09 October 2015, ©. Leeham Co: Last week an Airbus A320neo prototype with Pratt & Whitney’s (PW) GTF had a problem while testing hot and high conditions at Al-Ain airport in Abu Dhabi. The engine suffered a rubbing problem and PW and Airbus decided to replace the engine before returning the A320neo to Toulouse.
I had the opportunity to discuss what happened with PW people at ISTAT this week and decided it makes for a good follow up to our two other engine Corners to write about what happened and how serious it was.
The problem was compressor blades rubbing against the compressors stator wall. PW knew that this engine individual could have that problem. They saw when assembling the engine it was a bit tight in the compressor area. PW said they told Airbus there was a risk with this particular unit, and sure enough, there was rubbing to be seen when they boroscope checked the engine after the test.
Here what it was all about and what to do about it.
Turbofans and how they function
As I have written in previous Corners, turbofan engines are air pumps. The dominant pumping is done by the fan (90%) with about 10% of thrust coming from the gases coming out of the core. The core gives shaft power to drive the fan. It does this by compressing air and then mixes it with fuel in the combustor and burns it to get hot gasses with high pressure. These then generate shaft hp in the turbines which drives the fan and compressors via shafts.
Both the compressors and turbines are made of a rotating part (compressor/turbine rotor with blades) and a fixed part (compressor/turbine stator with blades), see Figure 1. The compressor gains pressure for each rotor/stator combination. The turbine does the reverse; it uses the high pressure gases from the combustor to extract shaft hp and the pressure and temperature falls for each stage.
To have the compressor’s pressure gain using the minimum of shaft hp and the turbine pressure fall generating the maximum of shaft hp, it is important these stages having no leaks of air in the direction of lower pressure. The main leak opportunities are at blade tips, Figure 1.
Air would leak past the tip and into the stream on the blade’s back side and therefore back to the previous stage in the compressor, thus lowering the stage’s compression ratio. And it would leak past the turbine blade to the next stage without pushing the turbine blade to generate shaft hp.
Therefore, the tip clearances of compressor and turbines are made as small as ever possible. It is a play with tolerances and sometimes one gets the tolerance too tight in production and there is rubbing, blade-tip to stator shroud.
The other area where air can slink through is at the blade roots. The root area is sliding past the inner part of the stator on both sides of the blade. The middle figure in Figure 1 shows the forward seal of a turbine stage. To seal this area, rotating seals are used, here shown as knife edge seals. Other solutions use different brush structures, all trying to stop air slinking past the seal.
Rub or no rub
There are different methods to get to minimal leaks over the blade tips. Turbine stages have high stage pressure drops and calls for tight solutions in an area with high thermal movements and stresses. For turbines, Rolls-Royce has been using blades with tip shrouds, Figure 1. This pushes the problem from tip leaks to a rotating seal problem, which is easier to handle when the blades grow with temperature and centrifugal forces. The drawback are heavy blades, which require a beefier disc to compensate, the engine gets heavier.
The alternative are blades which are allowed to create their own tip clearances by deliberate rubbing. These blades have tips equipped with hard metal implants that scratch their needed clearance from a stator shroud made with a forgiving material. This is often complemented with a stator shroud which can contract and expand through an active tip clearance control system. This way of getting the high pressure turbines to have minimal leaks is used by PW, GE and now also Rolls Royce for the Trent XWB 97k’s high pressure turbine.
For the low pressure turbines (LPT), normal tips with close clearances are used, aided by control of the tip clearances by controlling the stator thermal expansion through throttled cooling tubes passing over its surface. The cooling uses air from an early compressor stage. Once again Rolls Royce uses shrouded blades instead of stator cooling for the LPT.
For the compressors, the stage pressure gain is smaller than for a turbine stage (1-2 versus a fall of up to 5 for a high pressure turbine stage) and it is not worth the trouble with shrouded blades, rubbing tips or throttled cooling of the long compressor stators. Here the technology is to run close tip clearances once operating temperature has been reached (up to 700°C at the last stages).
The manufacturers therefore produce compressors with the closest gaps possible between the blade tips and the stator shrouds. Sometimes it gets to close as in the PW Al-Ain case. PW recognised it was a close case when assembling the engine and told Airbus to check it during the tests.
Clearances don’t scale with size
Compressors need to shrink the dimensions of later stages as the air gets more compressed and takes less space. Subsequently, the blades get smaller. For small engines, it is not possible to shrink the tip clearances in proportion. Large tip leaks are the result. This makes small axial compressors hard to make efficient, especially for the last stages.
Therefore small engines (turbonfan as well as turboshaft) use radial compressors for the last compressor stages, Fig 2.
A radial compressor has a larger stage pressure gain and therefore replaces several of the last axial stages. It also has larger dimensions and is therefore easier to manufacture for acceptable efficiency.
The large radius (it decides the core’s diameter for this engine) is why it is not used on earlier stages or when the core’s dimensions makes axial stages effective.
To have an engine rub its blade tips against the outside walls is not good unless it is designed to do it (turbine example). But it is in most cases a matter of getting manufacturing tolerances right and engine testing is where to find the last fractions of these tolerances. The Al-Ain problem was therefore with high probability a manufacturing problem.
This is different to PW’s last problem on the Bombardier CSeries. There the rear bearing compartment’s rotating seals did not make oil stay put in the compartment. You use a deliberate compressed air leak over the rotating seal, pushing air into the compartment, to stop oil from travelling past the seal. PW needed a fix to get that right.
For the rubbing, the fix should be that they open up their manufacturing tolerances a tad.