Investigation of Convective Heat Transfer to Supercritical Carbon Dioxide with Direct Numerical Simulation

For fluids at supercritical pressure, the phase change from liquid to gas does not exist. In the meanwhile, the fluid properties change drastically in a narrow temperature range. With supercritical fluid as working fluid in a heated pipe, heat transfer deterioration and recovery have been observed, which correspond to the turbulent flow relaminarization and recovery. Direct Numerical Simulation (DNS) of supercritical carbon dioxide flow in a heated vertical circular pipe at a pressure of 8 MPa is developed with the open-source code OpenFOAM in the present study. Forced convection cases and mixed convection including upward and downward flow have been considered in the simulation. The change of the mean flow and turbulence statistics has been analysed in detail. In the forced convection, flow turbulence is attenuated due to acceleration from thermal expansion, which leads to a peak of the wall temperature. Buoyancy has a stronger impact to the flow. In the upward flow, the average streamwise velocity distribution turns into an M-shape profile because of the “external” effect of buoyancy. Besides that, negative buoyancy production caused by the density variation reduces the Reynolds shear stress to almost zero, which means that the flow is relaminarized. Further downstream turbulence is recovered. This behaviour of flow turbulence is confirmed by visualization of turbulent streaks and vortex structures. The observation of the flow turbulence of this can help to develop advanced turbulence models for applications in nuclear or conventional energy generation technologies.