Optical and numerical study on the effect of wall impingement on passive jet ignition characteristics of methane/air mixture

Pre-chamber turbulent jet ignition is a promising high-efficiency combustion technique to achieve stable lean-burn operation for spark-ignition engines. Wall impingement by a high momentum turbulent jet could exert a critical influence on jet ignition behavior. In this work, the passive jet ignition characteristics of premixed methane/air mixture under wall impinging condition was investigated based on optical experiments in a constant volume chamber and numerical simulations. Two ignition modes were found for flat wall impinging cases across a range of non-dimensional impinging distances (H/D) from 5.7 to 20. In mode 1, ignition initiates from near wall region. Under lean conditions of λ = 1.3, compared with that in the free jet case, the ignition delay time characterized by a 5 % mass fraction burned (MFB-05), decreases by 3.3 ms at H = 30 mm and increases by up to 6.6 ms at H = 20 mm. In mode 2, ignition initiates from the jet root. The ignition delay times are shorter for all impinging distances (H = 20, 30, 40 mm), with MFB-05 decreasing by up to 3.4 ms. Simulations reveal that when the wall is located in the jet developing zone with H/D below 7, wall impinging deteriorates the ignition, which is attributed by higher turbulent kinetic energy (TKE) and lower Damköhler (Da) number near the stagnation zone, resulting in extended ignition delay. Conversely, as H/D increases, ignition is promoted by the stagnation effect with a shorter ignition delay. Additionally, the effect of wall shapes was analyzed. Using sharp-tip V-shaped wall, the stagnation region with high TKE is minimized. A blunt-tip V-shaped wall shows enhanced impinging but no obvious increase of Da number at stagnation point. Using M-shaped wall, the ignition mode with a “lift-off” flame was found. Besides, ignition is more favorable in the secondary jet zone with active radicals and higher Da number.