Probability density functions of product concentration in a turbulent reacting coflow jet of NH3 and HCl

The distribution of product in a turbulent reacting coflow jet was investig ated in which dilute HCl issued into a coflowing stream of dilute NH3. Mass concentrations of the product, NH4Cl aerosol, were inferred from Mie scatt ering measurements. Probability density functions (PDFs) of the product exh ibited a preferred concentration, or mode, that remained constant across th e jet while the flow was in an early stage of reaction, indicating that pro duction occurred in large scale structures that extended across the jet. In contrast, in regions where the reaction had progressed to a significant de gree, the mode marched to lower concentrations with radial distance from th e centerline, such that the PDF resembled that of a passive scalar. In thes e regions, radial distributions of the mean and rms fluctuations of product concentration were similar to those of mixture fraction, measured using la ser induced fluorescence of acetone. In regions where reaction was still in the early stages, product distributions differed markedly from mixture fra ction distributions. The PDFs suggested that product was formed at the edge s of large structures of rather uniform reactant concentration. A two-dimen sional image of Mie-scattered laser light from the aerosol product supports this interpretation by revealing long, narrow regions of product. (C) 2000 American Institute of Physics. [S1070-6631(00)00111-2].