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Optical Characterization of Dimethyl Ether (DME) forLaser-based Combustion DiagnosticsÖ. ANDERSSON a , H. NEIJ a , J. BOOD a , B. AXELSSON a & M. ALDEN aa Department of Combustion Physics , Lund Institute of Technology , Box 118, Lund, S-22100, SwedenPublished online: 20 Jan 2011.

To cite this article: Ö. ANDERSSON , H. NEIJ , J. BOOD , B. AXELSSON & M. ALDEN (1998) Optical Characterization ofDimethyl Ether (DME) for Laser-based Combustion Diagnostics, Combustion Science and Technology, 137:1-6, 299-322, DOI:10.1080/00102209808952055

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Optical Characterization of DimethylEther (DME) for Laser-basedCombustion Diagnostics

b. ANDERSSON-, H. NEIJ, J. BOOD, B. AXELSSON

and M. ALDEN

Department of Combustion Physics, Lund Institute of Technology,Box 118, S-221 00 Lund, Sweden

(Received 4 April 1998; In final form 17June 1998)

Optical characteristics of dimethyl ether (DME) are presented, with emphasis on laser-basedcombustion diagnostics. OME is a well-known substance which has excellent properties as fuelfor compression ignition (CI) engines. It is also believed to have suitable properties for laserdiagnostics in CI engines, but reports of its optical properties are sparse in the literature. OM Ehas therefore been investigated by flame-emission, optical absorption, laser-induced fluores­cence (UF), Raman spectroscopy, and rotational CARS. A preliminary evaluation of thepotential for measuring NO in a OME flame is also presented. The Raman cross section ofOME is more than twice as large as that of methane. DME absorbs in the VUV, but oneabsorption band extends into the UV where many tunable lasers radiate. This tail is displacedtowards longer wavelengths with increasing temperature. Excitation at 193nm yields astructured fluorescence between 350-550nm. The DME rotational CARS signal is -10 timesweaker than that of nitrogen, and the non-resonant susceptibility is 9 times that of nitrogen.

Keywords: Laser diagnostics; dimethyl ether; alternative fuels; absorption spectroscopy; laser­induced fluorescence; Raman scattering; rotational CARS

INTRODUCTION

During the last two decades, laser spectroscopic techniques have beenproven to be powerful tools for investigating combustion processes ininternal combustion (IC) engines. Ie engine combustion is characterized byturbulent conditions, unsteady fluid dynamics, complex chemical kinetics,

*Corresponding author.

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300 o. ANDERSSON et at.

and non-equilibrium thermodynamics. These processes are extremelycomplex and difficult to model. Laser methods provide non-intrusivemeasurements with high spatial and temporal resolution. In many cases theyalso provide the possibility for two-dimensional imaging of temperature andconcentration fields, making them ideal tools in the development andvalidation of combustion models (Eckbreth, 1996). There are numerousexamples of the application of laser diagnostics in engine research, see e.g.,Eckbreth (1996) and references therein.

The absorption bands of many species of combustion interest reside in theUV spectral region, as is the case e.g., for O2, NO, and many radicals likeOH. Unfortunately, the hydrocarbon components of many fuels have broadabsorption bands in the same wavelength region. As a result unwantedsignal interference from fuel or depletion of the laser beam may arise,making optical measurements almost impossible. Even in cases when fuelinterference is not present, combustion intermediates in the flame, especiallypartially burned hydrocarbons and PAH, may exhibit similar effects duringflame propagation (Knapp et aI., 1996). Particulates formed during com­bustion may also be a cause of beam depletion and fouling of windows fittedfor optical access to the combustion chamber, especially in compressionignition (CI) engines (Brugman et af., 1997).

Much interest is presently attracted to CI engine combustion as a result ofthe tightening emission standards, especially for oxides of nitrogen (mainlynitric oxide) and particulates. There is an inherent dilemma in thesimultaneous reduction of both these emissions, since the nitric oxide(NO) production is favored by long residence times and high temperatures,conditions under which soot is oxidized in the cylinder. The investigation ofCl engine combustion by means of optical methods is limited due to theoccurrence of particulates and the fact that diesel fuel absorbs heavilythroughout the UV region. Lately, there have been reports on the use ofdimethyl ether (DME) as a substitute fuel for diesel (Sorensen andMikkelsen, 1995; Fleisch et af., 1995). Operating CI engines with OMEinstead of diesel is claimed to reduce NO emissions by 75% and virtuallyeliminate soot, meeting the California Ultra Low Emissions vehicle (ULEV)regulation for medium duty vehicles, with a thermal efficiency equivalent todiesel operation (Sorensen and Mikkelsen, 1995; Fleisch et af., 1995). Thisobviously makes DM E interesting both as an alternative fuel and as an aidfor making optical investigation of combustion processes in CI engines.

OM E is the simplest ether compound and has the chemical structureCH)-O-CH). Thus, it has no carbon-carbon bonds and has excellentauto-ignition characteristics (Fleisch et af., 1995). OME is a well-known

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DIMETHYL ETHER CHARACTERIZATION 301

substance and is for example used as propellant for spray cans, since it isnon-toxic and environmentally benign (Sorensen and Mikkelsen, 1995;Fleisch et al., 1995). Recently it has been found that OME can be producedfrom natural gas in large quantities at low cost (Hansen et al., 1995). Thesefacts imply that OME may become important as an alternative fuel in thefuture.

From a diagnostic point of view, OME is interesting mainly for tworeasons. First, it is important to investigate the mechanisms behind itsexcellent performance as a fuel. Second, since it is believed that OME yieldsless spectral interference than diesel, experiences from OM E measurementsmight be used to improve the diesel combustion process. Some physicalproperties of OM E are collected in Table J.

Even if OM E is a well-known substance, little information about itsoptical characteristics has been found in the literature. Some measurementsof the vacuum ultraviolet (VUV) absorption spectrum have been found.Early measurements were performed using hydrogen or deuterium lamps aslight sources, whereas more recent investigations have been performed usingsynchrotron radiation (Harrison and Price, 1959; Hernandez, 1963; Pacey,1973; Koizumi et al., 1986; Suto and Lee, 1988; Bremner et al., 1991).However, the information in these reports is not detailed in the region above190 nm, where most tunable laser sources radiate. Investigations of thevibrational Raman spectrum of OME have been reported (Taylor andVidale, 1956), but none have been found that estimates the Raman crosssection. No investigations of flame emission or laser-induced fluorescence ofOME have been found.

TABLE I Properties of dimethyl ether at 20°C and I atm, except" at 25°C and I atm

Property Reference

FormulaMolecular weightLiquid densityVapor pressureBoiling pointHeat of vaporizationFlash pointSelf ignition temperatureFlammability limitsLower heating valueLaminar flame speed

CHJ-O-CH,46.07gjmol

0.67 kgjl5.\ bar

- 24.9°C410kJjkg-41°C235°C

3.4%-27% by volume"28.430 MJjkg

45.6cmjs (at ¢ ee 1.19"·)

(CRC, 1982)(CRC,1982)

(Kapus and Ofner. 1995)(Begild Hansen et al., 1995)

(CRe, 1982)(Kapus and Ofner, 1995)

(Boglid Hansen et al., 1995)(Boglid Hansen et al., 1995)

(Glassman, 1987)(Sorensen and Mikkelsen, 1995)

(Wagner and Dugger, 1955)

•• tjJ is the fuel/air mass flow ratio at maximum flame speed. divided by the fuel/air ratio at stoichiometricconditions.

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The current work aims at presenting some optical properties of OMErelevant to laser-based combustion diagnostics. However, a full spectro­scopic evaluation is out of the scope of this work and was not attempted.

FLAME EMISSION MEASUREMENTS

Flame emission spectroscopy is one of the simplest methods in opticaldiagnostics of combustion processes, yielding information on locations ofradicals, soot etc. Flame emission spectra from a laminar diffusion flamewere recorded, using OME as fuel. For comparison, spectra were alsorecorded using methane as fuel. The experiments were performed in alaminar counterflow diffusion burner consisting of two concentric pairs ofnozzles, mounted opposite to each other in the vertical direction. The fueland air issued from the inner nozzles of the upper and lower burner head,respectively. To stabilize the flame, a coflow of nitrogen was set up in theouter nozzles. The two opposite flows of fuel and air met in a stagnationplane, where a reaction zone was formed. The laminar diffusion flameexhibited two luminous discs; a blue disc on the fuel lean side of thestagnation plane and a yellow disc on the fuel rich side.

The flame emission from a 300 urn wide region around the stagnationplane was imaged at right angles by an acrornatic lens onto the entrance slitof a Jarrel-Ash Monospec 18 spectrograph (300 lines/rum, dispersion 21 tun!mm). The spectrum at the output of the spectrograph was registered with animage intensified CCO camera (Princeton Instruments ICCD 576S/RB-T),having 374 by 576 pixels, each 25 urn wide.

The spectrally resolved flame emissions are shown in Figure I, where thesolid line represents the methane/air flame and the dotted line represents theDM E/air flame. The methane/air flame exhibits the well-known emissionsfrom OH, CH and C2 radicals. It is clear that the OME/air flame shows aless luminous emission from CH and C2 radicals, while the emission fromOH is nearly the same. One explanation for the apparently lowconcentrations ofCH and C2 is the tendency of DME to form formaldehydein atmospheric pressure flames. The reaction then continues by hydrogenabstraction, resulting in minute concentrations of CH and C2 (Oagautet al., 1996). The flame emission was investigated at various heights in thediffusion flame. Apart from a significantly lower emission from flameradicals in the DM E/air flame, no other differences were found between thefuels.

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12

10~

::i.;~ 8z-."icQ.)

6.5c0'Vi 4'"·6~

2

DIMETHYL ETHER CHARACTERIZATION

CH: A't.- x'n

C:A'n .x'n, . .L\v=0

liv=+l

303

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)

FIGURE I Flame emission spectra from the reaction zone of a laminar counterflow diffusionflame. The spectra have not been corrected for the detector spectral response. Solid line:methane/air flame. Dotted line: DM E/air flame.

ABSORPTION MEASUREMENTS

For successful diagnostics in a combustion environment it is essential toknow in what spectral region the fuel absorbs radiation. From previousinvestigations it is known that the absorption spectrum of DME resides inthe vacuum ultraviolet (VUV) region, but has a tail on the long-wavelengthside of 190 nm (Harrison and Price, 1959; Hernandez, 1963; Pacey, 1973;Suto and Lee, 1988; Bremner et al., 1991). Tunable laser radiation is mainlyavailable above 190nm. Absorption measurements were therefore per­formed in a flow rig and in a low pressure cell, to investigate how importantthis tail might be in practical diagnostics. A Cathodeon deuterium lamp(R07) was used for generating continuous radiation over the range 170­700 nm, with an intense UV continuum (Saunders et al., 1978). Thisreference radiation passed a laminar flow of DME (purity 99.8%) and wasthen imaged onto the slit of alSO mm spectrograph (Acton ResearchSpectra Pro-ISO, 300 lines/rum, blaze 300 nm) equipped with the same CCDcamera that was used in the emission measurements. The stream of DMEissued from a nozzle with a diameter of 10 mm and could be heated from300 K to 550 K by means of heating bands applied on the supply pipes.

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304 b. ANDERSSON et al.

,

The exit temperature was monitored by a thermocouple which intermittentlywas inserted into the stream of DME.

A reference spectrum was acquired by recording the emission from thedeuterium lamp that was transmitted through the laboratory air. A DMEspectrum was acquired in the same fashion, by transmitting the lightthrough the DME flow. The absorption spectrum was obtained by takingthe intensity ratios of the DME and reference spectra at each wavelength.

The absorption properties of DME were investigated for wavelengthsbelow 450 nm, but absorption was only detected below 205 nm. In Figure 2,the absorption spectrum of DME is shown for various temperatures. Thespectrum was obtained using long exposure times and binning of the CCDchip in one direction, and thus appears to be noiseless. Since the absorptionextends into the VUV, only the tail of the first band is shown. Forcomparison, the absorption spectrum of heated methane is also shown inFigure 2. Evidently, the radiation is strongly absorbed by DME in thisregion. Furthermore, a broadening of the spectrum towards longerwavelengths was observed with increasing temperature, which to theauthors' knowledge has not been reported before. With the present spectralresolution, the absorption spectrum appears to be featureless. Theabsorption measurements were repeated within a more narrow temperature

-300K.DME_340 K. DME _

390K, DME .. , ..44OK,DME_._. _

500K.DME_ .. _550 K, DME _._._.

390 K. methane ------

195 200 205 210 215

Wavelength (nm)

220 225 230

FIGURE 2 Absorption spectra of DME recorded in flow rig for temperatures 300-550K.The absorption spectrum of methane is also shown, as reference.

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DIMETHYL ETHER CHARACTERIZATION 305

interval in a low-pressure cell, containing DME at pressures between 7 and10mbar. The same results were obtained, but the more controlled geometryallowed an absorption cross section to be estimated. A least squares fityielded a cross section of7.2 ± 1.5Mbarn at 192nm (one barn is 10 -24 em 2).

Investigations of the absorption spectrum of DME by various methodshave been found in the literature. The first band is reported to extend fromabout 178nm to a location that varies between 188nm and 196nm in thereports (Harrison and Price, 1959; Hernandez, 1963; Suto and Lee, 1988;Bremner et al., 1991). In a pyrolysis experiment performed at 1180K a tailextending beyond 218nm has been found (Pacey, 1973), which is inagreement with our measurements. Maximum absorption has earlier beenreported to occur at 184nm (Harrison and Price, 1959; Suto et al., 1988;Bremner et al., 1991). The combined sensitivity of the detector andspectrograph used here is expected to fall off rapidly below 190nm, andbelow 202 nm the reference radiation from the deuterium lamp is heavilyabsorbed by the Schumann - Runge bands of molecular oxygen (Ackermanand Biaume, 1970). Maximum absorption was thus not obtained, but thecross section determined at 192nm compares well with the formerlyreported 6.5-9Mbarn around 184nm (Harrison and Price, 1959; Sutoet al., 1988; Bremner et al., 1991). Thus, the results reported here agree withpreviously published data.

LASER-INDUCED FLUORESCENCE MEASUREMENTS

The fluorescence properties of DME are of great importance for evaluatingthe prospects of laser diagnostics. Strong laser-induced fluorescence signalsfrom the fuel would indicate a potential for fuel concentration measure­ments. On the other hand, the absence of fluorescence from DME wouldindicate a potential for interference-free measurements of other speciesoccurring in a flame. To the authors' knowledge, there are no previousreports on investigations of DME by laser-induced fluorescence in theliterature. The fluorescent properties of DME were therefore investigatedupon excitation at 193, 230 and 248 nm.

Excitation at 193 nm

The flow rig described above was used to create a laminar flow of DME. Theunpolarized radiation from an ArF excimer laser (Lambda Physik EMG 150MSC) configured for broadband operation at 193- 194nm was focused into

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a horizontal sheet by means of a 300 mm cylindrical lens. The laser sheetwith a width of 4 mm was passed through the center of the DME flow. Dueto the strong absorption of the laser wavelength (see Fig. 2), the laser sheethad to be focused at the entrance side of the DME flow. The focal regionwas imaged at right angles onto the horizontal slit of the Acton Researchspectrograph used in the absorption measurements, equipped with the sameCCD camera. By changing the charging voltage of the laser, the laser energyat the flow rig could be varied in the range of 0.1-1 mJ.

In Figure 3a, the spectrally resolved fluorescence signal from DME at300 K is shown. No filters were used, since the scattered laser radiation issufficiently suppressed by absorption in molecular oxygen between the flowrig and the detector. A strong Raman signal from DME is seen at 205 nm,and a featureless fluorescence signal extends up to approximately 250 nm.The shoulder on the short wavelength side of the Raman peak results fromelastic scattering of the laser. A structured fluorescence spectrum appears at350 nm and extends towards longer wavelengths. The structured fluores­cence signal is shown in greater detail in Figure 3b, where the intense peak

10

.-.. 8doj-....-0'v; 6c:~.5

"u 4c:"u'""...0::l 2

G:

0

200 250 300 350 400

(a) Wavelength (11m)

FIGURE 3 (a) Dispersed signal from DME after excitation at 193nm. The fluorescenceintensity is low compared to the Raman scattering. (b) The fluorescence spectrum, shown ingreater detail. Solid line: DME at 300 K. Dotted line: DME at 543 K, where the intensity hasbeen multiplied by a factor of 10. At 387nm, the second order of the scattered laser radiation issuperimposed on the fluorescence spectra. None of the spectra have been corrected for thedetector spectral response.

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DIMETHYL ETHER CHARACTERIZATION 307

6r-------TT-----------------,

5

:i0;~ 4-C'iiic:~.5 3..,uc:1l 2 x 10

j '::V~'fij "350

.'f.

,400

. ,;.~

,450

T~300K

T=543 K

500

-

-

550

(b) Wavelength (nm)

FIGURE 3 (Continued).

at 386 nm is the second order of the scattered laser radiation. Thefluorescence extends from 350 to 550 nm and exhibits a broad featurelessbackground with a superimposed, banded structure. Investigations of thefluorescence were performed at different temperatures in the range of 300­543 K. Figure 3b shows the LIF signal for the temperatures 300 K and543 K, respectively. At the higher temperature, the LIF signal hasdiminished by one order of magnitude, and the superimposed vibrationalbands are less pronounced in relation to the broad unstructured back­ground. The origin of the fluorescence is unknown, and it could in principleemanate from laser-produced species other than DME. The structure wasnot observed in reference spectra recorded in air, and thus it does notoriginate from oxygen or nitrogen. Suto et al. (1988) report fluorescencefrom CH 30 in the 300-450nm region upon illuminating DME with157.5nm synchrotron radiation. However, the threshold energy fordissociation of DME into CH 30 is claimed to be 7A6eV (166nm), whichis far above the energy used here. When the laser power was increased by afactor of ten, the fluorescence intensity increased proportionally. Thus, thefluorescence seems to originate from a one-photon absorption, which mightindicate that the fluorescence signal emanates directly from an absorption inthe DME molecule.

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308 O. ANDERSSON et al.

The structured L1F signal is rather weak. The integrated fluorescencesignal from 350 to 550 nm is approximately of the same order of magnitudeas the Raman signal from DME, taking the spectral sensitivity of thedetector into account.

Excitation at 230 nm

Wavelengths around 230 nm are of interest in combustion diagnostics sinceboth CO and 0 can be excited via two-photon transitions in this wavelengthregion, and the NO I-bands reside around 226 nm. The same flow rig wasused to create a laminar flow of DME. A Nd:YAG pumped OPO (OpticalParametric Oscillator) from Spectra Physics (MOPO-730) was used forexcitation. The horizontally polarized radiation at 230 nm had a linewidth of0.2 em-1 and a pulse width of 6 ns. The laser beam passed through a laminarflow of DME at 300 K. A 500mm cylindrical lens was used to focus theradiation into a 7 mm wide, horizontal sheet. The signal from themeasurement area was collected at right angles by the above detectionsystem. A weak fluorescence signal was detected at 280-320nm, as can beseen In Figure 4, where also the Raman signal from DME at 246.4 nm isshown. The horizontal polarization of the laser greatly exaggerates the

4rr----------------------,

I

250

-,275

~

I

300 325 350 375

-

-

400

Wavelength (nm)

FIGURE 4 Dispersed signal from DME after excitation at 230nm.

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DIMETHYL ETHER CHARACTERIZATION 309

fluorescence intensity in proportion to the Raman peak. This faintfluorescence is thus too weak for measurement purposes. Since theexcitation wavelength is almost out of bounds of the absorption band inFigure 2, it is likely that the signal will not increase significantly withtemperature.

Excitation at 248 nm

Examples of species of combustion interest that can be excited around248 nm are H20 , OH, and NO. A KrF excimer laser (Lambda PhysikEMG ISO MSq was used to generate unpolarized, broadband radiation at248- 249 nm. The laser radiation was focused into a laminar flow of DMEby means of a 300mm cylindrical lens, thereby generating a horizontal,10mm wide laser sheet. The radiation emitted from the focal region wasresolved spectrally by the detection system described above. DME wasinvestigated at temperatures in the range of 300- 543 K using laserexcitation energies in the range of 3-120mJ. No LIF emission could bedetected.

VIBRATIONAL RAMAN CROSS SECTION

Spontaneous Raman scattering is one of the most commonly exploited laserdiagnostic techniques for measuring species concentrations. The processoccurs on a typical time scale of 10-12 s, which can be compared to 10-8 swhich is a typical lifetime of the excited state in the LIF process (Eckbreth,1996). As a result, the Raman scattered intensity is insensitive to collisionalquenching, and therefore yields more reliable concentration data than doesLIF. However, Raman scattering is an intrinsically weak process and its useis therefore often limited to major species.

No value for the vibrational Raman cross section of DME has beenfound in literature. To evaluate the possibilities for using spontaneousRaman scattering from DME in combustion diagnostics, its Ramanproperties were therefore investigated. Due to the v4 dependence of theRaman scattered intensity on frequency, the signal increases when using aUV laser source. The 248 nm radiation from the tunable KrF excimer laserdescribed above was therefore used. A variable filter, consisting of twodielectric mirrors, was used to vary the laser intensity. To avoid gasbreakdown the laser beam was focused softly at the center of a low pressurecell, using a f= 150mm cylindrical lens. The quadratic cell was equipped

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310 O. ANDERSSON et al.

with suprasil quartz windows and was optically accessible from all sides.The detection system consisted of the above mentioned CCD camera and150mm Acton Research spectrograph, this time equipped with a 1200lines/mm grating. The Raman scattered light was collected and imaged on thespectrometer slit with an achromatic lens. To suppress elastically scatteredlaser radiation, a liquid butyl acetate filter was placed in front of thespectrometer (Hargis Jr, 1981).

From preliminary measurements it was known that the Raman shift ofDME nearly matched that of methane. A relative cross section was thereforedetermined by comparison to the known Raman cross section of methane,since this would minimize errors resulting from a wavelength dependence ofthe detection efficiency. Fixed amounts of either DME or methane wasadded to the cell (about 0.20 bar), and Raman spectra were acquired byaveraging the signal from 400 laser shots. The intensity ratio of theintegrated intensities under the Raman peaks of DME and methane,respectively, was then used to calculate a relative Raman cross section. Aseries of 14 measurements was performed at pulse energies between 14mJand 45 mJ, to confirm that the signal was linearly dependent on laserintensity.

In agreement with earlier measurements (Taylor and Vidale, 1956), theRaman shift was found to be approximately 2900cm- l

, probablyoriginating from the C-H stretch (see e.g., Svanberg, 1991). A faint peakwith a shift of about l450cm- 1 was also observed, but not furtherinvestigated. Knowing the pressures of the two gases, the relative crosssection at 2900cm- 1 vibrational frequency could be determined bycomparison with the two methane peaks at 2915cm- 1 and 30l7cm- l

,

which were not resolved by the spectrograph. The cross section was found tobe 2.2 ±0.2 times greater than that of methane. The relatively large standarddeviation results from taking ratios when evaluating the cross section.

ROTATIONAL CARS

Coherent anti-Stokes Raman spectroscopy (CARS) is today a well­established method for temperature and major species concentrationmeasurements in combustion processes (Eckbreth, 1996). The theorybehind CARS, as well as numerous examples of successful applications, aredescribed in several review articles, and references therein (Greenhalgh,1988; Eckbreth, 1988).

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DIMETHYL ETHER CHARACTERIZATION 311

In dual-broadband rotational CARS (Alden et al., 1986), the variant ofCARS used here, one narrowband and two broadband laser beams arefocused to an intersection point where the CARS signal is generated as alaser-like beam. The physical basis for temperature measurements using thistechnique is that the spectrally resolved signal mirrors the temperature­dependent molecular distribution over rotational levels. Since the rotationalRaman transitions of most molecules of interest in combustion are sited inthe same frequency range, simultaneous temperature and relative concen­tration measurements are possible for species having sufficiently largerotational Raman cross sections. Dual-broadband rotational CARS has forexample been applied for temperature measurements in spark-ignitionengines (Bengtsson et al., 1994; Bood et al., 1997). Simultaneous tem­perature and relative oxygen concentration measurements have beendemonstrated in different mixtures of oxygen and nitrogen (Martinssonet al., 1996). A more detailed description of the method regarding agreementbetween experiments and theory is given elsewhere (Martinsson et al.,1993).

The aim of the present study was twofold; on one hand to analyseexperimental rotational CARS spectra from pure DME, on the other handto investigate the potential for temperature measurements in mixtures ofnitrogen and DME. Since DME is a non-linear polyatomic moleculecharacterised as an asymmetric top belonging to point group C2v (Herzberg,1991), calculation of its complete rotational CARS spectrum is a difficulttask and beyond the scope of the present investigation. However, rotationalCARS spectra of some polyatomic molecules such as ethylene and sulphurdioxide have been modelled (Magens, 1993).

Rotational CARS spectra were recorded using both pure DME and purenitrogen as well as in three different mixtures of them; 20% DME + 80%N2, 40% DME + 60% N2, and 60% DME + 40% N2 . All measurementswere carried out at room temperature and atmospheric pressure. Tocompensate for the differences in probability of generating anti-Stokesphotons at different frequencies, and for the differences in sensitivity of thedetector pixels, each spectrum was corrected by taking intensity ratios ateach pixel with a completely non-resonant CARS spectrum. The non­resonant spectrum was recorded in a flow of pure argon. The result frommeasurements in a constant flow of pure DME is shown in Figure 5a. Thepractically unresolved spectral structure on the low-frequency side(below ~ 50cm- l

) indicates resonant rotational transitions. The spectrumalso exhibits a strong non-resonant contribution. As it can be assumed thatthe high frequency side of the spectrum in principle consists entirely of non-

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312 O. ANDERSSON et al.

1614 a)

10~ 12

b):i :i~ 10 ~~ 8 ~·Vi ·Vi

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Raman shift (ern" J Raman shift (em")80

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050 100 150 200 250 50 100 150 200 250

Raman shift (cm") Raman shift (em:' J

FIGURE 5 Experimental rotational CARS spectra recorded in four different mixtures ofnitrogen and DME: a) 100% DME, b) 60% DME, c) 40% DME, and d) 20% DME. Eachspectrum consists of 300 accumulated single-shot spectra. In all spectra the correspondingbackground has been subtracted and each spectrum is also divided by an argon spectrum (thesame argon spectrum in all cases).

resonant signal, and since intensity ratios are taken with an argon spectrumat each pixel, the non-resonant susceptibility can be roughly estimated. Bycalculating the square root of the mean value (accounting for the use of aneutral density filter) over the spectral region between 225 and 250cm- 1 thenon-resonant susceptibility is estimated to be approximately seven times thevalue for argon. Using the values for argon and nitrogen in Rosasco andHurst (1985) the non-resonant value of OME is about nine times the valueof nitrogen, Rotational-CARS spectra of four different mixtures of OM Eand nitrogen are shown in Figure 5. A comparison of the spectra recordedin pure nitrogen and pure OM E indicate about a factor of ten weakerrotational CARS signal for OME than for nitrogen (based on the ratiobetween the highest peaks in each spectrum),

ln order to qualitatively verify the spectral features of OME observed inthe experiments the relative rotational line strengths were calculated. Theline strength is given by:

(I)

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DIMETHYL ETHER CHARACTERIZATION 313

where bJJ' is the Placzek-Teller coefficient PJJ' is the population differencefactor between the rotational levels J and J I given in Eq. (2):

{. [F(J,K)] [F(J1,K

1)]}b.PJJ' ()( gl(2J + I) exp - kT - exp - KT (2)

where gl is the statistical weight factor due to the nuclear spin degeneracy,F (J, K) is the energy of rotational level J, K (where J is the total angularmomentum quantum number and K is an orientation quantum number), k

is the Boltzmann constant, and T is the temperature. The followingrotational constants were used: AD = 1.2938em?", Do = 0.3355 cm- I

,

Co = 0.2964cm- 1 (Herzberg, 1991). Since Dor::;Co the DME molecule canbe treated approximately as a symmetric top. The term values for a rigidsymmetric top are given by Weber (1979):

I [I] 2F(J, K) =:2 (Do + Co)J(J + I) + AD -:2 (Do + Co) K (3)

The most intense Raman transitions are then governed by the selectionrules:

b.K = 0, b.J = + I (for K"I 0), b.J = +2 (4)

From Eqs. (3) and (4) the Rand S-branch transition frequencies might becalculated from the expressions:

b.F(J + I, K) r::; (Do + Co)J R-branch

b.F(J + 2, K) r::; (Do + Co)(2J + 3) S-branch

(5)

(6)

These expressions show that every second rotational line is an overlappingline from both S-branch and R-branch transitions. Figure 6 shows simulatedrelative rotational line strengths of DME at 300 K and 1.0atm (without anystatistical weights). This figure clearly shows that the resonant rotationaltransitions occur in the low frequency-region (from Ocm- I to about50cm- l

) which was also observed experimentally (Fig. 5a). Moreover, thespectrum is dominated by the S-branch transitions. An energy separationbetween two neighboring S-branch transitions of 1.26cm- l

, calculated fromEq. (6), and assuming the same linewidth as for the isolated nitrogen lines(1.0cm- 1 FWHM) with a gaussian lineshape implies a substantial overlap ofthe rotational lines. Furthermore by also taking the intermediate R-branch

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1.0

0.8 I-

--:­~

~ 0.6 -

C'iii<:OJ 0.4C......

O. ANDERSSON er al.

-

-

-

::: -.111111111111o 10 20 30 40

Raman shift (em")

50 60

FIGURE 6 Simulated rotational line strengths of DME at 300 K, without any statisticalweights due to nuclear spin degeneracy. The lower peaks represent Rebranch transitions and thehigher peaks correspond to overlapping R- and S-branch transitions.

transrtions into account, the spectral lines are expected to be nearlyunresolved.

In order to investigate the potential for temperature measurements inmixtures of DME and nitrogen, the experimental spectra shown in Figure 5were evaluated using a computer code written for evaluation of rotationalCARS spectra (Martinsson et al., 1993). Only molecular data for purenitrogen were used as input data, and the non-resonant susceptibility wasfitted in all evaluations. The spectrum recorded in pure nitrogen, 20% DME(Fig. 5d) , and 40% DME (Fig. 5c) gave the temperatures 290, 287, and293 K, respectively. The spectra corresponding to 60% DME (Fig. 5b) and100% DME (Fig. 5a) consisted of a too high degree of resonant DME­transitions for reasonable evaluation. Thus, spectra consisting of up to 40%DM E were possible to evaluate with a good temperature accuracy, i.e., thetemperature dependence on the non-resonant susceptibilty is very weak.

Simultaneous relative concentration measurements are not feasible at themoment. As mentioned previously DME is difficult to model and severalmolecular parameters such as the complete polarisability tensor androtational Raman linewidths are to our knowledge not available. Moreover,since the rotational lines of DME are not possible to resolve with our

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DIMETHYL ETHER CHARACTERIZATION 315

spectrometer (f = 1.0m, 600 linesjmm) the sensitivity might be quite poor.However, since the vibrational Raman cross section is shown to becomparatively strong (see previous section in this paper) one may expect astrong vibrational CARS signal. A possible approach for makingsimultaneous temperature and concentration measurements for DMEfractions up to 40% could therefore be to combine rotational andvibrational CARS using a double-folded BOXCARS configuration(Bengtsson et al., 1995). In such an experiment the shape of the rotationalCARS signal is used for temperature measurements, and the vibrationalCARS signal is used for evaluation of the DME fraction by measuring theratio between the DME and nitrogen signals. However, this approach needsaccurate calibration measurements and the dependence of both therotational and vibrational CARS signal intensities on temperature has tobe known (Dreier et al., 1988).

DETECTION OF NO IN A DME FLAME

Since it is not clear why DME produces less NO in engines than ordinarydiesel fuel, NO spectroscopy is of particular interest in diagnostics of DMEflames. In order to investigate the diagnostic potential in such environments,NO was detected in a laminar, premixed DMEjair flame using L1F. Theflame was set up using a water-cooled bunsen burner with a nozzle diameterof 10mm. Bronkhorst mass flow controllers were used to regulate the OMEand air flows. NO was excited in the A-X(O, 2) bandhead just below 248 nm,using a tunable KrF excimer laser. The excitation wavelength was not withinthe range where the laser normally produces narrowband radiation. In orderto maintain narrowband operation, the laser was optically reconfigured as isdescribed by Griinefeld et at. (1996). The pulse energy was 20 mJ, and toavoid optical breakdown in the flame the beam was focused I mm behindthe flame, using a 150mm spherical quartz lens. Signal from themeasurement region was imaged with a 100mm achromatic lens on thehorizontal slit of the 150mm Acton Research spectrograph equipped with a300 lines/rum grating and the image intensified CCO camera, both describedabove. Since the beam intersected the central cone of the flame, the signalfrom three distinct flame regions was imaged on the slit: the preflame zone(containing unburned DME and air), the reaction zone, and an outer zoneconsisting of products from the reaction zone and a diffusion flame if thecombustion is fuel-rich in the premixed reaction zone (see Fig. 7). In thediffusion flame, mainly CO and Hz from the premixed flame are combusted.

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The images acquired on the CCO chip thus contained spatial informationalong the direction parallel to the slit and spectral information perpendi­cular to the slit, as is depicted in Figure 7. The two-dimensional images thusobtained could easily be divided into three regions corresponding to thethree zones. Three normalized spectra were obtained by summing the signalover the width of each region, and dividing by the region's width in pixels.Thus, the relative signal contribution from each of the three environmentscould be determined. In Figure 8 three such spectra acquired in a slightlyrich flame (1) = 1.25) are shown. The lower spectrum corresponds to thepreflarne zone and contains the Raman peaks from OME, oxygen, andnitrogen. Although the spectral resolution is poor, the signal to noise ratio(SN R) is high and the spectrum is free from spectral interferences. Themiddle spectrum corresponds to the premixed reaction zone and contains abroad, noisy fluorescence feature, probably originating from combustionintermediates. The Raman signal from OME can hardly be seen, but is stillpresent at 267 nm. In the top spectrum, acquired in the outer flame, the A­X(O, 3), A-X(O, 4), and A-X(O, 5) bands of NO are clearly visible. The SNRis not as high as in the reactant-zone spectrum and a broad background withthe same spectral distribution as in the reaction-zone spectrum is observedalso here. The same measurements were repeated for stoichiometries

)

Spe:tralJ:eS:)Jut:i:mFIGURE 7 Schematic illustration of the flame on the bunsen burner. with the threeinvestigated regions indicated.

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DIMETHYL ETHER CHARACTERIZATION 317

75

300

300

290

290

280

280

270

270260

250

50

25

50

100

250r------------------,

150

300

200

100

250 260 270 280 290 300

Wavelength (nm)

FIGURE 8 Three normalized spectra recorded in each of the three flame regions depicted inFigure 7. The top spectrum is recorded in the outer flame and shows (from left to right) the A­X(O,3), (0,4), and (0,5) bands of NO. The middle spectrum is recorded in the premixed reactionzone and contains a broad, noisy fluorescence. The lower spectrum is recorded in the preflamezone and shows (from left to right) the Raman peaks of oxygen, nitrogen, and OM E.

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318 O. ANDERSSON et al.

between 0.8 < cP < 2.4. The only resulting difference was an increase influorescence in the reaction zone and increasing Raman signal from DME,as cP increased.

As can be seen in Figure 8, the Raman signal from DME is relativelylarge as compared to the NO fluorescence. One reason for this is of coursethat the Raman spectrum was acquired in the cold reactant flow where thedensity is almost an order of magnitude higher than in the flame. It is alsoknown that DME produces 75% less NO in a CI engine as compared todiesel fuel (Sorensen and Mikkelsen, 1995), and it is thus not unlikely thatthe same is valid in an atmospheric pressure flame. Although the Ramansignal from DME clearly does not interfere with the NO bands, the spectrafrom the reaction zone and the outer flame indicate that totally interference­free detection of NO is difficult to achieve in the displayed wavelengthregion. However, comparison with a methane flame set up in the sameburner shows that the interfering fluorescence is of the same magnitude inthe two flames. The numerous examples of successful NO measurements inmethane/air flames (see e.g., Heard et al., 1992) imply that this backgroundfluorescence should not pose a significant problem. If absolutely inter­ference-free detection is to be undertaken in a non-stable flame, the broadbackground fluorescence will be difficult to account for by backgroundsubtraction. However, the intensity of the background fluorescence is lowerat shorter wavelengths. It is thus likely that anti-Stokes detection of the NOfluorescence in the A-X(O, 0) and A-X(O, 1) bands (blue-shifted from thelaser wavelength) will be successful. A significant portion of the fluorescenceis emitted in these bands and this detection scheme has been successfullyapplied for quantitative concentration measurements of NO in runningspark-ignition engines (Schultz et al., 1996; Hildenbrand et al., 1998).

SUMMARY AND DISCUSSION

Although the peak of the first absorption band of DME resides around184nm, significant absorption can also be expected in the long-wavelengthtail above 190nm. At 192 nm the room-temperature absorption cross­section is 7.2 ± 1.5Mbarn. Even in a lean fuel-air mixture, typical for CIengines, the high absorption cross section will result in complete depletion ofa 193nm laser beam over a fraction of the cylinder diameter. It is thusimportant to use a laser wavelength in a spectral region where the mixture isoptically thin. The edge of the absorption band is displaced towards longer

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DIMETHYL ETHER CHARACTERIZATION 319

wavelengths with increasing temperature at a rate of about 0.02 nm/K,within the investigated temperature interval.

A free flow of DME in air shows several fluorescence bands in the 350­550 nm region, upon illumination at 193nm. The LIF signal is of the sameorder of magnitude as the spontaneous Raman scattering from DME anddecreases with temperature. Excitation at 230 nm yields a faint fluorescencein the 280 - 320 nm region. Excitation at 248 nm yields no detectablefluorescence. Detecting DME by LIF is impractical using 193nm excitationdue to the strong absorption and relatively low signal intensity. Excitationat longer wavelengths yields even weaker signals. When measuring othercombustion species than DME, this might be a favorable property.

A relative spontaneous Raman cross section of DME is determined to be2.2 ± 0.2 times the value for methane at 248 nm. The vibrational Ramanshift ofDME is 2900cm- J

, and the Raman line is thus well separated fromthe laser wavelength. These properties imply that spontaneous Ramanscattering may be an especially useful diagnostic technique in environmentscontaining DME. The use of Raman scattering for two-dimensionalimaging measurements of methane in a multi-pass configuration has beenreported (Webber et al., 1979), and DME can probably be detected in thesame manner.

The rotational CARS signal from DME is about ten times weaker thanthe signal from molecular nitrogen. DME's non-resonant susceptibility isabout nine times greater than that of molecular nitrogen. Temperaturemeasurements by rotational CARS can be performed in DME/air mixturescontaining up to 40% DME (a stoichiometric DME-air mixture consists ofabout 7% DME). Simultaneous relative concentration measurements arenot feasible at the moment, for several reasons: DME is a difficult moleculeto model, the polarizability tensor is to our knowledge not available, and asthe rotational lines of DME are not possible to resolve with thespectrometer used (f = 1.0m, 600 lines/mm) the sensitivity might be quitepoor. However, since the vibrational Raman cross section is shown to becomparatively strong, one may expect a strong vibrational CARS signal. Apossible approach for making simultaneous temperature and concentrationmeasurements for DME fractions up to 40% could therefore be to combinerotational and vibrational CARS using a double-folded BOXCARSconfiguration. In such an experiment the shape of the rotational CARSsignal is used for temperature measurements and the vibrational CARSsignal is used for evaluation of the OM E fraction by measuring the ratiobetween the DME and nitrogen signals. However this approach needsaccurate calibration measurements and the dependence of both the

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rotational and vibrational CARS signal intensities on temperature has to beknown.

As the interest in DME as a fuel increases, the interest in investigations ofits behavior in practical combustion applications will increase. The fact thatDME combustion is virtually smokeless in CI engines opens up newpossibilities for optical diagnostics in these environments. Results presentedhere are meant to serve as an indicator of what approaches might be suitablein such investigations. For example, if NO is to be detected using LIF in aDME fueled CI engine, the reported absorption characteristics imply thatexcitation in the sometimes exploited D-X(O,I) band at 193nm is lessfavorable than excitation in the -y-bands. The current investigation alsoallows for choice of a detection scheme that excludes interference of Ramanscattering from DME. Interference from combustion intermediates seems tobe less of a problem in DME flames as compared to traditional fuels. Ifspray characteristics are to be studied, Raman scattering seems to be a betterchoice for DME detection than LIF, unless a fuel tracer with suitablefluorescent and thermodynamic properties is found. Determination of theflame position can be realized by detecting various radicals, and a suitabledetection scheme can be chosen on basis of the results presented here.Preparations for laser diagnostic investigations in a DME fueled direct­injection CI engine are presently underway at our laboratories.

Acknowledgements

This work was financially supported by the Swedish Board for Industrialand Technological Development (NUTEK) and the Foundation forSrategic Research (SSF).

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