Dynamics of Bubble Motion and Bubble Top Jet Flows From Moving Vapor Bubbles on Microwires

Abstract

Rapid bubble sweeping along heated wires was observed during subcooled nucleate boiling experiments on very fine wires with jet flows emanating from the tops of the vapor bubbles for a variety of conditions. This paper presents experimental results with a numerical analysis of the physical mechanisms causing the experimentally observed bubble motion and jet flows. The results show that the moving bubble creates a nonuniform temperature distribution in the wire by cooling the wire as it moves along the wire with significant heat transfer in the wake behind the bubble. The results verify that the bubble motion is driven by the temperature difference from the front to the back of the bubble, which causes Marangoni flow. The Marangoni flow then thrusts the bubble forward along the wire with the calculated bubble velocities agreeing well with experimental measurements. In addition, the temperature difference from the bottom to the top of the bubble creates a vertical component to the Marangoni flow that results in the jet flows from the bubble tops. Comparisons with experimental observations suggest that the condensation heat transfer at the bubble interface is restricted by noncondensable gases that increase the surface temperature gradient and the resulting Marangoni flow. The numerical results also show that the heat transfer from the wire due to the Marangoni flow is significantly larger than the heat transfer due to the evaporation under the bubble.