Surge/Compressor Stall
 

Sorry if this is a dull Planky question but:

Could someone give me a relatively simple answer about the differences if any between Surge and compressor stall. Also how does it affect/effect the engine with an axial flow or centrifugal compressor power unit?

Thanks for your time!:confused:

My interpretation as follows;-

Engine surge in relation to a jet/turbine is a fairly non technical word that can describe any type of undemanded, usually intermittent, power increase/decrease, possibly due to a faulty fuel control unit not regulating the fuel flow correctly. Occasionally, the word surge may be used in the event of a compressor stall. In my view this is an incorrect use of the word and may be misleading to an engineer trying to fix an engine problem

Compressor stall is when the flow of air through the compressor becomes unstable, the results of which will vary depending on the engine type and the severity of the stall. Typical symptoms can include a popping noise from the compressor or maybe a roaring noise. This can be accompanied by rising combustion chamber temperature and reduction in power. In severe cases, the engine can flame out or overtemp. Most modern engines are fairly reliable and will have devices fitted to minimise the chance of compressor stall. The typical helicopter engine consists of a multi stage axial compressor pumping air into a centrifugal compressor prior to the combustion chamber. This configuration does have problems as usually the axial compressor produces too much air for the centrifugal compressor at the lower rpm ranges. Manufacturers will usually fit a bleed valve between the two compressors which will be open at lower rpms to dump excess air. As the engine rpm increases, the valve will close allowing the total air flow into the centrifugal compressor.

Other causes of compressor stall are damaged or dirty compressor blades, recirculation of hot exhaust gases being sucked into the compressor when hovering downwind, FOD, incorrectly set up fuel control units, poor intake design etc, etc.

I'm sure there are very good written references which will describe surge and stall in much greater technical detail than I can but I hope this gives you some help.

I'll have a stab at it............

To some airframe manufacturers a stall constitutes a 'sudden stoppage' occurance with the accompanying (expensive) inspections and component overhauls.

Faulty bleed valves, over-fuelling and mis-matched airflows between compressors and turbines are common causes. The 500ER had a bad rap in some quarters from stall/surge characteristics of the engine. A PNG operator mandated that only matched modules from the test rig were swapped as a pair after many years of heartache. I am sure there are others here that have experienced this.

But having just re-read your query, my understanding is that there is very little difference between the two terms, I suspect that if you wanted to split hairs a stall is more to do bleed valve problems whilst the surge is 'flow reversal' for want of a term.

I stand to be corrected........

:)

Roundwego, I think you nailed it

No difference for engine manufacturers between the two terms, the boundary of operating conditions that define where compressor stall occurs is called the "surge line" by all engine manufacturers I have dealt with.

As is often the case, we have several terms for one thing, then define the percieved difference, then get wrapped up.

The compressor stalls or surges when the local angle of attack exceeds the critical value, and believe it or not, the conditions look an awful lot like rotor stall on the main rotor.

The easiest way to get stall/surge is to accelerate the engine too fast, so the N1 speeds up faster than the airflow can adjust. Such acceleration stalls are solved by limiting the N1 acceleration of the engine, and also by relieving the compressor pressure by dumping the compressor discharge air (bleed valves and belly bands).

Other ways to get engines to stall is to disrupt the aerodynamics of the blades (erosion, damage, dirt, ice) or to distort the inflow to the engine (hard to do at helo speeds, but not impossible).

Stalls hurt the engine in several ways: 1) the compressor blades get badly stressed when they see wild angles of attack, and odd make-breaks of the flow, so fatigue damage or even static blade failure are possible 2) the engine temperature in the hot section can get sky high since fuel is flowing but air is stagnating and 3) the cooling air that is passed into the combustor and even the hollow blades is disrupted, so these areas can get hot spots and perhaps even burn through.