November 18, 2016, ©. Leeham Co: In our series on modern turbofan airliner engines, we will now go deeper into the compressor part. Last week, we covered the fundamentals of compressors. As compressors and turbines use the same principles, we also covered the fundamental working principles of turbines.
We also described that compressors are temperamental parts, which can protest to wrong handling with violent “burps” (burst stalls with the combustion gases going out the front of the engine) or end up in a rotating stall where it simply stops working.
Turbines, on the other hand, are your robust companions. Aerodynamically they just work, albeit more or less efficiently dependent on what one asks them to do (mechanically it can be very different; we recently saw a turbine disintegrate with large consequences on an American Airlines Boeing 767 in Chicago). More on the turbines later.
In the GasTurb cross section of a two shaft turbofan in Figure 1, the engine has both an axial and a radial compressor. We will consider why engine designers combine these two for certain engine types.
Deeper dive into compressors
Compressors are divided into the two types which are common: axial compressors, which predominate by larger engines, and radial compressors, which are common for smaller engines.
The dominance of the axial compressor for larger engines is because it can conveniently be stacked stage by stage to achieve the pressure gain which is needed. Today’s design target for long range engines is 60:1 pressure ratio at Top of Climb (ToC) with minimum dimensions and weight. Each stage gains around 1.3:1 in pressure ratio for such engines.
Radial compressors gain more per stage, up to 6-8:1 per stage, but are difficult to cascade. The compressor accelerates the air along the impellers radial vanes, Figure 2. The air speed can be close to the speed of sound when it enters the diffusor to be slowed down and gain pressure.
The collector or Volute then ducts the gas in the axial direction again, often to a reverse flow combustion chamber as seen in Figure 1.
Figure 3 is a cut through of the Safran Silvercrest engine for the new Dassault and Cessna mid-size business jets. It’s a top, modern engine in the 11,500lbf thrust class. It contains all the compressor types which are used in modern turbofans. We will therefore use it to go through each type and their characteristics.
The engine has a titanium wide cord fan followed by a booster axial compressor. It’s called a booster because its sitting on the same shaft as the fan and therefore rotates at a relatively low speed. The pressure gain is therefore modest; it “boosts” the rest of the compressor chain.
Booster blades are most often mounted on a prolonged fan hub. The booster blades have tangential speeds which are the same as the inner part of the fan. As the pressure gain in a compressor stage is achieved by diffusing air which has been accelerated in the blade section and the booster blades have a low tangential speed (we can assume a 4,000 RPM shaft speed; a 0.3m blade tip radius giving a blade tip speed of M0.37 or 125m/s), booster stages have very low pressure gains, around 1.10 to 1.15 per stage. The booster on the Silvercrest is four stages and would probably deliver a pressure gain of around 1.6-1.8.
The next compressor is the four stage high pressure axial compressor with a 20,000 RPM shaft speed. The compressor radius and shaft RPM is optimized for a high axial and radial compressor pressure gain. We can assume the four stage axial unit to have a gain of around 2.5 and the radial unit of around 6. Total pressure gain of inner part of fan, booster, axial and radial high pressure stages is 38.5:1 at ToC (Top of Climb, the aerodynamically most demanding point for a turbofan, where the compressor spins the fastest).
Why finish with a radial stage?
As an axial compressor compresses the air, the density increases and the air takes less place. To keep the axial speed around M0.5, the compressor duct gets narrower and the blades shorter, Figure 4.
As blade dimension shrinks, the leak losses around the tips increases. Blade tip clearances don’t scale with dimensions. At one point, the dimensions get too small for an efficient axial stage. The radial compressor does not have dimensional constrains in the same way and is therefore suited as the last compressor stage for smaller engines.
The diffusion for a radial compressor takes place at a large radius (Figure 2). It requires quite a swan neck duct to get the air collected at the center for an additional radial stage. Cascaded radial compressors are therefore seldom seen. It’s normal for the collector of a radial stage to route the air axially at constant radius to the start of a reverse flow combustion chamber. This is also true for the Silvercrest.
Seals and compressor stability
The axial stages for the high pressure compressor for the Silvercrest are milled from solid billets of titanium, so-called Blisks (Bladed disks). Blisks avoid the air leak path around the blades fixing on the rotor, thereby gaining efficiency.
Other leakage paths in Figure 4 are from the downstream side of a stator vane around the vanes seal with the rotor drum and back to the area before the vane (red path). This forces every stage to have rotating seals between the static vanes and the rotating rotor drum. Rotating seals that seal well for the 10,000 to 20,000 missions that modern airliners fly before engine overhaul is challenging technology.
Another tricky area is the compressors function at different RPMs. At close to the design point, i.e., at ToC, cruise and takeoff, the flow through the compressor is as planned. When reducing the thrust, the compressor spins slower and the axial speed of the flow reduces. The flow axial speed in the rear stages is kept up by the air not being so compressed; it takes more space and the speed in the narrow duct increases.
At the front of the compressor, the lower RPM slows the axial speed of the air and there is no significant change in compression. As described in the last Corner, the lower axial air speed means higher angles of attack for the front rotor blades and these approach stall.
There are three ways to counter this:
In practical engines, all these methods are used in unison to make the fickle compressors behave over a wide range of RPMs and therefore thrust settings.
The front of the compressor sees temperatures of the air flow after the fan. The temperature after the fan is around 25°C hotter than the intake temperature. As the stages pass, the air gets hotter until it can no longer be handled by the preferred material for Blisks, titanium. This means that the last stages in the compressor must be made of temperature-resistant Inconel or other super alloys.
For today’s high pressure engines, the temperature at the last compressor stages is on the limit for what can be handled with the materials available today. Just like the entry temperature to the turbine can be a limiter for engine performance, the so-called T3 temperature, Figure 1, can also limit the engines performance. T3 is approaching 700°C for the most modern long range engines at takeoff thrust on a hot summer day.
Unlike for combustor/turbines, there is no way to cool the compressor by routing cooling air into the blades, because there is no air available with lower temperature at the high pressure required. It’s possible to cool the compressor air via a pre-cooler heat exchanger protruding into the cold fan stream. This is looked at for the next generation engines.