The Influence of the Axial Rub Added in the Radial Rub on the Wear of the Seal Fins during the High Speed Rub of Labyrinth-Honeycomb Seal

Labyrinth-honeycomb seals are a state-of-the-art sealing technology commonly used in aero-engine interstage seal. The undesirable severe rub between the seal fins and the honeycomb due to the clearance change may induce the cracking of the seal fins. A pervious study investigated the wear of the seal fins at different radial incursion rates. However, due to the axial thrust and mounting clearance, the axial rub between the seal fins and the honeycomb may occur. Hence, this paper focuses on the influence of the axial rub added in the radial rub on the wear of the seal fins. The rub tests results, including rubbing forces and temperature, wear rate, worn morphology, cross-sectional morphology and energy dispersive spectroscopy results, are presented and discussed. Overall, the participation of the axial rub leads to higher rubbing forces, temperature, and wear rate. The tribo-layer on the seal fin is thicker and the cracks are more obvious at high axial incursion rate. These phenomena indicate the axial rub has a negative influence on the wear of the seal fins and should be avoided.

A Study for Rotordynamic and Leakage Characteristics of a Long-Honeycomb Seal With Two-Phase, Mainly Air Mixtures

This paper investigates the impact of the oil (silicone oil PSF-5cSt) presence in the air on the leakage and rotordynamic characteristics of a long-honeycomb seal with length-to-diameter ratio L/D = 0.748 and diameter D = 114.656 mm. Tests are carried out with inlet pressure Pi = 70.7 bars, pressure ratio (PR) = 0.35 and 0.25, inlet liquid volume fraction (LVF) = 0%, 3.5%, and 7%, and shaft speed ω = 10, 15, and 20 krpm. During the tests, the seal is centered. Test results show that leakage mass flow rate m˙ increases (as expected) as inlet LVF increases. Increasing inlet LVF makes direct stiffness K increase more rapidly with increasing excitation frequency Ω. Increasing inlet LVF has a negligible effect on K at low Ω values, but increases K at high Ω values. The value of effective damping Ceff at about 0.5ω is an indicator to the system stability since an unstable centrifugal compressor rotor can precess at about 0.5ω. Increasing inlet LVF increases the value of Ceff at about 0.5ω, reducing the possibility of subsynchronous vibrations (SSVs) at about 0.5ω. San Andrés's model is used to produce predictions. The model assumes that the test fluid in the seal clearance is an isothermal-homogenous mixture. The model adequately predicts m˙, K, and the value of Ceff at about 0.5ω.

A Study for Rotordynamic and Leakage Characteristics of a Long-Honeycomb Seal With Two-Phase, Mainly Air Mixtures

This paper investigates the impact of the oil (silicone oil PSF-5cSt) presence in the air on the leakage and rotordynamic characteristics of a long-honeycomb seal with length-to-diameter ratio L/D = 0.748 and diameter D = 114.656 mm. Tests are carried out with inlet pressure Pi = 70.7 bars, pressure ratio PR = 0.35 and 0.25, inlet liquid volume fraction LVF = 0%, 3.5%, and 7%, and shaft speed ω = 10, 15, and 20 krpm. During the tests, the seal is centered. Test results show that leakage mass flow rate ṁ increases (as expected) as inlet LVF increases. Increasing inlet LVF makes direct stiffness K increase more rapidly with increasing excitation frequency Ω. Increasing inlet LVF has a negligible effect on K at low Ω values, but increases K at high Ω values. The value of effective damping Ceff at about 0.5ω is an indicator to the system stability since an unstable centrifugal compressor rotor can precess at about 0.5ω. Increasing inlet LVF increases the value of Ceff at about 0.5ω, reducing the possibility of sub-synchronous vibrations SSVs at about 0.5ω. San Andrés’s model is used to produce predictions. The model assumes that the test fluid in the seal clearance is an isothermal-homogenous mixture. The model adequately predicts ṁ, K, and the value of Ceff at about 0.5ω.

Influence of Hole-Pattern Stator on Leakage Performance of Labyrinth Seals

The honeycomb seal shows promising characteristics in many turbine machines for the leakage control and rotor stability enhancement. However, the cost of honeycomb seal is relative high due to its complexities in manufacture and installation process. The hole-pattern seal has a very close leakage and rotordynamic performance with honeycomb seal, and also the manufacture and installation of hole-pattern seal are easier than the honeycomb seal, which attract the researchers and designers in recent years. In the published literature, there have been many papers dealt with the rotordynamic coefficients measurements, but very few researchers concentrated on the leakage control performance for the labyrinth seal with hole-pattern land. In this paper, the experimental tests were carried out to obtain the leakage rates versus pressure ratios at four clearances for the straight-through labyrinth seal with smooth stator, straight-through labyrinth seal with hole-pattern stator, stepped labyrinth seal with smooth stator and the stepped labyrinth seal with hole-pattern stator. The flow fields in the seal chambers were also visualized at different clearances and pressure ratios. Moreover, the CFD tool was also implemented to predict the leakage performance in labyrinth seals, and the numerical results were compared with the measurements. The results show that, the stepped labyrinth seal with hole-pattern land performs better leakage control characteristic than the straight-through labyrinth seal with hole-pattern land, and the stepped labyrinth seal with smooth land performs better leakage control than the straight through labyrinth seal with smooth land. For the stepped labyrinth seal, holes in the stator increase the effective clearance thus increase the leakage rate in seal. However, the straight-through labyrinth seal with hole-pattern land almost has the identical leakage performance with the smooth configuration.

Experimental and Numerical Study of Honeycomb Tip on Suppressing Tip Leakage Flow in Turbine Cascade

In this paper, the effect of a novel honeycomb tip on suppressing tip leakage flow in turbine cascade has been experimentally and numerically studied. Compared to the flat tip cascade with 1%H blade height, the relative leakage flow in honeycomb tip cascade reduces from 3.05% to 2.73%, and the loss also decreases by 8.24%. For honeycomb tip, a number of small vortices are rolled up in the regular hexagonal honeycomb cavities to dissipate the kinetic energy of the clearance flow, and the fluid flowing into and out the cavities create aerodynamic interceptions to the upper clearance flow. As a result, the flow resistance in the clearance increased and the velocity of leakage flow reduced. As the gap height increases, the tip leakage flow and loss changes proportionally, but the growth rate in the honeycomb tip cascade is smaller. Considering its wear resistance of the honeycomb seal, a smaller gap height is allowed in the cascade with honeycomb tip, and that means honeycomb tip has better effect on suppressing leakage flow. Part honeycomb tip structure also retains the effect of suppressing leakage flow. It shows that locally convex honeycomb tip has better suppressing leakage flow effect than the whole honeycomb tip, but locally concave honeycomb tip is slightly less effective.

Experimental and Numerical Analysis of the Secondary Flow Across the Interphase Balance Drum of a High Pressure Back-to-Back Centrifugal Compressor

With the major aim of gathering information on the machine lateral stability in high pressure-high density conditions, and of assessing the prediction capabilities of the in-house design tools and overall process, a back-to-back centrifugal compressor has been instrumented and tested in several operating conditions. The present paper focuses on the secondary flows across the interphase balance drum of the back-to-back compressor, where the sealing is accomplished with a honeycomb seal. The compressor interphase section has been instrumented with dedicated special probes for the clearance measurement associated to pressure and flow angle probes in order to characterize pressure distributions and swirl variations depending on the specific operating range. The experimental data acquired over the machine operation have been compared with a three-dimensional steady-state numerical analysis results obtained from the simulation, carried out with a Reynolds averaged Navier-Stokes (RANS) approach, of the flowfield in the complex interphase secondary system composed by the impeller cavities and the honeycomb seal. This paper addresses the comparison between numerical results and experimental data, which allowed the matching of models with experiments in terms of pressure distribution and the complex flowfield. Finally, all the data have been used to validate an in-house one-dimensional flow network solver for pressure distribution and leakage flow calculations along cavities and seals. Results have shown a general good agreement between measured data and calculation output. In particular, computational fluid dynamic analysis provided detailed pressure and velocity distributions that allowed gaining insight in the physics of such a complex region. The one-dimensional model has been demonstrated to be a fast and reliable tool to well predict local pressure variations inside cavities and seals and, consequently, the residual axial thrust.