Erwin Lindermeir

Nonintrusive optical measurements of aircraft engine exhaust emissions and comparison with standard intrusive techniques

Nonintrusive systems for the measurement on test rigs of aeroengine exhaust emissions required for engine certification (CO, NO(x), total unburned hydrocarbon, and smoke), together with CO(2) and temperature have been developed. These results have been compared with current certified intrusive measurements on an engine test. A spectroscopic database and data-analysis software has been developed to enable Fourier-transform Infrared measurement of concentrations of molecular species. CO(2), CO, and NO data showed agreement with intrusive techniques of approximately ?30%. A narrow-band spectroscopic device was used to measure CO(2) (with deviations of less than ?10% from the intrusive measurement), whereas laser-induced incandescence was used to measure particles. Future improvements to allow for the commercial use of the nonintrusive systems have been identified and the methods are applicable to any measurement of combustion emissions.

Calibration of a Fourier transform spectrometer using three blackbody sources

A procedure to calibrate a Fourier transform spectrometer is presented. Blackbody sources of three different temperatures are used to eliminate errors in the calibration that result from the limited accuracy of the temperature measurement of the calibration sources. With three spectra of blackbodies it is possible to assume that the temperatures are unknown variables as are the parameters of the functions that describe the spectrometer. A nonlinear Gaussian balancing calculation is used to determine these unknown variables and to minimize the influence of noise. A comparison between the results obtained with this method and a conventional calibration procedure is presented.

Thermal IR radiative properties of mixed mineral dust and biomass aerosol during SAMUM-2

Ground-based high spectral resolution measurements of downwelling radiances from 800 to 1200 cm−1 were conducted between 20 January and 6 February 2008 within the scope of the SAMUM-2 field experiment. We infer the spectral signature of mixed biomass burning/mineral dust aerosols at the surface from these measurements and at top of the atmosphere from IASI observations. In a case study for a day characterized by the presence of high loads of both dust and biomass we attempt a closure with radiative transfer simulations assuming spherical particles. A detailed sensitivity analysis is performed to investigate the effect of uncertainties in the measurements ingested into the simulation on the simulated radiances. Distinct deviations between modelled and observed radiances are limited to a spectral region characterized by resonance bands in the refractive index. A comparison with results obtained during recent laboratory studies and field experiments reveals, that the deviations could be caused by the aerosol particles’ non-sphericity, although an unequivocal discrimination from measurement uncertainties is not possible. Based on radiative transfer simulations we estimate the aerosol’s direct radiative effect in the atmospheric window region to be 8 W m−2 at the surface and 1 W m−2 at top of the atmosphere.

Observation of NO and NO2 in the young plume of an aircraft jet engine

For the first time the emission indices of NO and NO2 were measured simultaneously in-flight by a non-intrusive spectrometric measurement setup. Moreover the method described here is capable of determining the (static) temperature of the exhaust gas and emission indices for H2O and CO at the same time with a precision of 1–4%. After a very brief description of the experimental setup, results of measurements are shown. They reveal mean emission indices for EI(NO2) of 1 g/kg and 3–5 g/kg for EI(NO) (estimated accuracy: 20–25%). The ratio of concentrations [NO2]/[NOx] of 12–22% is by a factor of approximately two higher than most values cited for other engines. Based on the bulk mean properties of the plume it is shown that the temperature determination is accurate to several Kelvin.

Calibration of a Fourier transform spectrometer using three black body sources

A procedure to calibrate a Fourier transform spectrometer is presented. Blackbody sources of three different temperatures are used to eliminate errors in the calibration arising from the limited accuracy of the temperature measurement of the calibration sources. With three spectra of blackbodies it is possible to assume that the temperatures are unknown variables, as are the parameters of the functions describing the spectrometer. Nonlinear Gaussian balancing calculation is applied to determine these unknown variables and to minimize the influence of noise. A comparison between results obtained with this method and a conventional calibration procedure is presented.

Evaluation procedure for nonintrusive exhaust gas analysis of jet engines using FTIR spectroscopy

A method for the non-intrusive determination of temperature and concentrations of the exhaust gases of aeroengines will be presented. A MIROR-type FTIR spectrometer is used to measure spectra of the IR radiation emitted by the hot gases in the exhaust. New evaluation software, specially developed for this application, is described. The software permits line-by-line radiative transfer modeling of the radiance emitted by the exhaust of these engines. Least squares fitting routines are then used to match the measured with the modeled spectrum, thereby determining the unknown quantities, i.e., temperature and species concentrations. Results of measurements aboard aircraft are presented. The achieved accuracy in temperature is estimated to be better than 1 percent. Comparison of the values determined for the NOx emission index with results from correlation models show deviations of 15 to 20 percent and are thus within the accuracy limits claimed for both methods.

FTIR airborne measurement of aircraft jet engine exhaust gas emissions under cruise conditions

A flight qualified Fourier transform infrared spectrometer has been built by applying the MIROR principle (Michelson interferometer with rotating retroreflector) where an eccentrically rotating retroreflector generates optical path differences. The unique optical design is especially suiting the rough environment of airborne missions. The purely optical and passive method in no way influences the gases as sample collecting procedures are likely to do. It is able to deliver true space/time resolved spectra of several species in the exhaust plume simultaneously. First measurements aboard a civil jet aircraft have been performed, successfully collecting spectra of the infrared radiation emitted by the hot exhaust gases just behind the engines nozzle. The spectra were radiometrically calibrated and column densities of trace gases in the plume and in the for- and background as well as gas temperatures were calculated applying inversion algorithms. From these then emission indices (mass pollutant per mass kerosene) of the engine for specific trace gases were determined.

Spectroscopic Measurements from Space with the FOCUS Sensor System to analyse Gas and Smoke Properties of high Temperature Events

In 1998 the European Space Agency (ESA) selected the proposal FOCUS as the first and only Earth observation experiment for the Early Utilization Phase of the International Space Station (ISS). FOCUS consists of imaging sensors in the visible and infrared spectral region as well as an infrared Fourier transform (FTIR) spectrometer. It aims at autonomous detection and geolocation of high temperature events (HTE) on Earth like forest and bush fires, volcano eruptions or coal seam fires. Furthermore HTEs and their emissions are to be analyzed with respect to their spatial extension, temperatures, gas and smoke composition and distribution. This will be achieved by a combination of spectral and imaging data of the FOCUS sensor system and analyses based on unique fusion of these data. FOCUS is scheduled to be launched with ISS utilization flight UF-3 in September 2004.

Evaluation of FTIR emission spectrometry for the determination of turbine exhaust composition in test beds

The capability of taking non-intrusive species measurements in a jet plume of a modified mid-size low by-pass aero-engine running on a sea level test bed at several thrust levels was demonstrated. Also conventional intrusive measurements were performed with a spatially resolved method using a traversing single-point sampling probe which fulfills ICAO standards. The FTIR spectrometry measurements included both emission and absorption mode with a multi-path reflection compartment as well as the single emission mode. Due to the lack of a common/unique definition for the exhaust plume diameter it was found that the column density was the best measure to compare the different techniques. The FTIR engine measurement results for CO2, CO, and NO have been proven to be in agreement with the intrusive data within plus or minus 30%. Several error sources during the radiometric radiance calibration were identified which lead to uncertainties in the FTIR retrievals, namely (1) incomplete knowledge of the optical surface emissivities, (2) incomplete knowledge and inhomogeneities of the optical surface temperature, and (3) undefined instrumental drifts and non-linearities during the calibration.

Determination of exhaust composition in turbine test beds by FTIR emission spectrometry

A modified mid-size low by-pass aero-engine running on a sea level test bed was used for measurements with non-intrusive demonstrator systems and currently used gas sampling analysis techniques. A novel open-path White mirror system was developed and installed in the test bed to enhance the sensitivity of non-intrusive FTIR spectrometry. A comparison was made of the different measurement techniques at several engine thrust levels i.e. gas concentrations. This included the emission and absorption mode of the FTIR-spectrometers with the multi-path reflection compartment as well as the single emission mode. A new calibration procedure with a hot cell filled with CO (temperatures 300 to 750 K) was developed and used to calibrate the FTIR instruments. Retrieval results from FTIR measurements were obtained by using a rectangular and Gaussian distribution profile of temperature and gas concentrations in the plume. The FTIR measurement results for CO2, CO, and NO have been proven to be in agreement with the intrusive data. The deviations were generally in the order of plus or minus 30%, i.e. comparable to the day-to-day variations of the engine emissions. NO2 could be detected in the absorption mode only.