Study on the temperature characteristics of the primary side of the casing type once-through steam generator under single parameter transient conditions

This study investigates the influence mechanisms of primary-side inlet temperature and flow rate on the axial temperature distribution of heat transfer tubes under transient conditions, based on a single-channel primary circuit numerical model of the once-through steam generator (OTSG). The results demonstrate that the temperature variation rate governs the coupled thermal inertia-flow inertia effects. In low-slope regions, the system exhibits quasi-steady-state characteristics with less than 2% variation in evaporation section length. Conversely, high-slope regions trigger rapid dry-out zone expansion, where void fraction exceeding critical thresholds induces flow regime transition. This initiates a positive feedback loop between void growth and heat transfer deterioration, significantly amplifying local overheating risks. Flow inertia regulates phase change reconstruction processes: rapid pump startup induces 0.3L evaporation front migration with temperature fluctuations 2.3 times greater than slow startup, whereas gradual startup limits migration to 0.1L with controllable temperature fields. The research reveals synergistic interaction mechanisms among thermal inertia, flow inertia, and latent heat absorption during transients, providing theoretical foundations for OTSG safety operation and transient control strategies. Key findings highlight that thermal inertial effects dominate at low perturbation rates, while flow inertia becomes decisive in rapid transients through momentum-driven phase redistribution.