Requirements of LES in Modelling Natural Circulation Loops: Preliminary Study in a Pipe Geometry

At Politecnico di Milano, the experimental facility DYNASTY has been built to study natural circulation of fluids with internal heat generation, and the reliability of passive safety strategies in the Generation IV molten salt reactor. In a preliminary step prior to the simulation of the entire facility with computational fluid dynamics and large-eddy simulation, in this work LES is applied to pipes with flow conditions similar to those found in DYNASTY. The rather low Reynolds number, typical of the laminar to turbulent transition region, and the focus of most of the available literature on channel flows, explain the need for further understanding of LES requirements and predictive capabilities in such conditions. An incompressible adiabatic flow at Re = 5300 is predicted in a cyclic pipe section with numerical solution meshes of increasing quality and refinement, and using the WALE sub-grid scale model. Comparison with direct numerical simulation results demonstrates good accuracy in predicting the turbulent flow field, and frictional pressure drops, essential for the stability features of natural circulation loops, are also successfully determined. As expected, accuracy of the results depends strongly on the grid refinement. More importantly, refinement levels appropriate to achieving a desired accuracy, but maintaining the model's computational tractability on the large spatial scales and long temporal transients of natural circulation loop studies, are identified.