Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
models and subsequent atmospheric modeling downstream of the engine exit.
7.6.4.4. Reactive Species (O, OH, HO 2, SO, H 2, H, N, CH)
Current chemical models assume the radicals N and CH to be negligible for current technology
engines, and their influence on the course of post-combustion chemistry has been ignored. The
validity of this assumption has not been assessed, however, and as combustion temperatures
increase in future engines these constituents (as well as the others) may become more
important. O and OH are considered to be the dominant oxidizing species within current
modeling assumptions (HO 2 levels remain less important), and orders of magnitude change are
possible in these reactive species throughout the turbine and nozzle. Their evolution is very
sensitive to chemical kinetics and species initialization at the combustor exit, and they are
influenced by flow nonuniformities in the turbine and nozzle to a similar extent as the oxidation
products. Prediction of their levels with sufficient accuracy for assessment needs is beyond
currently validated capabilities, and the existing experimental database is insufficient to validate
models. Furthermore, recent modeling studies (Yu and Turco, 1997, 1998) have implicated
chemi-ions-a product of flame ionization in the combustor-as important in the growth of volatile
particles after the exhaust leaves the engine. No measurements of ion levels at the engine exit
Figure 7-30: Emissions characteristics of a fan engine
versus load at sea level and in flight at 11 km.
have been identified; few measurements downstream of the exit exist (Arnold et al., 1994, 1999), and only simple estimates of the recombination inside the engine
have been attempted to date, so the uncertainty in the emission levels of chemi-ions is very high.
Pressure and temperature and residence times change over relevant ranges of operating conditions and engine cycles, and the chemical processes are expected to
change as a result. Generalizations regarding the exhaust constituents described above are not expected to change significantly over cruise conditions and even over
the broader operating conditions representative of the landing/take-off cycle. However, changes in exit speciation are expected for the oxidative products and radical
species that depend non-linearly on combustor exit concentrations and local conditions. Only limited research has been carried out on this topic (Hunter, 1982; Brown
et al., 1996; Lukachko et al., 1998).
Table 7-7 provides a simplified status summary of the confidence attached to current modeling and measurements of emissions emerging from engines. Reducing
uncertainties in the oxidation products of secondary species and in reactive species will require additional new measurements, as well as increased physical
understanding of exhaust species oxidation in the turbine and nozzle.
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