Liquid-Phase Fuel Penetration in Diesel Sprays 980809
The maximum axial penetration distance of liquid phase fuel
(i.e., the “liquid length”) in an evaporating diesel
spray was investigated over a wide range of conditions using Mie-scattered
light imaging. The parameters varied in the investigation included: the
injection pressure, the orifice diameter and aspect ratio, the ambient gas
temperature and density, and the fuel volatility and temperature. The
experiments were conducted in a constant-volume combustion vessel with
extensive optical access. Fuels were injected with an electronically
controlled, common-rail diesel fuel injector.
The dominant trends observed were: (a) Liquid length decreases linearly with
orifice diameter and approaches zero as the orifice diameter approaches
zero. (b) Injection pressure has no significant effect on liquid length. (c)
Liquid length decreases with increasing ambient gas density or temperature,
but with a declining sensitivity to each one as they increase. (d)
Decreasing fuel volatility or fuel temperature increases liquid length. (e)
Finally, the liquid length of a multi-component fuel is controlled by its
lower volatility fractions.
Two major conclusions were drawn from the observed trends. First,
vaporization in a diesel spray, like diesel combustion after the premixed
burn, is controlled by air entrainment into the spray
(i.e., by turbulent mixing). Atomization and local
interphase transport processes, such as droplet evaporation, do not limit
the rate of vaporization. Second, evaporation in a diesel spray occurs to a
large extent through a batch distillation-type process with higher
volatility components evaporating first and lower volatility components
controlling the liquid length.