• buoyancy
• CFD
• deterioration
• enhancement
• FLUENT
• heat transfer
• STAR-CCM+
• supercritical pressure
• SWIRL
• validation
• water

The present work, partly made during a three month stage at the University of Aberdeen (UK), is aimed to investigate the heat transfer problem in heated channels with water at supercritical pressure. The analysis is performed with three different Computational Fluid Dynamics codes (SWIRL, an “in-house” CFD code, FLUENT and STAR-CCM+, general-purpose CFD codes). The aim of this work is to evaluate the performances of different low-Reynolds number turbulence models in predicting mixed convection heat transfer of fluids at supercritical pressure, with particular attention to the features that are affected by the modifications of the turbulence field due to influence of flow acceleration and buoyancy.
Several simulations are performed and the predicted results are compared with the data obtained by the experimental facility in the Nuclear Engineering Laboratory at the University of Manchester. The simulated test section is 2 m long, with a diameter of 25.4 mm (1”). Different operating conditions are imposed for water at 25 MPa, in both downward and upward flow.
Mainly, the analysis is conducted using the Yang-Shih turbulence model and the SWIRL CFD code in simple two-dimensional geometry. Results on a broad-spectrum of boundary conditions are achieved and a better understanding of the heat transfer behaviour was developed by the analysis of the dimensionless velocity and the dimensionless turbulent kinetic energy, for different axial locations along the pipe. Deterioration and enhancement phenomena are pointed out considering the ratio between the Nusselt number in the mixed convection flow and the Nusselt number of forced flow, while the difference between the effects of buoyancy and flow acceleration is pointed out by the Buoyancy parameter Bo*, introduced by Jackson, et al. (1979).
Some numerical aspects are also pointed out. The low-Reynolds number k-e turbulence models adopted in the work, are generally able to predict both heat transfer enhancement and deterioration phenomena even if, in some upward flow cases, wall temperature over prediction was obtained from the simulations, due to the large change in fluid properties when temperature reaches the pseudocritical value. So, the predicted effect of buoyancy is much greater than suggested by a buoyancy parameter based on bulk temperature. Buoyancy and the consequent laminarization effects mainly govern the occurrence of deterioration in the considered experimental data, but other mechanisms contribute to the occurrence of this phenomenon.