Magnetohydrodynamics and Heat Transfer in a Free-Surface, Flowing Liquid Metal Experiment

Abstract

This thesis examines magnetohydrodynamic (MHD) effects and heat transfer in free-surface, liquid metal flow. The interaction of the flow with the applied magnetic field introduces an anisotropy to the fluid motion, resulting in starkly different behavior depending on the orientation and strength of the applied magnetic field. The transition from a hydrodynamic state to an MHD state was experimentally found to be characterized by the interaction parameter, <italic> N = &sigma L B<super>2</super>/&rho v0 </italic>, crossing above 0.5.

The experiment consisted of a channel 1 m in length, up to 16 cm in width and 1 to 2.5 cm in depth situated within an electromagnet capable of producing a uniform magnetic field up to 2.7 kG. The fluid velocity was measured with an array of 25 potential probes that resolved the spanwise velocity profile, while waves on the surface of the metal were monitored via a diagnostic that tracked the motion of a laser beam reflected off of the free surface. Lastly, the temperature response of the fluid was recorded with an array of 32 thermocouples embedded in the bottom of the channel and a mid-wavelength infrared camera used to image the free surface.

Three distinct sets of experiments were performed to investigate the effects of the applied magnetic field on the dynamics within the flow, studying: 1) changes to vortices in the wake of a cylinder, 2) behavior of surface fluctuations due to the turbulent flow, and 3) vertical heat transfer resulting from heat deposition on the free surface. The first set of experiments examined the wake of a cylinder inserted into the flow with its axis parallel to the magnetic field. Measurements with the potential probes indicated that vortices in the wake became laminarized as the field was increased due to the alignment of the injected vorticity with the applied field. The second set of experiments showed that surface fluctuations were suppressed at high magnetic fields, but the precise nature of the damping depended on the orientation of the applied field. Lastly, when heat was injected at the free surface via a resistive heater, the temperature response of the fluid was found to vary considerably depending on the strength and direction of the applied field.

In all experiments, it was observed that the angle between the vorticity of structures in the flow and the applied magnetic field dictated the macroscopic behavior of the fluid. This work provides the first experimental results of several phenomena associated with free-surface, liquid metal flows in an applied magnetic field. Further, the results have direct implications for proposed flowing liquid metal divertors for fusion reactors, and offer insight into the nature of turbulent MHD flows.