Study on microbubble dynamic behaviors at vertical micro-nano hybrid surfaces based on open capillary microgrooves heat sink

Titanium (Ti) nano-coating is deposited on the open rectangular microgrooves heat sink to form a micro-nano hybrid surface. A visualization experimental study on microbubble dynamic behaviors at the vertical micro-nano hybrid surface is performed. Experimental investigations on the influences of nano-coating thickness and microgroove geometry on bubble behaviors of micro-nano hybrid surfaces are systematically conducted. Experimental results indicate that the formation of micro-nano hybrid surface can significantly affect the bubble behaviors and heat transfer characteristics, which can be attributed to combined effect of micro-configuration restriction and larger specific surface area as well as the stronger solid-liquid interaction resulting from nano-coating. As the increasing thicknesses of Ti nano-coating, the bubble period decreases and the bubble departure frequency increases. The phenomena are more notable at lower heat flux. Heat transfer coefficients (HTC) for various surfaces are measured to further verify the heat transfer enhancement effect of the micro-nano hybrid surfaces. The maximum of HTC improvement is 61% compared to the microgroove surfaces. For different Ti nano-coating thicknesses, the improved HTCs are mainly attributed to the increase in the nucleate site density of bubble from the enhanced porosity and surface roughness. For the micro-nano hybrid heat sinks, the smaller microgroove sizes could reduce the equivalent diameter of bubble-burst. Compared with microgroove surfaces, the variation of micro-configuration dimension has a more marked influence on equivalent diameter of bubble-burst for micro-nano hybrid surfaces. Under the condition of the same microgroove geometry, the bubble-burst equivalent diameter of micro-nano hybrid surface is significantly smaller than that of microgroove surface. This paper reports the improved two-phase heat transfer characteristics at vertical micro-nano hybrid surfaces and demonstrates the effects of surface characteristics in micro- and nano-scale on two-phase heat transfer, which provide a new insight on effectively enhancing boiling heat transfer.