Numerical Study On Structure Optimization Of A Packed Bed Heat Storage Device For Flue Gas Waste Heat Recovery

Heat storage technology is an effective way to adjust the load and time scale mismatch between heating equipment and heat users.It is also conducive to improving energy efficiency and protecting the environment.With the development of heat storage technology,the advantages of phase change heat storage over sensible heat storage gradually appear,such as higher energy storage density.By comparing different heat storage unit forms,spherical encapsulated heat storage unit has the advantages of high energy storage density,simple preparation,etc.,and has been widely used in latent heat storage packed bed.In this paper,the waste heat recovery of flue gas is taken as the goal to build a heat storage system,and the heat storage and release characteristics of the packed bed constructed with the heat storage unit of spherical encapsulated phase change materials are numerically simulated.The heat storage and release characteristics of the optimized single ball and integral packed bed are simulated,and the heat transfer process is analyzed by setting the corresponding variables according to the instability of the flue gas recovery process.The main contents are as follows:First,a physical model of spherical phase change heat storage unit with a certain amount of air was established,and the heat transfer characteristics of the phase change material in the phase change unit was analyzed.Based on the Solidification and Melting model in Fluent software,the Volume of Fluid(VOF)model is introduced,and the change of the free interface between the PCM and air inside the sphere is obtained by considering the change of the volume of the PCM.In the heat storage process,the melting trend of phase change materials is to melt against the direction of the heat transfer fluid flow.In the process of heat release,the free interface between phase change material and air moves downward in the shape of an inverted trapezoid.The main heat transfer mechanism of phase change materials at different stages was analyzed by observing the changes of flow field and heat flux inside the sphere.Secondly,based on the single sphere heat transfer analysis,the influence of the characteristic size d/D on the heat transfer process of small spheres was studied by using the Nusselt number,surface heat flux,and resistance coefficient as analysis indicators.In the new structure,the characteristic sizes of the small spheres were 0.15,0.25,and 0.35,respectively.During the charging process,the average melting rate of the PCM increased by about 20% with the increase of the characteristic size.During the discharging process,the average solidification rate of the PCM increased by about 40.24% with the increase of the characteristic size.This is mainly because the larger the characteristic size d/D,the greater the contact area between the heat transfer fluid and the small sphere,and the higher the degree of natural convection heat transfer enhancement.Finally,the heat storage and release process of a packed bed made of spherical phase change capsules was simulated.The influences of the structure of heat storage unit,the arrangement of phase change spheres,the inlet velocity of heat transfer fluid and other factors on the liquid phase rate,thermocline,heat storage efficiency and heat storage were investigated.The study shows that the spherical phase change capsule with center pore improves the heat storage and release efficiency and reduces the heat storage and release time at each temperature measuring point in the axial direction of the packed bed compared with the integral packed bed filled with the capsule with ordinary structure.In the spherical capsule filled bed with center hole,the heat storage and release time of the anterorow arrangement was shortened by 4.34%(A2)and12.4%(D2),respectively,compared with that of the intercalated arrangement.In the packed bed filled with spherical capsules with central pore,the difference of heat storage time between the two arrangements is greater with the distance from the entrance of the packed bed.The inlet velocity was increased from 0.1m/s to 0.5m/s,and the heat storage time was shortened by 286 min.