Experimental and numerical investigation of convection heat transfer of CO2 at supercritical pressures in a vertical tube at low Reynolds numbers

Convection heat transfer of supercritical pressure CO2 in a vertical tube at low Reynolds numbers (less than 2500) was investigated experimentally and numerically. The tests investigated the effects of inlet temperature, pressure, mass flow rate, heat flux, buoyancy and flow direction on the heat transfer. For the lower heat flux of 4.49 kW/m2, the numerical results corresponded well to the experimental data (within 8%). However, for heat fluxes higher than 13.7 kW/m2, the numerical results for the convection heat transfer coefficient using laminar flow were much less than the experimental data over most of the tube (e.g., difference larger than 74% for x/d=15) due to the strong influence of buoyancy and the decrease of the dynamic viscosity with temperature along the tube which results in an early transition from laminar to turbulent flow. The numerical results for the convection heat transfer coefficient using turbulent flow corresponded much better with the experimental data. The convection heat transfer coefficient increases with increasing heat flux and then decreases with further increases in the heat flux for both upward and downward flows. The flow direction significantly influences the heat transfer. For high heat flux (e.g., 61.0–94.0 kW/m2) upward flow, the local wall temperature varies in a complex, nonlinear fashion, while for downward flow the local wall temperature increases monotonically and the heat transfer is strongly enhanced by buoyancy.