External reactor vessel cooling (ERVC) is the key technology for In-Vessel Retention (IVR) to ensure the safety of a nuclear power plant (NPP) under severe accident conditions. The thermal margin of nucleate boiling heat transfer on the reactor pressure vessel (RPV) lower head is important for ERVC and of wide concern to researchers. In such boiling heat transfer processes, the reactor vessel wall inclination effect on the heat transfer coefficient (HTC) and critical heat flux (CHF) should be considered. In this study, experiments were performed to investigate the effects of heater material and surface orientation on the HTC and CHF of nucleate boiling. Copper and stainless steel (SS) surfaces were used to perform boiling tests under atmosphere pressure. The orientation angle of both boiling surfaces were varied between 0° (upward) and 180° (downward). The experimental results show that the surface orientation effects on the HTC is slight for both the copper surface and the SS surface. In addition, the relationship of measured CHF values with the inclination angles was obtained and it shows that the CHF value changes little as the inclination angle increases from 0° to 120° but it decreases rapidly as the orientation angle increases towards 180° for both boiling surfaces. The material effect on CHF is also observed and the copper surface has higher CHF value than the SS surface. Based on the experimental data, a correlation for CHF prediction is developed which includes both the surface orientation effect and the heater material effect.

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