Experimental Investigation of Flutter in Midstage Compressor Designs

Abstract

A comprehensive flutter test program was performed at subsonic/transonic Mach numbers in a nonrotating Annular Cascade rig. This rig was designed to investigate negative incidence choke flutter and positive incidence stall flutter. The objective of the program reported here was to establish choke flutter criteria for use in design of axial flow gas turbine airfoils which operate in the subsonic/transonic aerodynamic environment. To realize this objective, a systematic and controlled experiment was conducted which allowed the documentation of flutter boundaries over an extensive range of aerodynamic and aeromechanical parameters. The data presented are from an experimental investigation of blade flutter related to compressor midstage rotor blades with emphasis on negative incidence and choke flutter. Flutter characteristics were established for three midstage airfoil designs that provide a range of solidity and flexural reduced velocity. Effects of inlet test conditions on the flutter boundaries were evaluated for each of the five configurations tested. Results indicate that at Af/M max =0.5, a 25% increase in design reduced velocity produced a 13% decrease in flutter incidence margin and a 23% increase in solidity resulted in a 16% decrease in flutter incidence margin. Effect of increasing inlet pressure is destabilizing with less effect at high inlet pressures. Inlet temperature is also destabilizing; a 30% increase in temperature resulted in an 18% decrease in flutter incidence margin.b h M Nomenclature half-chord, in. airfoil lateral deflection, in. fl°w incidence normalized by the maximum flow incidence obtained in the investigation cascade inlet axial Mach number at inner flowpath relative Mach number normalized by the maximum relative Mach number obtained in the investigation cascade test rig inlet total pressure, psia cascade test rig inlet total temperature, °R first flexural mode reduced velocity inlet air velocity relative to cascade, fps cascade inlet flow angle relative to axial plane, deg cascade inlet flow angle relative to axial plane that causes cascade to choke, deg = cascade inlet total pressure, P 01 , divided by standard day pressure (14.7 psia or equivalent) = total temperature, T 01 , divided by standard day temperature (518.6°R or equivalent) = solidity = air foil twist deflections, rad = first flexural mode natural frequency, rad/s

Periodicity, Superposition, and 3D Effects in Supersonic Compressor Flutter Aerodynamics

“…Finally, an assessment of the importance of unsteady 3D effects is attempted using the 3D method. (1,2,3). Its analysis is a complicated aeromechanical problem (4,5,6).…”

Periodicity, Superposition, and 3D Effects in Supersonic Compressor Flutter Aerodynamics

“…Hence, the interblade phaseangle of a traveling-wave cannot be arbitrary. For an N-blade disk there are Ν possible interblade phase-angles, j3 r = 2ΤΓΓ/Ν ; r = 0,..., Ν -1 (2) and hence, Ν possible traveling-waves, for each blade mode. As shown by Lane /15/ the travelingwave assembly modes of all the blade modes form a linearly independent vector basis which spans the bladed disk degrees-of-freedom space.…”