Numerical Investigation of the Impacts of Key Spacer Elements on Thermal Hydraulics of Subcooled Flow Boiling in a Single Fuel Rod PWR Channel

Subcooled flow boiling in the subchannels of pressurized water reactors (PWRs) is a critical phenomenon that significantly impacts their performance and safety. This research provides a computational fluid dynamics (CFD) evaluation of subcooled flow boiling in a single fuel rod channel representative of PWR conditions. While prior CFD studies often assume a simplified spacer grid geometry by focusing solely on the impacts of mixing vanes, this work addresses this gap by incorporating a complex spacer geometry, including dimples and springs, along with mixing vanes. The well-known Eulerian two-fluid model (Eulerian-TFM) linked with the Rensselaer Polytechnic Institute (RPI) wall boiling model is utilized for simulation. Moreover, the coalescence and breakup of bubbles in the bulk subcooled zone are modeled by coupling the interfacial area transport equation model, which enables the precise prediction of interfacial transfer terms and phase distribution. The key purpose of the present research is to assess the impacts of key spacer features (mixing vanes, dimples, and springs) on the critical thermal-hydraulic parameters of the coolant, such as pressure drop, secondary swirling flow, vapor volume fraction, wall superheat, heat transfer coefficient (HTC), etc. at near-prototypical reactor conditions. The CFD models used are also validated against experimental data at PWR-like ultrahigh-pressure conditions, addressing a gap in prior literature and demonstrating the model’s reliability. The key findings reveal that the inclusion of dimples and springs in the spacer resulted in a 10.3% greater reduction in average vapor volume fraction and a slight increase in bulk liquid temperature than the presence of mixing vanes only but at the cost of suppressing peak secondary swirling flow intensity (SFI) by 9.3% and incurring an 11.3% higher pressure drop. Furthermore, their presence resulted in a less pronounced wall superheat drop and HTC increase immediately downstream of the spacer, compared to the presence of mixing vanes only.