A Comparison of Static and Rotordynamic Characteristics for Three Types of Impeller Front Seals in a Liquid Oxygen Turbopump

Abstract

Annular seals are widely used to minimize the leakage, and improve efficiency and rotordynamic stability in turbomachinery. In a liquid oxygen turbopump, the operating conditions of annular seals are harsher with the high-pressure cryogenic propellant. Researches show that the primary leakage of a liquid oxygen turbopump is from the front and rear shoulder seals of the impeller. Moreover, the impeller shoulder seal sustains the largest pressure drop and highest swirl velocity in machine. Therefore, the shoulder seal plays an important role in rotor stability and successful full load operation of the turbopump, due to the significant seal fluid response forces. Previous literature shows that the hole-pattern seal has excellent performance in rotordynamic characteristics, and its leakage rate shows a very weak sensitive to the rotating speed. For helical-groove seal, however, the leakage shows a significant decrease with the increasing rotating speed and the roughness on stator surface can enhance the “pump effect” of helical-groove. The static and rotordynamic characteristics of annular seals with cryogenic liquid oxygen are limited in the previous literature. To make use of the advantages of both types of seals and address the sealing challenge in liquid oxygen turbopump, three types of liquid annular seals were designed for the front shoulder seal of a liquid oxygen turbopump, which include a hole-pattern seal (HPS), a double helical-groove seal (DHG), and a hybrid damper seal (HDS, with hole-pattern on stator and helical-groove on rotor). Computational fluid dynamics (CFD), as an important method to solve fluid dynamic problems, has been widely used to solve fluid governing equations. To assess and compare the leakage and rotordynamic characteristics of the present three types of annular seals under the liquid oxygen medium, a steady three-dimensional CFD method was developed to predict the seal leakage, based on the Frozen Rotor interface model. A transient three-dimensional CFD-based perturbation method was also proposed to predict the seal rotordynamic characteristics, based on the subdomain method, mesh deformation technique, and multiple-frequency elliptical-orbit whirling model. The accuracy and reliability of the present numerical methods were demonstrated based on the experimental data of leakage rates and rotordynamic coefficients for an experimental hole-pattern seal and an experimental helical-groove seal using water as working fluid. The leakage rate and rotordynamic coefficients of these three types of annular seals were calculated and compared under six variable load-operating conditions of a liquid oxygen turbopump, focusing on the seal leakage rate, effective stiffness, and damping. The numerical results show that compared with the traditional hole-pattern seal and helical-groove seal, the novel hybrid damper seal possesses the much less leakage flow rate and much better rotordynamic characteristics, so is a desired seal scheme for the impeller shoulder seal.