Stability analysis of a parallel channel-loop system with single-phase natural circulation under asymmetric conditions

Abstract Flow instability can cause mechanical oscillations in the equipment as well as periodic changes in the thermal stress, both of which have a great impact on the safety of the reactor. In addition, flow instability can interfere with the control system, making it difficult to capture stable parameters. In this study, to examine the change in the stability of a parallel channel-loop system with natural circulation under asymmetric conditions, the single-loop natural circulation system is taken as the starting point, and the control equations are obtained by using a dimensionless analysis method. The Fourier expansion of displacement term in the control equations is carried out to obtain the Jacobian matrix representing the single-loop natural circulation system. Based on this matrix, the Jacobian matrix model of parallel channel-loop system is constructed. According to the model, the natural circulation stability of parallel channel-loop under different load and resistance difference is analyzed, and the effects of geometric characteristics, coolant and power on the stability boundary under asymmetric condition are examined. The results show that there are two critical Reynolds numbers, and the system becomes unstable when the load difference introduced by the left and right loops is greater than the critical Reynolds numbers. The area of stable region can be increased by increasing the aspect ratio and heating zone length, and reducing the pipe diameter and cooling zone length. Furthermore, the stability boundary of natural circulation is sensitive to the aspect ratio and the length of heating zone. In addition, the stability can be improved within the allowable range of the natural circulation by choosing liquid metal coolant and increasing the pressure drop, and this model cannot accurately describe the changes in the stability boundary at low power. Overall, the above results can serve as useful reference for the design of the parallel channel-loop systems and can be used to improve the stability of natural circulation. The stability boundary can be used to preliminarily judge the stability distribution and the variation trend of the parallel channel-loop system with a certain degree of reliability.