Analysis of the unstable behaviour of a single-phase natural circulation loop with one-dimensional and computational fluid-dynamic models

This paper discusses the results obtained using one-dimensional and three-dimensional computational fluid-dynamic codes for the prediction of the dynamic behaviour observed in experiments carried out in a single-phase natural circulation apparatus. The loop is made of glass and is equipped with vertical and horizontal heaters and coolers that can be separately operated, thus obtaining different working configurations. An in-house program, capable of linear and non-linear stability analysis of one-dimensional loops, with arbitrary configurations of heat sources and sinks, and a transient thermal–hydraulics system code were adopted for the purpose of highlighting capabilities and limitations of one-dimensional models in predicting the involved phenomena. Both linear stability maps and transient flow evolutions have been calculated by the programs, obtaining information on both the linear and the non-linear dynamic behaviour of the addressed system, as predicted by one-dimensional, cross-section averaged balance equations. A computational fluid-dynamics code has been also adopted for simulating the system dynamics in the different loop configurations. In particular, the CFD code was effective in showing the origin of pulsating instabilities observed with horizontal heater and cooler, which could in no way be predicted by the one-dimensional models. Abbreviations: CFD, computational fluid-dynamics; HHHC, horizontal heater horizontal cooler; HHVC, horizontal heater vertical cooler; RNG, renormalised group theory; VHHC, vertical heater horizontal cooler; VHVC, vertical heater vertical cooler; 1D, one-dimensional; 3D, three-dimensional.