Clearance

[rguil]

Does anyone have experience with reducing compressor blade to case clearances using abraidable coatings, ie... Aluminum, Honeycomb...? I am looking for the effects of different clearances on compressor performance or possible surge due to excessive turbulence caused a negative clearance. I am actually trying to acheive the optimal clerance but am experimenting with extremes on both ends of the spectrum. Any help?

 

[insideman]

About a zillion years ago GE experimented with soft copper tips on compressor blading. Worked fine, increasing efficiency. But eventually the rubbing on the casing cracked it. End of experiment....

 

[borjame]

The blade tip clearance is just about critical to the performance of any turbine, too much clearance and your stall margin becomes too small. With abradables, you WANT a negative clearance to start or a negative clearance at your aero design point. The abradables allow the designer to take out all the radial build tolerances with a radial interference fit at the blade tip. This will give a close to zero gap (assuming concentricty between the compressor disk/blade and the case) at the blade tip after the engine is started and the abradable is eaten away by the blade tip. The blade tip is usually coated also to prevent damage to the blade, but that's a whole other problem.

 

[WKTaylor]

rguil...

Most very high performance military turbine engines have fixed abradable seals around the blade path. I have seen high density light weight honeycomb core potted with the bonding adhesive used for this purpose.

NOTE: although 0.0000001" blade-tip clearance is desirable, it is also impractical. Typical maintenance practices call for a STATIC [no-load] clearance all-around between the blades and case-half-seals... otherwise undesirable damage during assembly/installation and start-up WILL OCCUR. High precision blade and disk diametrical measurements are taken of the assembly; then the outer-case abraidable seal(s) are machined to provide a very slight static clearance. The engine is then started and slowly advanced to full power. At this time, the blades stretch and distort due to centrifugal loads, air loads, thermal loads, etc... up-to a maximum. During the "break-in" period leading up to full power, the blade tip-path "grows" until it "gently" and deliberately abrades into the seal... setting the final clearance [and pattern]. A final visual inspection [Borescope] is used to confirm that the seals have been uniformly abraded around the circumference... and they are otherwise undamaged.

NOTE: the Blade tip path is usually "set" slightly outside of the desired airflow-path, by a small amount, to minimize airflow distortion at the very tips and enhance sealing.
Regards, Wil Taylor