Research On Gas-Solid Heat Transfer Characteristics And Optimized Design Method In Particle Packed Structure

Particle packed bed is a typical multi-phase complex flow and heat transfer system,which is widely used in industry.During the process of heat transfer and flow performed by this system,the characteristics of the fluid are unstable,the stacking structure is uneven,the mechanism of heat transfer is complex and the heat transfer law is unclear.At present,no unified theory and model description has been formed.In this paper,Taking the particle packed structure as the research object and establishing the perfect heat transfer model,numerical simulation and experiment are used to analyze the heat transfer characteristics of the complex particle packed structure in different heat transfer states(unsteady state and steady state).Furthermore,the optimization models are constructed and the optimization methods of the pakced structure and heat transfer system are discussed.The main research work and results are as follows:1.Based on the theoretical analysis of the heat transfer in the particle packed structure,the mathematical descriptions of the particle packed structure are established under two heat transfer states(the unsteady heat transfer mathematical model and the steady heat transfer mathematical model)by using the local non-heat equilibrium model,as well as the corresponding definite solution conditions.The thermal theories such as heat conduction inside the particles,convective heat transfer by gas-solid,wall effect,thermal dispersion effect and wall heat loss are introduced into the heat transfer research of packed bed.The applicable scope and accuracy of the heat transfer model are improved in particle packed structure.2.An unsteady state heat transfer experiment for a fixed particle bed is carried out to verify the accuracy of the unsteady state heat transfer model for the particle fixed bed and the simulation results.The simulation results show that there is a significant temperature difference between two phases in the fixed bed,which proves the limitations of the local thermal equilibrium model,and it is found that wall effect seriously affects the temperature distribution in packed beds with low D/d.In addition,the effects of particle diameter,particle position,inlet wind speed and other factors on the heat transfer law of the particle packed structure are discussed.A steady state heat transfer experiment of particle packed structure is carried out.The initial conditions and boundary conditions are provided for solving the steady state heat transfer model of particle packed structure,and the accuracy of the model and the results is verified.The influence of thermal dispersion on heat transfer is analyzed by simulation,and the distribution of gas-solid temperature field of the particle packed structure under steady state heat transfer is obtained.The analysis finds that the particle moving speed has an important effect on the heat transfer in grate cooler.3.Based on the analysis of heat transfer characteristics,the particle packed structure is optimized.Four kinds of particle packed structures are constructed by used discrete element method(DEM),namely Randomly-UM,Randomly-CM,Radially-LM,Axially-LM.Through the DEM-CFD coupling.The flow and heat transfer processes of different packed structures are studied by means of simulation and experiment.It is found that the wall effect seriously affects the distribution of the flow field and temperature field in packed bed with low tube-to-particle diameter ratios,resulting in the higher velocity and lower temperature near the wall than in the central region.By comparing different models,It is found that the radial porosity distribution of Radially-LM is more uniform,so that the flow field and temperature field are more evenly distributed,and the influence of wall effect is weakened.Compared with other models,Radially-LM has less flow loss and better heat transfer performance.Finally,the dimensionless correlation of the Radially-LM is fitted.4.The heat transfer characteristics and entropy generation optimization model of the unsteady particle packed heat transfer system are established and analyzed.The effects of particle diameter,air flow,and pressure drop are discussed.Due to the common influence of convective heat transfer resistance and heat transfer resistance of particle,the comprehensive heat transfer coefficient of the particle packed structure varies non-linearly with particle diameter,and there is an optimal particle diameter that maximizes the comprehensive heat transfer coefficient and minimizes the heat transfer time.In addition,particle diameter and air flow have important effects on entropy generation.Based on the proposed unsteady entropy generation model,the multi-objective genetic algorithm is used to optimize the heat transfer system.Under different fan power,the best particle diameter and inlet air flow are obtained.When the fan power is 300W,the optimal particle diameter and air apparent velocity are 0.014m and 2.4m/s respectively.The optimization model provides guidance for the optimization design in particle packed unsteady state heat transfer system.5.The energy flow model of particle packed steady state heat transfer system is established,and the heat transfer system is optimized.Based on the study of the heat transfer characteristics of the particle packed steady-state heat transfer system,it is simplified into a cross-flow heat exchanger network,and the energy flow algorithm is adopted.The its energy flow model based on the inlet temperature of hot and cold media is derived.The equivalent thermal resistance network model and global constraints are established by Kirchhoff’s law.According to this mathematical model,the multi-objective genetic algorithm is used to optimize the heat transfer system with the improved entropy generation numbers caused by heat transfer and viscous dissipation as the targets.After optimization,the fan power consumption is reduced by about 25.44%,and heat recovery Efficiency is increased by 11.35%.In addition,it is found that the change of local heat load distribution will affect the optimal operating parameters.The optimization result of clinker diameter shows that when the diameter is 0.02m,the total fan power consumption is the lowest,which is more economical.The optimization model provides guidance for the optimization design in particle packed steady state heat transfer system.