Aircraft Propulsion, 2nd Edition

7.5 Chemical Kinetics

So far our study of the chemical reactions involved an equilibrium state where the forward and reverse reaction rates were equal. However, the time it takes to reach the equilibrium state was not explicitly studied. To study the (time) rate of formation of product species in a chemical reaction, or equivalently the rate of disappearance of reactants in a reaction, is the subject of chemical kinetics. As a propulsion engineer, why should you be interested in chemical kinetics? The short answer is that it gives you an important characteristic timescale of the problem, which may affect the size and efficiency of your combustion chamber. For example, you will need to design the combustion chamber (as in a supersonic combustion ramjet) such that the residence time of species in the chamber is longer than the characteristic chemical reaction timescale for the heat of reaction to be fully realized. Also, we learn that the reaction rate depends on the pressure of the mixture, which at high altitudes (low pressures) may become too slow to effect a combustor relight. Finally, the study of chemical kinetics helps combustion engineers meet the low-pollution design objectives of low NO_x, low CO, soot, and (visible) smoke dictated by the regulatory organizations (e.g., EPA and ICAO).

The cornerstone of chemical kinetics is the law of mass action, which for the following generic reaction

may be written as

that depicts the forward reaction rate. The reaction rate coefficient ...