| Thermal energy storage technology based on the application of phase change materials(PCMs)is one of the advanced technologies to promote the sustainable development of buildings.Its application in cooling,heating and coupling with building ventilation technology significantly improves the indoor thermal environment of buildings and reduces the energy consumption of air conditioners.However,the thermal conductivity of PCMs is generally low,which seriously limits the overall heat transfer performance of the energy storage system and is difficult to meet the requirements of building system regulation,making it difficult to apply and popularize phase change energy storage technology in the field of building energy conservation.This paper studied the enhancement of heat transfer properties of PCMs in a horizontal tubular phase change energy storage unit.The main contents of this study are as follows.(1)A two-dimensional numerical model of the energy storage unit was established based on the enthalpy-porous medium method,and the accuracy of the established model was verified..The heat transfer mechanism in the heat storage process experienced a short-term thermal conduction-dominated heat transfer,and natural convection became the main heat transfer method.For a certain Gr number when the Ste number increased,faster melting can be achieved;for a certain Ste number,the complete melting time of PCMs increased with the increase of the Gr number,the maximum value was 57.5%(Ste=0.3 13),and the time of natural convection was delayed,the trend of the average heat transfer coefficient of the inner wall of the shell was slowed down,and the peak valuewas decreased.In the case of low Ste number(Ste<0.093),natural convection had little effect on the melting of PCMs.(2)Nanocomposite PCMs with MWCNT mass fractions of 0 wt.%,0.05 wt.%,0.10 wt.%,0.20 wt.%,0.50 wt.%and 1.00 wt.%were prepared.Adding MWCNT to paraffin can improve the thermal conductivity of NePCM,and the improvement effect increased with the increase of nanoparticle loading..At the same temperature,the increase in viscosity was greater than that in thermal conductivity,and it increased with the increase in the concentration of nanoparticles.It was found that the thermal conductivity of nanocomposite PCMs with 1.00 wt.%MWCNT increased by 5.78%on average at the measurement temperature,while the viscosity increased by 52.8%.(3)The effect of nanoparticles on the heat storage process of nanocomposite PCMs showed that the thermal conductivity and viscosity increase caused by nanoparticles jointly affect the melting rate of NePCM in the energy storage unit..Under each working condition,the heat transfer effect of NePCM was weakened due to the increase of viscosity,and the melting rate was slowed down between different nodes.(4)The melting process of PCM by nanoparticles mainly depended on the thermal conductivity and Ra number of NePCM.When the heat transfer temperature difference was 20℃,adding 0.05wt%nanoparticles improved the melting rate of nanocomposite PCMs.When the heat transfer temperature difference was 40℃,adding 0.10wt%nanoparticles had the most obvious effect on improving the melting rate of nanocomposite PCMs.The increase ratio of thermal conductivity and the decrease in the Gr number were proposed as the heat transfer enhancement indicators for NePCM,indicating that nanoparticles can only promote the heat transfer process of NePCM when the energy storage system had a low Gr number. |
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