## 1 Introduction
The homogenization method [14], the thermoelectric analogy model [15], and the mathematical model [16] have been developed for many years, and they have different advantages depending on the purpose. The Representative Volume Element (RVE) method was developed based on the homogenization method, and it has attracted much attention due to the fact that it could introduce many information of the micro/meso-structures. This model was firstly studied by [PERSON] [17]. A simple way to apply the RVE model is to use the minimal structural repeating unit with a periodicity hypothesis. For example, [PERSON] et al. [18] investigated the effective thermal conductivity and thermal resistance of a 2-D woven fabric using the finite element method. An RVE model including two warp yarns and two weft yarns was applied based on the scanning electron microscope, and the geometrical size of the RVE was 0.862 mm \(\times\) 0.862 mm \(\times\) 0.5 mm. [PERSON] et al. [19] studied the thermal conductivity of 3-D woven C/C composites at high temperature numerically and experimentally. The multi-scale method was applied, including the microscale model and the mesoscale model, in which the mesoscale model was an RVE model including two warp yarns, two weft yarns, and one yarn in the thickness direction. The geometrical size of the RVE was 1.96 mm \(\times\) 1.96 mm \(\times\) 0.76 mm, and the thickness of the experimental sample was 5 mm. [PERSON] et al. [20] built an RVE model with a geometrical size of 2.724 mm \(\times\) 2.724 mm \(\times\) 5.843 mm to predict the thermal conductivities of three-dimensional four-directional braided composites. Similar works can also be found in other references [21; 22; 23]. All of the studies mentioned above used an RVE model where the geometrical structure was based on the periodicity hypothesis and the global thickness of the RVE models was in the range of 0.5 mm to 3 mm. However, the thickness of the hot components, such as the turbine vane, was small (2-3 mm). Therefore, for the thermal analysis of a CMC component and the prediction of the thermal conductivity of the corresponding material, the RVE model may be improper. The periodicity hypothesis, which is the foundation of the RVE model, is unsatisfied. Some researchers tried to build a model with a full size in the thickness direction. For example, [PERSON] et al. [24] numerically and experimentally investigated the thermal conductivity of 2.5-D angle-interlock woven composites. A full-size model with a geometrical size of 8.8 mm \(\times\) 8.46 mm \(\times\) 6.26 mm was built. In this model, the thickness of the fiber bundles was nearly 0.3 mm. The results showed that the thermal conductivity in the thickness direction based on the full-size model agreed well with the experimental data, and the relative error was about 4.1%. Although the full-size model could describe the geometrical characteristics more accurately, it needed much more meshes than the RVE model, due to the big differences between the sizes of the microstructure and the macroscopic components. This limited the application of the full-size model in the thermal analysis of the CMC components with large sizes. Therefore, there is a critical thickness for the application of the RVE model or the full-size model, which need to be studied considering both the accuracy and the efficiency. Accordingly, the present work aims to investigate the ATC prediction model of the 2.5-D braided ceramic matrix composites, taking into account the actual thickness of the high-temperature components. An RVE model with a periodic boundary and a full-size model based on the actual thickness were built to study the effect of the composite thickness on the estimation accuracy of the effective thermal conductivity. The validation experiment was carried out to verify the estimation method of the effective thermal conductivity.