Studies on Diesel sprays under non-reacting and reacting conditions

Doktorsavhandling, 2017

Methods for reducing engine-out emissions are urgently needed to mitigate climate change and air pollution. In diesel engines, the quality of fuel-air mixing and the subsequent combustion process strongly affect fuel efficiency and engine-out emissions. However, fuel-air mixing, the subsequent combustion processes, and their dependence on the operating conditions are not yet fully understood. This thesis aims to address this deficiency by analyzing the effects of various orifice geometries and high injection pressures on the characteristics of diesel sprays. The thesis briefly reviews the fundamental physics governing the flow of pressurized fuel through the internal nozzle of a diesel injector, the subsequent formation of the liquid and vapor spray, and the turbulent diffusion combustion processes. The experimental work presented in this thesis can be divided into three main parts. The first focuses on the effects of geometry-induced cavitation on the liquid/vapor phase spray and injection rate evaluation. To this end, a light absorption and scattering technique (LAS) was used to measure the distributions of liquid and vapor sprays formed using nozzles with various geometries. It was found that the vaporization of the diesel spray was controlled by turbulent air mixing. The effects of geometry-induced cavitation on the spray properties were mainly due to differences in the fuel mass flow rate, spray momentum and spreading angle. In addition, the injection rates of cavitating and non-cavitating nozzles were evaluated using the momentum flux measurement method. It was found that failure to account for cavitation caused the injection rate to be overestimated for the cavitating nozzle but not for the non-cavitating nozzle. The second part of the experimental campaign investigates the effect of the injection pressure and nozzle geometry on soot formation and oxidation. A two-dimensional laser extinction method was used to measure time-resolved soot concentrations and soot volume fractions; OH* chemiluminescence imaging was used to measure the lift-off length and measure the distributions of the OH radicals qualitatively; soot luminosity images were used to identify the sooting area in the soot shadowgraph images. It was found that the equivalence ratio in the jet center at the lift-off length (ϕ_cl), which is influenced by the operating conditions, played a critical role in soot formation. Reductions in ϕ_cl thickened the OH zone in the upstream region of the jet, reducing the volume corresponding to the maximum soot volume fraction. The expansion of the OH zone also helped reduce the sooting zone’s width. However, under high sooting conditions (e.g. ϕ_cl>3.5), the sooting zone width in the downstream jet was independent of ϕ_cl.  The third part of the thesis investigates combusting and non-combusting sprays formed from different blends of ethanol with diesel fuel. Using 0%-20% ethanol blended with diesel fuels, liquid/vapor phase spray images were captured, the ignition delay was measured, the lift-off length was measured, and natural soot luminosity images were captured. It was found that the differences in the fuels’ composition did not significantly affect the liquid/vapor phase sprays. However, as the ethanol content of the fuel was increased from 0% to 20%, the lift-off length increased and the detectable soot luminescence decreased. This indicates that soot formation declines as the fuel’s ethanol content increases.