Multi-mode absorption spectroscopy

A method of absorption spectroscopy is reported that uses a comb of laser modes that is frequency tuned over the mode spacing. Absorption of individual modes in a gaseous absorber is detected by reduction in the total transmitted power as the modes are tuned yielding a signature characteristic of the absorbing species. Analysis of the signal provides information on the species concentrations, temperature and pressure. The potential of the method for rapid detection of multiple transitions with high resolution and wide spectral range is demonstrated using a multi-mode diode laser to detect multiple transitions in the A-band of molecular oxygen around 760 nm. Application of the technique to gas-phase thermometry with a precision of ±2 % is reported.     

Multi-mode absorption spectroscopy, MUMAS, using wavelength modulation and cavity enhancement techniques

Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band $b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-}Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band b¹\varSigma _g⁺? X³\varSigma _g^-b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-} of molecular oxygen at 760 nm. Direct MUMAS and WMS-MUMAS signals were recorded using a White cell for air and pure oxygen for pressures in the range 0 to 1 bar. CEAS-MUMAS signals were recorded with and without WMS in an open enhancement cavity containing laboratory air. Enhancement of the signal-to-noise ratio has been obtained demonstrating the potential for increased detection sensitivity for gas-sensing applications of MUMAS.     

Two-wavelength mid-IR absorption diagnostic for simultaneous measurement of temperature and hydrocarbon fuel concentration

A two-wavelength mid-IR laser is used for time-resolved absorption-based measurements of temperature and n-heptane concentration in shock-heated gases. The novel difference-frequency-generation laser provides tunable mid-IR light from nonlinear conversion of near-IR light, enabling access to the strong hydrocarbon absorption bands between 3.3 and 3.5 μm associated with the CH stretching vibration. This laser was modified to alternate between two mid-IR wavelengths at 200 kHz, providing 5 μs time resolution for simultaneous monitoring of temperature and concentration in reactive flows and combustion systems. Temperature-dependent absorption spectra of n-heptane are first measured in a cell from 298 to 773 K using an FTIR spectrometer. These spectra are used to select candidate pairs of wavelengths with good sensitivity to temperature and concentration and to provide accurate temperature-dependent absorption cross-sections at the selected wavelengths. Laser absorption measurements of shock-heated n-heptane are then used to extend the cross-section data to 1300 K and to investigate the sensor accuracy and noise characteristics. The temperature and concentration inferred from the measurements are compared to known post-shock conditions, with a 4.5% RMS deviation from the calculated temperature and 1.7% RMS deviation from the calculated concentration. Finally, at high temperatures, the sensor is used to monitor decomposition of n-heptane, illustrating the potential of this diagnostic for hydrocarbon kinetics experiments in shock tubes. This new sensor concept should prove useful for simultaneous, time-resolved temperature and hydrocarbon concentration measurements in a variety of combustion and propulsion applications.     

遗传规划用于非分散红外吸收光谱的CO浓度测量

非分散红外吸收光谱法(NDIR)是CO浓度测量的主要手段之一.设计了NDIR的开放式CO气体测量系统及连续测量实验装置,对10种不同浓度的标准CO气体进行测量实验,分析了获取的40个实测数值.将遗传规划(GP)用于CO光强采样数据的建模,得到验证集的相关系数为0.999 7,验证集的相对误差最大值和平均相对误差分别为4...     

The measurement of relative concentration profiles of NH2 using laser absorption spectroscopy

Laser absorption spectroscopy has been used to measure relative concentration and translational temperature profiles of amidogen (NH₂) in premixed ammonia-oxygen flames operating at low pressure. The narrow-linewidth laser absorption technique used in the study is described, and the details of the spectroscopy of NH₂ necessary in the interpretation of the absorption spectra are discussed. Results are presented which illustrate the effects of mass flowrate and fuel equivalence ratio. The magnitude of the oscillator strength required to bring these measurements into agreement with flame calculations is of the same order of magnitude as previous estimates for oscillator strengths of other vibration-rotation lines in the same electronic band. Because of the high spectral resolution of the laser absorption technique, the previously unmeasured spin splitting of the doublet used in the analysis was obtained as a by-product of the research.     

X-ray absorption spectroscopy and high pressure

Abstract

The difficulties of the adaptation of high pressure to x-ray absorption are presented. The advantages of the energy-dispersive geometry are discussed as well as the future improvements expected with the new synchrotron radiation sources.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Development of diode laser absorption spectroscopy method for determining temperature and concentration of molecules in remote object

We develop a method of diode laser absorption spectroscopy for noncontact measurements of the temperature and pressure of a gas mixture under unsteady-state conditions at low signal-to-noise ratios. The method is based on the measurement of the absorption spectra of water molecules and approximation of experimental spectra by spectra simulated based on spectroscopic databases. Different approximation algorithms of experimental spectra are tested, such as fitting by individual contours and fitting by a part of the simulated spectrum. We reveal that, at small signal-to-noise ratios, the approximation of experimental data by a simulated spectrum yields more correct data on the temperature of the mixture compared to the fitting by individual contours. For the examined temperature range of 300–1300 K, the determination error of the gas temperature in the test cell proved to be approximately three times lower than upon fitting by individual contours. The developed method of recording and processing spectra is used to measure the temperature, water vapor concentration, and total pressure under the unsteady-state combustion conditions of an air-hydrogen mixture in a supersonic flow.     

基于时分复用技术的吸收光谱气体温度在线测量研究

可调谐半导体激光吸收光谱技术是一种具有高灵敏度、高选择性的非接触式气体在线测量技术。通过直接扫描多条H2O特征谱线并结合最小二乘算法实现对开放环境气体温度的在线测量。利用HITRAN光谱数据库详细讨论了边界效应对气体温度浓度测量的影响,计算结果表明,扫描多特征谱线并结合最小二乘算法可有效减小边界效应对开放环境气体温度测量的影响。实验中采用时分复用技术同时扫描了7 444.36,7 185.60,7 182.95和7 447.48cm-1四条H2O特征谱线,对管式炉573～973K范围内不同工况下的气体温度进行了测量。吸收光谱测量结果与热电偶信号的最大温差小于52.4K,温度测量最大相对误差为6.8%。     

基于吸收光谱法的CO_(2)正压测量技术分析北大核心CSCD

为了研究可调谐半导体激光吸收光谱(TDLAS)技术在气体检测过程中,正压条件下的相邻谱线影响问题,建立基于积分吸光度的压力测量模型。以CO_(2)为研究对象,在室温、高纯度条件下进行了范围为(1~2)atm的压力测量仿真和实验。实验结果显示:随着压力的增大,相邻吸收谱线相互影响程度加剧,吸光度曲线偏离零基线的程度加大;实验测量结果在1.25 atm处最大相对偏差为4.94%,在2.00 atm处最小相对偏差为0.73%,平均相对偏差为3%。     

Simultaneous measurement of absolute OH concentration,temperature and flow velocity in a flame by photothermal deflection spectroscopy

We demonstrate the use of photothermal deflection spectroscopy for a simultaneous measurement of absolute minority-species concentration, temperature and flow velocity in a flame from the analysis of a single data trace. The ability to make absolute concentration measurements, as well as the ability to measure three important combustion parameters simultaneously, may make this technique quite useful in many circumstances. Received: 27 August 2002 / Revised version: 26 September 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-479/575-4580, E-mail: rgupta@uark.edu     

基于激光吸收光谱技术的燃烧场气体温度和浓度二维分布重建研究

基于可调谐半导体激光吸收光谱技术和代数迭代算法（ART）实现燃烧场温度和浓度二维分布重建．采用时分复用技术,在1kHz扫描频率下分别扫描H2O的两条吸收谱线,7205．25和7416．05cm^-1,对温度分布在300-1100K范围内的气体温度场进行了重建．研究了投影角度和投影光线数目对温度场和浓度场重建结果的影响,并将温度场重建结果与热电偶测量结果进行比较,结果表明,采用四个投影方向时,温度场重建结果与热电偶测量结果除中心低温区域外基本符合．当光线数目减少时,通过在两条光线间增加虚拟光线,代入到迭代算法中,增加光线数目,提高了温度场和浓度场的重建效果．但此方法受到燃烧场温度梯度大小的影响,即在两条光线之间气体温度梯度较大,增加虚拟光线提高温度场重建效果不明显．     

Distributed optical-fibre sensors for the measurement of pressure,strain and temperature

An analysis has been made of fully distributed optical-fibre measurement sensor (DOFS) systems, with particular reference to the measurement of pressure, strain and temperature in industrial environments. The analysis derives relationships between the primary system parameters; these relationships allow appropriate trade-offs to be made in order to meet any given set of industrial requirements; they also allow quantitative comparisons to be made between different kinds of DOFS system. The various physical phenomena which may be used in DOFS are considered, leading to the conclusion that, for performance likely to be generally attractive for industrial application, systems which use optical phase modulation by the measurand should be used. A review of systems which have already been studied illustrates the application of the analysis, and highlights the practical problems which are encountered in attempting to implement DOFS ideas. The proposed way forward is to begin research on two novel approaches to DOFS which use phase modulation and simplified signal processing. Implications in respect of the component technology and the industrial economics are considered for these novel system approaches.     

Analysis of algebraic reconstruction technique for accurate imaging of gas temperature and concentration based on tunable diode laser absorption spectroscopy

An improved algebraic reconstruction technique(ART) combined with tunable diode laser absorption spectroscopy(TDLAS) is presented in this paper for determining two-dimensional(2D) distribution of H_2O concentration and temperature in a simulated combustion flame.This work aims to simulate the reconstruction of spectroscopic measurements by a multi-view parallel-beam scanning geometry and analyze the effects of projection rays on reconstruction accuracy.It finally proves that reconstruction quality dramatically increases with the number of projection rays increasing until more than 180 for 20 × 20 grid,and after that point,the number of projection rays has little influence on reconstruction accuracy.It is clear that the temperature reconstruction results are more accurate than the water vapor concentration obtained by the traditional concentration calculation method.In the present study an innovative way to reduce the error of concentration reconstruction and improve the reconstruction quality greatly is also proposed,and the capability of this new method is evaluated by using appropriate assessment parameters.By using this new approach,not only the concentration reconstruction accuracy is greatly improved,but also a suitable parallel-beam arrangement is put forward for high reconstruction accuracy and simplicity of experimental validation.Finally,a bimodal structure of the combustion region is assumed to demonstrate the robustness and universality of the proposed method.Numerical investigation indicates that the proposed TDLAS tomographic algorithm is capable of detecting accurate temperature and concentration profiles.This feasible formula for reconstruction research is expected to resolve several key issues in practical combustion devices.