Flow instability of supercritical carbon dioxide in Converging–Diverging tube

Abstract

In promising energy conversion system of supercritical carbon dioxide (SCO2), flow instabilities easily occur in this system when it operates near the critical pressure and pseudo-critical temperature due to drastic change of the thermal properties. The converging–diverging tube (CDT) is widely used in enhance heat transfer because of its unique advantage. However, there is no research on instability, and the mechanism of density wave oscillations (DWOs) in CDT remains unclear. Accordingly, in this paper, we studied the instability mechanism and occurrence conditions of DWOs for CDT. We found that the critical trans-pseudocritical number Ntpc of the CDT was reduced by 20.6% compared to that of the straight tube due to the feedback delay effect. This was because the frictional resistance increases per unit mass of flow led to a decreasing flow velocity. The effects of the flow direction on the DWOs in the CDT and straight tubes were different due to the buoyancy changing. The critical Ntpc changed little at different pressures due to the small differences of the average value of βPCp. Finally, we obtained the dimensionless instability boundary map of the CDT to distinguish and identify the regions where instability occurs. This work helps clarify the mechanism of DWOs, identifies the differences between the DWOs in CDTs and straight pipes, and guides the safe operation of CDTs.