Title

Experiments on dispersion and evaporation of droplets in a planar turbulent heated mixing layer

Date of Completion

January 1996

Keywords

Engineering, Aerospace|Engineering, Mechanical

Degree

Ph.D.

Abstract

A comprehensive experimental investigation of the two-dimensional, droplet-laden, turbulent mixing layers at isothermal and non-isothermal conditions has been conducted. The isothermal gaseous mixing layer was characterized using hot film anemometry and the effect of shear was assessed. The results showed good agreement with previous investigations.^ The droplet-laden mixing layer was then studied and measurements of the droplet field were conducted using a single component phase doppler particle anemometer providing a full characterization of the two-phase mixing layer in terms of velocity, volume flux and droplet concentration. The higher nonlinear spread rate of the droplet-laden mixing layer was attributed to the size-dependent droplet dispersion and was correlated to the local Stokes number and relevant time scales of both the droplets and the flow. Droplet dispersion mechanisms inside the mixing layer were analyzed and supported by a full documentation of the evolution of size-based probability distributions across the layer at different shear.^ An experimental study targeting dispersion and evaporation of droplets in a differentially-heated mixing layer followed. Differential heating was found to alter droplet and flow time scales ending with a Stokes number modification. The combined effect of small droplet disappearance and large droplet size reduction, shifted more droplets into the dispersed Stokes regime. The interplay between dispersion and evaporation at different heating and shear, was completely described by the evolution of the pdf histograms of droplet size distribution across the differentially heated mixing layer. High shear appeared to balance out the heating effect on size based growth, overwhelm it for intermediate shear and fall short of it at low shear.^ Visualization using Mie scattering from water droplets and laser induced fluorescence from acetone vapor gave a qualitative confirmation of all the features of the mixing layers under investigation. ^