Pool Boiling Inversion and Hysteresis with Femtosecond Laser Processed 304 Stainless Steel Surfaces for Heat Transfer Enhancement

In this paper, experimental results of pool boiling inversion and hysteresis on functionalized 304 stainless steel using distilled water at atmospheric pressure are presented. The surface was modified using femtosecond laser surface processing to create hierarchical micro/nanostructures. Boiling curves were generated once steady-state was reached, with steady-state defined as having a temperature slope less than 0.25 degrees Celsius per minute. The steady-state criterion was examined by allowing the surface to have extended boiling times at specified heat fluxes. In separate runs, the surface boiled for four hours at heat fluxes of 25, 40, 60, and 70 watts per square centimeter once the steady-state criterion was reached. No change in superheat was seen for the 25 and 40 watts per square centimeter heat fluxes over the four hour period. Drastic reductions in superheat of 6.4 and 4.9 degrees Celsius were seen for the 60 and 70 watts per square centimeter heat fluxes, respectively, resulting in higher heat transfer coefficients when compared to a run without extended boiling. It was found that at heat fluxes below the pool boiling inversion point, the superheat did not change with extended boiling times and the steady-state condition was adequate. The opposite was true for heat fluxes above the pool boiling inversion point. The decrease in superheat at higher heat fluxes was attributed to the change in nucleation dynamics, which are the cause of the pool boiling inversion phenomenon. These changes in the nucleation dynamics are suspected to cause a delay in the activation of nucleation sites, requiring longer times to achieve true steady-state conditions. To illustrate the difference in times, lower heat fluxes reached true steady-state in 15 to 20 minutes whereas higher heat fluxes took nearly three hours. Hysteresis of the boiling curve was examined by decreasing the heat flux once it reached about 105 watts per square centimeter. Hysteresis occurred during each run and resulted in a more efficient boiling curve when compared to the increasing heat flux portion. The hysteresis curves for each run were nearly identical regardless of the extended boiling at varying heat fluxes. These results highlight the need for careful consideration and understanding of the steady-state criterion, especially with surfaces that result in pool boiling inversion. These results further confirm that combining pool boiling inversion with hysteresis can result in stable and repeatable enhancement in the boiling curve, regardless of the thermal history of the surface.