Turbulence, Heat and Mass Transfer 5 K. Hanjalić, Y. Nagano and S. Jakirlic (Editors) Turbulent Natural Convection in Horizontal Coaxial Cylindrical Enclosures: LES and RANS Models Y. Addad 1 , D. Laurence 1 , and M. Rabbitt 2 1 The University of Manchester, School of Mechanical, Aerospace and Civil Eng., M60 1QD, UK, yacine.addad@manchester.ac.uk , dominique.laurence@manchester.ac.uk 2 British Energy plc, Barnwood, Gloucester, GL4 3RS, UK, mike.rabbitt@british-energy.com Abstract - Numerical investigations of turbulent natural convection between concentric, cylinders has been carried out. Large Eddy Simulations (LES) have been performed on three different geometries. The first two consider a single inner cylinder at gap-based Rayleigh numbers of 1.18×10 9 and 2.38×10 10 and an aspect ratios R o /R i =4.87 and 3.37, respectively. The last LES run considers three internal cylinders. It reveals that the flow patterns in the decay heat pen are more complex than the one internal concentric cylinder cases. The two-equations RANS models predictions for this test case are less satisfactory than in the simplified test cases, while second moment closure results are closer to the LES predictions. 1. Introduction In several Advanced Gas Cooled Reactors (AGCR), boiler penetrations run horizontally through the thickness of the reactor pressure vessel. The length of these penetrations is long compared with their diameter. At the bottom and top of the boilers, the penetrations contain tubes that carry water and steam respectively into and out of the boilers. In both cases the tubes are colder than the ambient pressurised carbon dioxide gas. The number of tubes within the penetrations ranges from 3 to 44, and tube size varies significantly. The tubes are supported along their length by a number of support plates, also intended to act as gas baffles. However, an angular gap exists between the plates and the outer penetration sheath tube to accommodate expansion and bowing of the tubes. Because of the high gas pressure, these gaps do not offer a significant resistance to the flow reaching the end of the penetrations. The gas flows within the penetrations are generally long range thermosyphons, driven by temperature differences (Figure 1). The rate of heat transfer of the thermosyphons crucially governs how far into the penetrations the thermosyphons reach. Computational simulations are used to study the thermal fields of the penetrations. The turbulence model used to calculate the gas flows is a vital component of the predictions. The accuracy of current predictions in such situations is not currently quantified. Thus, the work described in this study has been undertaken to help define reliability of turbulence RANS model for use in undertaking calculations of this type of flows driven by temperature differences in horizontal penetrations. For the purpose of validation studies natural convection in “infinite” annular cavities is considered. This allows periodic conditions in the pipe-axis direction for LES and 2D simulations with RANS. The early work of Bishop et al. [1] observed that transition start at a Rayleigh number of approximately 10 5 for a relative gap width between the concentric cylinders of 1.35. Later on, Bishop [2] performed an experimental study of turbulent natural convection of helium in a horizontal annulus at cryogenic temperatures and gap-width Rayleigh numbers ranging from 6×10 6 to 2×10 9 . The expansion number βΔT ranged from 0.25 to 1.0 for a constant Pr=0.668 and diameter ratio of 3.36. Then, McLeod and Bishop [3]