| Energy shortage is one of the most serious social problems that human beings are facing.Latent heat thermal energy storage(LHTES)technology can reduce energy discontinuity of the time and instability of the intensity.And it has the advantages of high energy storage density,easy control of the process,approximate isothermal of charging and discharging process and so on.Thus,LHTES has great application value in the field of electric peak shaving,solar energy utilization,waste heat recovery and aerospace.The latent heat storage performance of phase change materials(PCM)and the heat transfer characteristics in the phase change process are the important research topics of latent heat thermal energy storage technology.The thermal characteristics of Al-Cu alloy with Cu content in 7.4%?51.7%,including phase change temperature,phase change latent heat,specific heat,thermal diffusivity and thermal conductivity,has been investigated by differential scanning calorimetry(DSC)and laser flash apparatus(LFA).The internal mechanism of the effect of metallurgical structure on Al-Cu alloy thermophysical properties has also been analyzed.The results indicate that when Cu content is in the range of 7.4%?51.7%,the phase transition temperature of Al-Cu alloy is within 524.4 ? to 645.9 ?.With the growth of Cu content,the mass latent heat of Al-Cu alloy decreases and the volume latent heat increases.Al-7.4%Cu has the maximum melting/freezing mass latent heat,which are 339.6 kJ kg-1 and 343.5 kJ·kg-1 respectively.Al-51.7%Cu has the maximum melting/freezing volumetric latent heat with 1179 MJ·m-3 and 1143 MJ·M-3 respectively.When the temperature rises from 25 ? to 500 ?,the specific heat of Al-Cu alloy keeps increasing.On the other hand,when Cu content rising from 7.4%to 51.7%,the specific heat of Al-Cu alloy decreases.The specific heat of Al-7.4%Cu is 0.85 J g-1·K-1 at 25 ?,and reaches the maximum 1.08 J g-1·K-1 at 500 ?.Besides,the thermal conductivity of Al-Cu alloy decreases with the increase of Cu content,but its thermal conductivity is still as high as 104 W·m-1·K-1,even if Cu content reaches 51.7%.Al-Cu alloy have a good thermal stability,after 300 times of accelerated melting and solidification cycle(for 18000 min),phase transition temperature and latent heat of Al-Cu alloy were changed in a small range.Our results reveal that Al-Cu alloy,as high-temperature phase change material,has great potential of application in the field of solar thennal energy storage.The melting and solidification process of Al-Cu alloy was simulated numerically in square heat storage unit which has an isothermal circular heat exchange tube inside.The melting and solidification behaviors of phase change material(PCM),and the effect of aspect ratio and the position of the central heat exchange tube on the melting and solidification behaviors of PCM was studied.And the effect of heat storage area coefficient was discussed.The results show that for a square heat storage unit with 20 mm diameter of the center heating tube and 6400mm2 cross-sectional area,the melting and solidification time of PCM are first decreasing and then increasing with the increase of aspect ratio.Especially,when the aspect ratio is about 1.56,the melting time of PCM is shortest,and this aspect ratio is defined as the optimum melting aspect ratio.When the aspect ratio is about 1,the solidification time of PCM is shortest,and this aspect ratio is defined as the optimum solidification aspect ratio.Besides,with heat storage area coefficient increasing,the optimum melting aspect ratio of heat storage unit presents an increasing trend,and the optimum solidification aspect ratio of heat storage unit remains unchanged.In addition,with the decrease of heat exchanger tube position y,the melting and solidification time of PCM are first decreasing and then increasing.When the melting/solidification time of PCM is the shortest,the position of the central heat exchanger tube y is defined as the optimum position of the melting/solidification heat transfer tube.Then,the optimum position of the melting heat transfer tube is between-1/8H and-1/16H,and the optimum position of the solidification heat transfer tube is about 0.With heat storage area coefficient increasing,the optimum position of the melting heat transfer tube of heat storage unit presents a decreasing trend,and the optimum position of the solidification heat transfer tube of heat storage unit remains unchanged.Combining the aspect ratio and the position of the central heat exchange tube,the effect of it on melting and solidification process was studied.Under the condition of the optimum melting aspect ratio(K=1.56)and the optimum solidification aspect ratio(K=1.56),the effect of the position of central heat exchanger tube on the melting and solidification processes were compared. |
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