Experimental investigation on the effect of oxidation on subcooled boiling under highly subcooled conditions

• Effects of oxidation on CHF and heat transfer in subcooled boiling were clarified. • Repeated subcooled boiling increased roughness and wettability, increasing CHF. • Formation of Cu₂O/CuO oxide layer due to oxidation deteriorated heat transfer. • Oxidation of the heating surface reduced oscillation frequency in MEB. • Oxidation shifted dominant frequency peaks lower in MEB region. Surface oxidation is an inherent phenomenon in boiling that fundamentally alters the heating surface through changes in chemical composition, roughness, and wettability, thereby significantly affecting heat transfer performance. This study experimentally investigates the effects of surface oxidation on both nucleate boiling and microbubble emission boiling (MEB) under highly subcooled conditions. The results indicate that while oxidation increases surface roughness and enhances wettability, thereby increasing the critical heat flux (CHF), the progressive accumulation of insulating oxide layers (Cu₂O/CuO) introduces considerable thermal resistance, ultimately decreasing the heat transfer coefficient over repeated boiling cycles. In nucleate boiling with high heat flux, the maximum bubble diameter and interval time exhibit minimal variation across multiple cycles, suggesting that heat transfer deterioration is primarily due to oxide-induced thermal resistance rather than changes in bubble dynamics. In MEB, surface oxidation reduces bubble oscillation frequency and decreases the spatial-temporal uniformity of oscillations. Acoustic measurements further show that the sound pressure level and amplitude of the high-frequency peak increase with oxidation in nucleate boiling. In MEB, oxidation shifts the dominant frequency peaks toward lower frequencies. Although surface oxidation has minimal impact on the correlation between SPL and heat flux in MEB, it significantly alters the relationship between SPL and wall superheat. This study provides insight into the effects of repeated boiling cycles on subcooled boiling, emphasizing the importance of considering surface conditions when designing and optimizing boiling heat transfer applications under highly subcooled conditions.