CRDS-CARS-PLIF技术精确定量测量火焰OH浓度实验研究

The Study on Precise and Quantitative Measurement of Flame OH Concentration by CRDS-CARS-PLIF

在预混甲烷/空气燃烧的平面火焰炉上,采用脉冲式光腔衰荡光谱技术(cavity ring-down spectroscopy,CRDS)实现了对OH分子浓度的定量测量。根据光腔衰荡吸收光谱理论,选取OH的A2Σ+-X2Π(0,0)电子跃迁带中的P1(2)吸收谱线构搭建了一套激光波长在308.6 nm的脉冲CRDS实验装置。脉冲CRDS装置中的衰荡光腔是由一对反射率为99.7%的高反射镜组成且其衰荡腔的腔长为270 cm,并测量空腔(光腔中无火焰)的衰荡时间为2.33μs。通过理论分析影响浓度精确测量的实验参数,分别采用平面激光诱导荧光(planar laser induced fluorescence,PLIF)、相干反斯托克斯拉曼散射(coherent anti-stokes Raman scattering,CARS)和脉冲CRDS三种技术精确测量OH的有效吸收长度、高温火焰的温度和有效的光腔衰荡时间。当在平面火焰炉上燃烧预混的甲烷(1.1 L·min^(-1))和空气(15 L·min^(-1))且在距离炉面高度为6 mm时,采用PLIF技术测量的有效吸收长度比直接选用燃烧器炉面直径作为吸收长度的精度提高7.1%,室温下利用CARS技术测量的温度要比热电偶测量的温度精度提高45%,衰荡光腔内有火焰且选用非OH吸收波长时测得的光腔衰荡时间要比采用空腔时测得的光腔衰荡时间精度提高21.6%。因此,通过以上多种测量技术相结合的方式精准测量各实验参量,最后得到OH分子数密度在距离炉面高度为6 mm时达到最大值(3.59×10^(13)molecules·cm^(-3))且OH浓度精度要比于未修正的OH浓度提高了35.6%。另外,在不同当量比下(Φ=0.7~1.1),OH粒子数密度都会随着距离炉面高度的增加而减少,通过曲线拟合发现OH浓度随着距离炉面高度的增加呈e指数衰减。在同一燃烧高度的富氧燃烧状态下,OH浓度随着当量比的增加而增加;当甲烷流量保持恒定时,富氧燃烧状态下的OH浓度要高于低氧燃烧状态下的OH浓度。在燃烧场中,采用这种多光谱技术相结合(CRDS-CARS-PLIF)的精准测量方式不仅能够实现对OH浓度精准的定量测量提高了测量精度,还可为定量测量其他燃烧产物分子的浓度提供技术支撑,对研究燃烧化学反应起着至关重要的作用。

In this paper,a pulsed cavity-ring down spectroscopy(CRDS)is employed to measure the quantitative concentration of the OH radical in a plane flame burner with premixed methane/air.By analyzing the cavity ring-down absorption spectrum theory,we select the Pi(2)absorption line spectrum of the electronic transition band OH Az Z+-X,II(O,O)and build a set of the pulsed CRDS experimental device with a laser wavelength of 308.6 nm.The device of the pulsed CRDS is composed of a pair of mirrors with a reflectance of 99.7%,the cavity length of the ring-down cavity is 270 cm,and the ring-down time of the empty cavity(without a flame in the optical cavity)is 2.33μs.By analyzing the experimental parameters that affect the precise measurement of concentration,we use Planar Laser Induced Fluorescence(PLIF),Coherent Anti-Stokes Raman Scattering(CARS),and the pulsed CRDS to measure the effective absorption length of OH,high temperature of the flame,and cavity ring-down time.When the premixed methane(1.1 L·min^(-1))and air(15 L·min^(-1))are burned in a flat flame burner,and at the height of 6 mm from the burner surface,the precisely measured effective absorption length by PLIF is 7.1%higher than that of directly choosing the diameter of the burner surface as the absorption length,the measured precision of the temperature by CARS is increased by 45%than that measured by the thermocouple under room temperature,the measured precision of the optical cavity ring-down time with flame in the cavity and non-OH absorption wavelength is improved by 21.6%than that measured time of cavity ring-down without a flame in the cavity.By combining the above measurement techniques to measure all experimental parameters precisely,we obtain that the number density of OH molecules(3.59 X10^(13) molecules·cm^(-3))can reach the maximum value when the height from the furnace burner is 6 mm,and the precision of OH concentration is 35.6%higher than that of the unmodified OH concentration.Under different equivalence ratios(@=0.7~1.1),with the increase of the height from the burner surface,the number of OH particles gradually decreases,and the curve fitting shows that the OH concentration decreases in an e-exponential decay.At the same combustion height,the concentration of OH increases with the increase of equivalent ratios.When the methane flow rate is kept constant,the OH concentration in the oxygen-rich combustion condition is higher than in the low-oxygen combustion condition.In the combustion field,the precise measurement method with the multi-spectral technology(CRDS-CARS-PLIF)can achieve the precise quantitative measurement of OH concentration and provide technical support for the quantitative measurement of the concentration of other combustion product molecules,which plays a crucial role in the study of combustion chemical reactions.