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. Romat Laboratoire d’Etudes Aérodynamiques Poitiers,France

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 with

Linear stability analysis Dispersion relation between ω and k, found by Reitz 1 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 , 1986

Linear stability analysis Dispersion relation between ω and k

Numerical treatment of the dispersion relation Dimensionless growth rate Dimensionless wave number ka Initial charge density σ 0 = C/m² The electrical charges destabilize the jet With electric charges:  Higher growth rate  Smaller diameter droplets

Experimental device pumpliquid tankpressure 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

Dv[90]=350 µm 90% of the total volume is occupied by droplets which have a maximal diameter of 350 µm

Experimental results Droplet size distribution for a driving pressure of 120 bars at z=30 mm, x=0 mm (center of the jet) Potential 0kVPotential -25kV Volume cumulate distribution (%) Diameters µm Volume of the class diameter (%)Volume cumulate distribution (%) Diameters µm Volume of the class diameter (%)

Potential 0 kV -25 kV Diameter droplet for Dv[75] 320 µm 160 µm Dv[75]=320 µm 75% of the total volume contain droplets with maximal diameter of 320 µm Potential0 kV - 25 kV Volume of class diameter of 20 µm 2.5% 5% Volume of class diameter of 200 µm 4% 6% Volume of class diameter of 350 µm 12% 1%

Experimental results Droplet size distribution for a driving pressure of 120 bars at z=30 mm, x=5 mm (edge of jet) Potential 0kVPotential -25kV Volume cumulate distribution (%) Volume of the class diameter (%) Diameters µm

Potential 0 kV -25 kV Diameter droplet for Dv[75] 300 µm 160 µm Dv[75]=300 µm 75% of the total volume contain droplets with maximal diameter of 300 µm Potential0 kV - 25 kV Volume of class diameter of 20 µm 6% 9% Volume of class diameter of 200 µm 8% 1%

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