Alex Stein

Computational Analysis of Stall and Separation Control in Centrifugal Compressors

A three-dimensional compressibleNavier ‐Stokes code has been used to model thesteady and unsteady e owe eld within a low-speed centrifugal compressor cone guration tested at NASA Lewis Research Center. Near-design conditions, the performance map, pressure e eld, and the velocity e eld were in good agreement with measured data. At off-design conditions, the calculations show that e ow reversal e rst occurs near the blade leading edge. If leftunchecked,thereversed-e owregiongrowsspatiallyandtemporally.Injectionofairupstreamofthecompressor face was found to modify the local e ow near the blade leading edge and to suppress rotating stall and surge. Even a moderate amount of air, typically around 5% of the total mass e ow rate, was sufe cient to extend the useful operating range of the compressor.

Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

A Navier-Stokes solver for simulating unsteady viscous e uid e ow in turbomachinery components has been developed and used to study e uid dynamic phenomena that lead to instabilities in centrifugal compressors. These studies indicate that large e ow incidence angles, at reduced e ow rates, can cause boundary-layer separation near the blade leading edge. High-pressure jets upstream of the compressor face are studied as a means of controlling compressor instabilities. Steady jets are found to alter the leading-edge e ow pattern and effectively suppress compressorinstabilities.Itisalsoobservedthatyawedjetsaremoreeffectivethanparalleljetsandthatanoptimum yaw angle exists for each compression system. Pulsed jets can yield additional performance enhancements and lead to a reduction in external air requirements for operating the jets. Jets pulsed at higher frequencies perform better than low-frequency jets. HE stable operating regime of current generation centrifugal compressors is limited at reduced mass e ow rates by two e uid dynamic phenomena: rotating stall and surge. The occurrence of these phenomena dee nes the surge limit, also referred to as stall line. When operating a compression system under these conditions, stronge uctuationsandlimitcycleoscillationscancauselargevibra- tions, leading to fatigue and damage of the entire compression unit. Unsteady e uctuations, caused by rotating stall or surge, may lead to excessive heating of the impeller blades and can produce additional periodicloadson theimpeller,resultinginincreasedoperating noise levels. If the e uctuations are left unchecked, even a complete e ow re- versal is possible, a situation that a compressor cannot tolerate. Because of the severity of these hazardous conditions, centrifugal compressors are conservatively designed to operate well below the peak pressure rise point. A safety margin of 10 -20% is generally introduced between the surge line and the design operating condi- tion. Most compressor control systems currently used in industry are based on this strategy, called avoidance control. Signie cant progress has been made in the past several years to develop reliable compressor control schemes. These schemes em- ploy fast-response-stall-detection devices, in most cases circumfer- entially distributed over thecasing walls thatmeasure e owe eld data and send e ltered signals to a controller unit. When a compressor approaches a stall-like condition, the presence of growing insta- bilities, sometimes called stall precursor waves, is measured and signaled back to the controller unit before the actual stall process begins. The control mechanism is then activated by a direct link be- tween the controller and a set of high-bandwidth actuation devices. The type of control system is often categorized by the choice of a particular actuator.

Numerical Studies of Stall and Surge Alleviation in a High-Speed Transonic Fan Rotor

The off-design performance characteristics of a transonic axial compressor called NASA Rotor 67 have been analyzed using a three-dimensional time accurate Navier-Stokes analysis. Rotating stall inception in this highspeed compressor has been studied. The effects of external disturbances such as inlet stagnation pressure distortion on rotating stall have also been simulated. The calculations show that at off-design conditions, reversed flow first forms in the tip clearance region. Its interaction with the shock over the blade leads to compressor stall, and at lower mass flow rates, to surge. It is demonstrated that stall can be eliminated, and stable operation restored, by the use of bleed valves located on the diffuser walls.

Recent progress in compressor stall and surge control

i ABSTRACT , A “review of recent research activities by the authors in thearea .of compressor stall and surge control is described. New ,multi-mode models for rotating stall and surge ha+e been developed, which extend the classical single mode Moore-Greitzer model to handle the nonlinear interaction between multiple azimuthal modes of instability. Linear controllers have been developed and tested, which open and close bleed ports by an amount proportional to the rotating &ll amplitude. Fuzzy logic controllers, which use heuristic laws rather thari.a reduced order model, are also described. The issue of robust control, i.e., controlling a system when the underlying opera@ characteristcs of the .._ system are not known, is briefly addressed Computational studies &red at ‘under&id@ the physics of compressor instabili&s ‘a& given Exper&mtal verification of the