On ignition mechanisms of premixed CH4/air and H2/air using a hot turbulent jet generated by pre-chamber combustion

An experiment was developed to investigate the ignition mechanisms of premixed CH4/air and H2/air mixtures using a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition process inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produced a jet comprising of only hot combustion products from pre-chamber combustion. The latter produced a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and spark position on the ignition mechanisms and probability. A global Damköhler number was defined to remove parametric dependency. The limiting Damköhler number, below which ignition probability is nearly zero, was found to be 140 for CH4/air and 40 for H2/air. Lastly, the ignition outcomes were plotted on the turbulent premixed combustion regime diagram. All non-ignition cases fell within the broken reaction zone regime, whereas flame and jet ignition mostly fell within the thin reaction zone regime. These results can provide useful guidelines for future pre-chamber design and optimization.