Development of an annular wall friction tester and testprocedure, suitable for equipment design and powdercharacterisation

Annular, ring or torsional shear testers are commonly used in bulk solids handling research for the purpose of powder characterisation or equipment design. This paper reports from a DEFRA sponsored project which aims to develop an industrial powder flow-ability tester (based on an automated annular shear tester) that is economic to buy and quick and easy to use in trained but unskilled hands. A necessary part of the development of the annular shear tester was to design a wall friction cell so that a complete ‘flow’ characterisation of a powder could be undertaken. Whilst the Jenike type linear wall friction tester is the accepted standard tester, a review of the literature turned up several different designs for torsional type wall friction testers. However, from the literature review it was apparent that what appears to be a simple test gives very varied results and that there is a need to standardise the wall friction test procedure to get reproducible results. This paper reports the effects on the measured wall failure loci, and effective wall friction angle of changes in the testing procedure such as: • the technique used to prepare the wall sample prior to testing, to produce a dirty surface representative of the material in service in a hopper or chute, • consolidating the powder over an increasing versus a decreasing stress range, • assumption of a family of over-consolidated failure loci versus a single critically consolidated locus, • and shearing the powder over the wall versus shearing the wall over the powder