Gas Turbines - Compressors — Page 232, Lesson 305

I want to walk you through the axial flow compressor now, because we've just been looking at the centrifugal type. Let me start with the key limitation we just saw on the centrifugal side: because of how the centrifugal compressor works, engine compression ratios greater than 15:1 are not considered possible with it. That's a hard ceiling for that design. And right at the elbows of the compressor outlet casing, cascade vanes are fitted. Those vanes let the air be turned through large angles with the minimum of loss, and they're also used to complete diffusion — that is, to finish converting the air's velocity into pressure. Now, the axial flow compressor. Its principle is basically the same as the centrifugal: it converts kinetic energy into pressure, which is potential energy. The difference is the means it uses to achieve that conversion. Let me build the picture from the hardware up. The axial flow compressor consists of several rows of rotating blades, called rotor blades, of aerofoil section. Interspersed with those are rows of stationary diffuser blades, called stator blades, also of aerofoil section. So you have alternating rows: rotor, stator, rotor, stator, all the way along the compressor. A stage consists of one row of rotor blades, fastened to discs on a rotor drum, followed by a row of stator blades, which are fastened to the compressor outer casing. So the rotors are mounted on the rotating drum, and the stators are fixed to the stationary casing around them. Now here's the geometry that does the work: on both the rotor and the stator, the spaces between the blades form divergent passages. Divergent means the passage widens out in the direction of flow. That widening is what slows the air down and raises its pressure. Let me trace what happens in each part. In the rotor, which is turned continuously at high speed by the turbine, mechanical energy is added. That mechanical energy is converted into both kinetic energy — that's velocity energy — and potential energy, which is pressure energy. So the rotor speeds the air up and starts compressing it. Then, within the stator, the pressure is increased by the conversion of the kinetic energy into pressure energy. The stator takes that high-velocity air, slows it down through its divergent passages, and the velocity energy becomes pressure. That's the diffusion process, the same job the diffuser did in the centrifugal compressor. So to tie it back to what you already know: simply stated, the rotor stages do the same job as the impeller in a centrifugal compressor, while the stator stages can be compared to the diffuser in a centrifugal compressor. That's the cleanest way to think about it. Now, the trade-off. The pressure rise across each stage is only quite small — the ratio is about 1.1 or 1.2 to 1. To give you a feel for that, in the first stage the pressure might only increase by about 3 psi. That's tiny. So as a consequence, to gain the compression ratios demanded by modern engines, many stages may be used on the same spool, and an engine may have up to three spools. Each spool is a separate rotating assembly, and you stack stages along each one to build up the total compression. And this method is so effective that in an engine like the RB 211, compression ratios as great as 35:1 can be attained. To put that in perspective, in that engine the pressure rise over the last stage alone can be as much as 80 psi. So even though each individual stage gives you only a small rise, stacking many stages gives you a total ratio far beyond what the centrifugal compressor could ever reach. Let me just make sure the picture is complete. The rotor blades are aerofoil section, the stator blades are aerofoil section, the passages between them are divergent, and the whole thing converts kinetic energy into pressure energy — same principle as the centrifugal, different mechanism. The rotor adds mechanical energy and converts it to velocity and pressure; the stator converts that velocity back into pressure. Small rise per stage, many stages, up to three spools, and you get ratios like 35:1 in the RB 211. That's the axial flow compressor in full.