Research on characteristic of flow instability in a two-phase natural circulation loop with parallel once-through steam generators

Abstract

When there are two sets of parallel once through-steam generators(OTSG) in the natural circulation loop, the thermal-hydraulic characteristics are complicated. Flow instability often occurs in such loops. In order to alleviate the harm caused by flow instability and enhance the natural circulation capacity, it is necessary to study the characteristics of flow instability in depth. In this paper, a thermal-hydraulic computational model is developed for a two-phase natural circulation loop with two parallel OTSGs based on the RELAP5 program. And the model is validated. The thermal-hydraulic parameters of the loop were calculated for different decay heat conditions. The flow instability of the loop is found to be characterized by a clear division into three zones in the decay heat interval. The concept of “load-flow ratio” is proposed to analyze the critical point of flow instability under different operating conditions. The reason for the flow instability zones of the loop is explained. The conclusions of the relevant research can provide a basis for an in-depth study of the characteristics of flow instability and the development of operational schemes to mitigate flow instability.