Experiments and modeling of droplets motion induced by turbulent air flow on inclined surfaces

Experiments were performed to measure the velocities of droplets propagating on the bottom of an upward inclined pipe when exposed to air shear flow. Effects of the droplets' shape, liquid properties, air velocity, and the pipe inclination were examined. It was found that the droplet velocity increases with the air velocity and droplet length, and reduces with the pipe inclination and the liquid viscosity. A mechanistic model that considers the stretched shape and forces acting on a moving droplet was established. The model uses a correlation for the drag coefficient that was derived based on the experimental data of the droplet velocity. It enables the prediction of the measured droplets’ velocities and the effect of the various parameters on the droplet motion. Based on data from the literature, a different drag coefficient correlation for a stationary droplet, yet unstretched, is also presented. The different models for stationary and moving droplets imply a hysteresis phenomenon regarding the air critical velocity. Namely, the air velocity required to displace a stationary droplet is larger than the air velocity below which the drop will stop its movement. However, the hysteresis becomes less notable with larger droplets.