Grinding in an air classifier mill — Part I: Characterisation of the one-phase flow

• Published in: Powder technology 2011-07, Vol.211 (1), p.19-27
• Main Authors: Toneva, Petya, Epple, Philipp, Breuer, Michael, Peukert, Wolfgang, Wirth, Karl-Ernst
• Format: Article
• Language: English
• Subjects: air; angle of incidence; Applied sciences; CFD; Chemical engineering; Classifiers; Comminution; Computational fluid dynamics; Exact sciences and technology; Fluid flow; Fluids; Grinding; Hydrodynamics of contact apparatus; Impact mills; Mathematical models; Mills; Miscellaneous; Multiphase flow; PIV; prediction; Preserves; Solid-solid systems
• ISSN: 0032-5910; 1873-328X
• DOI: 10.1016/j.powtec.2011.03.009

In this and the related second paper [1], we present an in-depth study of the two-phase flow and the stressing conditions of particles in an air classifier hammer mill. This type of mill belongs to the mostly used mills at all. In order to develop a predictive grinding model not only the material's reaction to the applied stress but also the stressing conditions within the mill, e.g. impact velocity, incidence angle, number of stress events, have to be known. The latter are strongly affected by the interactions between the fluid and the solid phase within the mill. Systematic flow investigations in the vicinity of the impact elements and in the region of the internal classifier have been performed by Particle Image Velocimetry (PIV) and by numerical predictions of the fluid flow in the complete mill using a commercial CFD solver. Different pin geometries have been studied at various peripheral velocities of the grinding disk and the classifier. The classifier velocity does not influence the velocity profiles near the impact elements in the main flow direction and vice versa, the flow in the grinding zone has little influence on the classification. The velocity profile in front of the impact element, where the comminution process takes place, is constant with time and preserves a characteristic form independent of the operational conditions. An in-depth study of the one-phase flow in an air classifier mill is presented. The fluid velocity profile in front of the impact elements is constant with time and preserves a characteristic form independent of the operational conditions. The flow pattern within the classifier can be separated in three characteristic regions concerning the vortex type formation. The figure below shows the calculated flow pattern at the impeller wheel classifier (peripheral velocities of the grinding disk and the classifier 100 m/s and 30 m/s, respectively, classifier wheel rotates counter-clockwise, fluid velocity relative to the classifier peripheral velocity). [Display omitted] ► The classifier velocity does not influence the flow near the impact elements. ► The flow in the grinding zone has no influence on the classification. ► The velocity profile at the front side of the impact elements is constant with time. ► Characteristic velocity profiles are observed at the front side of the impact elements. ► The flow pattern within the classifier can be separated in three characteristic regions.