During a severe accident, ex-vessel cooling may pose a risk for larger-powered reactors. The current in-vessel retention (IVR) (through ex-vessel cooling) capability may not be sufficient for the larger-powered reactors, and critical heat flux (CHF) conditions may eventually lead to vessel failure. A manner in which the CHF can be increased is by applying a structured surface design on the outer surface of the reactor pressure vessel (RPV). A simple design proposed in this work is the pin–fin. An experimental investigation was performed to observe the effect of the pin–fin on CHF with a downward-facing heated surface in flow boiling conditions. A reduced pressure of approximately 0.05 MPa allowed for saturation at approximately 81 °C. A range of flow rates corresponding to mass flux of 202–1456 kg/m2 s were applied in the experiments. The results showed an increase in the CHF when compared to a bare surface. An average CHF enhancement of 61% was observed from the finned surface. An enhancement of approximately 19% was observed in the heat transfer coefficient. As seen in nanoparticle/nanofluid enhancement, an increase in the CHF also leads to an increase in the superheat. Even though an increase in the CHF had been observed, the CHF for the finned and bare surfaces occurred at approximately similar superheat.

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