Mean statistics of a heated turbulent pipe flow at supercritical pressure

Direct numerical simulations (DNS) of a heated turbulent pipe flow with a fluid at supercritical pressure are performed at a Reynolds number of Reτ=360, based on pipe diameter and friction velocity at the inlet. A constant wall heat flux is applied and the temperature range within the flow domain incorporates the thermodynamic region where large variations in thermophysical properties occur. The contribution of these property variations on the mean flow is studied. As compared to ideal gas heat transfer, additional terms appear in the mean flow governing equations. These terms can significantly affect the energy balance, because they modify the averaged wall heat flux and the enthalpy diffusion close to the pipe wall. Furthermore, the averaged thermophysical properties, especially the isobaric heat capacity cp, deviate significantly from those evaluated using the mean temperature or enthalpy. This is due to an averaging artifact called the Jensen inequality, caused by the enthalpy fluctuations and the non-linear dependence of thermophysical properties with respect to enthalpy. Turbulent statistics for one forced convection and two mixed convection cases in an upward flow are discussed. A decrease in turbulent kinetic energy is observed for the forced convection and the low buoyancy case, which cause heat transfer deterioration indicated by high wall temperatures. For the high buoyancy case the turbulence activity first reduces (heat transfer deterioration) and then increases due to turbulence recovery.