Toward a Grand Challenge in Powder Flows: The Effect of Material Properties, Boundary Conditions and Shear Rate on Fluctuations and Stress Fields in Flowing Powders.

Publication Reference: 
08-11
Author Last Name: 
Tardos
Authors: 
Professor Gabriel I. Tardos, Mr. Mehrdad Kheripour Langroudi
Report Type: 
ARR
Research Area: 
Powder Flow
Publication Year: 
2008
Publication Month: 
10
Country: 
United States

Executive Summary

The research is focused on the Mechanics of Powders and the ultimate goal of the work is to develop a quantitative description of active flows of fine powders. The study is centered on the “intermediate” regime of flow where both frictional and inertial effects are important. The main application is in the area of small-size, rough and/or cohesive powders that are industrially relevant.

The novelty of the project is to study a relatively large range of materials and flow geometries to gain meaningful insight. We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and elastic), used in an axial-flow Couette and in two Jenike-type shearing devices to measure stresses and their fluctuations as a function of geometry and shear rate.

From stress transmission tests in the Jenike-type devices, we found that a layer of granules (powder) transmits normal stresses only if sheared and large fluctuations are introduced for the case of large particles. When the particles are small, of the order of hundreds of microns or less, or are cohesive and form a cake upon compression, the fluctuations are diminished and stresses are transmitted without significant alteration.

The further goal to develop constitutive equations for the intermediate regime of flow was also undertaken. The axial-flow Couette device was used as a “rheometer” to characterize the flowability of powders, develop a constitutive equation and use it in a continuum-type theoretical model to predict flow patterns, velocity distributions and forces on boundaries such as stationary walls and moving pedals. The PI is collaborating with several groups of simulators (DEM and MDS) and mathematicians to implement these models and compare theoretical results to computations. The best results so far, were obtained for a relatively fast moving Jenike-type cell where large, spherical particles where sheared against a smooth wall. Further results in the Couette device using a continuous-type theory (on the lines proposed by the work of Tardos, 1997) are also very promising