Multi-channel nanosecond discharge plasma ignition of premixed propane/air under normal and sub-atmospheric pressures

Relight of jet engines at high altitude is very difficult due to the relatively low pressure and temperature of inlet air. Currently, advanced ignition technology for high-altitude relight in jet engines is urgently needed. Successful ignition is achieved only when the ignition kernel can propagate outwardly beyond the so-called critical flame initiation radius. At high altitude with low pressure, the critical flame initiation radius becomes large and it cannot be easily reached by the ignition kernel. Therefore, in order to achieve successful ignition at low pressure conditions, large ignition kernel should be generated. In this study, plasma assisted ignition using multi-channel nanosecond discharge (MND) is proposed to induce a large ignition kernel and to achieve successful ignition at low pressures. Ignition experiments for propane/air mixtures at different equivalence ratios (Ф = 0.8∼1.6) and under normal and sub-atmospheric pressures (P= 0.3∼1.0 bar) were conducted in a constant volume combustion chamber. The performance of three ignition methods, spark discharge, single-channel nanosecond discharge (SND) and MND, were assessed; and the advantages of MND for ignition at sub-atmospheric pressures were demonstrated. The ignition kernel development, ignition probability, minimum ignition energy, and flame development for these three ignition methods (spark, SND and MND) were measured and compared. It was found that compared to spark and SND, MND can generate a much larger ignition kernel with stronger flame wrinkling and has much higher ignition probability, especially at low pressures. Therefore, MND has the advantage in achieving successful ignition at low pressure. Besides, it was shown that though the ignition kernel evolution and ignition probability strongly depend on ignition methods, the subsequent flame propagation is not greatly affected by ignition and there is little change in the flame rise time for different ignition methods.