Air Entrainment Phenomenon due to a Free Falling Power Jet

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When powder is discharged from the bottom orifice of vessel under gravity, the ambient atmosphere is entrained into the powder jet. in this research, we experimentally and numerically examine the flow mechanism of the free falling powder jet. A laser Doppler velocimeter is used for the measurement. The particles used are glass beads with a mean diameter of 454 mum .It is found that the free falling velocity of the powder jet is larger than that of the single particle in the still air and increases with an increase in the mass flow rate of powder. This is because there exists a downward air stream in the powder jet by the entrained air. At a long distance from the orifice, the velocity profile of powder jet relative to the local free falling velocity of the single particle is similar to that of a single-phase free turbulent jet. Near the orifice a potential-like profile is observed. Based on the present measurement we estimate the axial velocity profile of powder jet and the entrained air flow rate. The spread of the powder jet is very small as compared with that of the single-phase turbulent jet. The numerical simulation is performed based on Lagrangian modeling for particles and Eulerian modeling for air flow. We consider particle-particle collision, drag force, gravity force and transverse force due to the particle spin and to the velocity gradient of air flow, and apply a laminar flow model. The present simulation well explains our measurement in terms of particle and air velocity profiles and jet spreading.