## 3 Results and Discussion
### Wall Thickness Analysis to Determine \(d_{\textit{mean}}\), \(d_{\textit{eq}}\) and msth  
In this section, the results obtained from the wall thickness analyses performed on the CT-scanned volumes of specimens N\({}_{1}\) and N\({}_{2}\) are discussed in detail. Figure 5a represents the cross-sectional view of specimen N\({}_{1}\) parallel to the channel direction, analysed in the background mode of the Sphere method within the confined ROI marked in blue, to determine the functional equivalent diameter \(d_{\textit{eq}}\), while Figure 5b represents the 3D rendering view of the same. On the other hand, Figure 6 illustrates the 3D rendering view of specimen N\({}_{2}\) as an example of the analysis performed in the material mode of the Sphere method. Figure 5: CT scanned volume of specimen N1. (**a**) Cross\(-\)sectional view of specimen N1 parallel to the channel direction, analysed in the background mode of the Sphere method within the confined ROI; (**b**) 3D rendering view of specimen N1 analysed in the background mode of the Sphere method. Figure 6: The 3D rendering view of specimen N1 analysed in the material mode of the Sphere method, revealing the microscale surface features in red. It can be seen that the wall thickness analysis using the Sphere method was performed within the confined ROI that is marked in blue in Figure 5a using the background and material modes in both specimens N\({}_{1}\) and N\({}_{2}\) to evaluate \(d_{\alpha q}\) and _msth_, respectively. When looking at the scale in Figure 6, it becomes evident that the microscale surface features exhibit heights to 100 \(\upmu\)m and are distributed in an anisotropic fashion along the length and circumference of the channel. Figures 7 and 8 show examples of the wall thickness histograms and the corresponding fitted cubic spline curve and Gaussian fits for the wall thickness analysis performed with the Sphere method in background and material modes to determine the equivalent functional diameter \(d_{\alpha q}\) and _msth_ of specimen N\({}_{1}\). From the peak analysis of the fitted curves, \(d_{\alpha q}\) was found to be 1.27 mm, and the corresponding _msth_ was found to be 24 \(\upmu\)m. Furthermore, to determine \(d_{mean}\), the same procedure was followed for the histograms obtained with the Ray method. Figure 9 represents the wall thickness histogram and the corresponding fitted Gaussian for the wall thickness analysis performed with the Ray method in background mode and a search angle of 15\({}^{\circ}\) to determine \(d_{mean}\). The geometrical mean diameter of the channel was found to be  
Figure 8: Wall thickness histogram with fitted Gaussian for _msth_ for specimen N\({}_{1}\). Figure 7: Wall thickness histogram with fitted cubic spline for functional equivalent diameter (\(d_{\alpha q}\)) for specimen N\({}_{1}\). 1.31 mm, which is 190 \(\upmu\)m smaller than the design value of 1.5 mm. The data range for the Gaussian fit was chosen around the centre of the main histogram peak considering the histogram data between 1100 \(\upmu\)m and 1700 \(\upmu\)m. The R-squared value of the fitted Gaussian curve over the CT data was 0.98 in both cases. The same approach was applied to the histograms of all ROIs of specimens N\({}_{1}\) and N\({}_{2}\) to determine the corresponding \(d_{eq}\), \(d_{mean}\) and _mtsth_, and the results are listed in Table 2. The ROIs for the full-channel section and straight and inclined sections are indicated as F, S and I respectively. From Table 2, it can be seen that the difference between \(d_{eq}\) and \(d_{mean}\) is consistent in specimens N\({}_{1}\) and N\({}_{2}\) irrespective of the ROI that is analysed. It should also be noted that \(d_{eq}\) and \(d_{mean}\) measured separately at the inclined and straight sections are equivalent to the ones measured along the entire length of the channel, indicating that the deviation from the nominal diameter is constant irrespective of the section of the channel.