The significance of the buffer zone of boundary layer on convective heat transfer to a vertical turbulent flow of a supercritical fluid

Heat Transfer and Hydraulic Resistance in Turbulent Flow in Vertical Round Tubes at Supercritical Pressures—Part II: Results From Numerical Simulation With Differential Turbulent Viscosity Models

The review of numerical studies on the turbulent flow and heat transfer of supercritical pressure (SCP) coolants in heated vertical round tubes, which were conducted using different differential turbulent viscosity models, is presented. The results of predictions are compared with the experimental data on wall temperature and heat transfer rate. It is shown that most often the turbulent viscosity models only qualitatively (but not quantitatively) predict the deteriorated heat transfer effects, which appear due do buoyancy forces and thermal acceleration effects at strongly variable physical properties of a coolant. At the same time, the regimes of normal heat transfer are successfully reproduced by “standard” k–ε and RNG models with wall functions (WFs), as well as by two-layer models. The conclusion is made that none of the presently known turbulent viscosity models can be confidently recommended for predicting any flow regimes and heat transfer of SCP coolants. Strongly variable properties of SCP coolant stipulate more strict demands for validating mesh independence of the obtained results and for an accuracy of approximation of the tabulated values of the coolant properties. It was ascertained that using more and more numerous calculation codes and the results from their application requires certain caution and circumspection. Sometimes, the energy transport equations were erroneously written for temperature or temperature variance rather than for enthalpy. Crying discrepancy in the predictions of different authors conducted using the same CFD codes and turbulence models and possible reasons for such a discrepancy are not analyzed.