5 Vane Wake And Rotor Blade Potential Field Effects On Seal Flow

The seal flow was characterized with data taken at three seal mass flow rates, referred to here as high (Cw= 1.2x104), midrange (Cw=9.2x103), and low (Cw=4.6x103). In all cases the momentum of the emergent flow, the presence of a vane wake, and the passing rotor potential-field impact the flow development along the hub wall downstream of the seal. To characterize these effects, data in measurement planes A and C are considered, where Plane A is completely removed from the vane wake convection path and Plane C intersects the vane wake convection path.

The radial velocity magnitude in the near-seal region of the flow field for the three mass flow cases is presented in Figure 5.1 for rotor position ¿=375 |j,s at vane Plane A. For the high flow case, shown in the top frame of Figure 5.1, a positive purge flow is observed exiting the seal. No gas ingress into the cavity was observed at any rotor position for this case. As the seal flow was reduced to the midrange point, a significant reduction in the mass flow leaving the seal is observed, with the strongest purge appearing at the seal leading edge. When the seal flow is reduced to the minimum value, air ingestion into the seal becomes the dominate phenomena during portions of the blade pass event, as shown in the bottom frame of Figure 5.1.

5.1 Rotor Potential Field Effects in the Absence of a Vane Wake

The effect of the rotor pass event on the axial velocity profiles near the rotor for each of the three seal mass flow rate events are presented in Figure 5.2. At axial locations upstream of the rotor, the velocity profiles for all three cases are relatively insensitive to rotor position. However, near the blade the deceleration of the flow as it approaches the potential field is evident. At times ¿=450 |s and t=600 |s, near the blade, the potential field has strongly affected the axial velocity at all radial positions. In the high flow case, the blockage caused by the penetrating seal flow has reduced the momentum of the flow from near the hub when compared to the other two cases. This low momentum fluid is most strongly affected by the approaching blade, as seen at time ¿=450 |s.

Figures 5.4 and 5.5 show the axial velocity profiles for the low and high seal flow cases at five rotor positions. In each frame, three different axial planes have been chosen, with x= 0.185 in. (4.7 mm) furthest from the rotor and x= 0.665 in. (16.9 mm) nearest the rotor. For the low flow case, Figure 5.3, there is acceleration of the flow near the hub when the measurement plane is located near the middle of the rotor passage. As the potential field increases, t > 300 |s, there is uniform velocity reduction and the 'bulge' in the profiles shrinks, until by t=600 |s all profiles are similar. From an examination of Figure 5.1 this behavior is not unexpected, as the strength of the adverse pressure gradient has become sufficient to produce a negative purge flow (seal ingress) which is removing low momentum fluid near the endwall and flattening the profile.

As shown in Figure 5.4, the high seal flow rate case exhibits dramatically different behavior than that observed in the previous case. Here, as previously noted, the seal flow emerges with high radial momentum and penetrates deeper into the mainstream flow. This results in hub blockage and low momentum flow in an extended region to approximately r=5 mm from the hub. Other than this effect, the profiles are quite nominal until t=450 |s, where there is a large axial velocity deficit forming at radial locations higher than r=5 mm. This deficit then disappears again by t=600 |s. This behavior is best explained by observing the streamlines presented in Figure 5.5 for time t=450 |s. At the high flow rate (left frame), we see the start of horseshoe vortex roll-up approaching the seal region. This would introduce both local acceleration on the top surface due to clockwise rotation and significant blockage near the hub for the approach flow. This results in a large increase in radial velocity near the seal, and continuity arguments can thus defend the deficit in velocity observed in the profile of Figure 5.4.

Supporting these arguments for the effect of rotor blade potential field on the purge flow, velocity magnitude and vorticity contours for two blade times are presented: a time when the blade potential field is near minimum, t=300^,s, and a time when the blade leading edge is aligned with the measurement plane, /=675^s.

The velocity field for the measurement plane outside of the vane wake (Plane A) and with the minimal effect of the rotor potential field (t=300^s) is shown in Figures 5.6 and 5.7 for both the high and low seal flow rate respectively. The corresponding results for the normal vorticity are presented in Figures 5.8 and 5.9. The evidence of the seal flow can be realized by a low momentum region near the hub wall just downstream of the seal gap as well as by a region of high normal vorticity at the downstream edge of the seal. In the high seal flow case, the influence of the purge air is much more significant, whereas with the low seal flow rate little disruption downstream of the seal is seen.

The velocity magnitude contours of Figure 5.6 for the high seal flow show that in the absence of a vane wake, the strong seal flow causes an area of low momentum fluid immediately downstream of the seal. Additionally, analysis of the vector field demonstrated that the upstream flow turns radially upward due to the seal flow generated blockage. This effect is amplified by the presence of the rotor, as shown in Figure 5.10, where the rotor blade potential field effects are increased (t=675^s). As the blade passes, the low momentum area along the hub wall is slowed yet further by the blade potential field (i.e. the blade "bow" wave). Vector information reveals that the flow at the wall reverses, creating a clockwise recirculation region extending from x=0.590 to 0.787 in. (x=15 to 20 mm) on the axial scale. This is most easily seen in the corresponding vorticity contour given in Figure 5.11. This recirculation can be attributed to a rotor blade leading edge horseshoe vortex which exists within an area 0.197 in. (5 mm) in diameter at its greatest.

The growth of this structure with rotor pass and the interaction with the roll-up on the lee side of the purge flow is clearly portrayed in the streamline history of Figure 5.12. The instantaneous axial-radial streamlines for the high seal flow case at ten rotor positions are shown, demonstrating the evolution of the secondary flow structures during the rotor pass event. The image plane movement relative to the rotor is indicated by the arrow, beginning with the image plane centered between rotor blades at the top center frame and moving counter-clockwise as the image plane approaches the blade which occurs at the bottom center frame. This structure increases the endwall blockage and subsequent radial deviation of the seal approach flow. As the blade nears, the seal flow interacts with the rotor blade horseshoe vortex which grows until near the blade the vortex swallows the emergent seal flow. This behavior was not observed at lower seal flow rates.

When the two streamline patterns of Figure 5.5 are compared, the enhancing role of the seal flow in the secondary flow structure becomes obvious. At low seal flow rates, only a small vortex structure is seen near the blade. This may well be due to an invigoration of the boundary layer that occurs when the seal flow is introduced with positive flow but low radial momentum, or negative flow, over a portion of the blade pass cycle. When the seal flow rate is increased such that it possesses sufficient radial momentum to disrupt the hub boundary layer, it encourages roll-up and results in a large horseshoe vortex structure dominating the near-hub flow. However, even without the blade potential effect, Figure 5.8 shows that the purge flow still has a distinct impact on the flow profile downstream of the seal.

5.2 Wake Interaction with Seal Flow

A significant alteration of this velocity field occurs when a vane wake crosses the hub seal. For the measurements made in the presence of a vane wake (Plane C) and with high seal flow rate, an area of increased axial velocity over the seal is observed. This high flow condition is likely due to the interaction of the emergent seal flow and the convected wake structures which carry opposite signs of rotation. Higher momentum in the flow near the wall downstream of the seal is observed for both blade positions, as shown by a comparison of the velocity contours of Figures 5.13 and 5.14 to those of the corresponding no-wake case (Figures 5.10 and 5.6). The wake structure is clearly visible from the negative vorticity observed over the seal in Figures 5.15 and 5.16.

Due to this effect, the seal flow penetration into the mainstream is reduced, resulting in increased axial momentum downstream of the seal and less blockage to the approach flow. The seal flow appears to remain close to the wall and the blade potential effect is reduced to the degree that the size of the horseshoe structure which develops with the blade pass is significantly reduced, as is the magnitude of normal vorticity upstream of the rotor leading edge. However, as shown by the seeded seal flow images of Figures 5.17 and 5.18 for this case, the purge fluid actually does not remain as close to the wall as implied by the velocity and vorticity data. Instead, within 3mm axial distance from the seal, rapid mixing entrains seeded seal fluid into the main stream. The mixing process is enhanced by the rotor pass event as shown in Figure 5.17.

The low seal flow case evaluation of velocity magnitude contours shows that in the absence of a vane wake, there is very little boundary layer disruption that can be attributed to the seal flow. Figures 5.19 and 5.7 present this no-wake case (Plane A), which shows that in the immediate area of the seal there is a slight reduction in axial velocity. This reduction is more noticeable during the blade pass event. In Plane C, with the vane wake, (Figures 5.20 and 5.21) a larger magnitude of axial velocity over the entire measurement field is seen. This is likely due to flow under turning that occurs in the intra-stage space far from the vane exit, as it is an effect that is evident in both the high flow and low flow cases for all rotor positions.

It would be reasonable to expect the same influence of the wake for the low seal flow rate case as that for the high seal flow rate case, but surprisingly Figures 5.22 and 5.23 shows otherwise. In this low flow case, the field of high negative normal vorticity associated with the wake appears to be considerably diminished in size. This suggests that the magnitude of the seal flow itself contributes to the size of the wake-generated structure and the resulting degree of influence that the seal - wake interaction has on the downstream flow. In this low flow case, the structure appears to do more than keep the seal flow close to the hub wall, it may act to reinvigorate the platform boundary layer. In fact, referring back to the velocity magnitude plots shown in Figures 5.20 and 5.21 for the wake case, an area of increased axial velocity downstream of the seal gap coincides with the region of high negative normal vorticity. Velocity profiles plotted for discrete axial locations around x=12mm showed an increasingly smaller boundary layer thickness with increasing axial position.

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