EXPERIMENTAL AND DISCRETE ELEMENT MODELLING INVESTIGATION OF FLOW BEHAVIOUR OF COHESIVE POWDERS UNDER VARYING CONSOLIDATION STATES

GRANULAR MATERIALS FIND USE IN MANY INDUSTRIAL PROCESSES, SUCH AS MIXING, COATING, GRANULATION, MILLING, TRANSPORT, & DISCHARGE. FOR THESE PROCESSES TO WORK EFFICIENTLY & WITHOUT STOPPAGES, IT IS NECESSARY TO EVALUATE FLOWABILITY OF THE POWDERS FOR DESIGN & OPERATION OF THE BULK SOLIDS HANDLING EQUIPMENT. FLOWABILITY OF POWDERS IS GENERALLY EVALUATED USING THE METHODOLOGY PROPOSED BY JENIKE, WHICH IS OFTEN REFERRED TO AS THE SHEAR TESTING METHODOLOGY. THE SHEAR TESTING METHODOLOGY INVOLVES FIRST SHEARING A POWDER SAMPLE UNDER A DEFINED NORMAL STRESS TO A CRITICAL SHEAR CONSOLIDATION STATE CHARACTERIZED BY CONSTANT SHEAR STRESS & BULK DENSITY, FOLLOWED BY MEASURING THE SHEAR STRENGTH OF THE POWDER BY SHEARING THE POWDER SAMPLE UNTIL FAILURE UNDER A LOWER NORMAL STRESS. HOWEVER, THERE ARE MANY HANDLING SCENARIOS IN INDUSTRY IN WHICH THE POWDERS MIGHT NOT BE SUBJECTED TO FLOW REGIMES LEADING TO CRITICAL SHEARING STATES, SUCH AS IN DOSING, POWDER FEEDERS FOR TABLETING, AND SMALL-SCALE SILOS FOR STORAGE & DISCHARGE. IN SUCH SCENARIOS WHERE INTERNAL SHEARING DEFORMATION OF THE POWDERS IS LIMITED DURING FLOW, THE POWDERS MAY NOT ACHIEVE CRITICAL CONSOLIDATION CONDITIONS & INSTEAD BE ONLY PARTIALLY CONSOLIDATED, IT IS PLAUSIBLE THAT THE JENIKE METHOD OF FLOWABILITY ASSESSMENT MAY NOT APPLY. THIS STUDY CONSIDERS THE POSSIBILITY OF POWDERS TO BE PARTIALLY CONSOLIDATED & INVESTIGATES THE IMPACT OF CONSOLIDATION STATE ON POWDER FLOWABILITY. A FLOWABILITY CHARACTERIZATION PROCEDURE TO TEST POWDER SPECIMENS UNDER VARYING CONSOLIDATION STATES WAS DEVELOPED USING THE SCHULZE RING SHEAR TESTER (RST). FLOWABILITY OF TWO COHESIVE POWDERS AT STEADY & PARTIALLY CONSOLIDATED STATES WERE CHARACTERIZED USING RST, AND FLAT-BOTTOM MODEL SILO EXPERIMENTS WERE CONDUCTED TO STUDY THE INFLUENCE OF CONSOLIDATION IN POWDER FLOW. THE RST RESULTS SHOW THAT THE EXTENT OF SHEAR DEFORMATION & THE RESULTANT CONSOLIDATION STATE IMPACTED THE YIELD LOCI & STRENGTH OF THE POWDERS, AND IT MARGINALLY IMPACTED THE FLOW FUNCTION AND THUS THE FLOWABILITY, THE LATTER FINDING WAS ALSO CONFIRMED BY THE SILO EXPERIMENTATION RESULTS. THIS STUDY SHOWS THAT THE POWDER FLOWABILITY EVALUATED AT CRITICAL CONSOLIDATION IS A GOOD APPROXIMATION OF THE FLOWABILITY AT PARTIALLY CONSOLIDATED STATES. TO STUDY THE PARTICLE LEVEL PHENOMENA UNDERLYING THE BULK BEHAVIOUR OBSERVED DURING SHEARING & FLOW, THIS RESEARCH FURTHER INVESTIGATED THE POWDER FLOW IN SHEAR TESTS & IN THE MODEL SILO USING THE DISCRETE ELEMENT METHOD (DEM). AS IT IS COMPUTATIONALLY PROHIBITIVE TO SIMULATE POWDER FLOW PROBLEMS AT THE REAL PARTICLE SCALE FOR MICRON-SIZED PARTICLES, A MESO-SCALE OR COARSE-GRAINING APPROACH WAS ADOPTED TO SCALE UP THE PARTICLE SIZE TO REDUCE THE NUMBER OF PARTICLES IN THE SYSTEM & TO SIMPLIFY THE PARTICLE SHAPE REPRESENTATION. PRIOR COARSE-GRAINING STUDIES HAVE NOT BEEN ABLE TO REPRODUCE THE EXPERIMENTAL FLOW FUNCTION WHICH IS OBTAINED BY CONDUCTING THE SHEAR TEST AT VARYING LEVELS OF PRE-SHEAR NORMAL LOAD, REQUIRING STRESS HISTORY DEPENDENT/SENSITIVE BEHAVIOUR OF THE MODELLED PARTICLE SYSTEM. IN THIS THESIS, THE MESO-SCALE COHESIVE DEM MODEL OF THE SHEAR TESTING TO REPRODUCE THE FULL FLOW FUNCTION & TO STUDY PARTICLE LEVEL PHENOMENA WAS DEVELOPED USING THE EEPA CONTACT MODEL & CALIBRATED USING EXPERIMENTAL DATA. THE CALIBRATED DEM MODEL OBTAINED BY THE ADOPTED MESO-SCALE & CALIBRATION APPROACH IS ABLE TO PRODUCE THE STRESS HISTORY DEPENDENT FLOW BEHAVIOUR FOR CRITICAL & PARTIAL CONSOLIDATION STATES & THE FULL FLOW FUNCTION FOR THE CRITICAL STATE. THE BULK DENSITY, VELOCITY & BULK STRESSES WERE EVALUATED FROM THE DISCRETE PARTICLE DATA & USED TO STUDY PARTICLE LEVEL PHENOMENA SUCH AS SHEAR STRAIN EVOLUTION, & PRINCIPAL PLANE ORIENTATION IN THE LOCALIZED ZONES IN THE POWDER BED. THE COARSE-GRAINING TECHNIQUE WAS FOUND TO BE A USEFUL MODELLING TOOL TO MODEL COHESIVE POWDERS FOR CONSERVING OR REPRODUCING THE FLOW CHARACTERISTICS OF THE ORIGINAL SYSTEM WHILST REDUCING THE COMPUTATIONAL COSTS.