Liquid length and vapor penetration of conventional, Fischer-Tropsch, coal-derived, and surrogate fuel sprays at high-temperature and high-pressure ambient conditions

摘要

Considering liquid fuels from a variety of feedstocks and refinement processes, evaporating sprays of various fuels are investigated in a constant-volume chamber at ambient and injection conditions typical of a diesel engine. For diesel sprays, the liquid phase of the spray reaches a maximum penetration distance soon after the start of injection, while the vapor phase of the spray continues to penetrate downstream. This liquid-phase penetration distance or in short, the "liquid length", depends upon the ambient and fuel-injector conditions. To clarify fuel effects on liquid length and vapor penetration, in this study we performed simultaneous high-speed imaging of Mie-scattering and schlieren at approximately 50-kHz framing rates. Experiments were performed at specified ambient gas temperature and density in an environment with 0% oxygen. This non-reacting condition was selected intentionally to isolate the mixing and vaporization processes from more complex effects of combustion. Fuels with a wide range of boiling points and densities were examined, including a conventional No. 2 diesel, low-aromatics jet fuel, world-average jet fuel, Fischer-Tropsch synthetic fuel, coal-derived fuel, and a two-component surrogate fuel. Results show that the liquid length increases with increasing fuel density for fuels of similar boiling-point temperature. Considering a mixing-limited vaporization process, entrained ambient mass per fuel mass is inversely proportional to the fuel density, and thereby, high fuel density causes a decrease in ambient-to-fuel ratio, which increases liquid length. Consequently, both boiling point and density of fuel affect liquid length. In contrast to the liquid-length result, no correlation between fuel density and vapor penetration was found. This is because the momentum flux, which governs the jet penetration, is not a function of the fuel density. Although the liquid length varies, the similarity in vapor penetration for all fuels investigated shows that the fuel density and boiling point do not significantly affect the total entrainment and mixing into the spray.