| The pellets have the properties of high compressive,uniform quality and low energy consumption and have been used in the iron and steel industry on a large scale.Grate is a thermal equipment for drying,dehydration,preheating and oxidation of pellets,the distribution of temperature field in the grate has a direct effect on the quality of pellets and the effective utilization of heat energy.The drying and convection heat transfer in the grate is affected not only by temperature field,but also by multi-physics coupling such as airflow velocity field and pressure field.Therefore,this dissertation focused on the up-drought drying of grate,and investigated the influence of multi-field coupling of inlet temperature,airflow velocity,air pressure and thickness of pellet material layer on the temperature field of up-drought drying.Firstly,the research characteristics of iron ore sintering and pellet bed drying preheating at home and abroad are comprehensively analyzed.The pellet material layer is abstracted as an isotropic porous medium and adopted the local non thermal equilibrium model to describe the convective heat transfer between pellet and hot air flow.Multi-physical field coupling mathematical model for up-drought drying of grate machine is established based on heat transfer theory,porous media theory,computational fluid dynamics theory and drying dynamics theory,including gas control equation,equation of pellet material layer control,simplified model of grate bed,surface drying model and internal drying model of pellets.Secondly,reasonably simplified the heat and mass transfer process of pellets,established physical model of two-dimensional unsteady heat transfer in air drying section of pellet grate and discretized the region.Compiling the calculation function of the source term in a mathematical model based on Ansys Fluent software's own user-defined function(UDF).Compiled and added to Fluent software,the mathematical model was simulated,the temperature distribution,airflow velocity distribution and pressure distribution in the up-drought drying of grate were obtained and analyzed theoretically.The numerical simulation results are verified by the test data of pelletizing plant,the maximum error is 8.6%,which indicates that the mathematical model can be applied to engineering practice.Lastly,the factors such as inlet air temperature,airflow velocity,air pressure and pellet material layer were selected as orthogonal test factors,and determined the level value of each test factor according to the theory of orthogonal test.The orthogonal test was carried out by orthogonal test with four factors and three levels,the results were analyzed by range analysis.The results show that the main factor affecting the temperature field in the lower part of the grate bed is the air temperature of the inlet,followed by the thickness of the pellet material layer,and then the velocity and pressure of the air flow;the main factor affecting the temperature field of grate plate is inlet air temperature,followed by air velocity and then the thickness of pellet material layer and air pressure;the main factor affecting the temperature field of pellets is inlet air temperature,followed by the thickness of pellet material layer,and then the air pressure and airflow velocity;the main factor affecting the temperature field in the upper part of the pellet is inlet air temperature,followed by the thickness of pellet material layer,and then the air velocity and air pressure.The research of this topic has provided theoretical basis for the accurate control of the temperature field in a grate.It has reference value for reducing energy consumption,improving pellet production quality and operational life span of chain grate in modern pelletizing enterprises in China. |
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