An experimental study of two-phase flow instability in a multi-loop natural circulation system

Abstract

In this research, an experiment was carried out in a multi-loop natural circulation system to study its response behaviors under single-phase and two-phase flow instability conditions. The experimental facility consists of primary and secondary natural circulation loops, as well as an external forced circulation loop, which serves as a final heat sink to the system. The secondary loop is meant to simulate passive residual heat removal system found in the advanced reactors and some small modular reactors. The experiment was carried out under low-pressure conditions and constant heating powers. Within single-phase region, the thermal hydraulic parameters were found to vary linearly with the heating powers. By gradually increasing the power beyond the single-phase region, oscillation of the thermal hydraulic parameters was observed in both natural circulation loops. Critical conditions for the occurrence of instabilities in both natural circulation loops were determined. Flow-induced instabilities causing vibrations and acoustical noises were observed in the shell and tube heat exchanger connecting the two natural circulation loops. At higher power, fully mature oscillations were observed and characterized. In addition, effect of temperature rise in the system heat sink was also investigated. The oscillation period of the observed oscillations is about twice the boiling channel residence period. The oscillation amplitudes increase with increasing heater’s inlet temperature and heat sink temperature for the primary and secondary loops respectively. However, the influence of the heater’s inlet temperature on the flow instability is observed to be non-linear. This paper can contribute in the identification and determination of threshold values of thermal hydraulic parameters that are capable of triggering flow instabilities, which are important in the design and safe operations of natural circulation systems. It can also provide some useful information about how temperature rise in the system heat sink can influence the occurrence and amplification of flow instability in a natural circulation system.