Physical modelling of particle distribution for stir cast metal matrix composite using sand-water and sand - aqueous CaCl_2 slurry system

摘要

Suspension characteristics of particle is a very important aspect in many industrial processes such as waste water treatment, composite making by stir cast technique, mineral and chemical engineering processes. In some cases, homogeneous mixing is desirable, in other cases, either floatation or sedimentation is desirable. Although apparently the phenomenon seems to be very simple, but tailoring the particle movement as desired is not easy. The complexity arises because the particle suspension characteristics depends upon a number of factors viz. particle shape, size and size distribution, specific gravity, its concentration; nature of liquid i.e. its specific gravity, viscosity and surface tension; vessel and stirrer geometry and also the design and process variables. Moreover various forces such as gravity, buoyancy, centrifugal, viscous drag and frictional forces due to collision of particles jointly act on a particle under dynamic rotating motion of the fluid. There is a tendency of hindered settling of particles when its amount is relatively higher than a critical value. In the present study, sand-water slurry system was taken and stirred by a propeller type of stirrer in a cylindrical shaped perspex vessel of diameter 0.10m X 0.15m height, the average particle size being 200 microns of mostly sub-angular to rounded shape. Some studies were also done using a bigger vessel of diameter 0.20 m X 0.30 m height with sand-water and sand-aqueous CaCl_2 (density 1330 Kg/m~3) systems. Various factors such as particle size and distribution, its amount, speed of rotation and power input to stirrer, its location in the vessel, direction of rotation of stirrer were varied and their influences on air-entrainment, piping, layering (single phase zones of particles), sedimentation and overall particle distribution were investigated which is a different approach in comparison with the earlier investigations. The distribution was assessed qualitatively (visually and photographically through perspex) and quantitatively by measuring the concentration and sieve analysis of the samples drawn from different locations along the height of the slurry column. Finally a suspension model in terms of particle size distribution; was made under various conditions studied. The degree of uniformity of particle distribution is found to be strongly dependent upon stirrer speed as well as the distance between the vessel bottom and the stirrer base. Particle distribution was found to be better in sand-aqueous CaCl_2 slurry than in sand-water system. This suspension model can be extended to metallic system considering non-dimensional parameters such as Reynolds Number, Archemides Number, Lyaschenko Number, Fronde Number. Up-scaling is also possible using various similitude criteria.