|Aviation and the Global Atmosphere|
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Heightened interest in engine emissions has had a considerable effect on the approach to combustor development over the past 25 years. As with other key fields of gas turbine technology, earlier dependence on semi-empirical and semi-analytical models has been lessened by advances in CFD methods of modeling and analyzing complex flow processes (Mongia, 1994, 1997a). Nevertheless, the complexity of the problems dictates that experimental verification is still required to demonstrate that the combustor meets all emissions, operational performance, and durability requirements. As experience and confidence in these approaches grow, the high costs of combustion system development might be contained-or even reduced slightly. At present, however, modeling is not expected to completely replace experimental testing, especially in the case of more advanced, low-emission concepts for which experience is limited. Thus, the pursuit of advances in combustor technology will remain a major part of the cost of any new engine. A summary of current modeling capabilities, limitations, and potential improvements is given elsewhere (Mongia, 1997a).
It is important to note that the intrinsic complexity of the combustion process and limitations of current analytical tools lead to considerable uncertainty in prediction of emissions from new combustor designs. Mongia (1997a) estimates that even with analytical models that have been "anchored" to measured data-a process to systematically calibrate a model so that it reproduces measured emissions results-prediction accuracy is only about ±15% for NOx and ±30% for CO and HC. The uncertainty in prediction is even higher for smoke.
One of the clear lessons that emerged from early emissions reduction programs was that changes to reduce NOx emissions could produce adverse effects on other performance characteristics, leading toward tradeoffs in design. Tradeoffs can be attributed to the engine cycle selected or the combustor design itself. Tradeoffs based on engine cycle reflect changes in overall pressure and bypass ratios, which, in turn, affect fuel burn rates and the conditions at which combustion occurs (see Section 18.104.22.168). Tradeoffs generated by design changes affecting the combustor can influence major combustor performance parameters, including operability, reliability, durability, efficiency, and noise-all of which must be taken into account during engine development.
More fuel efficient, high bypass engines reduce not only CO2 and H2O, but also HC and CO emissions. For a given combustor design, however, NOx formation rates rise as a result of higher air pressures and temperatures. Thus, despite the reduction in the amount of fuel that can form NOx, the increased formation rate can result in a net rise in the mass output of NOx. This result is shown clearly in Figure 7-17 (Rudey, 1975). Many earlier researchers have suggested that the only way the more basic tradeoff between CO2 and NOx could be altered was by finding satisfactory ways of reducing fuel-rich zones without compromising stability, and reducing the residence time of burning gases in the combustor without compromising exit temperature profiles or pattern factors.
This important NOx-CO2 (fuel) tradeoffs issue has been summarized as follows:
Improved low-emission combustor technology can alter the precise tradeoff between CO2 and NOx. Theoretically, this technology can reduce NOx levels at any pressure ratio. However, for any currently available aircraft engine combustor technology, there will still be some tradeoff between CO2 and NOx, although it will be at lower NOx levels. Even with very advanced combustor technologies that minimize NOx formation by premixing fuel and air to control combustion temperatures, there is a tradeoff between CO2 and NOx as a result of high combustor exit temperatures associated with advanced, highly efficient engines.
A further NOx-related issue arises from a common practice adopted by industry. In recent years, there have been numerous occasions when operators have sought to increase the capacity (passenger numbers or weight) or range of an existing airplane. A highly cost- and time-effective response to such requirements is to increase the thrust level of an existing engine type. This response has led to a type of development usually referred to as a "throttle push" or "throttle bend" of the core engine. This approach invariably entails increasing overall engine pressure ratio and results in higher combustor inlet pressures and temperatures, thus higher NOx levels at high power levels. However, analytical studies (ICCAIA, 1997d) of engine growth characteristics have shown that the rate at which the mass formation rate of NOx increases as the "throttle bend" technique is used to increase thrust is similar to data measured on a wide range of engine families over a significant range of engine operating pressure ratios (ICAO, 1995a). It is also worth noting that, in practice, the required thrust at cruise and measured cruise SFC remain essentially constant for the "throttle push" versions, resulting in increases in fuel efficiency levels as payloads and number of passengers have risen.
Current combustors can contribute little more to fuel efficiency because at power levels above "idle" the energy conversion level is virtually 100% (see Tables 7-4 and 7-5). However, superimposed on all combustor design considerations is the continuing underlying requirement that low emissions features must not compromise basic combustion requirements or have any significant effect on engine performance. In recent years, this requirement has imposed difficult problems in introducing new emissions reductions features. Table 7-4 provides a list of basic requirements, and Table 7-5 highlights options and compromises that designers have had to face.
Figure 7-18 provides a qualitative indication of the development process as engineers work to reconcile the combustion system constraints listed in Tables 7-4 and 7-5 and the operational requirements of the engine.
Low-emissions combustors currently fall into two categories. The first category is composed of existing combustors, which have incorporated relatively minor changes to liner and/or fuel nozzle designs to improve emissions. Recent examples of changes that have been quite successful in reducing NOx emissions are shown in Figure 7-19.
The NOx emissions of state-of-the-art combustors are 20-40% lower than those of older combustors. Figures 7-20, 7-21, and 7-22 provide evidence of some of the progress that has been made in recent years to avoid what had earlier appeared to be an unavoidable link between lower NOx emissions and relatively high CO and HC emissions. In these examples, relatively minor changes to the combustor's airflow pattern and the location of fuel injection resulted in reduced CO/HC without a NOx penalty.
Although this category of improvement entails relatively minor changes, the development and engine recertification process remains a long one. Because safety is the overriding concern in aviation, the time and cost to introduce a change can be considerable. Safety considerations can 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 NOx 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 NOx technology, which rely on low temperature, lean combustion to achieve low NOx. 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 NOx, CO2, 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.
The incorporation of such a staged combustion system into an existing engine type requires changes to several parts of the high-pressure section of the engine, including the compressor outlet diffuser, combustor case, and inner structure. Major changes in the fuel control and fuel delivery system are also required. Additional modifications to the turbine may also be needed to accommodate changing temperature patterns during staged operation. Thus, the center and aft sections of the engine, which account for a major fraction of the cost of an engine, may be significantly different from the same engine with a current technology combustor. There may also be increases in weight, maintenance cost, and fuel burn. Several in-service engine models incorporate single burning zone unstaged combustors that employ limited fuel staging to maintain lean blow out performance at low power and operate at reduced NOx levels at high power.
Retrofitting an older engine model with one of these advanced combustors is technically feasible. However, it could involve not only replacement of the existing combustor but also replacement of almost all other elements of the engine core. Estimates suggest (ICCAIA, 1997c) that retrofit could incur a cost of about one-third the price of a new engine, even if it were accomplished during a standard hot section overhaul. In some cases, aircraft systems and components such as cockpit indication elements, auto throttle, flight management computer, and FADEC interfaces could be affected. It now seems more likely that both categories of combustion system improvement will be considered only for application in new production engine units.
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