Environmental and climate effects as well as econom-ical aspects of aircraft and industrial gas turbine op-eration require the development of gas turbines withincreased efficiency and reduced exhaust emissions.1New gas turbines must be shown to meet the envi-ronmental requirements laid down by internationalregulatory bodies, and industrial gas turbines mustalso meet environmental requirements specified inthe area where they will operate. In the aviation
sector, the International Civil Aviation Organizationis responsible2 for defining environmental regula-tions. For measurements of the exhaust compositionduring the whole development cycle of a new gasturbine, i.e., for the studies of the effects of designparameters on the emission, and for the measure-ments of the final product, less-expensive methodsthan the currently used intrusive methods2 are re-quired.
Nonintrusive measurement techniques used forthese objectives are based on, e.g., Fourier-transforminfrared (FTIR) spectrometry for temperature, CO2,CO, NO, NO2 , and total unburned hydrocarbon mea-surements (emission and absorption spectrometry);narrowband IR spectroscopy for temperature; CO2and CO measurements; and laser-based analysis ofparticles (laser-induced incandescence). These tech-niques were shown to have the potential to overcomethe disadvantages of currently used gas samplingtechniques.3 In addition to offering the capabilities ofconventional gas species determination systems,these techniques have the following benefits:
Y avoid the costly design and manufacture of thecomplex sampling and analysis devices,
Y exclude chemical changes that can occur dur-ing sampling and transport of the sample to the anal-ysis system,
Y avoid the need to demonstrate the representa-tiveness of rake samples, and
Y reduce the risks of turbine damage during mea-surement.
When this research was performed, K. Schäfer ([email protected]) and C. Jahn were with the Fraunhofer Institute for Atmo-spheric Environmental Research, Kreuzeckbahnstrasse 19, Garmisch-Partenkirchen 82467, Germany. They are now with the Institute forMeteorology and Climate Research, Atmospheric Environmental Re-search, Forschungszentrum Karlsruhe GmbH, Kreuzeckbahnstrasse19, Garmisch-Partenkirchen 82467, Germany. P. Wiesen and K.Brockmann are with the Institute for Physical Chemistry, Ber-gische Universität Wuppertal, Gaussstrasse 20, Wuppertal 42097,Germany. When this research was performed, J. Heland was withthe Institute for Physical Chemistry, Bergische Universität Wup-pertal, Gaussstrasse 20, Wuppertal 42097, Germany. He is nowwith DB Systemtechnik, TZF 74.4, Deutsche Bahn AG, Völcker-strasse 5, München 80939, Germany. When this research wasperformed, O. Legras was with the Research Unit, Auxitrol S.A., 5Allee Charles Pathe, 18941 Bourges Cedex 9, France. He is nowwith the Division of Research and Development, ULIS UncodedInfrared Detectors S.A., BP 27, Veurey Voroize 38113, France.
Received 9 June 2004; revised manuscript received 8 November2004; accepted 9 November 2004.
These advantages do not exist if other optical anal-ysis metrology systems4 such as photoacoustic spec-troscopy5 and Raman spectroscopy6 are applied,which require exhaust sampling for gas analysis incells. Further alternatives such as laser-inducedfluorescence7,8 and cavity ringdown spectroscopy8
need highly-sophisticated laser equipment. Low op-erational costs, easy handling, and versatility arenecessary to provide the possibility for an early char-acterization of turbine emissions that represent im-portant information for turbine development.
Our motivation for the research presented here isthe improvement of detection limits for nonintrusiveNO and NO2 mixing ratio measurements in turbineexhaust. During earlier studies3 it was shown thatthe sensitivity of passive FTIR spectrometry for thedetection of NO2 was not sufficient. Better detectionlimits are expected by use of absorption spectrome-try. One possibility would be use of lasers.9 Tuneablediode lasers are often used for absorption spectrom-etry.10,11 However, normally the measurements mustbe taken at a reduced gas pressure to obtain highsensitivity, which is obviously difficult to realize incombustion exhaust in conventional test beds. Alter-natively, a multicomponent measurement in exhaustis possible by use of a globar and a multipass open-path White mirror system for high-resolution absorp-tion spectrometry with a FTIR spectrometer todetermine the mixing ratios of CO2, CO, NO, NO2,and unburned hydrocarbons.3 IR spectroscopy ofgases and vapors makes use of the physical principlethat these molecules exhibit characteristic spectralstructures in the IR, originating from energy transi-tions between specific vibrational and rotational lev-els and the corresponding absorption and emission ofradiation.
The objectives of the research presented here arethe development and test of a prototype of a multi-pass open-path FTIR spectrometry system for nonin-trusive monitoring of the composition of gas turbineexhaust in engine test beds. The sensitivity study forthe detection of NO and NO2, the design of the mul-tipass open-path White mirror system, and the ex-perimental results are described. Conclusions aredrawn for operational use of the method.
2. Experimental Setup
A sensitivity study to estimate the limit for the de-tection of NO and NO2 with the White mirror systemwas performed on the basis of simulations of NO andNO2 spectral signatures in absorption and emissionmeasurement modes in the mid-IR with the aim tostudy the influence of real NO and NO2 mixing ratiodistribution profiles on the spectra simulations.Gaussian and rectangular mixing ratio distributionsof NO and NO2 across the exhaust plume with equalcolumn density as well as different H2O distributionswere investigated. In addition, continuum absorptionwas considered. The calculations were performed fortwo measurement modes: multipass absorption with32 and 16 passes as well as single emission consid-ering different realistic power settings of an aero-
engine, namely, 65%, 82.5%, and 91% NH, whereNH is a definition of the power setting relative to themaximum high-pressure spool rotation speed. Thespectral regions of interest for the retrievals are forNO2 at 1597–1601 cm�1 and 1629–1634 cm�1 and forNO around 1897, 1900, and 1903 cm�1.
A multipass open-path White mirror system wasused in a previous study to reach high sensitivity forthe detection of the different species of interest.3 Thelimiting experimental condition was that the stan-dard White system12 had a cone-shaped beam profilebetween the White mirrors, which lead to integrationof different zones of the plume. For this study a newWhite system, called the Kriesche system, with aparallel IR beam was developed to overcome this dis-advantage by means of monitoring a defined smallcylindrical section of the turbine plume.
The multipass absorption mode sensitivity studieswere continued by laboratory investigations. The in-strumentation was also tested in the real environ-ment at different test beds. The White system wasinstalled inside the turbine test beds and required nooperation of measurement equipment outside the testbed as in a previous study.3 For the measurements, aNicolet FTIR spectrometer (type Magna 560 and 550)was used. This spectrometer was adapted to the Kri-esche system by specially designed transfer optics.
A. Sensitivity Study for the Investigation of NO and NO2
Detection in Turbine Exhausts
The objective of this paper is to study the influence ofreal NO and NO2 mixing ratio distribution profiles onsimulated spectra.
The task was to simulate spectra for the singleemission mode as well as the multipass absorptionmode (with the open multireflection mirror system asdescribed above). The simulations were performed onthe basis of the Forschungszentrum Karlsruhe soft-ware MAPS13–15 with a spectral resolution of0.2 cm�1. The radiation transfer through an exhaustplume was calculated by this line-by-line software onthe basis of the HITRAN96 and HITEMP spectralline database.16,17
The propagation of monochromatic IR radiation isdescribed by the equation of radiative transfer.18 Inthe active absorption measurement mode the IR ra-diation is generated by an IR radiation source (glo-bar). This radiation is absorbed by the molecules onthe way to the detector. In the case of homogeneousopen-path absorption, the radiation damping of an IRlight source is used and the incoming radiation I�� ina certain spectral interval �� [in W�m�2 sr cm�1�] isgiven by the Beer–Lambert law18:
I�� � �B��(TGB)���(L), (1)
where TGB is the temperature (in K) of the source(e.g., globar with emissivity ε) and B�� is the radiationof the source as a Planck function in the spectralinterval �� [in W�m�2 sr cm�1�]. ��� is the transmit-tance (values between 0 and 1, no dimension) along
path L from the source to the detector in the spectralinterval ��. The optical depth is applied from thetransmission: D � exp������. The mixing ratio of theabsorbing species is obtained from the measured IRspectra, yielding path-averaged results along path L.
In the passive emission measurement mode the IRradiation from hot exhaust gases is detected. The hotmolecules emit IR radiation at specific wavelengthsand show characteristic signatures at the same IRwavelengths as in the case of absorption. To measure,turbine exhaust at airports, for examples this passivemode is used.14 In this configuration the thickness ofthe exhaust plume also determines the radiation in-tensity. The radiation transfer can be approximatedby a three-layered radiative transfer problem: a back-ground with radiation Ib, the plume layer with radi-ation Ip and transmittance �p, and a foreground withradiation If and transmittance �f. The received radi-ation I can be described by the corresponding radia-tive transfer equation (index �� is removed forsimplicity)14,15;
I � Ib�p�f � Ip�f � If. (2)
The plume radiation Ip is given by
Ip � B(Tp)�1 � �p), (3)
where Tp is the plume temperature. The plume radi-ation is calculated according to Eq. (3). All quantitiesin Eq. (2) are path-averaged quantities.
1. Input DataThe input data of spatial distribution of temperatureand H2O, NO, and NO2 mixing ratios in the exhaustplume profile are given in Table 1. The type of profileis the same for all parameters and can be described asa Gaussian profile with a shoulder. This profile is anapproach to the profile determined by former intru-sive measurements at the research engine TRACE.3The a priori NO profile was also assumed to be of thesame shape as the CO2 profile. Furthermore, the H2Oprofile was assumed to be of the same shape as theCO2 profile because H2O and CO2 are the main com-bustion products.
The peak mixing ratios of the H2O and the CO2mixing ratio profiles were almost identical and weretaken as equal. In addition, for a different set of sim-ulations, a rectangular profile with the same totalcolumn density across the whole plume profile as forthe Gaussian profile with a shoulder was used. Wecalculated the input values of the NO and NO2 mixingratio profiles from these Gaussian H2O profiles witha shoulder by using different peak values of the mix-ing ratio and consequently of the column density andthe total column density (Table 1).
No foreground influence was investigated. Thebackground temperature was 283K.
2. Simulations of Optical Depths andTransmittancesWe first calculated the optical depths and transmit-tances for the absorption mode for a single pass. Theplume model for temperature and column density of
Table 1. Input Values of Gas Mixing Ratio Profilesa
GAS
NH of 65% NH of 82.5% NH of 91%
C (ppm) q �1017 cm�2� Q �1017 cm�2� C (ppm) q �1017 cm�2� Q �1017 cm�2� C (ppm) q �1017 cm�2� Q �1017 cm�2�
aq is the peak column density in the line of sight perpendicular to the plume at the plume axis with a peak mixing ratio C. Q is the totalcolumn density across the whole plume profile in this line of sight: Gaussian H2O profile with shoulder, NO and NO2 mixing ratio profilescalculated from Gaussian H2O profiles with a shoulder by use of different peak values of the mixing ratio and consequently of the columndensity and total column density.
Table 2. Plume Model of Temperature T and H2O Column Density q for 15 Layers Symmetric to the Plume Axisa
NH of 65% NH of 82.5% NH of 91%
T (K) qH2O �1017 cm�2� T (K) qH2O �1017 cm�2� T (K) qH2O �1017 cm�2�
the gases with 15 layers as given in Tables 1 and 2was used. Then the calculations were performed for16 and 32 passes through the plume.
Figures 1–6 show the graphs of optical depths andtransmittances in three spectral regions �1595–1605 cm�1, 1627–1637 cm�1, 1895–1905 cm�1) forthe power setting NH of 65%. The optical depths areshown for 32 passes, and the transmittances areshown for single pass as well as for 16 and 32 passes.The absorption signatures of single gases as well asthe gas mixture are shown in the graphs of the opticaldepth (Figs. 1, 3, and 5). For all power settings a clearabsorption by NO at 1897, 1900, and 1903 cm�1 andby NO2 at 1597–1601 cm�1 and 1629–1634 cm�1 isvisible. These signatures are enhanced in the multi-pass absorption mode at 32 passes in comparisonwith 16 passes (Figs. 2, 4, and 6). For example, at aNH of 65% the typical transmission is 0.4 in compar-ison with 0.2.
3. Consideration of Radiation ContinuumThe radiation continuum is a wave-number-independent radiation background in gaseous emis-sion spectra. This type of radiation cannot becalculated by line-by-line software codes with aspectral line database alone because it is caused (a)by the overlapping of far wings of single spectrallines and (b) by aerosol radiation emission. Gaseslike CO2 and H2O show significant continuum ab-sorption or emission in certain spectral rang-es.14,15,18,19 Furthermore, uncertainties in thecalculation of instrumental line shapes cause radia-tion continuum effects.
Existing algorithms and software packages such asLOWTRAN cannot be used to determine this radiationbecause these programs are not valid for the hightemperatures in turbine exhausts.
Briefly, a determination of this radiation for ex-haust investigations is possible by analyzing thespectral regions where the simulation of transmis-sion of gases is equal to unity, i.e., without gaseousemission or absorption signatures. These spectral re-gions should be near the spectral regions, which areused for gas mixing ratio retrievals. Under the as-sumption that the radiation continuum is the productof the Planck function of the exhaust gas layer, andthe difference of unity and continuum transmission[see Eq. (3)], it is possible to calculate the continuum
Fig. 1. Optical depths of NO2 and interfering compounds (CO2
and H2O) in the spectral region 1595–1605 cm�1 for the powersetting NH of 65%. The absorption signatures for 32 passes ofsingle gases as well as of the gas mixture (sum) are given.
Fig. 2. Transmissions of NO2 together with H2O and CO2 in thespectral region 1595–1605 cm�1 for the power setting NH of 65%.The absorption signatures for single pass (total transmission) aswell as for 16 and 32 passes are given.
Fig. 3. Optical depths of NO2 and interfering compounds (CO2
and H2O) in the spectral region 1627–1637 cm�1 for the powersetting NH of 65%. The absorption signatures for 32 passes ofsingle gases as well as of the gas mixture (sum) are given.
transmission from the measured radiation.14 Thecontinuum transmission must be calculated for eachmeasurement and each spectral region for the re-trieval of gas mixing ratios. Examples for spectralregions with a gaseous transmission equal to unityfor the different spectral regions, which are used forthe retrieval of NO and NO2 mixing ratios, are1899.5, 1599.6, and 1636.7 cm�1.
B. White Mirror System with a Parallel Beam
A White mirror system and an instrument rack weredesigned taking into account the following consider-ations:
Y The detection of NO2 for different plume mod-els and for different power settings as given above ispossible with a minimum of 16 passes. A maximum ofapproximately 30 passes of the IR beam through theexhausts may be needed for maximum sensitivity ofthe FTIR measurements.
Y The field of view of the FTIR instruments in theplume should be as narrow as possible to improve thespatial resolution.
Y All instruments are implemented in the Whitemirror racks next to the exhaust nozzle of the tur-bine.
Y The White mirror system should be as flexibleas possible for the work in test beds that have differ-ent sizes and that are used for turbines with differentexhaust nozzle configurations and diameters.
1. DesignThe new White system, called the Kriesche system, ischaracterized by a parallel IR beam. It is shown sche-matically in Fig. 7.
The basis of the calculation of such a White systemis the throughput of the spectrometer. The through-
put ��1� of a given spectrometer is defined as theamount of light collected and transformed into a par-allel beam by the first mirror Q behind the aperture.Mirror Q collimates the beam, and the diameter ofthe parallel beam is equal to ØQ. This light is passingthrough the spectrometer. Losses due to absorptionon the mirror surfaces were not taken into consider-ation. The throughput can be calculated by
�I �Øs
2ØQ2
fg2 I1, (4)
Fig. 4. Transmissions of NO2 together with H2O and CO2 in thespectral region 1627–1637 cm�1 for the power setting NH of 65%.The absorption signatures for single pass (total transmission) aswell as for 16 and 32 passes are given.
Fig. 5. Optical depths of NO and interfering compounds (CO2 andH2O) in the spectral region 1895–1905 cm�1 for the power settingNH of 65%. The absorption signatures for 32 passes of single gasesas well as of the gas mixture (sum) are given.
Fig. 6. Transmissions of NO together with H2O and CO2 in thespectral region 1895–1905 cm�1 for the power setting NH of 65%.The absorption signatures for single pass (total transmission) aswell as for 32 passes are given.
where Øs is the diameter of the aperture, ØQ is thediameter of the opposite mirrors Q, and fg is the focallength of mirror Q. I1 is the emitted light of the globarin arbitrary units.
To avoid losses of light in the White system, thethroughput of the White system should be the sameas the throughput of the spectrometer. In the case ofa parallel beam, the diameter of the images on thefield mirror F need to have the same size as theillumination on the opposite mirror G.
ØG � ØF. (5)
The throughput of the White system is defined as
�I � �W �ØG
2ØF2
R2 , or �W �ØW
4
R2 , (6)
where R is the radius of curvature of the field andopposite mirrors.
For a given throughput, the diameter of the beamcan be calculated by
ØE � ��WR2�1�4. (7)
The focal length fE of mirror E is given by
fE �RØE
ØG, (8)
where ØE is the diameter of the beam of mirror E.With the help of mirror E, an image of mirror Q is
generated on the opposite mirrors. The distance bQ
between mirror E and mirror Q to generate the imagesize ØW can be calculated with
1bQ
�1fE
�1
R � fE. (9)
The diameter of such a system depends on the aper-ture of the spectrometer (influence on the throughput
�1) and the size of mirror Q. A reduction of theseparameters will lead to a smaller diameter of theparallel beam in the White system.
The base length, i.e., the distance between the fieldand the opposite mirrors, was chosen to be 4000 mmbecause this is a typical test-bed and exhaust plumedimension. These calculations lead to a transfer passlength of 10, 168 mm to receive a parallel beam in theKriesche system.
2. Adaptation of the Kriesche System to theSpectrometer and the Selected Measurement ModesFor the measurements, a Nicolet FTIR spectrometer(type Magna 560) with the following characteristicswas used: a noise-equivalent spectral radiance withan InSb detector at 3.0 10�6 W �cm2 sr cm�1� and anoise-equivalent spectral radiance with a HgCdTedetector at 1.7 10�6 W �cm2 sr cm�1�. This spec-trometer was adapted to the Kriesche system by spe-cially designed transfer optics. In the Nicoletspectrometer the IR beam is focused in the probeposition. Figure 8 shows schematically the side viewof the spectrometer with the focused beam in theprobe position.
The transfer optics from the spectrometer to theWhite system itself was positioned in the samplecompartment of the Nicolet Magna 560 spectrometer.It consists of a solid ground plate with transfer mir-rors and can easily be placed and fixed inside thespectrometer for absorption measurements.
The probe position was decided to be the best placeto couple the IR beam out of and, after passingthrough the Kriesche system, back into the spectro-meter.
The beam coming from the interferometer is fo-cused in the sample position. With the transfer opticsin the sample position, this focus is shifted somewhatupwards. The beam is made parallel afterwards andguided to the Kriesche system. The beam coming outof the Kriesche system is treated the same way andguided to the detector inside the spectrometer. Figure8 shows this arrangement in the sampling position ofthe Nicolet spectrometer.
Fig. 7. Schematic setup of a Kriesche system with a parallel beam. See text for details.
The FTIR spectrometer can be used in four basicmeasurement modes:
Y single emission mode,Y emission mode with the White mirror system,Y absorption mode with the White mirror system
with a nonmodulated globar source, andY absorption mode with the White mirror system
with a modulated globar source,
The characteristics of the different modes are thefollowing:
(a) Single emission mode. The single emissionmode is the most simple measurement mode: Theradiation of the plume is directly collected at theentrance of the spectrometer.14 The plume tempera-ture and mixing ratios of CO2, CO, and NO can bedetected. Nevertheless, the signal-to-noise ratio withthis configuration does not allow the detection of lowmixing ratios of NO and NO2 with enough accuracy.This configuration needs a radiometric calibrationbefore measurements.
(b) Emission mode with the White mirror system.In this case, the radiation is collected by the spec-trometer after several passes through the plumethrough the White mirror system, enabling a highersignal-to-noise ratio for low mixing ratios. Becausethe reflectivity of the mirrors is not uniform in space,in wavelength, and in time,3 the emission mode withthe White mirror system does not seem to be suitablefor the measurements.
(c) Absorption mode with the White mirror sys-tem with a nonmodulated globar source. For the ab-
sorption mode with a nonmodulated source, fourmeasurements are needed for the evaluation of thetransmission: (1) radiation source on, turbine on; (2)radiation source off, turbine on; (3) radiation sourceon, turbine off; and (4) radiation source off, turbineoff. Therefore measurements with this mode aretime-consuming; and because of the noise in the sin-gle measurements, they are of low accuracy.3 There-fore this mode is not used here.
(d) Absorption mode with the White mirror sys-tem with a modulated globar source. The absorptionmode with a modulated globar source installed in thespectrometer has the advantage that only two mea-surements give the transmission and it is faster thanthe mode described in (c): measurement source on,turbine on; and measurement source on, turbine off.Since only two measurements are needed, the theo-retical error in this configuration should be smallerthan with the mode (c). Another advantage of thismode is that no spectral calibration is needed.
Therefore the two measurement modes selected atthe beginning of the experimental research describedin this study were the single emission mode and theabsorption mode with the White mirror system withthe modulated globar source of the spectrometer (alsocalled multipass absorption mode).
The multipass absorption mode induces use of theWhite mirror system (Subsection 2.B.1) enabling severalpasses through the plume; and an adaptation of theWhite mirror system to the Nicolet spectrometer, allow-ing the globar radiation to be coupled out of the spectrom-eter, to pass through the plume through the mirrorsystem, and then to be coupled into the spectrometer.
For the measurements in the single emission mode,two additional mirrors were placed in the oppositemirror rack. With these two mirrors the emission ofthe plume was transferred to the emission port of theNicolet spectrometer. Figure 9 shows the setup of themirrors for the single emission mode.
The change between the single emission mode andthe multipass absorption during one power setting of
Fig. 8. Transfer optics in the probe position of the Nicolet Magna560 spectrometer.
Fig. 9. Mirror setup for the single emission mode.
the turbine was computer controlled from outside thetest bed. For this switch three adaptations to theFTIR spectrometer were necessary:
Y an optical system to collect IR radiation,Y a movable mirror to switch the mode (IR radi-
ation comes from the optical system and not from theKriesche mirror system and the transfer optics),
Y a second detector to enhance the wave-numberrange and the number of substances for themeasurements.
Figure 10 shows these adaptations to the NicoletMagna 560 spectrometer.
The analytical instrumentation, the Kriesche mir-ror system, and the long transfer pass were inte-grated into two solid racks. The larger rack (oppositemirror rack) contains the analytical devices like theFTIR spectrometer. The opposite mirrors of the Kri-esche system are also placed in this rack. The fieldmirror and mirrors to fold the long transfer pass�10, 168 mm� were placed in a smaller rack (field mir-
ror rack). Other instruments were integrated insideand on top of this rack. The setup of the instrumen-tation in the two racks in an engine test bed at Qi-netiQ, Farnborough (former Defence and EvaluationResearch Agency), is shown in Fig. 11.
C. Laboratory Experiments to Determine Detection Limitsfor NO and NO2
To estimate the limit for the detection of NO2 and NOwith an original White mirror system in multipassabsorption mode, laboratory investigations were car-ried out. The spectra were collected with a Nicoletspectrometer (Magna 550, 50 coadded scans, spectralresolution 0.125 cm�1). The White system was placedin a 405-liter glass reactor and had an optical pathlength of 50 m. This setup has been described in de-tail elsewhere.20
The spectra in the range between 1580 and1620 cm�1 shown in Fig. 12 were taken at room tem-perature and atmospheric pressure with a NO2 mix-
Fig. 11. Instrumentation in an engine test bed at QinetiQ, UK.On the left side is the opposite mirrors rack, and on the right sideis the field mirror rack.
Fig. 10. Adaptations to the Nicolet Magna 560 spectrometer to change between multipass absorption and single emission mode.
Fig. 12. Laboratory absorbance spectra of 3 ppm NO2 (solidcurve) and 800 Pa H2O (dotted curve) between 1580 and 1620 cm�1
ing ratio of 3 parts per million (ppm) and a water-vapour partial pressure of 800 Pa. This ratio is whatone would typically expect in turbine exhaust emis-sions.2 Clear signatures of NO2 are visible. In thespectral regions around 1598 and 1605 cm�1, NO2signatures were observed that are not interfered bywater absorption. These absorption signatures wereused for the estimation of the detection limit of NO2(see subsection 2.A.2).
The spectrum shown in Fig. 13 was taken at roomtemperature and atmospheric pressure with a NOmixing ratio of 4 ppm NO and 800 Pa H2O between1880 and 1920 cm�1. The NO absorption spectra weretaken under the same conditions as the NO2 spectra.The lines at 1897, 1900, and 1903 cm�1 were used toestimate the detection limit of the Kriesche systemfor the detection of NO (see Subsection 2.A.2).
D. Measurements in Engine Test Beds
The standard procedure of turbine exhaust composi-tion measurements for certification purposes is de-scribed by the International Civil Aviation Orga-nization.2 The sampled exhaust probe of the turbinein the test bed by intrusive measurement systems istransferred to the analytical instruments through asingle transfer line. Care must be taken to avoidchemical reactions in the transfer line. The temper-ature is maintained at �423 5� K to avoid conden-sation of water and volatile hydrocarbons. At the endof the transfer line the sample is analyzed for CO2and CO by nondispersive infrared absorption, for un-burned hydrocarbons by a heated flame ionizationdetector, and for NOx with a chemiluminescenceanalyzer. Each instrument is calibrated with a cali-bration gas traceable to national or internationalstandards.
During the first experiments with the nonintrusiveequipment in an engine test bed, high noise was ob-served in all IR spectra at the higher power settingsof the turbine. During these measurements thestrongest vibration was observed at a frequencyof 200 Hz. Even at a power setting of less than
5000 kilopound thrust, an acceleration of more than1 g �9.81 cm s�2 acceleration� was observed. The vi-brations at a higher frequencies (400 and 600 Hz)were in resonance with a frequency at 200 Hz. It wasfound that the frequencies did not shift for differentpower settings, thus leading to the assumption thatthese were resonance frequencies of the mirror racks.The corresponding isolation of the racks for low fre-quencies was difficult because the vibrations wereproduced by the turbine and were transferred to theracks by the air and the floor. To prevent the mirrormounts from these oscillations during the followingexperiments, each mount was stabilized with threeadditional suspensions. Further vibration tests dur-ing the other campaigns have shown that these mod-ifications of the mirror and the instrument rack led toa significant reduction of the propagation of noisevibration in the system. No vibrations of the racksand the mirror mounts were observed during theother trials.
The influence of sound pressure at high noise levelson the FTIR spectrometers was unclear at the begin-ning of the measurements in the engine test beds.Laboratory investigations were carried out to test theinfluence of sound pressure on a Nicolet FTIR spec-trometer. Sound levels of �100 dB were produced ina frequency range from 200 to 400 Hz. In Fig. 14 acomparison of the FTIR background spectrum col-lected with and without this acoustic noise is shown.The noise influence on the background spectra of theFTIR is clearly visible. Much higher noise levels (upto 140 dB) in a broader frequency range are prevail-ing in a test bed during cruise and takeoff conditionsof the turbine. The spectra measured during the lab-oratory investigations showed that the FTIR spec-trometer was sensitive to noise even at lower noiselevels and in a defined narrower frequency range.Accordingly, sufficient noise isolation of the opticalinstruments was a prerequisite for the following mea-surement campaigns.
A much more serious problem was the radiationemission of the exhaust plumes of some turbines.
Fig. 13. Laboratory absorption spectra of 4 ppm NO (solid curve) and 800 Pa H2O (dotted curve) between 1880 and 1920 cm�1 atatmospheric pressure and room temperature.
During the measurement campaigns in which tur-bines with very high exhaust temperatures, such as1000 K, were studied, the spectra taken in the mul-tipass absorption mode were covered by a high radi-ation background noise. Simple calculations showedthat the energy of the IR radiation emitted by theplume at temperatures higher than �750 K is muchhigher than the radiation of the IR light source (glo-bar) used in the spectrometer. Therefore radiation ofthe plume leads to the saturation of the detector. Toovercome these saturation problems, it was neces-sary to decrease the nonmodulated radiation by add-ing optical filters and an aperture at the entrance ofthe spectrometer. Filters for the spectral regions ofCO and NO detection were used. These filters re-duced the emission radiance intensity of the plumeand it was possible to detect CO and NO under take-off conditions, which was not possible by the othernonintrusive methods under this condition. On theother hand, modern civil turbines with high by passratios have low exhaust temperatures, so these prob-lems will not occur.
3. Results of Simulations and Measurements
A. Simulations and Retrieval of Spatial Distributions
The spectral regions for the mixing ratio retrievalsfrom emission and absorption spectrometry are given
in Table 3. The NO signatures at 1897, 1900, and1903 cm�1 are characterized by the same relativevariation in dependence from the spatial distributionof the NO mixing ratio. The same is valid for the NO2
signatures at 1600 and 1630 cm�1.The calculations show that in the single emission
mode the Gaussian distribution of NO and NO2 in theexhaust plume produces up to 30% higher line inten-sities than the rectangular distribution for the samecolumn density. Otherwise use of a rectangular apriori profile for the inversion of the spatial distribu-tion of NO and NO2 mixing ratios in the exhaustplume gives a higher column density than use of theGaussian a priori profile.
This dependence of line intensities is much smallerthan the corresponding influence of different profilesfor the distribution of H2O mixing ratios. The H2Odistribution profile can be retrieved from lines withdifferent lower state energies, i.e., lines with differenttemperature dependence.
Consequently, information about the spatial distri-bution of NO and NO2 mixing ratios in the exhaustplume can be retrieved only if
Y the signal-to-noise ratio of the measurement isvery high (much higher than a factor of 10),
Y the radiometric calibration of the measure-ments is very precise (accuracy much better than10%),
Y the aerosol and continuum radiative effects aretaken into account, and
Y the retrieval procedure of the H2O profile isvery accurate (accuracy much better than 10%).
In the multipass absorption mode the capability for adistinction between the different mixing ratio profilesis better at high power levels.
B. Laboratory Investigations
The experiments described in Subsection 2.C wereperformed at room temperature. The NO2 absorptionat 1599 cm�1 shows an absorbance of 0.35 corre-sponding to 3 ppm NO2 (see Fig. 12). The noise ob-served in the spectra corresponds to absorption of�0.01. For a signal-to-noise ratio of 3 to 1, this leadsto a detection limit of 0.26 ppm. The noise during thetest-bed measurements is much higher than in thelaboratory study. Assuming a 20 times higher noise,the detection limit will be 5.2 ppm for the laboratorysystem.
The optical path length in the laboratory setup wasfg. The Kriesche system can be aligned between 6 and32 passes. Taking an average plume diameter of 1m
Fig. 14. Comparison of the background spectrum collected by theFTIR spectrometer (given in millivolts, i.e., arbitrary units) with-out acoustic noise (top) and with a noise level of 100 dB (bottom) ina frequency range from 200 to 400 Hz.
Table 3. Spectral Regions for the Mixing Ratio Retrievals of H2O, NO,and NO2
in combination with 30 passes, the Kriesche systemwill have a 30-m optical path length; and thereforethe detection limit for NO2 will be approximately9 ppm.
The NO line at 1900 cm�1 shows an absorbance of0.091, and the line at 1912 cm�1 shows an absorptionof 0.081 for a mixing ratio of 3 ppm NO (see Fig. 13).The noise observed in the spectra corresponds to ab-sorption of 0.003. Under the same assumption as forNO2, a detection limit for NO can be estimated. Thedetection limit for the absorption at 1900 cm�1 wasfound to be 6 ppm, and for the line at 1912 cm�1 itwas 7.4 ppm.
Taking the behavior of the detector at lower reso-lution into account, the detection limit for NO can bebetween 2 and 3 ppm. The estimated detection limitsfor NO lines that were not interfered by water ab-sorption are summarized in Table 4.
C. Results of Measurements in Engine Test Beds
According to Subsection 2.B.2, the FTIR spectrome-ter was used during engine test-bed measurements inthe three modes:
Y single emission mode to determine the CO mix-ing ratio at idle and cruise turbine conditions (thespectra were too noisy at takeoff conditions),
Y multipass emission mode, andY multipass absorption mode to measure low CO
and NO mixing ratios (see Figs. 15 and 16).
During the turbine condition 91% NH (takeoff), thefollowing column densities for CO were measured:1.62 1016 cm�2 by the intrusive method with amoving probe (19 points on a horizontal line) and1.7 1016 cm�2 by the Nicolet FTIR spectrometerused with the Kriesche system in multipass emissionmode. This is a difference of 5%.
Examples of calculated profiles obtained from spec-tra measured with the FTIR spectrometer linked tothe Kriesche system for CO are given in Fig. 15 and
for NO in Fig. 16. Figure 16 shows that the nonin-trusive measurement of the NO mixing ratio is 35%higher than the intrusive measurement. One reasoncould be a chemical transformation of NO into NO2 inthe tubes used for intrusive measurements.
The three important conclusions of the engine test-bed measurements are
Y A low CO mixing ratio can be measured with aFTIR spectrometer linked to a White mirror system.
Y A NO mixing ratio can be measured with thesame optical system (spectrometer and White mirrorsystem).
Y The multipass absorption mode seems a bitmore sensitive than the multipass emission mode be-cause the lines appear more clearly. Furthermore,the absorption mode needs no calibration and theemission mode does; i.e., in the absorption mode,smoke in the exhausts does not affect the retrievals inthis measurement mode. Moreover, to improve thesignal-to-noise ratio in the absorption mode, there isthe possibility to increase the intensity of the back-ground light source (globar).
Three different numbers of passes were tested: 10,18, and 32. The optimum number was found to be 18passes through the plume. Absorption was too strongwith 32 passes, leading to noisier spectra, and 10passes did not give a long enough path to obtain clearsignatures.
NO2 mixing ratios could not be determined becauseof the noise problems described in Subsection 2.D,
Table 4. Estimated Detection Limits for Different NO Lines WithoutWater-Vapor Interference
aCalculated under the assumption of a signal-to-noise-ratio of 2to 1 and 20 times higher noise in the spectra measured in theturbine exhaust gas.
bDetection limit estimated by taking into account a higher sen-sitivity of the detector at a lower resolution.
Fig. 15. Inversion results of intrusive measurements and mea-surements taken with the Kriesche system for a CO mixing ratioacross the plume versus the distance to the plume axis at 91% NH(takeoff turbine conditions).
Fig. 16. Inversion results of intrusive measurements and mea-surements taken with the Kriesche system for a NO mixing ratioacross the plume versus the distance to the plume axis at 91% NH(takeoff turbine conditions).
which could not be solved during these investiga-tions.
4. Summary and Conclusions
The objective of this study was the improvement ofdetection limits for NO and NO2 mixing ratio mea-surements in turbine exhausts by FTIR spectrome-try. It was known from previous research that thesensitivity of passive FTIR spectrometry for detectionof NO2 mixing ratios in turbine exhausts is not suf-ficient. According to sensitivity studies, multipassmode measurements and a White mirror system werefound to be essential for the retrieval of NO2 abun-dances in gas turbine exhaust plumes.
The laboratory studies supported the definition ofexperimental parameters in this study. It was foundthat the detection limits for all other compounds (COand NO) are better in the absorption measurementmode in comparison with the single emission mode.The minimum detection limits from laboratory mea-surements for the multipass absorption mode for atypical gas turbine plume of 50 cm in diameter, aFTIR spectrometer with 0.25-cm�1 resolution, andwith noise-equivalent spectral radiance values be-tween 1.7 and 3.0 10�6 W�cm2 sr cm�1� are approx-imately 100 ppm for CO2,3 4 ppm for CO,3 6 ppm forNO, and 9 ppm for NO2.
Simulations of NO and NO2 spectral signatures inall modes (absorption and emission spectrometry) inthe mid-IR showed that information about the spatialdistribution of mixing ratios in turbine exhaustplumes can be retrieved.
A novel multipass reflection system that is basedon the White mirror system (Kriesche system) with aparallel IR beam to monitor a defined small cylindri-cal section of the turbine plume was developed. AFTIR spectrometer was adapted to the White mirrorsystem by specially designed transfer optics to focusthe IR beam in the probe position. The FTIR spec-trometer was used in three different modes: singleemission mode without the White mirror system,multipass absorption mode, and multipass emissionmode. This instrumentation was tested in the realenvironment at different turbine test beds. Low NOand CO mixing ratios could be measured with anoptimum of 18 passes through the plume in the mul-tipass absorption mode. The requirements to detectspatial distribution of NO and NO2 mixing ratios inthe exhaust plume as the signal-to-noise ratio, theradiometric calibration, the consideration of aerosoland continuum radiative effects, as well as the accu-racy of the retrieval procedure of the H2O profile weredefined.
The authors acknowledge the European Commu-nity for financial support of this research under theIndustrial and Materials Technologies Programme(Brite-EuRam III) through contract BRPR-CT98-0618 and scientific officer Reiner Dunker for supportthroughout the project.
We thank the partners within this project who co-operated with us to use engine test beds and provided
us with intrusive measurement results, namely,Rolls Royce plc (Roger Burrows), Fiat Avio (PasqualeDi Martino), Snecma (Nadine Harivel), and the De-fense Evaluation and Research Agency (which is nowQinetiQ) (Chris Wilson), as well as the Physical Lab-oratory of the University of Reading (Moira Hilton)who provided filters for the FTIR spectrometer.
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