Static and Rotordynamic Characteristics for a Novel Helical Damper Seal Operating in a Liquid Hydrogen Turbopump

In this paper, two damper seal schemes were designed and assessed for application at the inter-stage location in a cryogenic liquid hydrogen turbopump, focusing on the improvement of the sealing capability and rotor dynamic stability, including a conventional hole-pattern damper seal (HPS) and a novel helical damper seal (HDS). This novel helical damper seal (HDS) was designed with the hole-pattern on stator and the helical grooves on rotor, derived from the conventional straight-through hole-pattern seal (HPS) with the same sealing clearance, diameter, axial length, hole diameter and depth. To enhance the seal net damping capability at high inlet preswirl condition, a straight swirl brake was also designed at seal entrance for each one of these two damper seals. A comprehensive assessment and comparison on the static and rotordynamic characteristics was conducted for the present conventional HPS seal and the novel HDS seal. The leakage flow rates, rotordynamic force coefficients, cavity dynamic pressure, and swirl velocity developments were calculated and analyzed for two damper seals with/without inlet swirl brake, using a proposed transient computational fluid dynamic (CFD)-based perturbation method, based on the multiple-frequency elliptical-orbit stator whirling model in a multiple reference frame. 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, using an in-house code based on the National Institute of Standards and Technology (NIST) reference fluid properties database (REFPROP). Results show that the present novel HDS seal obviously leaks less (by 54%–57%) than the HPS seal across the whole load range, thus possessing obviously better sealing capability, due to the combined contributions of the “energy dissipation effect” of the static hole-pattern textured stator and the “pumping effect” of the rotating helically-grooved rotor. From a viewpoint of rotor dynamic stability, vibration model and unbalance sensitivity analysis, the present novel HDS seal without inlet swirl brake is an ideal seal scheme for the present inter-stage position of the LH2 turbopump, producing the lower effective stiffness and the higher effective damping, especially at high load conditions.