A Visual Study Of Bubble Dynamic Behavior On Surface With Micro-nano Composite Structures

Micro-nano composite structures refers to one type of composite structure which includes micro and nano structures inside. This composite structures has not only features of micro scale structure, but also special properties of nano-scale structures. So, the micro-nano composite structures has more unique effects of heat transfer enhancement. Study on flow enhancement and phase-change heat transfer on surface with micro-nano composite structures is rising now. This study has important academic meaning for revealing mechanism of flow enhancement and phase-change heat transfer on surface with micro-nano composite structures. It also makes for developing and enriching phase-change heat transfer theory. As for application in engineering, research achievement of this subject can provides theoretical basis, principles and methods for design of micro-cooling system for electronic devices with high heat flux. It also has great significance for solving bottleneck problem of heat dissipation in electronic industry and promoting technical process in electronic industry of our country.In this experimental study, after a progress of magnetron sputtering on wafer with micro grooves, the surface with micro-nano composite structures is then formed. Using surface with micro-nano composite structures as the heat sink, visual study of bubble dynamic behavior and experimental research of heat transfer characteristics are conducted.The visual study of bubble dynamic behavior on surface with micro-nano composite structures is first conducted. When there is nucleate boiling occurring on surface of the vertically placed heat sink, the bubble dynamic behavior is then under observational study. High-speed photography and movement analyzation technology are used to the bubble observational study. One high-speed camera (Phantom V5.1) is used to take and record pictures. And its accompanying software (PCC 2.3) can playback bubble pictures frame by frame to extract and process information. After the process, each bubble dynamic behavior parameter including bubble growth time, bubble waiting time, bubble period, bubble departure frequency, change of bubble size, bubble departure diameter and others can be got. Results of experimental statistics indicate that with increase of thickness of titanium nano-coating, bubble growth time decreases, bubble waiting time increases, bubble period decreases, bubble departure frequency increases, bubble departure diameter increases, boundaries during growth stages of bubble are more and more indistinct at the same time. For these results of bubble dynamic behavior, size effect and surface effect of nano-materials are introduced to discuss and explain them.Secondly, heat transfer characteristics of surface with micro-nano composite structures are also studied in this experiment. For the study on saturated nucleate boiling on surface with micro-nano composite structures, the whole thermal resistance system in the heat transfer process is analyzed, where heat transferred from heat source to surrounding water vapor via the surface with micro-nano composite structures. Then, the analysis of thermal resistance is simplified according to order of magnitude. Curve of nucleate boiling heat flux to wall superheat (q-Δ7) can be drawn, as well as curve of nucleate boiling heat transfer coefficient to wall superheat (ho-ΔT). Experimental results show that the boiling heat transfer curve can significantly shift to left with increase of thickness of titanium nano-coating. This indicates that under the same heat flux, the wall superheat decreases and the boiling heat transfer coefficient increases on surface with micro-nano composite structures, compared with those on the microgrooves surface without titanium nano-coating.