Capillary and Boiling Limits of Micropillared Thermal Wicks

University Of Texas At Austin, Austin TX

Investigators

Abstract

PI: Carlos Hidrovo Proposal #: 1134104 This project aims to understand the governing physics behind capillary limit flow in microstructures with phase change, with the objective of developing thermal wicks capable of dissipating upwards of 1 kW/cm2. The transformative aspect of the project resides in providing new insights into the effects that microstructure geometry has on thermal-phase change capillary flow systems. Furthermore, this understanding will lead to the development of novel wicking structures based on vertical micropillared arrays for heat pipe and vapor chamber applications capable of low temperature heat fluxes not seen before. Specifically, the project will tackle and elucidate the governing physics behind the capillary flow within arrays of vertically etched silicon micropillars, particularly related to phase change heat transfer applications. The following questions will be addressed by the project: (1) How does microstructure affect capillary flow? (2) What are the key parameters that control capillary pumping capabilities in such wicking structures? (3) How are heat transfer and phase change processes coupled to the capillary flow? (4) Can MEMS technology, in the form of micropillared wicks, be implemented as a means of exceeding current heat pipe and vapor chamber heat flux dissipation capabilities? In order to answer these questions, the following specific tasks have been identified: (i) Introduction of a novel experimental setup capable of simultaneously obtaining thermal and capillary flow data for different wick samples. This system will be used to assess the thermo-hydraulic performance of silicon based micropillar samples. (ii) Fabrication of silicon based micropillared wicks, with precisely controlled microstructure of varying geometry. (iii) The experimental results will be complemented by and validated against compact models that will capture the relevant physics associated with the capillary flow, phase change (boiling), and thermal transport in these systems. (iv) The coupled experimental and modeling results will be used as a design tool towards optimization of ideal microstructure geometries for the silicon micropillar array samples. The intellectual merit of the project includes elucidating the impact that micro-geometry have on phase change capillary systems. Since both the models developed and the experiments conducted will be done on specially designed samples with known geometry, the underlying physics will have a much clearer context. This will allow for better understanding of the key parameters that affect capillary flow in thermal systems with phase change. Lessons learned from this research will carry over to the understanding of capillary flow with phase change on non-regular and non-uniform structures, such as fractals. The broader impact of the project includes having significant relevance in the general area of capillary flow in porous media, with major implications for fields such as geology, hydrology and manufacturing. The problems to be tackled here present rich engineering, physics, and materials challenges, great cross-disciplinary projects for the graduate and undergraduate students involved. The PI is a young leader in experimental thermal fluids, MEMS and optical diagnostics. The results of the project will provide new classroom materials for courses that the PI teach or is currently developing, as well as outreach activities geared towards elementary school audiences. Importantly, the PI will initiate an undergraduate summer internship program, aimed at underrepresented UT freshmen and sophomores to truly prepare them as multidisciplinary leaders of future engineering challenges.