Flow and Heat Transfer CFD Analysis in the Section of THELMA for Wall Surface Temperature Determination

Thermal-Hydraulics Experimental Laboratory for Multiphase Applications (THELMA) has been built in the Reactor Engineering Division of Jozef Stefan Institute. The main apparatus in the THELMA laboratory is a unique test section, resembling closely a single fuel rod, which, among other things, allows visual observations of boiling phenomena including the Critical Heat Flux (CHF) in annulus geometry. Two fluids are involved in the test section: a primary fluid is the object of investigation (boiling, condensation, etc.) and a secondary fluid, which provides a controlled temperature of the rod's wall. The flow and thermal conditions are monitored with flow meters and a number of miniature thermocouples embedded in the wall as well as installed within the secondary fluid path. However, despite being numerous and miniature in size, they are still limited in resolution as well as the insight into the flow and heat transfer behaviour along the test section. Hence, a Computational Fluid Dynamics (CFD) study is performed as a design support and for better understanding of flow and temperature distribution inside the test section. A series of CFD analysis is carried out for a wide range of single-phase test conditions, i.e. different flow rates, primary fluid temperatures and secondary fluid temperatures. Steady-state Reynolds Averaged Navier-Stokes (RANS) simulations are performed with a Conjugate Heat Transfer (CHT) approach using ANSYS Fluent CFD code. Obtained results are compared for different low-Reynolds turbulence models, which resolve the momentum and thermal boundary layer. The predicted temperatures at the surface of the fuel rod have been carefully examined and the uncertainty of such CFD predictions is estimated as well. Moreover, a simple correlation is derived from the obtained CFD predictions, which will be used for correction of the measured wall temperature for determining an accurate temperature at the surface of the rod.