Anomalous thermal behaviour of liquid metals in confined Rayleigh–Bénard convection

This study examines anomalous thermal transport in confined liquid metal Rayleigh–Bénard convection via direct numerical simulations over Rayleigh numbers italic Ra element of left bracket 10 Superscript 5 Baseline comma 10 Superscript 8 Baseline right bracket Ra ∈ [ 10 5 , 10 8 ] $\\textit{Ra} \\in [10^5, 10^8]$ . A phenomenological hybrid model is introduced to capture the full transition from the near-wall boundary layer to the non-monotonic core, and the presence of an inverse mean temperature gradient in the bulk is shown to originate from the spatial organization of dynamic thermal blobs. For temperature fluctuations, a spatial characterization of the temperature standard-deviation profile ( sigma Subscript upper T σ T $\\sigma _T$ ) reveals a distinctive bimodal structure, with a secondary peak emerging away from the wall. Through variation of spanwise confinement, no-slip sidewalls are found to constrain the development of corner vortices, reorganizing them into a coherent spiral-like precession. This three-dimensional flow topology induces intense spanwise velocity fluctuations, which are associated with the transport and trapping of boundary-layer thermal plumes in the core region, thereby supporting the formation of the anomalous near-core sigma Subscript upper T σ T $\\sigma _T$ peak. At the largest Rayleigh numbers considered, the strengthening of turbulent mixing and large-scale circulation weakens this coherent corner-vortex activity, leading to the attenuation of the bimodal signature. Furthermore, we show that such quasi-two-dimensional confinement modifies the macroscopic heat transfer scaling, producing a steeper effective exponent ( italic Nu tilde italic Ra Superscript 0.332 Nu ∼ Ra 0.332 $\\textit{Nu} \\sim \\textit{Ra}^{0.332}$ ) compared with classical predictions – a phenomenon closely tied to the morphological condensation of thermal plumes.