Experimental and numerical investigation on the dripping behavior of high-concentration viscous solid waste

The dripping behavior of high-concentration viscous solid waste entering a boiler directly affects combustion efficiency and operational stability. This study investigates droplet formation through a combined experimental and computational approach. A recirculating pipeline system with adjustable outlet diameters and discharge velocities was constructed, and a two-phase volume of fluid model incorporating a Herschel–Bulkley constitutive law was implemented to simulate the transient dripping process. Using high-concentration coal slime as a representative viscoplastic material, the simulations accurately reproduce the experimentally observed necking evolution and final droplet geometry, with length deviations of approximately 3.2%. Results show that the droplet formation interval decreases sharply with increasing velocity and follows a shifted inverse relationship. As the velocity increases, droplet morphology transitions from short and wide to elongated and slender due to two coupled mechanisms: (i) stretching induced by the preceding droplet and (ii) a downward migration of the pinch-off location that keeps the slime near the outlet in a yielded state immediately after detachment. Consequently, the difference between the initial droplet and subsequent steady-state droplets increases with velocity. The effect of pipe diameter on the droplet formation interval is negligible, with variations within 5%. However, at a fixed flow velocity, larger diameters produce greater reductions in maximum droplet diameter due to the formation of an extended tapered yielding zone near the droplet tip. This study provides a validated numerical framework and mechanistic insight into the dripping behavior of yield-stress slurries, offering predictive capability for engineering slime-feeding systems.