Gas Turbine Compressor Stall

[koolraj09]

Hi all,
I just wanted to understand the gas turbine stall phenomenon. Have been going through a lot of materials lately, but none seems to give me an intuitive feel about what exactly happens when we say that the compressor stall. Hope to get an idea here. 
My question is fundamental....when we say that a compressor stalls..whats happening to the airflow...? Does if slow down or go fast or completely stops or does it continue as it is.... An analogy might be very helpful. Again I am unable to get an intuitive feel about it...I know the equations and airfoil stall and performance curve etc etc...

Any help is appreciated,
Thanks!

[gidge348]

Ok, this is my simplified description of a turbine stall.

Firstly all the turbines, compressors, stators etc in a turbine engine are basically small wings and act just like wings (air foils).

This is a video of a air foil working and stalling by increasing the angle of attack www.youtube.com/watch?v=6UlsArvbTeo

Notice how the air stops sticking to the upper side of the air foil, so like putting your hand out the window of a car. Hold it almost horizontal and you can feel lift. Hold it vertical and there is no lift, just a lot of drag.

In a turbine engine there a few movable "wings", but most are fixed, but the same thing (stall) can happen if we increase "or" decrease the speed of the air or increase "or" decrease the density (pressure) of the air from the design parameters.

All this separated turbulent air wont do any work (or very little) in an engine and so it is said to have stalled, same as an aircraft air foil.

This is over simplified others I am sure are a lot smarter than me may be able to explain it better.

Cheers Ian..

[finiteparts]

Ian's description is spot on, so all I will add here are a few clarifying elements.

The big picture of what you are trying to do in a compressor is that you transfer mechanical energy from the rotor into the fluid as an increase in flow velocity, thus it's kinetic energy is increased. Then, to get the pressure rise, you want to convert the flow kinetic energy into potential energy (or static pressure) by reducing it's flow speed in the diffuser.

Near the surface of the compressor blades or any flowpath component (diffuser wall, vane, combustor wall, etc) there exists a boundary layer were the fluid adjusts from a zero velocity at the surface to the free stream velocity at some distance away from the surface. Ideally, this boundary layer is thin because it is composed of fluid particles shearing against each other due to their differences in velocity. Fluid shear is a loss mechanism...it converts fluid kinetic energy into random molecular motion, which is also called heat. So, we want to design fluid passages where the flow is smooth and controlled so that as much of the fluids kinetic energy is flowing out of the impeller and can be recovered in the diffuser.

So now lets get to the "stall"...When the fluid is going through the compressor impeller, it must first be turned from the relative inlet flow angle to a radial direction, this is done in the inducer. The inducer metal angle is fixed to one angle, but changes in the rotational speed, compressor back pressure, inlet air density, etc change the inlet mass flow and thus the angle at which the flow meets the inducer blades. So at any other operating condition other than the design point, the inlet flow will approach the inducer blades at some angle, the incidence angle. Like a wing, if the incidence angle gets too large, the flow will separate flow the surface. It may reattach a little downstream, which is termed a separation or recirculation bubble. But, as the flow becomes more unstable, the detached flow will get larger and potentially stay separated through the whole impeller passage. Stalling is the same thing as saying separated flow. The flow near the surface is reversed and the streamlines within the passage are forced away from the surface by the recirculating flow.




Flow separation can happen anywhere in the engine, but it is most likely in components with diffusing flows, like the compressor system. There are many mechanisms of stalling in the compressor, but they all are based on flow separation. Flow separation at the leading edges of the diffuser vanes (in a vaned diffuser) can cause stall...flow separation in the impeller due to the cross passage pressure differential (suction side to pressure side) being to large can be a big one...



...and the list goes on. It can happen in a single vane passage or the whole impeller can stall...several passages can stall and migrate from passage to passage in a rotating stall... when the flow stalls, it creates a blockage that reduces the effective area through the compressor which can cause the incidence flow angle to change, reduce the mass flow through the compressor, etc...so you can see, there are a lot of mechanisms that can trip the flow and cause separation.

Generally, in a stall the flow is reduced and can sometimes go backwards, since the pressure downstream of the compressor is momentarily higher than what the compressor can push in...the magnitude of the flow effects depend on the magnitude on the amount of separation.

Here is a good video...

www.youtube.com/watch?v=MQWYhsYfMxE

I hope this helps!

~ Chris