Flow Instability of Supercritical Carbon Dioxide in Converging–Diverging Tube

Abstract

The supercritical CO2 energy conversion system is one of the most promising technologies. However, flow instabilities easily occur in this system when it operates near the critical pressure and pseudo-critical temperature. The previous research mainly concentrated on straight pipes, yet there have been few reports of enhanced tubes. Accordingly, we study the instability mechanism and conditions under which instability occurs in an enhanced tube type—the converging–diverging tube (CDT). A transient model based on the finite volume method is presented to explore the phenomenon when supercritical CO2 transitions from the sub-critical state to the supercritical state in the CDT. When the heating power exceeded the critical value, the working medium transitioned from a pseudo-critical state to a supercritical state, which caused the working medium to experience large physical property changes, particularly, drastic changes of the density in the tube, which led to flow instability. Therefore, the trans-pseudocritical number Ntpc was introduced to reflect the degree of density variation at the corresponding heating power. The critical Ntpc was used to replace the critical heating power to represent the critical threshold at which instability occurred. In this work, we focused on exploring the effects of four parameters on the flow instability in the CDT: the inlet temperature (300-315 K), inlet pressure (7.5–12.0 MPa), pressure drop (550–3300 Pa), and flow direction. Finally, we obtained the dimensionless instability boundary map of the CDT to distinguish and identify the region where instability occurs.

Keywords: Supercritical CO2, Converging-diverging tube, Instability mechanism, Flow oscillations