Numerical study on the pretreatment of ammonia by nanosecond pulsed discharge for combustion enhancement: Effects of pretreatment uniformity

The nanosecond (NS) pulsed discharge for ammonia combustion assistance is numerically investigated with a special focus on the effects of discharge pretreatment uniformity. A method to mimic spatially non-uniform discharge pretreatment is established. Using NH₃/O₂/He and NH₃/O₂/N₂ mixtures as examples, the ignition delay time (IDT), the laminar burning velocity (LBV), and the extinction strain rate (ESR) of mixtures pretreated by different discharge pretreatment methods were analyzed under a wide range of pretreatment uniformity. The results indicate that for the premixed NH₃/O₂/He (or N₂) pretreated by NS discharge pulses, with the increase of the pretreatment non-uniformity, the IDT initially increases slightly, reaching a maximum and then decreases rapidly by 80 %, while the LBV and the ESR first decreases slowly, reaching a minimum and then increases rapidly by 10 %. The initial increment of IDT with the non-uniformity can be attributed to the deterioration of discharge-induced chemical effect but the rapid decrease of IDT as the discharge pretreatment becomes highly non-uniform is largely determined by the combustion-related chemical effect. The effects of pretreatment uniformity on LBV and ESR can be largely due to the thermal effect. When the oxidant is pretreated by the NS discharge, the minimal IDT, the maximal LBV and ESR can be observed under uniform conditions owing to the chemical effects and thus a uniform discharge pretreatment is preferred. Furthermore, a comparative analysis of the three pretreatment methods under identical energy input reveals that under uniform pretreatment, the direct pretreatment of pure NH₃ is optimal, since it can generate more H₂ to enhance the combustion. However, under highly non-uniform conditions, the discharge pretreatment of premixed fuel/oxidant mixtures is the most efficient for combustion enhancement.