Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The LPP concept has the likely potential of reaching the lowest levels of NO x . The intent of premixing is to provide the combustion zone with a very lean, uniform fuel/
air mixture that is just above the flame extinction limit. This approach results in a low flame temperature with enough residence time to complete combustion and
produce low NO x . Maintaining uniform fuel/air mixtures throughout the combustor is critical because NO x increases rapidly with any local fuel/air maldistribution. In
practice, premixing is achieved with large numbers of small-diameter premixers. Design challenges with this concept include flashback or auto-ignition in the premixer,
maintaining combustion near the lean extinction limit over the entire engine cycle operating span, potential fuel clogging of small-diameter fuel injectors, and complexity
of the design because of fuel staging requirements.
The RBQQ concept is a derivative of an axially staged combustor and presents the more stable combustion configuration. The fuel/air mixture of the primary
combustion zone is fuel-rich, thus producing low flame temperatures and low NO x . In a second stage, air is quickly introduced to mix with the partially reacted fuel.
Combustion is completed in a final stage at lean conditions. Most of the NO x is produced in the second stage and is a function of the uniformity and time it takes to
dilute the reacting mixture. Design challenges with this concept include indirect cooling of the primary combustion zone-which may require high-temperature ceramic
materials currently under development-and an advanced second stage that produces nearly instantaneous, uniform mixing of reacting gas and air. Furthermore, this
design may require engine power-related control of air and/or fuel staging for practical implementation.
There are three active programs in which this research is being conducted:
● U.S. NASA High Speed Civil Transport (HSCT), with a goal of supersonic cruise NOx of EI(NOx ) = 5
● Japanese Supersonic/Hypersonic Transport (HYPR), with a goal of NOx emissions below EI(NOx ) = 5 at Mach 3 cruise
● EU Low NOx III, to develop ultra-low NOx combustor technology for a second-generation civil supersonic transport aircraft.
Under laboratory and component test cell conditions, very low levels of NOx below EI(NOx ) = 5 have been achieved at simulated engine operating conditions of
pressure, temperature, and fuel flow with combustor sectors. This result gives credence to the view that EI(NOx ) = 5 could well be achieved in engine tests scheduled
after the year 2000. The intrinsic outputs from combustion of conventional fuels are more difficult to alleviate. With kerosene as a fuel, for every ton of fuel burned, 3.2
tons of CO2 and 1.2 tons of water are produced.
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