Researches on Preparation,Characteristics and Applications of High-performance Micro/Nano-sized Phase Change Material Emulsions

Author:Wang Fang Zuo

Supervisor:fang xiao ming

Database:Doctor

Degree Year:2019

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Pages:118

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Improving the specific heat capacity of fluids is an important ways to enhance heat transfer.Dispersing organic phase change materials(PCM)which can absorb or release a large amount of latent heat during phase change into conventional heat transfer fluids(HTFs)is an effective way to improve their specific heat capacities.Phase change material emulsion(PCME)prepared by directly dispersing the PCM into water,has the advantages of larger specific heat capacity,simple preparation process and low cost.However,poor stability,large supercooling and low thermal conductivity are the main problems restricting the practical applications of micro-sized PCMEs.Moreover,nano-sized PCME has good stability due to its small droplet size,and explore its application performance is a prerequisite to promote the practical process.This paper aims at preparing high-performance micro-sized PCMEs with good stability,little supercooling and high thermal conductivity,together with investigating the thermal management performance of the lithium-ion battery liquid cooling system employing the nano-sized PCME as a novel working medium.The mainly results are as follow.In order to obtain the PCMEs with good stability and little supercooling,a mixed polymeric emulsifier consisting of polyvinyl alcohol(PVA)and polyethylene glycol-600(PEG-600)was first explored for preparing high-performance paraffin/water micro-sized PCMEs.After the effects mass ratio of PVA to PEG-600,the mass ratio of the mixed emulsifier to the paraffin and the homogenization rate on the size distribution,viscosity and dispersion stability of the 20 wt%PCMEs were systematically investigated.The optimal emulsification conditions have been determined to be 50:50 for the mass ratio of PVA/PEG-600,1:5 for the mass ratio of mixed emulsifier/paraffin and 10000 rpm for the homogenization rate.The separation rate of 20 wt%PCMEs was 13.1%after being experienced 50 heating-cooling cycles,indicating that the 20 wt%PCMEs had good dispersion stability.Furthermore,the as-prepared PCMEs containing different mass fractions of paraffin exhibit no supercooling and their maximum specific heat capacities are 1.51-2.18times as high as that of water,but their thermal conductivities are lower than that of water.Moreover,the pumping power consumption of the PCME shows a drastic reduction as compared to that of water at the same heat storage capacity.When the heat storage capacity is26 kW,the power consumption of the 20 wt%PCME is only 33.2%of the water.The results demonstrate that the PCMEs show great potential for use as a novel HTF in thermal energy storage(TES)systems.Aiming at obtaining graphite nanoplatelets(GNPs)decorated PCMEs with good stability,little supercooling,enhanced thermal conductivity and improved photo-thermal conversion performance for direct absorption solar collectors(DASCs),GNPs were dispersed into paraffin/water micro-sized PCMEs stabilized by the mixed polymeric emulsifier.The effects of the amount of GNPs and paraffin mass fraction on the phase change temperature,phase change enthalpy,supercooling,thermal conductivity as well as the photo-thermal conversion performance of GNPs decorated PCMEs were investigated systematically.The results show that the thermal conductivity and optical absorption of GNPs decorated PCMEs increase with the mass fraction of GNPs,but decrease with the increase of paraffin mass fraction.To avoid the evaporation of the water included in a PCME to weaken the incident light consequently,its optical absorption property should match with its thermal conductivity.The 0.07 wt%GNPs decorated 20 wt%PCME has the best photo-thermal conversion performance,and its relative heat storage capacity is 164%higher than that of water.Additionally,the 0.07 wt%GNPs decorated 20 wt%PCME exhibits good thermal stability and thermal reliability.To overcome the problems of poor stability,large supercooling and low thermal conductivity of cold storage PCMEs,graphite nanoparticles decorated PCMEs were prepared by dispersing graphite nanoparticles into OP10E/water PCMEs.The effects of the amount of graphite nanoparticles on the supercooling,thermal conductivity,stability as well as thermal reliability of graphite nanoparticles decorated PCMEs were studied systematically.The results show that the degree of supercooling for the PCME decreases from 9.9 ~oC to almost 0 ~oC after the dispersion of the graphite nanoparticles at an optimal concentration of 2 wt%,and its enthalpy almost remains the same.The thermal conductivity of the PCME containing 2 wt%graphite nanoparticles increase by 88.9%compared with the pure PCME.It is indicated that the graphite nanoparticles can eliminate the degree of supercooling and enhance thermal conductivity of the PCME without affecting its heat storage density.The PCME containing 2wt%graphite nanoparticles has good dispersion stability and thermal reliability,since no obvious changes in these properties have been found for the PCME after being stored for 30days or being experienced 300 heating-cooling cycles test.The nano-sized PCME was first used as a novel working medium of the lithium-ion battery liquid cooling system,and its application performance was studied.Specifically,the nano-sized OP28E/water PCMEs containing different mass fraction of OP28E were prepared by an ultrasonic method.The effects of the mass fraction of the nano-sized PCMEs on the maximum temperature rise,maximum temperature difference and pressure drop of the battery thermal management system(BTMS)were investigated.Additionally,numerical simulation was carried out to optimize the flow rate of the nano-sized PCME.It is found that the as-prepared PCMEs have average particle sizes of less than 200 nm and good dispersion stability.The maximum apparent specific heat capacities the NPCMEs containing 10 wt%and20 wt%OP28E are 2.3 times and 4.7 times as higher as that of water,respectively.The NPCMEs are Newtonian fluids,and their apparent viscosities are less than 5.11 mPa·s,meeting the transportability requirements for pumping in practical applications.The maximum temperature and maximum temperature difference of the battery pack using 10 wt%NPCME as the coolant are 1.1°C and 0.4°C lower than that using water as coolant at the flow rate of 200 mL·min~-11 and the discharge rate of 2 C.The total pressure drop of 10 wt%NPCME is just 1.6%higher than that of water.The numerical simulation results show that the maximum temperature and maximum temperature difference decrease with increasing flow rate,and the BTMS performance of 10 wt%nano-sized PCME is better than that of water.