III a I MEI 111 111 I EFFECT OF FLOW INSTABILITIES AND SELF-SUSTAINED OSCILLATIONS ON LABYRINTH SEAL LEAKAGE RESISTANCE

ABSTRACT Two flow instabilities involving a bifurcated flow pattern were discovered for the throughflow jet in a stepped labyrinth seal. A bifurcation stability map was developed showing which combinations of tooth clearance and step height occur in which of the three flow regimes. These instabilities, along with selfsustained flow oscillations, were experimentally explored to obtain a preliminary understanding of their effect on seal leakage. Computer-captured visualization videos were used to measure the throughflow angle oscillation amplitudes, frequencies and mean flow trajectory angles. For small tooth clearances, the intermediate step height case, which exhibited the sharpest flow deflection and largest oscillation amplitude, gave the highest leakage resistance. Further, for larger tooth clearances, the large step height cases, located farthest on the stability map into the Oscillatory Bifurcated regime, gave the highest resistance. Thus, for large clearances the oscillating nature of the Oscillatory Bifurcation flow pattern appears to give enhanced leakage resistance via increased turbulent mixing. INTRODUCTION The primary objective of labyrinth seals is to restrict fluid flow between regions of high and low pressure by providing a highly frictional flow path through a series of cavities. An alternative objective is to control internal flow rates of coolant in areas where cooling specific components is vital. A common type of labyrinth seal is the stepped labyrinth, shown i