Look at input flow rates. The compressor can only draw as fast as the smallest oriface in the inlet aprroaching the speed of sound. Stalling is a different issue.

For high speed flights the compressor is protected from the sound shock waves (to not appear on the compressor blades - with deadly consequences) by adjusting the intake area so as the speed of sound and the sound wave to happen in that area, away from the compressor blades. This regulates also the draw for different regimes of the compressor/engine.

@Jeigh S. Shelley

Can you please be more explicit. I figured out that stalling occurs due to flow separation but surging ??? the answer is a little elusive.

Let's start by being very careful with vocabulary. When you are talking with technicians in the feild, they will sometimes use the words "stalling", "surging", "starving" and others all interchangably to mean that the pump is not preforming properly. When you read the textbooks, these words have different very specific meanings. "stalling" is, as you said, flow separation on the low pressure side of the blade. Starving is trying to spin the compressor faster than the inlet conditions will allow. Both stall and starve are steady-state problems. "Surge" is a time dependant problem. A stall, or other flow anomally, starts in one location, then propgates to another location in the compressor. The time dependancy may be very rapid or slow. Intermittant poor performance results.

turbine

Thanks guys

Stall resembles the onset of surge as illustrated by many people including mine in my book :

Aircraft Propulsion and Gas Turbine Engines, CRC Press, Feb 2008 (Two-chapters allocated for centrifugal and axial compressors).

There are two types of stall; namely, positive stall and negative stall depending on the angle with which relative speed impact the rotor blade. From compressor map as the mass flow rate decreases, then for constant rotational speed, pressure ratio increases and compressor is subjected to positive stall and surge could start. Control systems for compressor is immediately activated and then either variable stator vanes or bleed system is used to decrease the incidence angle and avoid separation or subsequent surge. Decrease of mass flow rate is encountered in many cases of compressors used in aircraft like FOD,...etc

OK, now I am confused. We were taking about turbines and now we are discussing Compressors. The vocabulary s different in the different applications. Try some of these websites:

for car engines: http://forums.hybridz.org/index.php/topic/77587-what-is-compressor-surge-explanation-within/

for pipline applications: http://pipelineandgasjournal.com/compressor-surge-control-design-and-modeling-performance-verification

It is my understanding that compressor surge doesn't actally start in the compressor. It starts in the inducer due to starving or stall in the inducer, or pre-compressor components. Stall in the inducer, caused by flow separatation, results in inconsistent flow coming out of the inducer and into the compressor. It is this inconsistency, a time dependency of the flow rate which may be chaotic, that gets magnified in the compressor, and is called "compressor surge" even though it isn't actually caused by the parts in the compressor..

1- Turbines never surge or stal!!!!!!!!!!!l.

The reason is very simple, their geometry is convergent duct, thus velocity increases and pressure decreases.

Compressors have divergent duct geometry. Thus velocity decreases and pressure increases leading to a possible separation and surge.

2- Surge never happens to ducts whether upstream or downstream compressors.

3- Kindly please refer to fundaental books on Compressor design

Yes - exactly! you put it quite succintly!

Stalling and suuging are both a unstable operation condion in a axial compresor, stall occurs normally when airflow seperates from the convex sides of the compressor blades, this meams that the airflow and the speed of the rotor deos not match.if stall develops further, it may cause a compressor surge. When suging, the airflow through the compressor will reverse and comes out from the front of the compressor. Compressor surging can cause vibration and compressor blades damage.

And it occurs when engine is in high angle of attack. and when unsteady and unstable flow is in motion on first stage, stall will occur, and by developing this phenomenon to next stages it cuases surge on the compressor.

I would like to add that whereas stall is a local instability, caused by high incidence, surge is a system instability caused by the unstable operating curve (ie. rising pressure ratio as a function of mass flow) of the compressor at low mass flow ... which is precisely the consequence of the stall. Surge implies a general mass reversal, and is a function of the compressor operating curve and downstream "reservoirs" or plena.

Stall in compressors is usually not a stationary phenomenon, occcuring for all blades at the same time, but an unsteady phenomenon in which some blades will be stalled, and others not. This pattern then rotates around the rotor. This phenomenon is called rotating stall.

Hi everybody, to understand these two phenomena, we have to understand the airflow over an aerofoil/airfoil. An airfoil stalls when the air flow separates "completely" on the suction side (top side). On this side the air is initially accelerated from the leading edge (LE), to cause reducing pressure gradient. As the flow reaches the maximum thickness of the airfoil, it starts to slow down, causing pressure to increase, or a positive pressure gradient starts to occur. This pressure gradient is unstable as the airflow is still positive (LE towards Trailing Edge (TE)). This unstable conditions forces a flow separation - there exist reverse flow over the airfoil surface past separation point. If the angle of attack of the airfoil increases, this separation point will move forward towards the LE, making the flow separation ares bigger. It will come to a point that the whole flow over the airfoil separates, starting from the LE. This condition is called stall.

As for stall in compressors, a single blade or a small number of blades, may stall due to many reasons - mainly micro disturbance of flow going into the compressor. This is normally local phenomena. A stable rotating stall can occur on specific blade(s) (while the other blades on the rotor operates normally) which causes rotating "channel flow" through the engine creating havoc in it.

Stall can be detrimental to an engine - vibration (blade failure), vibration (whole engine), possible flame out.

A surge is a "rotor" phenomena. Or it is a "global" occurrence, of which all the blades in a rotor stall. When this happens, the flow through the rotor becomes choked (no overall flow), and sometimes reverses direction. This is also bad for an engine, as the engine would be starved of oxygen causing flame out. The choking can be unstable - the flow can restart, and re-choke continuously, creating very violent vibrations to the engine. I have heard of a rotor got its whole blades torn due to surge.

I call pilots and aero-engine designers to give us more insight of these two phenomena, for all of us to understand how severe their effects can be.

Although I discussed these in and aero-engine environment, these phenomena also occurs in stand alone compressors. And as such the consequences may be less worrying.

Thank you

Wan

Sirs,

I edit a book for a major publisher for «Compressor Surge Operation, Instrumentation and Control».

Can you share a novel paper of yours or your research team for the above publication?

Professor Dimitrios VENTZAS (SMISA), GREECE,

Professor D.E. Ventzas (SMISA)

TEI of Larissa

Dpt of Informatics and Telecommunications

Tel. +30 69xxxxxxxxxxx

skype = dimitrios.ventzas

[email protected] http://teilar.academia.edu/DimitriosVENTZAS

Surging in a centrifugal or axial compressor occures when it is deprived of a minimum flow rate required through it at a perticular pressure ratio for stable operation.

For example, if a compressor is rated to deliver 1000 Nm3/min with a pressure ratio of 5; then may be at around 600 Nm3/min flow rate (depending on design) at same pressure ratio, it may surge. This can happen when consumption of gas at discharge side is reduced below the required stable flow rate at same pressure ratio. At lower pressure ratio, stable flow rate requirement is also lower. On this basis only the surge curves for compressors are drawn by practically surging the compressor at different pressure ratios and flow rates. Automatic operation of anti-surge control valve is always set above the surge curve to prevent surging.

An excellent reference on this topic is Surge and rotating stall in axial flow compressors—Part I: Theoretical compression system model

EM Greitzer

Journal of Engineering for Gas Turbines and Power 98 (2), 190-198

Stall refers to local instability, indicating flow recirculation in the domain. Even during the BEP and Choke, there can be presence of Stall. On the other hand, Surge is the final step of the stall. When the stall cells propagate it becomes the surge. You can have a better understanding of stall if you have a look at the velocity diagram in the impeller. At lower mass flow rate the meridional velocity is low and since the rotating speed of the impeller is constant the incidence angle changes dramatically causing a change in velocity direction and small recirculation in the impeller blade passage. If this recirculation propagates, it will oppose the impeller rotation, which can lead to a reduction in impeller speed. This phenomenon is known as the Surge.