Ph.D. Proposal Presentation by Timothy Koehler
Monday, November 28, 2005

(Dr. Said Abdel-Khalik, Chair)

"Experimental and Numerical Investigation of Thermocapillary Effects in Thin Liquid Layers"

Abstract

Thin liquid layers have been proposed for heat extraction and protection of the solid surfaces of divertors in magnetic fusion reactors. Conceptual designs of plasma-facing components use stationary and flowing liquid layers as a renewable first wall for reactor chambers in order to remove heat and shield solid surfaces from damaging radiation while maintaining acceptable plasma purity levels. Liquid-protected components have the potential to make fusion more commercially attractive by increasing reactor lifetimes and decreasing failure rates. The results of this doctoral research will help enable designers to identify the parameter ranges for successful operation of such protection schemes.

Previous work on thermocapillary effects has primarily focused on thin films of very small aspect ratios that lie within the limits of asymptotic analyses. This proposed research will investigate the thermocapillary behavior of thicker liquid layers above non-uniformly heated solid surfaces, a regime where inertial forces are non-negligible. A numerical model utilizing the level contour reconstruction method is used to follow the evolution of the liquid free surface above an axisymmetric non-isothermal surface. By varying the governing dimensionless parameters (viz. the Marangoni, Prandtl, and Bond numbers), the corresponding maximum allowable surface temperature gradient is determined. This maximum gradient will in turn be used to generate generalized non-dimensional charts that provide the design limits to avoid film dry-out and hot-spot formation for various operating conditions. Building upon the previous two-dimensional long-wave analysis by Tan et al . (1990), an analogous axisymmetric analysis is also performed. Experimental validation of the numerical simulations are performed using layers of silicone oils of various viscosities and surface tension temperature coefficients on a horizontal flat plate. A radial temperature gradient with the temperature decreasing from the center to the edge is imposed on the surface of the plate. A needle contact technique is used to obtain height profiles for various Marangoni numbers. The results of this investigation will allow designers to identify operating windows for successful implementation of liquid-protected plasma–facing components.