Experimental Investigation On The Convective Heat Transfer Characteristics Of Microencapsulated Phase Change Material Slurries In Mini-channel

Lithium-ion batteries are prone to thermal runaway under harsh conditions such as high ambient temperature and high charge and discharge rate,which leads to safety problems of electric vehicles.Therefore,the thermal safety of lithium-ion batteries is one of the main factors restrict the development of electric vehicle industry.In order to avoid the accumulation of heat in the battery,this paper aims at the battery thermal management using liquid as the heat transfer medium.Combining the characteristics of the microencapsulated phase change material(During the phase change process,the temperature is approximately constant but the latent heat of the phase change is large),two kinds of microencapsulated phase change material slurries were prepared,and the experimental bench to test the convection heat transfer in the mini-channel was built.The effects of Reynolds number,Stephen number and capsule mass fraction on the convective heat transfer characteristics and temperature distribution of the mini-channel were studied.The main contents and conclusions are as follows:(1)Prepared the microencapsulated phase change material slurries by three kinds of microencapsulated phase change materials(Paraffin/Silica,Eicosane/Polymethy methacrylate,Paraffin/Urea formaldehyde resin).Due to the thermal instability of paraffin/urea formaldehyde resin,stable microencapsulated phase change material slurries with different mass fractions(5 wt.%,10 wt.%,20 wt.% and 30 wt.%)were prepared by Paraffin/Silica and Eicosane/Polymethy methacrylate respectively.The relative error of thermal conductivity between measured value and calculated value by Maxwell formula was less than 3%,the thermal conductivity increased with temperature.In accordance with Vand's viscosity formula,when the mass fraction was greater than 20 wt.%,the viscosity of slurries increased exponentially.(2)The effects of Reynolds number,Stephen number and mass fraction on the convective heat transfer characteristics were investigated.An experimental bench for laminar convection heat transfer test was built with a straight tube of inner diameter of 6 mm as the test section.It was verified that the relative error of thermal equilibrium and accuracy of the experimental bench were under 9%.The results showed that increasing Reynolds number or mass fraction could enhance the convective heat transfer performance of composite fluid.When the Reynolds number was 80 and the Stephen number was 13.9,the average surface heat transfer coefficient of the slurries with mass fractions of 20 wt.% was 64.1% higher than that of the base fluid.(3)In order to investigated the effect of tube diameter and tube shape on the convective heat transfer characteristics of phase change material slurries,the convective heat transfer characteristics of the phase change material slurries in straight tube,U tube and helically coiled tube with inner diameters of 3 mm and 6 mm were tested.The results showed that reducing the diameter of the tube could effectively improve the convective heat transfer intensity.Take U-bend tubes and helically coiled tubes as examples,the average surface heat transfer coefficients of U-tube and helically coiled tube with inner diameter of 3 mm were 103.4% and 124.7% higher than those of U-tube and helically coiled tube with inner diameter of 6 mm.In U-bend tubes,secondary flow occurred at elbow of tubes when flow rate increased,which destroyed the temperature boundary layer of heat transfer wall and enhanced heat transfer performance.