# A Novel Way to Enhance the Spark Plasma-Assisted Ignition for an Aero-Engine Under Low Pressure
## 3 Results and Discussion
### Spark Characteristics of the SI and the MDJS  
Discharge images (including spark volume, spark penetration depth and activated area development) of the SI and the MDJS from a side view at different pressures ranging from 0.12 bar to 0.26 bar are shown in Figure 5. At first, the conventional SI discharge in Figure 5 shows a fine filament, whereas the MDJS shows a much brighter arc discharge after an ionization path among EC1 and EC2 is created.  
As is shown in Figure 6, the MDJS discharge is visualized as a jet-enhanced spark: the external spark was pushed forward by the high-temperature jet which was rapidly heated and pressurized by the strong pulsed spark between the internal electrode couple EC1, which resembles a cambered arc. However, the conventional spark igniter generated a much lower spark plasma penetration depth and a relatively smaller spark plasma activated area. Figure 6 a,b shows that the spark plasma penetration depth of the SI is 10.9 mm, whereas that of the MDJS is approximately 19.1 mm. A higher spark plasma  
Figure 4: Separation of spark discharge for the MDJS: (**a**) electrical circuit and measurement points; and (**b**) total arc discharge energy of the SI and the MDJS at different pressures.  
Figure 3: Schematic of the model swirl combustor.  
penetration depth of the initial spark kernel generated by the MDJS can not only prolong the service life of the igniter but also help increase the probability of contact with the flammable hydrocarbon mixture of stoichiometric ratio, which contributes to improving the secondary ignition ability of the igniter.  
In addition, Figures 7 and 8 show that a higher spark plasma penetration depth and a larger activated area at a faster rate, particularly before 40 \(\upmu\)s, were observed using the MDJS. This means the MDJS generates a much larger plasma volume and a fast growing spark kernel in comparison to the conventional SI. At the beginning of the discharge, the MDJS generated a spark plasma penetration depth which was approximately 3 times higher than that of the SI, and an activated area which was approximately 2.5 times larger than that of the SI. Then, the spark plasma penetration depth of the MDJS increased at a faster rate than the SI over time at 0.26 bar, and the activated area showed a similar trend. When the pressure was lowered to 0.12 bar, the MDJS still showed a deeper spark penetration depth and a larger activated area, which was not as obvious as at the higher-pressure state (0.26 bar). That occurred because the air was so thin that the high-temperature gas inside the cavity could not be expelled outside the orifice as fast as it could under the higher-pressure state (0.26 bar). At 100 \(\upmu\)s, the spark plasma penetration depth of the MDJS was increased by 49% and 103% at 0.12 bar and 0.26 bar, respectively. In addition, the activated area generated by the MDJS was increased by 39% and 112% at 0.12 bar and 0.26 bar, respectively.  
Figure 5: Spark plasma volume and activated area development from a side view of the SI and the MDJS at different pressures.  
Figure 6: Detailed structure of (**a**) the conventional spark igniter (SI) and (**b**) the multichannel discharge jet-enhanced spark (MDJS) at 20 \(\upmu\)s in an ambient pressure of 0.26 bar.  
Because the MDJS could not only generate a much higher arc discharge energy and a higher spark penetration depth but also create a much larger plasma volume as well as an activated area in comparison to the SI, it has the potential to achieve a successful ignition more easily in a fuel-lean state in an aero-engine combustor.