Material-driven boiling heat transfer: thermal effusivity and surface morphology interplay

This study evaluates boiling heat transfer performance, specifically heat transfer coefficient (HTC) and critical heat flux (CHF), for six hydrophilic nuclear relevant alloys: 304SS, 316SS, FeCrAl, Inconel600, Monel400, and Zr4. These alloys exhibit relatively low thermal effusivity and low surface roughness with Ra approximately 0.1 to 0.2 μm by polishing. Experiments are conducted under saturated pool boiling and vertical upward flow boiling conditions at a mass flux of 400 kg/m²/s and inlet subcooling of 5 °C. Recognizing limitations of conventional roughness metrics, we introduce concavity percentage, derived from Hsu's nucleation theory, to quantify the size distribution of thermodynamically active nucleation sites as the ratio of total concave area to nominal surface area. Results reveal that material thermal effusivity dominantly and linearly enhances HTC under high heat flux and flow boiling conditions characterized by thin thermal boundary layers. This enhancement occurs primarily through improved heat diffusion in the thermal boundary layer, accelerating bubble growth and reducing departure diameters. Conversely, CHF correlates strongly with surface morphology, specifically concavity percentage, rather than thermal effusivity or wettability. Even slight concavity percentage increases produce marked CHF enhancements by controlling bubble dynamics and liquid replenishment, as observed for 316SS and Inconel600 during flow boiling. This work establishes direct causal links between material-driven thermal effusivity and surface morphology with boiling performance while isolating wettability effects. Optimized boiling surfaces should combine high thermal effusivity (maximizing HTC) with high concavity percentage (maximizing CHF), particularly for advanced nuclear components like ATF claddings. © 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.