Tunable Diode Laser Absorption Spectroscopy Technique Research Articles

Tunable Diode Laser Absorption Spectroscopy Based Temperature Measurement with a Single Diode Laser Near 1.4 μm.

The rapidly changing and wide dynamic range of combustion temperature in scramjet engines presents a major challenge to existing test techniques. Tunable diode laser absorption spectroscopy (TDLAS) based temperature measurement has the advantages of high sensitivity, fast response, and compact structure. In this invited paper, a temperature measurement method based on the TDLAS technique with a single diode laser was demonstrated. A continuous-wave (CW), distributed feedback (DFB) diode laser with an emission wavelength near 1.4 μm was used for temperature measurement, which could cover two water vapor (H2O) absorption lines located at 7153.749 cm−1 and 7154.354 cm−1 simultaneously. The output wavelength of the diode laser was calibrated according to the two absorption peaks in the time domain. Using this strategy, the TDLAS system has the advantageous of immunization to laser wavelength shift, simple system structure, reduced cost, and increased system robustness. The line intensity of the two target absorption lines under room temperature was about one-thousandth of that under high temperature, which avoided the measuring error caused by H2O in the environment. The system was tested on a McKenna flat flame burner and a scramjet model engine, respectively. It was found that, compared to the results measured by CARS technique and theoretical calculation, this TDLAS system had less than 4% temperature error when the McKenna flat flame burner was used. When a scramjet model engine was adopted, the measured results showed that such TDLAS system had an excellent dynamic range and fast response. The TDLAS system reported here could be used in real engine in the future.

Design of an electronic system for laser methane gas detectors using the tunable diode laser absorption spectroscopy method

In this paper, a new electronic system is designed for methane gas laser analyzers using the tunable diode laser absorption spectroscopy (TDLAS) technique. This electronic system is presented in such a way that, based on this technique, optical wavelength and stability confirm the power of the laser light source. The proposed design includes current and temperature control circuits, amplifier circuits, and laser sensor circuits. This system leads to the control of laser light power. Due to the high cost of a laser sensor distributed feedback diode (DFB) and the impossibility of purchasing it for the actual implementation of the proposed electronic system, the design and simulation stage of this system was performed in the proteus simulator environment at normal atmospheric temperature and constant control flow conditions. The simulation results show that the proposed new electronic system based on the TDLAS technique detected the amount of leaking methane gas by generating a wavelength of 1653.72 nm related to the DFB laser sensor and displaying it on display during calculation. The test of optical wavelength stability, optical power, and methane gas wavelength generation by the laser sensor in the proteus simulator environment at different distances is excellent and remarkable. These results show that if we buy a laser sensor and build a gas analyzer device, we can achieve perfect results by using the device with the provided technique.

Measurement Technique of Oxygen Concentration in Narrow Channels of PEFCs Based on Transmission Laser Absorption Spectroscopy

Oxygen transport at cathodes of polymer electrolyte fuel cells (PEFCs) is one of the key factors for determining their power generation performance. To maintain their stable operations, it’s necessary to establish a technique for real-time monitoring of oxygen concentration inside cathode channels of operational cells. In the previous works, the measurements of oxygen distribution within gas channels of operating PEFCs have been conducted using gas chromatograph (GC) [1], oxygen-quenching of fluorescence and phosphorescence [2], and tunable diode laser absorption spectroscopy (TDLAS) [3]. GC techniques provide better accuracy for gas detection in flow channels, however, it takes a long time (at least a few minutes) to collect a gas sample and identify a species. On the other hand, TDLAS enables the non-invasive in-situ monitoring of chemical species inside cells with a high time resolution of 100 milliseconds. Fujii et al. [3] applied the single-ended TDLAS sensor to measure the oxygen concentration in a PEFC channel. In their experiments, since the laser beam is injected perpendicular to the flow direction, the optical path length is extremely short and the sensitivity is insufficient. The detection sensitivity of oxygen is relatively low due to the weak infrared absorption. Therefore, the optical path needs to be lengthened to measure its concentration with high accuracy. In this study, to solve the above-mentioned problem, we developed the fiber-optic transmission TDLAS system and applied it to the quantitative measurement of oxygen concentration in the narrow channel of a simulated cell.Fig. 1 shows the experimental apparatus for measuring the oxygen concentration in a simulated flow cell using the fiber-optic transmission TDLAS system. TDLAS is one of the high sensitive absorption spectroscopy techniques which measure the absorbance of a chemical species and identify its concentration using infrared diode laser. The triangular modulated beam emitted from a DFB laser diode (wavelength: 764 nm) is directly injected into the straight narrow channel (width and depth: 1.0 or 1.5 mm, length: 50 mm) through the single-mode optical fiber. To collimate the laser beam at the channel inlet, a GRIN lens is attached at the top of the pitch fiber. The injected beam passes one or two channels along the flow direction. In the case of passing two channels, the beam transmitted through the first channel is turned 180 degrees by a triangular prism at the bend section (length: 5 mm). The returned beam is passed through the second channel and detected at a photodiode. The output signal is transmitted to a lock-in amplifier, and then the second harmonic (2f) absorption spectrum can be obtained by phase-sensitive detection. In the experiments, the mixed gas of oxygen and nitrogen was supplied to the channel at 100 ml/min. The oxygen concentration in the channels was adjusted to 0 to 100% by controlling each gas flow rate. This measurement was performed at 25 oC and 1 atm.Fig. 2 presents the 2f absorption signals of oxygen in the narrow channel of the simulated flow cell. The incident laser beam passed two straight channels (channel width and depth: 1.5 mm, optical path: 105 mm). The oxygen mole fraction was varied from 0 to 100%. Although the TDLAS measurements were performed inside the narrow channels, the optical fringe noise was hardly observed and the 2f spectra were clearly obtained. It was also noted that the peak-valley height of 2f spectrum increased with an increase in oxygen mole fraction. The oxygen concentration in fuel cell channels can be quantitatively estimated from only the 2f spectrum data using the fiber-optic transmission TDLAS technique.

Effects of CO2 and N2 Dilution on the Combustion Characteristics of H2/CO Mixture in a Turbulent, Partially Premixed Burner.

Herein, the influences of CO2 dilution, N2 dilution, and CO2/N2 (in which half of the N2 is replaced by CO2) dilution on the combustion characteristics of a turbulent, partially premixed CO/H2–air flame were experimentally investigated in terms of the flame structure, flame temperature, and CO and CO2 concentrations in flames. TDLAS (tunable diode laser absorption spectroscopy) technique and an infrared gas analyzer were used for such purposes. CO2 dilution not only increases more momentum but also reduces the reaction rate. This results in a much longer flame length than that under N2 dilution. Compared with N2 dilution, the LH (axial length of the high-temperature reaction zone) values for the same levels of CO2 dilution and CO2/N2 dilution are much longer. The highest CO concentration in the CO2 diluted flame is higher than that in the CO/H2 flame and that in the CO2/N2 diluted flame is higher than in that the N2 diluted flame. The sizes of the main chemical reaction zone in CO2 and CO2/N2 diluted flames are larger than that in the N2 diluted flame. The inflection points in the rates of variation of the flame temperature and the CO and CO2 concentrations verify that CO2 dilution creates lower intensities and lower rates of chemical reactions, compared with N2 dilution.

A Remote Sensor System Based on TDLAS Technique for Ammonia Leakage Monitoring.

The development of an efficient, portable, real-time, and high-precision ammonia (NH3) remote sensor system is of great significance for environmental protection and citizens’ health. We developed a NH3 remote sensor system based on tunable diode laser absorption spectroscopy (TDLAS) technique to measure the NH3 leakage. In order to eliminate the interference of water vapor on NH3 detection, the wavelength-locked wavelength modulation spectroscopy technique was adopted to stabilize the output wavelength of the laser at 6612.7 cm−1, which significantly increased the sampling frequency of the sensor system. To solve the problem in that the light intensity received by the detector keeps changing, the 2f/1f signal processing technique was adopted. The practical application results proved that the 2f/1f signal processing technique had a satisfactory suppression effect on the signal fluctuation caused by distance changing. Using Allan deviation analysis, we determined the stability and limit of detection (LoD). The system could reach a LoD of 16.6 ppm·m at an average time of 2.8 s, and a LoD of 0.5 ppm·m at an optimum averaging time of 778.4 s. Finally, the measurement result of simulated ammonia leakage verified that the ammonia remote sensor system could meet the need for ammonia leakage detection in the industrial production process.

Real‐Time Plume Velocity Measurement of Solid Propellant Rocket Motors Using TDLAS Technique

AbstractAccurate measurement of plume velocity plays an important role in investigation of rocket motors, and the plume velocity is an indicator of energy level of solid propellants. A method based on laser Doppler effect using tunable diode laser absorption spectroscopy (TDLAS) is employed in this study. The absorption spectrum of water vapor feature at 1391.7 nm is utilized to measure the plume velocity. Three solid propellants, double‐base (DB), composite modified double‐base (CMDB), and nitrate ester plasticized polyether (NEPE) i. e., are burned in a motor with an internal diameter of 50 mm and their plume velocities are measured with scanned‐WMS‐2f technique. Afterburning reactions in the plume are numerically simulated. The simulations can provide insight into the nuances in measured TDLAS results and give confidence in the result. The results show that TDLAS is a good method to measure plume velocity. The relative standard deviation of the measurements is less than 4 %. The difference between the measured velocity and the simulated one is less than 8.25 %. Additionally, it is found that NEPE has the largest plume velocity, and the velocity of DB is the lowest. Uncertainties of velocity measurement using TDLAS are analyzed. Measurement uncertainty comes from velocity non‐uniformity in the direction perpendicular to the plume direction. Furthermore, it can be found that it is better to put the measurement position close to the nozzle exit to reduce measurement errors.

Experimental investigation of entropy waves generated from acoustically excited premixed swirling flame

Entropy waves play an important role in the production of indirect combustion noise and thermoacoustic instability. The characteristics of entropy waves from swirling flames have not been systematically investigated. Here, a premixed methane/air swirl burner was built with upstream acoustic excitation from a loudspeaker. A joint infrared imaging and tunable diode laser absorption spectroscopy (TDLAS) thermometry measurement was carried out to investigate the entropy waves generated in this burner. The infrared imaging technique provides qualitative images of the distribution, propagation and dissipation of entropy waves while the TDLAS technique provides quantitative measurement of temperature fluctuation. Time resolved measurements of the swirling flame bulk velocity, CH* chemiluminescence, gas temperature and infrared images were simultaneously obtained, demonstrating that entropy waves were generated from the premixed swirling flame under external acoustic excitation. Entropy waves were shown to be greatly influenced by the amplitude and frequency of acoustic waves. They also showed dissipation as the entropy waves propagate downstream according to the attenuated temperature fluctuation and infrared radiation intensities. Simultaneous high speed infrared imaging and particle image velocimetry measurements showed that the temperature non-uniformities arise from engulfment and mixing through periodic vortex roll-up.

Simple calibrated nonlinear excitation regime two-line atomic fluorescence thermometry.

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a promising two-dimensional (2D) thermometry technique for turbulent sooty flames. However, the complexity of calibrating three system parameters and expensive instruments restricts the application of the current NTLAF technique. Here we propose a simple and cheap NTLAF measurement approach based on a one-parameter model and tunable diode laser absorption spectroscopy (TDLAS) calibration. Using this methodology, only one system parameter, instead of three as in traditional NTLAF, is to be calibrated by path-averaged temperature acquired by the TDLAS technique. As a demonstration, instantaneous 2D thermometry data of a homemade burner were acquired using this approach, with measurement uncertainty of ∼4.5% and deviation from both reference TDLAS results and Raleigh scattering measurement results less than 50K, typically within 20K. This approach offers a novel simplified NTLAF solution for noncontact, in-suit, high-resolution 2D temperature measurement and is expected to greatly improve the compatibility of the NTLAF technique in scientific research and engineering applications.

Validation of wavelength modulation spectroscopy techniques for oxygen concentration measurement

Tunable Diode Laser Absorption Spectroscopy (TDLAS) based gas sensing technology, has widespread applications, ranging from use in biomedical industry to aerospace applications. An experimental set-up was established to measure concentration of oxygen gas in the range 0–100% using TDLAS technique. The oxygen absorption at 760.241nm was scanned with a tunable DFB laser, and wavelength modulation spectroscopy was used to obtain the harmonics (1f, 2f, 3f & 4f) of the oxygen absorption signal. The modulation parameters such as the modulation voltage, modulation frequency, reference phase, time constant of lock in amplifier, the tuning voltage, and the tuning frequency were optimized to obtain the harmonics of high amplitude and narrow half width. Keeping the experimental parameters constant, the oxygen concentration measurements were obtained by the following three methodologies, viz, (i) using only the 2nd harmonic, (ii) using the 2nd and 4th harmonics and, (iii) using the 1st and 2nd harmonics. The results of these measurements were compared and the merits and drawbacks of these methods are discussed.