Convection in a water droplet colliding with a solid particle

The convection rate (velocity) fields in a droplet colliding with a stationary particle (steel ball) are constructed using particle image velocimetry. The collision process is shown to lead to both a change in the nature of the velocity field in the droplet (most vectors change their original direction) and an intensification of the convection velocity by a factor of 3–10. The convection rate is shown to decrease as the ratio of droplet to particle diameters increases. A new correlation for the convection rate as a function of the initial diameter of the droplet is suggested. A phase field method, implemented in COMSOL Multiphysics, is used for numerical simulation of the collision between a droplet and a spherical solid particle. The difference between the predicted and observed average values of convection velocities is shown to be less than 12%. Potential applications of the results to water droplets that contain unregulated impurities with diameters of up to 500 μm, and to the development of thermal water treatment technologies, are discussed.