Liquid and vapor spray structure in high-pressure common rail diesel injection

摘要

Diesel spray structure is studied using optical diagnostics. A single-hole common rail diesel injector is used to enable high injection pressures up to 150 Mpa. The spray is observed in a high-pressure, high-temperature cell that reproduces the conditions existing in a combustion chamber of a diesel engine during injection. The liquid phase of the spray is studied using laser-induced exciplex fluorescence (LIEF). The vapor phase is studied using an innovative measurement technique for which a high-boiling-point tracer is added to the fuel. The vaporized part of the spray is visualized by tracer droplet light scattering of a laser sheet. The technique is made quantitative with an appropriate calibration method. These two diagnostic methods are then used to build a database of liquid-phase visualizations and of quantitative two-dimensional distributions of fuel vapor concentration, with varying injection parameters. A detailed analysis of the structure of sprays in done. It is found that the liquid spray exhibits an area of high optical density at the outlet of the nozzle, with an abrupt transition to a fully atomized spray within a few nozzle orifice diameters. The structure of the fully atomized spray exhibits and area of high optical density at the outlet of the nozzle, with tan abrupt transition to a fully atomized spray within a few nozzle orifice diameters. The structure of the fully atomized spray is comparable to a bundle-like structure, and is found to be the consequence of transient phenmena occurring in the nozzle. The structure of the vapor plume is similar, showing evenly distributed areas with steep gradients at the edges. This result is deducted from single-shot measurements and is found to contrast significantly with the more progressive structure of multipulse averages. This structure of the fuel vapor is expected to be responsible for the location of autoignition that occurs at multiple points nearly simultaneously. Finally, the effect of injection pressure is discussed. An increase of injection pressure is found to enhance the atomization at the nozzle outlet, resulting in a more distributed vapor phase, bence resulting in better mixing.,