Proposition and investigation of the composite with modified structure

5.3.1 Description of the aluminum foam with

modified structure

Although the average energy storage power of the composites with high porosity is relative low, they own the large energy storage capacity. Thus, the research works in this section focus on the improvement of the energy storage performance of composites with high porosity. As discussed above, the solid paraffin remained in the right bottom corner reduces the melting rate of PCM at last stage and deteriorates the energy storage performance of the composite. This problem is the same as the constant power condition. Considering the different

heat transfer characteristics of the composite under two heating condition, the modified methods for constant temperature condition are also different with the approaches in Section 4.4.

Recently, some researchers start to concern this problem, and they propose a new kind of structure: the gradient metal foam. Yang et.al [84] proposed the metal foam with linearly changed porosity, and the numerical results demonstrated that this structure could enhance the phase change process in PCM. Furthermore, this group also added a metal fin at the bottom of metal foam, and it is found that the transfer rate of this structure is larger than that of the metal foam without metal fin [132]. In this study, combining these two methods, aluminum foam with modified structure is proposed, which is defined as finned metal foam with graded porosity (FFGP). The FFGP structure is depicted in Fig. 5.9. Three metal foams with different porosities constitute the gradient metal foam. These three parts have the same size and they are connected with metal fin at the bottom. The PCM is filled in the gradient metal foam.

5.3.2 Validation of model of the aluminum foam with

graded porosity

The modeling method is according to the process in Section 4.2. The finite volume method (FVM) is employed to discretize the governing equations. For the numerical analysis of the thermal behavior of metal foam with graded porosity, the modeling methods applied in previous works include the local equation model and Lattice Boltzmann Method (LBM). To author’s knowledge, the paper that uses the FVM with two equations model to study the heat transfer of the PCM embedded in metal foam with graded porosity is relatively rare. In order to valid the model, it is necessary to compare the numerical results obtained by FVM with the experiment or numerical data in other research works. Fig. 5.10 shows the variations of liquid fractions predicted by LBM [85] and FVM the melting process. It could be seen that the two curves have the same tendency and they are in accordance with each other.

Fig. 5.10. Comparison of liquid fractions between the numerical results obtained by LBM [85] and FVM

Besides, based on the conditions in Ref.[15], the comparison of solidification process is also performed between experimental and numerical results, as shown in Fig. 5.11 and Fig. 5.12. It is observed that the liquid fraction and the evolution

of the interface obtained by two equation model agree well with the experiment results. Thus, these comparison results indicate that the model applied in this study is adequate and reliable.

Fig. 5.11. Comparison of liquid fractions between the experimental data [15] and numerical results by FVM

Fig. 5.12. Comparison of the interface locations between the experimental photos [15] (left) and numerical results (right)

5.3.3 Melting characteristic of PCM in FFGP

structure

Fig. 5.13 presents the comparison of interface evolutions of the PCM embedded in uniform metal foams and FFGP structure. Two kinds of structures have the same metal fraction and PPI value. As discussed above, the circulation flow in uniform metal foam makes PCM in right bottom corner melt at last, as shown in Fig. 5.13(a). Because the driving force of the natural convection decreases at last stage, the PCM in this position will melt slowly. In our previous research [152], this phenomenon could reduce the energy storage performance.

The melting process of PCM saturated in FFGP structure is depicted in Fig. 5.13 (b), which is different with the uniform one. Due to the addition of the metal fin, the PCM could melt from both horizontal and vertical directions. It is observed that the evolution process of the vertical interface is similar to that of the uniform structure. For the horizontal interface, its form is smooth at beginning. As the interface rising up gradually, the shape becomes wavy and rough, as presented at 2000s and 4000s in Fig. 5.13 (b). The phenomenon indicates that the natural convection develops between the metal fin and the interface. The result of velocity vector could visualize the circulation in the liquid, as illustrated in Fig. 5.14. Except for the large circulation flow between heat source and vertical interface, some small flows exist in the liquid near the metal fin, which contributes to the heat transfer between metal fin and solid PCM. In addition, the velocity of the fluid adjacent to the vertical interface in FFGP structure is larger than that in uniform porosity structure, which demonstrates the metal foam with graded porosity could benefit the development of natural convection. Finally, comparing the amounts of solid PCM of two configurations at 6000s in Fig. 5.13, it is concluded that the FFGP structure improves significantly the melting process of

PCM.

Fig. 5.13. Melting process of PCM (a) in uniform metal foam (b) in FFGP

Fig. 5.14. Velocity vectors of liquid PCM (a) in uniform metal foam (b) in FFGP

5.4 Optimization of the structural parameters