| Fusion blanket is the key functional component for power extraction, tritium fuel breeding, and radiation shielding. As the most important plasma facing component, the blanket will be confronted the extremely harsh fusion environment with high energy particle irradiation, gradient magnetic field, high thermal loads, complex mechanical loads, erosion and corrosion. This multi-physics environment field will cause a series of structure materials performance decline problems, like high neutron damage, hydrogen and helium bubbles.etc, which will have serious impact on the safety of the fusion reactor.To solve above problems, a blanket design with flowing liquid metal as the First Wall has been first proposed by N. Christofilos in 1971 and developed by M. Abdou at UCLA under the APEX (Advanced Power Extraction) project in early 2000s, Compare with the conventional solid first wall blanket, liquid first wall blanket have promising advantages higher thermal and neutron wall loads tolerance, higher outlet temperature, and longer service life span, due to the inherent of liquidity and continuous renewing of the liquid working medium.In the existing liquid metal first wall concepts the flow stability of the liquid metal is still a severe problem under the complicated nuclear environment in the fusion reactor vacuum vessel. The Lorentz force will be generated when the liquid metal flow cross the high magnetic field, which result in high flow drag caused by MHD (magneto hydrodynamics) effect and significantly change the flow state of liquid FW layer.After taking account of the relative merits of the exiting liquid first wall designs, this thesis put forward a control strategy of liquid first wall. The goal is to keep the good radiation performance of the thick liquid wall concept and reduce the MHD effect to enhance the flow stability. To achieve the goals the design method is to inject the thick liquid layer at the top of the vacuum chamber in toroidal direction to the backing structural wall. As it flows along the curved wall the fluid adheres to the structural wall by means of centrifugal and inertial forces. The thickness liquid layer would submerge the helical channels which are set along the magnetic field on the backing wall. In this case, it will establish two-direction liquid metal streams:one is bulk layer which flows in the helical channels along the toroidal direction of the vacuum chamber; the other one is surface free layer which flows above the flow channel walls. The bulk layer would carry out the most neutron nuclear heat and the induced electric potential can be reduced by avoiding the liquid metal from cutting the magnetic field line strongly. As the result, the MHD effect would be minimized in principle. Under the action of gravity, the surface free layer flow downstream without the support of channel walls and the vertical velocity would increase rapidly so it can avoid the free surface layer being overheated.In this work, the flow stability of liquid first wall with spiral channels was simulated with MHD module and VOF model of CFD software FLUENT. The MHD effect was simulated by MHD module, and the liquid metal free surface was captured by VOF model. The design of liquid first wall with spiral channels achieved the design goal of reducing MHD effects and thinning effects. The results shows the design of spiral channels could form continuous stable liquid surface, though some tiny bubble would appear in the inlet area, which was formed by the jet flow near the inlet. The variation of the thickness of liquid first wall along the spiral channels was small, without notable thinning effects. The electric potential difference induced by the flowing liquid metal in the spiral channel was smaller than 1V, and the area with large electric potential difference was focused on the free flow outside the channel and the jet near the inlet. |
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