Abstract
Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NOx produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them.
Highlights
Diesel engine performance and emission characteristics are largely determined by fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics
There have been experimental studies concerning the effects of nozzle orifice geometry on global injection and spray behavior [5,6,7,8,9,10,11,12,13]
Han et al [11] compared conical and cylindrical nozzles and found that the primary breakup region is strongly influenced by nozzle geometry
Summary
Diesel engine performance and emission characteristics are largely determined by fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Benajes et al [9] conducted an experimental study to analyze the influence of conical and cylindrical nozzle orifices on injection rate behavior of a common-rail fuel injection system at maximum needle lift under steady state conditions in a cavitation test rig. The literature review indicates that while the effect of orifice geometry on the injector flow and spray processes has been examined to some extent, its influence on engine combustion and emissions is not well established. Extensive validations of fuel oxidation chemistry and soot model will be presented
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