Simulation Of Heat Transfer Of Bubbling Bed Based On A Dynamic Dual Grid Method

The bubbling fluidized bed can continuously generate bubbles during the fluidization process,which can enhance the mixing between particles and the heat transfer between the gas and solid phases.Therefore,the bubbling fluidized bed with excellent heat and mass transfer performance and continuous operation characteristics has received extensive attention in the field of energy and chemical industry.However the multi-scale nonlinear interphase interaction law between the gas-solid two phases is not fully understood yet,which leads to more difficulties in the design and amplification of the equipment.Therefore,the research of the gas-solid two-phase flow and heat transfer characteristics will help to the control and optimization of related industrial processes.Aiming at the problem that the single-grid method limits the accuracy of the gas-solid two-phase flow and heat transfer calculation results in the fluidized bed,a CFD-DEM numerical calculation model based on the dynamic dual grid method is established to improve the accuracy of the calculation results.Through the comparison of mixing experiments and simulation studies to fluidized bed,the accuracy of the dynamic dual grid method CFD-DEM model is verified.On this basis,the particle flow characteristics in the stagnation zone of the rectangular spouted bed are analyzed,the bubbling fluidized bed of different initial accumulation states and different material particles are researched.First,the Fortran language was used to improve the original CFD-DEM program of the research team,the CFD-DEM model of gas-solid two-phase flow based on the dynamic dual grid method was established.The radial mixing experiment and numerical simulation research were carried out,at the same time,the calculation results were compared with the calculation results of the single grid method.The calculation time of the two grid models and the characteristics of the stagnation zone in the bed were analyzed.The dynamic dual grid method is used to simulate the flow process in the stagnant zone of a spouted bed under different inlet gas velocities and different initial accumulation heights.The results show that the results of fluidized bed mixing experiment and numerical simulation are highly consistent.Compared with the traditional single grid model,the dynamic dual grid CFD-DEM model established in this paper can simulate the flow state of spherical particles more accurately.The calculation accuracy is increased while the calculation time is increased by about 20%.The dynamic dual grid method can effectively solve the contradiction in the calculation of physical quantities in the CFD-DEM coupling calculation.When the initial accumulation height remains unchanged,as the inlet speed increases,the stagnation zone height decreases,but there is no obvious change in the speed and the extension speed to the nozzle.When the air inlet speed is unchanged,with the increase of the initial accumulation height,the particle falling speed in the stagnation zone also increases,but the extension speed to the nozzle gradually slows down.Then,based on the gas-solid two-phase flow CFD-DEM model with the dynamic dual grid method,to solve the problem of the partial uniformity of the particle temperature in the bubbling fluidized bed,the flow and heat transfer study of the bed under different initial accumulation states were carried out.The particle velocity,collision frequency,the overall particle temperature of the bed,the particle temperature distribution characteristics in the upper regions,stagnation zone,near-wall area were analyzed.The heat transfer coefficient and heat transfer related parameters also were analyzed.The results show that in the initial fluidization stage of the bed layer,the particle velocity and collision frequency are higher,the area with higher collision frequency is located near the two side walls.At the same time,the gas-solid two-phase temperature difference and the particle temperature rise rate are also larger,the particle mixing and heat transfer are also more intense.As time goes on,the particle velocity and collision tend to be stable,the particle temperature rise rate and total heat transfer rate also continue to decrease.The average temperature of the particles in the three bed layers changes close to the same throughout the fluidization process,rising by 70℃in 0-10s.The bed can be divided into upper and lower regions along the average expansion height,the temperature of the particles in the upper region is significantly lower than that in the lower region,the temperature difference between the upper and lower regions of the bed is about 5℃at 10 s.The particle temperature near the wall increases along with the height increasing of the bed,which is opposite to the temperature change trend of the upper and lower areas of the bed and will enlarge the temperature difference between the upper and lower areas of the bed.The temperature of the particles in the stagnation zone is lower,the temperature difference is 10℃compared with the average temperature of the bed.As the number of particles accumulated in the implantation layer increases,the heat transfer coefficient and heat transfer of the particles inside the bed show a continuous decreasing trend.Among the three heat transfer methods,the convective heat transfer accounts for the largest proportion,accounting for approximately 96%of the total heat exchange rate at 10 s,the proportion of radiant heat transfer and heat transfer has been increasing over time,of which the proportion of radiant heat exchange has increased from 0.3%to 3.5%,the proportion of conduction heat has increased from 0.01%to 0.1%.Finally,using the CFD-DEM model based on the dynamic dual grid method,the flow and heat transfer simulation calculations of the three explicit heat storage material particles including MgO,Al and Si O₂ are carried out to research heat transfer characteristics of different materials in a bubbling fluidized bed.The particle velocity,collision frequency,particle temperature distribution characteristics of the entire bed,upper regions,lower regions,and the stagnation regions are analyzed.The heat transfer coefficient and heat transfer related parameters are analyzed.The heat storage and release characteristics of different materials are also analyzed.The research results show that the temperature and heat transfer coefficient of the particles in the bed increase with the change of MgO,Al and Si O₂ materials,while the heat transfer of the particles decrease.For the three materials,the proportion of convective heat transfer in 0-5 s,the value changes close to each other,in 6-10 s,the convective heat transfer of MgO material is the largest.As time goes by,the ratio of radiant heat transfer and heat transfer continues to increase.In the process of heat storage and release,the temperature rise of Si O₂ material is the fastest,its heat release is also fast,but there is a large temperature difference in the heat storage and release process of the bed,while the temperature rise of MgO material is slower,its heat release is slower,but the temperature difference of the bed particles is smaller.