骨架中空对泡沫铜内融化过程的影响

Effects of hollow skeleton on melting process in copper foam

将多孔介质和相变材料复合是提高固液相变储能系统传热性能的有效措施.本文通过微型计算机断层扫描(micro computed tomography,Micro CT)三维重构得到泡沫铜的数值结构,采用格子Boltzmann方法对填充泡沫铜复合相变材料的方腔融化过程进行孔隙尺度模拟研究,讨论不同Rayleigh数以及热导率下,泡沫铜骨架中空对泡沫铜内融化过程的影响.结果表明,中空骨架泡沫铜相比实心骨架泡沫铜,融化前期换热强度更低、相变材料的融化更慢、储能效率η更高.与泡沫铜骨架相比,通过骨架中空区进入方腔的热流量可以忽略不计;随Fourier数的增大,泡沫铜的传热增强效率ζ会因为导热和自然对流的竞争而出现先上升后下降然后再上升的现象;当Rayleigh数减小时,储能效率η提高,传热增强效率ζ随Fourier数的变化趋于平缓,中空骨架泡沫铜和实心骨架泡沫铜对应的传热增强效率ζ的差距减小;泡沫铜骨架和相变材料热导率之比越大,融化结束时刻储能效率η越低,中空骨架泡沫铜和实心骨架泡沫铜对应的传热增强效率ζ的差距越小.

The compositing of porous medium and phase change material is an effective way to improve the heat transfer performance of solid-liquid phase change energy storage system.In this paper,we reconstruct the three-dimensional numerical structure of the copper foam by using the micro computed tomography,and then conduct the pore-scale numerical simulation of the melting process in a cubic cavity filled with the phase change material comprised of the copper foam via the lattice Boltzmann method.The effects of the hollow skeleton on the melting process are discussed in detail under different Rayleigh numbers and ratios of thermal conductivity of the copper foam to that of the phase change material.The results show that the hollow skeleton copper foam possesses a lower average Nusselt number along the left wall at the early stage of the melting process,a slower melting rate,and a higher energy storage efficiency than the solid skeleton copper foam.Comparing with the skeleton region of the copper foam,the heat transfer rate entering the cubic cavity through the hollow region of the skeleton is almost negligible.Because of the competition between heat conduction and natural convection,the heat transfer enhancement efficiency of copper foam first increases,then decreases,and then increases again with the increase of the Fourier number.When the Rayleigh number decreases,the energy storage efficiency increases,and the natural convection also weakens.Meanwhile,the fluctuation of the heat transfer enhancement efficiency decreases as the Fourier number increases,and the gap of the heat transfer enhancement efficiency between the hollow skeleton copper foam and the solid skeleton copper foam becomes smaller.When the ratio of the thermal conductivity of the copper foam skeleton to that of the phase change material increases,the energy storage efficiency is relatively high at the early stage of the melting process but becomes relatively low when the melting process is completed.With a larger thermal conductivity ratio,the heat transfer rate entering the cubic cavity through the skeleton region of the copper foam becomes dominant,which reduces the effect of the hollow skeleton on heat transfer,and thus the gap of the heat transfer enhancement efficiency between the hollow skeleton copper foam and the solid skeleton copper foam becomes relatively small.