École Nationale Supérieure de
Techniques Avancées

Abstract

The SMR (Small Modular Reactor) aspire to use passive safety principles, as the extraction of residual heat from the reactor to a surrounding water pool. It represents an important heat transfer, which has to be ensured by natural convection. The safety studies must demonstrate that the choice of natural convection is enough to avoid a rise of temperature threatening the components of the reactor. They are based on experiment and numerical simulation. But the SMR configuration implies important Rayleigh numbers, up to Ra = 10e15, and we currently do not have access to heat transfer correlations above Ra ≈ 10e12. Given the difficulties of a direct experiment, the choice was made for an experiment similar in terms of Rayleigh number with cryogenic helium, allowed by the existence of a cryostat at CEA Grenoble. At the same time, LES simulations were performed by Songzhi et al. [18] in order to prepare this experiment. Their huge cost in calculation time and resources points towards the realization of RANS simulations, more economical. We have chosen the two-layer k − ε turbulence model to simulate different cases of natural convection. Firstly, we have proceeded to the validation of the model, thanks to the experiment of Tsuji and Nagano [12]. The two-dimensional simulation correctly predicts the values of mean velocity and mean temperature in the boundary layer. It also gets the correlation between Rayleigh and Nusselt numbers in the laminar and turbulent regimes. However, the laminar-turbulent transition is delayed compared to the experiment. Besides, the three-dimensional simulation does not reduce the difference with experimental data. It may indicate a deficiency of the chosen turbulence model. Secondly, we conducted a RANS simulation with the same configuration as the LES simulation using cryogenic helium. The results of mean velocity and temperature are physically consistent, but show important deviation compared to the LES. The heat transfer correlation established for the turbulent boundary layer is found, but not the one for laminar boundary layer. The transition from laminar to turbulent regime does not appear either.