火焰的速度测量是燃烧力学和流体诊断中的研究内容之一, 对于燃烧成分分析和推进动力学具有重要研究意义。 激光多普勒速度测量学测量精度高, 但由于测量过程本身的复杂性以及其在低速测量中的误差增大使得该方法的应用受到限制, 因而光热偏转光谱法在面向低中速流体测量时具有较好的实用价值。 光热偏转光谱法测速使用一束泵浦光入射到被测介质中, 由于流体介质中成分吸收光后形成热透镜分布, 当一束探测光入射到介质中时, 由于热透镜移动使探测光产生偏折, 通过测量探测光相对泵浦光的高度和信号偏转对应的飞行时间, 可得到流体的速度。 通过自建泵浦探测光热偏转实验装置, 使用单脉冲能量为20 mJ、 波长为355 nm的泵浦光和功率为2 mW、 波长为632.8 nm的连续探测激光对煤油火焰的不同位置进行了速度测量, 测量装置的空间分辨率为2×10^-5 cm³。 对距煤油灯芯高度5, 8和11 mm处的火焰平面进行了速度测量, 得到了火焰对应的水平速度分布, 发现在接近火焰下方的位置, 同一水平面的火焰外部速度高于内部速度; 在接近火焰上方位置, 同一水平面的火焰内部速度高于外部速度; 同一平面的速度分布接近于抛物线形分布。 对距火焰中心±2 mm的三个竖直平面进行速度分布测量, 得到了对应竖直面的速度分布, 发现竖直中轴线上靠近火焰底部的点速度慢于两侧, 上部的速度快于两侧, 同上述水平速度分布测量得到的结论一致。 实验所测得的火焰速度在0.2~1.5 m·s^-1之间。 通过使用单脉冲能量分别为20, 40和60 mJ泵浦激光, 分析了介质击穿在速度测量过程中引入的误差。 通过进一步优化系统信噪比, 光热偏转光谱法作为流体速度测量的有力工具将可实现对温度和浓度等参数的测量并用于燃烧诊断。

The measurement of flame velocity is one of the research interests on the combustion process and fluid diagnosis, as it is of key research significance for the analysis of combustion composition and propulsion dynamics. In spite of its high accuracy, laser Doppler velocity measurement has limited application due to the complexity of the measurement process and increased error rate in low-speed measurement, hence giving rise to the use of photothermal deflection spectroscopy for low and medium-speed fluid measurement. Photothermal deflection spectroscopy is based on the detection of the thermal lens on the tested medium. As components in the fluid medium absorb light and form a thermal lens distribution, when a probe beam is an incident on the medium, it is deflected due to the movement of the thermal lens. Fluid velocity is obtained by measuring the height of the probe beam relative to the pump beam, as well as the flight time corresponding to the signal deflection. This paper adopted a self-built pump device to study photothermal deflection. A pump beam with a single pulse energy of 20 mJ and wavelength of 355 nm, and He-Ne laser probe with the power of 2 mW were used to measure velocity at different positions of kerosene flame. The device has a spatial resolution of 2×10^-5 cm³. Velocity was measured at planes with distances of 5, 8 and 11 mm from the kerosene wick, to obtain the distribution of horizontal velocity corresponding to the flame. The external velocity of the flame at the same horizontal plane was found to be higher than the internal velocity near the bottom of the flame; in the position near the top of the flame, the internal velocity of the flame at the same horizontal plane was higher than the external velocity; velocity distribution along the same plane was close to parabolic distribution. Velocity distribution of the three vertical planes ±2 mm away from the center of the flame was measured, to obtain the distribution of the corresponding vertical planes. Velocity on the central vertical axis near the bottom of the flame was found to be slower than that of both sides, and the velocity of the upper part of the flame was faster than that of both sides. This was consistent with the conclusions obtained from the earlier measurements of horizontal velocity distribution. The flame speed measured in the experiment ranged from 0.2 to 1.5 m·s^-1. This paper used pump beams with single-pulse energy of 20, 40 and 60 mJ, to analyze the errors introduced by dielectric breakdown during velocity measurement. Results indicate that a larger error is introduced by higher laser energy. A velocity error of 0.1 m·s^-1 was introduced by the 40 mJ beam, while a velocity error of 0.6 m·s^-1 was introduced by the 60 mJ beam. With the further optimization of signal-to-noise ratio, photothermal deflection spectroscopy shall enable the measurement of parameters, such as temperature and concentration, thus making it a powerful tool for fluid velocity measurement and combustion diagnostics.