Lateral fluid motion in nucleate boiling through asymmetric surface structures

Abstract

This thesis presents a feasibility study of a means to passively effect liquid motion parallel to a heated surface though surface geometrical modifications. Such a passive system is beneficial for electronics cooling applications as it reduces the pumping equipment normally required in flow loops and is desired for space applications, where launch costs greatly restrict the weight of onboard systems. The surface geometry considered was a repeated array of asymmetric silicon ratchets with reentrant cavities located on the shallow face. A serpentine thin film heater provided heat to the surface. A complete experimental facility in which to perform experiments was designed and constructed as part of this work to comply with requirements set by microgravity flight services in anticipation of future experiments in microgravity.

Experiments were performed using deionized and degassed water at atmospheric pressure at subcoolings of 5°C and 20°C. Applied area averaged heat flux was varied between 2 W/cm² and 19 W/cm² at these conditions. Magnified high-speed videos were used to resolve bubble behavior near the surface. A preferential bubble growth and departure direction was confirmed for both subcoolings and lateral fluid motion was confirmed in the high subcooling condition. Repeatability was confirmed with separate experiment performed 58 days later. Tracking of bubbles was accomplished using a custom bubble-tracking algorithm, designed to resolve only bubbles within a two-dimensional plane normal to the viewing direction. Instantaneous velocities of individual bubbles parallel to the surface were shown to be in excess of 600 mm/s immediately following departure, and liquid flows with mean velocities between 25 mm/s and 35 mm/s parallel to surface were observed in the plume farther from the surface. A simplified semi-empirical model of bubble growth phase is proposed to explain the observed mean liquid velocities.