Measurement of Flow Pattern Within a Rotating Stall Cell in an Axial Compressor

Effective active control of rotating stall in axial compressors requires detailed understanding of flow instabilities associated with this compressor regime. Newly designed miniature high frequency response total and static pressure probes as well as commercial thermoanemometric probes are suitable tools for this task. However, during the rotating stall cycle the probes are subjected to flow direction changes that are far larger than the range of probe incidence acceptance, and therefore probe data without a proper correction would misrepresent unsteady variations of flow parameters. A methodology, based on ensemble averaging, is proposed to circumvent this problem. In this approach the ensemble averaged signals acquired for various probe setting angles are segmented, and only the sections for probe setting angles close to the actual flow angle are used for signal recombination. The methodology was verified by excellent agreement between velocity distributions obtained from pressure probe data, and data measured with thermoanemometric probes. Vector plots of unsteady flow behavior during the rotating stall regime indicate reversed flow within the rotating stall cell that spreads over to adjacent rotor blade channels. Results of this study confirmed that the NASA Low Speed Axial Compressor (LSAC) while in a rotating stall regime at rotor design speed exhibits one stall cell that rotates at a speed equal to 50.6 percent of the rotor shaft speed

Tip-Clearance Actuation With Magnetic Bearings for High-Speed Compressor Stall Control

Magnetic bearings are widely used as active suspension devices in rotating machinery, mainly for active vibration control purposes. The concept of active tip clearance control suggests a new application of magnetic bearings as servo-actuators to stabilize rotating stall in axial compressors. This paper presents a first-of-a-kind feasibility study of an active stall control experiment with a magnetic bearing servo-actuator in the NASA Glenn high-speed single-stage compressor test facility. Together with CFD and experimental data a two-dimensional, incompressible compressor stability model was used in a stochastic estimation and control analysis to determine the required magnetic bearing performance for compressor stall control. The resulting requirements introduced new challenges to the magnetic bearing actuator design. A magnetic bearing servo-actuator was designed which fulfilled the performance specifications. Control laws were then developed to stabilize the compressor shaft. In a second control loop, a constant gain controller was implemented to stabilize rotating stall. A detailed closed loop simulation at 100% corrected design speed resulted in a 2.3% reduction of stalling mass flow which is comparable to results obtained in the same compressor by Weigl et al. (1998) using unsteady air injection. The design and simulation results presented here establish the viability of magnetic bearings for stall control in aero-engine high-speed compressors. Furthermore the paper outlines a general design procedure to develop magnetic bearing servo-actuators for high-speed turbomachinery.United States. National Aeronautics and Space Administration (Grant NAG3-1457

Robustness of rotating stall control for axial flow compressors.

The useful range of operation in axial flow compressors is limited by aerodynamic flow instabilities such as rotating stall. Feedback control has been pursued to address the rotating stall problem in axial flow compressors in order to extend the stable operating range and to improve engine performance. These controllers guarantee the stability of the bifurcated operating solution near the stall point. However, how robust these controllers could be is still not clear. In this thesis, after presenting an introduction to the problem and providing the mathematical background of bifurcated systems and bifurcation stabilization, local robust analysis is applied systematically to evaluate feedback control design for rotating stall. Different combinations of perturbation in the system model have been considered. In particular, the set of admissible uncertainty is characterized analytically in terms of feedback control gain so that it is possible to compare the robustness of controllers with different feedback gains. We are then able to use this knowledge as a selection criteria to choose a more robust controller from a set of existing control laws. The nonlinear feedback controller is also studied for any possible advantage over the linear control laws.Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2002 .T34. Source: Masters Abstracts International, Volume: 41-04, page: 1160. Adviser: Xiang Chen. Thesis (M.A.Sc.)--University of Windsor (Canada), 2002

Dynamic control of rotating stall in axial flow compressors using aeromechanical feedback

August 1993Includes bibliographical references (pages 250-252)Dynamic control of rotating stall in an axial flow compressor has been implemented using aeromechanical feedback. The control strategy developed used an array of wall jets upstream of a single stage compressor which were regulated by locally reacting reed valves. These reed valves responded to pressure perturbations in the flow that were associated with small amplitude perturbations that precede rotating stall. The control strategy was designed such that the combined system of compressor plus the reed valve controller was stable in previously unstable operating conditions. A 10% decrease in the stalling flow coefficient was achieved using this dynamic feedback control strategy, and the stable flow range was extended with no noticeable change in the steady state performance of the compression system.The experimental demonstration is the first use of aeromechanical feedback to extend the stable operating range of an axial flow compressor, as well as the first use of locally reacting feedback and dynamic compensation techniques to stabilize rotating stall in an axial flow compressor. The design of the experiment was based on a two-dimensional model of the rotating stall dynamics which incorporated the effect of aeromechanical feedback. The physical mechanism responsible for rotating stall in axial flow compressors was examined with focus on the role of dynamic feedback in stabilizing compression system instability. The effectiveness of the aeromechanical control strategy was predicted, and experimentally demonstrated, to be a function of a set of non-dimensional control parameters that determine the interaction of the control strategy and the rotating stall dynamics.Predictions based on linear stability analyses and non-linear numerical simulations agreed qualitatively with the steady state and time resolved experimental data. During the experimental investigations, large amplitude, one-dimensional acoustic oscillations were observed in the compression system with aeromechanical feedback stabilization. Based on these observations, the role of the compression system parameters in the acoustic oscillations was examined analytically and a method was developed to reduce these oscillations. The mechanism responsible for the generation of self-excited acoustic oscillations, and the implications for dynamic control of compression system instabilities was also examined

A survey of instabilities within centrifugal pumps and concepts for improving the flow range of pumps in rocket engines

Design features and concepts that have primary influence on the stable operating flow range of propellant-feed centrifugal turbopumps in a rocket engine are discussed. One of the throttling limitations of a pump-fed rocket engine is the stable operating range of the pump. Several varieties of pump hydraulic instabilities are mentioned. Some pump design criteria are summarized and a qualitative correlation of key parameters to pump stall and surge are referenced. Some of the design criteria were taken from the literature on high pressure ratio centrifugal compressors. Therefore, these have yet to be validated for extending the stable operating flow range of high-head pumps. Casing treatment devices, dynamic fluid-damping plenums, backflow-stabilizing vanes and flow-reinjection techniques are summarized. A planned program was undertaken at LeRC to validate these concepts. Technologies developed by this program will be available for the design of turbopumps for advanced space rocket engines for use by NASA in future space missions where throttling is essential

Fluid machines: Expanding the limits, past and future

During the 40 yr period from 1940 to 1980, the capabilities and operating limits of fluid machines were greatly extended. This was due to a research program, carried out to meet the needs of aerospace programs. Some of the events are reviewed. Overall advancements of all machinery components are discussed followed by a detailed examination of technology advancements in axial compressors and pumps. Future technology needs are suggested

Vibration exciting mechanisms induced by flow in turbomachine stages

The quasisteady computer analysis of the perturbated centrifugal impeller passage flow was reviewed. A total of 115 stage calculations were used to define the fluid damping coefficient, delta sub fluid. Results indicate that the average total damping coefficient per stage needed for stability is delta sub total 1.85

A theory of post-stall transients in multistage axial compression systems

A theory is presented for post stall transients in multistage axial compressors. The theory leads to a set of coupled first-order ordinary differential equations capable of describing the growth and possible decay of a rotating-stall cell during a compressor mass-flow transient. These changing flow features are shown to have a significant effect on the instantaneous compressor pumping characteristic during unsteady operation, and henace on the overall system behavior. It is also found from the theory that the ultimate mode of system response, stable rotating stall or surge, depends not only on the B parameter but also on other parameters, such as the compressor length-to-diameter ratio. Small values of this latter quantity tend to favor the occurrence of surge, as do large values of B. A limited parametric study is carried out to show the impact of the different system features on transient behavior. Based on analytical and numerical results, several specific topics are suggested for future research on post-stall transients

End wall flow characteristics and overall performance of an axial flow compressor stage

This review indicates the possible future directions for research on endwall flows in axial flow compressors. Theoretical investigations on the rotor blade endwall flows in axial flow compressors reported here include the secondary flow calculation and the development of the momentum integral equations for the prediction of the annulus wall boundary layer. The equations for secondary vorticity at the rotor exit are solved analytically. The solution includes the effects of rotation and the viscosity. The momentum integral equations derived include the effect of the blade boundary layers. The axial flow compressor facility of the Department of Aerospace Engineering at The Pennsylvania State University, which is used for the experimental investigations of the endwall flows, is described in some detail. The overall performance and other preliminary experimental results are presented. Extensive radial flow surveys are carried out at the design and various off design conditions. These are presented and interpreted in this report. The following experimental investigations of the blade endwall flows are carried out. (1) Rotor blade endwall flows: The following measurements are carried out at four flow coefficients. (a) The rotor blade static pressures at various axial and radial stations (with special emphasis near the blade tips). (b) The hub wall static pressures inside the rotor blade passage at various axial and tangential stations. (2) IGV endwall flows: The following measurements are carried out at the design flow coefficient. (a) The boundary layer profiles at various axial and tangential stations inside the blade passage and at the blade exit. (b) Casing static pressures and limiting streamline angles inside the blade passage