Enhanced Boiling Heat Transfer and Critical Heat Flux Characteristics of Fe 3 O 4 Nanofluid Under Downward-Facing Heating Conditions

To enhance the safety margin of in-vessel retention under external reactor vessel cooling conditions, this study investigates the pool boiling heat transfer characteristics of Fe3O4 nanofluids on downward-facing heating surfaces. The focus is on their influence on heat transfer coefficient (HTC) and critical heat flux (CHF). Pool boiling experiments were conducted using Fe3O4 nanofluids at varying concentrations (0 to 10.0 mg/L) to assess the effect of nanoparticle concentration on thermal performance. Results show that the nanofluid at 7.5 mg/L concentration yields optimal enhancement, with CHF increasing by approximately 21% compared to reverse osmosis water. However, a further increase to 10.0 mg/L leads to performance degradation due to excessive nanoparticle deposition. Contact angle and surface roughness measurements reveal that Fe3O4 nanoparticles form a porous and hydrophilic deposition layer on the heated surface during boiling, significantly improving surface wettability and increasing the number of active nucleation sites. These surface modifications show strong correlation with the observed improvements in CHF and HTC. The study further elucidates the critical role of nanoparticle-induced changes in surface wettability and microstructure in enhancing boiling heat transfer performance.