|Aviation and the Global Atmosphere|
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7.6. Turbine and Nozzle Effects on Emissions
This section describes what is known and not known about changes in various exhaust constituents that occur downstream of the combustor, in the air passages of the turbine and exhaust nozzle. The discussion centers on chemical species that are expected to have the highest potential for impact on the global atmosphere (see Chapters 2 and 3).
The discussion begins in Section 7.6.1 with a review of the factors that have generated interest in chemical processes in the turbine and exhaust nozzle. Section 7.6.2 provides a brief overview of functional requirements and constraints for aircraft turbines and nozzles, and how these requirements have led to particular design choices. Section 7.6.3 describes the relevant chemical and fluid mechanical effects. Section 7.6.4 describes what is known and not known regarding chemical changes in the turbine and exhaust nozzle.
Many engine exhaust species of interest for environmental impact assessment exist in trace amounts, typically tens of ppbv to tens of ppmv. Despite their relatively small concentrations, these trace species emissions can result in perturbations of chemical species in the atmosphere that may induce significant atmospheric effects. Chapters 2 and 3 discuss these issues in relation to atmospheric ozone and cloudiness, respectively. Numerical simulations and a limited number of experiments suggest that several of these trace species can undergo considerable change within the nonuniform, unsteady flow fields of the turbine and nozzle prior to injection into the atmosphere (Hunter, 1982; Harris, 1990; Brown et al., 1996; Lukachko et al., 1998). However, there is a high degree of uncertainty regarding the extent of this change and its dependence on engine technology and operating conditions. Indeed, estimates for the extent of sulfur oxidation (SO2 ' SO3 + H2SO4) range from 0.4 to 45% or more depending on the modeling assumptions and experimental data considered (Hunter, 1982; Harris, 1990; Arnold et al., 1994, 1999; Frenzel and Arnold, 1994; Miake-Lye et al., 1994; Fahey et al., 1995b; Brown et al., 1996; Kärcher et al., 1996; Hanisco et al., 1997; Lukachko et al., 1998; Miake-Lye et al., 1998). However, most of the predicted conversion efficiencies are less than 10%, with an experimentally derived lower limit of 0.34% (Curtius et al., 1998) (see Section 18.104.22.168). More important, such variations in estimates of the extent of sulfur oxidation within the engine and subsequent changes in aerosol formation lead to changes in predicted column ozone depletion by a fleet of supersonic transport aircraft by as much as a factor of 2 (Weisenstein et al., 1995; Danilin et al., 1997). Large uncertainty regarding trace species processes and the perception that intra-engine changes may be important-along with the desire for a more detailed and complete characterization of engine exhaust emissions to support downstream plume, wake, and atmospheric modeling efforts-have only recently motivated a more detailed study of intra-engine trace species chemistry (Dryer et al., 1993; Brown et al., 1996; NRC, 1997; Lukachko et al., 1998). There is relatively little research yet reported in this area; however, new measurement and modeling capabilities are currently being developed.
Sections 7.4 and 7.5 discuss the principal elements and functions of the components of a gas turbine engine. They describe how, after exiting the combustor, the engine core flow passes into the turbine then through the exhaust nozzle (Figure 7-7 et seq.). Within these components, the principal engineering constraints are associated with maintenance of the structural integrity of the parts exposed to the high-temperature environment downstream of the combustor and limitations on weight.
Figure 7-24 shows an example of a modern turbine stage. The design of the flow passages ensures maximum operating efficiency and meets engineering integrity requirements of the compressor and combustor. As explained in Section 7.4, the achievement of high thermal efficiency, hence low fuel consumption, means that turbines operate in gas flows that are several hundred Kelvin above the melting point of the materials employed (see, e.g., Kerrebrock, 1992). Cooling of all structures exposed to the flow path is therefore essential. The requirement for cooling adds significant complexity to the blades and vanes because of the need for small internal passages through which air bled from the compressor is channelled. This air is usually injected through small holes in the surfaces of the various components (as shown in Figure 7-24) to form a protective, cooler boundary layer. Air injected in this manner can account for as much as 25% of the flow through the core of the engine. The trend for increased temperature and pressure is expected to continue in the future, as discussed in Section 7.4.
Figure 7-25 shows the mean temperature and pressure history as a function of time through the aft end of a typical engine, to illustrate pressure and temperature ranges in which trace species chemistry occurs. Turbine inlet temperatures vary within the flight cycle from 1200 to more than 2000 K, and pressures vary from 0.8-4.5 MPa. The gases remain in residence within the turbine and nozzle for approximately 5-10 ms. Combustor residence times have decreased (currently around 5 ms) as a result of efforts to reduce NOx, and the time the exhaust gases spend within the turbine and nozzle can thus be longer than that in the combustor. At the engine exit, the temperatures and pressures typically range from 200-600 K and 0.02-0.1 MPa, respectively, depending on the particular engine technology and the operating conditions. Note that some of the temperature change within the turbine results from the addition of cooling air, as discussed above.
Significant variations around the baseline, one-dimensional flow conditions described above exist because of spatial and temporal flow nonuniformities at the combustor exit and throughout the turbine. There are many reasons for this nonuniformity: Combustion turbulence, the combustor exit temperature profile needed to maintain turbine blade life, fuel injector-induced hot spots, introduction of cooling flows, and viscous boundary layers and wakes. Together these effects lead to turbine inlet conditions-which may vary locally at any instant in time-between those associated with combustor inlet conditions and those resulting from stoichiometric combustion.
Downstream of the combustor-in the turbine and exhaust nozzle-the evolution of any particular species within the turbine and exhaust nozzle generally depends on local temperature and pressure, concentrations of other species, and variations of these parameters over time. The multitude of unsteady three-dimensional fluid mechanical effects and the large number of chemical species that interact with one another make this region a complex physical and chemical system. A full understanding and prediction of its behavior has not yet been attained.
Chemical models developed for application to the post-combustion expansion process must apply to a wide range of flow parameters, as discussed previously (Miake-Lye et al., 1993; Brown et al., 1996; Lukachko et al., 1998). The availability of kinetic data applicable to the entire post-combustion range is limited because little research has spanned the wide gap in parameters between combustor and atmospheric conditions for many of the relevant reactions. An example of a chemical mechanism currently employed (Lukachko et al., 1998) consists of 25 species coupled through 74 reactions representing contributions from gaseous SOxO, NOy, HOx, and COx chemistry (Westley et al., 1983; Tsang and Hampson, 1986; Tsang and Herron, 1991; DeMore et al., 1994; Yetter et al., 1995). Because of gaps in the kinetic data, interpolation of available rates introduces uncertainties into current model results.
Beyond limitations in the range of applicability of basic kinetic data for identified reactions, the overall kinetic mechanism has yet to be validated against experimental data. Thus, the kinetic mechanism may not be complete, and missing mechanisms have yet to be positively identified. In particular, heterogeneous processes (e.g., involving soot and volatile particles) have not been adequately addressed by current models or measurements (see Chapter 3). In addition, recent in-flight (Fahey et al., 1995b; Hanisco et al., 1997; Anderson et al., 1998; Hagen et al., 1998; Miake-Lye et al., 1998; Pueschel et al. 1998) and engine test cell measurements (Wey et al., 1998) have indicated that additional SOxO oxidation is occurring that cannot be explained by SOxO reactions currently accounted for in mechanisms used to date.
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