You Must be Choking!

Sorry about that.  Some things are just naturally irresistible!

The point of it is that I want to talk about ‘stall’ and ‘surge’.

Why?

Because there seems to be some confusion in the ranks. There have been questions posed that would lead me to believe that these are two aspects of a compressor that are not fully understood.

Firstly, are we talking about axial flow compressors or centrifugal flow compressors?

Both, really, but we will come to the centrifugal flow compressors later since they are somewhat different in their ‘cause and effect’.

Shall we take out an axial flow compressor blade and take a peek at it?

Hold it so that you are looking down from the tip to the root.

What do you see?

Yes, several things:

1.     It has an aerofoil shape, the same as a wing.

2.     It is ‘twisted’.

If we take out two blades and put them together, by holding the base of each blade firmly abutting (touching) each other, then we see something else.

That the gap between the blades, in the direction of airflow, is divergent – it gets bigger as the gas leaves it.

These designs in the blades are there to do one thing – to ‘do work’ on the air.

The turbine supplies energy to the compressor and the compressor uses that energy to bunch up the air.

Now we have to be very careful.

The flow of air through the compressor has to be (almost) uniform.

Not in terms of velocity but in terms of mass flow.

If we have 500 kgs per second going in the front of the compressor then we absolutely must have 500 kgs per second coming out of the back into the combustion section.

That is vital.

The velocity of the gas is, more or less, irrelevant and will depend on the rpm of the compressor and not (as many seem to believe) the speed of the aeroplane through the air.

Note that I have just suggested that the rpm of the engine is, for most engines, a variable.

Note that the cross-section of the blade is an aerofoil.

Aerofoils do well at an optimum angle of attack of four degrees (4°). This will only happen at one rpm.

That means that there is a compromise somewhere.

Listen to a jet engine starting up.  At some point in the start sequence you will, very likely, hear a rasping noise from deep within it.

This is a ‘stall’ occurring. It will always do this. You will never get a nice airflow over all the blades all the time to create that lovely angle of attack.

There are going to be times when the airflow through the engine does not match the speed of the blades (rpm). Start-up is one of those times. Every time.

If the angle of attack of the air on a blade is bad – naughty air! It may produce turbulence down the back of the blade.

Turbulence means that the blade is no longer ‘doing work’ on the air. But the turbine is still sending energy down the shaft to the compressor (remember that!).

It is unlikely that only one blade will stall.

It is more likely that a whole stage (row) of blades will stall. It may be the ones in the middle, the little ones at the back or, even, those hulking great big ones at the front.

If the conditions that created that breakdown of airflow over that stage are restored to normal, then the blades will continue to work.

If conditions do not return to normal there is a danger that other stages will also stall.

Why?

Because the stage that has stalled is doing no work on the air, which means that the air being received by the next stage is not at the best condition for it.

Similarly, the air leaving the stage in front of the stalled row is not being accepted by the stalled stage – slowing it down.

Air that changes speed is not at the same angle of attack that it was previously. This is why we cannot suddenly change the rpm of the engine because it will rapidly change the angle of attack on the blades.

We now have an engine that is in a state of stall.

The compressor is still receiving lots of energy to do the work with but it is doing less work. More stages stalling will reduce the workload even more.

How do we know that this stall is occurring?

Because the exhaust temperature will be going up, the engine is likely to be vibrating (because the gas is turbulent) and it is very likely that you will hear it.

What to do?

Close the throttle and watch the instruments. If the engine slows to idle rpm and the temperature comes down then run the engine for three minutes at cooling rpm (not necessarily idle – the RR Viper engine cools at a slightly higher rpm than ground idle), and shut it down to investigate the cause.

It may be that the temperature continues to climb (burning fuel with less load on the compressor and reduced air) and the rpm stays where it is.  Now all you can do is shut-off the fuel and hope that the bearings don’t suffer too much.

This last is what is called a ‘locked in’ stall.

But what if all the compressor stages stall?

Then the compressor is doing no work on the air at all. The high pressure air at the back can now come to the low pressure at the front of the compressor. It will do this suddenly and, almost, explosively.

You will hear a loud bang. If you are outside the aeroplane it will feel as if you have been punched all over by a giant fist. If you are very close to the engine it may knock you over.

If you are inside the aeroplane at the controls, you may see the exhaust temperature and rpm begin to rise. The rpm will only go up for a moment but the temperature will keep going up. This is a really good time to shut the fuel supply off. Of course, it may just be a ‘pop’ surge where engine ‘clears its throat’, as it were. Things may go back to normal immediately but it is still worth investigation.

Apart from discovering the source of the problem, what are we looking for after a surge?

Check the oil (SOAP) and MCD’s (Magnetic Chip Detectors) as well as the oil pressure filter and scavenge strainers in case the bearings have been damaged.

(The SOAP sample results will have to be compared with the previous SOAP results to see if there is a sharp up swing in the contaminant level.

NB: SOAP = Spectrometric Oil Analysis Programme. This is done by a lab unless you work for a very big major airline that has it’s own kit!)

Do a borescope check – especially on the rear stages where possible.

Check the turbine for ‘spatter’ – it will look like the sky at night with little ‘stars’ of melted metal all over the turbine blades and, often, the exhaust.

Naturally, look down the intake. Several times we needed to go no further than a quick peek past the first stage on co-axial (twin spool or dual-axial) engines.

So what about centrifugal compressors?  Do they stall? Yes. They do.

Try and stay with this because it gets a bit tricky.

Between the rotating part of a centrifugal compressor and the static part there is a gap.  Not a big gap but it is there.

The reason for the gap is that the air coming off the impeller vanes is quite thick, it has been compressed, it is hot and it is moving quite fast.

As each impeller vane on the rotating bit goes past a diffuser vane on the static bit there will be a ‘thud’ as a lump of compressed gas gets caught between the two. The air behind the impeller vane is not quite so ‘thick’ so the sudden increase in viscosity is really noticed by the diffuser vanes.

(Listen, if you get the opportunity, to a RR Derwent engine on, say, a Gloster Meteor going overhead at low level; it sounds like a V16 piston engine because of all the ‘thuds’ as each impeller vane’s air hits each of the diffuser vanes.)

The gap has to be wide enough – remember the gap? For the air to ‘spread out’ a bit before going into the diffuser to lessen this ‘thud’ because too much ‘thud’ creates too much drag – wasted energy. Too small a gap = too big in the ‘thud’ department.

But!

As the aeroplane goes up into the clouds, and beyond, the air gets thinner.

At some point the air will be so thin that, even with the impeller working very hard to compress it, it will slip around the gap. If air is going around in circles it will block off the air trying to get out of the impeller.

The compressor will now stall.

It is unlikely to surge (but it can happen) but the conditions of the stall are exactly the same as for an axial flow compressor.

So what causes a centrifugal compressor to stall?

Low air density - altitude.

There you have it.

Surge and stall made easy. I hope.

Now my brain has stalled, my temperature is going up and work is ceasing.  Time for bed after a quick refuel!