Two-Phase Flow and Heat Transfer in a Once-Through Steam Generator During Auxiliary Feedwater Injection

A phenomenological model is developed for the thermal-hydraulic processes on the secondary side of a once-through steam generator during auxiliary feedwater injection. Based on experimental observations, the flow of auxiliary feedwater in the secondary side is modeled as a turbulent falling film on the tubes, in direct contact with a countercurrent flow of steam, that receives heat from the primary side. Conservation equations for the falling film and steam on the secondary side, and for the primary-side coolant, are derived. Boiling in the falling film, evaporation and/or condensation at the falling film-gas interphase, and countercurrent flow limitation in the tube support plate passages are modeled. Numerical solution of the conservation equations provide the axial variation of flow rates and temperatures in the primary and secondary sides. Model predictions are successfully compared with the available experimental data.