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Atomization and Droplet Breakup Modeling of Diesel Sprays

Franz X. Tanner
 

Figure 1: Schematic representation of a high pressure-driven, solid-cone diesel spray.

Figure 2: Schematic representation of drop breakup mechanisms (Liu and Reitz, 1993): Bag breakup: 6<We<80, stripping (shear) breakup: 80<We<350, catastrophic (surface wave) breakup: We >350.

• Fuel sprays are the result of high pressure-driven liquid fuel jets injected through a nozzle orifice into a combustion chamber. The jet atomizes, and then the fuel evaporates and mixes with air to form a reactive compound which, as a consequence of increased gas pressure and temperature, ignites and thus initiates the combustion process.
• Fuel jet atomization and droplet breakup are multi-phase phenomena which strongly influence
• the ignition and combustion, hence the pollution formation
• the optimal fuel efficiency.
• The atomization process is a consequence of
• inner-nozzle disturbances, mainly due to cavity formation at the sharp nozzle inlet edge, which are responsible for the formation of a fragmented liquid core.
• aerodynamic forces which break the liquid fragments into tiny droplets.
• The breakup of liquids due to aerodynamic forces is the result of instabilities on the phase-dividing interface:
• Rayleigh-Taylor (R-T) instability: Inertia of the denser fluid opposes the system acceleration
• Kelvin-Helmholtz (K-H) instability: Viscous forces caused by the relative motion of the fluids
• Drop breakup mechanisms are classified w.r.t. the drop Weber number, We (c.f. figure 2)

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Franz X. Tanner
1999-02-04