Data Processing Algorithm for Diagnostics of Combustion Using Diode Laser Absorption Spectrometry

Temperature Measurements by Wavelength Modulation Diode Laser Absorption Spectroscopy with Logarithmic Conversion and 1f Signal Detection

A new version of a sensor for temperature measurements in the case of strong laser intensity fluctuation was developed. It was based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation, logarithmic conversion of the absorption signal, and detection of the first harmonic of the modulation frequency. The efficiency of the technique was demonstrated under experimental conditions with excess multiplicative noise. Temperature was evaluated from the ratio of integrated absorbance of two lines of the water molecule with different lower energy levels. Two algorithms of data processing were tested, simultaneous fitting of two spectral ranges with selected absorption lines and independent fitting of two absorption lines profiles. The correctness of the gas temperature evaluation was verified by simultaneous measurements with a commercial thermocouple. An error in temperature evaluation of less than 40 at 1000 K was achieved even when processing a single scan of the diode lasers.

Gaussian process regression for direct laser absorption spectroscopy in complex combustion environments

Tunable diode laser absorption spectroscopy (TDLAS) has been proved to be a powerful diagnostic tool in combustion research. However, current methods for post-processing a large number of blended spectral lines are often inadequate both in terms of processing speed and accuracy. The present study verifies the application of Gaussian process regression (GPR) on processing direct absorption spectroscopy data in combustion environments to infer gas properties directly from the absorbance spectra. Parallelly-composed generic single-output GPR models and multi-output GPR models based on linear model of coregionalization (LMC) are trained using simulated spectral data at set test matrix to determine multiple unknown thermodynamic properties simultaneously from the absorbance spectra. The results indicate that compared to typical data processing methods by line profile fitting, the GPR models are proved to be feasible for accurate inference of multiple gas properties over a wide spectral range with a manifold of blended lines. While further validation and optimization work can be done, parallelly composed single-output GPR model demonstrates sufficient accuracy and efficiency for the demand of temperature and concentration inference.

Advanced Fiber-Coupled Diode Laser Sensor for Calibration-Free 1f-WMS Determination of an Absorption Line Intensity

A new scheme for a calibration-free diode laser absorption spectroscopy (DLAS) sensor for measuring the parameters of harsh zones is proposed. The key element of the scheme is a micro-prism retroreflector (MPRR). The MPRR facilitates an increase in the mechanical stability of the sensor and a decrease in the background thermal radiation in the hot areas of a tested zone. Reduction in the broadband thermal emission allowed the application of a differential logarithmic conversion (LC) technique for elimination of the residual amplitude modulation and other sources of non-selective attenuation of the probing laser beam. LC allows the use of a 1f-wavelength modulation spectroscopy (WMS) detection scheme. Combination of LC and a 1f-WMS algorithm provided a new modification of calibration-free DLAS, which could be particularly useful for probing harsh zones with pronounced strong turbulence and high levels of acoustic and electrical noise. The influence of the experimental parameters and characteristics of the main electronic components of the recording and processing system on the accuracy of the integral line intensity determination is investigated theoretically and experimentally. The proposed optical scheme of a DLAS sensor and algorithm for the data processing allowed the integral intensity of an absorption line to be obtained. The potential for the scheme was exemplified with a single water vapor absorption line at 7185.6 cm^-1. Simultaneous detection of several absorption lines and data processing using the developed algorithm provides the final goal of a DLAS sensor-determination of temperature and partial pressure of a test molecule in a probed gas volume. The developed scheme allows the spatial multiplexing of the radiation of different diode lasers (DLs), which can be used if various test molecules are to be detected, or absorption lines of a test molecule are detected over different wavelength intervals.

An ultra-sensitive detection system for sulfur dioxide and nitric oxide based on improved differential optical absorption spectroscopy method

A highly sensitive detection system for sulfur dioxide (SO₂) and nitric oxide (NO) was developed via deep ultraviolet differential optical absorption spectroscopy (DUV-DOAS). The wavelength range of 200-230 nm was used which was rarely used before as result of severe cross sensitivity to SO₂ and NO, in this work, this problem was overcame. A system detection limit (DL) of 60 ppb for SO₂ has been reached which was among the best ones. Meanwhile, a novel method based on spectrum superposition theory was proposed to decompose the differential optical density (DOD) of NO from that of gas mixture in cross sensitive band. The advantage of this method is that the most sensitive absorption peak of NO was used, which cannot be used by conventional methods due to the cross sensitive to SO₂. A system DL of 7 ppb for NO has been achieved which is among the best ones reported before. Furthermore, the effect of gas temperature and humidity on concentration retrieval has also been studied, gas temperature and humidity compensation models have also been proposed. The experimental results show that the compensation models succeed in compensating the deviation caused by gas temperature and humidity. The environmental adaptability of the system has been enhanced. This work achieves the aim of monitoring ultra-low concentration of SO₂ and NO in a complex environment simultaneously.

Two-Dimensional Temperature Measurement in a High-Temperature and High-Pressure Combustor Using Computed Tomography Tunable Diode Laser Absorption Spectroscopy (CT-TDLAS) with a Wide-Scanning Laser at 1335-1375 nm

Tunable diode laser absorption spectroscopy (TDLAS) technology is a developing method for temperature and species concentration measurements with the features of non-contact, high precision, high sensitivity, etc. The difficulty of two-dimensional (2D) temperature measurement in actual combustors has not yet been solved because of pressure broadening of absorption spectra, optical accessibility, etc. In this study, the combination of computed tomography (CT) and TDLAS with a wide scanning laser at 1335-1375 nm has been applied to a combustor for 2D temperature measurement in high temperature of 300-2000 K and high pressure of 0.1-2.5 MPa condition. An external cavity type laser diode with wide wavelength range scanning at 1335-1375 nm was used to evaluate the broadened H₂O absorption spectra due to the high-temperature and high-pressure effect. The spectroscopic database in high temperature of 300-2000 K and high pressure of 0.1-5.0 MPa condition has been revised to improve the accuracy for temperature quantitative analysis. CT reconstruction accuracy was also evaluated in different cases, which presented the consistent temperature distribution between CT reconstruction and assumed distributions. The spatial and temporal distributions of temperature in the high-temperature and high-pressure combustor were measured successfully by CT-TDLAS using the revised spectroscopic database.

1.6 MHz scanning rate direct absorption temperature measurements using a single vertical-cavity surface-emitting laser diode

This paper presents 1.6 MHz scan rate, non-intrusive, time-resolved temperature measurements of a normal shock reflection from a plane end wall within a shock tube. A vertical-cavity surface-emitting laser (VCSEL) was used to conduct tunable diode laser absorption spectroscopy with water vapor as the probe species. The results are compared with analytical predictions. Temperatures measured with this technique agree within a single-scan standard deviation of ±33  K with calculated temperatures at a VCSEL modulation frequency of 800 kHz, which is sufficiently rapid enough to be used to investigate highly transient shock wave interaction processes.