The effects of Damköhler number in a turbulent shear layer

Abstract

A chemical reaction for which the reaction rate can be varied in studied in a fully developed, two-dimensional, turbulent mixing layer. The layer is formed between two nitrogen streams, one carrying low concentrations of fluorine and the other hydrogen and nitric oxide. For fixed concentrations of fluorine and hydrogen and for nitric oxide concentrations that are small fractions of the fluorine concentration, the heat release is fixed but the overall reaction rate is controlled by the nitric oxide concentration. Therefore, for fixed flow conditions, the nitric oxide concentration determines the ratio of the reaction rate to the mixing rate and hence a Damköhler number. For large values of this ratio, the amount of product, at a given downstream location, measured by the mean temperature rise, is independent of the reaction rate, i.e., the reaction is mixing limited. As the reaction rate is reduced and the amount of product declines, other effects are: (1) the mean temperature profile, which is initially somewhat unsymmetrical because the hydrogenfluorine freestream concentration ratio is set at a large value, becomes symmetrical, and (2) the ramplike instantaneous temperature traces within the large structure gradually become more like square waves. Based upon two choices for the mixing rate, numerical values of the Damköhler number are proposed to quantify fast and slow chemistry regimes.

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