An Investigation on Phase Transitions in a Supercritical CO<sub>2</sub> Dry Gas Seal
Cong Zhang, Jinbo Jiang, Xudong Peng, Xuan Zhang
Abstract
Dry gas seals with extremely low leakage are the preferred sealing technology in the supercritical carbon dioxide (SCO2) Brayton cycle. However, dry gas seals are subject to a significant risk of CO2 phase transitions on the seal face, which has yet to be numerically investigated. A comparison study of SCO2 dry gas seals with two typical inlet temperatures is presented in this article. The seal clearance, leakage rate, and gas mass fraction under different inlet pressures and inlet temperatures are analyzed, along with further insights into the CO2 expansion process. Some operating and design considerations for CO2 dry gas seals to avoid going through the liquid and two-phase states are finally discussed. The results indicate that liquid CO2 and two-phase CO2 appear on the seal face when operating near the critical temperature. Vaporization of CO2 always occurs near the inner radius of the seal face. The CO2 expansion processes can be decomposed into two slowly decreasing stages and a rapidly decreasing stage. The starting point of the rapidly decreasing stage plays an essential role in CO2 phase transitions. Both low pressure and high temperature help avoid CO2 on the seal face transitioning to the liquid and two-phase states.