A linear stability analysis for natural-circulation loops under supercritical conditions

Experiments were conducted in a rectangular supercritical carbon dioxide (SCCO 2 ) natural-circulation loop at Argonne National Laboratory (ANL) in order to verify the stability margin as suggested by some previous investigators. Although a one-dimensional transient computational model developed at University of Wisconsin, Madison, predicted the development of instabilities for the SCCO 2 loop, which had good agreement with some previous work, the experiments conducted at the ANL SCCO 2 loop exhibited stable behavior under similar conditions. In order to bridge the gap between the numerical predictions and experimental results by distinguishing between the numerical effects and physical effects, a linear stability approach is adopted in the present study. The linear stability analysis has been conducted for three model natural-circulation loop geometries employing water or carbon dioxide as the working fluid. The results for the supercritical water loops displayed flow stability for a more accurate equation of state (EOS); however, the analysis indicated the presence of instabilities for a less accurate EOS. Furthermore, this analysis still predicts the presence of instabilities for the SCCO 2 loop similar to our transient numerical predictions. We additionally note that the stability margin for both water loops and the SCCO 2 loop does not correspond with proposed stability criteria from a previous analysis. These two final points suggest the phenomenon is a more complex function of both fluid properties and loop geometry.