PARTICLE DISPERSION IN THE DEVELOPING FREE SHEAR-LAYER .1. UNFORCED FLOW

An experimental investigation of the dispersion of small particles in a developing, high-Reynolds-number, turbulent, plane mixing layer is presented. Flow visualizations, laser attenuation and diffraction techniques as well as hot-wire anemometry are used to describe the evolution of the mean and instantaneous particle and gas flow fields. It is shown that the large scale turbulent motion existing in the mixing layer plays a central role in the dispersion of the particles. The mean particle concentration field is shown to be the result of a statistical distribution of streaks produced by the large-scale coherent component of the turbulent motion in the mixing layer. Furthermore, for every particle size, the spreading of the particle concentration thickness is found to occur at a smaller rate than the one characterizing the momentum of the turbulent carrier gas. Large particles are shown to initially disperse into the mixing layer less effectively than the small ones. However, when both downstream and cross-stream coordinates are non-dimensionalized with a characteristic length proportional to the square of the droplet diameter, a universal, particle-size independent dispersion field is found to exist.