Numerical investigation of critical heat flux in single rod channel under extremely low flow conditions

Abstract

Critical heat flux (CHF) is a crucial thermal parameter influencing reactor safety and efficiency, particularly under extremely low flow conditions. This study analyzes the mechanisms of CHF in a single rod channel under flow conditions ranging from 50 to 300 kg m⁻²·s⁻¹,and system pressures of 2–15 MPa, using advanced numerical models, including the Eulerian two-fluid model, interfacial interaction model, and wall boiling model. The simulations show high accuracy, with deviations from experimental data within ±20 %, validating the computational framework. Under these low-flow conditions, CHF is primarily associated with the depletion of the thin liquid film adjacent to the heated surface, where insufficient liquid supply leads to dry-out and severely impairs heat transfer. The parametric analysis reveals that CHF increases with higher inlet subcooling, larger pipe diameters, and higher mass flow rates, while it decreases with longer channel lengths. Pressure has a non-monotonic effect: at lower pressures, CHF increases with pressure, whereas at higher pressures, CHF decreases as pressure increases. These analyses provide deeper insights into the CHF mechanisms under extremely low flow conditions, helping to optimize reactor thermal design and improve safety protocols. This research contributes to the field of thermal-hydraulics in nuclear reactors, offering practical implications for mitigating risks and enhancing energy system performance.