Experimental Study On Flashing-Induced Instabilities In An Open Natural Circulation System

TL;DR

This paper experimentally investigates flashing-induced instabilities in an open natural circulation system, revealing flow pattern transitions and conditions leading to instability, which are critical for safety in nuclear and passive cooling systems.

Contribution

It provides detailed experimental analysis of flow pattern evolution and instability conditions in open NCS, introducing the flash number as a key stability indicator.

Findings

Flow pattern transitions from bubble to annular flow during flashing.

Instability occurs when the flash number at the riser outlet is between 4 and 5.

Churn flow length can exceed 40% of the two-phase regime.

Abstract

The natural circulation system (NCS) uses gravity pressure drop caused by density differences in the loop to generate the driving force without any external mechanical devices, which has been widely applied to the design of the nuclear reactor system and the passive safety system due to its simple structure, high intrinsic safety, and strong heat discharge capacity. However, the low-pressure condition can lead to a two-phase flow and make the flow characteristics in the NCS more complex. Flashing-induced instability occurring in the open NCS will cause the system structural vibration as well as mechanical damage and bring safety problems. The study on flashing-flow behaviors in an open NPS has been conducted experimentally in this paper. High-speed camera, thermal needle probe and wire-mesh sensor were adopted to record the flow pattern and measure the void fraction in the polycarbonate visualization riser section. In the start-up process, with the inlet temperature in the riser section increasing, the open NCS has experienced single-phase stable flow, intermittent oscillation between single-phase and two-phase, high subcooling two-phase stable flow, flashing-induced instabilities flow, and low subcooling two-phase stable flow. The flow pattern evolution of flow flashing goes through bubble flow, cap-slug flow, churn flow and wispy annular flow, in which the length of churn can account for more than 40% length of the two-phase regime. The flash number Nflash is used to divide the region of flashing-induced instabilities. It is found that the open NCS is in a stable two-phase flow when the flash number at the outlet of the riser section N_{flash,out} = 4\sim 5.