## 5 Results and Discussion
### Effect of the Air Flow Velocity
Figure 10: The average flame area under the different air flow velocities. Figure 9: The flame swirl motion under air flow velocity of (**a**) 1.4 m/s and (**b**) 2 m/s. Figure 12 shows the vorticity in the combustion chamber. It indicates that the vortex clusters of the air flow with negative and positive values cross-distribute downstream of the intake manifold burner and move downward along the air flow direction. Therefore, the flame is affected by the negative-value vortexes and the positive-value vortexes simultaneously, leading to the complex flame pulsation and swirl motion. The vorticity under intake air flow of 2 m/s is higher than that under 1.4 m/s, leading to stronger air swirl motion and more violent flame stripping. Figure 11: The diesel vapor length with time. The preheating effect of the intake manifold burner is evaluated by the temperature rise of the air flow. Temperature rise in the combustion chamber and the exhaust pipe are measured by thermocouples. Figure 13 shows the preheating effect of the air flow velocity on the temperature rise in the combustion chamber. With the increase of the distance from the intake manifold burner, the temperature rise increases first and then decreases. This is because the outer flame locates at the position with the highest temperature rise. As the air flow velocity increases from 1.4 to 1.8 m/s, the position of the highest temperature rise shifts to 185 mm away from the intake manifold burner, and the temperature rise shows a significant increase. When the air flow velocity increases from 1.8 to 2.46 m/s, the position of the highest value remains constant, while the temperature rise shows a slight increase within 230 mm. Excessive heat is carried away by the air flow due to the forced heat convection, neutralizing the boost effect of the increased oxygen concentration and leading to a smaller temperature rise. The maximum flame length is about 25 cm in all the cases. Thus, the temperature rise within 250 mm reflects the flame temperature, while the temperature rise from 275 to 365 mm shows the air flow temperature. An obvious temperature rise from 275 to 365 mm is shown in Figure 13, indicating that though the promotion on combustion is neutralized by the forced heat convection, a better preheating effect can still be obtained. Figure 14 presents the temperature rise in the exhaust pipe under the fuel flow rate of 13 mL/min and air flow velocity of 2 m/s. When the distance from the intake manifold increases from 420 mm to 1120 mm, the temperature rise of \"40-Y\" and \"20-Y\" decreases due to the thermal convection and dissipation. While the temperature rise of \"0\" and \"\(-\)20-Y\" shows an increasing trend, indicating that the lower region receives the heat transfers from the high temperature zone. However, the temperature at the point with 40 mm below the circle center, that is to say, the temperature rise of \"\(-\)40-Y\", shows very slight fluctuation because most heat is dissipated before it arrives. The heat mainly concentrates in the upper zone due to the thermal buoyancy effect of the flame, then the heat transfers to the cold air, leading to decreasing temperature difference of adjacent points. Thus, the heat distribution becomes more uniform with the increasing distance from the intake manifold burner. The temperature rise of the points located symmetrically at the horizontal axis is similar. Therefore, the temperature rise of \"\(-\)40-X\" and \"\(-\)20-X\" can be expressed by the points of \"40-X\" and \"20-X\". The temperature rise of \"20-X\" and \"40-X\" is much higher than that of \"0\" point, indicating that the heat concentrates in the zone closest to the exhaust pipe wall. With the increasing distance from the intake manifold burner, the temperature  
Figure 13: The effect of air flow velocities on temperature rise in the combustion chamber under the fuel flow rate of 10 mL/min. rise becomes more uniform due to the heat transfers from high temperature zone to low temperature zone, which is consistent with the results at the \(y\)-axis.