Engine design speed [Archive]

Keith.Williams.

21st Mar 2004, 18:30

Olendirk,

Your question appears to indicate that you actually have a jet engine that stalls whenever it is runnning at anything other than its design speed. If this is the case then,it is a very sick engine and is in urgent need of attention.

If you are asking the more general question of why jet engines are prone to stall then I will attempt to explain.

The blades of a jet engine compressor are essentially just small wings. By moving through the air, they generate a lift force, which draws are into the engine and accelerates it rearwards.

Each stage of the compressor is made up of a row of rotating blades followed by a row or stationary blades. The spaces between adjacent blades form divergent ducts, which decelerate and compress the air. So the air is accelerated and compressed in the rotors, then decelerated and compressed in the stators. The overall results is a more-or-less constant airspeed with increasing pressure.

The angle of attack of each blade is determined by the relative magnitudes of the airflow due to rotation of the blades, plus the axial flow of the air through the comressor. If the angle of attack becomes too large, the blades will stall. If this occurs, the turbulent air flowing from the blades in one stage, is likely to cause stalling of the stages behind it. This reduces the effectiveness of the compression process, and in extreme cases results in a total breakdown of airflow through the engine.

Angles of attack are likely to be greatest when the axial flow of air through the engine is low, such as when starting or operating at very low RPM.

To prevent the stalling of the first row of rotor blades when operating at low RPM, many engines use variable angle inlet guide vanes or VIGVs. These are non-rotating blades immediately in front of the first stage rotor blades. By changing their angle they can control the angle at which the airflow approaches the first stage rotor blades. At low RPM they are angled to give the air a swirl, so that it always approaches the rotor blades at an acceptable angle of attack. In this position, the VIGVs also limit the amount of air that can be drawn into the engine.

As RPM increases there is less risk of stall and airflow must be increased to achieve maximum thrust. So the VIGVs gradually change their angle becoming more aligned with the airflow. This process increases the size of the spaces between the VIGVs, thereby increasing the mass of air flowing into the engine.

A second problem arrises from the fact that the air reduces in size as it is compressed. If for example the compressor multiplies the air pressure by factor of 10, the air shrinks to about 1/10 of its original volume. Unless the cross-sectional area of the compressor is gradually reduced from the front to the rear, this reduced volume air will slow down, causing the rear stages to stall.

So to allow for the reducing volume of the air, the diameter of jet engine compressors gradually reduces from the front to the rear. But the pressure rise achieved by the compressor is related to its RPM. At high RPM there will be a large increase in pressure and a large decrease in air volume. But at low RPM the pressure rise and volume reduction will be much less.

The cross-sectional areas of the compressor cannot be changed after it has been built, so the designers must select a shape that matches the rate at which the air will shrink at some fixed RPM. This might for example be at cruising RPM or at take-off RPM. But whenever the engine is running a lower RPM, the air will not be shrinking fast enough to match the reducing cross-section of the compressor. In effect the air becomes to big to continue flowing through the engine. Similarly, when running at a higher RPM the air is shrinking too fast, so it effectively become too small to fill the space at the rear of the compressor.

If the air becomes too small this will reduce the overall efficiency of the engine, but is unlikely to cause serious problems. But if the air becomes too big, it can cause the rear stages of the compressor to become choked. In extreme cases this will cause a breakdown of airflow, resulting in stalling of the blades of the rear stages. In the most severe cases this results in a total breakdown of aiflow, followed by the high pressure air surging forward from the rear to the front of the compressor. This surging process can cause a flame out and considerable damage to the engine.

This problem is greatest when the engine is starting, running at low RPM, or attempting to accelerate very rapidly. The principal solution involves fitting anti-surge bleed valves. These are essentially holes in the side of the compressor, which can be gradually closed as RPM increases. At low RPM they are open allowing any excess volumes of air to escape from the compressor. As RPM increases, the pressure rise increases causing the shrinkage rate of the air to better match the size of the compressor. So the bleed valves gradually close enabling all of the air to go through the compressor.

The design speed is the RPM at which the rate of volume change of the air matchse the rate of cross-sectional area change of the compressor. Although the risks of stall and surge increase as RPM moves above or below this optimum, such problems should not be evry-day occurences. In order to achieve certification, an engine must be able to demonstrate the ability to operate safely over a wide RPM range.