemical properties of the reactive species. The mechanism may be quite complex: a current, widely used methane/air model containing reactions involving only hydrogen, carbon, and oxygen5 (i.e., excluding NO formation) contains 30 species and 177 reactions, plus the inverse reactions. Although most of these steps must be included in the model, sensitivity analysis shows that a much smaller number dominate the prediction of anyone flame property such as flame speed or ignition time. The natural gas combustion chemistry model known as GRI-Mech5 has been developed using a different concept of flame model development. Each reaction rate coefficient needed for input has been carefully chosen (not averaged) following a critical review of the available literature. Because each of these has some experimental uncertainty, there will be a resulting uncertainty in the predic~op of any property of a flame or related system by the full model. Thus the entire model is optimized against a set of selected laboratory combustion property measurements: flame speeds, shock tube ignition delay times, and species profiles. These span a wide range of temperature, pressure, and composition, ensuring an optimization valid for many conditions and applications. For comparison with the measurements and results described here, this GRI-Mech H/C/O mechanism has been augmented by NO formation chemistry, using reactions largely chosen from Miller and Bowman,2 and the addition of reactions for up to three carbons (Le., ~H8)' QUANTITATIVE LASER-INDUCED FLUORESCENCE MEASUREMENTS As described above, LIP has many features that make it very attractive for the study of flame chemistry. It has been applied to the detection of many flame intennediates important in combustion chemistry. Table 1 contains those species containing one to four atoms, composed of H, Ct Nt 0, and S (the species occurring naturally in fuels), which have been detected by LIF. In addition to these, there are many other combustion-related species that can be measured, including metal atoms and their compounds, molecules present in specialized combustion situations such as boron- or chlorine-containing radicals, and some polyatomic, partially oxidized hydrocarbons. The experimental configuration for most LIP studies is quite simple, illustrated in Fig. 1. The laser beam is directed into the flame, and the fluorescence emitted at a right angle is focused through a filter onto a photodetector. The filter may be at a particular wavelength (color or interference futer) or may be scannable (a monochromator). The electronic signal from the detector is usually amplified, processed, and stored in a computerized data acquisition system which can 4 |
