Quantitative visualization of buoyancy-driven secondary flow in laminar supercritical n-decane under asymmetric heating

Buoyancy-driven flow phenomena in supercritical-pressure fluids under asymmetric heating are of critical importance for regenerative cooling systems, yet direct experimental evidence remains scarce. This study experimentally investigates the convective heat transfer and flow field of n-decane at 2.5 MPa in a horizontal channel subjected to bottom-wall heating under the heat flux of 0-125 kW/m² with Reynolds number of 100-500. A Z-type schlieren system combined with high-speed imaging is employed to visualize the flow field, and the Farnebäck optical flow method is applied to quantitatively resolve the two-dimensional velocity field from the image sequences. Under the present experimental conditions, the buoyancy force factor Gr / Re ² serves as a practical indicator. When Gr / Re ² < 0.5, the velocity profile remains symmetric and secondary flow is confined to the near‑wall region. When Gr / Re ² > 0.5, the high‑velocity core shifts toward the heated wall and the buoyancy‑affected region expands into the core flow with a significantly modified velocity profile. When Gr / Re ² > 1.0, vigorous secondary flow develops with pronounced upward migration of fluid parcels that can penetrate through the core flow to reach the top‑wall vicinity. Besides, a new quantitative metric of spanwise acceleration for evaluating buoyancy intensity is introduced. The findings provide quantitative experimental evidence of buoyancy-driven secondary flow characteristics in laminar supercritical-pressure channel flows, and establish an optical-flow-based diagnostic framework that bridges qualitative visualization and quantitative flow field analysis for heat transfer studies of supercritical-pressurized fluids.