Air Entrainment Phenomenon due to a Free Falling Power Jet

当粉末在重力作用下从容器的底部孔口排出时，周围的大气被带进粉末射流。在本研究中，我们对自由落体粉末射流的流动机理进行了实验和数值研究。使用激光多普勒测速仪进行测量。结果表明，在静止空气中，粉末射流的自由落体速度大于单个粒子的自由落体速度，且随粉末质量流量的增加而增大。这是因为在粉末喷嘴中存在被夹带的空气向下流动的气流。在距离孔板较远的地方，粉末射流相对于单个颗粒局部自由落体速度的速度分布与单相自由湍流射流的速度分布相似。在孔口附近观察到了一种类似电势的轮廓。在现有测量的基础上，我们估算了粉末射流的轴向速度分布和夹带风量。与单相湍流射流相比，粉末射流的扩散很小。数值模拟采用颗粒的拉格朗日模型和气流的欧拉模型。我们考虑了颗粒与颗粒的碰撞、阻力、重力和由于颗粒自旋和气流速度梯度引起的横向力，并应用了层流模型。目前的模拟很好地解释了我们在颗粒和空气速度分布以及射流扩散方面的测量。

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.