Position of Unsteady Boundary Layer

Compressor at maximum effi ciency (m=6.4 kg/s). The next figures de­

scribe the development of the unsteady boundary layer along the rotor and stator blades at maximum compressor efficiency (nmax) using t-s-diagrams of the bridge output voltage e and the skewness ^3 (Figures 10, 11) as well as line plots of e, rms and ^(Figures 12 — 15). The boundary layer on compres­sor blades can be described by two typical paths, which occur periodically: the wake-induced path with the regions A, B, and C, and the path between two wakes that consists of the regions A, D, E and F. The laminar region (A) occurs at the leading edge of the blade and becomes transitional downstream (E, F).

The beginning of the transition is dominated by a calmed region (D) along the non-wake induced path. This region causes a delay of the transition process in the transitional region E and is generated by turbulent spots in the transi­tional region (B). Further downstream a turbulent region can be identified for x/c >63%. Line plots through the two paths are shown in the Figures 12 and 13 for the rotor, which compare, among other things, the beginning of by-pass transition. At the wake induced region the incoming wakes cause instabili­ties, which generate turbulent spots and, therefore, results in an earlier induced transition (Fig. 13: x/c=38%).

Line plots for a stator vane along the two typical paths are shown in the Figures 14 and 15. The distributions were measured using the surface hot wire technique. The transition points (^3=0) show different positions along the two paths due to the incoming wakes, induced by the IGV stage.

Compressor at reduced mass flow (m=5.85 kg/s). The consequences of a mass ft>w reduction on rotor and stator are shown in Figures 16 — 23. The rotor blade Campbell-diagrams of quasi wall-shear stress (e) and skewness in­dicate that transition above a separation bubble that begins at x/c=25% chord has occured, see Figures 16,17. A turbulent reattachment takes place between x/c=70-75% and is clearly modulated by the IGV-wakes. In the line plots (Fig­ures 18,19) the beginning of the separation area is indicated by a local rms – maximum. The global maximum at 60% chord, on the other hand, describes the region of laminar-turbulent transition. A typical zero-skewness (transition point) could not be detected due to the sensor positions that are directly placed on the wall. The mass flow reduction causes free-stream conditions which are similar to an increase of the angle of incidence. This leads to a destabiliza­tion of the boundary layer and results in a separation due to a local pressure increase.

Turbulent boundary layer reattachment can be identified by means of look­ing at cross-correlations of the last few array sensors, see Figure 20. The pre­sented correlations use the last sensor of the array for reference. The results show no correlation with the sensor at x/c= 67% but a clear amplitude-increase and a positive time shift at 89.1% chord, which indicates a reattached boundary layer. The correlation at x/c=74.1% shows an increased amplitude tendency but no significant amplitude peak due to the sensor position, which is near the reattachment line.

While the rotor boundary layer is separated above a blade region of approx­imately 40-50% of the chord, investigations on the stator vane (Figures 21 and 22) reveal that the boundary layer remains attached and turns turbulent between x/c=20-40%, modulated by the incoming wakes.

Compressor at Stall. A Rotating stall is characterized by one or more cells of reduced, and often reversed, fbw travelling with an angular velocity which is much smaller than that of the rotor stages. While a few investigations of stator vanes have been carried out in the past, simultaneous surface sensor measurements on rotor blades were performed for the first time. Figure 23 shows ensemble averaged power spectra at 6 different streamwise positions at the rotor blade with an excited blade frequency at f=1665Hz. It can be seen that the frequency does not dominate in each spectrum due to highly unsteady flow conditions.

A sequence of instantaneous boundary layer distributions at the stator is shown in Figure 24. Due to the stall cells moving with a reduced angular velocity compared to the rotating speed, it appears that the boundary layer phenomena results in a backwards movement in the t-s-diagrams. A common transition mode such as by-pass transition or the transition above a laminar separation bubble will not arise. The boundary layer development along the chord is controlled by turbulence effects generated near the leading edge.

2. Summary

The results presented in this paper were obtained in a highly-loaded, 1,5- stage, axial low-speed compressor. Typical wake and non-wake induced paths, as well as regions of laminar, transitional, and turbulent boundary layer flows were periodically detected on rotor and stator blades using surface sensor ar­ray techniques. In particular, the results obtained by the novel surface hot wire imply that the technique has the potential for shear stress measurements in unsteady flows. Investigations of the boundary layer development on a rotor blade were obtained by means of a telemetry system and miniature constant – temperature anemometry fixed to the rotor. Using such a data acquisition sys­tem, it can be shown that the boundary layer at the design point and at maxi­mum efficiency is based on a by-pass process. A mass flow reduction causes a laminar boundary layer separation in connection with a transition above a bubble, as well as a turbulent reattachment near the trailing edge.

Acknowledgments

The reported work was performed within the AdComB research project that is funded by the 5th European Framework. The project has been supported by Rolls-Royce Deutschland Ltd. and Co. KG, and carried out at the Pfleiderer – Institute of the Technical University of Braunschweig. The 16-channel minia­ture constant-temperature anemometry ring used for the measurements at the rotor blade was developed by the M. Baumann engineer-bureau.

Rotor-Stator Interaction in a Highly-Loaded Single-Stage Axial Compressor 609

References

Adamczyk, J. J.,"Wake Mixing in Axial Compressors" ASME 96-GT-29, 1996 Swoboda M., Teusch R., Guemmer V., Fottner L., Wenger U., "Experimental Investigation of Boundary Layer Transition in Compressor Cascades at Unsteady Flow Conditions", STAB Proceedings, 1998, pp. 474-482

Rohkamm, H., Wulff, D., Kosyna, G., Saathoff, H., Stark, U., Gummer, V., Swoboda, M., Goller, M., "The Impact of Rotor Tip Sweep on the Three – Dimensional Flow in a Highly – Loaded Single-Stage Low-Speed Axial Compressor: Part-II Test Facility and Experimental Results", 5th European Conference on Turbomachinery – Fluid Dynamics and Thermody­namics, Prague, Czech Republic, 18.-21. March 2003.

Lakshminarayana B., "A Method of Measuring Three-Dimensional Rotating Wakes Behind Tur­bomachinery Rotors", Journal of Fluids Engineering, June 1974, Vol. 96, pp. 87-91 Sturzebecher, D., Anders, S., Nitsche, W., "The Surface Hot-Wire as a Means of Measuring Mean and Fluctuating Wall Shear Stress", Experiments in Fluids, Vol. 31, 2001, pp. 294-301

Figure 1. Measurement set up

Figure 4. Instrumented stator vanes Figure 5. Rotor with anemometry-

circuits and telemetry transmitting – component

Figure 17. Rotor: Campbell-diagram of the skewness (m = 5.85kg/s)

Figure 23. Rotor at stall: power spectra at different streamwise positions

Figure 24. Stator at stall: t-s-diagram sequence

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