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Theoretical and experimental study (linear stability and Malvern
granulometry) on electrified jets of diesel oil in atomization regime
L. Priol, P. Baudel, C. Louste, H. RomatLaboratoire d’Etudes Aérodynamiques
Poitiers,[email protected]
Summary
• Introduction
• Linear stability theory for charged jets– Dispersion relation– Numerical results
• Experiments– Experimental device– Experimental results (laser granulometry)
• Conclusion
Introduction
• The dynamic of fuel droplets: a critical importance in the behaviour of liquid fuel disintegration in combustion system
• Effects of electric charges, velocity, viscosity and electrostatic forces on the stability of an electrified jet
• Measurement of the size distribution of droplets with Malvern’s Spraytec ®
Decrease particle size by 10%
Decrease unburned carbon by 35%
Linear stability analysis
• Disturbance on the surface of an axisymmetrical liquid jet
U)cos()exp(0 kzt
IR with
Linear stability analysis
• Dispersion relation between ω and k, found by Reitz1
)()(
)()(
)(
)(1
)(
)(
)(
)(2
)(
)(2
10
0122
222
2
1
2
0
122
2222
21
1
1
0
122
0
121
2
kaKkaI
kaKkaI
kl
klk
kiU
kaI
kaI
kl
klak
a
k
laI
laI
kaI
kaI
kl
kl
kaI
kaIk
1 R.D. Reitz and F.V. Bracco, Mechanism of breakup of round liquid jets, Encyclopedia of Fluid Mechanics, Gulf Pub., N.J., vol. 3, pp.233-249, 1986
Linear stability analysis
• Dispersion relation between ω and k
)()(
)()(
)(
)(1
)(
)(
)(
)(2
)(
)(2
10
0122
222
2
1
2
0
122
2222
21
1
1
0
122
0
121
2
kaKkaI
kaKkaI
kl
klk
kiU
kaI
kaI
kl
klak
a
k
laI
laI
kaI
kaI
kl
kl
kaI
kaIk
kakaK
kaK
kaI
kaI
kl
kl
laI
laI
kaI
kaI
kl
kl
kaI
kaI
kl
kk 1
)(
)(
)(
)(
)(
)(
)(
)(
)(
)(
0
1
0
122
22
1
1
0
122
0
122
2
10
220
22110
20 1
4 kl
Numerical treatment of the dispersion relation
•Dimensionless growth rate
•Dimensionless wave number ka
•Initial charge density σ0 =3 10-4 C/m²
3
1aR
The electrical charges destabilize the jet
With electric charges:
Higher growth rate
Smaller diameter droplets
Experimental device
pump
liquid tank
pressure fluctuation absorber
recirculation pump
manometer
gate
Injector
Reception tank
Electrification system
Electrified high pressure jet
Diesel oil inlet
needle connected to the high voltage source
grounded counter electrode
x
z
Pressure 70 bars, Potential -30kV
Malvern granulometry experiments
•Malvern measures droplets from 1 µm to 250 µm
•Measurements based on Mie’s diffraction theory
•Real time spray measurement at up to 2500 Hz
•Laser beam diameter of 10 mm
Experimental results
Real time evolution of different “diameters” of droplets for p=80 bars, z=40 mm and x=0 mm
Injection of charges
Dv(90)=182.73Dv(90)=351.46
Experimental results
• Droplet size distribution for a driving pressure of 120 bars at z=30 mm, x=0 mm (center of the jet)
Potential 0kV Potential -25kV
Vol
um
e cu
mul
ate
dist
ribu
tion
(%)
Diameters µm Vol
um
e of
th
e cl
ass
diam
eter
(%
)
Vol
um
e cu
mul
ate
dist
ribu
tion
(%)
Diameters µm Vol
um
e of
th
e cl
ass
diam
eter
(%
)
Experimental results
• Droplet size distribution for a driving pressure of 120 bars at z=30 mm, x=5 mm (edge of jet)
Potential 0kV Potential -25kV
Vol
um
e cu
mul
ate
dist
ribu
tion
(%)
Vol
um
e cu
mul
ate
dist
ribu
tion
(%)
Vol
um
e of
th
e cl
ass
diam
eter
(%
)
Vol
um
e of
th
e cl
ass
diam
eter
(%
)
Diameters µm Diameters µm
Conclusion
• Dispersion relation with electrostatic forces shows the destabilizing role of the electric charges in the jet
• For an electrical charged jet the theory predicts a decrease of the droplet mean diameter
• Theoretical results confirmed by experimental measurements obtained by laser granulometry
Better atomization of a jet thanks to the
injection of electric charges