Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
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 SO x O, NOy,
HO x , 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.
Figure 7-27: Influence of wakes and cold blade
surfaces on chemistry for a single blade row in the
turbine (Lukachko et al., 1998).
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 SO x O oxidation is occurring that cannot be explained by SO x O reactions currently accounted for in mechanisms used to date.
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