Terrestrial Investigation of Vapor Bubble Characteristics in Nucleate Pool Boiling of a Dielectric Fluid on an Asymmetric Microstructured Metal Heat Sink (Pre-cursor to Microgravity ISS Experiments)

Abstract

This experimental, terrestrial study is part of a larger effort to dissipate increased heat fluxes through enhanced pool boiling in spacecraft electronics prior to an extensive study to be conducted on the International Space Station under pristine microgravity conditions. The absence of buoyancy forces in microgravity causes vapor bubbles to grow to a very large size, leading to premature critical heat flux (CHF). Using an engineered surface modification, namely an asymmetric sawtooth ratchet, to create mobility of the vapor mass can alleviate this problem. The stainless steel (SS 316L) test surfaces were fabricated using powder bed fusion, a metal additive manufacturing process. Boiling experiments were conducted in the upward-facing and downward-facing orientations in the designed terrestrial setup. Apart from the surface orientation, parameters such as cavity density (1-mm or 2-mm apart), sawtooth structure (60°-30° or 75°-15°) were also studied. The downward-facing orientation explored in this study is the bridge between the terrestrial and microgravity experiments, as buoyancy forces do not detach vapor bubbles from the surface. Vapor mobility was observed in the downward-facing configuration for the asymmetric sawtooth structure explored in the study. A thin liquid film was observed between the microstructure and vapor bubbles as they slid along the microstructure. The non-uniform nature of this liquid film is explored using high-speed imaging at the crest and trough of the sawtooth. A model to predict liquid film thickness for the downward-facing surface is proposed, based on modifications from a previous NASA Zero-G flight model. The proposed asymmetric saw-tooth microstructure is a potential technique to induce motion of vapor bubbles across electronic components when reduced buoyancy forces do not detach vapor bubbles from the surface.