Experimental and computational study comparing conventional diesel injectors and diverging group hole nozzle injectors in a high temperature pressure vessel and a heavy-duty diesel engine

Group hole nozzles (GHN) are injector tips with clusters of small holes used in place of a single large hole. In this work, GHN with two holes per cluster and a diverging angle of 15° between the holes were studied using experiments and computational fluid dynamics (CFD) and comparisons were made with conventional diesel injectors. First, experiments were performed in a high temperature pressure vessel (HTPV) to understand the free spray behavior. HTPV experiments revealed that the diverging GHN had slower penetration with a more dispersed spray and a shorter lift-off length relative to the conventional injector. The GHN also exhibited higher natural luminosity compared to the conventional injector, which is indicative of increased soot formation with the GHN. CFD model validated with HTPV experimental data was used to study the sensitivity to the angle between the holes and the hole count per cluster. Results showed that soot formation increased as the divergence angle increased from 0° to 15°. Furthermore, as the divergence angle increased, increasing the hole count per cluster increased the soot formation significantly. Following the HTPV study, the injectors were tested on a single-cylinder heavy-duty diesel engine at a high-load operating condition. Results showed that the diverging GHN injector produced a lower mixing-controlled combustion rate compared to the conventional injector. The slower burn rates resulted in lower NOx and higher soot emissions for the diverging GHN injector. CFD simulations of the engine experiments predicted the slower burn rate seen with the diverging GHN injector. In-cylinder visualization of the CFD results showed that slower penetration with the diverging GHN results in reduced mixing upon surface interaction, and an inability to utilize the fresh air in the outer regions of the combustion chamber which resulted in richer local equivalence ratios and higher soot emissions compared to the conventional injector.