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

Radial supercritical carbon dioxide (sCO2) turbomachinery is being explored as viable option for a sub-megawatt (sub-MW) power generation. However, at these scales, parasitic losses arising from disc friction and leakage through the passage between the impeller disc and the casing significantly degrade the performance. Since sCO2 Brayton cycles operate in a closed loop, the leaked working fluid is required to be recompressed and introduced back into the cycle with minimal compression work. This paper proposes a novel labyrinth seal architecture incorporating a fluidic barrier integrated with a reinjection mechanism to mitigate leakage losses. The design utilizes high-pressure sCO2 from the cycle itself as a barrier medium, thereby eliminating the need for auxiliary compression systems. Comprehensive CFD simulations are performed to investigate the influence of radial clearance, jet width, injection pressures, and rotational speed on the performance of the seal. Investigations reveal a strong dependence of optimal jet width on 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 increase of ~85-90% in leakage flow, 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.