Supersonic Jet Sampling for Combustion Diagnostics

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  • [ l i e NOTES

    Supersonic Jet Sampling for Combustion Diagnostics*

    W. B. WHITTEN Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

    Index Headings: Laser spectrometry; Supersonic nuzzle; Combustion diagnostics.

    The purpose of this note is to describe a novel appli- cation of supersonic molecular beam spectrometry 1-3 to the study of combustion processes in piston engines. In this technique, combustion gases are cooled by expan- sion from a nozzle integral with the cylinder head wall into an adjacent vacuum chamber. The rotationally cooled combustion products are analyzed by laser-in- duced fluorescence spectrometry with gigaHertz spec- tral resolution. Because of the rapid passage of the sam- ple into a relatively collision-free state, it should be possible to isolate and identify intermediates as well as the final reaction products. Time resolution can be ob- tained by synchronizing the pulsed laser excitation source to the engine cycle.

    This note presents some preliminary results obtained with a one-cylinder gasoline engine. The effectiveness of the technique was explored by the measurement of the fluorescence excitation spectrum of NO2 expanding with the combustion gases from the nozzle. The NO2 was added to the intake air supply of the running engine. Spectra were also obtained for NO~ seeded in either Ar or N2 expanded through the same nozzle, by the intro- duction of the gases through the spark plug hole with the engine stationary at top dead center (valves closed).

    How suitable the present method is for combustion studies depends on how well the gas emitted from the nozzle represents the reactions within the cylinder and also on the degree of rotational cooling that can be ob- tained in a mixture of polyatomic gases with rather ex- treme initial conditions. The first of these questions has been addressed by E. L. Knuth and coworkers, who have used a mass spectrometer in conjunction with molecular beam sampling to study combustion products. 4-~ Knuth 6

    Received 23 May 1985. * Research sponsored by the Office of Energy Research, U.S. Depart- ment of Energy, under Contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

    has presented a criterion for determining which is pre- dominantly sampled: the bulk gas within the chamber or the boundary layer in which the combustion is quenched by the chamber walls. When the nozzle di- ameter is greater than the boundary-layer thickness, hy-. drodynamic focusing concentrates the bulk gas near the center of the expansion, thus permitting its measure- ment. For nozzle diameters less than the boundary-layer thickness, however, it is primarily the boundary-layer gas that is sampled. The thickness of the boundary layer has been observed by Daniel 7 to fall in the 50 to 100 ~m range during the combustion period at full throttle. The thickness increases as the pressure and temperature are reduced to as large as 400 ~m as the throttle is closed. 7

    The present experiment was directed towards the sec- ond question, concerning the cooling that can be ob- tained. The temperature, T, of an ideal gas undergoing a supersonic expansion can be expressed by 1

    T [ (7 -1 ) ] -1 ~o = 1+- -~ - - -M2 (i)

    where T is the temperature of the reservoir, 7 is the ratio of the specific heat at constant pressure and volume, Cp/Cv, and M is the Mach number. The Mach number can be expressed approximately 8,1 by

    M = A(X/D) ~-1. (2)

    In this expression, the constant, A, depends on the na- ture of the gas, having the values 3.26, 3.65, and 3.96 for

    equal to 1.67, 1.40, and 1.29 respectively2 The quan- tities X and D are the distance from the nozzle and the nozzle diameter. High-resolution spectral measurements are usually carried out by seeding the sample molecules into an expanding monatomic gas such as Ar. 1-3 The car- rier gas in the present experiment is a mixture of com- bustion products, dominated by N2 at about 70%. We can estimate how the cooling will be affected by other than monatomic gases by assuming a fixed X/D ratio in Eq. 2. For an X/D ratio that gives a cooling from 300 K to 5 K with Ar carrier gas, the above equations predict a temperature of 20 K with a diatomic gas such as N2. Thus, substantial cooling should still be expected with air and combustion products as the carrier gas. Smalley et al2 showed that a rotational temperature of 30 K could be obtained in a triatomic expansion of neat NO~, while a temperature of 3 K was obtained with NO2 seed- ed in Ar.

    The engine used in this experiment is a Briggs and Stratton 3-hp, 4-cycle gasoline engine. The cylinder head

    104 Volume 40, Number 1, 1986 ooo3-7o28/86/4ool.olo45zoo/o APPLIED SPECTROSCOPY 1986 Society for Applied Spectroscopy



    EXCiTATiON ~ '~ '~ '~ I I

    O0 -RING I



    Fro. 1. Exploded view of the engine cylinder head, nozzle, and vac- uum manifold. A standard gasket (not shown) is used between the cylinder head and engine block.

    was machined flat and threaded to accept a stainless steel nozzle and Viton O-ring. Nozzles were made by the predrilling of 10-32 socket-head cap screws to a thick- ness of 0.03 cm. A small aperture was formed in the remaining material by laser drilling with the focused output of a frequency-doubled YAG laser. The nozzle used for the reported measurements has a nominal di- ameter of 50 ttm.

    A diagram of the expansion chamber is shown in Fig. 1. The vacuum manifold is bolted to the modified cyl- inder head with an O-ring seal and connected to a cham- ber pumped by a 15-cm oil diffusion pump and fore- pump. With the engine running and with the 50-t~m nozzle in place, the chamber pressure was 2 10 -4 Tort. Quartz windows attached to tubes leading from the manifold with Tort-Seal epoxy cement provide optical access to the sample. Laser excitation was from a Quan- ta-Ray PDL-1 pulsed dye laser focused above the nozzle. Fluorescence was monitored by a 1P28 photomultiplier with a Corning 2-59 filter to block scattered laser light. The laser was synchronized to the engine by a perforat-


    co .



    LI 0 Z ILl

    bOo Ld~ Cd 0

    __J L

    I I P(2) ~ R(2)

    p(4) R(0)ll (4)

    o I I 16850 16845 16860 16875

    FREQUENCY (cns -~) FIG. 2. a (upper), Fluorescence excitation spectrum of NO2 in At. Engine stationary, b (middle), Fluorescence excitation spectra of NO~ in N2. Engine stationary, c (lower), Spectrum on NO2 expanded in combustion gases, engine running, with NO~ introduced at air intake.

    ed disk mounted on the drive shaft, light-emitting diode, and photodiode trigger circuit. The laser flashed on al- ternate cycles (every fourth revolution) to make the rep- etition rate compatible with the laser power supply. The photomultiplier output was integrated by a boxcar av- erager, digitized, and stored on a small computer. The laser frequency calibration was established by the mea- surement of the optogalvanic spectrum of Ne. The fie- quencies are accurate to about 1 cm -1.

    A portion of the fluorescence excitation spectrum of N02 in Ar injected through the spark plug hole of the stationary engine is shown in Fig. 2a. This band has a vibronic origin at 16,849.8 cm-M The rotational assign- ments are due to Smalley et al., ~ with only peaks from the K = 0 manifold being labeled. The spectral resolu- tion is limited to 0.25 cm -1 by the dye laser which was used with no intracavity etalon for this scan.

    A rotational temperature of 8 K is estimated for this spectrum from comparison with a series of stick spectra for various temperatures. The precision is about 10%. While the rotational temperature is not a rigorously de- fined quantity, since the sample is not in thermal equi- librium, it is a convenient way of expressing the degree of concentration of spectral intensity in the low-angular- momentum transitions.

    A similar scan with N2 as the carrier gas is shown in Fig. 2b. The cooling is noticeably less pronounced, with


  • increased intensity at higher angular momentum. The rotational temperature is now estimated to be 18 K in qualitative agreement with the expectations mentioned earlier.

    Results with the engine running and the NO2 cooled by the expanding combustion gases are shown in Fig. 2c. The important feature of this curve is the close resem- blance to Fig. 2b. The rotational cooling is as good as can be obtained with pure N2 as the carrier gas. The relative intensities for this scan are represented by a rotational temperature of 16 K with an uncertainty now of 20% because of the increased noise.

    The present work demonstrates that it is possible to obtain a rotationally cooled sample of combustion gases with a simple continuous-flow nozzle mounted directly in the cylinder head of an internal combustion engine. Despite the polyatomic nature of the expansion carrier gas, rotational temperatures as low as 16 K can be ob- served. Fluorescence excitation spectra with this degree of cooling show substantial simplification with respect to those of samples at or above ambient temperature.

    The apparatus for this work was assembled for pre- liminary measurements and is far from optimum. Much of the noise with the engine running is due to photo- multiplier microphonics and fluctuations in the engine revolution rate. Because the 50-#m nozzle diameter is comparable to or smaller than the wall quench distance and because of the recessed nozzle aperture, it is prob- able that gases sampled in these measurements were pri- marily from the boundary layer. For practical experi- ments, the nozzle aperture should be flush with the combustion chamber wall or even in a conical intrusion. 6 The interaction of the laser with the expanding molec- ular beam would be easier to accomplish with a hemi- spherical combustion chamber geometry. If necessary, a differential pumping scheme could be used to permit the use of larger-nozzle apertures. It should not be difficult to combine the present laser-induced fluorescence mea- surements with a mass spectrometer interface such as developed by Knuth and coworkers. 4-6 Laser ionization techniques could be explored for further resolution en- hancement if desired. 1

    1. R. E. Smalley, L. Wharton, and D. H. Levy, J. Chem. Phys. 63, 4977 (1975).

    2. R. E. Smalley, L. Wharton, and D. H. Levy, Accounts Chem. Res. 10, 139 (1977).

    3. D. H. Levy, Ann. Rev. Phys. Chem. 31, 197 (1980). 4. W. S. Young, W. E. Rodgers, C. A. Cullian, and E. L. Knuth, in

    Rarified Gas Dynamics, Seventh Symposium, D. Pini, C. Cercig- nani, and S. NociUa, Eds. (Editrice Tecnico Scientifica, Pisa, 1971), Vol. I, p. 647.

    5. W. S. Young, Y. G. Wang, W. E. Rodgers, and E. L. Knuth, in Technology Utlization Ideas for the 70's and Beyond, F. W. Forbes and P. Dergarabedian, Eds. (American Astronautical Society, Tar- zana, 1971), p. 281.

    6. E.L. Knuth, in Engine Emissions, Pollutant Formation and Mea- surement, G. S. Springer and D. J. Patterson, Eds. (Plenum, New York, 1973), p. 319.

    7. W. A. Daniel, in Sixth Symposium (International) on Combus- tion (Reinhold, New York, 1957), p. 886.

    8. H. Ashkenhas and F. S. Sherman, inRarified Gas Dynamics, Fourth Symposium, J. H. de Leeuw, Ed. (Academic, New York, 1966), Vol. II, p. 84.

    9. R. E. Smalley, D. L. Ramakrishna, D. H. Levy, and L. Wharton, J. Chem. Phys. 61, 4363 (1974).

    10. D. M. Lubman and M. N. Kronick, Anal. Chem. 54, 660 (1982).

    Polarization Fluorometry to Determine Cell Density with Fluorochrome for DNA

    TSUTOMU ARAKI* and MASAOKI YAMADA Laboratory for Cytochemistry, Department of Anat- omy, School of Medicine, Tokushima University, Ku- ramoto, Tokushima 770, Japan

    Index Headings: Determination of cell number; Polarization flaorome- try; Hoechst 33258 fluorochrome; Background correction; Fluorescence for DNA.


    There is a need for a rapid and sensitive technique to estimate a small number of cells. The total number of both living and dead cells suspended in a solution can be measured directly with the use of the Coulter counter or a flow-cytometer, but both of these measurements are inaccurate for small bacteria, such as an Escherichia coli cell (0.6 #m diameter x 2 ~m length). The bacterial density is determined by the measurement of the tur- bidity of a cell suspension with a nepherometer or, more conveniently, with a spectrophotometer. However, for- eign particles (e.g., dust, protein, or lipid) often increase turbidity and result in an error in the assay. Fluoromet- ric assay for DNA content provides a sensitive estima- tion of the cell density free from turbidity changes. For example, Hill and Whatley 1,~ used an antibiotic drug (mithramycin) as a labeling fluorochrome to measure cell DNA density with a lower limit of 0.5 #g. Recently, more sensitive quantification of DNA has been achieved with a bibenzimidazole dye, Hoechst 33258 (H33258). 3-5 This dye reacts with the adenine-thymine-rich region of DNA, resulting in a high quantum efficiency for the de- termination, s Downs and Wilfinger 7 applied H33258 to quantify DNA measurement, and they achieved detec- tion limits as low as 4 ng for extracted DNA from pi- tuitary cells.

    We used H33258 in a fluorometric assay to determine bacterial cell density. In the ordinary assay of cellular DNA, one extracts the DNA from cells to increase the dye's accessibility to DNA, and adjusts the concentra- tion of the dye relative to the amount of DNA to obtain spectral linearity. Linearity must be established since the fluorescence spectra and quantum efficiency of the DNA-H33258 complex change 4 depending on the con- centration ratio of DNA to dye.

    To simplify the assay procedure, we have added H33258 directly to the cell suspension without extrac- tion of DNA. A background fluorescence due to unreact- ed free dye was removed optically, without removal of the dye, by the detection of the polarized component of the fluorescence. Because the cells were immersed in an excess volume of dye solution throughout the assay, the

    Received 7 February 1985. * Author to whom correspondence should be sent.

    106 Volume 40, Number 1, 1986 0oo3-7026/66/4o0~-o~o652.00/o APPLIED SPECTROSCOPY 1986 Society for Applied Spectroscopy


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