The effect of rounded labyrinth teeth tips and worn abradable lands has been found to give substantially increased leakage, which is well known to give reduced machine efficiency. Very little information concerning this exists, and some of the first measurements and visualization movies for stepped labyrinths are provided here to give an enhanced understanding of this phenomenon. A unique, very large-scale seal test facility was used. Glitter, and alternatively fluorescein dye, was employed as the flow tracer material. The flow visualization movies were digitally stored on the hard drive of a computer. Large decreases of leakage resistance due to the presence of worn teeth as well as rub-grooves were found. For the cases considered, the leakage resistance decrease for the large step height configurations were 85 percent, 55 percent and 70 percent for the small, medium and large pre-rub clearances, respectively. It was also found that the resistance varied with wear geometry, in order from highest to lowest resistance, as: (a) ungrooved-unrounded-teeth, (b) ungrooved-rounded-teeth, (c) grooved-unrounded-teeth and (d) grooved-rounded-teeth. Further, a substantial tooth tip recirculation zone was visually observed only for the grooved-unrounded-teeth cases, and it was shown to be the mechanism by which the unrounded teeth give this configuration a higher resistance than do the rounded teeth.

For the first time flow visualization images and leakage measurements were obtained and analyzed for an enhanced understanding of the effect of rub-grooves on straight-through labyrinth seal leakage. It was found that leakage resistance decreases sharply with increasing rub-groove axial width. Further, for smalltooth clearances with medium-or large-width rub-grooves, the groove gives an approximate 50 percent drop in leakage resistance. At the medium clearance, the presence of medium-or large-width rubgrooves increases and decreases the resistance at low Re and high Re, respectively. At the large clearance, the presence of grooves increases the resistance by some 200 percent and 40 percent for low Re and high Re, respectively. However, large clearance applications generally give very high Re where the 40 percent increase occurs.In addition, a vena contracta effect from the throughflow jet penetration into the rub-grooves was found on the flow pattern and on the leakage resistance. When penetration occurs, the trajectory angle of the jet entering the subsequent labyrinth cavity is sometimes dramatically altered, giving less kinetic energy carry-over to the subsequent tooth clearance. NOMENCLATUREc Clearance between tooth tip and opposing surface (excluding any groove depth) [cm] p Fluid density [kg/m ] K Leakage resistance coefficient P Pressure [kPa] Re Reynolds number based on clearance Vc Bulk fluid velocity at the clearance [m/s] It Dynamic fluid viscosity [N-s/m ]

The effect of rounded labyrinth teeth tips and worn abradable lands has been found to give substantially increased leakage, which is well known to give reduced machine efficiency. Very little information concerning this exists, and some of the first measurements and visualization movies for stepped labyrinths are provided here to give an enhanced understanding of this phenomenon. A unique, very large-scale seal test facility was used. Glitter, and alternatively fluorescein dye, was employed as the flow tracer material. The flow visualization movies were digitally stored on the hard drive of a computer. Large decreases of leakage resistance due to the presence of worn teeth as well as rub-grooves were found. For the cases considered, the leakage resistance decrease for the large step height configurations were 85 percent, 55 percent, and 70 percent for the small, medium, and large pre-rub clearances, respectively. It was also found that the resistance varied with wear geometry, in order from highest to lowest resistance, as (a) ungrooved-unrounded-teeth, (b) ungrooved-rounded-teeth, (c) grooved-unrounded-teeth and (d) grooved-rounded-teeth. Further, a substantial tooth tip recirculation zone was visually observed only for the grooved-unrounded-teeth cases, and it was shown to be the mechanism by which the unrounded teeth give this configuration a higher resistance than do the rounded teeth.

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 self-sustained 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.