Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
the optimization of current single-stage combustor technology. This approach involves
further improvements in fuel injection uniformity, better fuel/air mixing, reduction in
combustor liner coolant flow (making more air available for combustion), and
decreases in hot gas residence time. Such changes would have minimum impact on
engine cost of ownership. The first examples of this type of technology now exist in
manufacturers' product plans for the next 5 to 10 years.
● Reductions in NOx levels to 50-70% below CAEP/2 standards are being explored
using multi ple burning zones in radial and axial configurations (Figure 7-23). These
concepts permit local temperature and residence time in the combustor to be
controlled and optimized at each engine operating condition, to minimize NO x and
other emittants (Bahr, 1992; Segalman et al., 1993). In low-power operations, a single
stage is fueled and optimized for stability. At high-power conditions, one or both stages,
configured for lean burning, are fueled. Generally, radial staged designs are shorter
and lighter but larger in diameter, making it more difficult to achieve a uniform exit
temperature profile at off-design conditions. They are also more difficult to cool. Axial
staged systems are longer and have a larger number of fuel injectors. In both cases,
the increased complexity and weight of these combustors is expected to increase the
cost of ownership, as described by DuBell (1995). A major effort is needed to minimize
the cost, complexity, weight, and performance of these concepts. Reduction in NOx levels to 85-90% below CAEP/2 standards are focused on emissions from supersonic
aircraft at cruise conditions. Such work presently forms part of the U.S. supersonic
transport program. Combustors known as lean premixing prevaporizing (LPP) and rich
burn quick quench (RBQQ) are being studied. In the former, the burning zone is fed
with a lean and homogeneous fuel/air mixture. Premixing and prevaporizing takes
place in a premix duct outside the combustor. The RBQQ combustor consists of three
zones: A primary rich burning zone; a dilution zone, to rapidly reduce the rich mixture
to a lean one without recirculating dilution air into the primary zone; and a lean
reburning zone (DuBell, 1995). These combustors uniquely apply to the supersonic
engine with its relatively low engine pressure ratio and its requirement for long periods
at a single, high-speed cruise operating condition. Application of these concepts to
future subsonic engines would pose special problems because of higher pressure
ratios. For example, the LPP combustor will have to overcome the greater risk, at very
high pressures, of "flashback" or upstream burning-which, if undetected, could damage
the combustor. Similarly, the RBQQ combustor-with its high fuel concentration in the
primary burning zone-may well result in the generation of large amounts of soot and
smoke in high-pressure operations. However, some of the features found in these
concepts may be suitable for higher pressure ratio subsonic engines. Partial premixing,
coupled with moderately rich sector burning, represents one such concept. Multiple
burning zones, together with variable geometry to control local fuel/air ratios, would be
another. A concept incorporating several of these features (shown in Figure 7-23b) is
being pursued under the European Union's Targeted Research Action "Efficient and
Environmentally Friendly Aero-Engines." A radially staged configuration currently
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Figure 7-24: Illustration of complex flow passages and blade
cooling schemes in a typical turbine stage (after Rolls Royce, 1992).