TABLE OF CONTENTS For fast equilibrium chemistry (Section 5.4), an equilibrium assumption allowed us to write the concentration of all chemical species in terms of the mixture-fraction vector c( x, t) = c eq( ?( x, t)). For a turbulent flow, it is important to note that the local micromixing rate (i.e., the instantaneous scalar dissipation rate) is a random variable. Thus, while the chemistry may be fast relative to the mean micromixing rate, at some points in a turbulent flow the instantaneous micromixing rate may be fast compared with the chemistry. This is made all the more important by the fact that fast reactions often take place in thin reaction-diffusion zones whose size may be smaller than the Kolmogorov scale. Hence, the local strain rate (micromixing rate) seen by the reaction surface may be as high as the local Kolmogorov-scale strain rate.
In combustion systems, locally high strain rates can lead to micromixing-induced extinction of the flame - an important source of pollutants in turbulent combustion. The non-equilibrium effects caused by fluctuations in the micromixing rate can be modeled using the concept of laminar diffusion flamelets (Peters 1984; Peters 2000). In this model, the instantaneous scalar dissipation rate of the mixture fraction appears as a random variable in a reaction-diffusion equation, thereby directly coupling the local reaction rate to the local micromixing rate. Although the method has been extended to complex chemistry, it is most easily understood in...
