Since its introduction in the mid-sixties, Laser Doppler Velocimetry (LDV) has become one of the most widely used methods for the measurement of flows. Its remote and essentially non-intrusive nature provides an invaluable tool for a variety of difficult measurement situations which would be otherwise inaccessible. The high spatial resolution and rapid temporal response afforded by this technique are well suited to the determination of spatial and temporal details of flow fields, as well as characterization of turbulence. Advances in the understanding of the properties of LDV signals, accompanied by technological advances in coherent laser sources, detectors of high sensitivity and low noise, optical fabrication techniques and high-speed digital signal processing architectures have resulted in systems of increased accuracy and flexibility. As will be shown, recent progress in solid-state lasers and photo-detectors has been beneficial insofar as the compatibility of this method with the unique and severe constraints inherent in microgravity combustion science experiments

Greenberg, Paul S.

English

NASA Technical Reports Server

N96- 15590
LASER DOPPLER VELOCIMETRY AND FULL-FIELD SOOT VOLUME FRACTION
MEASUREMENTS IN MICROGRAVITY
Paul S. Greenberg
NASA-Lewis Research Center
Cleveland, Ohio
Laser Doppler Velocimetry
Since its introduction in the mid-sixties, z laser doppler velocimetry (LDV) has become one of the most widely used
methods for the measurement of flows. Its remote and essentialiy non-intrusive nature provides an invaluable tool
for a variety of difficult measm'ement situations which would be otherwise inaccessible. The high spatial resolution
and rapid temporal response afforded by this technique are well suited to the determination of spatial and temporal
details of flow fields, as well as characterization of turbulence. Advances in the understanding of the properties of
LDV signals, accompanied by technological advances in coherent laser sources, detectors of high sensitivity and low
noise, optical fabrication techm__Lquesand high-speed digital signal processing architectures have resulted in systems
of increased accuracy and flexibility. As will be shown, recent progress in solid-state lasers and photo-detectors have
been beneficial insofar as the compatibility of this method with the unique and severe c.onstraints inhe.rem in
microgravity combustion science experiments.
In brief, LDV relies on the doppler shift of optical radiation imparted by the velocity of the fluid flow. 2 Generally,
this requires the inm3duction of small seed particles into the flow to serve as optical scattering centers. The size and
mass of these seed particles must therefore be carefully considered to insure that their velocities are suitably
indicative of the velocity of the fluid medimn in which they are entrained. This property is usually expw_.d as the
aerodynamic or hydrodynamic diameter of the particle 3 and is defined as the diameter of a unit density sphere
possessing the same settling velocity as the particle in question. For the case of spherical particles,this expression
simplifies to d_ = a_o)m, where a is the actual diameter and P is the mass density. The frequency response of
this equivalent particle is then evaluated for its ability to respond to flow-field ac_erations. The relatively low
velocities, moderate tmbulence intensities, and large spatial scales in most microgravity combustion phenomena allow
the use of larger pa_tieles. The result is large scattering cross sections, and thus acceptable signal-to-noise ratios in
the presence of conzpaet, low power laser sources and modest optical collection efficiencies. As an exaznple, for a
band-limited turbulent specwa of 104 Hz., a 1.0 micron particle yields a value of (Vp/Va) = 0.98, where Vpis the
particle velocity and vt is the actual velocity of the fluid flow. At a maximum frequency Of 103 Hz., this ratio is
still achievable by the use of 5.0 micron particles.
Rudimentary analysis of a laser doppler signal demonswates that the frequency of the incident radiation is doppler
shifted by an amount _'1 = K. v, where K = ks- ki, where v is the velocity of the entrained particle, the latter
quantities representing the wave vectors of the scattered and incident radiation, respectively. The most common
physical realization of an LDV system is the so called dual-doppler, or fxinge configuration. This is shown
schematically in figure 1. The net result of the two incident beams can be thought of as producing a set of periodic
fringes throughout the spatial volume defined by their intersection. Particles passing through this intersection region
and traveling wansverse to the on'entation of these fringes produce a scaUered signal whose periodicity is given by:
The signal resulting from the passage of a single particle is referred to as a doppler burst; a burst produced f_m a
247
https://ntrs.nasa.gov/search.jsp?R=19960008424 2020-06-16T05:22:04+00:00Z
f = [v.g./IK I] 2 (sine)/x_ (_)
typical LDV system is shown in figure 2. For most systems encountered in practice, the fa'inge spacing results in
observed dual-doppler frequencies on the order of 105 to 106 Hz./meter/second.
The system represented in figure 1 is configured in a coaxial, backscatter arrangement. This affords two principal
advantages:1)thesystemissingle-ended(i.e.thetransmissiona d detectionopticsareon thesame sideofthetest
section),and 2)itiseasiertomaintainatignmcntofthetransmissiona d demctionsamplevoltt_es.The latteris
atmq)utablebothtothemore compactgeometryand tothefactthattheincidentand scatteredlightraversethesame
opticalpath.Thiseliminatesproblematicbeam steeringeffectsproducedby refractiveindexgradientsthatareoften
presentintheflow-felditself.The disadvantageofa coaxialbackscatterconfigurationarisesfrom thefactthat
differentialscaReringcrosssectionsarepeakedintheforwarddirection,thusoverallcollectionefficiencyisreduced.
Again,for thevelocityfieldstypicalof microgravitycombustionphenomena,thisisnot outweighedby the
advantagesofa backscatterconfiguration.
As previouslyindicated,technologicaladvancesinsolid-statelaserdiodesand avalanchephoto-detectors(APD's)
haverecentlybeenutilizedtoconstructcompact,mechanicallyrobustLDV systems.Suchsystemsofferextremely
modestelectricalpower consumptionaswell.Diode lasersourcespossessingreasonablecoherenceand geometric
radiationpropertiesarenow routinelyavailableon a commercialbasis.Activetemperaturecontrolcircuitryis
required,however,toprovidemodal stability.Incontrast,more conventionalLDV systemsthatusegasdischarge
lasers are too large, fragile, and consume too much power for reduced gravity applications.
HighperformanceAPD's and associatedpreamplifiersarenow similarlyavailable.Althoughphotomultipliertubes
(PMT's) providehigherabsolutegains,APD's are shown to be more suitableforthispurposedue to their
substantiallyargerquanttunefficiencies(QE) (=80% foran APD vs.=14% fora PMT). Becausephotonnoiseis
givenby (n/QE)ta ,wheren isthemean photonarrivalrate,APD's providea superiorexcessnoisefactorinthis
application,providinga three-foldimprovementin signalto noiseforpeak scatteredfluxesapproaching100
microwatts.APD's alsorequirehighvoltagepower supplies;incorporatingthisintoa compact packagewhile
providingthenecessarydegreeofshieldingand isolationrequirescarefulconsideration.Additionalimprovcmcnt
insignaltonoiseratioisachievedby coolingthedetector.Compact LDV systemsofthistypearepresentlybeing
utilizedintheMicrogravityCombustionDiagnosticsDevelopmentLaboratoryattheNASA-Lewis ResearchCenter.
Shown infigure3 isvelocityfieldatafrom anon-reactinggasjetobtainedwithsucha device.The dopplerburst
shown infigure2 was alsoobtainedwiththissystem.The particulardeviceusedtocollectthisdatameasures65
mm indiameterand 200 mm inlength,and consumesapproximately12wattsofelectricalpower.Containedwithin
this package are the diode laser source with active temlxn-atnre control circuitry, the APE) with the required high
voltage DC-DC converter, a low noise preamplifier, and a 50 ohm line driver.
The relatively modest f numbers associated with these compact systems (=f/6) decreases the spatial resolution of
the sample volume in the axial direction. The axial extent of the sample volume is on the order of 1.5 mm for
present systems, limiang the ability to perform measurements in close proximity to test section walls or other
boundaries. Scasholtz, et al4 demonstrated that cousiderable improvement can be obtained in this regard through
improvements in the transmitting optics, and confocal rn_ng.
Significant advances have also beenmade in the area of dedicated signal processors. To provide on-line estimates
of velodty spectra at bandwidths typical of LDV signals, early counter-type processors relied on zero-crossing signal
detection. Over the past several years, high speed digital signal processing arckitectmes have enabled the calculation
and subsequent parameter estimation of complete spectra, either in the form of Fourier spectra or temporal
autocorrelation functions. Processors of this type have yielded a ten-fold improvement in the accuracy of the
resulting velodty estimates, and are able to operate at lower overall signal to noise ratios. 5 A system of this type
is being utilized in mierogravity combustion science research at the Lewis Research Center, and provides several
248
uniquefeaturesrelevantothisapplication.Specifically,thisprocessorhasbeenconfiguredintheformoftwo 16-bit
ISA cardsforuseina conventionalpersonalcomputer.Thisaffordsconvenienceand flexibilityforutilizationn
the LeRC drop tower and aircraft facilities; it is also anticipated to be advantageous for space Right applications.
Also included is the capability to function as a digital transient recorder. This is extremely valuable for drop tower
and aircraft studies, allowing raw data to be acquired and archived for subsequent post-processing.
Soot Volume Fraction Imagin_
Soot is of fundamental interest from the standpoint of fuel and combustor efficiency, hardware longevity, and its
relationship to public health. A comprehensive understanding of soot formation, aggregation, and oxidation
mechanisms is, however, far from complete. Details concerning precursor chemistry, morphology, and overall
production rates (i.e. volume fractions) remain active areas of study. For more than a decade, optical extinction
methods have been utilized as an important tool for the determination of soot volume fractions in combustion
applications. 6'7 More recently, investigations have focused on providing detailed, spatially resolved measurements
of soot particle concentration fields, s'9 These techniques have benefitted f_m the availability of coherent,
monochromatic sources (i.e. lasers), owing to their well defined spectral properties, critical to interpreting the
spcctrally dependent properties of carbonaceous soot itself and to the advantages afforded in optical beam
conditioning and manipulation.
The analysis of optical extinction data involves the determination of local soot volume fractions from a set of
integrated line-of-sight measurements. Assumptions required to validate the inversion of this data include parallel,
chord-like ray trajectories, negligible refraction effects, no significant absorption or scattering from media other than
soot particles, an absence of radiant emissions corresponding to the wavelength of the laser source, and a knowledge
of the spoctrally dependent scattcaing and absorption properties of the soot itself, and the optical path length through
the SOOt-containing medium. The latter is usually handled by the selection of a combustion phenomenon possessing
axisymmetry. Well known Abel transformation equations may then be invoked.
Given the above assumptions, the extinction of optical radiation due to the presence of soot may be expressed as:
cu--A=-k_ (2)
ds
where k_ is the optical extinction coefficient evaluated at the wavelength of the source. Integration of equation (2)
along chord-like paths through the flame yields the relation:
(3)
In the Rayleigh scattering limit, the volume fraction is then related to the extinction coefficient by:
f, = _qJ(6_E(m)) (4)
where E(m)= -Ira (m2-1)/(m2+2)), m being the complex refractive index for soot particles. Spatially localized values
of fv are then obtained from equations (3) and (4) via tomograpttie inversion.
To date, studies have retied on sequential point-by-point interrogation of the soot field. Such methods are not well
suited to investigations involving nourepoatable time dependent or transient phenomena. In the context of
microgravity combustion science, where constraints on overall experiment duration and/or available expendable
resources are often encountered, this limitation can be difficult to overcome.
249
To address this need, a full-field, or image based technique was developed, w It provides the capability to perform
measurements at 2.5 x I05 simultaneous spatial locations at a _h rate of 30 Hz. The total field of view
corresponding to this sample set is determined by the receiving optics. An appredation for the benefits of this
enhanced capability is immediate when viewed in the context of aircraft or drop tower tests; by using a point
measurement teclmique, it would be difficult to obtain more than a few selected data points during the overall
experiment run time. Given the desire to reach some reasonable approximation of steady-state conditions, the
validityoftemporalcorrelationsbetweentheseindividualdatapointswouldbequestionable.Additionally,thevalue
ofan image ofthecompletesootfieldtoqualitativeunderstandingisimmense.
Soot volume fraction data corresponding to a 3.85 cc/scc i_minsr ethylene diffusion flame is shown in figure 4. In
this case, the burner is stabilized with an annular co-fiow of air. Data obtained with this method are observed to
be in dose agreement with previously published expcriment_ results obtained via pointwise measurements, s The
present limitation of this technique is absolute sensitivity. The high temporal bandwidths required to sustain the
described data ram of 7.5 megabytes per second provides a large noise equivalent power (NEP) as well. If overall
data rate is preserved, a signal to noise ratio of one corresponds to a line-of-sight extinction value of roughly two
perc.cnL
Apparatus to perform soot volume fraction imaging has also been implemented within the relatively confined volume
of a 2.2 second drop package. This does not include the provision for on-line data storage, which is presently
accommodated by telemetric link. A comparison of soot volume fractions corresponding to a 1.5 mm laminar gas
jet diffusion flame with a 50/50 [partial pressure] acetylene/nitrogen mixuae operating under both normal and
reduced gravity conditions is shown in figures 5 and 6. It can be readily observed that the soot shell occurs at
significantly larger radii, and that the maximum value of volume fraction is approximately twenty percent larger
under reduced gravity. Under these same conditions, the wansmitivity is reduced by over forty percent, indicating
a significant increase in overall soot production.
References
I. Yeh, Y., and Cummins, H. 7.., "Localized Fluid Flow Measurement with a He-Ne Laser Spec_meter," Applied Physics Letters, Vol. 4,
No. I0, 1964, pps. 176-178.
2. Edwards, 1LV., Angus, J. C.., French, M. J., and Dunning, J. W., "Spectral Analysis of the Signal from the Laser Doppler Flowmeter: Trine-
Independent Systems," Journal of Applied Physics, Vol. 42, No. 2, 1971, pps. 837-850.
3. Agarwal, J. IC, and Fingerson, L M., "Evaluation of Various Particles for Their Suitability as Seeds in Laser Velocirnetry," Laser
Velocimetry and Pax'tide Sizing, Proceedings of the Third International Workshop on Laser Velocimetry, Purdue University, Hemisphere
Publishing Coqxxafion, 1978.
4.
5.
Seasholtz, R. G., Obefle, L G., and Weikle, D. H., "Olximization of Frlngo-Type Laser Anemometers for Turbine Engine Component
Testing," AJ.AA/SAE/ASME 20 m Joint Propulsion Conference, Cindnnati, OH, June 11-13, 1984.
Hepner, T. E., "State-of-the-Art Laser Doppler Valocimeter Signal Processors: Cafibration and Evaluation," A/AA 32 mlAerosi)ace Sciences
Meeting, Reno, NV, January 10-13, 1994.
6. Haynes, B. S, and Wagner, H. G., "Soot Formation," Progress in Energy and Comixmion Sdence, 7, 1981, pps. 229-273.
7. T'ien, C. L., and Lee, S. C., "Flame Radiation," Progress in Energy and Combustion Science, 8, 1982, ppe. 41-59.
8. Santcco, R. J., Semerjiau, H. G., and Dobbins, R. A., "Soot Particle Measurements in Diffusion Flames,* Combestion and Flame 51, 1983,
pps. 203-218.
9. Gore, J. P., and Faith, G. M., "Structure and Radiation Properties of Turbulent Ethylene/Air Diffusion Flames," Twenty-F'wst Symposium
(International) on Comim_on, The Combustion Institute, 1986, ppe. 1521-1531.
10. Grenaberg, P. S., Ku, L C., Lin, IL -C.., K6ylfi, O. 0., and Faeth, G. M., "Soot Volume Fraction Imaging," nmauscript in preparation.
250
Figure 1: Schematic of dual-doppler LDV in backscatter configuration
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Figure 3: Velocity Field of Re 1438 Non-reacting Jet
251
?- 
Fgum 4: Absorbance and Soot Volume Ftaactioos: 3.85 d s e c  laminar ethylene d ~ o n  Rame 
Fgum 5: Absorbanca and soot Vdune Fmctions: 23 dsec  kminar ace-n diffusion flame: Normal Gtavity 
F y m  6: Absorbance and Soot Vdume Fractions: 2 3  d s e c  laminar gatylene/nibogen diffusion flame; ReducedGtavity 
252 


Laser Doppler Velocimetry and full-field soot volume fraction

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