Anatomy of Complex Reaction Systems. Combustion Reaction Mechanisms from Ignition Delay Times

Assa Lifshitz Emeritus Faculty Member Department of Physical Chemistry The Hebrew University of Jerusalem Jerusalem, Israel One of the very useful approaches to the understanding of the kinetics and mechanism of complex combustion systems is the measurement and modeling of the induction period that precedes the ignition of a fuel in a shock tube. When a mixture of a fuel and oxidant is subjected to shock heating, it ignites, following an induction period known as the ignition delay time. This delay is the outcome of the exponential character of the overall reaction rate resulting from various chain branching reactions and adiabatic temperature rise during the course of the reaction. The delay time which is a readily measurable quantity is a function of the initial temperature, pressure and composition of the reaction mixture. The measurement of its dependence on the reactant concentrations and temperature provides a very powerful basis for modeling and understanding the oxidation mechanism. The high potential of this methods was recognized by many combustion kineticists and a very large volume of experimental results and kinetics modeling have been published. The following picture is a typical pressure record showing the reflected shock heating and the ignition process. It is useful to summarize the dependence of the ignition delay times on the composition of the system and on the temperature in a simple parametric relation that can later serve as a basis for computer modeling. It has been shown in the past in numerous ignition studies behind shock waves that the general relation between the induction times and the concentrations is very similar to the relation between a rate of a chemical reaction and the concentrations, that is, tignition = A Ði Ci âi where tignition is the ignition delay time, Ci is a concentration of a component i, and âi is a parameter somewhat similar to an empirical reaction order. It has also been shown that the concentration independent parameter can be presented as, A = 10á exp [E/RT] an expression very similar to a rate constant (except that A decreases with temperature). The parameters E and âi are determined by a complex kinetic scheme. They are experimentally determined quantities and provide a very useful means to summarize the experimental results in a quantitative manner. After establishing an empirical relation as above and determining the parameters, one can perform computer experiments under conditions similar to the laboratory experiments and try, for a given reaction scheme to reproduce these parameters. One then arbitrarily varies the magnitude of the various rate constants in the kinetic scheme and examines the influence of such variations on the magnitude of the ignition delay time and its dependence on the concentrations and on the temperature. From the results of this type of experiments, the role of each reaction in the overall mechanism can be elucidated. By employing such methods, many interesting combustion schemes were analyzed in the past and details of the kinetics and an understanding of the oxidation mechanisms were achieved. We will present and discuss several such systems.

Location: Seaver Science Library, (SSL) Rm 150

Audiences: Everyone Is Invited

Contact: April Mundy