Natural convection around the upper dome with and without upstream flow from the subjacent cylinder

Some SMRs (Small Modular Reactors) adopt the steel containment vessel with cylindrical wall and dome, which work as the ultimate heat sink. The estimation of the natural connective cooling capability is crucial to the nuclear safety. This study investigated the influence of the upstream flow from the subjacent cylinder on the natural convection of the dome. Experiments were conducted using four types of domes (Ra_Db = 1.68 × 10¹¹ and Ra_Db = 1.08 × 10¹³ with truncation angles of θ = 90° and θ = 30°) combined with five different cylinder heights of 2.10 × 10¹⁰ ≤ Ra_H ≤ 2.63 × 10¹². To achieve high Rayleigh number, mass transfer experiments using copper electroplating system were carried out based on the analogy. For the Ra_Db = 1.68 × 10¹¹ domes, the flows were observed using Particle Image Velocimetry for all cases and velocities and vortex were analyzed. Without the upstream flow, the natural convective flow developing along the upper dome showed decrease in heat transfer but after flow separation, the heat transfer was enhance and then saturated due to vorticity generation. The upstream flows entering the dome region separated from the surface at the ‘separation point’ of each dome, forming the outer plume. Consequently, the flow patterns and heat transfer at the dome surface and within the inner plume remained identical to those of the dome without the upstream flow except for the leading edge of the dome where the presence of the upstream flow blocked the encounter of fresh fluid.