Investigation into Fluidization and De-Aeration Characteristics of Powders and Yield Stress Determination

Abstract

The study has two major purposes; one is to investigate into the fluidization and de-aeration behaviour of fine powders (belonging to Geldart group A and C) to determine the minimum fluidization velocity and propose a model by considering basic properties to predict the minimum fluidization velocity and the other is to determine the yield stress of fine powders at different depths of spindle immersions and to compare the shear stress behaviour of fine powders with bingham plastic fluids at varying strain rates. Initially, a model to predict the minimum fluidization velocity has been proposed by considering the experimental data adapted from the literature and by inputting the basic particle parameters. Necessary improvements in the experimental set up have been made to obtain more certain results. Experimental investigations were conducted on different fly ash materials collected from different fields of ESP hoppers from different powder plants and the proposed model was checked for validation. From the obtained experimental data the air retention capability of the fine powders has been analyzed. Shear stress behavior with varying strain rates have been investigated initially for mustard powder and mustard paste at two different depths of spindle immersion. Further investigation is conducted on fly ash from different fields of ESP hoppers of different power plants under fluidized and un-fluidized conditions. The model developed for predicting minimum fluidization velocity was found to be well suitable for fine powders, especially for powders that fall under A/C boundary. It has been found that the model shows large deviation from the experimental results for very cohesive powders with mean particle size d50 8 μm. The fluidized powders behave same as non-Newtonian bingham plastic fluids (mustard paste) with varying strain rates. The yield stress value of fine powders increases with increasing depths of spindle immersion. Shear stress value is lowered at high speeds of spindle rotation. Furthermore, investigation into shear stress behavior of fine powders is required to involve this as a parameter for accurate modelling of fluidized dense phase flow.