Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
sulfur oxidation (SO 2 ' SO 3 + H 2 SO 4 ) 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 3.2.2.2). More
Figure 7-26: General categorization of chemical
processes in the turbine and nozzle (Lukachko et al.,
1998).
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.
7.6.2. Aircraft Turbine and Nozzle Design
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.
7.6.3. Chemical and Fluid Mechanical Effects
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 NO x , 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
http://www.ipcc.ch/ipccreports/sres/aviation/104.htm (2 von 3)08.05.2008 02:43:38