The Meaning of Activation Energy and Reaction Order in Autoaccelerating Systems

Global reaction mechanisms and rate constants are commonly used in computational fluid dynamics models that incorporate chemical reactions to study aviation fuel thermal and oxidative thermal stability. Often these models are calibrated using one set of conditions, such as flow rate and temperature. New conditions are then calculated by extrapolation using the global expressions. A close inspection of the origin of global oxidation rate constants reveals that in systems that undergo autocatalysis or auto inhibition, a simple overall global activation energy and reaction order are not good descriptors of the reaction process. Furthermore, pseudo-detailed chemical kinetic modeling of a fuel that experiences autocatalysis shows that the observed reaction order for oxygen consumption varies with initial oxygen concentration, extent of reaction, and temperature. Thus, a simple global rate expression used to describe oxygen consumption in an autoaccelerating system is insufficient to allow extrapolation to different temperature or time regimes.