Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
aviation, the time and cost to introduce a change can be considerable. Safety considerations can combustor.
also constrain application of new combustor designs to new engines. For example, a new
combustor design is often introduced as a package that includes modifications to the combustion
chamber, fuel nozzles, and engine control. Positive steps must be in place to prevent intermixing
of new and old components during maintenance. Of course, other requirements-such as durability, weight, maintenance, and cost-must also be balanced during
product introduction. This first technology "category" can also include improvements in other engine components, such as the engine turbomachinery, with or without
concurrent combustor changes. Such improvements alone can lead to better fuel efficiency along with lower peak cycle temperatures, thereby reducing NOx and CO2 emissions and improving durability.
The second category of near-term advances involves major changes such as introduction of "staged" combustors (see Figure 7-23). Staging was introduced to
improve or provide an additional degree of freedom between operational and emissions requirements. Thus, the high-power stage of a combustor optimized for low
NO x does not have to cope with low-power stability requirements, which are dealt with by bringing in other parts of the staged combustor when needed. Staged
combustors, however, do require more complex control systems, incorporating fail-safe operation, to ensure that transient engine performance is not affected. This
requirement is especially crucial for the most recent advances in low NO x technology, which rely on low temperature, lean combustion to achieve low NO x . Staged
systems can present problems in achieving acceptable combustor exit temperature profiles, with associated losses in turbine efficiency, thus fuel efficiency. They are
also heavier. Together, the complex interaction of improvements and penalties translates into a form of tradeoff between NO x , CO 2 , and HC/CO; work in this field
continues to define designs that minimize penalties and maximize benefits.
The only example of a staged combustor in aircraft service today is the dual annular combustor (DAC), which is shown in Figure 7-23a. The DAC is a staged system
that incorporates two separate combustion zones. The pilot stage provides good operational performance required at low power. The main stage provides low NOx emissions at high power. Low NOx emissions are achieved with lean fuel/air mixtures, which reduce flame temperatures, and high throughput velocities, which reduce
the residence time available to form NOx . Relative to current state-of-the-art NOx levels discussed above, a single annular combustor in an engine having a pressure
ratio of approximately 30 achieves about 30% reduction in LTO NOx emissions, as shown in Figure 7-19, and NOx levels are about 40% below CAEP/2 standards
(ICAO, 1993). However, these improvements do not come without some tradeoffs. For example, Figure 7-20 (Mongia, 1997b) compares CO and NOx emissions for the
conventional baseline combustor and the DAC. Both combustors fall on approximately the same line, indicating an apparent increase in CO with reduced NOx . The
DAC system is also more complex. To obtain requisite staging capability, the engine must be equipped with a full authority digital electronic control (FADEC) system.
The FADEC system must deal with increased complexity of engine operation to accommodate the various staging modes and associated engine responses. The
added complexity invariably involves additional development effort for any new application to ensure the achievement of acceptable ground-level and altitude starting,
combustion efficiency, and turbine inlet temperature patterns. The NOx benefit is reduced in higher pressure ratio engines because of the increased competition for
airflow to meet the conflicting requirements of durability and emissions.
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