The Study On Fluid Flow And Heat Transfer Performance In Packed Bed

Particle packed bed is very important in many engineering applications because the structure is simple and the operation is convenient,however,the low heat transfer coefficient and the high pressure drop limits its further application in the heat transfer field.So it is great significance to effectively change the flow characteristics,enhance the heat transfer performance and improve the heat exchange between gas and solid particle in the bed for guiding the application of the packed bed in the heat transfer field.In this thesis,the experimental method is adopted to study the characteristics of fluid flow in the packed bed,in which the study mainly focused on the flow characteristics of packed beds filled with particles with the same diameter,and the packed beds filled with particles with different diameter respectively.The results show that when the ratio of tube diameter to the particle diameter is between 8 and 13.3,the wall effect of packed bed filled with particles with the same diameter is obvious.While for the packed beds filled with particles with different diameter,compared with random mixing of particles with different particle sizes,the radial distribution of multi particle size can improve the pressure drop greatly,and the permeability of the bed is smaller as well,which means there exist small fluid flow channels,and the distribution of the fluid in the bed is uniform,and the wall effects can be restrained in some extent.It can be founded from the ratio Xi of inertia loss to the total pressure drop that the flow state is between laminar flow and transitional flow within the fluid velocity studied in the bed,and there is certain influence of arrangement of particles on the critical velocity that flow state transform from laminar flow to transitional flow;moreover,the inertia loss increase with fluid velocity,which leads to the great pressure drop difference between the different arrangements of particles.In order to explore the heat transfer characteristics in the packed bed and reveal the influence of flow characteristics on the heat transfer performance of the bed,numerical simulation method is adopted to study the performance of packed bed with diameter 40 mm and height 180 mm.Taking the overall performance and entransy dissipation as the evaluation criterion,the three-dimensional numerical simulation is carried out to analyze the flow and heat transfer characteristics of packed bed under different particle size distribution and particle size ratio.The flow and heat transfer process between gas and solid particles is solve by Computational Fluid Dynamics(CFD)coupled with Discrete Element Method(DEM).The gas is treated as continuous phase and the particle is treated as discrete phase.The results show that,the wall effect is obvious in the packed beds filled with particles with uniform particle size;which leads to terrible heat transfer performance in the core region of the beds and low gas temperature at outlet.Compared with the uniform particle size distribution,radial distribution of multi particle size can effectively improve the heat transfer performance because it changes the velocity and temperature field,reduces the equivalent thermal resistance of convection heat transfer process,and the temperature of outlet gas increases significantly,at the same time,the entransy dissipation increases with the outlet gas temperature,which means more irreversible loss.And for the multi particle size packed bed with the same particle size distribution,the result shows that the distribution thickness along the radial small particles also has a certain influence on the heat transfer process.