图表示不同一次氮气压力下焰流速度值与轴向距离的关系。如图所示，速度曲线在拉瓦尔喷嘴扩张段和喷口自由射流区前段发生起伏波动。这是由于拉瓦尔管的喉径与喷管直管的口径较小，气体因压力梯度过大表现为欠膨胀状态，导致两个区域段激波的产生。此外，气体在拉瓦尔扩张管之后至0.1 m喷涂距离之前具有较好的速度保持性，有助于该距离段粒子的飞行与加速。在0.1 m喷涂距离之后，焰流速度的下降幅度随喷涂距离的变远而不断增大。

The graph represents the flame flow velocity values versus axial distance at different primary nitrogen pressures. As shown in the figure, the velocity curve fluctuates in the expansion section of the Laval nozzle and the front section of the free jet area of the nozzle. This is due to the throat diameter of the Laval tube and the small diameter of the nozzle straight tube, the gas behaves as under-expansion due to the excessive pressure gradient, resulting in the generation of excitation waves in the two regional sections. In addition, the gas has good velocity retention after the Laval expansion tube until the 0.1 m spraying distance, which contributes to the flight and acceleration of particles at this distance. After 0.1 m spraying distance, the decrease of flame velocity increases with the distance of spraying.