Equivalent Absorption Coefficient Research Articles

Radiation enhanced B-Number

Radiative heat transfer plays a crucial role in most fire hazard related combustion processes. The existing classic and modified mass transfer B-Numbers have deficiencies for some burning cases of fires because they do not involve external radiation and/or assume flame transparent to thermal radiation. This study extended the concept of the mass transfer B-Number using the one-dimensional steady-state diffusion flame model. An analytical expression of the universal mass transfer B-Number BR was derived, which applies to semi-transparent flames with external radiation. Compared with the classic mass transfer B-Number, in the numerator denoting the net heat generated and held by the combustion system, BR has an additional term that represents radiative heat which is being held and transferred within the semi-transparent flame; as well as in the denominator denoting the convective heat needing for maintaining the combustion, the contribution from the radiative heat is deducted from the total heat required for vaporizing a unit mass of fuel at the fuel surface. BR increases as external radiation and/or ambient oxygen concentration raise. In order to examine its physical significance, new parameters were defined and illustrated, such as the mass transfer equivalent absorption coefficient, specific radiant heat feedback and radiation factor, that demonstrate interactions of heat and mass transfers and radiative absorption and emission within inhomogeneous flames. Effects of the external radiation and ambient oxygen concentration on these parameters were also evaluated. Increases in the radiation factor as the ambient oxygen concentration rises theoretically validated enhanced radiative heat transfer in rich oxygen combustion.

Sub-ppb detection of formaldehyde with cantilever enhanced photoacoustic spectroscopy using quantum cascade laser source

A novel cantilever enhanced photoacoustic spectrometer with mid-infrared quantum cascade laser was applied for selective and sensitive formaldehyde (CH2O) gas measurement. The spectrum of formaldehyde was measured from 1,772 to 1,777 cm−1 by tuning the laser with a spectral resolution of 0.018 cm−1. The band at 1,773.959 cm−1 was selected for data analysis, at which position the laser emitted 47 mW. In univariate measurement, the detection limit (3σ, 0.951 s) and the normalized noise equivalent absorption coefficient (3σ) for amplitude modulation (AM) were 1.6 ppbv and 7.32 × 10−10 W cm−1 (Hz)−1/2 and for wavelength modulation (WM) 1.3 ppbv and 6.04 × 10−10 W cm−1 (Hz)−1/2. In multivariate measurement, the detection limit (3σ) can be as low as 901 pptv (1,773.833–1,774.085 cm−1, 15 spectral points each 0.951 s) for AM and 623 pptv (1,773.743–1,774.265 cm−1, 30 spectral points each 0.951 s) for WM. Because measurement time increases in multivariate measurement, its application is justified only when interferents need to be resolved. Potential improvements of the system are discussed.

All optical quartz-enhanced photoacoustic spectroscopy

A trace gas sensor based on all optical quartz-enhanced photoacoustic spectroscopy (QEPAS) is designed and demonstrated. An extra detection light beam is added to the traditional QEPAS setup to convert the amplitude of vibration of the quartz tuning fork, which is proportional to the concentration of the target gas, into the variation of the detection light intensity. As a result, there are not any electrical components near the measurement position. Such a design makes it immune to electromagnetic interference and provides a compact sensor head, which can be used in a tiny space or an adverse environment. Using the new setup to measure the water concentration in air, the obtained noise equivalent absorption coefficient is 1.13×10-6 cm-1W/√Hz. The approaches to further optimizing the system and improving the sensitivity are also discussed in detail.

A Novel QEPAS with Microresonator in the Open Environment

An improved quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing system for trace gas detection is proposed. The optical fiber Fabry–Perot (F–P) demodulation method is used to replace the conventional electrical one in the QEPAS system. The experimental QEPAS system, which has a microresonator consisting of two stainless steel tubes with a length of 2.3 mm and an inner diameter of 0.9 mm, is implemented to detect the absorption of water vapor in the open environment. The structure parameters of the quartz tuning fork (QTF) are optimized in order to make the sensing system work more stably and reliably. Demonstration experiments are carried out. The vibration signal of the QTF was picked up by the optical fiber F–P demodulator and the conventional electrical scheme at the same time. Normalized noise equivalent absorption coefficients of \(4.05\times 10^{-7}\,\text{ cm}^{-1}~{\cdot }~\mathrm{W}~{\cdot }~{\text{ Hz}}^{-1/2}\) and \(2.40\times 10^{-6}\,\text{ cm}^{-1}~{\cdot }~\mathrm{W}~{\cdot }~{\text{ Hz}}^{-1/2 }\) are obtained, respectively. The experimental result demonstrates that the sensitivity of the improved QEPAS sensing system with an optical fiber F–P demodulator is about 5.9 times higher than that of the conventional QEPAS system.

Femto-FT-CEAS applied to carbon disulfide around 1.54 μm

A femto–OPO laser fitted to a high-finesse cavity and a Fourier transform spectrometer to perform femto-FT-CEAS (for femto-Fourier transform-cavity enhanced spectroscopy) was used to record high overtones of carbon disulfide with a minimal rms equivalent absorption coefficient of 1.2×10−11cm−1Hz−1/2 per spectral element. New vibrational states are reported for 12C32S2 and 12C32S34S and new or improved rotational constants are obtained, for the v1v2l2v3=3003, 2203, 0114 and 4003 states in 12C32S2 and 3003 state in 32S12C34S.

Quartz enhanced photoacoustic spectroscopy with a 338 μm antimonide distributed feedback laser

A system for gas sensing based on the quartz-enhanced photoacoustic spectroscopy technique has been developed. It makes use of a quantum well distributed feedback (DFB) laser diode emitting at 3.38 μm. This laser emits near room temperature in the continuous wave regime. A spectrophone, consisting of a quartz tuning fork and two steel microresonators were used. Second derivative wavelength modulation detection is used to perform low concentration measurements. The sensitivity and the linearity of the Quartz enhanced photoacoustic spectroscopy (QEPAS) sensor were studied. A normalized noise equivalent absorption coefficient of 4.06×10(-9) cm(-1)·W/Hz(1/2) was achieved.

Ultra-sensitive carbon monoxide detection by using EC-QCL based quartz-enhanced photoacoustic spectroscopy

A quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide detection at ppbv levels was developed with a 4.65 μm external-cavity quantum cascade laser operating both in continuous wave (cw) and pulsed modes. A 23-fold enhancement of the measured CO signal amplitude was obtained when water vapor, acting as a catalyst for vibrational energy transfer, was added to the targeted analyte mixture. In the cw mode, a noise-equivalent sensitivity (NES, 1σ) of 2 ppbv was achieved at a gas pressure of 100 Torr, for 1-s lock-in amplifier (LIA) time constant (TC), which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of $1.48\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ . In the pulsed mode, the determined NES and NNEA were 46 ppbv and $1.07\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ , respectively, for a 3-ms LIA TC at atmospheric pressure with a laser scan rate of 18 cm-1/s and a 50 % duty cycle. An intercomparison between cw and pulsed QEPAS-based CO detection is also reported.

The Surface Absorption Coefficient of S304L Stainless Steel by Nd:YAG Micro-Pulse Laser

The surface absorption coefficient of S304L stainless steel during Nd:YAG pulse laser welding process was derived by using the inverse engineering technique. The thermal-elastic-plastic finite element method is employed to calculate the temperature distribution, melting pool depth and shape with specific heat source. The derived melting pool depths and diameters with different values of absorption coefficient were compared with experimental results. In this study, the least square error method is used to obtain the equivalent absorption coefficient. The results show that the equivalent absorption coefficient of S304L stainless steel is in the range of 0.1~0.25 for low power intensity Nd:YAG impulse laser welding.

A new k-interval selection technique for fast atmospheric radiance calculation in remote sensing applications

A new technique is proposed to generate the k-interval parameters, including the number of k-intervals, the equivalent absorption coefficients, and the quadrature weights when using the correlated k-distribution method for the computation of spectrally integrated three-dimensional (3D) atmospheric radiance. The main difference between the proposed technique and the traditional exponential sum fitting technique is that only quadrature weights are involved in the optimization process. To avoid the ill-conditioned problem in the proposed technique, the absorption coefficients with high value are dealt with by the delta log( k) (Δlog( k)) technique instead of involving them in the fitting procedure. The performance of the proposed technique is illustrated by radiance calculation results of cloudless and cloudy atmosphere for three different band settings. Results show that there are less relative errors with the proposed optimization technique than with the Δlog( k) technique under the same number of k-intervals. However, as the absorption becomes stronger, the performance of the proposed technique gradually decreases to the Δlog( k) technique. The relative root-mean-square error (RMSE) of radiance for 3D cloudy atmosphere can be controlled in less than 2% when the number of k-intervals is less than 10 particularly for weak absorption band, the RMSEs are less than 1% with only 6 terms.

Application of a broadband blue laser diode to trace NO_2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450 nm. A detection limit of 18 ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7 mW at a 1 s integration time, which corresponds to a 1 σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1) W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.

Trace gas detection based on off-beam quartz enhanced photoacoustic spectroscopy: Optimization and performance evaluation

A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized. Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity. The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated. The sensor performance was evaluated by determining the detection sensitivity to H(2)O vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient (1σ) of 6.2×10(-9) cm(-1) W/Hz(1/2) was achieved.

Determination of partition coefficients of three volatile organic compounds (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone oil mixtures

The main objective of this work was to propose a model able to predict the partition coefficient of odorous or toxic gaseous pollutants (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone oil mixtures. Experimental measurements using a static headspace method were carried out for pure water ( H voc,water), for pure silicone oil ( H voc,solvent) and for mixtures of varying composition ( H voc,mixture). The dramatic decrease in the partition coefficient ( H voc,mixture) with oil addition clearly showed a deviation from linearity, which was more pronounced for increasing H voc,water/ H voc,solvent ratios. Moreover, experiments using a dynamic absorption method underlined that the absorption capacity of a biphasic water/silicone oil mixture can be classed as the absorption capacity of a pseudo-homogeneous phase whose physical properties (molecular weight and density) can be calculated from the physical properties of water and solvent, and balanced using the “equivalent absorption coefficients” H voc,mixture/ H voc,water and H voc,mixture/ H voc,solvent. An “equivalent absorption capacity” concept is then proposed, which should be useful to design absorption units using two-phase liquid mixtures for the treatment of industrial air loaded with volatile organic compounds.

A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source

A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry-Perot quantum cascade laser (QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene (C(60)). As a first test of the system, high resolution spectra of the nu(8) vibrational band of CH(2)Br(2) have been acquired at approximately 1197 cm(-1). To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of CH(2)Br(2) were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of 1.4 x 10(-8) cm(-1). Spectral simulations of the band indicate that the supersonic source produces rotationally cold (approximately 7 K) molecules.

Off-beam quartz-enhanced photoacoustic spectroscopy

An off-beam (OB) detection approach is suggested and experimentally investigated and optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). This OB-QEPAS configuration, very simple in assembly, not only allows for use of larger excitation optical beams and facilitating optical alignment but also provides higher enhancement of photoacoustic signals than previously published results based on the common on-beam QEPAS under the same experimental conditions. A normalized noise equivalent absorption coefficient (1sigma) of 5.9 x 10(-9) cm(-1)W/Hz(1/2) was obtained for water vapor detection at normal atmospheric pressure.

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10−8 cm−1 W/Hz1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.

Relaxation effects and high sensitivity photoacoustic detection of NO2 with a blue laser diode

A detection limit of 200 ppt of NO2 in N2 at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10-9 cm-1W/Hz0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ < 4 μs. Mixing O2 into the NO2 containing gas results in a decrease of the photoacoustic signal. A simple model shows that this effect can be attributed to an increased vibrational-vibrational relaxation of NO2 to O2.

Continuous wave optical parametric oscillator for quartz-enhanced photoacoustic trace gas sensing

A continuous wave optical parametric oscillator, generating up to 300 mW idler output in the 3–4 μm wavelength region, and pumped by a fiber-amplified DBR diode laser is used for trace gas detection by means of quartz-enhanced photoacoustic spectroscopy (QEPAS). Mode-hop-free tuning of the OPO output over 5.2 cm-1 and continuous spectral coverage exceeding 16.5 cm-1 were achieved via electronic pump source tuning alone. Online monitoring of the idler wavelength, with feedback to the DBR diode laser, provided an automated closed-loop control allowing arbitrary idler wavelength selection within the pump tuning range and locking of the idler wavelength with a stability of 1.7×10-3 cm-1 over at least 30 min.Using this approach, we locked the idler wavelength at an ethane absorption peak and obtained QEPAS data to verify the linear response of the QEPAS signal at different ethane concentrations (100 ppbv-20 ppmv) and different power levels. The detection limit for ethane was determined to be 13 ppbv (20 s averaging), corresponding to a normalized noise equivalent absorption coefficient of 4.4×10-7 cm-1 W/Hz1/2.

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.

QEPAS based detection of broadband absorbing molecules using a widely tunable, cw quantum cascade laser at 8.4 μm

Detection of molecules with wide unresolved rotationa-lvibrational absorption bands is demonstrated by using Quartz Enhanced Photoacoustic Spectroscopy and an amplitude modulated, high power, thermoelectrically cooled quantum cascade laser operating at 8.4 mum in an external cavity configuration. The laser source exhibits single frequency tuning of 135 cm-1 with a maximum optical output power of 50 mW. For trace-gas detection of Freon 125 (pentafluoroethane) at 1208.62 cm-1 a normalized noise equivalent absorption coefficient of NNEA=2.64x10(-9) cm?(-1)W/Hz(1/2)was obtained. Noise equivalent sensitivity at ppbv level as well as spectroscopic chemical analysis of a mixture of two broadband absorbers (Freon 125 and acetone) with overlapping absorption spectra were demonstrated.