Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in the convection term of the volume fraction equation. Thus, in this study, we focused on an interface compression (IC) method for such a VOF approach; this technique prevents numerical diffusion issues and maintains boundedness and conservation with negative diffusion. First, on a sufficiently high mesh resolution and without the IC method, the validation process was considered by comparing the amplitude growth of the interfacial wave between a two-dimensional gas sheet and a quiescent liquid using the linear theory. The disturbance growth rates were consistent with the linear theory, and the validation process was considered appropriate. Then, this validation process confirmed the effects of the IC method on numerical diffusion, and we derived the optimum value of the IC coefficient, which is the parameter that controls the numerical diffusion.

Pool scrubbing is an important filtering process that prevents radioactive aerosols from entering the environment in the event of severe accidents in a nuclear reactor. In the process of transporting aerosol particles using bubbles, bubble hydrodynamics plays a crucial role in modeling pool scrubbing and significantly affects particle removal in a bubble. Our future research strategy is to apply the three-dimensional Computational Fluid Dynamics (CFD) approach to understand the detailed bubble interaction, which is difficult to be assessed experimentally. This study validates the applicability of the CFD simulation to bubble hydrodynamics at the flow transition from a globule to a swarm region. Two types of solvers based on the Volume Of Fluid (VOF) and Simple Coupled Volume Of Fluid with Level Set (S-CLSVOF) methods were used to capture the gas–liquid interface in the CFD simulation. We use the experimental data for validation. As a result, the VOF and S-CLSVOF methods accurately predict the bubble size and void fraction distributions. In addition, we confirmed that the bubble rise velocity of the S-CLSVOF method almost agrees with the experimental results. The validated code is expected to play a critical role in evaluating the constitutive equations in the stand-alone pool scrubbing code – SPARC-90.

In pressurized water reactor accident scenarios, the injection of water from the emergency core cooling system (ECCS) (ECC injection) might induce a pressurized thermal shock (PTS), affecting the reactor pressure vessel (RPV) integrity. Therefore, PTS is a vital research issue in reactor safety, and its analysis is essential for evaluating the integrity of RPVs, which determines the reactor life. The PTS analysis comprises a coupled analysis between thermal–hydraulic and structural analyses. The thermal–hydraulic approach is particularly crucial, and reliable computational fluid dynamic (CFD) simulations should play a vital role in the future because predicting the temperature gradient of the RPV wall requires data on the transient temperature distribution of the downcomer (DC). Since one‐dimensional codes cannot predict the complex three‐dimensional flow features during ECC injection, PTS is one reactor safety issue where CFD simulation can benefit from complement evaluations with thermal–hydraulic system analysis codes. This study reviewed from the viewpoint of the turbulence models most affecting PTS analysis based on papers published since 2010 on single‐ and two‐phase flow CFD simulation for the experiment on PTS performed in the Rossendorf coolant mixing model (ROCOM), transient two‐phase flow (TOPFLOW), upper plenum test facility (UPTF), and large‐scale test facility (LSTF). The results revealed that in single‐phase flow CFD simulation, where knowledge and experience are sufficient, various turbulence models have been considered, and many analyses using large eddy simulation (LES) have been reported. For two‐phase flow analysis of air–water conditions, interface capturing/tracking methods were used in addition to two‐fluid models. The standard k–ε and shear stress transport (SST) k–ω models were still in the validated phase, and various turbulence models have yet to be fully validated. In the two‐phase flow analysis of steam–water conditions, many studies have used two‐fluid models and Reynolds‐averaged Navier‐Stoke (RANS), and NEPTUNE_CFD, in particular, has been reported to show excellent prediction performance based on years of accumulated validation.

Turbulent flow and heat transfer characteristics in a square duct of 100 mm height with 45 degree square ribs of 10 mm height was analyzed numerically by using algebraic Reynolds stress model including the fixed turbulent Prandtl number and algebraic turbulent heat flux models. In this research, analytical results were compared with the experimental and predicted data reported by Bonhoff et al. [1], which were measured and analyzed turbulent flow fields at Reynolds number 5×10^4 based on bulk velocity and duct height by means of a PIV system and a Reynolds stress model. Analytical results obtained by algebraic Reynolds stress model showed a good agreement with the experimental data of streamwise and spanwise mean velocity profiles at the same Reynolds number of the experiment. As for secondary flow in the duct, analytical results showed relatively good agreement with the experimental data. Moreover, temperature distributions were obtained by using the fixed turbulent Prandtl number and algebraic turbulent heat flux models. Temperature profiles of algebraic turbulent heat flux model were not similar to velocity profiles. Considering algebraic turbulent heat flux model has potentiality to reproduce anisotropic temperature, these results suggested that correlation between momentum and heat transfer was not recognized in this turbulent field. As a result of this study, it was verified that the presented method was able to predict turbulent flow in duct with ribs through the comparison of calculated results with the experimental data.