By Bjorn Fehrm
December 02, 2016, ©. Leeham Co: We will now look at the combustor area in our series on modern turbofan engines. There is a lot of activity in this area, as it sets the level of pollution for the air transportation industry for some important combustion products.
We will also finish off the compressor part of our series by looking at the bleeding of cooling air for the engine and for servicing the aircraft with air conditioning and deicing air.
The amount of air which is tapped from compressor stages for cooling and other purposes can exceed 20% of the core flow (some of the flow paths are shown in Figure 1). At that level, it has a marked influence on the performance of the engine.
Compressor bleed air
A turbofan engine has the need for compressed air for many purposes:
The air is always tapped at the earliest possible place in the compression chain which has the appropriate pressure level. The least amount of energy is then spent on the air and it has the lowest possible temperature.
Figure 2 describes tapping of the air from the main compressor of the CFM56-5 engine for the Airbus A320 series (note the four variable stator guide vanes and the knife edge seals at the beginning/end of the compressor drum and at each vane end).
The CFM56-5 has up to three active clearance control systems:
All these mix colder air from, e.g., the booster area with hotter air from different parts of the compressor and routes the mix to the rotor drum (see arrows below the compressor disc ends) or casings that should be temperature-manipulated.
Controlling the temperature of the drum or casings controls the mechanical expansion of the metal and achieves the appropriate tip clearances for the blades of the section.
As can be seen in Figure 2, air is tapped from several places on the compressor. This is easy to achieve because each compressor stage can be opened to the outer casing at several places around the compressor.
Figure 3 shows a compressor which has holes leading to casing pockets with outlets A and B at the sixth and ninth stage of the compressor (air is tapped from one or the other dependent on compressor RPM and therefore pressure level).
In Figure 2, one can see that air enters the rotor drum by a gap at the first stator vane. This air is then mixed with air from the fifth stage under the control of the RACC valve. Air is tapped in many more places. For the CFM56, air is even tapped at the beginning and end of the combustor, in a cold area pockets, for cooling of the high-pressure turbine stator and rotor. Figure 1 shows some main paths for secondary air in an engine.
Bleeding air lowers the performance of the compressor and hence the engine. When bleed air is tapped for cabin air conditioning and de-icing, it increases the fuel consumption of the engine by up to 5%.
The bleed air tapped from the high-pressure compressor has to high temperature to be routed around the aircraft. It’s therefore pre-cooled in a pre-cooler which uses the fan’s airstream for cooling. Figure 4 shows the bleed air plumbing and pre-cooler for the GEnx-2B for the Boeing 747-8i compared to the same engine (in GEnx-1B variant) when used in the 787 (which does not use bleed air for aircraft systems).
Modern combustors are made as rings with fuel injectors and combustor cans housing the flames, Figure 4. The air is slowed down to around M0.2 at the entrance to the combustor by the divergent entry nozzle seen in the figure. The low axial speed of air is necessary to guarantee that the flame is stable and that, e.g., a re-light of the engine can be reliably done while in the air.
The combustor is an area where a lot of research is done. The high compression ratio of modern engines creates a potential high level of NOx emissions. Research programs like the European ACARE aims to define combustors with low levels of NOx, Figure 5.
The emission of CO2 is 100% coupled to the fuel consumption of the engine. With lower Specific Fuel Consumption (SFC, consumption per unit of thrust), the level of CO2 emissions are reduced accordingly.
This is not the case with NOx. Reduction of NOx levels require the division of the combustion into a rich pilot flame area (for reliable function over the power spectrum) and a lean burn area for low NOx generation.
The different OEMs have different programs to achieve low NOx combustors. Figure 6 is from a presentation by the Rolls-Royce-affiliated Loughborough University in the UK. It shows how combustors develop from classical types (lower left) to new types with clearly divided flame areas of rich and lean combustion.
Here are the design requirements according to Loughborough for a modern combustor:
The term AFR stands for Air Fuel Ratios. “Good temperature traverse quality” will guarantee that the exit nozzle from the combustor, which acts as the inlet nozzle for the high-pressure turbine, has a gas flow with an even temperature distribution so that the nozzle and turbine blades will not be subject to “hot spots.”
These parts of the engine are living a life on the verge of melting from temperatures as high as 1,700°C. Any “hot spots” on top of the high average temperature would kill the life of the components. More on turbines in the next Corner.
Note also the words “thermo-acoustic properties”. Resonant acoustic buzz is a major problem in combustors as it means high frequency variable loads on the structure in the combustor. This leads to fatigue cracks and the breakdown of e.g. the combustor’s can insert.