Study On The Effect Of The Parameters Of Particle Stacking Characteristics On The Heat Transfer Of Packed Bed

With the development of society,people’s dependence on energy is increasing,resulting in a rapid decrease in the earth’s fossil energy reserves.Large output of high-temperature solid particles in industrial production of China,which contains abundant waste heat resources.If it can be effectively used,it will be of great significance to improve energy efficiency,reduce unit energy consumption and promote the construction of ecological civilization.The waste heat recovery process of high-temperature solid particles is comprehensively affected by particle properties,particle spatial structure,heat exchanger structure and operation parameters,and there is still a lack of systematic research.In this paper,the direct heat transfer characteristics of packed bed are taken as the research object,and the steady-state heat transfer model of regular packed bed is established.Simple cubic stack,oblique cube stack,wedge tetrahedron stack and rhombohedron stack were selected as the basic research conditions.The usability of the model was verified by using the self-built steady-state heat transfer experiment system with simple cubic stack.Through the analysis of the factors affecting the stacking structure,a series of heat transfer models are established on the basis of the above stacking structure.The effects of four regular stack structures,bed thickness,particle contact angle and particle contact number on heat transfer characteristics of packed bed were studied by numerical simulation.The main conclusions are as follows:(1)The particle stacking method has a significant impact on the heat transfer characteristics of the packed bed.The apparent thermal resistance of the oblique cube stack is the largest,which is 1.77 times that of the rhombohedron stack.Due to the different particle accumulation methods,the difference in the gas-solid area ratio leads to a trend of group changes in the wall heat transfer ratio.The ratio of radiation heat transfer at the hot wall is 0.37～0.53,and the effect of radiation is relatively large and cannot be ignored.The ratio of radiation heat transfer on the gas-solid coupling surface of the particle layer is all above 0.5(except for the particles attached to the cooling wall),and the highest near the hot wall is 0.95.It is incomplete to express the above influence by only using the porosity.It is necessary to comprehensively consider the influence of the heat transfer distance,particle contact angle,and particle contact number of the packed bed with different accumulation methods.(2)The apparent thermal resistance of packed beds with different accumulation methods has a nearly linear relationship with the number of intermediate particle layers.Among them,the slope of the oblique cube stack is the largest,and the apparent thermal resistance is increased by about 3.2 times.The rhombohedron stack with the smallest slope also increased by 2.3 times.As the number of intermediate particle layers increases,the temperature difference between the adherent particles and the wall decreases.The radiation heat transfer of the hot wall takes up a relatively large proportion,all greater than 0.34,and it first increases and then decreases as the number of intermediate particle layers increases.(3)The apparent thermal resistance of the packed bed has a cosine function relationship with the contact angle(α)in a plane parallel to the overall direction of the heat flow,and the change is small in the range of 0°-25°.As α increases,the apparent thermal resistance is reduced by 2.5%.The heat flux of the gas-solid coupling surface of the particle layer increases,and the proportion of the radiation heat transfer of the gas-solid coupling surface decreases.The ratio of radiation heat transfer at the wall does not change much.The apparent thermal resistance of the packed bed is in a trigonometric function relationship with the contact angle(β)in the plane perpendicular to the overall heat flow direction.When β increases from 0° to 30°,the apparent thermal resistance increases by10.1%.Compared with the particle contact angle α,the change caused by β is more obvious.With the increase of β,the gas-phase heat transfer area on the wall is reduced by14%,and the particle stacking structure is closer,resulting in it more difficult for radiation heat transfer to pass across the particle layer.(4)The decrease in the number of particles in contact has a very obvious effect on the apparent thermal resistance of the packed bed,which can increase by up to 30%.The decrease in the number of particles near the cold wall has a greater impact.Radiation heat transfer makes up for part of the reduction in heat transfer caused by the lack of particles in high-temperature areas.The impact of missing particles in the central spherical layer is minimal.The combination of the missing two particles shows that the apparent thermal resistance increase caused by the missing particles in the close-packed layer at the same time is the most obvious.The closer the missing two particles are,the more obvious the weakening effect on the radiation shielding in the high-temperature zone,which is conducive to the radiation heat exchange with the lower temperature particle layer,thus resulting in the apparent thermal resistance of the packed bed smaller.