## 4 Results and Discussions
### Influence of the Thickness  
Although the full-size model can obtain more accurate results in comparison to the RVE model, it needs too many computing resources when the geometric size is large. The influence of the thickness on the ETC and the critical thickness that was suitable for the RVE model are analyzed in this section.  
Figure 10(a) shows the variation curve of the k\({}_{Z}\) with increasing the thickness L\({}_{Z}\). It can be seen that k\({}_{Z}\) firstly decreased and then tended to be constant. When L\({}_{Z}\) reached 18.4 mm, the relative variation of the k\({}_{Z}\) was only 0.51%, which could be treated as convergent. When the L\({}_{Z}\) was equal to 2.8 mm and 18.4 mm, the corresponding k\({}_{Z}\) values were 3.91 W/(m\(\cdot\)K) and 3.53 W/(m\(\cdot\)K), respectively, and the relative variation was 10.76%. The results show that the thickness had a big influence on the ETC k\({}_{Z}\) of the full-size model. The reason for this was that the volume fraction of the fibers V\({}_{f}\) increased with increasing L\({}_{Z}\), as shown in Figure 10(b). For example, when L\({}_{Z}\) was equal to 2.8 mm and 18.4 mm, the corresponding V\({}_{f}\) values were 39.61% and 51.92%, respectively. The variation of V\({}_{f}\) changed the percentage of the components with different thermal properties, which further affected the ETC of the full-size model.  
At the same time, according to the results in Section 4.1.3, the ETC k\({}_{Z}\) based on the RVE model was 3.42 W/(m\(\cdot\)K), as shown by the red dashed line in Figure 11. It can be seen that the line based on the  
Figure 11: The curves of the k\({}_{Z}\) and the volume fraction with the L\({}_{Z}\) increasing.  
Figure 10: A comparison between the boundary part and the periodic internal structure.  
full-size model tended to converge on the red dashed line of the RVE model. When L\({}_{Z}\) changed from 2.8 mm to 18.4 mm, the relative variation between the results based on the full-size model and RVE model varied from 12.53% to 3.12%. The results indicate that when the thickness L\({}_{Z}\) was large, the ETC k\({}_{Z}\) based on the RVE model and the ETC k\({}_{Z}\) based on the full-size model were close to each other. The reason for this was that when the L\({}_{Z}\) was large enough, the periodicity was approximately satisfied in the z-axis direction for the full-size model, leading to a small difference with the periodic RVE model and that the \(F_{f}\) of these two models were close to each other. For example, \(F_{f}\) of the RVE model and \(F_{f}\) of the full-size model with L\({}_{Z}\) = 18.4 mm were 54.34% and 51.92%, respectively, with a relative variation of only 4.45%. For a full-size model with L\({}_{Z}\) = 2.8 mm, this relative variation reached 27.11%.  
For the operating condition discussed in this section, when L\({}_{Z}\) was bigger than 18.4 mm, the relative variation was smaller than 5%, so the ETC k\({}_{Z}\) based on the RVE model could represent the result based on the full-size model. The corresponding Lz of 18.4 mm was determined as the \"critical thickness\", which was nearly 31 times greater than the thickness of the RVE model.