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Journal of Electrostatics 63 (2005) 899–904

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Laser granulometry measurements on electrifiedjets for different lengths of injector

Laurent Priol�, Pierre Baudel, Christophe Louste,Hubert Romat

Laboratoire d’Etudes Aerodynamiques, Boulevard Pierre et Marie Curie, Teleport 2, BP 30179,

86962 Futuroscope-Chasseneuil, France

Available online 23 March 2005

Abstract

It is well known that the electrification of jets of dielectric liquid can totally change the

characteristics of the atomization. Due to the presence of electric charges in the liquid the

stability of the jet changes and, therefore, mechanisms of creation of the droplets change as

well. Moreover since the droplets become charged they repulse each other and disintegrate in a

different way from non-charged droplets. In this article, we test the electrification system for

aeronautical applications. We present the results of the measurements done on electrified jets

of fuel (electrification of the liquid by injection of charges in the injector) with a Malvern

system. The mean velocity of the liquid ranges between 40 and 100 m/s, the applied voltage can

vary between 0 and 30 kV and the ratio L/D (length/diameter) of the injector varies from 10 to

2. The histogram and mean diameter graphs recorded live are given and analysed in terms of

the ratio L/D.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Electrified jets

see front matter r 2005 Elsevier B.V. All rights reserved.

.elstat.2005.03.052

nding author. Tel.: +33 5 49 49 69 42; fax: +33 5 49 49 69 68.

dress: laurent.priol@lea.univ-poitiers.fr (L. Priol).

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L. Priol et al. / Journal of Electrostatics 63 (2005) 899–904900

1. Introduction

The optimization of combustion in thermal engines has become a field of greatinterest for research and development. The goal is to improve the atomization of thefuel in order to obtain the desired spray characteristics that would limit the emissionsof pollutants. An attractive approach consists in using electrically charged sprayswhich have the interesting characteristic of being self-dispersing and avoid theformation of big droplets.

Two decades ago Kelly [1] started to work on electrostatic sprays. Later heestablished a theory which has been confirmed by his experimental work. Otherscientists like Huebner et al. [2] and Schneider et al. [3] had already worked on thetheory of the stability of charged liquid jet in the 1970s. More recently,Balachandran et al. [4] and Yule et al. [5] also carried out experiments on theelectrification process we have used for this article.

In the present paper we present the work done on diesel oil high-speed jets chargedwith an electrification system composed of a needle brought to a high potentialplaced inside the injector. We describe the experimental device and then compare theresults obtained on the size distribution of the droplets with a laser granulometrydevice on two different injectors.

2. Experimental setup

This experimental device is described in Fig. 1. It is basically composed of ahydraulic closed system. The diesel oil flows through injector (6) by means of thepump (1). The pressure is fixed at 70 bar for all the experiments. The flowrate is3 ml/s. The liquid is electrified inside the injector and then goes through the orifice ofthe nozzle. It arrives in a collector vessel (5) and discharges there. The liquid isrecycled with a small pump (8) which sends it back to the initial tank (2).

The details of the injector are also seen in Fig. 1. The electrical part of the injectoris composed of a needle connected to the high-voltage source and of a groundedcounter-electrode. The diameter of the needle is 5 mm and the radius of curvature ofits extremity is 30 mm. The needle is placed at 0.5 mm from the counter-electrode.The diesel oil is a dielectric and its electrical conductivity is about 300 pS/m.

Fig. 1. General view of the experimental device and electrification system.

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L. Priol et al. / Journal of Electrostatics 63 (2005) 899–904 901

In our study we use two different injectors. If L is the length and D thediameter of the canal of the nozzle, the ratio L/D of the first injector isequal to 2 whereas the ratio of the second is equal to 10. The potential of theneedle that injects electric charges in the liquid is negative and can be fixedup to �30 kV.

The measurement of the size of the droplets is done by a Malvern’s Spraytecsystem which has been designed for real-time high-speed measurements. It workswith a time-resolution measurement of 400 ms controlled by the software. Themeasurement system is based on Mie’s theory. The lighted droplets deflect the lightfrom its initial direction and the intensity of the deflected light and the angle ofdeviation give the size of the droplets. The Malvern system therefore gives the size ofthe droplet and the volume cumulate distributions. The histograms are obtainedafter a single or multiple scan depending on the accuracy we need. All themeasurements are done at z ¼ 50 mm from the nozzle and at x ¼ 0 mm from thecentre of the spray. Since the laser beam has a diameter of 10 mm, the Spraytec takesinto account the entire jet.

3. Comparison of different jets without charges

First, we compare the droplet size distributions of two different injectorswithout injection of electric charges (Fig. 2): the potential is 0 kV. For L=D ¼ 10the jet is less atomized than for L=D ¼ 2: In the latter case the jet has moredroplets with diameters under 100 mm. For L=D ¼ 2; 25% of the total volume isoccupied by droplets which have a maximal diameter of 65 mm whereas for L=D ¼

10; 25% of the total volume is occupied by droplets which have a maximal diameterof 90 mm.

The shape of an injector influences the behavior of the jet it produces. A jet with ahigh ratio L/D is less atomized than a jet with a small one. When L=D ¼ 10 the jethas a dense central area.

100010010Diameters µm

10

50

100

Vol

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cum

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n (%

)

L/D = 2L/D = 10

Fig. 2. Comparison for L=D ¼ 2 and 10 without charges.

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L. Priol et al. / Journal of Electrostatics 63 (2005) 899–904902

4. Effect of the injection of electric charges

In this paragraph, we inject electric charges in the liquid in both injectors. Thepotential of the needle varies from 0 to �30 kV. We can observe the effect of theelectric charges on the droplet size distribution in Fig. 3.

With the ratio L=D ¼ 2 (Fig. 3), the jet has more small droplets with the injectionof charges but the effect is not very significant. The maximal diameter of dropletswhich occupied 25% of the total volume changes from 65 to 50 mm.

For a small L, the jet is already atomized and the electric charges have only a littleeffect on the droplets because we are far from the Rayleigh’s limit. We will have toincrease the amount of electric charges into the jet in order to see a more significanteffect on the diameter of the droplets.

With L=D ¼ 10 (Fig. 3), the decrease of the diameter of the droplets is greater thanin the previous case and we can notice that the effect of the injection of charges is notlinear.

The size distribution is the same for �20 and �30 kV. In Fig. 3 we can see that themaximal diameter of droplets which occupied 25% of the total volume changes from90 mm for a potential of 0 kV to 50 mm for the potential of �30 kV. Like Yule et al.[5], we found that the jet is better atomized with charges than without.

For this kind of injector the injection of charges has a real effect. The electriccharges acts on the atomization of the dense area in the centre of the jet.

5. Comparison of different jets with charges

The last experiment is the comparison of the droplet size distributions for twodifferent injectors with injection of electric charges (Fig. 4). The potential is �30 kV.

We can see that the behaviour of the two injectors is the same. There is nosignificant difference between the curves L=D ¼ 2 and 10.

With the Malvern’s Spraytech we have also the value of the transmission of thelight of the laser beam through the jet. A weak value of the transmission means thejet is dense: the light cannot pass through the jet. With a charged jet, the transmissioncoefficient is about 25% for the L=D ¼ 2 and about 40% for L=D ¼ 10: This means

100010010

Diameters (µm) L/D = 10

01100010010

Diameters (µm) L/D = 2

1

50

100

Vol

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n (%

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0

50

100

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n (%

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0 kV-20 kV-30 kV

0 kV-20 kV-30 kV

Fig. 3. Effect of the potential for L=D ¼ 2 and 10.

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1000100101

Diameters µm

0

50

100

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cum

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n (%

) L/D = 2

L/D = 10

Fig. 4. Comparison for L=D ¼ 2 and 10 with charges.

L. Priol et al. / Journal of Electrostatics 63 (2005) 899–904 903

that even if the size distribution looks the same for both injectors the density of thejet is different. This can be easily observed to the naked eye: for L=D ¼ 2 the jet ismuch more dispersed than for L=D ¼ 10: In fact, we think that the injection of thecharges changes the primary breakup mechanisms and atomizes the dense area ofthe centre of the jet but acts weakly on the diminishing of the diameter of thedroplets already existing. For L=D ¼ 2; this central zone is not very dense (the jet isalmost totally already atomized) and the effect of the electrification is not veryimportant. For L=D ¼ 10 since the central zone is very dense (the jet is not totallyatomized) and the electrification can play an important part in the atomizationprocess. The reason why we have the same distribution of droplets and a differentdensity is not well understood and complementary experiments will be done soon toclarify this point.

6. Conclusion

The geometrical characteristics of an injector, particularly the ratio L/D, isdetermined for the droplet size distribution. In the presence of electric charges thecharacteristics of the distributions of droplets are the same for L=D ¼ 2 and 10 butfor the first ratio the jet is more dispersed than for the second one. The part of theelectrical effect (coulombic repulsion) and the one of the mechanical effect (injectionof impulse in the injector due to the motion of ions) on the atomization has not beenclearly understood so far. It will be investigated soon.

References

[1] A.J. Kelly, Low charge density electrostatic atomization, IEEE Trans. Ind. Appl. IA-20 (2) (1984)

267–273.

[2] A.L. Huebner, H.N. Chu, Instability and breakup of charged liquid jets, J. Fluid Mech. 49 (1971)

361–372.

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[3] J.M. Schneider, N.R. Lindblad, C.D. Hendricks, J.M. Crowley, Stability of an electrified liquid jet, J.

Appl. Phys. 38 (1967) 2599–2605.

[4] W. Balachandran, D. Hu, A.J. Yule, J.S. Shrimpton, A.P. Watkins, A charge injection nozzle

for atomization of fuel oils in combustion application, IEEE–IAS Annual Meeting, 1994,

pp. 1436–1441.

[5] A.J. Yule, J.S. Shrimpton, A.P. Watkins, W. Balachandran, D. Hu, Electrostatically atomized

hydrocarbon sprays, Fuel 74 (1995) 1094–1103.