Gas Turbines - Compressors — Page 237, Lesson 312

All right, let's pick up where we left off with the compressor. We've already dealt with the first rotor stage, but the airflow can still run into trouble further down the compressor when the engine isn't running at its optimum design speed. To fix this, some engines are fitted with variable stator vanes. Here's the key idea. These vanes can be pivoted automatically. As the compressor speed is reduced from its optimum design value, the vanes are progressively closed. Why? To maintain an acceptable angle of attack onto the following rotor blades. Think of it this way: when the rotor slows down, the air coming at the next set of blades changes direction. If you don't adjust the stator vane angle, the air hits the blade at a bad angle, which can cause the flow to break down. By closing the vanes, you keep that airflow hitting the blades at the right angle, preventing stall further down the compressor. That's the variable stator vane system in a nutshell. Now, let's move on to the second problem: compressor bleeds. I want you to follow this chain of events carefully, because it's a cause-and-effect sequence. When the engine slows down, its compression ratio decreases. That means the air isn't being squeezed as hard. Because of that, the volume of air in the rear of the compressor becomes greater. Now, that excess volume causes choking in the rear of the compressor. Choking is when the flow can't get through fast enough, so it backs up. This choking causes a decrease in the mass flow — that's the actual weight of air moving through the engine per second. And here's the knock-on effect: the decrease in mass flow causes a decrease in the velocity of the air in the front of the compressor. Slow air at the front increases the tendency to stall. So you have choking at the back, slow air at the front, and a stall risk building up. The solution is a compressor bleed valve. This valve is introduced into the intermediate stages of the compressor — that's the middle section, between the front and the rear. At low rpm, or during engine acceleration, the valve can be opened to allow some of that excess volume of air to escape. It literally bleeds air off. What does that do? It has two effects. First, it increases the velocity of the air in the earlier stages of the compressor — because there's less air backed up, the flow speeds up again. Second, it reduces the choking effects in the rear of the compressor — because you've given the excess air somewhere to go. This combination ensures that compressor stall is less likely to occur while the bleeds are open. But — and this is important — there are disadvantages. Opening compressor bleeds, whether they're being used as stall preventive measures or to supply air for aircraft services, decreases the mass flow through the engine. That's the trade-off. You're deliberately dumping air overboard to protect the compressor, but that air is no longer going through the engine to produce thrust. So you lose performance while the bleeds are open. Let me just make sure you've got the full picture. Variable stator vanes handle the airflow angle problem by pivoting to maintain the correct angle of attack. Compressor bleeds handle the volume problem by venting excess air from the intermediate stages at low rpm or during acceleration. Both are anti-stall measures, but the bleed system costs you mass flow. That's the price of keeping the compressor stable.