Effects of successive pool boiling cycles of SiO2 nanofluids on boiling surface stability: Role of concentration-dependent stability and post-stabilization dilution

Boiling surface stability is essential for ensuring the safety and long-term performance of boiling-based thermal systems. While previous studies have addressed nanofluid boiling under repeated operation, this work focuses on finding the concentration threshold required for surface stabilization and explains how the sequence of nanofluid concentrations and post-stabilization dilution governs the evolution of SiO₂-deposited surfaces during successive pool boiling-cooling cycles. Nanoparticle concentration (0.01-2 wt.%), particle size (20-30 and 60-70 nm), and concentration sequencing effects are investigated. The results show that low concentrations (≤0.5 wt.%) of SiO 2 nanofluid fail to stabilize surfaces within the accessible experimental range, whereas stable and repeatable behavior is achieved only at concentrations ≥1 wt.%. A surface stabilized with 2 wt.% SiO 2 nanofluid achieves critical heat flux (CHF) values of about 1300 kW/m² and 1250 kW/m² for 20-30 and 60-70 nm nanoparticles, respectively, with wall superheats of about 52°C. CHF is enhanced by 18% and 13% for 20-30 nm and 60-70 nm nanoparticles compared to deionized water, and this enhancement is preserved even after reducing the nanofluid concentration. On the 2 wt.%-stabilized surface, the boiling heat transfer coefficient (BHTC) rises with nanofluid concentration, and larger particles (60-70 nm) provide higher BHTC, with enhancements of approximately 8%, 11%, and 15% at 2%, 1%, and 0.5 wt.%, respectively. Successive boiling-cooling cycles delay the onset of nucleate boiling (ONB) by about 75-85% compared with the first cycle. The findings define a concentration-dependent stabilization threshold and propose a practical dilute-after-stabilization approach to achieve durable nanofluid boiling performance.