Pool-Boiling Enhancement on Periodic Micro/Nano Ripple Structured Surfaces Fabricated by Femtosecond Laser Treatment

The improved pool boiling was investigated by the periodic micro/nano ripple structure created on the metal surface by femtosecond laser processing. Pure copper was used as the base metal substrate. The change of the sample surface morphology according to the laser irradiation intensity was examined through SEM and confocal laser microscopy. Pool boiling tests were performed to compare the three laser fabricated samples and the bare copper surface using deionized water as a working fluid at atmospheric pressure. Each sample's pool boiling heat transfer coefficient (HTC) and critical heat flux (CHF) were compared. Bubble nucleation on all surfaces was visualized with a high-speed camera. At the onset of nucleate boiling (ONB), each sample's superheat, bubble departure diameter, and bubble growth period were measured and compared. The most efficient periodic micro/nano ripple structured surface reduces the superheat at ONB by 6.13 [[EQUATION]] C compared to the bare copper surface. The enhanced surface structure reduces the bubble departure diameter and bubble growth period by 60.5% and 55.58% compared to the bare surface, respectively. In addition, the HTC and CHF are enhanced by 178.50% and 39.07%, respectively. It is confirmed that the combination of increased nucleate site density, high nucleation site activation, and wicking effect in the periodic micro/nano ripple structure significantly improves pool boiling heat transfer performance.