Visual Experiment For The Effect Of Surface Wettability On Single Bubble Pool Boiling

Boiling heat transfer is an efficient and stable heat transfer method,which is widely used in electronic components,nuclear reactor operation and other industrial fields.In the process of pool boiling,the stage of nuclear boiling occurs from the beginning of bubbles on the heat transfer surface to the formation of vapor film.Nuclear boiling has the advantages of small surface temperature difference and strong heat transfer,so it is the key research object in boiling heat transfer process.There are complex phase transition phenomena in boiling process,which mainly occur at the solid vapor liquid interface of bubble contact line,and the heat transfer effect in boiling process is affected by bubble behavior.The change of the wettability of the heat exchange surface will also affect the surface tension at the gas-solid-liquid interface and directly affect the growth and detachment of bubbles.Boiling in isolated bubble pool plays an important role in revealing the influence of bubble growth behavior on boiling heat transfer.In conventional machined surface boiling experiments,multiple bubbles are often generated at the same time,which causes the mutual shielding and merging of bubbles.Therefore,the main purpose of this study is to realize the isolated bubble growth by making a surface with artificial cavity heating.The influence of artificial cavity size and surface wettability on the dynamic characteristics of single bubble in the process of single bubble pool boiling was explored to reveal the influence of bubble behavior on the heat transfer of nuclear boiling.In this paper,cylindrical cavities with diameters of 110 μm and 167 μm were formed on the heated surface of copper by mechanical micromachining,and the insulated and sealed surfaces around the heat exchange surface were replaced by epoxy resin coating to realize the process of isolated bubble pool boiling.The changes of the bubble base circle radius,volume,contact angle and other morphological parameters during the bubble growth process were recorded by a high-speed camera,and the change of the flow field around the bubble in the process of growth and separation was obtained through PIV experiment.Two types of wettability heat transfer surfaces with static contact angles of 40.3° and 69.3° were further fabricated.Morphological parameters and heat transfer properties of single bubbles on different wettability heat transfer surfaces were compared.The experimental results show that the apparatus can realize the isolated growth of bubbles in the process of nuclear boiling.The bubble detachment frequency increases with the increase of the wall superheat,and it can be concluded that the bubble detachment frequency increases with the increase of the drag force of the wake under different superheat.The experimental comparison of artificial holes of different sizes shows that the diameter of artificial holes has a great influence on the bubble behavior.The bubble growth period on the heat exchange surface with larger diameter is shorter,the contact Angle at the moment of detachment is larger,and the volume is smaller.Surface wettability has an important influence on bubble behavior,and it is difficult to generate bubbles on the heat exchange surface with contact angle of 40.3°.The waiting time of bubble formation on different wettability heat transfer surfaces increases with the increase of bubble growth time.The bubble waiting time of the heat exchange surface with static contact angle of 69.3° is far less than that of the heat exchange surface with contact angle of 40.3°,and the growth time is far longer than that of the heat exchange surface with contact angle of 40.3°.