Mean–local decoupling in convective–radiative heat transfer of annular cavities: Effects of scale, confinement and curvature

Coupled natural convection and surface radiation in confined annular cavities can produce strong heat-transfer partitioning and wall-flux non-uniformity, yet geometry can affect mean and local responses differently. Gas-turbine shutdown heat soak provides a representative application because decaying forced cooling leaves residual heat redistribution governed mainly by solid conduction, buoyancy-driven flow, and wall-to-wall radiation. This study establishes a three-dimensional numerical model of a concentric annular cavity containing clean air and gray-diffuse surfaces under representative non-uniform shutdown wall temperatures. A unified dimensionless framework evaluates Nu c , Nu r , the radiative heat-transfer fraction, and top–bottom non-uniformity while independently varying the overall scale ratio λ , aspect ratio Γ , and dimensionless curvature χ . At Δ T = 100 K, increasing λ from 1 to 16 raises Nu c from 14.05 to 92.30 and Nu r from 24.93 to 397.49, while the radiative fraction increases from 0.640 to 0.812. Under the common conductive normalisation, geometrically similar scaling increases both equivalent Nusselt numbers. The near-16-fold increase in Nu r is dominated by characteristic-length scaling and does not imply a comparable increase in area-averaged radiative heat-flux density. Increasing Γ strengthens radial confinement and suppresses both equivalent Nusselt numbers, while non-uniformity changes non-monotonically as the main circulation and end-region recirculation reorganise. Increasing χ promotes vortex migration and near-wall fluid renewal, enhancing Nu c more strongly than Nu r . The separated mean and local responses provide a heat-transfer basis for evaluating geometry-dependent thermal-load non-uniformity in confined radiating cavities.