Wall flow function measurements to assess wall cohesion and adhesion

The build up of cohesive powder on the surfaces of inclined chutes is a common industrial problem. The paper presents a technique for characterising wall cohesion / adhesion, using an annular wall friction tester. Traditionally wall friction measurements are undertaken for silo design over a range of reducing stresses and over-consolidation effects are ignored, these are the conditions of interest for flow in a converging hopper. The wall cohesion can be determined from these measurements, as the intercept of the wall locus with the shear stress axis. With a Jenike wall friction tester determining this property requires significant extrapolation of the locus as lowest test stress attainable is relatively high (at approximately 0.7kPa). However, for low stress build up of a cohesive powder on a chute the above approach is inappropriate. The technique presented uses a standard flow function test procedure, to measure a family of over-consolidation wall failure loci over a range of increasing stress that can be used to produce a wall flow function, i.e. wall cohesion presented as a function of consolidation stress level. The paper presents case studies from our previous industrial projects where wall flow functions have been determined for a range of powders.