Aerodynamic and Mechanical Vibration Analysis of a Compressor Blisk at Surge

Abstract

Typically surge events in compressors are investigated only from the aerodynamic point of view (aerodynamic instability or lack of surge margin). For this assessment various methods exist during the design phase. For the analysis of the structural impact of surge in this phase the situation is more challenging, because an analytical prediction of the blade loading during surge is difficult to obtain. In this paper a combined analysis of aerodynamic and structural aspects of surge in compressor rotor blades of advanced axial flow compressors with state-of-the-art numerical procedures is presented and compared to extensive strain-gauge measurements. The unsteady aerodynamic excitation of a compressor rotor blade during surge is determined with a numerical procedure which allows calculating the unsteady flow field in a compression system. Blade rows of the compressor are simulated by appropriate loss/deviation characteristics covering both the normal and the unstable operating regime of the compressor, including reversed flow conditions. Elements adjacent to the compressor such as inlet pipes, exit volumes and throttles are modeled as required to include their impact on the systems dynamic behavior. The unsteady flow field within the system is determined by solving the unsteady conservation laws for compressible, inviscid flow. Blade forces are determined from the change of aerodynamic momentum across a blade row. The resulting forces in axial and circumferential direction are used as an input for a direct transient Finite-Element stress analysis. The resulting forces are applied to the blades. The results of the Finite-Element analysis are compared with experimental results from a compressor test rig. Stress is measured by strain-gauges in various positions on the blades. In addition, transient pressure is recorded. Measurements are taken during normal operation of the compressor as well as during surge. It is shown that the procedure is able to predict the vibration stress level of the blades satisfactorily.