Non-Oberbeck–Boussinesq effects on glycerol-filled two-dimensional Rayleigh–Bénard convection in strongly confined cavities

Although geometric confinement and non-Oberbeck–Boussinesq (NOB) effects are known to influence convection, their combined impact in high-Prandtl fluids remains poorly explored. This study investigates the joint effects of confinement and NOB on flow organization, symmetry, and transport using 180 direct numerical simulations of two-dimensional Rayleigh–Bénard convection in glycerol (Pr=2547). Simulations cover aspect ratios 0.05≤Γ≤0.3, Rayleigh numbers 107≤Ra≤1010, and temperature differences between the hot θ̃ h and cold θ̃ c walls (Δθ̃=θ̃ h−θ̃ c) up to 50 K. Four flow regimes emerge Γ–Ra plane, showing that the critical Ra for the onset of convection decreases as Γ increases. Transitions between periodic and chaotic states exhibit re-entrant, non-monotonic dependence on Ra, highlighting the critical role of the aspect ratio in modulating the route to turbulence. While confinement alone induces minor thermal asymmetries, NOB effects are the dominant cause of symmetry breaking. This is evident in the center temperature, which varies non-monotonically with Ra in narrow cavities, but depends nearly linearly on Δθ̃ in wider ones. A fundamental contrast between heat and momentum transport is revealed: at high Ra, Nu becomes independent of Γ, whereas Re remains strongly dependent on Γ. This is evidenced by the steep scaling of Nu∼Ra0.48 in the narrowest cavity (Γ=0.05) compared to the classical Nu∼Ra0.3 in wider cells. Furthermore, NOB effects enhance Re by up to 25% without erasing this geometric dependence. For the most confined cases, NOB effects on Nu and Re vary non-monotonically with Δθ̃, whereas in the widest cavity (Γ=0.3), they consistently suppress Nu and enhance Re.