Publications

Shadowgraph/schlieren imaging techniques have often been used for flow visualization of reacting and non-reacting systems. In this paper we show that high-speed shadowgraph visualization in a high-pressure chamber can also be used to identify cool-flame and high-temperature combustion regions of diesel sprays, thereby providing insight into the time sequence of diesel ignition and combustion. When coupled to simultaneous high-speed Mie-scatter imaging, chemiluminescence imaging, pressure measurement, and spatially-integrated jet luminosity measurements by photodiode, the shadowgraph visualization provides further information about spray penetration after vaporization, spatial location of ignition and high-temperature combustion, and inactive combustion regions where problematic unburned hydrocarbons exist. Examples of the joint application of high-speed diagnostics include transient non-reacting and reacting injections, as well as multiple injections. Shadowgraph and schlieren image processing steps required to account for variations of refractive index within the high-temperature combustion vessel gases are also shown.

Diesel injection parameters effect on liquid-phase diesel spray penetration after the end-of-injection (EOI) is investigated in a constant-volume chamber over a range of ambient and injector conditions typical of a diesel engine. Our past work showed that the maximum liquid penetration length of a diesel spray may recede towards the injector after EOI at some conditions. Analysis employing a transient jet entrainment model showed that increased fuel-ambient mixing occurs during the fuel-injection-rate ramp-down as increased ambient-entrainment rates progress downstream (i.e. the entrainment wave), permitting complete fuel vaporization at distances closer to the injector than the quasi-steady liquid length. To clarify the liquid-length recession process, in this study we report Mie-scatter imaging results near EOI over a range of injection pressure, nozzle size, fuel type, and rate-of-injection shape. We then use a transient jet entrainment model for detailed analysis. Results show that an increased injection pressure correlates well with increasing liquid length recession due to an increased entrainment wave speed. Likewise, an increased nozzle size, with higher jet momentum and faster entrainment wave, enhances the liquid length recession. A low-density, high-volatility fuel does not decrease the strength of the entrainment wave; however, it decreases the steady liquid length causing the entrainment wave to reach the liquid spray tip earlier, which ultimately results in faster liquid length recession. A slow ramp down in injection rate causes a weaker entrainment wave so that the liquid length recession occurs even prior to injector close.

Unlike conventional diesel engines, which have a negative ignition dwell, many strategies for low-emissions diesel combustion operate with a positive ignition dwell mode, where the ignition delay exceeds the injection duration. Although nitrogen oxides and particulate matter emissions can be reduced by operating with a positive ignition dwell, unburned hydrocarbon and carbon monoxide emissions typically increase. Sources of these emissions can stem from characteristics of the fuel spray after the end of injection, which may differ significantly from the main injection period where most spray models have been developed. To provide fundamental details of spray mixing during the end-of-injection transient, we have studied liquid-phase spray penetration and evaporation using simultaneous high-speed shadowgraph and Mie-scatter imaging for a single-hole, common-rail injector. Experiments were conducted over a wide range of ambient temperature and density in a constant-volume vessel. The experiments show that during the injection-rate ramp-down, the liquid penetration decreases (recedes towards the injector) from the quasi-steady-state distance for most diesel conditions. A transient jet entrainment model, coupled with the assumption of mixing-limited spray vaporization and direct measurement of the vaporized jet spreading angle, shows that this behavior is caused by a slower fuel delivery interacting with an increased rate of ambient entrainment during the injection-rate ramp-down. This increased mixing travels downstream as an "entrainment wave", permitting complete vaporization at distances closer to the injector than the quasi-steady liquid length. The position of the entrainment wave relative to the quasi-steady liquid length determines how far, and how quickly, the liquid recedes towards the injector. The tendency of recession increases with increasing ambient temperature and density because the transit time of the entrainment wave to the liquid length is shorter than the injection-rate ramp-down transient. Alternatively, the liquid-length recession is zero for conditions with low ambient temperature or density because the entrainment wave does not reach the quasi-steady liquid length until after the end of the injection-rate ramp-down. Copyright © 2008 by the Japan Society of Mechanical Engineers.

Abstract not provided.

Abstract not provided.