Mechanical and Civil Engineering Seminar
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Mixtures of a very large number of chemical species qualify as chemically-complex (CC) fluids. Such CC fluids are encountered in pharmaceuticals, chemical engineering processes (e.g., household cleaners), energy release processes (e.g., fuel combustion) and in exotic planetary atmospheres. Because when studying CC fluids it is often inefficient, or even impossible, to take into account all the chemical species, it is valuable to devise methods in which only a small, but representative, subset of species is considered. The requirement for this subset is that it should duplicate the behavior of the entire mixture for a desired purpose. In the context of combustion chemistry, it is shown how the number of species in a mixture can be reduced by 99% while faithfully duplicating the temporal evolution of hydrocarbon oxidation chemical reactions. The methodology by which this is achieved works well when the species spatial distributions are uniform. However, in the more complex situation where spatial uniformity no longer holds, the methodology must be reconsidered. To study the effect of spatial non-uniformity, a Direct Numerical Simulation (DNS) database of high-pressure fluid flows realizations -- at otherwise identical initial dynamic conditions -- has been created for nominally the same fluid by neglecting or retaining trace species, thus leading to investigating the influence of trace species in a mixture. The DNS is conducted in the geometry of a three-dimensional temporal mixing layer where two counter-flowing streams of different initial composition undergo mixing and ultimately transition from a laminar to a turbulent state. The characteristics of these fluids at turbulent transition are identified. Challenges in the possibility of accurately representing the composition of the nominal fluid by reducing the number of trace species when species spatial distributions are non-uniform are indicated.