EXPERIMENTAL REDUCTION OF TRANSONIC FAN FORCED RESPONSE BY IGV FLOW CONTROL

S. Todd Bailie, Wing F. Ng,

Department of Mechanical Engineering Virginia Polytechnic Institute & State University Blacksburg, Virginia

William W. Copenhaver U. S. Air Force Research Lab Propulsion Directorate Wright-Patterson AFB, Ohio

Abstract The main contributor to the high-cycle fatigue of compressor blades is the re­sponse to aerodynamic forcing functions generated by an upstream row of sta­tors or inlet guide vanes. Resonant response to engine order excitation at certain rotor speeds is especially damaging. Studies have shown that fbw control by trailing edge blowing (TEB) can reduce stator wake strength and the amplitude of the downstream rotor blade vibrations generated by the unsteady stator-rotor interaction. In the present study, the effectiveness of TEB to reduce forced blade vibrations was evaluated in a modern single-stage transonic compressor rig. A row of wake generator (WG) vanes with TEB capability was installed upstream of the fan blisk, the blades of which were instrumented with strain gauges. Data was collected for varied TEB conditions over a range of rotor speed including one fundamental and multiple harmonic resonance crossings. Sensitivity of res­onant response amplitude to full-span TEB flowrate, as well as optimal TEB fbwrates, are documented for multiple modes. Resonant response sensitivity was generally characterized by a robust region of substantial attenuation, such that less-than-optimal TEB flowrates could prove to be an appropriate design tradeoff. For TEB fbwrates beyond the optimal region, attenuation decreased due to over-filling of the wake deficit. The fundamental crossing amplitude of the first torsion mode was reduced by as much as 85% with full-span TEB at 1.1% of the total rig fbw at that speed. Similar reductions were achieved for the various harmonic crossings, including as much as 94% reduction of the sec­ond leading edge bending mode resonant response using 0.74% of the rig flaw for full-span TEB. Thus the results demonstrate the effectiveness of the TEB technique for reducing rotor vibrations in the complex flaw environment of a modern, closely-spaced transonic stage row.

161

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 161-173. © 2006 Springer. Printed in the Netherlands.

1. Introduction

The occurrence of high cycle fatigue (HCF) failures in military turbine en­gines has increased dramatically in the past 25 years. HCF has been cited as the cause of 56% of USAF engine-related Class-A failures between 1982 and 1996. Additionally, HCF-related maintenance costs are estimated at more than 400 million dollars per year [1]. In response to this alarming trend, many recent research efforts have focused on understanding and mitigating HCF problems in turbine engines.

Compressor and turbine rotor blades are particularly prone to HCF damage, as they experience continuous forced excitation. This excitation is primarily the result of unsteady fbw interactions with neighboring blade rows [2, 3]. As rotor blades pass through the wakes shed by upstream stator vanes, they experience highly unsteady aerodynamic loading, inducing forced blade vibra­tions. Because potential flow interactions present an additional forcing func­tion, downstream vanes can also excite rotor blade vibration. Both of these aerodynamic excitations occur at the fundamental vane passing frequency (VPF) and its harmonics, thus corresponding to fixed engine orders.

Operation at a resonance crossing, where the rotor speed is such that an engine order excitation coincides with a blade natural frequency, can be espe­cially damaging. Because load cycles are rapidly accumulated at the high ro­tational speeds typical in turbomachinery, high-amplitude resonant vibrations can quickly lead to fatigue failure. Thus it is a standard, and often iterative, de­sign practice to try to locate blade modes such that resonance crossings do not occur near the intended steady operating speed(s) of the rotor. However, rotor blades will experience multiple resonance crossings during engine run-up and run-down [2]. Accumulation of damage is consequently inevitable and, in the event that this damage exceeds material limits, a failure will occur.

Accordingly, a specific objective of the National HCF Science & Technol­ogy Program has been to damp resonant stress by 60% in fans [1]. There are two basic approaches to alter a system’s forced response: either the system itself (that is, in this case, the fan blades) or the input to the system (the forc­ing functions) can be modified. While simply thickening the rotor blades can increase damage tolerance, the associated weight and dynamic load penalties are often unacceptable. Many research efforts have attempted to modify the system by adding damping, in various forms, to the rotor blades. While this approach has produced promising results, implementation without reducing the structural integrity of the rotor remains a challenge [4].

The forcing function can be modified in several ways. One such method, employed by von Flotow et al. [4], is to selectively impose an additional forc­ing function, out of phase with the original modal excitation, to produce a can­celing effect. This technique can be highly effective, but must be calibrated to target specific modes, and as such would generally be implemented only after a fatigue problem has been diagnosed. Also, to produce and control the addi­tional forcing function, engine system complexity and performance penalties are typically increased.

An alternate method, which can be implemented without a priori knowledge of the critical resonance crossings, is to attempt to reduce the amplitude of the dominant forcing function, which is the set of wakes shed by the upstream vane row. Waitz et al. [5] discussed the feasibility of various fbw control techniques for wake-reduction on curved airfoil shapes. The study concluded that the trailing edge blowing (TEB) technique should be more effective than boundary layer suction for reduction of viscous wakes. In this technique, air is ejected from the trailing edge of the wake source to reduce the mean velocity deficit and turbulent velocity flictuations in the wake region. Morris et al. [6] studied TEB effects in a linear cascade of stator vanes, and subsequently in a 17-inch (43.2 cm) scaled fan rig, which had a rotor instrumented with strain gauges. Rotor blade stress reductions of as much as 90% were reported with TEB massfbw for twenty vanes at 1.4% of the total rig fbw. However, shock interaction was not present, as the rotor was operated at subsonic conditions [7].

The present study applies the TEB technique in a modern compressor rig. The compressor is a highly loaded transonic design with close stage row spac – ings. Thus strong shock-wake interactions are present, which have been ob­served to substantially increase wake depth [8]. The effects of full – and part – span IGV trailing edge blowing on rotor blade forced response have been eval­uated. The present paper discusses findings from the full-span TEB experi­ments.