Hefei

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1 2017 3rd International Conference on Computer Science and Mechanical Automation (CSMA 2017) ISBN: Experimental Study of Broadening Coefficients for the v3 Band of CO by Tunable Diode Laser Spectroscopy Dong CHEN 1, Peng-Fei FANG 1, Yin-Xiu WANG 1 and Zhao-Li JIA 2,* 1 School of Instrument Science and Opto-electronic Engineering, Hefei University of Technology, Hefei School of mathematics, Hefei University of Technology, Hefei dchen@hfut.edu.cn, iazhaoli@hfut.edu.cn Keywords: Broadening coefficient, High temperature, Tunable diode laser absorption spectroscopy, CO. Abstract. A diode laser sensor based on absorption spectroscopy is developed for measurement of self-broadening coefficients for the three lines of v3 band of CO at high temperature. the spectra of CO at cm -1, cm -1 and cm -1 at temperatures ranging from 323K to 873K with different pressures are recorded. The line widths are acquired by the line shape fittings based on oigt function, and the collitional broadening coefficients and temperature dependences of the three lines are also derived. Introduction Over the last decades, high resolution absorption spectroscopy using diode laser-based sensors has been employed for the detection of various parameters in difficult, real-world environments[1,2]. This technique can provide fast, remote, sensitive, non-intrusive, and reliable measurements of the parameters, which are very attractive for the in-situ combustion diagnostic. Reliable spectroscopic parameters, such as line strengths, self-broadening coefficients, air-broadening coefficients, and temperature exponents, are important in absorption spectroscopy for accurately studying species properties, such as temperature, concentration, speed, and their corresponding field distributions. However, spectroscopic parameters widely used are mainly theoretical calculation results, and there exist considerable errors compared with the results in actual situations, especially at high temperature region. The primary motivation for measuring CO is for emissions-monitoring and combustion control applications. CO is a key indicator of combustion completeness and a regulated pollutant, so measurements of its presence can be useful for controlling combustion and monitoring the emissions from industrial combustors. While significant advances have been made in the experimental determination of CO line parameters at room temperature for the fundamental, first and second overtone bands[3,4], and detailed theoretically predicted results can also be found in a number of molecule spectroscopy databases like HITRAN, GEISA etc.[5], the improved CO line parameters at elevated temperatures of 700 to 2100K, which are important for laser spectroscopy sensors application in combustion diagnostics, are still required. In this paper, the high resolution spectra of CO at temperatures ranging from 323K to 873K are recorded by using a distributed feed-back (DFB) diode laser based spectrometer combined with high temperature gas absorption cell. The self-broadening coefficients and temperature exponents are deduced by oigt profile fitting of the recorded spectra, and the results are also compared to theoretical prediction. Theory For the high resolution molecular spectrum at elevated temperature, Doppler and pressure broadening are considerable; the line shape is given by the oigt function, which is the convolution of Gauss function and Lorentz function. For convenience of the line shape numerical fitting, the 241

2 expression given by Whiting[6] is often used, and through fitting the measured absorbance, the line width can be acquired. x 1 Φ ( ) ( ) ( ) ( 2 ) ( ) ( 2.25 ν = Φ ν0 1- x exp x x exp y 2 ) y y where S( T ) is the line strength, line width(fwhm), respectively. S( T ) Φ ( ν ) = + + ( ) 0 v 2 x x x v v L 0 = v v y = v 2 2 ( ) 0.5 v = v v + v (2) L L D v, vd and v are the oigt, Doppler and collisional broadening L v can be calculated using D T M 7 vd = v (3) 0 where v 0 [cm -1 ] is the linecenter frequency, T [K] is the sample temperature, and M [a.m.u] is the molecular weight of the absorber species. Pressure linewidth is proportional to pressure broadening coefficient γ ( T) and sample pressure. vl v = P X γ (T ) (4) L where X is the mole fraction of component, and γ [cm -1 atm -1 ] is the collisional broadening coefficient due to the th component. The temperature dependence of collisional broadening coefficient can be expressed as where T 0 is the reference temperature and n temperature dependence. n T0 γ ( T ) = γ (T0 ) (5) T (1) represents the corresponding coefficient of Experimental Details For these studies, a near infrared tunable diode laser spectroscopy system combined with a heated three-zone quartz optical cell is utilized. Fig. 1 shows the schematic diagram of the setup. The diode laser used for the experiments is a DFB InGaAsP diode laser from NEL Corporation. The distributed feedback structure constrains the laser to operate in a single longitudinal mode, make it extremely narrow in line width (<3MHz). The diode laser is controlled by ILX Lightwave LDC- 3900, which includes temperature and current control in the same package. A superposition of a linear voltage ramp (100 Hz frequency and 1 amplitude) is applied to the laser current driver to scan the laser frequency across the CO absorption features, and the laser output is separated into two beams by a 1 2 fiber beam splitter. The main beam (90% of beam power) is collimated and directed through The three-zone quartz optical cell is hermetically sealed and separated by wedged quartz windows into three sections. The 20cm long middle zone, used as test cell, is fill with CO gas at known pressure for line strength and collisional broadening parameter measurements, the two 30cm long zones on both sides, used as buffer cells, are filled with high purity nitrogen to prevent unwanted absorption along the optical path. The three-zone quartz optical cell is temperature 242

3 controlled by a heating furnace(model GSL-1700X, MTI) with a heating section length of 55cm.Three type-b thermocouples(b220) with an accuracy of ±0.75% are mounted along the middle test zone in equally spaced intervals to measure the temperature of the test gas. As such the temperature deviation along the middle test zone is less than 1% during the measurements. The pressure in the middle test zone is measured using a capacitance manometer(model CDG800, TamaGawa) with an accuracy of ±0.12%. The transmitted laser intensity is measured by an InGaAs PIN detector. The other laser beam passes through an ring interferometer(model FFP-I 1580, Micron Optics) for the wavelength variation monitoring during the laser scanning, the free-spectral range (FSR) of the ring interferometer is cm-1 with an uncertainty of ±0.05%, the output beam is monitored by another InGaAs PIN detector. The spectral data are collected using a 16Bits DAQ card (Model MP422, WWLAB) and processed by a LabIEW programmed PC. Detector Ring Interferometer DFB Type-B Heating Furnace Thermocouples Uniform Test Section 3-zone Quartz Cell Lens Detector Laser Controller Beam Cllimator Splitter Function Generator P acuum Gauge acuum Pump Figure 1. The experimental setup. Results and Discussion Scanned-wavelength direct absorption is used to measure the line strength of 3 lines of CO transitions located at cm -1, cm -1 and cm -1. The lines are select for their potential use in combustion diagnostics application, because this region has little interference from maor gaseous products of combustion, e.g. H 2 O, CO 2. Fig.2 shows the direct absorption signal, baseline fitting and etalon signal. A third-order polynomial was fit to the nonabsorbing regions of the intensity scan to infer the baseline laser intensity, I0, over regions of absorption. Fig.3 shows the conversion relationship between laser time domain and frequency domain Transmission Signal (m) I t I 0 Etalon FSR Sampling Points Figure 2. Direct absorption signal, baseline fitting and etalon signal. 243

4 Relative Frequency Sampling Data Figure 3. The conversion relationship between laser time domain and frequency domain. The spectral absorbance is calculated using Eq. (4) after the baseline emission is subtracted from the recorded laser intensity. Fig.4 shows the absorbance of cm -1 at different temperatures. Spectroscopy parameters were extracted from the data by fitting oigt line shapes to the measured absorbance profiles according to Eq. (1). Fig.5 shows the oigt fitting and residuals of cm -1. The best-fit oigt profile yields a maximum residual of 0.2% Absorbance P=0.579atm T=573K L=20cm Experiment oigt Fit Residual Std= Figure 4. oigt fitting and residuals of cm -1 line. The spectra of pure CO at a series of temperatures and different pressures are recorded. Then according to Eq.2 and Eq.5, the self-broadening coefficients γ are obtained through the relationship between collisional widths and pressures. Wavenumber (cm -1 ) self 244

5 0.010 Absorbance P=0.130atm P=0.363atm P=0.493atm P=0.658atm P=0.844atm P=0.912atm T=873K (a) Self-broadening linewidth(cm -1 ) K 473K 573K 673K 773K 873K (b) Wavenumber (cm -1 ) Pressure(atm) Figure 5. The spectra of pure CO at temperature of 873K with different pressure (a) and the linear fitting of self-broadening line width with pressure at different temperature (b). Fig.5shows the spectra of pure CO at temperature of 873K with different pressure (a) and the linear fitting of self-broadening line width with pressure at different temperature (b). It is shown that the CO self-broadening line width has a good linear relationship with pressure, and the linear fitting gives the self-broadening coefficients. Fig.6 show the fitted self-broadening coefficients of cm -1, cm -1, and cm -1 lines at different temperatures, an exponential function is fit to the plots, then the temperature exponents n are derived from the best nonlinear fitting of Eq.(5) as 0.60, 0.62 and 0.65, respectively. Fig.5 also shows the self-broadening coefficients calculated based on classical theory and included in HITRAN database. As seen in the Fig.5, the plots show general consistency for the experimental and theoretical results at low temperature range, but large discrepancies between the measured and theoretical results are found in elevated temperature, the maximum deviation is about 10%. This result again validates that the classical theory[7] of rigid rotor is not sufficient to accurately describe the broadening at elevated temperature Self-broadening coefficient(cm -1 atm -1 ) (a) Temperature(K) 0.12 γ Ex γ Th Best fit of γ Ex Best fit of γ Th Self-broadening coefficient(cm -1 atm -1 ) (b) Temperature(K) γ Ex γ Th Best fit of γ Ex Best fit of γ Th Self-broadening coefficient(cm -1 atm -1 ) (c) Temperature(K) Figure 6. Comparison between experimental and theoreticalself-broadening coefficients of CO (a): cm -1, (b): cm -1, (c): cm -1 vs. temperatures. 245 γ Ex γ Th Best fit of γ Ex Best fit of γ Th

6 Summary In this paper, the spectra of CO at temperatures ranging from 323K to 873K are recorded by using a tunable diode laser based high resolution spectrometer. Through the oigt line shape fitting to the spectra of CO at cm -1, cm -1 and cm -1 at different pressures and temperatures, the self-broadening line widths, self-broadening coefficients and temperature exponents are acquired. The self-broadening coefficients of CO measured are general consistency with the theoretical results at low temperature range, while at elevated temperature, the measurements values can differ from the classical theory values by over 10% at high temperature range. Acknowledgment The research described in this paper is supported by the National Key Scientific Instrument and Equipment Development Proect of China (2012YQ220119), and the Special Proect of Anhui Science and Technology (15czz04124). References [1] M. Lackner, Rev. Chem. Eng., 23, 2, , [2] P.A. Martin, Chem. Soc. Rev., 31, 4, , [3] T. Drascher, T. F. Giesen, T. Y. Wang, J. Molec. Spectro.192, , [4] A. Campargue, E.. Karlovets, S.Kassi, J. Quant. Spectrosc. Radiat. Transfer, 154, , [5] L.S. Rothman, I.E. Gordon, Y. Babikov, J. Quant. Spectrosc. Radiat. Transf., 130, 4-50, [6] E.E Whiting, J. Quant. Spectrosc. Radiat. Transfer, 8, , [7] G. Herzberg, Molecular Spectra and Molecular Structure II: Infrared and Raman Spectra of Polyatomic Molecules Florida: Krieger Publishing Company,