• Heat transfer
• Supercritical
• Deterioration
• CFD
• STAR-CCM+

The present work is aimed to investigate the heat transfer problem in heated channels with water at super-critical pressure. The study is conducted with a commercial Computational Fluid-Dynamics code, STAR-CCM+, with the aim to observe the performance of different low-Reynolds number turbulence models in predicting heat transfer features in fluids at supercritical pressure, with a particular attention on the influence of buoyancy and acceleration.
Several simulations are been performed and the predicted results are been compared with the data obtained by Watts experimental facility (Watt, 1980), reported in Jackson (2009a), in the Nuclear Engineering Laboratory at the University of Manchester at the end of seventy-years. The relevance of these experimental data is mainly due to the deterioration phenomena shown in several upward flow conditions with wall temperatures below and above the pseudo-critical temperature. The computational domain of the test section is 3 metres long with a diameter of 25.4 mm (1”), in a simple two-dimensional axial-symmetric geometry, created with STAR-Design. Different boundary conditions of heat flux, mass flux and inlet temperature are imposed in both downward and upward flow for water at 25 MPa. The consequent heat transfer deterioration or enhancement phenomena are pointed out considering the ratio between the actually computed Nusselt number and the Nusselt number obtained in case of pure forced convection.
The numerical analysis was mainly conducted using the Standard LIEN k – ε turbulence model, available in the STAR-CCM+ code. Moreover, to better understand the numerical aspects of the model, a comparison of some of the results obtained by the LIEN k – ε turbulence model with those of other two models is performed; in particular, the AKN k – ε turbulence model and the SST k – ω turbulence model are considered. The results of these models are compared together and with experimental data, using as reference variable the predicted inner wall temperature.
Finally, the work reports results of a systematic comparison between different fluids at supercritical pressures, basing on similarity principles conceived to establish a rationale for fluid-to-fluid comparison. Simulations have been performed with NH3, CO2 and R23 implemented in STAR-CCM+ and the results are compared with the mentioned experimental data by Watts, in the aim to understand the capabilities of this theory.