Flow structure and heat transfer in conical Taylor–Couette flow under laminar regimes: effects of apex angle

Heat transfer enhancement under laminar flow regimes is required in processes involving high-viscosity fluids or fragile materials. To establish an effective strategy, this study numerically investigates thermo-fluid characteristics and heat transfer performance in conical Taylor–Couette (TC) flow with various apex angles ( α ) under low-to-moderate Reynolds number conditions. A radial temperature difference between the inner and outer conical surfaces (Δ T = T o – T i ) was imposed to examine buoyancy effects. The results demonstrate that both the magnitude and sign of Δ T significantly influence the flow structure. Positive Δ T induces a fluctuated TC flow due to the interaction between buoyancy-driven and meridional circulations, whereas negative Δ T promotes a stable helical mode. Under negative Δ T , the intrinsic influence of apex angle becomes clearer: as α increases, the helical motion and vorticity intensify. The total Nusselt number increases with increasing α up to 18° at the same Reynolds number ( Re il ≥ 155), but decreases at α = 24°. This behavior is associated with changes in the flow structure: moderate apex angles enhance coherent helical vortical motion and promote stable stagnation regions near the outer surface, leading to increased heat transfer. In contrast, at α = 24°, increased unsteadiness and temporal modulation of the vortex structures hinder the persistence of these regions, resulting in reduced heat transfer. Furthermore, at comparable power input, conical configurations outperform the cylindrical case, reflecting the influence of geometry-induced flow structures on transport under laminar conditions.