Experimental And Numerical Research On Heat Transfer Structure Optimization Of Composite Latent Heat Thermal Energy Storage

Latent heat thermal energy storage(LHTES)is promising technology that absorbs(releases)heat during the melting(solidification)process of phase change materials(PCMs).The technology plays an important role in many fields due to its safety,reliability,high latent heat and small temperature variation during the phase change process.However,the relatively poor thermal conductivity of PCMs constrains its further development.Currently,changing the physical parameters of PCMs and optimising the structure of LHTES are the focus of many researchers.The paraffin,copper foam and polypropylene were used as PCM,porous medium and container of composite LHTES.The experimental platform was established,and the melting process of PCM was investigated with the constant heating temperature.The temperature variations of thermocouples for composite PCM and pure PCM were recorded.The experiment result indicated applying foam metal could accelerate the temperature rise of PCM to reach a stable temperature as soon as possible and improve the uniformity of temperature distribution inside PCM.In the numerical simulations,the flow and heat transfer models of fluids in porous medium were investigated and the enthalpy-porosity model was used to simulate the phase change process of PCM.In addition,numerical method and empirical formulations concerning the effect of nanoparticles on PCM,as well as the Boussinesq approximation,were also taken into account in the present computational model.The accuracy of the model was demonstrated by comparing the simulated results with the experimental data.Furthermore,the accuracy of the phase change process for nanoPCM was further verified by comparison with other literature.By using porous media with different porosity distributions,parameters such as full melting time and average heat flux density of LHTES were analysed.The conclusions were summarized as follows:(1)Compared to pure PCM,the use of porous medium reduced the full melting time by 73.7% and increased the average heat flux density by 2.43 times.(2)Compared with uniform porosity,the full melting time was reduced by 4.26%,and the average heat flux density was increased by 3.54% as the porosity increased in the y-direction.(3)In order to further improve the heat transfer performance of LHTES,the porous medium was divided into two upper and lower parts with porosity distributions increasing along the x-direction and y-direction,respectively.Compared with uniform porosity,the full melting time and average heat flux density were further decreased by 6.78% and increased by 4.26%,respectively.The effect of nanoparticle and porous medium on the LHTES during the heat release process was numerical investigated.The effect of the volume fraction of nanoparticles(φ)and the porosity distribution of the porous medium were considered.The conclusions were summarized as follows:(1)Adding nanoparticles to the PCM increased the viscosity of the liquid PCM,leading to a decrease in convective heat transfer.However,the increase in thermal conductivity of the PCM was more pronounced.As the volume fraction of nanoparticles increased from 0 to 0.1,the full solidification time was decreased by 30.15%.Furthermore,increasing the nanoparticle content accelerated the reduction of temperature inhomogeneity,but did not reduce the maximum value of temperature inhomogeneity.(2)The effect of porous medium on the heat transfer performance of LHTES was higher than that of nanoparticles.In addition,as the porosity increases along the x-direction,the fastest solidification rate was achieved.(3)The combined use of nanoparticles and porous media could further improve the energy efficiency of LHTES.The energy efficiency of LHTES was highest when the porosity distribution was ε=2x+0.88 as well as φ=0.1.For the horizontal shell-and-tube TES,the temperature,velocity and average heat wlux density during the heat adsorption and release processes of the two-dimensional shell-and-tube TES were analysed by analysing the porosity and pore density distribution,the wall and initial temperatures,and the diameter ratio.In addition,a three-dimensional shell-and-tube TES model was established to analyse the effect of porous media with porosity distribution along the axial direction on the heat transfer of LHTES.The conclusion were as follows:(1)The heat transfer performance of LHTES was improved as the porosity increased along the outer diameter direction.For the the porosity increasing along the y-direction,the melting process was accerated,but the solidification speed was slow down due to the low thermal conductivity near the outer wall.(2)For the pore density increasing along the outer diameter direction,the heat convective near inner wall could be fully utilised during the exothermic process,and the large heat transfer area near the outer wall existed during the heat absorption process,which was the optimal pore density distribution to improve heat transfer efficiency of LHTES.(3)As the temperature difference between the inner wall temperature and phase change temperature increased,the average heat flux density was increased and the time required for complete solidification or melting decreased.(4)The change of initial temperature had a little effect on the time required for the complete phase change.However,as the initial temperature affects the total amount of absorbed(or exothermic)heat and the average heat flux density.(5)As the diameter ratio decreased,the melting(solidification)rate of PCM increased,the temperature of PCM proceeded less time to reach a stable temperature.(6)The heat transfer performance of LHTES was further improved when the porosity decreases along the flow direction of the HTF and the gradient value was small.A larger gradient value resulted in the reduction of the heat flux density near the local area and prolonged the time required for the complete phase change of the PCM.