Effect of Fluid Barrier in See-Through Labyrinth Seal for Supercritical CO2 Turbomachinery

Abstract The compact size and high efficiency of supercritical CO2 (sCO2) radial turbomachinery make it an ideal choice for sub-MW scale power generation. At these power scales, the primary concerns are the parasitic losses due to disc friction and leakage flow in the clearance gap between the back face of the impeller and the casing. Since sCO2 systems operate in a close cycle, the leakage mass needs to be reintroduced into the system to ensure satisfactory performance. Reinjection of the working fluid requires recompression work, which in the case of sCO2 Brayton cycles is significant, but is seldom accounted for in the cycle efficiency calculations. This paper proposes a novel labyrinth seal incorporating a fluid barrier and a reinjection system to effectively manage and reintroduce the leakage flow back into the cycle with minimal work. In the proposed seal configuration, high-pressure sCO2 from the system itself is used as the barrier gas, thus eliminating the need for a separate gas injection compressor. Comprehensive CFD studies are performed to understand the influence of radial clearance, jet width, injection pressure, inlet pressure, and rotational speed on the performance of the proposed seal arrangement. The flow field in the labyrinth gaps is examined for a set of geometric and operating conditions. Parametric studies reveal a strong relation between the optimum jet width and seal geometry. Throttling losses are dominant for seals featuring lower clearance, albeit with decreased leakage flow at the outlet. An increase in inlet pressure from 110 to 140 bar results in an astounding increase of ∼85–90% in leakage flow at the back side of the impeller, irrespective of seal clearances or injection pressures. In contrast, the leakage flow at the outlet remains constant for small clearances (≤ 0.1 mm) while marginally increasing by ∼3% for clearance of 0.2 mm. Sensitivity analysis is undertaken to understand the influence of geometric and operating parameters to help optimization of seal geometry. Finally, the efficacy of the new seal is presented through a study for a sub-MW scale sCO2 turbomachinery.