• CFD
• Supercritical fluids

This thesis is concerned with phenomena involved in heat transfer to fluids at supercritical pressure and in particular the ability of four equation RANS turbulence models to correctly reproduce observed behaviour.
Fluids at supercritical pressure exhibit property variations with temperature when crossing the so called "pseudo-critical" temperature and changing from a "liquid-like" to a "gas-like" condition. This impacts heavily on the effectiveness of the heat transfer. This can lead to either heat transfer impairment or enhancement. Such changes have presented difficulties for the two-equation k- and k- turbulence models which, in earlier studies, did not seem capable of dealing with the situation, especially when working close to the pseudo-critical conditions. The present study investigates the capabilities of some four-equation turbulence models, usually in combination with the Algebraic Heat Flux Model (AHFM) which provides an advanced relationship for modeling turbulent heat flux.
The models considered are AKN (1995), Deng et al. (2000), Hwang Lin (1999) and Zhang et al. (2010). Calculations are performed using an in-house code named THEMAT, as a versatile tool allowing the adoption of “mixed” models obtained by the combination of different model features.
Experimental data, for both carbon dioxide and water flowing in vertical circular pipes, are considered; the comparison of calculated and measured values is mainly performed on the basis of the wall temperature. Several analyses are conducted; the production terms of turbulent kinetic energy, due to both shearing and buoyancy, were evaluated especially in relation to the velocity distributions.
The experimental data considered allow a sufficiently broad range of operating conditions to be investigated; the capabilities of the four equation turbulence models are tested in many situations of interest and results helped in achieving a better understanding of the deterioration and enhancement phenomena involved.