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Static Instability of a Once-Through Steam Generator with a Modular Feedwater Line

Static flow instability of a once-through steam generator (OTSG) with a modular feedwater line is an instability related to the change of a flow direction in individual steam generating U-shaped channels operating at a given pressure difference. The nature of the static instability is close to a Ledinegg instability [1] related to the presence of multiple flows at the full hydraulic curve of a U-shaped channel. In this paper, the conditions for a reverse flow for the OTSG with a U-shaped modular feedwater line (MFL) are studied. From the results of the studies, it is revealed that the change of a flow direction in the MFL is due to a boiling of the feedwater in the downcomer branch of the U-shaped MFL and that multiple flows start in an area of the extremes corresponding to the minimum pressure difference of the hydraulic curves. Calculation models for predicting a threshold of a static instability for the OTSG of interest is proposed and the analysis results are compared with the experimental data.