Flow visualization and leakage measurements of labyrinth seals

Abstract

A large scale test rig is used to conduct an experimental investigation into the leakage resistance properties and flow characteristics of labyrinth seals. A novel test facility with multiple cavities that provides 2D, planar flow at a scale enlargement of 100: I is utilized. This facility uses water as the working fluid and allows real-time visual observation of experiments via large acrylic windows. Externally adjustable seal elements, also made of acrylic glass, allow each element to be moved to the desired radial and axial position. Two distinct flow visualization techniques are employed to assist in the identification of flow patterns which are beneficial to seal leakage resistance. Three classes of labyrinth seals are tested. Within each class, seals are tested over a range of Reynolds numbers, step heights, and axial offset positions (where applicable) for a fixed annular clearance. A permanent flow visualization record for each experiment is kept and analyzed in both real-time and slow motion. Statistical information is derived from the flow visualization record, and the time-mean flow field is mapped. When sudden changes in the leakage resistance properties of a seal occur, the flow visualization record is consulted to see what changes in the time-mean flow field have occurred. Furthermore, the statistical data provides information on the unsteadiness and oscillations in the instantaneous flow field. The experimental results imply that by using a combination of axial offset distance and step height, seal performance can be greatly improved. Inspection of the time-mean flow visualization images indicates that certain combinations result in greater serpentining. Seals that exhibit a moderate amount of serpentining generally had greater leakage resistance. Furthermore, it was found that larger step heights tend to amplify the effect that the axial offset has on serpentining. Conversely, however, it was observed that decreasing the clearance diminished the effect that axial offset has on leakage resistance. The statistical information obtained from the flow visualization seemed to have some correlation to the leakage resistance of a seal, however, the lack of a more comprehensive data set limits the conclusions that can be inferred.