Experimental investigation of the effect of duct structure geometry on macroscopic spray characteristics and air entrainment of duct fuel spray

The fuel-air mixing process can be improved via straight (ST) ducted fuel injection, but there is a risk of greater heat transfer loss due to prolonged spray tip penetration (STP) and spray impingement. In this study, we propose a convergent-divergent (CD) duct spray to produce an acceptable STP to improve the spray air entrainment (SAE) and spray cone angle (SCA). Three CD and two ST ducts were designed and installed in a constant-volume chamber. An optical Schlieren visualization system with a high-speed camera was used to examine the effects of duct geometry on the macroscopic spray parameters of SCA, STP, spray area (SA), SAE, and spray morphology. Compared with the free spray, the longest STP was noted for the ST2 duct spray, whereas the shortest STP was observed for the CD4.5 duct spray; the STP decreased by 14.5 %, whereas the SCA increased by at least two times for CD4.5 and CD6 duct sprays. A typical trade-off relationship between SCA and STP was confirmed for larger diameter CD duct sprays. When the injection pressure increased from 120 to 140 to 160 MPa, the STP of the ST2 duct spray increased by 1.7 % and 2.4 %, and that of the CD4.5 duct spray increased by 3.1 % and 7.0 %, respectively. At the late spray stage, the CD4.5 and CD6 duct spray had the better SAE ability, followed by the ST3 and CD3 duct sprays, ST2 duct spray exhibited slightly lower SAE values than the free spray. A typical mushroom-shaped spray head was observed near the outlet of the ST2 and ST3 duct sprays, while a cotton -shaped spray was observed for the CD duct sprays during the initial early spray stage. The development of the CD duct spray showed a left-right swing phenomenon with an S-shaped track, particularly for CD4.5 and CD6 duct sprays, which can promote the fuel-air mixing process with a larger SCA. The CD4.5 duct spray demon-strated considerable potential for improving the fuel-air mixture with a stronger SAE and larger SCA, as well as the ability to reduce the heat transfer loss with a shorter STP and weaker spray impingement.