## 5 Results and Discussion
### Effect of the Fuel Flow Rate  
Figure 18 shows the flame propagation under the fuel flow rates of 7.3 mL/min, 15.2 mL/min and 16.8 mL/min, respectively. The air flow velocity, ambient temperature and voltage are 1.4 m/s, 20 \({}^{\circ}\)C and 24 V, respectively. With the increase of diesel flow rate, the flame length increases, indicating that the combustion is enhanced. The flame stripping phenomenon is found under different fuel flow rates, denoted by a yellow dotted circle, leading to the abrupt decrease of flame area. The flame plume with an arc shape generates close to the intake manifold burner due to the air entrainment and vortexes, then the flame swirl and rolls in the combustion chamber, leading to the flame stripping. As presented in Figure 19, the AFA under air flow velocity of 1.4 m/s shows an increasing trend with the fuel flow rate increase from 7.3 to 16.8 mL/min, meaning that  
Figure 17: The flame penetration under 20 \({}^{\circ}\)C (NT) and \(-\)9 \({}^{\circ}\)C (LT). Figure 18: The flame propagation under the fuel flow rate of (**a**) 7.3 mL/min, (**b**) 15.2 mL/min and (c) 16.8 mL/min. the combustion is boosted. When the fuel flow rate ranges from 7.3 to 16.8 mL/min, the fuel quantity is low, leading to incomplete combustion. Thus, a higher fuel flow rate improves the lean fuel-air mixture and the subsequent combustion. However, the fixed power supply of the heating rods limits the amount of diesel evaporation. When the fuel flow rate increases from 16.8 to 18.9 mL/min, excess diesel makes the diesel evaporation and combustion deteriorate, leading to the decreasing AFA. Figure 20a shows the fuel vapor under the fuel flow rates of 10 mL/min and 15.2 mL/min, respectively. The fuel vapor under the higher fuel flow rate of 15.2 mL/min develops earlier and shows a notably higher amplitude than that under 10 mL/min. The velocity field under 15.2 mL/min shows similar results with that under 10 mL/min, as shown in Figure 20b. The vorticity in the combustion chamber under 15.2 mL/min shows a slight increase compared with that under 10 mL/min. Noteworthy is that, with the fuel flow rate increasing from 7.3 to 18.9 mL/min, an extremely small increase of occurrence probability is shown, indicating that the effect of the fuel flow rate is weak and nearly negligible. Though the periodic diesel evaporation and vortex clusters are enhanced by the increasing fuel flow rate, the corresponding effect on flame swirl motion is too weak. Thus, the flame swirl motion is mainly determined by the air motion, which determines the vortexes in the vicinity of intake manifold burner. Figure 19: The average flame area under the different fuel flow rates. Figure 20: (**a**) The diesel vapor length with time, (**b**) the velocity field and (**c**) the vorticity under the fuel flow rate of 15.2 mL/min. Figure 21 shows the preheating effect of the fuel flow rate on the temperature rise in the combustion chamber. When the air flow velocity is 1.4 m/s, the temperature rise of air flow shows a notable increase with the increasing fuel flow rate within 14.5 mL/min. However, the temperature rise of the flame shows a slight increase when the fuel flow rate increases from 14.5 to 15.2 mL/min, and the air flow temperature remains the same, indicating that the promotion effect of flow rate reaches its limit. Under the air flow of 2 m/s, when the fuel flow rate increases from 7.3 to 15.2 mL/min, the temperature rise increases significantly, and the position of the highest value shifts away from the burner, meaning the combustion is boosted and the flame length increases. Figure 22 present the temperature rise in the exhaust pipe under different fuel flow rates. When the fuel flow rate increases from 7.3 to 15.2 mL/min, the temperature rise of all the points increases significantly, indicating the promotion of the intake manifold burner combustion. For all the temperature measuring points, the temperature rise with different fuel flow rates shows the same trend as that under the fuel flow rate of 13 mL/min.