Coupling of chemical reaction with flow and molecular transport

During the last years the interest in the numerical simulation of reacting flows has grown considerably. Numerical methods are available, which allow to couple chemical kinetics with flow and molecular transport. However, the use of detailed physical and chemical models, involving more than 100 chemical species, and thus more than 100 species conservation equations, is restricted to very simple flow configurations like one-dimensional systems or two-dimensional systems with very simple geometries, and models are required, which simplify chemistry without sacrificing accuracy. In many chemically reacting flows chemical processes occur with time scales differing by many orders of magnitude (e.g., 10$^{-10}$ s to 1 s in combustion processes), whereas the time scales of flow, molecular transport, and turbulence usually cover a much smaller range of time scales. Based on local time scale analyses it is possible to decouple the fast (and thus not rate limiting) chemical processes. In this way the chemistry can be described in terms of a small number of governing reaction progress variables, and computations of complex reacting flow problems become possible. Examples for calculations with detailed and simplified chemistry are shown for various reacting flows, such as hypersonic reacting flows or combustion processes.