Investigation On Controllable Preparation Of Nanoencapsulated Paraffin Phase Change Materials And Stability And Thermophysical Properties Of Their Suspensions

Nanoencapsulated phase change materials?NEPCMs?suspension is one of the applications of the encapsulated phase change materials technique.As a novel latent functionally thermal fluid,NEPCMs has the advantage of big specific heat capacity,which can effectively improve the heat transfer capacity of traditional heat transfer fluid to reduce the pumping power of the system.However,NEPCMs prepared by traditional method has a nonuniform particle size and irregular appearance,as well as the poor dispersion stability in fluid,which is not conducive to its relevant investigation on the convective heat transfer.In this study,the NEPCMs with n-Octadecane as the core material and PMMA as the shell material were used as the research object.the preparation method of NEPCMs with narrow size distribution and regular appearance is presented and utilizing the steric hindrance of nanosheets to enhance the suspension stability of NEPCMs.Meanwhile,the effect of temperature on the hybrid nanosheet/NEPCMs suspension has investigated.The specific work is as follows:?1?The influence factors of the NEPCMs with uniform and controllable particle size and optimized performance were investigated.By adjusting the concentration and particle size of Zr P nanosheets and the dosage ratio between shell material and core material,the particle size of the NEPCMs can be controlled and its performance can be optimized.The results show that the uniform and controllable preparation of NEPCMs particle size between 207.4nm and1125nm can be achieved by changing the concentration and particle size of Zr P nanosheets.Based on the Zeta potential and particle size distribution analysis of emulsion,it can be known that the synergistic stabilization of surface modified Zr P nanosheets and nonionic surfactant at the oil-water interface of droplets can effectively reduce the probability of collision and fusion for droplets and improved the stability of emulsion droplets.In addition,when the ratio of shell to core ranges from 0.65 to 0.85,the prepared NEPCMs has an excellent thermal properties and thermal stability.?2?The dispersion stability and optimization factors of NEPCMs suspension were studied.The dispersion stability of NEPCMs suspension was improved by the steric hindrance of nanosheets.The results show that the dispersion stability of NEPCMs suspension is poor,because NEPCMs particles easy agglomeration and its density is less than the base fluid,which leads to the stratification phenomenon of NEPCMs suspension.The reason why the dispersion stability of NEPCMs suspension can be effectively improved by adding the nanosheets is that the steric hindrance of nanosheets delays the agglomeration of NEPCMs and the floating movement of their aggregates.The improvement effect of TiO₂ nanosheets is better than that of GO nanosheets.?3?The effect of temperature on the thermal properties of the hybrid TiO₂/NEPCMs suspension was investigated.The results show that the variation of temperature has not affect the dispersion stability of the hybrid suspension.The specific heat capacity of the hybrid suspension in the phase change temperature region increased with the increase of the NEPCMs concentration.With the increase of temperature,the thermal conductivity and viscosity of the hybrid suspension increase and decrease,respectively.The comparison with the thermal property of TiO₂ suspension and base fluid show that the main reason for this change is that the thermal property of base fluid is only affected by the variation of temperature.