Microscale LTNE analysis of porous Fe₃O₄ fouling evolution and heat-transfer degradation on PWR steam-generator tubes

Fe₃O₄ fouling on the secondary side of pressurized-water-reactor steam-generator tubes is an evolving porous deposit whose thermal effect depends on layer growth, pore closure, liquid replenishment, and local evaporation. In this work, a two-dimensional local thermal non-equilibrium (LTNE) model is developed to predict the 40-year evolution of a vapor-chimney–porous-fouling unit cell. Separate solid and liquid energy equations are coupled with Darcy replenishment, vapor-chimney evaporation, particle deposition and removal, dissolved-Fe transport, in-pore crystallization, and porosity-dependent transport properties. Under baseline conditions, fouling thickness increases from 1.0 to 21.7 μm, while mean porosity first rises from 0.39 to 0.48 and then decreases to 0.31. Fouling resistance consequently reaches 8.17 × 10⁻⁶ m² K W⁻¹ at 40 years. Comparisons using identical frozen fouling structures give peak vapor-chimney heat-transfer fractions of 0.244, 0.522, and 0.762 for the LTNE, local thermal equilibrium (LTE), and Short-type models, respectively, revealing strong sensitivity to thermal-equilibrium and chimney-boundary assumptions. For heat fluxes from 100 to 400 kW m⁻², higher heat flux advances near-wall densification and suppresses the early heat-transfer-enhancement stage. A 33-case joint multiparameter screening identifies particle deposition as the primary control on final thickness and resistance, whereas the critical porosity governs hydraulic closure. The final pore-liquid velocity correlates strongly with the vapor-chimney heat-transfer fraction (Pearson = 0.931; Spearman = 0.978), making it a more direct indicator of chimney evaporation than regional mean permeability. Thus, long-term fouling should be evaluated through coupled pore evolution, heat transfer, and liquid replenishment rather than thickness alone.