Numerical analysis of supercritical flow instabilities in a natural circulation loop

Abstract

Numerical studies have been carried out to investigate flow instabilities in a natural circulation loop with supercritical CO 2. For steady-state and dynamic analyses of the loop under supercritical conditions, a single-channel, one-dimensional model is developed. In this model, equations for the conservation of mass, momentum and energy are discretized using an implicit finite difference scheme. A computer code called flow instability analysis under super critical operating conditions (FIASCO) is written in FORTRAN90 to simulate the dynamics of natural circulation loops with supercritical fluid. Stability boundaries are determined by simulating the loop's time evolution following a small perturbation under different operating conditions. Stability threshold results substantially deviate from the results reported by previous investigators, and contradict some of the reported findings. The disagreement in results is most likely due to the undesirable dissipative and dispersive effects produced from the large time steps used in previous studies, thereby leading to a larger stable region than those found using smaller time steps. Results presented in this paper suggest that the stability threshold of a natural circulation loop with supercritical fluid is not confined to the near-peak region of the (steady state) flow-power curve. Results obtained for the range of parameter values used in this investigation always predict the stability threshold to be in the positive slope region of the (steady state) flow-power curve. Parametric studies for different operating conditions reveal the similarity of stability characteristics of flow under supercritical conditions with those in two-phase flows.