Heat transfer in a U-bend pipe flow at supercritical pressure

U-bend is widely used as a pipe fitting in heat exchangers, but the flow and heat transfer of supercritical fluid in and following a U-bend have not been well understood yet. This work numerically investigates a heated flow of supercritical R134a in horizontal U-bend tubes. After validating the model against experimental data from U-tube and straight tubes, numerical runs were performed at a constant pressure (1.05 times the critical pressure) with varying mass flux, heat flux, tube diameters, and bend curvatures. Results show that U-bend barely affects heat transfer in the preceding straight section, while significantly affecting flow and heat transfer in and downstream of the U-bend under mixed convection. Local wall temperatures vary sharply in the U-bend and fluctuate after the bend. Remarkable influence can extend to a far distance of 90d downstream. Compared to the straight tube, average heat transfer coefficients increases by 6% to 53% within the U-bend and up to 23% after the U-bend, respectively. The flow field gradually evolves due to the competition between buoyancy and centrifugal forces. The introduction of U-bend disrupts the original development of buoyancy-driven mixed convection. Secondary flows developed in and after the bend is so strong that they have almost an identical effect as the thermal development at the tube entrance. This is the underlying mechanism of better heat transfer performance in U-bend pipes.