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FTIR-Emission-Spectroscopy detects the thermal radiation of hot exhaust gases, yielding all information about its compounds during one measurement. Apart from the interpretation of smoke stack measurements, FTIR-Emission-Spectroscopy as a remote sensing technique was further developed for analyzing layered plumes, especially aircraft exhausts in a program of the German Science Foundation (DFG) on the effect of air traffic on the environment. The measurements shall be used as input data for model calculations and to validate the extrapolated emission data at flight altitude. The evaluation of the spectra with respect to the gas composition contains a line-by-line calculation of the transmittances of several layers of the exhaust plume (temperature- and concentration-gradients) followed by the radiative transfer through the medium towards the detector. The spectral input data are taken from the HITRAN 92 database. After the spectroscopic determination of the plume temperature and its profile from the CO2-band around 2400 cm-1, one obtains the total mass of the single gas species in the field of view of the spectrometer. Comparing the measured data for CO2 with the theoretical emission index from ideal stoichiometric combustion, one obtains the emission indices for the other measured species. Knowing the fuel consumption of the engine, one gets the emission rates of the compounds in g/s. Several engine types, old fashioned engines (no bypass) and modern JT8 and CFM56 bypass engines at different thrust levels have been analyzed. H2O, CO2, CO, and NO concentrations can be derived immediately from the measurements right behind the nozzle exits, where the temperature profile is known to be homogeneous. The retrieval of the measured data far behind the nozzle exit uses the multilayer plume model. Formaldehyde and other hydrocarbon species are seen in the spectra and shall be implemented in the computer code. Apart from future applications for the turbine development and the engine-status control after a certain flight time, this remote sensing system can deliver emission data of aircraft engines and the temperature decay of the exhaust plumes at all altitudes.
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