Estimation of Particle Rotation in Fluidized Beds by Means of PTV

Abstract

Fluidized bed granulation is a widely used process to produce pharmaceuticals, food and fertilizer. The achievable product quality, for example the uniformity of the formed layer or the size of the agglomerates, inherently depends on the particle dynamics in the bed. Generally, the solid phase velocity field is used to determine characteristic particle times (turn-over and residence time) in different zones of a spray fluidized bed. This information can be acquired using particle image velocimetry (PIV) together with digital image analysis (DIA) as for example in Börner et al. (2011, 2013). In order to derive certain quantities, such as particle mass flow rates, the measurement results are averaged in space and time, which is directly associated with a loss of information. However, an alternative is available when using the Lagrangian particle description (particle trajectories and local particle velocities) in terms of particle tracking velocimetry (PTV). Particle velocities are estimated on the basis of identifying individual particles, leading to exact particle mass flow rates. There are known shortcomings with this method for dense particulate flows, in particular ambiguities during particle identification. These problems have been fixed to a certain extent, for example using colored tracer particles (Natarajan et al., 1995, Bendicks et al., 2011) or recognition of flow pattern (Capart et al., 2002). The former approach yields a low resolution, but ensures reasonable results for particle velocities and associated trajectories. Additionally, it is possible to obtain information concerning the particle rotation, when using traces with an adequate color pattern (e.g. Zimmermann et al., 2011). Therefore, we used half-page blackened tracer particles in order to estimate particle velocities and particle rotation simultaneously and unambiguously within 2D-fluidized bed. Solid mass flow rates and particle residence times can be derived from the local particle velocities. At the same time, the observation of discontinuous particle tracks together with the quantification of the particle rotation can be used to foster the insight and understanding of particle-particle contact and the associated momentum transfer in dense gas-solid multiphase flows. Eventually, we can obtain local particle collision rates and corresponding changes of normal and angular momentum of individual tracer pairs.