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Instantaneous Measurement of Local Concentration and Vapor

Fraction in Liquid-Gas Mixtures by Laser-Induced Breakdown

Spectroscopy∗

Akihiro KIDO∗∗, Kenji HOSHI∗∗∗, Hiroto KUSAKA∗∗∗,Hideyuki OGAWA∗∗∗∗ and Noboru MIYAMOTO∗∗∗∗

Laser-induced breakdown spectroscopy (LIBS) with atomic emission excited with a fo-cused high-energy ND: YAG laser was applied to quantify the concentration and the vaporfraction of liquid-gas mixtures. With LIBS it is possible to quantify local concentrations ac-curately even in liquid-gas mixtures as the ratio of the number of fuel-borne hydrogen atomsto nitrogen or oxygen atoms in the ambient gas. The ratio has a strong linear relation with theratio of the peak emission intensities regardless of phase of the fuel. As the full width at halfmaximum (FWHM) of the emission peak from the fuel-borne hydrogen increases linearlywith the liquid fraction due to the Doppler shift with micro-explosions, the FWHM yields thefuel vapor fraction. Simultaneous, high-resolution measurements of equivalence ratios andvapor fractions in an intermittent fuel spray in a pressurized atmosphere were obtained withthis method. The results showed that the tip of the intermittent spray has a richer mixturewith a lower vapor fraction.

Key Words: Laser, Fuel Injection, Internal Combustion Engine, Laser-Aided Diagnostic,Atomic Emission, Equivalence Ratio, Vapor Fraction, Full Width Half Maxi-mum, Micro Explosion, Laser-Induced Breakdown Spectroscopy

1. Introduction

Understanding the mixture formation process in fuelsprays, including the evaporation and entrainment mech-anism is essential to improve combustion efficiency andexhaust gas emissions, and here quantitative measure-ments of local and instantaneous fuel concentrations arenecessary to describe the phenomena and characteristics.However, it is not easy to measure the fuel concentrationdistributions quantitatively as the fuel spray contains bothgas and liquid phases in a spatially complicated and un-

∗ Received 23rd February, 2006 (No. 04-1256). JapaneseOriginal: Trans. Jpn. Soc. Mech. Eng., Vol.71, No.706, B(2005), pp.1702–1707 (Received 22nd November, 2004)

∗∗ Division of Automotive Engineering, Hokkaido Automo-tive Engineering College, 2–1 Nakanoshima, Toyohira-ku,Sapporo-shi, Hokkaido 062–0922, Japan

∗∗∗ Toyota Motor Coop., 1 Toyota-cho, Toyota-shi, Aichi471–8571, Japan

∗∗∗∗ Division of Mechanical Science, Hokkaido University,N13, W8, Kita-ku, Sapporo-shi, Hokkaido 060–8628,Japan. E-mail: h-ogawa@eng.hokudai.ac.jp

steady structure. Several methods of measurement havebeen developed to determine the fuel concentration dis-tribution in a fuel spray, but there are no methods withsufficiently high spatial and time resolutions as well as ac-curacy to cover a wide range of fuel concentrations.

A field where gas and liquid coexist can be analyzedwith optical measurements of the absorbance and scat-ter at two wavelengths(1) and with an exciplex method(2).These methods have limitations, however, imposed bythe experimental conditions as a special tracer or a fuelwith tracer is necessary. Laser-induced breakdown spec-troscopy (LIBS) which has been applied to gas analysishas the advantage that it requires no special tracer, it hasno limitations on the wavelength of laser light, and hasa high accuracy over a wide range of mixture strengths,from lean to rich.

In the research reported here LIBS was applied toquantitative measurements of fields, where the gas andliquid components coexist, and the simultaneous mea-surement of fuel concentration and evaporation rate wasachieved. The experiments were expanded to an analysis

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of the evaporation and mixture formation processes in anintermittent fuel spray in a heated compressed vessel.

2. Theory of Atomic Emission

2. 1 Emission intensityThe number of excited atoms per the unit time and

unit section, R is given as follows(3):

R=N1∆zB12u(ν) [m−2s−1] (1)

Here, N1: Number density of lower level, 1; ∆z:Thickness of absorbance; B12: Einstein’s absorbance coef-ficient [m3J−1s−2]; u(ν): Energy density of excitation laser[Jm3Hz−1].

Defining the cross section of the incident laser, ∆S ,and the volume, ∆V =∆Z∆S , the absorbed energy, W, isgiven as:

W =R∆S hν=N1∆VB12u(ν)hν [Js−1] (2)

Here, h: Planck’s constant [Js−1], and ν: Vibrationnumber of excitation wavelength [s−1]

The atomic emission energy, E, is obtained from thequantum yield, η, and Eq. (2) as follows:

η=A21/(A21+K21) (3)

E=ηW =ηN1∆VB12u(ν)hν [Js−1] (4)

Here, A21: The Einstein spontaneous transition coef-ficient; and K21: The non-irradiative transition constant.

For pulsed laser excitation, the measured intensity ofthe atomic emission, I, is obtained by introducing the ex-citation period of the laser, ∆τ [s], the measurement solidangle, Ω [sr], and a constant that originates in the charac-teristics of the device, α, into Eq. (4) as follows:

I = α(∆Ω/4π)E∆τ

= α(∆Ω/4π)ηN1∆VB12u(ν)hν∆τ [J] (5)

When high power laser light is irradiated at themolecules, they dissociate to atoms. Therefore, Eq. (5)is rearranged to Eq. (6) using the dissociation rate, ξ, asfollows:

ξ=N1/N0 (6)

I =α(∆Ω/4π)ηN0ξ∆VB12u(ν)hν∆τ [J] (7)

Here, N0 is the number density of the molecule.Equation (7) is the basic equation for the measure-

ments of the atomic emission and clearly shows that theintensity, I, strongly depends on N0, ξ, and ∆V .

2. 2 Spectrum profileIt is reported that the Doppler broadening is enhanced

by micro explosions of droplets when a high energy laserbeam is introduced into the fuel spray(4). The quantityof the vibration number increase, ∆ν, due to the Dopplereffect is as follows:

∆ν= ν0Vz/c (8)

Here, ν0: the vibration number without the Dopplerbroadening, Vz: the velocity of the atom to the measure-ment direction, and c: velocity of light.

When it is approximately estimated that a liquiddroplet of normal hexane of 50 µm diameter becomes gasphase plasma at 5× 104 K with 3.2× 104 times the vol-ume of the liquid phase droplet due to micro explosions,Vz is estimated to be 105 m/s. Here, it is assumed that theplasma behaves like an ideal gas and that the micro explo-sion completes during the excitation laser pulse duration,8 ns. The Doppler shift of the excitation wavelength of hy-drogen atoms with the Vz is calculated to be about 0.5 nm,where the central wavelength of atomic emissions of His 656 nm. Consequently, the liquid fraction of the fuelcan be determined from the full width at half maximum(FWHM) of the H atom spectra which increases with in-creases in the liquid fraction.

3. Experimental Procedure

3. 1 Experimental apparatusIn this investigation a laser beam was focused with a

convex lens to obtain high energy densities, and the fo-cused region in a fuel spray was evaporated, broken down,excited, and emitted atomic emissions. Figure 1 shows theexperimental apparatus for the measurements of atomicemissions in the field where gas and liquid componentsfrom vaporizing fuel coexist. The test vessel has inlet andoutlet pipes for the charge gas, a T type 25 µm diame-ter thermocouple, a fan to agitate the inside of the vessel,quarts windows, and a common rail fuel injector. The tem-perature in the vessel is controlled with a heater. Normalhexane was mainly used as the test fuel.

The measuring field including the gas and liquid com-ponents from the fuel was irradiated by a focused beam ofan Nd: YAG laser with an 80 mJ/pulse at 1 064 nm and abeam diameter of 8 mm. The laser beam was condensedby a convex lens with a focal length of 70 mm. The scat-tered light signal from the focal point was gathered intothe optical tube and led to the spectrometer, with an opticalresolution of 0.1 nm, and a multi channel CCD detector.

Fig. 1 Experimental apparatus

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Fig. 2 Atomic emission spectra and plasma emissions

To establish the calibration curve for the FWHM ofhydrogen atom emission spectra and the liquid fraction, agas and liquid coexistence field with a preset liquid frac-tion is formed in a vessel as shown in Fig. 1. The gas andliquid coexistence field was formed with rapid decompres-sion in the vessel containing a gaseous fuel and nitrogenmixture at high pressure. The liquid fraction was calcu-lated with the fuel vapor pressure given by the Antoine’sequation. This experimental system was also used for themeasurements of the fuel spray.

3. 2 Quantification procedureFigure 2 shows typical atomic emission spectra with

a plasma emission which appears at the bottom of theatomic emission. In this measurement, the atomic con-centration is quantified only by the atomic emission, andthe plasma emission has to be subtracted from the mea-sured intensity(5). Here, two kinds of parameter, the heightof peak intensity, I, and the area of the band spectra, S,were used for the quantification. To quantify the liquidfuel fraction, the S parameter, was used as the spectrumbroadens with micro explosions.

The fuel concentration was quantified with the emis-sion intensity ratio of hydrogen and nitrogen and the liquidfraction was derived with the FWHM at the atomic emis-sion of hydrogen. Here, emissions at 656 nm and 871 nmwere used for hydrogen and nitrogen respectively. Theequivalence ratios were calculated with the assumptionthat the ambient nitrogen was the air.

4. Results and Discussion

4. 1 Atomic emission spectra from neat fuelsFigure 3 shows the atomic emission spectra from neat

dimethyl ether (DME, C2H6O) and neat dichloromethane(CH2Cl2). The atomic emission from oxygen in DMEand from chlorine in dichloromethane were measured at777 nm and 800 – 900 nm respectively(6), and the emis-sions of hydrogen and carbon atoms were observed atalmost the same wavelength, 656 nm. The emissionsat 656 nm can be regarded as almost exclusively fromhydrogen because the emission from carbon is quiteweak(7). Therefore, the equivalence ratio in the fuel spray

Fig. 3 Atomic emission spectra of DME and CH2Cl2

Fig. 4 Relation between emission intensity ratio and atomicnumber ratio

can be quantified from the intensity ratio with hydro-gen and nitrogen or oxygen. The quantification fromdichloromethane can also be performed from the intensityof chlorine.

4. 2 Calibration curve for equivalence ratios in hy-drocarbon mixtures

Figure 4 shows the relation between the equivalenceratios of propane – nitrogen mixtures and the emission ra-tios of hydrogen and nitrogen. The open circles show thedata from the height of peak intensity, IH/IN, and the solidcircles are from the area of the band spectra, SH/SN. Bothintensity ratios correlate well with the equivalence ratiofor the wide range of equivalence ratios, from below 0.1to above 10, and an equivalence ratio can be quantifiedfrom Fig. 4. For measurements of the lean mixture, IH/IN

is more appropriate than SH/SN due to the better linearityof IH/IN.

Figure 5 shows the relation between the atomic num-ber ratio of hydrogen and nitrogen, NH/NN, and the in-tensity ratio, IH/IN, for various kinds of fuels under at-mospheric pressure. In the experiments, the fuels wereintroduced from the evaporator to the flow cell with nitro-gen as the carrier gas, and the fuel concentration is cal-culated from the saturated pressure. The intensity ratio,

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Fig. 5 The calibration curve for the intensity ratio and atomicnumber ratio (Pa=0.1 MPa)

Fig. 6 The calibration curve for the intensity ratio andequivalence ratio in the hexane gas

IH/IN, correlates well with NH/NN regardless of the kindof fuel. This suggests that all molecules in the measuredregion dissociate completely and emit atomic emissions.

The relation between the atomic emission ratio,SH/SN, and the equivalence ratio for normal hexane isshown in Fig. 6, obtained by enlarging the hatched regionin Fig. 5. Here, the open and solid circles show the data forambient pressures of 0.1 MPa and 1.0 MPa respectively.While SH/SN for 0.1 MPa increases linearly with increasesin the equivalence ratio for a wide range of equivalenceratios, the increase in SH/SN for 1.0 MPa almost saturatesat an equivalence ratio of 5. This saturation can be ex-plained as quenching of hydrogen emissions is strongerunder richer and higher pressure conditions. However,for an equivalence ratio range below 5, SH/SN correlateswell with the equivalence ratio regardless of the ambientpressure. Thus, the intensity ratio, SH/SN, can be used toquantify the mixture strength independent of fuel propertyand ambient pressure except at pressurized and rich con-ditions.

4. 3 Calibration curve for the liquid fuel fractionDoppler broadening of atomic emissions, which is

caused by micro explosions, is the signal that enables cal-culating the liquid fraction. Gas and liquid fuel mixturesform with a preset liquid fuel fraction in the experimentalvessel as explained in section 3.1, and the mixtures were

Fig. 7 The calibration curve for FWHM and number of atomichydrogen in liquid fuel

Fig. 8 Relation between FWHM and equivalence ratio in thegas phase

excited with the focused laser beam, and the FWHM of thehydrogen emissions was measured. Figure 7 shows the re-lation between the FWHM and the number of hydrogenatoms in the liquid fuel. The FWHM increases linearlywith increases in the number of atomic hydrogen in theliquid fuel regardless of the overall equivalence ratio inthe vessel, φ.

Figure 8 shows the influence of the equivalence ratioon the FWHM for mixtures without a liquid phase. Theequivalence ratios affect FWHM little while the FWHMincreases with increases in the ambient pressure, maybecaused by the pressure broadening of the atomic emis-sion spectra. Like the equivalence ratio is measured fromthe atomic emissions, the liquid fraction can be simultane-ously quantified with Fig. 7 used as the calibration curvein the following experiments.

4. 4 Accuracy of the quantificationFigure 9 shows the relative error in the emission ratio

quantified with the emission intensity. Here, the relativeerror was defined as the ratio of the average value of tenmeasurements and the maximum deviation from the aver-age value. The relative errors in the emission ratios, IH/IN

and SH/SN, are less than 2.5% near the equilibrium ratio,but increase on both the lean and rich sides.

Figure 10 shows the relative errors in the liquid frac-

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Fig. 9 Relative error in the equivalence ratio quantified withemission intensity

Fig. 10 Relative error in liquid fraction quantified with FWHM

tion measurements with a hexane spray injected into a ni-trogen atmosphere. The error is estimated in the same wayas the emission ratio measurements in Fig. 9. The errorsfor ambient pressure of 0.1 MPa are shown with open sym-bols, and are about 5% regardless of the liquid fractionand the equivalence ratio. However, the error increaseswith increases in the ambient pressure. This may be dueto the difficulty of setting the liquid fraction in the vesselaccurately.

4. 5 Application to fuel spraysFigure 11 shows schlieren images of hexane sprays

at various elapsed times after the start of injection, ∆t, in-jected by a common rail injector with the injection pres-sure 40 MPa and the injection duration 2.06 ms. Here, theambient pressure was 1.0 MPa and the temperatures were60C and 190C. Evaporation of hexane is remarkablewith the 190C ambient temperature, as the temperatureexceeds the boiling point at 1.0 MPa.

Figure 12 shows the results of simultaneous measure-ments of equivalence ratio and vapor fraction in a hexanespray under the same conditions as in Fig. 11. The mea-surement position is 60 mm from the nozzle tip on the cen-tral spray axis, also shown in Fig. 11 with a solid circle.The equivalence ratio and the vapor fraction were quan-tified with the calibration curves shown in Figs. 6 and 7,and all data in Fig. 12 are averages of ten measurements.

Fig. 11 Schlieren images of hexane sprays

Fig. 12 Equivalence ratio and vapor fraction at a point in ahexane spray

Just after the tip of the fuel spray reaches the measuringpoint, the equivalence ratios increase, reach the maximumvalue at ∆t = 1.8 ms, and then decrease. The vapor frac-tions decrease before ∆t = 1.5 ms, then increase slightly,before slowly decreasing again. This phenomenon canbe explained by the characteristics of an intermittent fuelspray that shows a fuel rich region just upstream of thespray tip. With increases in the ambient temperature, φincreases with the vapor fraction, maybe due to the de-crease in ambient gas density with increases in ambienttemperature. The higher vapor fraction with higher ambi-ent temperature shows the increase in vaporization.

5. Conclusions

Laser-induced breakdown spectroscopy, LIBS, wasused to simultaneously quantify the concentration and thevapor fraction of liquid-gas mixtures with atomic emission

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sector profiles without seeding of additives. The resultsmay be summarized as follows:

( 1 ) Local equivalence ratio of even the liquid-gasmixtures was instantaneously quantified with the ratio ofthe number of fuel-borne hydrogen atoms to the numberof nitrogen or oxygen atoms.

( 2 ) The liquid fraction of a fuel spray was quantifiedwith the full width at half maximum, FWHM, of the hy-drogen emission as the FWHM linearly increases with in-creases in the liquid fraction due to a Doppler shift broughtabout by micro-explosions.

( 3 ) The method made it possible to obtain simulta-neous, high-resolution measurements of the equivalenceratio and vapor fraction in an intermittent fuel spray in apressurized atmosphere.

( 4 ) The tip of the intermittent spray has a richer mix-ture with lower vapor fraction than the rest of the spray.

Acknowledgments

The authors wish to express their appreciation to Mr.N. Yamanaka, a graduate student in Hokkaido Universityand Mr. T. Shimizu, an engineer of Mitsubishi Motor Co.,Ltd., for their cooperation in this investigation.

References

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( 2 ) Senda, J., Kanda, T., Kobayashi, M., Tanabe, Y. andFujimoto, H., Quantitative Analysis of Fuel Concen-tration Fields in Diesel Spray (1st Report, Quantita-tive Analysis of Fuel Vapor Concentration), Trans. Jpn.Soc. Mech. Eng., (in Japanese), Vol.63, No.607, B(1997), pp.1068–1073.

( 3 ) Yamamoto, M. and Murakami, S., Spectroscopic Mea-surement of Plasma, (in Japanese), (1995), pp.53–58,74–76, Gakkai Shuppan Center Co. Inc.

( 4 ) Yamazaki, H., Tsue, M. and Kadota, T., Evaporationand Combustion of Emulsified Fuel, Trans. Jpn. Soc.Mech. Eng., (in Japanese), Vol.58, No.546, B (1992),pp.582–586.

( 5 ) Miyamoto, N., Ogawa, H., Kido, A., Yoshioka, S. andTakahashi, T., Instantaneous Measurement of LocalEquivalence Ratio of Hydrocarbon Mixtures by Laser-Induced Breakdown Spectroscopy, Trans. Jpn. Soc.Mech. Eng., (in Japanese), Vol.67, No.658, B (2001),pp.1494–1499.

( 6 ) Harrison, G.R., Wavelength Tables, (1939), The Tech-nology Press, MIT.

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