Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.7.2. Engine Load and Emission Correlation
7.7.2.1. Emission Correlation Methods
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Establishment of an aircraft emissions inventory for a given flight traffic scenario requires a knowledge of the engine's emissions over the aircraft's total flight mission.
This inventory is relatively straightforward for CO 2 , H 2 O, and SO x O (in total) emissions because of their direct link with mission fuel burn. For several other emissions
that depend on engine power setting, combustor design, and flight condition, correlation methods have been developed to calculate the exhaust emissions for a
specific engine type using measured data taken during the engine certification process. This correlation is carried out primarily with species of emissions such as NO x ,
CO, HC, or soot by means of a semi-empirical correlation between emissions and principal combustion parameters, using measurement programs involving combustor
rigs and engine systems together with theoretical considerations of the main combustion processes. Exhaust emissions production processes generally are complex
because they involve unsteady physical processes as well as non-equilibrium chemical processes. A fundamental element in the development of formulas to correlate
measured and predicted emission indices is a model of the process based on the relationship of the emission index, chemical kinetic rates, and residence time in the
reaction zone. This model is of great value to manufacturers wishing to predict the emissions performance of a new or development combustor. For an existing design,
a reference correlation method is used to predict emissions on the basis of measured data. Because combustor inlet pressure (p 3 ) and temperature (T 3 ) are the main
parameters involved. For EI(NO x ), the "p 3 /T 3 " method leads to a relationship that, in its simplest form, is as follows:
EI(NO x )/EI(NO x ) ref = (p 3 /p 3ref ) n f(Humidity) for T 3 = T 3ref where n is an exponent in a range between about 0.3 and 0.6 derived from engine and
combustor rig testing.
The parameters p 3 and T 3 come either from measurements or from computational engine simulation. Measurement programs, especially for NO x emission indices,
reveal agreement (compared with testing) of better than 5% (AERONO x , 1995; Brasseur et al., 1998). Emissions of HC and CO depend on the completeness or
efficiency of the combustion process. Test data are well correlated using a combustor loading parameter, which takes account of the residence time of the burning
products and reaction time in the combustor reference volume. As an example, Figure 7-30 shows typical functions of emission indices of NO x and CO versus engine
load for a turbofan engine at different altitudes on the basis of a thermodynamic engine simulation. Unburned hydrocarbons will follow the same trend as CO, but on a
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