# Engineering Design of the European DEMO HCPB Breeding Blanket Breader Zone Mockup
## 6 Performance Analysis  
### Thermofluid-Dynamic Analysis  
To reduce meshing and computation time, only critical regions had been modelled in the 3D conjugated CFD thermofluid-dynamic analyses to assess the temperature fields at these critical regions and the flow distribution. The critical regions include the fluid domain from the inlet to outlet of the mockup, the electric heaters, the Tube-1, Tube-2, Tube-3, and Tube-4. The mesh of the model is shown in Figure 7, done by using ANSYS Meshing. Besides the wire electric heaters and Tube-1, all the other parts were meshed by structured hexagonal mesh. In the fluid domain, there were 10 layers of boundary layers. And the mesh size is fine enough to ensure that the y+ is about 1. Totally, there are about 50 million elements. The CFD software CFX was used for the calculations.  
The solid structure was made of stainless-steel SS 316, whose physical properties were provided by the supplier and are temperature-dependent. The helium properties were taken from the NIST [22] and implemented into the CFX setting-up as temperature- and pressure-dependent.  
Many CFD simulations were done. Here, only a representative case is shown, with a mass flow rate of 64 g/s and with a q\({}^{+}\) of 0.0006 (corresponding to a total heating power of 3370 W). The thermal contact conductance (TCC) between Tube-2 and Tube-3 is calculated to be about 2400 W/(m\({}^{2}\) K) assuming a stagnant helium gap of 0.1 mm.  
The velocity streamline of the coolant is shown in Figure 8. The temperature contour of the mockup is shown in Figure 9. The hotspots are located at the heaters and the Tube-1, which surrounds the heaters. Maximum temperature of the tube is 430.6 \({}^{\circ}\)C, well below the allowable operating temperature 550 \({}^{\circ}\)C. The total pressure drop between inlet and outlet is calculated to be about 390 Pa.  
Figure 7: Mesh of the critical region of the mockup.  
The average heat flux from Tube-3 towards coolant is shown in Figure 10. The heat flux at heating region between 50 mm and 400 mm is uniform. It can be seen that there are entrance and exit effects on the heat flux distribution, which should be taken into account when post-processing the experimental data in the future.  
The velocity contour of the coolant is shown in Figure 11. The velocity profile of 10 lines crossing the annular channel (location of the 10 lines shown in Figure 11) approaching the end of the test section is shown in Figure 12. It can be seen that the flow profiles at different lines almost overlap. The velocity profile comparison at lines 4, 9, 5, and 10 is shown in Figure 13. It is observed that the flow there is not symmetric, probably because of the T-junction at the outlet.  
Figure 8: Velocity streamline of the coolant.  
Figure 9: Temperature contour of the mockup.  
As the contact between Tube-1 and Tube-2 is ensured via brazing, a perfect contact between them is assumed. The TCC between different tubes will influence the heat transfer between them. Therefore, for the sake of model robustness, a scoping study varying the TCC values between Tube-2 and Tube-3 should be done. The scoping study considers four different TCC values between Tube-2 and Tube-3: 500, 1000, 1500, and 2000 W/(m\({}^{2}\) K), all below the nominal value 2400 W/(m\({}^{2}\) K). The temperature fields of the four cases are shown in Figure 14. It can be seen that the maximum temperature 467.6 \({}^{\circ}\)C on the mockup occurs at TCC = 500 W/(m\({}^{2}\) K), which implies that the mockup will be safe (T\({}_{\rm max}\) < 550 \({}^{\circ}\)C), even with a very low TCC.  
Figure 11: Velocity contour.  
Figure 10: Average heat flux of Tube-3 towards coolant at the heated zone.