Two-Phase Pool Boiling and Flow Boiling Heat Transfer Enhancement with Femtosecond Laser Processed Metallic Surfaces

In the present work, functionalized metallic surfaces are studied for pool boiling and flow boiling heat transfer performance. These metallic surfaces are fabricated with the use of femtosecond laser surface processing (FLSP). This technique uses a laser to create self-organized micro/nanostructures on metallic surfaces. These surfaces are beneficial for heat transfer enhancement because they increase the active surface area, increase surface roughness, and result in superhydrophilic and highly wicking surfaces. Multiple heat transfer experiments were conducted for both pool boiling and flow boiling. A series of stainless steel FLSP surfaces were fabricated and tested for pool boiling performance. This first set of samples was created with NC-Pyramid FLSP structures. These structures were known to have a thick nanoparticle layer that covers the core microstructure. With these FLSP surfaces, an increase in the CHF was measured as high as 141 W/cm2 compared to 91 W/cm2 for a polished reference sample. Although the CHF increased, the HTC generally decreased for most of the surfaces. The thick nanoparticle layer was found to act as an insulator and thus limiting the performance. This was confirmed by cross sectioning the microstructures as well as selectively removing the nanoparticle layer which resulted in an increase in the HTC. A new phenomenon referred to as secondary boiling effects was also studied. This focuses on a unique characteristic of FLSP boiling surfaces where the surface temperature drops as the heat flux increases near the CHF. FLSP surfaces with lower thermal conductivities result in secondary boiling effects due to a shifting nucleation dynamic associated with the geometry of the microstructures. Additionally, a series of copper FLSP pool boiling surfaces were tested. These surfaces were found to have drastically decreased CHF and HTC values. Unlike stainless steel FLSP surfaces, copper FLSP surfaces develop a very thick and porous oxide layer on the core microstructure. This oxide layer was studied and confirmed to be the cause of the poor heat transfer performance with copper FLSP surfaces and a method for selectively removing this oxide layer was proposed. Finally, stainless steel FLSP surfaces were applied to a single microchannel flow boiling configuration. Four unique FLSP surfaces were studied and evaluated for heat transfer performance. Three different flow rates were analyzed. Both the CHF and HTC values increase with increasing flow rate. Also, the FLSP surfaces did not result in a drastic change in the CHF compared to the polished surface but did significantly increase the HTC. At the largest flow rates, CHF values of about 250 W/cm2 and HTC as high as 300,000 W/m 2-K were measured, which was significantly higher than that found in pool boiling