探討具有雷射燒蝕的微米柱狀結構和電鍍多孔結構之銅表面對Novec-7100池沸騰的綜合影響

Abstract

本研究探討了飛秒雷射燒蝕的微米柱狀結構和電鍍多孔層一起施加在平坦銅表面上，對Novec-7100池沸騰熱傳的綜合效果。在微米柱狀結構的製造過程中使用了飛秒雷射，並在雷射燒蝕測試表面之後，對測試表面再進行了二步電鍍製程以進一步增強表面。在二步電鍍製程中，第一階段較大的電流密度產生的氫氣泡可以在測試表面上製造微小空腔。由於這兩種表面改質方法的支持，當柱狀結構的間距（溝槽寬度）為300μm且第一階段電流密度為500 mA/cm2時，獲得了最高的HTC（比光滑平坦表面增加206%）和CHF（比光滑平坦表面增加24.85%）的改進。HTC的增加可以歸因於再濕潤能力的增強、表面粗糙度、空腔數量和空腔尺寸的增加，並通過雷射共軛焦顯微鏡分析、掃描電子顯微鏡圖像和氣泡高速攝影圖像得到了解釋。此外，在第一階段電流密度較高的情況下，CHF值由於毛細能力的改善而增加；而在較低的第一階段電流密度的情況下，由於電鍍產生的孔徑不夠大，甚至可能減少氣泡浮力相較於孔徑黏附力的大小，導致氣泡停滯在表面上不離開，因而導致CHF值與光滑平坦表面相比減少。

This study investigated the combined effects of laser-textured micro-pin-fin and electrodeposited porous layer over a flat copper surface on pool boiling heat transfer of Novec-7100. The femtosecond laser was applied during the fabrication process of micro-pin-fin. After the laser texturing, a two-step electrodeposition process was employed on the test surface for further surface enhancement. The ability of the larger first-stage current density to generate hydrogen bubbles can create cavities on the boiling surface. Supported by these two surface-enhancing effects, the greatest improvement of HTC (206% increment compared with plain surface sample) and CHF (24.85% increment compared with plain surface sample) was achieved using a modified surface with a pin spacing (groove width) of 300 μm and a first-stage current density of 500 mA/cm2. The HTC increment can be attributed to the increased ability of circulation of the rewetting liquid, surface roughness, number of cavities, and cavity size, and elucidated by the analysis of laser confocal microscope, SEM images and high-speed images of bubbles. Furthermore, compared with the plain surface sample, the CHF values increased due to the improvement of the capillary wicking ability in the region of higher first-stage current density, and decreased due to the smaller pore size that could deteriorate the buoyancy of the bubbles compared with the adhesion force in the cavities, causing the bubbles to stagnate on the surface, in the region of lower first-stage current density.