Simulations of Stall and Stall Control in Turbocompressors
Simulations were found to agree with experimental observations.
Anumerical-simulation study of stall and stall control in radial and axial compressor stages of gas turbine engines has been performed. This and other similar studies are needed because even though the adverse consequences of stall are well known and rudimentary stall-warning and stall-control systems are in use, the scientific basis for predicting and mitigating stall is not yet established.
Gas turbine engines are the prime movers in many military ground vehicles, ships, and aircraft. The stable-operation envelope of a gas turbine engine is dictated in large part by the aerodynamic stability (tolerance to stall) of the engine compression system. Stall-detection and stall-control schemes involve reliance on measurement of smallamplitude stall-precursive disturbances. The uncontrolled growth of these disturbances leads to stall. In these schemes, the outputs of transducers that measure these disturbances are fed as inputs to stall-warning and stall-control systems.
Stall-control systems developed by collaboration between researchers at the Army Research Laboratory and the NASA Glenn Research Center have been demonstrated to extend the stable operating ranges of both high-speed axial and centrifugal compressors. These systems utilize local discrete injection of fluid at high relative total pressure to energize regions of low-momentum fluid, which are believed to play a critical role in destabilizing compressors. In an axial compressor, a system of this type injects the high-relative-total-pressure fluid at discrete locations around the annular flow cross section just upstream of the leadingedge tips of rotor blades to energize the low-momentum flow associated with a rotor-blade-tip leakage vortex.
At present, the causal link between the small stall-precursive disturbances and the fluid-mechanical processes occurring within compressor blade rows immediately prior to stall is not understood. The fluid-mechanical principles governing how stall-control extends the stable-operation range are also not understood. The instant study is part of a continuing effort to understand both the physical causes of stall and the physical basis for mitigation of stall by stallcontrol systems. Such understanding is essential for designing improved stallcontrol systems.
In this study, flows in turbocompressors were simulated by a use of the TURBO code, which was previously developed expressly for use in simulating the complex, three-dimensional, time-dependent flows in multistage turbomachines. TURBO solves the Reynolds-averaged Navier-Stokes equations of flow of a viscous, compressible fluid numerically by use of a finite-volume, implicit scheme. In this study, flows with and without stall control were simulated for a high-speed single-stage axial compressor and a high-speed centrifugal compressor stage for which experimental data were available.
The results of the simulations were found to closely approximate the corresponding experimental data. The results of the simulations for the axial compressor were interpreted as evidence of the capability of flow-simulating computer codes for predicting the onset of flow instabilities and their subsequent growth into fully developed rotating stalls. The results were interpreted as clearly showing multiple stall cells traveling at about 84 percent of the rotor speed and then coalescing into a larger single stall cell traveling at about 43 percent of the rotor speed. The figure shows the massflow disturbance associated with such a stall event. This simulated stall-cell behavior was found to be consistent with what has been observed experimentally.
The axial-compressor simulations also were found to predict the stall-suppressing effect of injecting high-relative-total pressure fluid upstream of the leadingedge tips of rotor blades to energize the fluid upstream of the leading-edge tips of the rotor blades at discrete locations around the annulus. More specifically, the results were interpreted as showing that this stall-control technique appears to suppress the growth of the instabilities, thereby enhancing compressor stability.
Similarly, the simulations for the centrifugal compressor stage were found to indicate the development of stalling passages with reverse flow regions, in agreement with experimental observations. The stability-enhancing effect of reverse-tangent injection (in which injected flow has velocity components both tangential to the impeller discharge flow and opposing the impeller rotation) at discrete locations around the annulus in the diffuser vaneless space was also found to be clearly predicted by the simulations, in agreement with the experiment.
This work was done by Michael D. Hathaway, Gregory P. Herrick, and Gary J. Skoch of the Army Research Laboratory, Jenping Chen of Mississippi State University, and Roberts S. Webster of the University of Tennessee. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Mechanics/Machinery category. ARL-0008
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