Mixed convective flow and heat transfer of supercritical CO2 in circular tubes at various inclination angles

We performed the numerical simulations of laminar mixed convective flow and heat transfer in a 0.5 mm diameter and 1000.0 mm length tube. The supercritical carbon dioxide in the tube was cooled at constant wall temperature. The inclination angles were in the range of −90° (vertical downward flow) to 90° (vertical upward flow). The velocity and temperature distributions, secondary flow, friction factor and heat transfer coefficient were plotted vs. inclination angles and gravity force magnitudes. The kinetic energy of secondary flow was introduced to quantify its effect on the heat transfer. It is found that under the mixed convective flow and heat transfer conditions, the horizontal flow display the largest heat transfer coefficients. The inclined flows at α = −30° and 30° also behave better heat transfer performance among various inclination angles. The effect of inclined angles on the heat transfer is decreased with decreases in the gravity force magnitudes. The combined parameter of Gr/Reb2 was used to quantify the buoyancy force effect on the flow and heat transfer.