Investigation of Rope Formation in Gas-Solid Flows using Flow Visualization and CFD Simulations

摘要

Coal is still one of the widely-used resources for power generation all over the world. Most of the relevant industries use pulverized coal as fuel which is delivered to the furnace by pneumatic conveying. Extensive use of coal has resulted in severe environmental problems due to emissions such as Carbon dioxide, Nitrogen and Sulphur compounds among others. It is postulated that if combustion efficiency is improved, this will lead to significant reduction in pollutant emissions. Combustion efficiency of pulverized coal power plants is influenced strongly by particle size distribution. Most industries use Cyclone Separators (or Classifiers) to separate the larger particles from the smaller ones as part of pre-combustion processes. The sizing and scaling of these classifiers are mostly based on empirical formulations. Detailed 3D numerical studies of these classifiers have not been successful in prediction of experimental observations, hence as such cannot be used as reliable tools for scale up studies. The main reason for this anomaly is believed to be failure of the models in capturing the dynamics of particle behavior in bends and ducts where particles form rope like structures with dense particle clusters. It is then imperative that more study is needed into the understanding of rope or cluster formation in gas-solid flows.;The main objective of the current study is to investigate the underlying mechanisms of rope formation phenomena. Gas-solid flow experiments have been performed in a vertical- horizontal 90o glass bend with high speed imaging of the rope formation. Also, several Computational Fluid Dynamics (CFD) simulations have been performed using the commercial CFD package Ansys FLUENT to capture the roping phenomenon, and results have well supported the experimental observations. Several factors affecting rope formation have also been studied. Roping is basically a type of particle clustering in the sense high particle concentration regions are formed in both these phenomenon. Simulations have been performed on Fluid bed risers to capture clustering phenomenon and also to study the role of vorticity in cluster and rope formation with an objective of developing a fundamental definition for roping. MFIX, a multiphase flow code developed by NETL has also been used to capture the roping phenomenon. These results showed that high particle concentration was found to be in low vorticity regions surrounded by clockwise and counter-clockwise vortices. It was observed that there is indeed a vortex roller effect behind the formation of ropes. These results can be used to provide direction in development of computational models to better handle the gas-solid flow dynamics in classifiers.