Study On Cold Storage Characteristics Of Dodecane/EG Shape-stabilized Composite Phase Change Material

With the development of economy,people's living standards are also improving,the demand for fresh food has become increasing.The cold storage technology(CST)has become got more and more attention.The CST can adjust the environmental temperature during the state change process of the working medium which can store or release the heat energy.The phase change material(PCM)which can store or release latent heat used as working medium in CST shows the advantages of high thermal energy storage density,approximate constant temperature during the phase change process and no driving force.At present,the phase change thermal energy storage has become one of the most concerned CSTs.PCM is the most important factor of the phase change CST.Dodecane,which has the phase change temperature of about-10 °C,becomes one of the preferred PCM for CST due to its suitable phase change temperature,high latent heat,no supercooling.However,the shortcomings of low thermal conductivity and liquid leakage of dodecane limit its application in CST.In view of the defects of dodecane,this work intends to synergetically enhance the thermal conductivity and prevent the liquid leakage of dodecane by expanded graphite(EG)which features the high thermal conductivity,porosity and light weight.The heat transfer properties of the prepared dodecane/EG composite PCM are also studied.The shape-stabilized PCMs with dodecane as PCM and EG as skeleton material used in CST system was developed firstly.The optimal content of EG in composite was confirmed as 16wt% by heat treatment and DSC testing.The DSC results show that the shape-stabilized PCM has a phase change temperature of-9.67 ?C and latent heat of 151.7 J/g.The microstructure of the shape-stabilized PCM was characterized by SEM,FT-IR and specific surface area analyzer,and the results show that the dodecane is uniformly stuffed in the pore structure of EG under the physical action.Thermal conductivity test result presents that the thermal conductivity of the shape-stabilized PCM is 15 times higher than that of dodecane.And the thermal conductivity enhancement of shape-stabilized PCM was also verified by the thermal performance test.The thermal conductivity and liquid leakage of dodecane were improved synergetically.The thermal conductivity of phase change material is an important parameter that affects its heat transfer characteristics.Based on the fractal theory,the prediction model of effective thermal conductivity of dodecane / EG shaped phase change material is derived and solved.According to the Sierpinski carpet model in fractal geometry,a series of fractal elements with different fractal dimensions were constructed for dodecane / EG shaped phase change materials.Based on the principle of self similarity and the theory of thermal resistance network,the general solution of dimensionless effective thermal conductivity of typical Sierpinski carpet fractal element is derived.Furthermore,the dimensionless effective thermal conductivity prediction model of dodecane/EG shaped phase change materials is obtained by comparing with the experimental data.The dodecane/EG shaped phase change materials were used in the phase change cold storage unit.The thermal storage and release characteristics of dodecane/EG shaped phase change materials were studied by numerical simulation.On this basis,the paper proposes to optimize the characteristics of phase change cold storage unit by using double-layer phase change material structure.The mathematical model of doublelayer phase change cold storage unit is established.The simulation results show that when the layer thickness ratio is the same,properly increasing the difference between the phase change temperature of outer layer phase change material and inner layer phase change material can achieve the same temperature control effect,but the larger the phase change temperature difference is,the lower the temperature control effect will be;when the phase change temperature difference is the same,the greater the layer thickness ratio of phase change material,when the phase change temperature difference can be extended to 3K.Therefore,the selection range of phase change materials can be expanded by changing the layer thickness ratio.