In the present study, experiments of downward-facing flow boiling were conducted to investigate oxidation effects of copper under atmospheric conditions. The copper surface was polished and then put into a drying oven for a period of 1 h, 3 h and 6 h under 300°C. Compared with a bare surface, a deterioration in the heat transfer coefficient and an enhancement in critical heat flux (CHF) was found in the oxidized case for three mass flux conditions. With the mass flux increasing, the CHF enhancement ratio decreased. After oxidation, the contact angle decreased. Although many previous studies tried to explain the CHF enhancement by the change of contact angle, it was difficult to be applied in the downward-facing flow boiling. The enhancement of CHF was associated with nucleation site density (NSD) change caused by the surface morphology change during oxidation. Fewer bubbles generated on the copper surface after oxidation. The number of bubbles generated on the surface was calculated by a python code and it showed an inverse relationship with CHF, which was explained by a model. Under higher heat flux, the vapor cover area decreased, which was a result of the reduction in NSD after oxidation. Microscope images showed the decrease of the number of cavities on the surface, which was likely to be the reason for the decreasing of NSD.
