Aeroelasticity at Reversed Flow Conditions—Part III: Reduction of Surge Loads by Means of Intentional Mistuning

Abstract

Compressor surge consists of four phases: (i) pressure rise, (ii) flow breakdown, (iii) blow-down, and (iv) flow recovery. During the blow-down phase reversed flow conditions exist, where a blade may accumulate hundreds of vibration cycles, depending on the surge volume and the vibration frequency. High vibration amplitudes and blade damages were observed in the past. In Part I (GT2011-45034) a compressor cascade was analyzed experimentally and analytically at steady reversed flow conditions. It has been shown that (i) the steady flow field can be predicted well by CFD analysis, (ii) the overall damping coefficient calculated by unsteady CFD compares reasonably well with measurements, and (iii) a blade may become unstable at certain reversed flow conditions. In Part II (GT2011-45035) the analytical procedures used in Part I were applied to the front part of a multistage HPC for reversed flow conditions. It was found that surge loads consist in reality of two physically different phenomena (i) the pressure wave during the flow breakdown leading to rather low blade stresses and (ii) flutter during the blow-down phase which may lead to very high blade stresses and damages during surge for some stages. As it is well known that intentional mistuning is a way to mitigate flutter, intentional mistuning is investigated in Part III of the paper at reversed flow conditions. At first, a CFD study of a single airfoil is presented showing the dependency of aerodynamic damping upon flow angle and pressure ratio over the airfoil at reversed flow conditions, including intentional mistuning studies. Secondly, an investigation is presented which shows experimentally and analytically that surge stresses can be reduced significantly by the use of intentional mistuning. In a multistage compressor test rig, one rotor stage, which experienced very high stresses during surge, was subjected to a cutback on every second blade, leading to significantly reduced surge stresses. Analytically, an aeroelastic eigenvalue analysis showed the same behavior.