| Axial compression systems are widely used in many aerodynamic applications. However, the operability of such systems is limited at low-mass flow rates by fluid dynamic instabilities. These instabilities lead the compressor to rotating stall or surge. In some instances, a combination of rotating stall and surge, called modified surge, has also been observed. Experimental and computational methods are two approaches for investigating these adverse aerodynamic phenomena. In this study, numerical investigations have been performed to study these phenomena, and to develop control strategies for alleviation of rotating stall and surge.; A three-dimensional unsteady Navier-Stokes analysis capable of modeling multistage turbomachinery components has been developed. This method uses a finite volume approach that is third order accurate in space, and first or second order in time. The scheme is implicit in time, permitting the use of large time steps. A one-equation Spalart-Allmaras model is used to model the effects of turbulence. The analysis is cast in a very general form so that a variety of configurations—centrifugal compressors and multistage compressors—may be analyzed with minor modifications to the analysis.; Calculations have been done both at design and off-design conditions for an axial compressor tested at NASA Glenn Research Center. At off-design conditions the calculations show that the tip leakage flow becomes strong, and its interaction with the tip shock leads to compressor rotating stall and modified surge. Both global variations to the mass flow rate, associated with surge, and azimuthal variations in flow conditions indicative of rotating stall, were observed.; It is demonstrated that these adverse phenomena may be eliminated, and stable operation restored, by the use of bleed valves located on the diffuser walls. Two types of controls were examined: open-loop and closed-loop. In the open-loop case mass is removed at a fixed, preset rate from the diffuser. In the closed-loop case, the rate of bleed is linked to pressure fluctuations upstream of the compressor face. The bleed valve is activated when the amplitude of pressure fluctuations sensed by the probes exceeds a certain range. Calculations show that both types of bleeding eliminate both rotating stall and modified surge, and suppress the precursor disturbances upstream of the compressor face. It is observed that smaller amounts of compressed air need to be removed with the closed-loop control, as compared to open-loop control. |
