Study On The Characteristics Of Flow And Heat Transfer Of Urea Slurry For Cold Storage

Using phase change energy storage technology,the cold storage air conditioning system can shift the peak load and fill the valley,effectively improve the energy utilization efficiency of the system,and alleviate the power shortage.Among them,the phase change cold storage slurry has good fluidity and high cold storage density,which can be used as both cold storage material and secondary refrigerant for cooling capacity transmission,thus reducing the amount of refrigerant charging in the system and improving the operation efficiency of the system.At present,urea slurry,a new type of slurry for cold storage,has attracted people’s attention due to its suitable phase transition temperature,large cold storage/cooling capacity and low cost.However,there are few relevant studies on its cold storage and heat transfer characteristics,and the basic data is insufficient.In this paper,based on the phase diagram of urea aqueous solution,the urea slurry with phase transition temperature of 5℃～12℃ was prepared,and its flow heat transfer characteristics and cold storage performance were experimentally studied.As the basis for the study of the characteristics of urea slurry,the thermophysical properties and crystallization characteristics of urea aqueous solution were firstly experimentally studied.The thermophysical parameters include phase change temperature,effective latent heat,density,kinematic viscosity and so on.According to the measurement results,the phase change temperature of urea solution with mass concentration of 43 wt.%～48 wt.%is 5℃～12℃,and the effective latent heat is 213.7k J/kg～223.2 k J/kg.When the mass concentration of the solution is greater than 32.5wt.%,the density of the urea solution increases with the increase of the concentration at the liquidus,and the kinematic viscosity of the solution decreases with the increase of the concentration.Then,the flow test section of urea slurry was built,and the effects of Reynolds number Re,slurry mass fraction X_u and pipe diameter D_in on the slurry flow characteristics were analyzed.The flow pressure drop of urea slurry increases with the increase of Re and X_u,and decreases with the increase of D_in.When analyzing the Non-Newtonian fluid characteristics of slurry,the pipeline friction coefficientlis introduced,in whichl/l₀ is all greater than 1.Under the condition of low Re and high X_u,the value ofl/l₀ is large,and the separation of slurry in the pipe is serious,which deviates far from the characteristics of Newtonian fluid.Then,the Power-Law model was used to logarithmically fit the shear stress and shear rate of urea slurry.The flow characteristic index n was greater than 1,and the urea slurry presented the characteristics of dilatant fluid.Under different working conditions,n decreases with the increase of X_u,and the consistency coefficient K’increases with the increase of X_u.Generally,when the pipe diameter increases,the n value increases and the K’value decreases.By introducing the modified Reynold number Re _MR and the empirical correlation formula of the friction coefficient of the Power-Law fluid,from the fitting results ofland Re_MR,it can be seen that the turning Reynolds number of urea slurry in 6 mm and 8 mm tubes are 2000～2300and 1800～2200,respectively.At the end of this paper,the heat transfer characteristics of urea slurry in a 6 mm horizontal copper tube are experimentally studied,and the effects of heat flux q,Re and X_u on the local heat transfer coefficient of slurry,i.e.h_local and Nusselt number ratio,Nu_exp/Nu₀,are analyzed.Among them,h_local decreases first and then increases slowly along the pipe flow direction,and increases with the increase of X_u,q and Re.Nu_exp/Nu₀is introduced.When Re is 2000,Nu_exp/Nu₀ is larger and changes obviously with X_u;when Re is 6000,Nu_exp/Nu₀ is close to 1 and changes slightly with X_u.To sum up,urea slurry has the advantages of low supercooling degree,suitable phase transition temperature,high storage density,low cost and good flow heat transfer,which has a broad application prospect in the field of cold storage air conditioning.