Turbulent mixing of passive and chemically reacting species in a low-speed shear layer

Abstract

A series of experiments has been conducted in a low-speed wind tunnel in which measurements were performed in a two-dimensional turbulent shear layer experiencing the mixing of both a passive and a chemically reacting species. The low-temperature air in the jet's primary flow was seeded with dilute concentrations of N2O4so that the dissociation reaction N2+ N2O4[harr ] 2NO2+ N2occurred in a near-equilibrium manner within the mixing layer owing to the turbulent mixing properties and the imposed temperature gradient. Mean and fluctuating values of velocity, temperature and NO2concentration were measured up to axial distances of 25 in. for jet velocities of 23 and 50ft/s (Rex[les ] 7 × 105) and for three primary temperatures (252, 273 and 305°K). Velocity and temperature measurements were performed with hot-wire probes, whereas a fibre optics light sensor probe was used to measure NO2concentrations. Local correlations between species and other fluid properties were obtained by positioning a hot-wire sensor within the light gap of the fibre optics probe and simultaneously recording output signals from both probes. A relatively complete set of turbulent statistics was measured for the non-reacting shear layer, including such results as temperature/species correlations, probability densities, filtered and unfiltered moving-frame velocities, skewness and flatness factors, spectra, velocity and temperature integral scales, intermittency factors for velocity, temperature and passive species, and conventional intensities. Some typical results from the investigation are as follows: the turbulent Schmidt and Lewis numbers were 0·5 and 1·0 respectively; the correlation between passive NO2concentration and temperature was approximately 0·95; dramatic changes consistent with equilibrium chemistry occurred in NO2concentration profiles with chemical reaction; velocity, temperature and concentration spectra were comparable over a 2½-decade range in wavenumber (k−2); spectra, probability densities, time-trace data and smoke-seeded shear-layer photographs indicate that, for axial locationsx=x0[ges ] 18·5 in. and for speedsu1[ges ] 23ft/s, undisturbed edge fluid rarely penetrates completely across the mixing region. Although not specifically addressed during the current study, measured results herein suggest that the turbulent motion for the present shear layer is characterized more by random and/or three-dimensionality effects than by large-scale two-dimensional coherent structures, as has been observed recently in other shear-layer investigations.