Modeling and Experimental Study of Variation of Droplet Cloud Characteristics in a Low-Speed Horizontal Icing Wind Tunnel

Variation of the characteristics of aerosol clouds created in icing wind tunnels is studied theoretically and experimentally. The characteristics of interest are the droplet size distribution, liquid water content, temperature, velocity, and air humidity, which are
among the most important factors affecting atmospheric icing. Several processes influence the trajectory, velocity, size and temperature of the droplets, such as collision, evaporation and cooling, gravitational settling, and turbulent dispersion. The authors have
developed a two-dimensional theoretical model that takes these processes into account, and predicts how they influence the changes in the characteristics of the droplet cloud during its movement in the tunnel. The most recent development pays special attention to
two of the possible collision outcomes, i.e. coalescence after minor deformation and bounce, together with the transition between them. Indeed, these outcomes are frequent when the relative velocity of the droplets is small, as is the case for a cloud formed after the injection of water droplets in the direction of air flow. An experimental study is also carried out with different thermodynamic parameters at different positions in the test section of the tunnel, which makes it possible to observe the evolution of cloud
characteristics under different ambient conditions. The droplet size distribution and liquid water content of the aerosol clouds were measured using an integrated system for icing studies, which comprises two probes for droplet size measurements and a hotwire liquid water content sensor. Droplet trajectories were observed using particle image velocimetry. The experimental results are also used to validate the model by comparing them to model predictions. Satisfactory agreement between the experimental and calculated results establishes the applicability of the model to determine the evolution of droplet size distribution and liquid water content in an aerosol cloud in the streamwise direction, together with their vertical variation.