Validation of NEPTUNE_CFD Module with Data of a Plunging Water Jet Entering a Free Surface

This work presents a validation of NEPTUNE_CFD against plunging water jet experiments by Iguchi et al., with sensitivity tests to turbulence modeling. NEPTUNE_CFD is the thermal-hydraulic two-phase computational fluid dynamics tool of NURESIM (European Platform for Nuclear Reactor Simulations) and is designed to simulate two-phase flow in situations encountered in nuclear power plants. Iguchi et al.'s flow configuration shares common physical features with the emergency core cooling injection in a pressurized water reactor uncovered cold leg during a small-break loss-of-coolant accident. This work contributes to the validation of the NEPTUNE_CFD code capability to predict the turbulence below a free surface produced by a plunging jet. In the experiment, the water was injected vertically down a straight circular pipe into a cylindrical vessel containing water. Mean velocity and turbulent fluctuations were measured below the jet at several depths below the free surface. The influence of several models on code predictions was investigated, and both standard and modified turbulence models were tested. A single-phase jet case was also simulated and compared with both measurements and two-phase calculations, to investigate bubble entrainment influence on turbulence prediction. The calculated mean velocity field was always in quite good agreement with the experimental data, while the turbulence intensity was generally good with some underestimation far from the jet axis region.