Experimental Study Of The Critical Suspension Of A Cylinder In A Stirred Tank

In recent years,anaerobic fermentation technology which can directly use biomass resources has been widely concerned.The raw material for fermentation is usually irregular and non-spherical.The raw material is stirred in an agitated anaerobic fermentation reactor to form solid-liquid suspension fluid,which could effectively promote the fermentation process.In addition,solid-liquid suspension plays an important role in nature and various industrial processes.The existing researches on solid-liquid suspension mainly focus on the macroscopic movement characteristics of solid particles,and some scholars have also studied the suspension of single spherical particles.However,the research on the movement of non-spherical particle is rare,and the research on its suspension characteristics is much less.In this thesis,the critical suspension characteristics of a single cylindrical particle in a square stirred tank driven by a disk are studied.Based on the experimental apparatus and method,the azimuth angle a and elevation angle ?are defined to describe the orientation of the cylinder.In the experiment,two high-speed cameras were used to capture the movement of the cylindrical particle from the side and the bottom simultaneously.Then the captured images were processed by MATLAB to obtain the information including particle's motion trajectory,rising speed,orientation,etc.The experimental results show that the critical impeller speed NLo does not increase monotonously with the increase of the cylinder's aspect ratio.For cylinders with the same diameter,the cylinder with large aspect ratio could be lifted off from the tank's bottom at a smaller impeller speed.For cylinders with larger diameter,the critical Shields number could be considered as the key factor to control their suspension.The experimental results show that the critical impeller speed NLo of the cylinder depends on its diameter and aspect ratio,and the further the cylinder protrudes in this laminar flow,i.e.the larger its diameter or the longer its aspect ratio,the stronger it feels the flow.The viscous Shields number representing the ratio of viscous force suspending the cylinder to net gravity force pulling the particle down can be considered as the key control parameter for the suspension of cylinder.The cylinder initially located at the center of the stirred tank's bottom will gradually deviates from the center due to the flow field's affect.And the deviation distance is approximately increase linearly with the increase of the cylinder's aspect ratio.The obvious deviation of the cylinder's position is one of the necessary conditions for its suspension.The max rising height and the max elevation angle ? decrease with the increase of the cylinder's aspect ratio.Due to the restriction of disk and the influence of cylinder's turning,there are two decelerations in the rising process of cylinder.