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Validation of Reaction Mechanisms

Currently, no known published mechanism is capable of accurately modeling mixtures containing all of the chemical species of interest to this study. Computational requirements and limitations cause most mechanisms to be designed for a particular application, and they are of uncertain value under off-design conditions. To evaluate the usefulness of the collected mechanisms under a variety of conditions, simulations using the mechanisms have been compared to experimental data from the literature. Data suitable for comparison with chemical kinetics computational results include shock tube induction times, flame induction distances, stirred reactor induction times, and flame species concentration profiles. To simplify the analysis, and because a large portion of the available data is in the form of shock tube induction time, we concentrated on comparisons with induction time measurements. For the sake of numerical analysis, the chemical reactions behind both incident and reflected shocks are modeled as constant-volume processes. This is a good approximation in most cases since the shocked mixtures are typically highly diluted with Argon and there is relatively weak coupling between the chemical reactions and the fluid motion.

In general, induction times are straightfoward to measure and there are abundant data in the published literature. However, there are a number of difficulties that we encountered:

1.
Induction time can be defined in a number of different ways for the purposes of both experimental measurement and numerical modeling (see Section 3.1.1).
2.
Data from different reactant concentrations are sometimes presented together without individual identification, making proper modeling difficult.
3.
Most validated reaction mechanisms are most accurate at lower pressures and higher temperatures than those encountered in detonations (within the induction zone).
4.
Each validation data set or each set of reaction rate parameters is useful for limited ranges of temperature, pressure, and species concentrations.
5.
Many investigators plot induction time data in such a way as to remove the pressure dependence (i.e. $\tau$[X]), and then plot data for a variety of pressures together. This makes it difficult to precisely compare experimental and computational results.

In Sections 3.1.3 to 3.1.9 below, a brief review of the validation effort is given for each simple fuel-oxidizer mixture. In some instances, different dilutions are examined separately. Each section consists of a list of references containing reaction mechanism or induction time data, discussion of these references, a description of the results of the validation study, and recommendations of appropriate conditions for use of the studied mechanisms. These recommendations are summarized in Table 11 and supporting figures are provided in Appendix [*]. These reviews are not exhaustive.

The thermodynamic condition within a fuel-air detonation typically varies from 1500 K and 40 atm (von Neumann state) to 3000 K and 20 atm (Chapman-Jouguet state). Since the conditions within the induction zone are approximately the von Neumann condition and primarily determine the length of the reaction zone, accurate modeling of this condition is most important.


 

next up previous contents
Next: Experimental Data Up: Chemical Reaction Kinetics Previous: Chemical Reaction Kinetics
Joe E. Shepherd
2000-01-17