Even with advanced cooling and single crystal fabrication, thermal barrier coatings (TBCs) are utilized. TBCs provide essential in-sulation and protection of the metal substrate from combustion gases. A ceramic TBC topcoat provides ther-mal resistance, and a metal bond coat provides oxidation resistance and bonds the topcoat to the sub-strate. GEPS developed an air plasma spray deposition process and associated software for robotic ap-plication. An e-beam test facility replicated turbine blade surface temperatures and thermal gradients to validate the process. The TBC is now being used where applicable throughout the GEPS product line.
Compressor
To meet H System. air requirements, GEPS turned to the high-pres-sure compressor design used in its CF6-80C2 aircraft engine. The 7H system uses a 2.6:1 scale-up of the CF6-80C2 compressor, with four stages added (bringing it to 18 stages), to achieve a 23:1 pressure ratio and 1,230 lb/sec airflow. The design incorporates both variable inlet guide vanes, used on previous systems, and variable stator vanes at the front of the compressor. These variable vanes permit airflow adjustments to accom-modate startup, turndown, and variations in ambient air temperatures.
GEPS applied improved 3-D computational fluid dynamic (CFD) tools in the redesign of the compressor flow path. Full-scale evaluation of the 7H compressor at GEPS’ Lynn, Massachusetts compressor test facility validated both the CFD model and the compressor performance.
H System. compressors also circulate cooled discharge air in the ro-tor shaft to regulate temperature and permit the use of steel in lieu of Inconel. To allow a reduction in compressor airfoil tip clearance, the de-sign included a dedicated ventilation system around the gas turbine.
Combustion
To achieve the single digit NOx emission goal, the H System. uses a lean pre-mix Dry Low NOx (DLN) can-annular combustor system similar to the DLN in FA-class turbine service. The H System. DLN 2.5 combus-tor combines increased airflow resulting from the use of closed-loop steam cooling and the new compressor with design refinements to produce both single digit NOx and CO emissions.
GEPS subjected full-scale prototype, steam-cooled stage 1 nozzle seg-
ments to extensive testing under actual gas turbine operating condi-
tions. Testing prompted design changes including application
of TBC to both the combustor liner and downstream transi-
tion piece, use of a different base metal, and modified
heat treatment and TBC application methods.
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