|Labyrinth seal leakage has often been the subject of investigations in the past because it influences the overall engine efficiency and heat balance. During transient operation of gas turbines, the rotor and stator expand differently. Hence, in labyrinth seals the radial clearance and axial position of the knifes change, which was quantified by Steward and Brasnett  using an X-ray visualization technique. These seal movements cause leakage changes. Rubbing of the knifes on the stator occurs and therefore abradable lands or honeycomb structures are deployed to prevent the danger of a seal failure. During the first engine cycles rub-grooves form on these stator lands (Fig. 1) and due to the new stator geometry, the leakage characteristic of the labyrinth seals with rub-grooves changes significantly compared to plain configurations ~e.g., Zimmermann and Wolff . |
The results from  showed that the knife tips of some seals even reside inside these rub-grooves during engine operation. However, the seal discharge behavior at negative nominal clearances has not been investigated in the past.
The objective of the present study is to analyze and quantify the influence of rub-grooves on labyrinth seal leakage for a wide range of configurations and operating conditions. To gain such a broad picture, the three labyrinth seal types commonly used in gas turbines are considered. For each seal type, a parametric study was conducted varying the clearance, the rub-groove geometry and the pressure ratio. In this parameter range negative nominal radial clearances are included, that correspond to engine conditions measured by Steward and Brasnett . To identify the flow mechanisms responsible for the altered sealing performance, flow visualizations were performed in a water-channel.
Following the introduction, the theoretical aspects needed to analyze labyrinth seal leakage are discussed. Then, the test facility and the seal geometry are described. Finally, the results of the present study are presented and compared with those found in the literature.