## 5 Results and Discussion
### Effect of the Air Flow Velocity
Compared with the case under the air flow velocity of 2 m/s, the temperature rise is lower under 10 m/s due to more heat dissipated in the air flow, as shown in Figure 15. With the increasing distance from the combustion chamber, the temperature rise of the vertical axis shows an overall decreasing trend because more heat is carried away by the high-speed air flow. The data of \"20-Y\" shows the highest temperature rise in the vertical axis, while the data of \"0\" and \"20-X\" show much higher temperature rise than that of \"40-X\", indicating that the warm gas mainly distributes in the upper region and center zone of the exhaust pipe. Compared with the case under 2 m/s, the warm air is concentrated in the zone closer to the central zone under 10 m/s due to the stronger heat convection and dissipation. Figure 14: The temperature rise at (**a**) \(y\)-axis and (**b**) \(x\)-axis in the exhaust pipe under the fuel flow rate of 13 mL/min and air flow velocity of 2 m/s. Figure 15: The temperature rise at (**a**) \(y\)-axis and (**b**) \(x\)-axis in the exhaust pipe under the fuel flow rate of 13 mL/min and air flow velocity of 10 m/s. The average temperature rise in the exhaust pipe at the distance of 1120 mm away from the intake manifold burner was used to calculate the combustion efficiency. The specific heat capacity of the exhaust gas is calculated through the equation shown below:  
\[c=\sum g_{i}\times c_{i} \tag{8}\]  
where \(g_{i}\) is the mass percentage of each component and \(c_{i}\) is the corresponding specific heat capacity. The combustion efficiency is calculated by the following equation:  
Figure 10.0  
where \(Q_{1}\) and \(Q_{2}\) are the heat of diesel and heating rods, respectively. \(Q_{3}\) is the heat loss along the exhaust pipe, \(m\) is the mass flow rate of the intake air, and \(\Delta t\) is the average temperature rise in the cross section. The average temperature rise and combustion efficiency under different air flow velocities are shown in Figure 10.1 With the air flow velocity increasing from 1.4 to 10 m/s, the average temperature rise first increases before decreasing, indicating that the combustion is promoted within 2 m/s. Excess air flow hinders the combustion when the velocity exceeds 2.64 m/s, and the stronger air flow leads to higher heat dissipation. Thus, lower air flow temperature rise is shown from 3.9 to 10 m/s. However, the combustion efficiency increases due to the increased air flow volume. The flame penetration under the intake air temperature of normal temperature (20 \({}^{\circ}\)C, NT) and low temperature (LT, \(-9\) \({}^{\circ}\)C) is shown in Figure 10.2 Lower intake air temperature has a negative effect on combustion due to the deteriorating diesel evaporation. The average temperature rise and combustion efficiency under low temperature are 96 \({}^{\circ}\)C and 33.7%, which are lower than that under normal temperature. The flame stripping phenomenon shows the same occurrence probability, indicating that the flame stripping is irrelevant to the intake air temperature. Figure 10.3 (**a**) The average temperature rise and (**b**) the combustion efficiency in the exhaust pipe with different air flow velocities.