The investigation of liquid jet breakup and spray development is critical to the understanding of combustion phenomena in liquid propellant rocket engines. Much work has been done to characterize low-speed liquid jet breakup and dilute sprays, but atomizing jets and dense sprays have yielded few quantitative measurements due to their high liquid load fractions and hence their optical opacity. Focus was on a characteristic of the primary breakup process of round liquid jets, namely the length of the intact-liquid core. The specific application considered is that of shear-coaxial-type rocket engine injectors in which liquid oxygen is injected through the center post while high velocity gaseous hydrogen is injected through a concentric annulus, providing a shear force to the liquid jet surface. Real-time x ray radiography, capable of imaging through the dense two-phase region surrounding the liquid core, is used to make the measurements. The intact-liquid-core length data were obtained and interpreted using two conceptually different methods to illustrate the effects of chamber pressure, gas-to-liquid momentum ratio, and cavitation

Burch, Robert

Cheung, Fan-Bill

Kuo, Kenneth

Woodward, Roger

English

NASA Technical Reports Server

N 94- {j
MEASUREMENT OF INTACT-CORE LENGTH
OF ATOMIZING LIQUID JETS BY IMAGE DECONVOLUTION
Roger Woodward, Robert Butch, Kenneth Kuo, and Fan-Bill Cheung
Propulsion Engineering Research Center
and
Department of Mechanical Engineering
The Pennsylvania State University
University Park, PA 16802
SUMMARY:
The investigation of liquid jet breakup and spray development is critical to the understanding of combustion
phenomena in liquid-propellant rocket engines. Much work has been done to characterize low-speed liquid jet breakup
and dilute sprays, but atomizing jets and dense sprays have yielded few quantitative measurements due to their high
liquid load fractions and hence their optical opacity. This work focuses on a characteristic of the primary breakup
process of round liquid jets, namely the length of the intact-liquid core. The specific application considered is that of
shear-coaxial-type rocket engine injectors in which liquid oxygen is injected through the center post while high-
velocity gaseous hydrogen is injected through a concentric annulus, providing a shear force to the liquid jet surface.
Real-time x-ray radiography, capable of imaging through the dense two-phase region surrounding the liquid core, is
used to make the measurements. The intact-liquid-core length data have been obtained and interpreted using two
conceptually different methods to illustrate the effects of chamber pressure, gas-to-liquid momentum ratio, and
cavitation.
TECHNICAL DISCUSSION:
The focus of this study is the measurement of the intact-core length of coaxial jets using X-ray radiography.
Two injector sizes are used, having liquid exit diameters and annular-gas-flow exit areas of 1) 4.8 mm (3/16 in) and
45 mm 2 (0.069 in 2) and 2) 2.4 mm (3/32 in) and 11 mm 2 (0.017 in2). The propellant simulants consist of a
solution of potassium iodide (KI) in water for the LOX and gaseous nitrogen or helium for the annular-flowing
hydrogen. Iodide is a X-ray absorber, thus allowing of line-of-sight imaging of the spray. A detailed description of
the test and image processing equipment, setup, and calibration has been given previously (1-3). Following Beer's
law, the measured radiance levels of the resulting image are a function of liquid thickness. Through the use of
calibration cells, the integrated liquid thickness at each point in the jet can be obtained.
In the course of this study, two different methods of data analysis have been used to determine the intact
core length from the X-ray images: a threshold criterion technique based on the integrated liquid thickness, and a
threshold criterion based on mean liquid volume fraction using a deconvolution technique. In previous studies by this
research group [ 1-3], the intact-core length was obtained from the processed images using a threshold value based on
an integrated liquid thickness corresponding to the end of the intact core. The threshold criterion was selected to be
the mean radiance of the 1.6 mm calibration cell for the larger injector and for the smaller injector, a radiance level
corresponding to one-fourth that thickness. The core length was measured directly from the image for each of ten
https://ntrs.nasa.gov/search.jsp?R=19940018556 2020-06-16T15:54:40+00:00Z
imagescorrespondingtothatoneflowcondition.Thesemeasurementswerethen averaged for one mean value at each
condition. Figure 1 presents dimensionless intact-core length (Lb/D) results obtained using this image threshold
technique versus dimensionless chamber pressure for a range of gas velocities and two sets of liquid velocities (v t =
15 and 27 m/s). Figure 2 presents similar results for the smaller injector over a range of gas velocities and two sets
of liquid velocities (v t = 30 and 60 m/s). These results are discussed in detail by Woodward [3].
40
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Figure 1. Effect of chamber pressure (gas density) on
intact-core length for larger injector using
liquid thickness threshold criterion length.
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Figure 2. Effect of chamber pressure on intact-oore
length for injector B (smaller injector).
The selection of the aforementioned threshold criterion is somewhat arbitrary since it is based on an
integrated liquid thickness over the entire jet cross section and not an actual core diameter (although such a technique
is largely dependent on the core diameter). The alternate technique, based on image deconvolution, considers the
combined x-ray path length through the gas as well as the liquid, accounting for the two-phase regions of the coaxial
spray. This technique requires the following assumptions to determine radial distributions of the mean liquid volume
fraction: axial symmetry of the jet and statistically steady flow. The 33 ms exposures used are of a sufficient length
of time for the jet to adequately meet these assumptions. The time-averaged intact-liquid-core and the radial boundary
is demarcated by the departure from unity of the liquid volume fraction.
As recommended by Dasch [4], to accomplish the jet image deconvolutions, the three-point Abel inversion
has been applied to the present problem since it is relatively easy to calculate and was shown to be more robust and
less noisy in controlled tests cases than other common methods. All deconvolution techniques are very sensitive to
noise present in the original line-of-sight projection data. To improve the smoothness of the x-ray absorption
profiles across the liquid jet in the normalized injection images, the entire series of ten images per test case is
averaged together to create one clean normalized image at that injection condition; and lastly, a spatial smoothing is
applied to this averaged image.
Todeterminemeanliquidvolumefractions,whichbydefinitionvaryfromzerotounity,thedeconvolution
resultsarenormalizedbycenterliner sultsveryneartheinjectorexitwheretheliquidvolumefractionisatleast
approximatelyunity.Theintact-liquid-corelengthis determinedfor eachimagebythresholdingtheresultant
deconvoluted-jetimageatameanliquidvolumefractionvalueof0.9.Thecorelengthisthenmeasuredintermsof
pixelsdirectlyfromthisimage.
Figure3presentsheintact-corel ngthresultsobtainedfromjet imagedeconvolutionsfortestsconducted
withthelargerinjector.Dimensionlesscorelengthisplottedversusdimensionlesschamberpressure.AsinFig.1,
twosetsofliquidjetvelocitydata(vt=15and27m/s)arerepresented.It isapparentfromFig.3thatthecore-length
curvesrepresentingconstantgasandliquidvelocityhaveashapeandtrendverysimilartotheirthresholdtechnique
counterpartsinFig.1.However,sincethesefiguresbeingcomparedareplottedatthesamescale,it isobviousthat
theresultsobtainedfromthejet imagedeconvolutionsareconsiderablyshorterthanthoseobtainedusingthe
thresholdtechnique.Thereductioni intact-corel ngthcomparedtothepreviousresultsisnotofaconstantfactor.
Onaveragefor theresultsreportedinFig.3,theintact-corel ngthsfromthedeconvolutedj t imagesare40%
shorterthanthoseobtainedusingthethresholdtechniquewiththe1.59mm(1/16in.)thresholdcriterion.This
reductionfactorvariesovera rangeof 20%to nearly70%for oneextremecase.Althoughtheintact-core
measurements resulting from the determination of liquid volume fraction distributions are much shorter than the
corresponding results from the threshold method, the single jet tests in the non-cavitating regime (at Pc = 4 atm and
above for v l = 15 m/s and at Pc = 10 atm and above for v l = 27 m/s) still indicate off-scale (> 30 liquid exit
diameters) core lengths. On the other hand, the v l = 30 m/s, Vg = 65 m/s core length result at an ambient pressure
condition of 15 atm that was previously off-scale now shows a measurable intact length of 19.8 injector diameters.
Although the deconvolution results presented in Fig. 3 match in general the breakup length trends exhibited
by the corresponding threshold method results, there is one data point that shows a major discrepancy. The point at
15 atm on the v l = 27 m/s, Vg = 150 m/s curve is below its neighboring points at 10 and 20 atm. Based on the
other v! = 27 m/s curve of Fig. 3 and the corresponding results of Fig. 1, the 15 atm point in question should be
near the peak of the curve. Also, the v! = 15 m/s, Vg = 65 m/s data point lies above the higher liquid velocity point
as shown so that another disagreement occurs with the reasoning that the larger liquid jet momentum associated with
the higher jet velocity should result in a longer intact core length. It is suspected that this stray data point is an
anomaly and that previous findings should not be refuted by this.
Analogous to Fig. 3, Fig. 4 is a plot of the intact-core-length-via-deconvolution results obtained this time
using the smaller injector. While Fig. 3 appeared as a squashed but similar version of the corresponding threshold
results, Fig. 4 bears little resemblance to the corresponding threshold technique results in Fig. 2. Some of the
indicated core lengths are shorter and some are longer than their counterparts determined by the previous method. No
consistent trend is exhibited by these deconvolution results for the smaller injector. It is likely that the measurement
of intact-core length by the deconvolution method is more unreliable for the smaller injector. This is quite
conceivable considering the lower x-ray attenuation levels associated with the smaller diameter jet and the fact that
there are only a few pixels across the liquid jet. The spatial resolution as well as the gray level resolution may be
3
insufficienttoproduceadecentdeconvolutionfthesmallerjet,especiallyconsideringthattheinjectionimagesof
thesmallerinjectoraresubjecttothesamenoiselevelseeninthelargerinjectorimages.
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Figure 3. Intact-core length results obtained using the
image deconvolution method for injector A tests.
Figure 4. Intact-core length results obtained using the
image deconvolution method for injector B tests.
Two conceptually different methods have been used to determine intact-liquid-core lengths for coaxial
injection at realistic flow rate conditions: 1) thresholding of jet images to reveal the core region corresponding to a
specified liquid integrated thickness and 2) deconvolution of time-averaged jet images to get mean liquid volume
fraction distributions. Intact-core results between these two methods agree in form but not in magnitude.
Theoretically, the deconvolution method is the more correct one to use; however, it is difficult to obtain an accurate
deconvolution due to sensitivity to noise in the line-of-sight image data. Hence, the intact-core lengths obtained
using this method are of uncertain accuracy and even questionable for the smaller injector. The threshold technique
was applied to the larger injector by using a 1.59 mm (1/16 in.) integrated liquid thickness as the criterion to
determine the extent of the core. The consistently shorter deconvolution results indicate that this criterion thickness
may be too thin. A major problem with the threshold technique is that different thickness criteria are needed for
different size injectors.
REFERENCES:
1 Woodward, R. D., Burch R. L., Kline M. C., Kuo, K. K., and Cheung, F.-B., "Measurement of Intact-Liquid-
Core Length of Rocket Engine Coaxial Injectors," Fourth Annual PERC Symposium, NASA-MSFC,
Huntsville, AL, September 1992.
2 Woodward, R. D., Burch R. L., Kuo, K. K., and Cheung, F.-B., "Real-Time X-Ray Radiography Study of
Liquid Jet Breakup from Rocket Engine Coaxial Injectors," The Third International Symposium of Special
Topics in Chemical Propulsion: Non-Intrusive Combustion Diagnostics, Scheveningen, The Netherlands, May
1993.
3 Woodward, R. D., Primary Atomization of Liquid Jets Issuing from Rocket Engine Coaxial Injectors, Ph.D.
Thesis, The Pennsylvania State University, Aug. 1993.
4 Dasch, C. J., "One-Dimensional Tomography: A Comparison of Abel, Onion-Peeling, and Filtered
Backprojection Methods," Applied Optics, Vol. 31, No. 8, March 1992, pp. 1146-1152.


Measurement of intact-core length of atomizing liquid jets by image deconvolution

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