Among this year's exhibits of German engine specialist MTU is a test rig for active control systems for high-pressure compressors (HPCs) in gas turbine engines.
Such test rigs have been used to explore two principal technologies - active tip clearance control for compressor blades and active surge control. This was part of the EU-sponsored New Aero Engine Core Concepts (NEWAC) research project, in which MTU participated along with other manufacturers such as Rolls-Royce, Snecma and Avio.
Minimising the clearance between the blade tips and compressor case lining is central to reaching high engine efficiency. But the clearance varies depending on engine performance. When the compressor spins fast on take-off, its rotating assembly slightly expands in diameter due to centrifugal forces. This needs to be accommodated by the tip clearance.
At lower cruise thrust settings, however, the excessive clearance leads to efficiency losses. The engineers are thus working on adjustable compressor case linings to ensure an optimal tip clearance in different flight regimes. A challenge in designing such systems is to make them fail-proof to avoid inadvertent closure of the mechanism, which could in the worst case jam the rotating parts.
Thus far, MTU has not earmarked the technology for any of its production HPCs. Whether it will find its way into service depends on the usual trade-off between potential fuel savings and penalties for additional weight, system complexity and maintenance costs, says Frank von Czerniewicz, project manager compressor technology at MTU. This, apparently, is not quite clear yet.
But active surge control technology will be implemented into the next generation of Pratt & Whitney's geared turbofan family. The engineers developed a system whereby bleed air - which is extracted further downstream in HPCs - is released on to the blades at the compressor entry. This moderates the air flow to prevent the compressor blades from stalling.
The bleed air is not constantly flowing into the gas path, but only released temporarily in flight situations where a compressor surge could occur. Von Czerniewicz says that compressors are designed not to surge throughout the flight envelope. But this "jack of all trades" approach means that potential efficiencies may be compromised in certain performance areas. Active surge control could thus provide the necessary safety margin in moments where more critically designed compressor aerofoils tend to surge.
MTU says that the technology will be introduced in the next GTF generation beyond the current the PW1000G family for the Airbus A320neo, Bombardier CSeries, Embraer's second-generation 175, 190 and 195, Irkut MS-21 and Mitsubishi MRJ regional jet.
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