State Diagrams of Gas/Solid Flows with Partial Phase Separation

• Main Authors: O. Molerus, K.-E. Wirth
• Format: Article
• Language: English
• Published: Hosokawa Powder Technology Foundation 2014-06-01
• Series: KONA Powder and Particle Journal
• Online Access: https://www.jstage.jst.go.jp/article/kona/9/0/9_1991022/_pdf/-char/en
• ISSN: 0288-4534; 2187-5537

Preferable operational conditions of gas/solid flows are often situated in the transitional regime between two well defined modes, typical examples being pneumatic transport in a horizontal pipe and circulating fluidized beds. The energetic optimum of horizontal pneumatic transport is situated in the transitional regime between strand flow and fully developed flow in which regimes with different physical factors (wall friction of sliding material or single particle wall impact, respectively) prevail. The circulating fluidized bed may be regarded as the transition from a bubbling fluidized bed to pneumatic transport vertically upwards. Experimental results indicate particle slip velocities which are quite a bit higher than predicted by single particle fall velocity, hence indicating strong phase separation in terms of clusters. In both situations, empirical correlations fail because they ignore the complicated structure of the system in question, i.e. being transitional. On the other hand, even very sophisticated modelling can fail, because it is mostly based on inherent a priori assumptions concerning the state of flow. As an alternative, comparably simple models of two phase flows in horizontal pipes and for circulating fluidized beds are presented. The models originate from very obvious visual observations and from considerations of momentum exchange between the two phases. The evaluation of the respective resulting sets of algebraic equations can be represented in terms of state diagrams for gas/solid flows with partial phase separation.