Experimental and numerical investigation on fuel/air mixing and macroscopic spray characteristics of free spray, convergent-divergent, and straight duct sprays under non-vaporizing conditions

In this study, the convergent-divergent (CD) and straight (ST) duct sprays were conducted for comparison with free spray to investigate the diesel macroscopic spray characteristics and mixing process. The schlieren visualization system with a high-speed camera was performed with a 0.22 mm single-hole heavy-duty diesel injector. The injection pressure varied from 120 via 140 to 160 MPa, with the ambient pressure being set at 40 bar. The experimental result showed that CD and ST duct spray had much higher spray tip velocity compared to free spray during the early spray stage. CD duct spray presented a more dispersed and more distributed spray morphology along with larger-scale flow vortexes and gas entrainments, indicating the promoted fuel-air mixing process. Large-eddy simulations (LES) were conducted to analyze the pumping effect, the local separation of the boundary layer, and the air entrainment of different sprays. Compared with free spray, a much stronger pumping effect was generated for ST duct spray followed by CD duct spray; Less flow interaction with the spray body inside the ST duct, retaining more energy for spray development; A much stronger flow interaction was noted as the external gas was sucked into the expanding section of the CD duct, produce the local air vortexes in the narrow inner wall space of the CD duct, leading to the promoted spray diffusion and development with a wider spray cone angle. These phenomena from CD duct spray were beneficial for achieving a more sufficient fuel-air mixing process.