Study On The Heat Transfer And Flow Characteristics Of Phase Change Cold Storage Packed Bed And Optimization Of Its Packing Mode

In recent years,with the vigorous development of economic and social construction,the level of building energy consumption in China is also increasing year by year.Air conditioning energy consumption is the main source of building energy consumption,and its load distribution has the objective attribute of periodicity and volatility.Cold storage air conditioning technology is a typical medium to coordinate the supply and demand of energy,which is of great significance to realize the "peak shifting and valley filling" of power and improve the efficiency of power grid.Phase change cold storage packed bed is the most core energy storage component of the cold storage air conditioning system.The existing packed bed generally uses spherical package as the basic energy storage unit,and its specific surface area is small,which is not conducive to the heat exchange of the packed bed.Due to its large specific surface area,red blood cell(RBC)shaped encapsulation provides a research idea for enhancing the heat transfer capacity of packed bed.In addition,the packing mode of capsule in packed bed will also has a direct impact on the flow and heat transfer performance of packed bed.Based on the above problems,considering the different structural characteristics of spherical and RBC shaped capsules,the flow and heat transfer performances of two kinds of capsules based cold storage packed bed are fully compared and analyzed,and the packing mode of capsules in the packed bed is deeply explored and optimized to improve the cold storage performance of phase change cold storage packed bed.Specifically,the research work of this paper mainly includes the following aspects:Firstly,the geometric characteristic parameters of spherical and RBC shaped capsules are extracted,and the capsule models based on the two packing structures are established respectively.Combined with the related derivative structures,their internal phase change heat transfer laws are studied by numerical simulation and experimental verification.It is found that the heat transfer ability of RBC shaped capsules can be enhanced from two aspects: increasing the heat transfer area and shortening the heat transfer path.The calculation shows that the specific surface area of RBC shaped capsule is 34% higher than that of spherical capsule,and the corresponding storage time is also saved by 51.11%.It can be proved that RBC shaped capsule has obvious advantages in the heat transfer ability of capsules.Secondly,based on the theory of lattice densest packing,two kinds of three-dimensional densest packing modes,face-centered cubic(FCC)and hexagonal closest packing(HCP),are proposed.Taking the simplified filling unit as the research object,their flow and heat transfer performance are compared with the conventional aligned dense layer(ADL)by numerical calculation.It is confirmed that the heat transfer performance of HCP and FCC filling unit is significantly improved compared with that of the ADL filling unit,and the FCC filling unit is better.But correspondingly,the flow resistance will inevitably increase.On this basis,the mathematical models of phase change cold storage packed bed based on three packing modes are established and the transient simulation is carried out.The results show that compared with ADL packed bed,the cold storage time of HCP and FCC packed bed is saved by 21.69% and 29.27%,the pressure drop loss is increased by 43.36% and 47.55% respectively.The energy storage density of them is both increased by 22.48%.Accordingly,18.35% space volume of both packed beds can be saved.Therefore,among the three packing modes,FCC packing mode has more obvious advantages.Finally,the mathematical model of the packed bed based on RBC shaped capsules is established.The flow and heat transfer mechanism of the packed bed is explored by numerical simulation,and the influence of changing the inlet temperature and Re of heat transfer fluid(HTF)on the flow and heat transfer performance of the packed bed is analyzed.On this basis,the packing mode of packed bed is optimized by changing the angle of attack θ and the arrangement structure of capsules.The results show that the one-dimensional staggered packed bed with θ = 0° has the highest comprehensive thermal performance coefficient JF,which can reach 1.25.So it is a relatively ideal packing mode.