ANALYTICAL AND NUMERICAL MODELING OF A THERMAL SWITCH VIA SHAPE CHANGING STIMULI-RESPONSIVE MATERIAL

Here the authors present a thermal switch design composed of a stimuli-responsive material (SRM) component paired with a structured material (SM) component for thermal insulation applications. A numerical model is used to assess the performance of a point versus area contact switch design in a vacuum environment where the emissivity is set to epsilon >= 0.00, with a conduction-based analytical model developed to validate the numerical model for when epsilon = 0.00. A point contact switch is governed by contact at a singular point in the ON state to enhance thermal transport compared to the OFF state. Conversely, when an area contact switch is in the ON state, thermal transport occurs across a cross-sectional area rather than at a point. For the numerical solution of the temperature scalar field, a finite element method (FEM) approach discretizes the governing equations that are solved via the Newton-Raphson method. For the conduction-based analytical solution, a trigonometric approach based on the geometry of the design is used to find the geometrical factor and volume fraction, whose product with the material thermal conductivity produces the switching ratio. Numerical analysis reveals that increasing the contact area of the switching component results in a significant increase in the switching ratio, especially at the geometric limits of the design. When epsilon = 0, the switching ratio of the point contact is found to be approximately r = 1.54, while the area contact has a switching ratio up to r = 134.25 as the contact area increases to its largest value. The conduction-based analytical model produces a switching ratio that ranges from r = 1.12 to r = 102.61 as the contact area increases to its maximum value. For epsilon > 0, r increases with epsilon for s(*) <= 0.20, but r decreases with epsilon for s(*) >= 0.60, after undergoing an inflection for 0.20 < s(*) < 0.60. This inversion occurs due to the radiative thermal loss to the ambient environment. With a conduction-only model, the switching ratios of the analytical and numerical models are approximately identical and follow the same trend with increasing contact area. An extended numerical model investigates the thermal contact resistance and mechanical forces that may limit the efficiency of the switch design. This work provides a detailed quantitative analysis on how contact area strongly affects the performance of a thermal switch design that utilizes a stimuli-responsive material as the switching mechanism.