Simulation of Experiments on Density Wave Oscillation and Ledinegg Instabilities in Helical-Coiled Steam Generators

Simulation studies using the TRACE computer code are performed for experiments related to density wave oscillations (DWOs) and Ledinegg instabilities in helically coiled steam generator tubes carried out at the SEIT (Società Informazioni Esperienze Termoidrauliche) facility in Italy. The simulation results show that the predictions of the onset of DWO–induced instabilities are in reasonable agreement with the experimental measurements. The TRACE predictions of the temporal behavior of local pressures and flow rates are also in qualitatively reasonable agreement under conditions that resulted in DWO instabilities. This includes good quantitative agreement on the measured oscillation period.

On the other hand, the agreement between the simulation predictions and experimentally measured conditions was not as good whenever Ledinegg instabilities and parallel channel asymmetries become prominent. At high system pressure (80 bar), and intermediate pressure (40 bar) with lower injection rates, both the measured and predicted instabilities are due to pure density wave conditions. At intermediate pressure and higher injection rates, higher heat input, or low pressure, the simulations showed that the system undergoes a superposition of DWOs and Ledinegg instabilities. Ledinegg instabilities that are predicted by TRACE at intermediate pressures were not observed in the experiments.

At the lowest pressure of 20 bar, the simulation results showed the onset of pure density wave instability at heat rates generally in agreement with the measurements. On the other hand, simulations at 20 bar are shown to result in Ledinegg instabilities at low power levels. These power levels are significantly below the threshold for DWO initiation, and therefore, similar Ledinegg instabilities are not reported for the experiments.

Among the reported experimental data and discussions, Ledinegg instabilities are only illustrated by measurements at 21 bar. TRACE simulations, also carried out at 21 bar for which experimental data have been reported, showed a progression of the instability mechanisms as a function of increasing heat rate that generally agrees with the measurements. The simulations progress from stability to large-amplitude oscillations following flow excursion–induced by Ledinegg instabilities that do not overlap due to large channel asymmetry, to less asymmetry, such that the oscillations partially overlap, to the fully overlapping large-amplitude oscillations that comprise the signature of the pure DWOs and instabilities.