Fluid flow and thermal characteristics in inclined tubes with transcritical carbon dioxide as working fluid

In this research work, the fluid flow and thermal characteristics in inclined tubes with transcritical carbon dioxide as working fluid were examined numerically in details. To investigate the drastic thermophysical variations of CO2 near the critical point, the predictions with temperature-dependent thermophysical properties were presented. It was done in inclined tubes with CO2 pressure being 8.0 MPa, the inlet temperature (To) 300 K and the wall temperature (Tw) 330 K with various inclined tube angles ranging from 90° (vertical upward flow) to − 90° (vertical downward flow). The velocity and temperature distributions, secondary flow, and friction factor coefficient were presented in detail. Predicted results showed that the effects of temperature-dependent thermophysical properties on the thermal and fluid flow characteristics in inclined tubes with transcritical carbon dioxide as working fluid are significant. Near the entrance, the CfReo behaves significant changes. For the vertical upward flow, the fluid near tube wall is lighter thus the buoyancy force is upward, decreasing the downward flow velocity in the core region but increasing the fluid velocity and its gradient near the wall. For the vertical downward flow, the downward buoyancy force accelerates the fluid velocities in the core region but decreases the fluid velocities near the wall, yielding the smallest CfReo among various negative inclination angles.