Combustion structure of free and wall-impinging diesel jets by simultaneous laser-induced fluorescence of formaldehyde, poly-aromatic hydrocarbons, and hydroxides

Abstract

The structure of combusting diesel jets in low-temperature conditions is studied using laser-induced fluorescence (LIF). A single-hole common rail diesel injector is used, which allows high injection pressures up to 120 MPa. Visualizations are performed in a high-pressure, high-temperature cell that is designed to reproduce the typical thermodynamic conditions in the combustion chamber of a diesel engine. Planar LIF of the hydroxide (OH) radicals (OH LIF) with excitation near 280 nm and LIF with excitation at 355 nm (355 LIF) are applied simultaneously. In addition, simultaneous 355 LIF imaging and spatially resolved single-shot spectral analysis are performed in order to identify spatially the contribution of formaldehyde and poly-aromatic hydrocarbon (PAH) fluorescence in the 355 LIF images. Also, the analysis of the pressure signal in the chamber and subsequent calculation of heat release rates are used to locate temporally each image within the different combustion stages. The combustion structure of a low-temperature freely propagating jet during those stages is then analysed in detail, using the information given by the simultaneous LIF techniques on the different reaction zones; this is then summarized by a conceptual model. Finally, the simultaneous LIF techniques are applied to a configuration where a jet impinges on a perpendicular wall. The results are used to analyse the effect of wall interaction on the combustion structure.