多层次复合微纳结构表面影响过冷沸腾气泡行为的微距观测

Experimental investigation on the effects of multi-layered micro/nano structures on behaviour of subcooled boiling bubbles with macrophotography

通过真空钎焊的方法,将微结构的金属网与纳米尺寸的金属粉末结合,从而在过冷沸腾换热表面生成微纳米多层次复合结构。对比采用不同规格网格制备的表面与光滑表面的微观形貌特征,发现所采用处理工艺成功地在材料表面形成了凹状空穴、孔隙结构、类纳米柱体等不同尺度的三层次复合微纳米结构。与水的接触角测量显示,复合结构表面在保持原表面亲水性的同时,在一定程度上降低了对水的浸润性。进一步通过微距摄影观测表明,复合结构表面气泡生成密度均大幅度高于光滑表面,较细的网格尺度更显著,但小于120μm后增加不明显。分析发现,复合表面会诱导复杂的流动不稳定性,在增加汽化核心的同时也有效地避免了过早形成气膜,对提高临界热流密度有利。上述研究结果可为设计更先进的相变换热表面提供参考。

The multi-layered micro/nano structures are generated on the subcooled boiling surface with microstructural metal mesh and the nanoscale metal powder through vacuum brazing. Comparing the surface topographic characteristics of the cases manufactured by metal mesh of different aperture width scales with those of smooth surface case, it is founded that three layers of composite micro/nano structures with cavity, pore and nano pillar structures at different scales are successfully generated on metal surfaces by the proposed technique. The comparison of contact angles with water shows that the composite micro/nano structures on surfaces keeping the hydrophilic state can reduce the hydrophily to some degree. Further macro-photographic measurements show that the bubble densities generated on composite micro/nano structured surface in subcooled boiling cases are several times higher than that on smooth surface, especially for the case manufactured with smaller aperture width scale metal mesh. However, there is a critical aperture width scale of metal mesh, i.e., 120μm. The case with smaller aperture width than the critical value presents subtler increase of bubble generation density. It is concluded that the composite micro/nano structures can induce more complex flow instability on boiling surface, provide much more boiling nucleus and effectively delay the vapor filming generation, which will be beneficial to CHF(critical heat flux) promotion. The results would provide some reference for the advanced heat transfer surface developments.