Sealing systems contribute significantly to the efficiency of turbomachinery. The most commonly used seal is the labyrinth seal. At small gap widths, which are important for low leakage mass flows, thermal and mechanical expansion of the rotor can lead to contact with the stator. During these so-called rubbing processes, friction energy is released, which in the form of heat can lead to a detrimental temperature increase of the rotor. Therefore, the stator is fitted with rubbing linings which oppose the rotor with a low resistance and are subject to targeted wear. Thus, the rotor is protected during operation and the smallest possible gap can be formed. State-of-the-art honeycomb structures are manufactured from thin metal foils and soldered onto the stator. Nickel-based alloys are used because they offer the necessary resistance to the high operating temperatures. Since the rotor of a turbomachine is one of the most critical components, it is of eminent importance to design the system consisting of the rubbing liner and the seal fin tip as robust as possible. Therefore, it is necessary to make an accurate prediction of the performance and service life of the seal.

In order to investigate the effects of the rubbing process, an existing test rig was modified to allow the controlled incursion of a honeycomb liner segment onto a labyrinth seal fin. The seal fin consists of Inconel 718. The honeycomb segment represents a small degree section of a honeycomb coating and consists of Haynes 214. The test rig enables the variation of the relative velocity in the contact (up to 165 m/s) as well as the incursion rate (up to 0.5 mm/s). Furthermore, different axial configurations of the seal fin with respect to the honeycomb structure are considered. The assembly process results in metal foil structures with single and double wall thickness. If the seal fin contacts a joining point of the metal foil and thus forums the largest possible contact area, we speak of the large contact. The so-called small contact is formed by the part of the honeycomb structure where the seal fin contacts the 60-degree oriented side wall and has the smallest possible contact area. During the experiment, the three-dimensional contact forces, the temperature of the rotor and the torque change are recorded at a high sampling rate up to 100 kS/s. The measurement of the geometry of the honeycomb structure using a laser scanner and the recording of the weight before and after the test enables the evaluation of the wear mechanisms. A distinction is made between wear due to material removal and plastic wear.

Thus, it is possible to quantify the influence of the relative velocity in the contact and the incursion rate on the load of the seal fin and the stator. In addition, a statement can be made how sensitively the system reacts to a change in the seal fin position relative to the honeycomb structure. The results for the honeycomb structure are also compared with previous results of a simplified setup, where individual metal foils were examined. With the results of this study we can contribute to the optimization of labyrinth seals and the development of new types of liner materials as well as geometries.


Experimental Investigation on the Rubbing Process of Labyrinth Seals Against Honeycomb Liners

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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