基于紫外可调谐激光吸收光谱技术的甲烷/空气平面预混火焰温度测量研究

Temperature Measurement of CH_4/Air Premix Flat Flame Based on the Absorption Spectroscopy Technology of UV Tunable Laser

火焰温度是燃烧领域最重要的宏观物理量之一,使用紫外可调谐激光吸收光谱技术,以火焰中的OH自由基作为测量对象对甲烷/空气平面预混火焰进行了温度测量。首先使用平面激光诱导荧光（PLIF）技术对甲烷/空气平面预混火焰不同燃烧工况条件下火焰中的OH基分布进行了测量,选取火焰中OH基分布均匀工况进行了紫外吸收光谱温度测量。通过LIFBASE仿真计算,综合考虑温度测量灵敏度、测量信噪比等因素,选择OH基A-X（0,0）吸收带中的P_1（2）和Q_1（8）两支谱线作为被测跃迁。测量时使用Nd∶YAG激光器泵浦染料激光器,经倍频后输出308～311nm紫外可调谐激光。通过染料激光器以0.4pm为步长进行激光波长调谐,分别扫描获得两条吸收谱线的吸收峰线型。对实验数据进行voigt拟合后,通过计算两条谱线的积分吸收值之比,获得了平面预混火焰中的温度信息。分别测量了燃烧器表面不同水平位置与燃烧器中心不同高度处的火焰温度。测量结果与文献报道的采用同样结构燃烧器,通过其他光谱技术获得的测量结果进行了横向对比。在OH基浓度较高的火焰锋面区域测温结果吻合度较高,验证了该技术测量结果的可信度。由于其测量对象与双线OH-PLIF测温的一致性,该技术未来可作为局部温度测量方法,进一步应用于对双线PLIF等二维火焰温度空间分布测量结果的标定当中。

The physics of combusting flows consists of a complex interaction between chemical reactions,fluid mechanics and radiation.Temperature is one of the most important parameters for the processes.Laser-based imaging techniques are frequently used to assess temperature information from reactive systems without perturbing the system under study.To verify the feasibility of the temperature measurement of UV tunable absorption spectroscopy technology the methane/air premix flat flame was selected as the target of test because of the combustion stability of this kind of flame.Before the temperature measurement the distribution of OH radical in the premix flat flame under different operating conditions were obtained by using planar laser induced fluorescence（PLIF）.At the low equivalence ratio the OH radicals distribute uniformly in the flame for the adequate oxygen in the premix gas.The condition with uniform distribution of OH in the flame was selected for the UV tunable absorption spectroscopy measurement.For the selection of absorption lines of the measurement the spectrum of OH A-X（0,0）band have been simulated by LIFBASE.Considering the slope sensitivity and SNR of the test the transitions P1（2）and Q1（8）were suitable for the temperature measurement of the flame.A dye laser pumped by a frequency doubled Nd：YAG laser was used to generated the UV laser.The dye laser was operated with the mixed dye of DCM and PM580 for high conversion efficiency at 310 nm.To investigate the transitions of Q1（8）and P1（2）of OH A-X（0,0）the laser was tuned from 309.225～309.255 and 308.625～308.655 nm separately with the step of 0.4pm,30 pulses were recorded at each step.The laser pulses reflected by the beam splitter were collected by detector A,and the pulses passed the flame were collected by detector B.The signal of these two detectors were recorded by the oscilloscope and acquired by the computer automatically.The line shape of these transitions can be obtained after fitting the experimental data with the Voigt function.The integral ratio between the fitting results of these two lines was calculated.Then temperature of the flame could be deduced by the integral ratio.The temperatures of different positions above the surface of burner and varied heights of the flame center were obtained by measuring the integrated absorption ratio of these two transitions.The test results of this method are compared with the report in reference,in which temperature of the burner with the same structure was measured by other ways.The results of these two tests agree well.It shows that this method has the potential to be a calibration for the two-dimension thermometry in flame such as two-line PLIF.