Size and velocity correlation for splashing droplets generated by jet impingement

Municipal drainage systems can transmit harmful microorganisms such as COVID-19 virus through wastewater as well as drain airflow with suspended tiny bio-droplets. The generation of tiny droplets in drainage systems can be simplified as the phenomenon of jet impingement and splashing, which is also common in industries, but the size and velocity correlation and the kinetic energy of splashing droplets remain unclear. This paper uses high-speed photography to study splashing from a jet impinging on a horizontal plate. Since the jet can break into successive drops before impingement, successive drops impingement and continuous jet impingement are defined and their splashing modes are revealed. Successive drops impingement and continuous jet impingement, respectively, produce splashing droplets with sizes smaller than 0.25 and 0.30 times the size of the impact drops and jet, and with the velocities up to 3.0 and 1.5 times the impact velocity. Correlations between size and velocity of splashing droplets are established: the range and maximum of velocity increase as diameter decreases, and the high velocity splashing droplets have ejection angles within 10 degrees to 20 degrees. For successive drops impingement, the percentage of total kinetic energy of splashing droplets relative to impact kinetic energy increases with impact Weber number and can reach nearly 50%; however, it varies little with impact Weber number for continuous jet impingement, reaching only 10%. Successive drops impingement produces more splashing droplets characterized by a combination of smaller size, higher velocity and larger ejection angle, resulting in a greater risk of producing airborne microorganisms.