3D LES of free convection from a side-heated vertical wall with cryogenic helium up to Ra ~ 10$^{15}$
Résumé
The passive safety concept of Small Modular Reactors (SMR) is based on the
extraction of residual heat from the reactor to a surrounding water pool. However, the
large scale of the reactor vessel (height≈15m) can lead to a rather intensive heat
exchange process mostly by natural convection (Ra $\approx 10^{15}$). Reliable heat transfer correlations exist to date only up to Ra $\approx 10^{12}$, with uncertainties in the extrapolation to higher Ra. To improve the understanding of natural convection at high Ra number and find a valid heat transfer correlation, the three-dimensional turbulent natural convection boundary layer along a side-heated vertical wall is simulated through LES
with the CEA in-house code TrioCFD. The simulation has taken into account the
local variations of the fluid properties. Near wall mesh discretization is enough refined
(y+~0.2) to resolve the thin boundary layer. Our preliminary analysis with water as a
working fluid have shown the ability of the computational model in recovering the
heat transfer behaviour2 at moderate Rayleigh number (Ra $\approx 10^{12}$). As the second step, cryogenic helium is selected as a working fluid for its special physical properties (low viscosity, high β,…), which allows to reach high Ra $\approx 10^{15}$ in meter-sized experiments.
The numerical results concerning the heat transport process show a good agreement
with available reference data. Moreover, the mean temperature and velocity in the
boundary layer are also presented. The vortex evolution in the boundary layer is
visualized to enhance the understanding of the turbulence developing phases. Current
work can shed more light on the understanding of the turbulent natural convection
along a vertical wall at high Ra numbers. The numerical simulations will be
complemented by a new experiment currently under development at
CEA aimed at reaching Ra $\approx 10^{15}$ using cryogenic helium to probe the boundary layer behaviour and statistics with μm-sized micro-fabricated hot & cold wires.