Doctor of Philosophy
School of Mechanical, Materials & Mechatronic Engineering
Grima, Andrew Phillip, Quantifying and modelling mechanisms of flow in cohesionless and cohesive granular materials, Doctor of Philosophy thesis, School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3425
Discrete element modelling is gradually becoming more popular as a validation tool by industry to simulate bulk flow in bulk material handling and processing equipment. Although discrete element method (DEM) is a popular numerical method in academia to model the complex interactions in particle systems, the demand for the application of DEM to industrial problems is high. The use of DEM simulations to evaluate the flow of bulk materials through belt conveyor transfer points is popular to assist designers to trouble-shoot and quantify the functionality of a design. DEM does not directly design bulk material handling and processing equipment or replace design know-how but DEM simulation is an alternative approach to model particle flow that has traditionally been analysed using analytical methods, empirical design rules and scale model physical prototyping. Calibrated DEM simulations provide additional confidence of the performance of equipment and improve the efficiency and life of equipment due to the significant amounts of quantitative data available to provide insight into the complex physical interactions that occur.
Recently there has been a growth of available commercial DEM programs such as EDEM®, which has been utilised in this study, but calibration and validation of DEM simulations still requires further investigation to adequately characterise and model cohesionless and cohesive bulk materials with realism. Insufficient calibration of DEM models can lead to unrealistic predictions making DEM a misleading design validation tool and may lead to poor performing equipment that can be costly to repair.
The purpose of this research was to develop techniques to calibrate DEM models based on physical bench scale tests which reproduce the bulk flow behaviours to be modelled on a larger industrial scale. Cohesionless and cohesive bulk materials including polyethylene pellets, coal and bauxite were examined to characterise and develop a set of realistic particle-to-particle and particle-to-boundary interaction parameters for DEM simulations. The accuracy of the characterisation and calibration procedures were examined by modelling the impingement of polyethylene pellets against a flat impact plate using a novel conveyor transfer research facility where experimental quantitative and qualitative data was compared against the DEM simulations and analytical methods.
Additional to the latter experimental test program, DEM simulations were conducted on a large scale industrial transfer point to model the flow of wet and sticky bauxite where results from the DEM models were evaluated against the limited quantitative information measured and observations from an existing transfer station.
The aim of this thesis was to provide industry with some validated techniques to develop a calibrated DEM model to simulate the flow of bulk materials through complex transfer points and examine the accuracy of the predictions and limitations of DEM. It has been found that some DEM parameters are sensitive, especially when modelling cohesive materials using a simple linear cohesion and the more complex Johnson, Kendall and Roberts (JKR) contact model requiring careful selection of contact values.
Analytical methods to evaluate the flow and trajectory of bulk material against a flat impact plate and through a straight and curved chute have been evaluated to verify the DEM predictions and experimental results. As the analytical methods are based on a 2-D analysis of flow or lumped mass analogy, limitations of the analytical methods to predict complex 3-D flow were discovered in this research that are frequently used by industry.
The thesis also investigates the measurement of wall friction angles between a bulk material and wall surface using the standard Jenike direct shear tester and a novel large scale wall friction tester (LSWFT) to examine the scale-up effects of the particle size distribution and shear cell. Materials that were examined on the latter machines included magnetite concentrate, polyethylene pellets and bauxite, which have different particle and bulk properties. Various wall materials were also examined including stainless steel 304-2B, alumina ceramic tiles, Matrox and Bisplate® 400. Findings from this test work showed there were variations between the measured wall friction angles between the two machines for various bulk material and wall sample combinations. The LSWFT displayed better capability and reliability to measure wall friction angles of bulk materials consisting of particles greater than 4 mm due to the larger shear cell size and greater shear strain. Accurate measurements of wall friction angles and modelling of the particle-to-boundary interactions in DEM simulations have shown to be imperative when designing equipment for reliable flow to occur.
It can be concluded that DEM calibration and adequately modelling non-spherical particles by clustering balls together are essential for successful and reliable application of DEM simulations to industrial applications.