Gas Turbines - Compressors — Page 237, Lesson 314

I want to walk you through the next stage of compressor design — the part where the engine protects itself from stall and surge. We've already covered the basic construction, so now we're looking at two things: how the engine actively controls the airflow, and how the designer maps out a safe operating region. Let's start with active clearance control. This is a later development, and its whole purpose is to control the airflow through the engine. The fundamental problem behind every case of stall is that the angle of attack of the airflow over the blade is no longer at its optimum value. Now, that angle of attack can be disturbed by changes in either of two things: the axial velocity of the airflow over the blades, or their rotational speed. If we can control the axial velocity over the whole of the engine speed range, then the chances of stall or surge happening are diminished. So how do we control that axial velocity? One method is to vary the size of the air annulus at the high pressure end of the compressor. The air annulus is the ring-shaped passage the air flows through between the rotor and the casing. And here's the clever part — varying that annulus size was considered technically impossible not too long ago. The way we achieve it is by cooling the compressor casing. When you cool the casing, it shrinks, and that shrinking reduces the clearance between the casing and the blade tips. The cooling medium most often used at present is air, which is introduced into tubing running through the exterior of the compressor casing. Now let's move to the compressor surge envelope. Compressor stall and surge are caused by an imbalance between the flow of air through the compressor and the pressure ratio. The designer has to ensure the relationship between pressure rise and rpm follows a path known as the working line, or design line. Built-in airflow control devices, such as bleed valves, allow a safety margin between the working line and the surge line. So the surge line is the boundary beyond which the compressor will stall, and the working line is where the engine actually operates — the bleed valves keep those two lines apart. Finally, let's look at the construction. The rotor shaft is supported in bearings and is coupled to the turbine shaft, so that minor variations in alignment are allowed for. The centrifugal load imposed on the compressor dictates that the rotor blades are fixed to a disc, which itself is fitted around the rotor shaft. So the blades don't mount directly on the shaft — they mount on a disc, and that disc fits around the shaft, because the centrifugal load at speed is enormous and needs that solid disc to carry it. Let me show you the bleed valve operation and the variable vanes, because they tie directly into this surge envelope idea. So to pull it together: active clearance control cools the casing to shrink the annulus and control axial velocity, and the surge envelope shows us the working line versus the surge line, with bleed valves providing the safety margin. That's the complete picture of how the compressor manages airflow and protects itself.