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AFUEL PCAYOSTRUCTURES

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SYMPOSIUM ON LONG RANGE AND SHORT RANGE

OPTICAL VELOCITY MEASUREMENTS

German-French Research Institute, ISL, Saint Louis,

15th - 18th September 1980

4C&S,?- 72- 7 37/9'

MfJEASUR T OF FEL SPRAY VAPORISATICkBY LAS TECNIOES.

by

A.J. Yule, C. Ah Seng, P.G. Felton, A. Ungut

and N.A. Chigier

Department of Chemical Engineering and Fuel TechnologyUniversity of Sheffield

Mappin Street, Sheffield SI 3JD, England

V 2u0 limitod.

-ASBUfT OF FUEL SPRAY VAPORISATIC

BY LASR TECHNIQUES

by

A.J. Yule, C. Ah Seng, P.G. Felton,

A. Ungut and N.A. Chigier

COMBUSTION AERODYNAMICS RESEARCH LABORATORY DTI cTA

*utUtif i,-J0 n_

Department of Chemical Engineering and Fuel Technology K- - - -University of Sheffield,i NYs,_~t~

Mappin Street, Sheffield Sl 3 JD, England S-t-r .. o..

p Availnbijity Cc-

Abstract o

Comparison of fuel spray structures in heated and in cold environments is made by

using a new laser tomographic technique and laser anemometry. The tomography

technique is shown to give accurate and rapid 'point' measurements of droplet

sizes and concentrations. Experimental results show acceleration of droplets to

the local gas velocity, preferential vaporisation of the smallest droplets and the

dispersion of droplets by the turbulence.

1. - Introduction

Recently there has been considerable interest in the use of light scatteringtechniques for making measurements inside two phase flows without disturbing theflow. These techniques have the potential for rapid data analysis, compared withthe relatively lengthy analysis times required for imaging techniques such as photo-graphy and holography. The authors1 have reported elsewhere on the analysis ofscattered light from a laser aAemometer for the measurement of particle sizes.We report here on entirely new methods for making measurements of droplet size dist-ributions and concentrations in sprays or aerosols, with good spatial resolution.The technique uses a measurement procedure and data analysis routines which aresimilar to those used in medical X-ray scanners by which line integrated data,

A A

across the volume of interest, are transformed into 'point' measurements byusing data inversion routines. Data derived by this technique are compared belowwith measurements made by spark photography. The technique is then used, in rcombination with laser anemometry, to measure the structure and vaporisation of afuel spray in a surrounding air stream.

2. Apparatus and Measurement Techniques

Laser Tomography

S: alar.e Abog LUghl Boom Maae .dF,om Et.ry Poi, of Spray

1! Wic a" UqMt Po.. V.*S,.d atEM. 1y to Spray

P R.sw. Light P *.r in 86m a tE..I Fom Spray

-xw

FIG 1. Fourier transform collection of scattered light from laser

beam traversing spray.

The laser tomographic technique makes use of Malvern Instrumentsf ST 1800 Particle

Size Meter. As shown in Fig 1 a 5mW He/Ne laser beam is passed through the sprayand the forward scattered light is collected by a Fourier transform lens system.The radial light power distribution is measured by using 30 concentric annulliphotodetectors and the particle size distribution is calculated from the powerdistribution by using an interfaced DEC PDP 8 computer. In its conventional usethis instrument rapidly gives an 'overall' size distribution which is effectively

representative of a 'line integral' of droplets within the beam across d completewidth of the spray. The tomographic technique uses a number of these line integralsthrough different parts of the spray and transforms these data into two-dimensionaldistributions of droplet size d4stributions and concentration over a cross-sectionof the spray. Tomography has been applied to several systems in recent years,particularly for medical X-ray brain and body scanners. Details of the generaltheory of the tomographic transformation if light scattering data have been qiven byYule et a12 .

IAA

For the particular case of axisymmetric, dilute sprays in which the totalextinction of light is small, the transformation is analogous to the Abeltransformation of spectroscopic data. Following the notation shown in Fig. 1, ,

P (w, r) (the scattered light power within the anglie__ originating from a unitelement of the spray at radius r,) can be related to Ps (w, p) (the totalmeasured light power for angle w for a complete scan of the spray at a perpen-dicular distance p from the spray centre). Measurements of P (, p) are madefor various values of p by traversing the spray across the laser beam. For eachscan, P is measured as a function of scattering angle w. These data are thentransformed by the relation,

P S(w, r) - Cp

r ff P2 - r2 )r

The transformed light scattering data P C(, r) are then used to calculate thelocal absolute volume distribution of droplets V (d) by an equation of the form

dP s (w, r) 3 . 3 P ax V X (d,u) d (d) _

2 dmin d

where X is the scattering function which is derived from geometrical optics andP is the illuminating power. Knowledge of the beam diameter permits the deriv-ation of the local volume concentration of droplets as a function of diameter.

Test Rig

The spray rig consisted of a twin, fluid atomiser spraying vertically downwardsinto a coflowing airstream which could be electrically preheated. In the experi-ments reported here, two atomisers were used. Atomiser A was a Spraying Systematomiser with liquid injected axisymmetrically and centrally into the atomising air.This atomiser was used to validate the tomography technique by making comparisonswith photography. Water was used with a flow rate 1.43 x 10i mm 3/s and airwith a flow rate of 6.8 x 104' mm 3/s.

Atomiser B was constructed from hypodermic tubing and was designed to cause minimumdisturbance to the secondary flow. This atomiser was used to inject keroseneinto the secondary stream for the study of fuel spray vaporisation.

Laser Anemome try

An OEI IDA system, with a Cambridge Consultants' tracker, was used to measure gasand droplet velocities for the sprays from atomiser B. Because the momentum of -

the liquid phase was everywhere very much less than that of the gas phase, it wasfound that gas velocity could be measured reliably by using the atomising air, butwithout kerosene injection.

3.- Results and Discussion

100

60

40 ___ -1-

U 20

0 30 60 90 120 150

Dianeter, pm

FIG. 2 Droplet size distributions obtained in spray by laser

tomography compared with measurements by spark photography.

We present here a sample of the results which can be seen more fully inReferences 2 and 3. Interpretation of data in terms of vaporising spray structureis also given in Ref. 3 Figure 2 compares size distributions measured in thespray from atomiser A by using the laser tomography technique and by spark photo-graphy with automatic image analysis. Size distributions are shown for thecentral and outer parts of the spray and also an overall size distribution isgiven for a line integral of the droplets in a complete width of the spray. Theagreement is seen to be good. The largest differences between the two techniquesoccur for measurements of the smaller droplets. Errors can be expected in thephotographic technique for small droplets. Figure 3 shows measurements oflocal droplet volume concentrations obtained by the two techniques. These andother data from the experiments have demonstrated the high degree of accuracy,speed and convenience of the lasertomography technique.

Figure 4 shows mean gas and mean droplet velocities measured in a spray fromatomiser B with 9 x 10 72 ml/s of kerosene and 1.52 x 102 ml/s of atomising air.

Droplet velocities represent ensemble averages of velocity for all dropletslarger than 5 jim approximately. The results clearly show the acceleration ofthe droplets until most droplets attain a velocity within a few percent of the b

local gas velocity at 100 mm downstream. Data3 for the same spray with highersecondary air temperatures show similar trends but in addition velocity decay

is slower for both gas and droplets due to entrainment of hot air.

CLR0 1.01

o ~ ~ ~ ~ ~ ~ -- -

w0.5

a 0 10 20 30 40

Radial distance. r(rnm)

FIG 3. Distributions of droplet volume concentration in a spraymeasured by laser tomography and photography.

11M

<- i-60 mm

a40mm ow *~

FIG. 4 Distributions of gas and droplet mean velocities in spray.

It can be seen from Fig. 4 that droplet velocities at the extreme outer edge of 1the spray are higher than the local gas velocity. The reason for this is seenby examination of Fig. 5, which shows radial distribytions of mass mean dropletdiameters measured by the laser tomography technique. Measurements are shownfor different distances downstream for the same spray but for two temperaturesof the secondary airstream. It can be seen that there is a sheath of verylarge droplets at the outer peripheries of the sprays and these droplets, withtheir relatively low drag/inertia ratios, are responsible for the largermeasured differences between mean gas and droplet velocities in these regions.Figure 5 shows that mean droplet sizes for the spray in the heated secondary flow,increase significantly with increasing distance downstream. This is cau§ed bythe preferential vaporisation of the smaller droplets. The size distributionmeasurements 3also show a narrowing of distributions with increasing distancedownstream which is alsoqualitatively expected.

1-. s -

-

P_

• ' ' 10100

,-Smm

l./. . x 6-mm

FIG. 5 Mass mean droplet diameters in sprays in 'cold' and heatedsecondary streams.

Figure 6 shows measurements of droplet volume concentration for the spray with twovalues of secondary flow temperature. The data clearly show the rapid vaporis-ation of the fuel spray at the higher gas temperature. It is seen that dlthoughthe outer sheath contains large droplets this region of the spray is very dilute :

•and contributes very little to the total volume of droplets on the total cross-section of the spray.

4. - Conclusions

A commercial line-of-sight light scattering, particle sizing instrument can becombined with tomographic data inversion and laser anemometry, to give measure-ments in sprays with good spatial resolution.

values~~~~~~~~~~ ~ ~ ~ ~ ~ ~ ofscnayfo eprtr.Tedt lal hwterpdvprs

Measurements of droplet sizes, concentrations and velocities are obtained .insufficient detail and with sufficient accuracy for the assessment of models offuel spray vaporisation and dynamics.

-20 -W 0 0 20040p

.1 A

I ::

-20 -1o 0 I0 20

'I .

FIG. 6 Droplet volume concentrations in sprays in cold andheated streams

5. Acknowledgement

The authors gratefully acknowledge support for this work by the NASA-Lewis Research

Center under Grant No. NSG - 7517. Support has also been provided by the ScienceResearch Council, The European Research Office of the U.S. Army and the AirForce Office of Scientific Research, U.S.A.F.

REFERENCES

1. Yule, A.J., Chigier, N.A., Atakan, S. &,Ungut , A., Particlesize and velocity measurement by laser anewometry, AIAA J. of

Energy, 1, pp 220 - 228, July 1977

2. Yule, A.J., Ah Seng, C., Felton, P.G., Ungut, A. & Chigier,N.A., A laser tomographic investigation of liquid fuel sprays,Proceedings of 18th Int. Syinp. on Combustion, University ofWaterloo, Ontario, Canada, August, 1980.

3. Yule, A.J., Ah Seng, C., Boulderstone, R., Ungut, A., Felton, C'P.G., & Chigier, N.A., Detailed investigation of a vaporisingfuel spray, NASA Contractors' Report, Grant No. NSG-7517,July 1980.

4. Yule, A.J., Cox, N.W., & Chigier, N.A., Measurement of particle

size in sprays by the automated analysis of spark photographs,Particle Size Analysis, ed. Groves, pp 61 - 73, Pub. by Heyden,London 1978.

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pr AD-A094 789 MEASUREMENT OF FUE1 SPRAY VAPOR.SATIWN BY LASER iLTECHNIQUES U SHEFFIELD UNIV (ENGLAND) DEPT OF CHEMICALENGINEERIHG AND FUE.. A 0 YULE ET AL. SEP AD

UNCLASSIFIED AFOSR-TR-8A-0092 AFOSR-77-3414 IG 20/S NL

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Pro~ccdings of Symposium on Long Range and Short Range Optical VelocityMeas-ivriets, (German-French Research Institute, 15-18th September 1980,ISL, SaiInt Louis

19. KEY WORDS (Contirnue an reverse ide If acoocmaw nd Identfy by bech nhmwb.,)

FUEL SPRAY STRUCTURESTOMCRAPHIC TECHNIQUE

Comparison of fuel spraf structures in heated and in cold environments is madeby using a new later tafgxaphic technique and laser anemometry. The tomographytechnique is shown to give accurate and rapid 'Point' measurements of dropletuizes and concentrations. Experimental results show acceleration of dropletsto the local gas belocity, preferential vaporisation of the smallest droplets andthe dispersion of droplets by the turbulence.

O D J A 7 1 1 4 3 9 1 7 1 " O F I : v S I s 6 " O ~ f a S E C u l lI T Y C L A S S I I C A T I O N O F T H I S P A G E 1 1 4 0 - 1 3 - 1 0 . , , '