Understanding Interaction between Reactive Jets in Pre-Chamber Ignition of Gaseous Fuel

In order to improve the ignition capacity and burning rate for spark-ignited engines, pre-chamber jet ignition is a promising technique to achieve fast premixed combustion and low pollutant emissions. However, few studies focus on the interaction between multiple reacting (i.e. flamelet) or reacted (i.e. radical) jets, its effect on ignition, exotherm and flow behaviors also remain to be revealed. This paper investigated two types of jet interaction under different pre-chamber structures, including the jet-crossing and unequal nozzle designs. Optical experiments under different conditions were conducted in a constant volume combustion chamber with CH₄ as fuel, using simultaneous high speed schlieren and OH^* chemiluminescence method. Meanwhile, computational fluid dynamics (CFD) simulations with CH₄ and NH₃/CH₄ blend fuels were carried out using Converge software to provide further insights of turbulent flow and ignition process. For the jet-crossing structure, it was found that ignition can be controlled in the jet-crossing zones under different initial conditions, with more repeatable and stable developments than the parallel structure case. The cross angles in the range of 20°-30° with respect to the pre-chamber axis show similar performances and more favorable for ignition control overall, while the cross region too closed to the nozzles reduce the overall burning rate. The simulation results show that local Da number around ignition zone is in the range of 0.2-0.4, and with the addition of NH₃, the flame regime is located in thickened and broken reaction zone during early jet evolution. For the unequal nozzle design with the diameter of 2 mm and 4 mm, the ignition delay time of the radical jet will be reduced by 60 % than the two equal nozzles with the diameter of 2 mm, because of the suppression of quenching in presence of the flame jet. And the flame expansion in horizontal direction is also improved. In addition, the ignition patterns are also influenced by different equivalence ratios due to the changes of temporal sequence of two jet evolution.