Effects of carbon steel surface oxidation on critical heat flux in downward-face pool boiling

Abstract

In this study, a downward-face pool boiling critical heat flux (CHF) experiment was performed using a carbon steel plate soldered to a copper base. The results of the experiment were analyzed and compared with those of a carbon steel and copper block experiment previously conducted at the same test facility. After polishing the surface using sandpaper, heat was applied to the surface by cartridge heating until CHF was attained. Due to the oxidation of the carbon steel, the surface changed gradually during boiling. The boiling cycle was repeated several times, and the CHF variations were observed. Detailed images of the heating surface behavior during boiling acquired by two synchronized high-speed cameras with different heat fluxes were analyzed. It was found that the more oxidized the surface became, the fewer bubbles were generated and the higher the CHF became; however, the bubble film departure frequency remained the same in all conditions. The contact angle and microscope surface images of the oxidation material were also obtained, and it was determined that the combined effect of the increase in wettability and decrease in nucleation site density was likely the reason that the CHF increased. This gradual oxidation process could be beneficial in actual situations since it can increase the upper limit of in-vessel retention external reactor vessel cooling.