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Title:Advanced geometries for dryout mitigation in temhd-driven liquid lithium systems
Author(s):Szott, Matthew Michael
Director of Research:Ruzic, David N
Doctoral Committee Chair(s):Ruzic, David N
Doctoral Committee Member(s):Allain, Jean Paul; Andruczyk, Daniel; Kapoor, Shiv G; Kersh, Mariana E
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Plasma facing components
Liquid metal
Thermoelectric magnetohydrodynamics
Heat flux
COMSOL Multiphysics
Multiphase flow
Level set
Abstract:The promise of nuclear fusion as an energy source is unparalleled, but the technological challenge is arguably the most difficult humanity has faced. As progress continues toward bringing sustained fusion power production to the grid, the conditions inside fusion devices are becoming more extreme. A common method of producing sustained fusion reactions is by heating isotopes of hydrogen until they are a plasma and confining them magnetically in a toroidal vacuum system. High plasma density, long confinement time, and extremely high plasma temperatures must be achieved in order to create efficient fusion. Plasma facing components (PFCs) must bear the brunt of these extreme conditions, which can result in a myriad of damage mechanisms on even the most resilient materials. One method of mitigating the damage in solid PFCs is through the use of liquid metals, specifically liquid lithium. Liquid lithium PFCs reduce erosion and thermal stress damage, prolonging device lifetime, and have been shown to enhance plasma performance, decrease edge recycling, and reduce impurities. Flowing open surface liquid metal concepts utilize flowing liquid lithium to provide a constantly refreshing PFC surface and can remove impurities from the device, though potential concerns include surface stability, wetting control, hydrogen retention, and heat flux handling. The Liquid Metal Infused Trench (LiMIT) concept pioneered at the University of Illinois harnesses the heat and magnetic fields already present in fusion devices to drive lithium flow via thermoelectric magnetohydrodynamics (TEMHD). Proof of concept testing at the Center for Plasma Material Interactions and larger scale testing in the HT-7 and EAST tokamaks and the Magnum PSI linear plasma device have shown sustained flow and improved plasma performance. Continued development of the system has focused on mitigating potential concerns, including defining stability criteria, enhancing ability to control lithium wetting and flow, and designing systems to recover the hydrogenic fuel species from lithium. Under high localized heat fluxes present in fusion devices, TEMHD forces can cause a depression of the lithium surface below the solid structures, minimizing the benefits of the flowing liquid system and risking damage. This is known as the lithium dryout phenomenon. This work adapts the standard LiMIT trench design to improve heat flux handling and eliminate the presence of lithium dryout on the free surface. Improvements to the design focus on extending the 1-D trench design to 2-D and 3-D flow channels, which result in post and foam structures. Using extensive COMSOL Multiphysics modeling and experimental testing, the propensity for TEMHD flow and the resistance to dryout in the face of high localized heat flux is investigated. The 3 post TEMHD designs exhibit effective TEMHD drive with maximum velocities on the order of 0.2 to 0.9 m/s, depending on the geometry and the peak heat flux applied. The addition of secondary flow channels improves dryout resistance, though swirling flow and eddies develop around the posts. Experimental testing verifies the usefulness of the crosstalk to distribute flow, and velocities match numerical modeling of the system. A disordered foam geometry and 3 ordered foam geometries are tested as concepts to improve capillary action and surface stability while still allowing TEMHD flow. While the internal structure of the disordered foam as manufactured did not prove compatible with liquid lithium, a new pipeline was developed to incorporate arbitrary geometries into TEMHD modeling. The ordered designs exhibit sustained TEMHD flow of slower maximum magnitude than the post geometries, between 0.05 and 0.35 m/s. This reduction in flow speed comes with improved resistance to dryout. Experimental testing of proof of concept cases showed velocities that matched numerical modeling. Electron beam testing of the foam proves heat flux handling capabilities of the designs and increases the operating regime of the LiMIT system by 127%, to 6.8 MW/m2, with no signs of dryout or impending damage. New capabilities of multiphysics modeling of the TEMHD systems were developed to capture the motion of the free surface. Using a level set multiphase model, TEMHD flow under low heat flux proof of concept conditions was replicated. Applying a high heat flux stripe to the free surface resulted in lithium dryout and pileup in the trench domain, which was reduced in the post and foam designs. The inclusion of surface tension in the model steadies the free surface against dryout. Surface tension values near that of liquid lithium, up to 0.3 N/m, were applied. The surface tension simulations displayed successful elimination of dryout in a 3 MW/m2 peak heat flux scenario, but the large surface forces induced spurious wave motion in the free surface.
Issue Date:2020-05-01
Rights Information:Copyright 2020 Matthew Michael Szott
Date Available in IDEALS:2020-08-26
Date Deposited:2020-05

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