Advanced Turbine Systems 先进的涡轮系统(4)

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Early entry of the 9H is part of the H System. development strategy
to reduce risk. The 9H incorporates critical ATS design features and pro-vided early design verification. Also, because ATS goals required ad-vancements in virtually all components of the gas turbine,
GEPS incorporated its new systems approach for the
H System.—the “design for six sigma” (DFSS)
design process. DFSS accelerated development
by improving up-front definition of perfor-mance requirements and specifications
for subsystems and components, and by
focusing the research and development
activities. Downstream, the benefits
will be improved reliability, avail-ability, and maintainability due to
integration of manufacturing and
operational considerations into the
DFSS specifications.

GEPS’ 400-ton MS7001H in transit to full-speed, no-load testing

Meeting the Technical Challenges Turbine
The need to address the conflict-ing goals of higher efficiency and lower NOx emissions required sys-temic changes. The major driver was to increase the firing temperature (temperature into the first rotating turbine stage) without exceeding the NO formation combustion tem-
x

perature of 2,800 oF. To do so, GEPS introduced closed-loop steam cool-ing at the first and second stage nozzles and turbine blades (buckets) to reduce the differential between combustion and firing temperatures. The closed-loop steam cooling re-placed open-loop air cooling that depends upon film cooling of the airfoils.
In open-loop air cooling, a sig-nificant amount of air is diverted from the compressor and is intro-duced into the working fluid. This approach results in approximately a 280 oF temperature drop between the combustor and the turbine rotor inlet, and loss of compressed air en-ergy into the hot gas path.  Alterna-tively, closed-loop steam improves cooling and efficiency because of the superior heat transfer character-istics of steam relative to air, and the retention and use of heat in the closed-loop. The gas turbine serves as a parallel reheat steam generator for the steam turbine in its intended combined-cycle application.
The GEPS ATS uses a firing temperature class of 2,600 oF, ap-proximately 200 oF above the most efficient predecessor combined-cycle system with no increase in combustion temperature. To allow these temperatures, the ATS incor-porates several design features from aircraft engines.
Single crystal (nickel superal-loy) turbine bucket fabrication is used in the first two stages. This technique eliminates grain bound-aries in the alloy, and offers supe-rior thermal fatigue and creep characteristics. However, single crystal material characteristics con-tribute to the difficulty in airfoil manufacture, with historic applica-tion limited to relatively small hot section parts. The transition from manufacturing 10-inch, two-pound aircraft blades to fabricating blades 2–3 times longer and 10 times heavier represents a significant challenge. Adding to the challenge is the need to maintain very tight airfoil wall thickness tolerances for cooling, and airfoil contours for aerodynamics.
　
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