Measured Flow Oscillations and Instabilities in Labyrinth Seal Cavities

Abstract

Two flow instabilities involving a bifurcated flow pattern were discovered for the throughflow jet in a stepped labyrinth seal cavity. These instabilities, along with self-sustained flow oscillations, were experimentally explored to obtain a preliminary understanding of this phenomenon. Computer-captured visualization videos were used to measure the oscillation amplitudes and frequencies, as well as the mean value, of the throughflow jet trajectory angle. It was found that, depending on the seal cavity tooth clearance and radial height of the step, the flow pattern of the throughflow jet leaving the step corner was either 1) nonbifurcated, 2) always bifurcated, or 3) oscillatory bifurcated. A bifurcation stability map was developed showing which combinations of tooth clearance and step height lie in which of the three flow regimes. It was also found that the intermediate value of step height in the presence of the small tooth clearance exhibited the sharpest flow deflection and largest oscillation amplitude, as well as the highest mean-flow leakage resistance. Furthermore, for larger tooth clearances, the large step height cases, located farthest on the stability map into the oscillatory bifurcated regime, gave the highest leakage resistance.