5.3.2 Translational Velocity Profile 5-107
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The University of New South Wales Faculty of Science School of Materials Science and Engineering Model Studies of Solid Flow and Size Segregation in Packed and Moving Beds Thesis by Shimin Wu Submitted in Partial Fulfillment of the Requirement of the Degree of DOCTOR OF PHILOSOPHY In Materials Science and Engineering July 2007 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Wu First name: Shimin Other name/s: Abbreviation for degree as given in the University calendar: PhD School: Materials Science and Engineering Faculty: Science Signature The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS COPYRIGHT STATEMENT 'I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed Date cD V I ^ AUTHENTICITY STATEMENT 'I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.' Signed Date CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material which is previously published or written by any person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of the university or other institute of higher learning, except where due acknowledgment is made in the text. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged. Date ACKNOWLEDGMENTS I would like to thank my supervisor Prof. Aibing Yu, my co-supervisors Dr. Haiping Zhu and Dr. Paul Zulli for their patience and helpful guidance in the undertaking of this project. I also thank Australia Research Council and BlueScope Steel for providing APAI Scholarship which made this research possible. I would like to extend my thanks to all members in the Center for Simulation and Modelling of Particulate Systems for their help and useful discussion. Finally I would like to thank my wife Xiaofeng Wen for her support and encouragement throughout these years. Ill ABSTRACT This work examines the fundamental behavior of granular materials in packed/moving beds under simplified blast furnace conditions. Such study has a significant impact on the development of new technology such as pulverized coal injection and the performance of blast furnace operation. The top part of a blast furnace covers ore and coke receiving hopper to shaft as a top down approach. The total process of burden distribution can be grossly subdivided into the following major parts: material flow through the charging device and free stream of material trajectory; surface profile and coke collapse; radial size segregation (radial distribution of void fraction); radial gas distribution; burden descent. Experiments have shown that a number of interesting phenomena appear in blast furnace operation. The most extensive phenomenon is the particle size segregation which happens in receiving hopper, central spout, burden formation process and deadman surface, and has been well developed. The most important phenomena in the raw materials discharging process are the coke collapse and gouging. The variation of coke collapse will significantly affect the blast furnace performance. The other phenomena include particle creep motion under surface flow, heap formation during coke/ore free flying and particle percolation during the descending process of burden materials. These phenomena involve rich granular dynamics which currently attract strong interest from a wide scientific and engineering community. However, previous work at this area, limited by the research techniques, is predominantly at large scales focusing on phenomenological descriptions, but rarely touching on the basic fundamentals governing these phenomena. A novel discrete element simulation at an individual particle level can overcome these problems. For this purpose, this work conducts a systematic study of these important phenomena, including crater formation, coke collapse, creep motion and particle percolation, by use of the discrete element method (DEM). The experiments and simulations conducted in the impact of a particle stream onto a particle bed using a 2D slot model suggest that the discrete element method simulation can reproduce the experimental results well under comparative conditions. It is shown that as a result of impact by the falling particles, the particles in the top central region of the particle bed have relatively large velocities and contact forces. The velocities and forces propagate into the bed, and reach the bottom of the base layer quickly. They then continue to propagate leftwards and rightwards to create a crater. The crater size is shown to be affected by the discharging rate, discharging height and materials properties, and be related to the ratio of the input energy from the falling stream to the inertial energy from the original packing. Fundamental understanding of coke collapse based on three different configurations: batch charging, self loading and load impact has been investigated. It was found that collapse process involves weight from the top, particle motion and impact from the top. Collapse can be observed in batch charging top if there is density difference. Static load triggers the initial avalanche. Coke collapse is a kind of continuous avalanche due to top layer particles spreading. The other material properties such as particle size are not found significant to coke collapse. The creep motion of particles in a slot model has been studied based on the results generated by DEM. Experiments are carried out to validate the numerical model. The flow profiles of the surface granular flow and the creep motion of particles in the pile are studied. It is shown that the mean velocity of the surface flow exhibits a linear relationship with depth, while that of the creep motion in the pile decays exponentially with depth and the characteristic length of the decay is on the order of the particle size. The existence of the creep motion can be attributed to the variation of the porosity distribution of the pile. The granular flow on 'frozen' static pile is also investigated to understand the effect of the creep motion on the surface flow. Percolation happens due to both gravity and strain, the strain effect is more significant when the bed starts to move and in the case of larger particle size ratio. The particle percolation rate is increasing with the decreasing of friction coefficient, and is independent of descending velocity while the friction coefficient is smaller than 0.001. More and faster percolation happens near the wall with larger descending velocity. The percolation is definitely directional and vertical penetration occurs much more easily than radial penetration. Rotation is important to percolation. Additionally, this work demonstrates the value of discrete element simulation as a tool for complementing experimental observations. TABLE OF CONTENTS TITLE PAGE I CERTIFICATE OF ORIGINALITY II ACKNOWLEDGMENTS Ill ABSTRACT IV TABLE OF CONTENTS VI LIST OF FIGURES X LIST OF TABLES XIV NOMENCLATURE XV CHAPTER 1 INTRODUCTION 1-17 CHAPTER 2 LITERATURE REVIEW 2-22 2.1 INTRODUCTION 2-23 2.2 BLAST FURNACE BURDEN DISTRIBUTION 2-24 2.2.1 Solid Flow Phenomena in Blast Furnace Top 2-25 2.2.2 Burden Distribution 2-26 2.2.3 Coke Gouging and Mixed Layer Energy 2-32 2.2.4 Coke Collapse Phenomenon 2-33 2.3 IMPACT DYNAMICS OF GRANULAR MATTER 2-37 2.4 GRANULAR SUFACE FLOW AND CREEP MOTION 2-40 2.5 PARTICLE SEGREGATION AND PERCOLATION 2-41 2.6 COMPUTER SIMULATION OF GRANULAR MATERIALS 2-45 VI 2.6.1 Classification of Simulation Methods 2-46 2.6.2 Discrete Element Method (DEM) 2-47 2.6.2.1 Governing equations 2-47 2.6.2.2 Contact forces between particles 2-49 2.6.2.3 Non-contact forces between particles 2-53 2.6.2.4 DEM application 2-55 CHAPTER 3 NUMERICAL INVESTIGATION OF CRATER PHONOMENON .3-56 3.1 INTRODUCTION 3-57 3.2 EXPERIMENTAL METHODS 3-58 3.2.1 Physical Modeling 3-58 3.2.2 Numerical Simulation 3-60 3.3 RESULTS AND DISCUSSION 3-62 3.3.1 Model Validity 3-62 3.3.2 Evolution of Velocity and Force Structure 3-65 3.3.3 Evolution of Energy 3-71 3.3.4 Effects of Geometry and Properties on Crater Size 3-74 3.4 CONCLUSIONS 3-77 CHAPTER 4 DEM STUDY OF COKE COLLAPSE 4-78 4.1 INTRODUCTION 4-79 4.2 METHODS 4-80 4.3 RESULTS AND DISCUSSION 4-83 4.3.1 Experimental Observation 4-83 4.3.2 Simulation Results 4-86 4.3.2.1 Model validation 4-86 VII 4.3.2.2 Batch charging 4-87 4.3.2.3 Self loading 4-91 4.3.2.4.