Gas Turbines - Compressors — Page 237, Lesson 311

Right, let's pick this up. We've just looked at what a compressor surge and stall actually are—the loud bang, the flame out, the whole drama. Now I want to walk you through the really important part: how we stop that from happening in the first place. You see, the fundamental problem is this. A compressor is designed to be at its most efficient right up near maximum rpm. That's what the design criteria aim for. But in real life, we have to throttle the engine back. We have to fly at part power. So we are inevitably going to operate the engine outside that optimum rpm and axial velocity range. And the moment we change the rotational speed of the compressor, we also change the axial velocity of the air flowing through it. And that combination—changing both of those together—is exactly what encourages the onset of stall and surge. So, because we're committed to doing this, the engine has to be fitted with methods to prevent it. Let me list the main ones for you, and then we'll dig into the first one in detail. The methods are: Variable Inlet Guide Vanes, or VIGVs; Variable Stator Vanes; Compressor Bleeds; Multi-spool Compressors; and Active Clearance Control. Now, let's focus on the Variable Inlet Guide Vanes. These are fitted to engines that have a particular problem with inherent compressor stall at low rpm, or during engine acceleration or deceleration. So they're not on every engine—only on ones that need that extra help. Here's the key construction detail. These vanes are fitted just in front of the first rotor stage. So they're the very first thing the air meets as it enters the compressor. And they can be automatically pivoted around their own axis. That's the whole point of them being "variable." What they do is vary the path of the airflow going into the compressor. And by doing that, they maintain the proper relationship between compressor rotational speed and airflow in the front compressor stages. That relationship is the thing that keeps the airflow at the right angle over the blades. Let me explain what happens at low compressor speeds, because that's when they really earn their keep. At low speeds, the VIGVs are angled to impart the greatest amount of swirl to the air. Now, "swirl" just means they spin the air around as it enters. Why do we want that? Because by swirling the air, we correct the relative airflow so that it arrives at the optimum angle of attack over the rotor blades. That optimum angle of attack is what prevents the flow from separating off the blades—which is what causes the stall in the first place. And critically, maintaining that optimum angle of attack allows a smooth and rapid engine acceleration. So the whole story of the VIGV is: sense that we're at low speed, pivot the vanes to add swirl, correct the airflow angle onto the rotor blades, and that gives you a clean, rapid acceleration without stalling. I've got a figure here that shows exactly how these vanes pivot as the compressor speed is reduced. Let me show you. That's the Variable Inlet Guide Vane. Next, we'll look at how the Variable Stator Vanes differ from these, and then the bleed valves.