Instabilities In Axial Compressors Research Articles

Operating Conditions Effect on Aerodynamic Instabilities in Axial Compressors

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.

Active Control of Compressor Instability and Surge by Stator Blades Adjustment

Introduction T HE inceptionof compressorinstabilityin generaloccursduring an adverse operation condition that may cause an increase in compressor blade incidence angle and, thus, higher  ow de ection. Reducing the compressor mass  ow at a given rotational speed past the design surge limit exhibits one of the typical scenarios that can force the compressor into unstable operation. The decreased axial velocity component caused by the mass  ow reduction changes the incidenceangle leadingto an increased ow de ection that resultsin a pressureincrease.The increasedpressure,however,may cause the velocity proŽ le within the boundary layer to approach its separation point. In this case, the  uid particleinsidethe bladesurfaceboundary layer is, roughly speaking, exposed to three different forces. The convective force tries to move the particle in the direction of the motion, whereas two coacting forces, the shear stress force and the pressure force, oppose the motion of the particle.These three forces are generally in a dynamic equilibrium condition, which can be disrupted if additional pressure buildup is triggered by an adverse off-design operating point that leads to increased  ow de ection. This situationleads to a major boundary-layer ow separationalong the blade surface, hub, and casing, which indicates the inception of compressor instability. The phenomenon of the  ow separation (stall) and its controlhas been well known in external aerodynamics for more than half a century, where Prandtl, Schlichting,1 and Betz2 suggestedseveraldifferentcontrolscenarios.Stabilityenhancement using casing treatment was suggested by several researchers and summarized by Greitzer. The method discussed in this Note deals with the aerodynamic control and prevention of axial compressor instability and surge. It is based on dynamically adjusting the stator blades to the compressor interstage unsteady aerodynamics. Although it is concentrated on the aerodynamiccontroland preventionof stall, the method does not deal with control speciŽ c procedures and algorithms. However, it can be combined with any multivariable control system to provide an active feedback control to prevent inception of compressor instability during a dynamic operation.

Coupled compressor-diffuser flow instability

The effect on axial compressor instability of the coupling between the compressor and a downstream component (such as a diffuser) is studied analytically and experimentally. It is shown that the presence of an exit diffuser can have an adverse effect on the point of inception of instability, i.e., can move the stall point to a higher mass flow than occurs without the diffuser. Conversely, an exit nozzle, or a downstream compressor operating away from stall, can exert a stabilizing influence. The analytical results are found to be in agreement with experiments carried out on a three-stage axial compressor.