Magnetohydrodynamics in free surface liquid metal flow relevant to plasma-facing components

Abstract

While flowing liquid metal plasma-facing components (PFCs) represent a potentially transformative technology to enable long-pulse operation with high-power exhaust for fusion reactors, magnetohydrodynamic (MHD) drag in the conducting liquid metal will reduce the flow speed. Experiments have been completed in the linear open-channel LMX-U device [Hvasta et al., Nucl. Fusion. 58 (2018) 01602] for validation of MHD drag calculations with either insulating or conducting walls, with codes similar to those used to design flowing liquid metal PFCs for a Fusion Nuclear Science Facility [Kessel et al., Fusion Science Technol. 75 (2019) 886]. We observe that the average channel flow speed decreased with the use of conducting walls and the strength of the applied transverse magnetic field. The MHD drag from the retarding Lorentz force resulted in an increase of the liquid metal depth in the channel that ‘piled up’ near the inlet, but not the outlet. As reproduced by OpenFOAM and ANSYS CFX calculations, the magnitude and characteristics of the pileup in the flow direction increased with the applied traverse magnetic field by up to 120%, as compared to the case without an applied magnetic field, corresponding to an average velocity reduction of ~ 45%. Particle tracking measurements confirmed a predicted shear in the flow speed, with the surface velocity increasing by 300%, despite the 45% drop in the average bulk speed. The MHD effect makes the bulk flow laminarized but keeps surface waves aligned along the magnetic field lines due to the anisotropy of MHD drag. The 3D fringe field and high surface velocity generate ripples around the outlet region. It was also confirmed that the MHD drag strongly depends on the conductivity of the channel walls, magnetic field, and volumetric flow rate, in agreement with the simulations and a developed analytical model. These validated models are now available to begin to determine the conditions under which the ideal liquid metal channel design of a constant flow speed and fluid depth could be attained.

Experimental, numerical and analytical evaluation of j × B -thrust for fast-liquid-metal-flow divertor systems of nuclear fusion devices

“…Finally, simulation results with a recently developed tool called FreeMHD [19] are also shown for these tests, showing agreement with the simplified analytical equations derived. More experimental validation of FreeMHD is found here [20]. The aim of this work is beyond finding the required flow speeds to avoid overheating in LM-divertor systems.…”

“…FreeMHD was previously validated with experiments under different conditions that alter MHD drag forces in a LM flow [20] subject to a toroidal magnetic field. For this work, further validation of FreeMHD is shown for magnetic fields normal to the free surface of the flow and externally applied currents.…”