激光诱导热光栅光谱测温技术研究

报道了激光诱导热光栅光谱测温技术的研究. 通过两束相干交叉的脉冲抽运光, 在NO₂/N₂混合气中诱导出热光栅, 一束满足布拉格散射条件的连续探测光在交叉区域激励出相干的热光栅信号, 经过空间和光谱滤波的信号光由光电倍增管探测, 并由数字示波器显示和存储. 该信号携带了丰富的流场信息, 通过频域分析, 对气体的温度进行了测量, 热光栅光谱技术测量的温度与热电偶温度符合得很好. 同时还利用热光栅光谱技术进行了气体声速的直接测量, 在一定的温度范围内, 测量结果与理论曲线基本一致, 显示了该技术具有较高的测量精度与多参数同时测量的能力. 对影响信号波形的因素进行了分析, 结果表明, 热光栅光谱测温技术在高压强环境下应用具有独特的优势, 是一种应用前景广阔的激光燃烧诊断技术.

Laser induced thermal grating spectroscopy thermometry technique

In this paper the laser induced thermal grating spectroscopy thermometry technique is investigated. Two coherent, pulsed pump lasers are crossed in NO₂/N₂ mixture to induce an interference pattern, owing to the resonant absorption and the subsequently quenching effect. The heat released into the bulk gas can modulate the local refractive index (temperature grating). Simultaneously, the sound wave induced by the electric field forms the standing wave (acoustic grating). These two effects mentioned above produce a thermal grating, and a continuous probe laser satisfying the Bragg scattering condition, generates a coherent signal in the crossed region. The spatial and spectral filtering signal is detected with a photomultiplier tube, and displayed with a digital oscilloscope. The signal carries plenty of flow field information. The gas temperature is obtained through frequency analysis. In order to increase the precision of temperature measurement, we calibrate the grating spacing at a known temperature in a pressurized gas cell. Then the temperature in a range of 300-500 K is measured by the laser induced thermal grating spectroscopy technique, and the thermocouple temperatures are recorded at the same detecting point simultaneously. Both of them agree well with each other, though some discrepancies are still existent. The difference is explained according to the heat radiation loss. We also use this technique to measure the gas sound speed directly, which is crucial to studying the gas behaviors at high pressures and the interaction between molecules. In a certain temperature range, the measurement result and the theoretical curve are nearly consistent, which shows the high precision and multi-parameter measurement ability of laser induced thermal grating spectroscopy. The factors influencing the signal waveform are analyzed, too, and the results demonstrate that the signal duration, the signal intensity, and the oscillation peaks increase with pressure increasing. As a consequent, the accuracy of measurement can be improved. Also, other gas dynamic parameters, such as the thermal diffusion rate and the heat conductivity, can also be measured by using this technique. The unique advantage of laser induced thermal grating spectroscopy thermometry technique provides us with a powerful diagnostic tool used in high pressure condition.