An experimental study on bubble dynamics and pool boiling heat transfer of grinding/laser-structured surface

In the present work, a novel technique is introduced to enhance thermal conductivity of the surface by grinding process with predefined location of diamond abrasive particles on wheel surface. Grinding process with modified wheel led to an increase the bubble nucleation site formation and departure frequency in surface channels. For a more detailed study, grinding results were compared with the surface produced by the laser with the same morphology. Although both the structured surfaces revealed better heat transfer coefficient in comparison with to unstructured surface (grinding by 455% and laser by 348%), abrasive-structured surface present 130% improvement comparing the surface produced by laser for wall superheat of 10 K. The cause of this phenomenon is the formation of an oxide layer during the laser processing, which has led to a decrease in the thermal conductivity of the surface. This phenomenon was clearly detected by X-ray-spectroscopy mapping analysis, and to better understand it, the heating surfaces reached two consecutive critical heat flux. The result showed that after the first achieving CHF, the sample surface oxidizes, which leads to a decrease in the CHF and an increase in wall superheat. The hypothesis of formation of the oxide layer on the surface after CHF was also proved by static contact angle testing, which the angle increased from 115 to 133° after boiling. This 13.5% improvement was due to an increase in surface roughness stems from the formation of the oxide layer. Increasing the hydrophobicity of the surface led to a problem to facile water refilling (surface rewet ability) and capillary wicking of the surface and to delay the nucleation site formation, bubble formation, its growth, and finally departure frequency. Graphical abstract