THE MEANING OF ACTIVATION-ENERGY AND REACTION ORDER IN AUTOACCELERATING SYSTEMS

Global reaction mechanisms and rate constants are commonly used in com putational fluid dynamics models that incorporate chemical reactions t o study aviation fuel thermal and oxidative thermal stability. Often t hese models are calibrated using one set of conditions, such as flow r ate and temperature. New conditions are then calculated by extrapolati on using the global expressions. A close inspection of the origin of g lobal oxidation rate constants reveals that in systems that undergo au tocatalysis or auto inhibition, a simple overall global activation ene rgy and reaction order are not good descriptors of the reaction proces s. Furthermore, pseudo-detailed chemical kinetic modeling of a fuel th at experiences autocatalysis showy that the observed reaction order fo r oxygen consumption varies with initial oxygen concentration, extent of reaction, and temperature. Thus, a simple global rate expression us ed to describe oxygen consumption in an autoaccelerating system is ins ufficient to allow extrapolation to different temperature or time regi mes.