Studies on flow instability of helical tube steam generator with Nyquist criterion

•Density-wave oscillation in helical-tube steam generators was studied.•The multi-variable frequency domain method was used for the modeling.•The flow stability was evaluated by the Nyquist stability criterion.•The calculated results are consistent with the experimental results.

The steam generator of the 10 MW High Temperature Gas-Cooled Reactor (HTR-10) in China consists of a series of helical tubes where water/steam flows inside and helium flows outside. It operates under middle pressure, which tends to cause the flow instability. Density-wave oscillation is the most common type of two-phase flow instability in the steam generators. This paper presents the research on flow instability for the HTR-10 steam generator. The drift flux model was used for two-phase flow analysis. The transfer matrix was obtained by using linearized perturbation and Laplace transformation on the conservation equations. The flow stability was evaluated by the Nyquist stability criterion. The results obtained from frequency domain method were compared and discussed with the results from the time domain method and the experimental results.