At this stage, there is no reliable information relating specifically to the design of supersonic propulsion combustion systems, but there is no reason to believe that the degree of inefficiency should be any different from subsonic types. Emissions arising from this form of inefficiency are discussed in Section 7.5. The fundamental nature of the proportion of CO2 and H2O emitted with conventional fuels and similar proportions of sulfur in available fuels suggests that these emittants will be produced in the same proportion to fuel usage as in the subsonic fleet, though at supersonic cruise they will be deposited in the stratosphere.
Assessments of advanced supersonic aircraft concentrate on long-range over-water routes but do not assume that such routes will be served exclusively by supersonic services. It is now assumed that supersonic flight will occur only over water. The combination of overland routes where sonic booms are unacceptable and fitting into daily cycles will preserve a place for long-range subsonic services to meet airport curfews and provide passengers with comfortable time zone changes.
Research programs to develop low NOx combustion systems for cruise are in place in Europe, the United States, and Japan. Two basic combustion concepts are being researched to produce ultra-low NOx levels at supersonic cruise conditions. These technologies-the Lean Premixed Prevaporized (LPP) and Rich Burn Quick Quench (RBQQ)-utilize both lean and rich combustion concepts such as those shown in Figure 7-42 on the previous page. As a result of low combustor operating pressures in supersonic transport applications, these concepts appear capable of achieving their full ultra-low NOx reduction potential while maintaining satisfactory durability and performance. In subsonic transports, the higher pressures dictate compromises in both concepts to avoid incipient flashback in the LPP and excessive soot production in the RBQQ.
The LPP concept has the likely potential of reaching the lowest levels of NOx. 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 NOx. Maintaining uniform fuel/air mixtures throughout the combustor is critical because NOx 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 NOx. 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 NOx 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:
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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