## 5 Results and Discussion
### Effect of the Air Flow Velocity  
Figure 8 shows the velocity field of slice A and slice B under intake air flow velocity of 2 m/s and 1.4 m/s. The power supply, ambient temperature and diesel flow rate are set to 24 V, 20 \({}^{\circ}\)C and 10 mL/min in this section, respectively. In the downstream zone close to the intake manifold burner, a negative velocity zone is generated because the air flow is hindered by the burner block. The negative velocity zone under the intake air flow velocity of 2 m/s, which is denoted by the black dotted line, shows higher velocity compared with that under 1.4 m/s, leading to stronger flame swirl motion. The velocity vector diagram of the intake manifold burner vicinity under 2 m/s is shown in Figure 8c. It shows that two swirl zones with opposite rotation directions, which are noted with the red arrows. In order to obtain the air flow direction, 30 3D streamlines that cross the vertical slice plane A are extracted. The streamlines are three-dimensional distributed in the combustion chamber, as shown in Figure 8d,e. Two vortexes with opposite rotation directions are generated in the vertical plane, which is noted with the red arrows. It is consistent with the vortexes in Figure 8c. While the vortex located below is found to be symmetrically distributed, and the contrary swirl direction is shown, which is noted with the yellow arrows. Because of the complex vortexes and air flow direction in the vicinity of the intake manifold burner, air entrainment and flame swirl motion are generated, leading to flame stripping and instability. The flame swirl motion is shown in Figures 9 and 10, as denoted by the red dotted lines. The fuel flow rate is 10 mL/min. The flame plume generates with a small arc shape. Then, the small arc develops into a big arc with time elapses. Under the air flow velocity of 2 m/s, the radian of the flame plume increases significantly to the negative velocity zone closest to the intake manifold burner and breaks into two parts eventually. Part I shortens rapidly and fuses with the flame in the vicinity of the intake manifold burner. Part II continues to move downstream and eventually fuses with the ambient flame. However, part I under the intake air flow velocity of 1.4 m/s fuses with ambient flame before it breaks up and makes it difficult to distinguish. Therefore, only part II can be seen. With the increase of air flow velocity, higher occurrence probability of the stripped flame is shown, indicating that the flame swirl motion is enhanced by the strong air flow. Figure 8: The velocity field under (**a**) 2 m/s and (**b**) 1.4 m/s; (**c**) the velocity vector in the vicinity of the intake manifold burner; the 3D streamlines crossing slice plane B in the combustion chamber under (**d**) 2 m/s and (**e**) 1.4 m/s. The diesel flow rate is 10 mL/min. The average flame area (AFA) under different intake air flow velocities during the steady stage is shown in Figure 10. During the steady stage, the flame is more stable than that at the developing stage. The AFA under the fuel flow rate of 10 mL/min increases first and then decreases with the air flow velocity increases from 1.4 to 3 m/s, indicating the hindrance effect of the excessive air flow on combustion. The increasing air flow provides more oxygen and boosts the mixture of air and diesel within 2.64 m/s, enhancing the combustion. In addition, the increasing air flow velocity enhances the air swirl motion and heat dissipation, leading to increased ignition difficulty and combustion deterioration from 2.7 to 3 m/s. Figure 11 shows the diesel vapor (C7H16) distribution in the combustion chamber. The diesel vapor length in the combustion chamber shows obvious periodic reciprocating oscillation, indicating that the diesel evaporation process is periodic. Compared with the case under higher air flow velocity of 2 m/s, the fuel vapor under 1.4 m/s develops earlier and shows lower amplitude. The periodic evaporation of the fuel vapor is believed to be one reason that leads to flame pulsation and instability.