Development of Mixing and Dispersion in an Isothermal, Droplet-laden, Confined Turbulent Mixing Layer

Abstract

This article concerns an experimental study of dispersion of liquid droplets in a confined, turbulent mixing layer. An ambient temperature air stream is seeded with dilute, polydisperse water droplets and it is brought into contact with another ambient temperature air stream at a different velocity in a planar, confined mixing layer confirguration. The gaseous mixing layer is first characterized using hot wire anemometry including mean and rms streamwise velocities and turbulence power spectra. Measured mixing layer parameters are found to compare favorably with those reported in the literature. Next, measurements of droplet field are conducted using a single component phase Doppler anemomenter. Time averaged measurements of droplet size, velocity, number density and volume flux at different velocity ratios are presented in the early part of spatially developing, confined mixing layers. Droplet dispersion is found to be strongly size selective in the early part with the small droplets being dispersed into the mixing layer much more effectively than the larger droplets. This process results in a buildup of small droplets inside the early part of the mixing layer. However, large droplets are eventually entrained into the mixing layer and this imbalance gradually disappears at farther downstream distances. The growth of the droplet concentration thickness in the layer is found to depend on the droplet size and the shear parameter λ = (Ud− Ua)/(Ud+Ua) across the layer with Ud and Ua being the droplet laden and other air stream velocities. When concentration thicknesses and the streamwise coordinate are normalized by a length scale proportional to particle diameter squared multiplied by δU=2Udλ/(λ + 1) the experimental data are found to collapse onto a universal trend accounting for different droplet sizes and mixing layer shear. While differential dispersion of droplets is affected by the shear, total liquid volume flux distributions do not reflect such differences due to the lesser contribution of small droplets to the total liquid volume flux.