Pool Boiling with Engineered Surfaces via Femtosecond Laser Surface Processing: Extraordinary Dielectric Heat Transfer Enhancement, Boiling Inversion Mechanism, and Microstructure Characterization

In this work, functionalized metallic surfaces are tested in a series of pool boiling experiments with efforts being made to increase critical heat fluxes (CHFs) and heat transfer coefficients (HTCs). Femtosecond laser surface processing (FLSP) was utilized as a functionalization technique to provide a single-step, scalable and permanent modification technique for enhancing heat transfer. FLSP leads to self-organized, quasi-periodic structure formations on the surface containing both micro and nanoscale roughness. FLSP surface modifications were applied to stainless steel to understand the nucleation dynamics associated with boiling inversion, a phenomenon in which the surface temperature reaches a maximum superheat, beyond which it decreases with further increase in heat flux. A sudden change in the nucleation dynamics was found occurring near the onset of boiling inversion. Functionalized copper surfaces were tested with and without a citric acid cleaning process to remove oxidation from the surface. The impact of the acid cleaning process was found to be dependent on the distribution of the oxides. For oxidation confined to the surface, CHF was enhanced after oxide removal. CHFs decreased after removal of oxides distributed in layers below the surface. A hybrid approach combining FLSP and copper hydroxide nanoneedles was also tested for boiling enhancement. Inclusion of the nanoneedles led to degraded performance due to the copper hydroxide decomposing into copper oxide, thereby adding insulating material to the surface. Additionally, FLSP applied to aluminum surfaces was tested using a dielectric fluid (PF-5060) as a working fluid. Laser fluence and pulse count were varied and led to significant enhancement across a range of laser parameters. The best performing surfaces achieved 680% and 110% enhancement in the maximum HTC and CHF, respectively. Enhancement was attributed to an abundance of nucleation sites found after laser processing. Finally, the impact of the nanoparticle layer on FLSP aluminum in PF-5060 was examined. CHF was found to consistently decrease after removing the nanoparticle layers which aided in liquid resupply