# A Novel Way to Enhance the Spark Plasma-Assisted Ignition for an Aero-Engine Under Low Pressure
## 3 Results and Discussion
### Effect of the MDJS on the Ignition Process and the Ignition Limit Extension  
In order to investigate the lean ignition limits of the SI and the MDJS and to minimize the effects of random elements that may be involved, data from five tests were obtained to plot one curve. The global equivalence was gradually reduced by decreasing the fuel flow rate, and this process was repeated at different air flow rates (AFRs). Figure 9 shows a representative distribution as well as the intensity of CH* radicals with the SI and the MDJS at 0.26 bar when \(\varphi=0.081\). A false-color enhancement technology was used when comparing the SI with the MDJS. As is shown in Figure 9, the flame kernel formed at the top wall is bright at \(\mathrm{t}=0.1\) ms and it begins growing in strength and propagating inside the combustor. By 22 ms, it has filled the combustor which indicates that the combustor has settled into a quasi-stable burning state. A larger initial spark kernel and a higher spark plasma penetration depth of the MDJS were observed. These were beneficial to the ignition performance because of the critical flame radius theory. Figure 8: Activated area of SI and MDJS at (**a**) 0.12 bar and (**b**) 0.26 bar. Figure 7: Spark plasma penetration depth of the SI and the MDJS at (**a**) 0.12 bar and (**b**) 0.26 bar. Moreover, it was observed that when \(t=6\) ms and \(t=11\) ms, a larger initial spark kernel and a higher spark plasma penetration depth of the MDJS helped to accelerate burning velocities. Figure 10 shows a quantitative comparison of the light intensity which indicates that the activated area of the MDJS was much larger at the beginning and grew faster than that of the SI. However, the activated area of the SI and the MDJS became the same due to the same equivalent ratio and under the same operating conditions. The total integrated discharge intensity of the MDJS was also greater than that of the SI and it showed a similar trend with the activated area. Figure 11 shows CH* chemiluminescence images of the SI and MDJS ignition processes when \(\varphi=0.04\) at 0.26 bar. The MDJS generates a much larger initial flame kernel which grows faster in strength and shows a greater flame propagation speed than that of the SI. Figure 9: CH* chemiluminescence images of ignition processes using the SI and the MDJS when \(\varphi=0.081\) at 0.26 bar. The comparison of the lean ignition limits of the SI and the MDJS with kerosene at 20 \({}^{\circ}\)C (local room temperature) and \(-30\)\({}^{\circ}\)C is shown in Figure 12. It is readily observed that the lean ignition limit expands with the increasing air flow velocity, a result which is different from those concluded in previous work. This occurs because the fuel flow rate must be raised to keep the equivalence ratio constant while the air flow velocity increases. A much higher pressure is loaded onto the fuel atomizer to maintain a larger fuel flow rate, and that leads to a higher pressure drop (\(\Delta p/p\)) between the  
Figure 11: CH\({}^{*}\) chemiluminescence images of SI and MDJS ignition processes when \(\varphi\) = 0.04 at 0.26 bar. Figure 10: Activated area and total integrated discharge intensity of the SI and the MDJS when \(\varphi\) = 0.081 at 0.26 bar. ambient air flow and the fuel atomizer, a situation which contributes to atomization and evaporation. For the same air flow velocity, a noticeable extension of the lean ignition limit is observed using the MDJS with normal and low-temperature kerosene. The lean ignition limit is extended 24% from 0.034 to 0.026 at 25 m/s with 20 \({}^{\circ}\)C kerosene and 17% from 0.075 to 0.062 at 12 m/s with \(-\)30 \({}^{\circ}\)C kerosene maximally. An improvement of the MDJS is expected via thermal, kinetic, and transport parameters [24]. The MDJS performs at a higher spark plasma penetration depth, which takes more energy to heat the gas and accelerate the combustion chemical reactions.