Effect of Micro-Hole Nozzle on Diesel Spray and Combustion

The influence of nozzle geometry on spray and combustion of diesel continues to be a topic of great research interest. One area of promise, injector nozzles with micro-holes (i.e. down to 30 μm), still need further investigation. Reduction of nozzle orifice diameter and increased fuel injection pressure typically promotes air entrainment near-nozzle during start of injection. This leads to better premixing and consequently leaner combustion, hence lowering the formation of soot. Advances in numerical simulation have made it possible to study the effect of different nozzle diameters on the spray and combustion in great detail. In this study, a baseline model was developed for investigating the spray and combustion of diesel fuel at the Spray A condition (nozzle diameter of 90 μm) from the Engine Combustion Network (ECN) community. Upon validation of parameters such as spray penetration, lift-off length, and ignition delay the baseline simulation was extended to study different nozzle orifice diameters. All simulations were performed using a constant-volume combustion chamber (CVCC) geometry with similar ambient conditions of pressure (60 bar) and temperature (900 K). It was shown that liquid length was shortened to a “minimum” level after nozzle diameter was reduced to 50 μm or lower. Moreover, the decrease in nozzle diameter enhanced spray atomization and gas entrainment to the fuel jet. This caused the location of the lifted flame to move closer to the nozzle tip. While local equivalence ratio along the spray centerline was significantly reduced with smaller nozzle diameters, the fuel/air ratio remained relatively unchanged (lean) at the jet boundary where the lifted flame stabilized. Higher injection pressures in the simulation also enhanced fuel-air mixing, resulting in shorter lift-off lengths and lower soot formation. The results from this study agree with literature findings of the trend in soot reduction with very small nozzle diameter. Potential improvements to the current models include different kinetic mechanisms for prediction of soot precursors such as benzene (C₆H₆), and better matching in terms of ignition and flame structure. A full engine combustion chamber simulation confirmed the soot reduction phenomenon associated with the smaller nozzle diameter.