Preliminary results from applying a Kerr-Fourier imaging system to a water/air spray produced by a shear coaxial element are presented. The physics behind ultrafast time-gated optical techniques is discussed briefly. A typical setup of a Kerr-Fourier time gating system is presented

Alfano, R. R.

Breisacher, Kevin

Galland, P.

Ho, P. P.

Liang, X.

Liou, Larry

Wang, L.

English

NASA Technical Reports Server

- N95. 70907
Picosecond Imaging of Sprays
7
L. Wang*, X. Liang*, P. Galland*
Kevin Breisacher and Larry Liou, NASA Lewis, Cleveland, Ohio
P. P. Ho* and R.R. Alfano*, CCNY IUSL (NASA/IRA), New York, N.Y.*
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Summary
Preliminary results from applying a Kerr-Fourier imaging system to a water/air spray
produced by a shear coaxial element are presented. The physics behind ultrafast time-gated
optical techniques is discussed briefly. A typical setup of a Kerr-Fourier time gating system is
presented.
Discussion
In attempts to characterize the spray of a simulated coaxial rocket engine injector t it
became clear that it was not possible to make drop size measurements any closer than 2 in.
downstream of the injector face. The applicability of dropsize correlations developed from
cold flow measurments 2 in. +downstr_ of _e injector face is debatable. The details of the
evolution from jet to spray (cdtcal to modeling) are lost. Ultrafast optical imaging provides
the possibility of making diagnostic measurements in dense sprays.
Various ultrafast time-gated optical imaging techniques 2+ such as Kerr-gate, holography,
streak camera, etc., have been used in attempts to identify hidden obects in turbid media for
use in medical diagnosis. Recently, a collaborative effort was established between researchers
at Lewis Research Center and the Institute for Ultrafast Spectroscopy and Lasers
(NASA/IRA) at the City College of New York to see if these techniques might be applicable
to making diagnostic measurements in the dense sprays encountered in liquid rocket engines.
When ultrashort laser pulses are incident upon a slab of scattering medium the transmitted
pulses consist of a ballistic component, a diffuse component, and a snake component. The
intensity of the ballistic component (consisting of photons traveling along a straight line path)
is attenuated exponetially with the thickness of the material they are traversing.This
attenuation puts a severe limitation on the use of ballistic photons in imaging applications.
The diffuse component consists of photons that have been scattered randomly in all directions
and have different path histories through the material. The snake component consists of early-
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arriving photons that have undergone only a few scatterings along an almost straight path
through the material. The snake photons form the early component of the diffuse photons and
carry information about the optical characteristics of the material they have traversed. Time
resolved detection techniques separate these snake photons from the rest of the diffuse
component and use them to construct images of objects in the material with different optical
characteristics.
As part of the collaborative effort between Lewis and CCNY, a spray rig (Figure 1.) has
been installed at the Mediphotonics Laboratory at CCNY. The spray rig consists of a single,
water/air coaxial element spraying into a plexiglass test section. The inner diameter of the
water tube is .078" and the inner and outer diameters of the air annulus .125" are .219" and
respectively. The dimensions are similar to those used in the test program of refernce 1. The
test section is approximately high and 3.75 by 8 inches. Provisions have been made for
optical access through two windows at the top of the test section near the injector face.
In the first phase of this effort a Kerr-Fourier picosecond imaging system developed by
CCNY will be used to image the water jet through the dense spray of the coaxial element.
The Kerr-Fourier imaging system is shown in Figure 2. The imaging system consists of three
main parts: a laser source, an optical Kerr gate (which acts as an ultrafast shutter), and a
detector. A picosecond mode locked Nd + glass laser laser system which emits a 1054 nm 10
ps laser pulse is used as an illumination source. Its second harmonic (527 nm)is used as the
gating source. The Kerr gate consists of a pair of calcite crossed polarizers and a lem CS2
Kerr cell. A cooled CCD camera system was used to detect the gated image.
Figures 3 and 4 show the preliminary results obtained with the ultrafast imaging system. All
flowrates quoted are approximate due to unsteadiness_ in the supply system. Figure 3 shows
an image of a water jet at approximately 1.5 em downstream of the injector face. The wat6r ....
and air flowrates were 10 gls and .8 gls respectively. The image system configuration used to
produce Figure 3 did not include a Kerr gate. The circular border corresponds to the 1 em : _:_
diameter region iUumunated. At this relatively low air flowrate, the jet remains largely intact
and its sinuous, helical structure is evident. The contrast obtained is at least as good as that
obtained with laser sheet imaging. A result obtained with the Kerr-Fourier imaging system is
displayed in Figure 4. The water and air flowrates were 5 g/s and 1.1 g/s respectively. In
Figure 4, the imaging region is at the injector face. The tip of the water tube is visible as it
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protruded several millimeters from the injector face. The tube was extended from the injector
face so that the imaging system would not encounter liquid hanging from from the injector
face due to recirculated drops. At this higher air flowrate, the stripping of liquid ligaments
and drops is evident. However, a contiguous liquid jet remains. Image contrast remains
relatively sharp but it is apparent that better resolution is required in imaging individual drops
or ligaments. CCNY is currently at work on a lens arrangment for increased resolution.
Further work will include attempts to image regions of the spray donstream of the jet at this
higher resolution.
References
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1) Zailer, M. Z. and Klem, M. D., "Shear Coaxial Injector Spray Characterization"
2) Wang, L., Ho, P.P., Liang, H. D., and Alfano, R. R., _"Ke-rr-Fourier imaging of hidden
objects in thick turbid medium", Optics Letters, 1993.
3) M. A. Duguay and Mattiek, Applied Optics, 10, 1971.
4) Alfano, R. R., Liang, X., Wang, L. and Ho, P.,"Time Resolved Imaging of Translucent
Droplets in Highly Turbid Media" ,Science, pp. 1913-1915, June 1994.
5) Rebane, A. and Feinberg, J., Nature, 351, 1991.
6) Yoo, K. M. and Alfano, R. R., Opt. Letters, 15, 1990.
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Figure 1. - Water / Air Spray Rig
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1. ML mode-locked picosecond laser
2. Beam splitter
3. Movable prism for varying delay time
4. Object to be imaged in turbid medium
5. Kerr cell
6. CCD
1054 nm
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Figure 2. - Kerr - Fourier Imaging Setup (CCNY)
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Figure 3. - Non - Kerr gated image
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Figure 4. - Kerr gated jet image
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equal access to programs, admission, and employment without regard to race, religion, sex, national origin, handicap, age or status as
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Picosecond imaging of sprays

Picosecond imaging of sprays

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