The Use of a Circumferentially Nonuniform Stator to Attenuate Harmful Aerodynamic and Mechanical Interactions in an Advanced Mixed Flow Splittered Rotor/Tandem Variable Stator LP Compressor

Abstract

A potential flow computer model that can handle blade row interaction problems has been used to analyze the circumferential static pressure distribution at the trailing edge plane of an advanced mixed flow splittered rotor low pressure compressor produced by a downstream tandem stator/strut system. The computer model is based on the Douglas-Neumann formulation and features a powerful automated optimization feature which can define a restagger pattern that will either minimize stator blade-to-blade loading differences or minimize the circumferential static pressure nonuniformity on a preselected axial plane. The latter approach was used presently to design a circumferentially nonuniform stagger angle distribution for the second row of the tandem stator that reduced the circumferential static pressure variation, and consequently its harmonic excitation, on the upstream rotor. Comparison between the predicted static pressure distribution for the baseline case (with uniform stagger angles) and the optimized design showed a dramatic reduction in the circumferential pressure variation. Fourier analyses of the pressure distributions confirmed a substantial decrease in the magnitude of the harmonic index thought to be responsible for the observed severe rotor vibration. The analytical results were confirmed by back-to-back engine tests of the baseline and restaggered tandem stators, where light probe measurements of splitter blade synchronous response showed a proportional decrease in rotor response for the optimized stator configuration. The restagger was implemented quickly and easily using simple modifications to the stator unison ring. With the high splitter blade deflections reduced to an acceptable level, the engine was able to safely accelerate through the former prohibited speed range and continue with the planned test sequence. The restaggered stator design and test effort, from initial problem statement through a successful demonstration test in the gas generator, was accomplished in less than 6 weeks.