Liquid-Phase Fuel Penetration in Diesel Sprays

Abstract

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.