Two layer heat transfer model for supercritical fluid flow in a vertical tube

Experimental heat transfer data in a supercritical vertical upward CO₂ flow were analyzed, based on the relationship between the wall heat flux and mass flux, buoyancy and flow acceleration effects, and specific heat variation across the turbulent boundary layer. These analyses indicated that the flow acceleration and significant specific heat variation in the boundary layer greatly influenced the heat transfer phenomena under the tested experimental conditions. A two layer heat-transfer model that sufficiently reflects both the effects of flow acceleration and specific heat variation was proposed to quantify the heat-transfer characteristics of supercritical fluids. This model was based on the thermal resistance behavior in the viscous sub-layer and the buffer layer. In our assessment of this model, the Nusselt number calculated from various experiments agreed with our data within a margin of error of ±30%. Also, the location of the peak inner wall temperature from experimental data almost coincided with the peak maximum thermal resistance in the viscous sub-layer, calculated using the proposed model.