Abstract
Natural circulation is exploited in nuclear systems to passively remove power in case of accident scenarios. In this regard, the DYNASTY experimental facility at Politecnico di Milano has been setup to increase the knowledge on single-phase, buoyancy-driven systems in the presence of distributed heating. In this paper, the development of a computational fluid dynamics (CFD) model of DYNASTY is presented, focusing on the capability of CFD to assess the dynamic behavior of the facility. The large eddy simulation (LES) model takes into account both the fluid and the solid regions, with heat generation and 3D heat conduction resolved in the pipe walls. The study, conducted using OpenFOAM, shows (i) the capability of reproducing stable and unstable transients of DYNASTY, (ii) new observations on the features of flow reversals during unstable transients, (iii) the suitability and the advantages of LES for the prediction of the specific features of natural circulation systems.
Highlights
Natural circulation is the result of the presence of density gradients in a fluid system, induced by temperature differences, which generate convective motion as a result of the action of buoyancy forces
The stable transient ð1 kW; 180 CÞ shows the convergence of global parameters to steady-state values, whereas the unstable transient ð5:3 kW; 240 CÞ is characterised by an oscillating mass flow rate that diverges with time until a periodic flow reversal with characteristic frequencies of oscillation is established
The present paper focused on the development of a singlephase computational fluid dynamics (CFD) model, including the modelling of the solid walls, for the study of natural circulation dynamics with a distributed heat source
Summary
Natural circulation is the result of the presence of density gradients in a fluid system, induced by temperature differences, which generate convective motion as a result of the action of buoyancy forces. Temperature distributions are achieved via local or distributed external heating/cooling applied to the pipes to introduce/extract heat from the fluid These facilities are important as they provide the possibility to observe the behaviour of natural circulation flows in a controlled environment, as well as the dependence between the system’s parameters and its stability, and in providing validation data for the numerous mathematical models that have been developed to predict the behaviour of NCLs. Once insight from simplified NCLs has been obtained, this can be applied to more complex systems, obtaining a priori information on the desired system behaviour before it is physically built. DYNASTY’s dynamic stability was studied through various models of increasing fidelity, from stability maps, to 1D system models (DYMOLA (Dassault Systèmes, 2019)) and CFD simulations based on RANS modelling (Cauzzi, 2019) These simulations demonstrated the possibility for the system to produce both stable and unstable transients, depending on the system’s setup parameters (amount of injected heat, heating distribution, cooling temperatures).
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