A Study of Combustion Structure in High Pressure Single Hole Common Rail Direct Diesel Injection Using Laser Induced Fluorescence of Radicals(Spray Technologies, Mixture Formation)

Abstract

The structure of combusting Diesel jets is studied using Laser Induced Fluorescence (LIF) of radicals. A single hole common rail Diesel injector is used which allows high injection pressures up to 120MPa. Visualization studies have been conducted in a high pressure, high temperature cell that is designed to reproduce the typical thermodynamic conditions which exist in the combustion chamber of a Diesel engine. Planar LIF of the OH radicals (OH LIF) with excitation near 280nm and LIF with excitation at 355nm (355 LIF) have been applied. OH LIF enables the localization of ground state OH which exists as the equilibrium product in high temperature regions and acts as a tracer to both the zones of reaction and the burned gas regions. The objective of 355 LIF is to excite the fluorescence of formaldehyde in order to observe and localize this intermediate species which is also present in the reaction zone. Since no determination of the origin of the fluorescence signal collected with excitation at 355nm was made in order to distinguish formaldehyde fluorescence from other molecules which might fluoresce at the same wavelength, the technique is referred to as 355 LIF, and special care has been taken when interpreting the results. Analysis of the pressure rise in the chamber was also monitored allowing the calculation of heat release rates and auto-ignition delays. Simultaneous visualizations of 355 LIF and chemiluminescence during the early stages of combustion showed that 355 LIF is useful in identifying the precursors of auto-ignition. Furthermore, simultaneous visualizations of 355 LIF and Mie scattering of the soot during the diffusion-limited combustion phase showed that 355 LIF is useful in identifying soot precursors during the latter stages of combustion. At this corresponding stage, OH was observed in the periphery of the jet. A comparison of the results obtained with these different techniques was performed in order to analyze the combustion structure. The presence of intermediate species in the upstream part of the jet was identified and allowed the localization of the main reaction zone in the upstream periphery of the jet, in the mixing zone. The ensemble of results obtained have been used to present a conceptual model of the combustion process.