Numerical investigation on flow and thermal performance of supercritical CO₂ in horizontal cylindrically concaved tubes

To improve the overall heat transfer of supercritical carbon dioxide (SCO₂) in a horizontal circular tube, this study proposes a novel kind of tube with cylindrically concaves and fillets on the circular surface. Several tubes are designed accordingly to investigate various parameters like θ (sectorial angle of concaves), r₂ (radius of concaves), r₃ (radius of fillets) and n (number of concaves) to observe their effects on the thermal performance. The heat transfer and pressure drop characteristics of SCO₂ in these tubes are conducted by applying a verified turbulence model at an operation pressure of 8.0 MPa, an inlet temperature of 323.15 K, a mass flow rate of 0.005652 kg/s and a heat power rate of 565.488 W. It is found that the heat transfer coefficient of all the designed cases are improved due to the secondary flow and the maximum value of the h/h₄/(f/f₄) is 1.134 compared with that of the standard case 4, while the friction factors are not increased but in fact even lower without raising the vortex pairs. Finally, the heat transfer coefficient at the vicinity of the pseudo-critical point is studied and it is noted that the heat transfer is also enhanced at this region.