Leakage and Rotordynamic Characteristics for Three Types of Annular Liquid Seals Operating in a Liquid Hydrogen Turbopump

In this paper, three types of annular liquid seals were designed for the inter-stage location of a cryogenic liquid hydrogen turbopump, in efforts to limit internal leakage flow and achieve rotor dynamic stability. These three seals represent the main seal types used in high-speed rotating machines, including a labyrinth seal (LABY), a fully-partitioned pocket damper seal (FPDS), and a hole-pattern damper seal (HPS). To enhance the seal net damping capability at high inlet preswirl condition, a straight swirl brake also was designed and employed at seal entrance for each one of these three seals, having the same sealing clearance, diameter and axial length. To present a comprehensive assessment and comparison on the leakage and rotordynamic characteristics, leakage flow rates, rotordynamic force coefficients, and cavity swirl velocity developments were calculated and analyzed for these three seals, using a proposed transient computational fluid dynamic (CFD)-based perturbation method based on the multiple-frequency elliptical-orbit rotor whirling model. To take into account of the thermodynamic effect and the weak compressibility of the liquid hydrogen, a table look-up procedure was implemented for the fluid properties of liquid hydrogen in supercritical region, based on the National Institute of Standards and Technology (NIST) reference fluid properties database (REFPROP). Results show that the present HPS possesses the least leakage and the largest effective damping throughout the entire subsynchronous frequency range. Therefore, from a rotordynamic viewpoint, the HPS seal with inlet swirl brake is the best seal scheme for the present cryogenic liquid hydrogen turbopump. When applying the present FPDS seal and HPS seal in high-pressure multiple-stage liquid hydrogen turbopump, the large seal stiffness effect must be carefully assessed based on the rotor vibration model and unbalance sensitivity analysis.