Leakage flow-induced excitation behavior and rotor instability in the high-speed supercritical CO2 scallop damper seals

Abstract

As an innovative type of damper seal, the scallop damper seal (SDS) has shown greatly improved sealing performance for the high-compactness supercritical CO2 (S-CO2) turbomachinery, but leakage-induced excitation forces have an impact on the rotor stability. In this research, we improve the rotordynamic solution based on the multi-frequency elliptic whirling model for the S-CO2 seals and investigate the leakage-induced excitation behavior of SDS depending on seal geometry and operating condition. By comparing the results of a labyrinth seal and a full-partition pocket damper seal, we prove that SDS has better rotordynamic performance for the S-CO2 compressor. The results show that increasing the pressure drop favors greater direct stiffness and effective damping of the SDS. Inlet preswirl changes the direction of fluid-response force to coincide with the rotor motion, which is detrimental to rotor stability. Changing the cavity depth has no significant effect on the dynamic characteristics of the SDS, but shortening the cavity length reduces the direct stiffness to below zero. Supplementing the number of circumferential cavities increases the direct stiffness and effective damping but results in SDS not being able to be machined directly by milling cutters. The design with 12 circumferential cavities is recommended for the 100-kW-class S-CO2 compressor.