Operating Conditions Effect on Aerodynamic Instabilities in Axial Compressors

Abstract

This work aims at analyzing the flow behaviour through an axial compressor and predicting the instabilities appearance with the help of Computational Fluid Dynamics using the FLUENT software. The aerodynamic instabilities produced in axial flow compressors are one of the most important issues for compressor designers causing energy losses within the compressor. In this paper we focus on the effect of the operating conditions on the instabilities inception. In order to well demarcate the conditions favouring the flow instabilities the flow around compressor blades is investigated in different operating conditions characterized by the inlet Mach number the flow attack angle and the compressor rotation velocity. Three cases are considered a 2D, 3D isolated blade, and a 2D stage cascade. An attempt has been made to study the effect of the aerodynamic instabilities on various flow parameters. Particularly, four principle parameters, viz. the static pressure, the velocity, the lift and drag coefficients, are considered in this analysis. The obtained results show that the compressor instability margin is encountered between two range of inlet flow velocity, where operating at low inlet Mach number, especially with higher flow angles can involve the stall of the blades and operating at higher inlet Mach number, produce shock waves, in both situations the flow endure a separation and creation of vortices which leads to an aerodynamic instability reducing compressor efficiency characterized here by the lift and drag aerodynamic coefficients. The results also show that the compressor blade complex geometry is a factor controlling instabilities, where those instabilities differ from the root to the tip of the blade. It emerges that the aerodynamic simulation over a compressor stage can make appear within the flow new phenomena such as the rotating stall which does not appear while analysing the flow over an isolated blade.

Keywords: Instabilities, Axial Compressor, CFD, Subsonic Flow