Development and Validation of a Tunable Diode Laser Absorption Spectroscopy System for Hot Gas Flow and Small-Scale Flame Measurement

“TDLAS (tunable diode laser absorption spectroscopy) is an important gas analysis method that can be employed to obtain characteristic parameters non-invasively by the infrared absorption spectra of tracer molecules such as CH4, H2O and O2. In this study, a portable H2O-based TDLAS system with a dual optical path was developed with the aim of assessing the combustion characteristics of flammable gases. Firstly, a calculation method of gas characteristics including temperature and velocity combining absorption spectra and a HITRAN database was provided. Secondly, to calibrate and validate this TDLAS system precisely, a pressure vessel and a shock tube were introduced innovatively to generate static or steady flow fields with preset constant temperatures, pressures, or velocities. Static tests within environment pressures up to 2 MPa and steady flow field tests with temperatures up to 1600 K and flow velocities up to 950 m/s were performed for verification. It was proved that this system can provide an accurate values for high temperature and velocity gas flows. Finally, an experimental investigation of CH4/air flames was conducted to test the effectiveness of the system when applied to small diffusion flames. This TDLAS system gave satisfactory flame temperature and velocity data owing to the dual optical path design and high frequency scanning, which compensated for scale effects and pulsation of the flame. This work demonstrates a valuable new approach to thermal hazard analysis in specific environments.”

2.3. The Experimental TDLAS System

The TDLAS system designed in this work mainly comprised an infrared laser, modulation unit, optical path, photodetectors, and high-speed data acquisition and processing module, as shown in Figure 2. An OEM VCSEL driver (VITC002 from Thorlabs, Newton, NJ, USA) with a temperature controller was applied for laser modulation. The functional parameters of the laser (VCSEL from Vertilas, München, Germany) and function signal generator (DG-1022 from Rigol, Beijing, China) are listed in Table 1.
Figure 2. Schematic of the TDLAS system designed in the present work.
Because of the advantages provided by analyzing water vapor, H2O-based TDLAS measurement assessments were used in this study but with more accurate experimental validation and a specially designed optical path intended for the monitoring of combustion processes. Water will produce intense absorbance bands in the near-IR region 1400, 1800, and 2700 [16,21]; and to avoid any interference by other species (such as C-H radical), 1392 nm was selected as the center wavelength for water vapor detection. Photodetectors (PN-2000 from Lightsensing Technologies, Beijing, China) with a response range of 900–1650 nm were used to determine the transmitted light intensity. Data were obtained using a data acquisition card (PCI-20612 from TDEC, Sichuan, China) with four channels, operating at 32 bits and a maximum rate of 50 MSa/s.

2.4. Experimental Design for Validation

The functioning of the TDLAS system was calibrated or examined in three ways, as shown in Figure 3, using a pressure vessel, a shock tube, and a co-flow combustion platform.
Figure 3. Experimental facilities for TDLAS tests, including (a) a pressure vessel, (b) a shock tube, and (c) a co-flow flame burner.
(1) Firstly, normal pressure and temperature were applied to the pressure vessel (Figure 3a) with standing air to calibrate the basic performance of the system. This vessel was made of stainless steel with optical glasses on both sides. The optical path in this device had a maximum length of 0.4 m and a 532 nm green laser was employed to adjust the path.
(2) Secondly, the shock tube was intended to provide determinable high-temperature and high-speed water vapor flow to permit the precision and response rate of the measurement system to be ascertained. As shown in Figure 3b, the shock tube was comprised a high-pressure section, a low-pressure section, a gas circuit, and an electronically controlled diaphragm. Prior to each test, the low-pressure section was charged with air to a preset pressure. Following this, the high-pressure section was also slowly filled with air until the diaphragm instantaneously ruptured to create a shock wave, thus producing a high-temperature/pressure, high-speed flow field.
(3) Finally, the calibrated TDLAS system was used for the CH4/Air flame temperature and hot gas velocity measurements, as shown in Figure 3c. A co-flow CH4/air burner was made to generate a stable diffusion flame with preset initial conditions [22,23]. High precision mass flowmeters (KM7100 from Alicat, Tucson, AZ, USA) were used to dispense the combustible gases. To avoid the disturbance by H2O absorption in the non-flame zone (i.e., a background signal resulting from atmospheric H2O), a beam splitter (50%:50%) was used to subtract the background interference. As noted, the flame width (absorption length about 2–3 cm) was relatively short, thus a reflector was added to obtain a stronger absorption signal.
The experimental conditions are summarized in Table 2. All the tests were repeated 20 times to ensure reproducible results.

3. Results and Analysis

3.1. Room Temperature Measurement by TDLAS

Absorption spectra of the contents of the pressure vessel (see Figure 3a) could be obtained on the basis of comparisons between the laser output and absorption line strength using the HITRAN [17] data, as shown in Figure 4 with an example at initial pressure of 1 atm. Figure 4a shows the voltage U variation of function signal generator output used for driving laser during a half cycle, and Figure 4b presents the transmitted light intensity after absorption. Furthermore, clear positions and strengths of absorption peaks could be found in Figure 4c. The line strength of water vapor vs. wavelength is plotted in Figure 4d with independent absorption line.
Figure 4. Variation in (a) laser driving voltage and (b) transmitted light intensity in a half circle with (c) peak positions and strengths, and (d) the line strength distribution calculated from the HITRAN [17] database.

To calculate the vapor temperature, the time-domain of transmitted light intensity (I vs. t) should be transformed to frequency-domain (that is, I vs. ν or I vs. λ) at first. Based on the approximately linear relationship between λ and U, two reference wavelength-time points were selected: (λ1�1t1�1) and (λ2�2t2�2). Then we obtained

This simplified linear fitting was considered a reasonable approximation over short time spans. Note that λ vs. t would not be a continuous function, due to the discreteness of λ. Furthermore, the wavelengths that were selected for these calculations (λ1�1 and λ2�2) should be a certain distance apart to reduce the error caused by uncertainties in determining the positions of the absorption peaks. Hence, λ1�1 = 1391.67275 nm and λ2�2 = 1395.00424 nm were selected in the present work for the purpose of wavelength calibration.
Consequently, as ν~1/λ�~1/�, the correlation between I and ν could be obtained by combining Equation (10) with the data in Figure 4, as shown in Figure 5a. The baseline in Figure 5a was fitted by using the polynomial I0=a0+a1ν+a2ν2+a3ν3�0=�0+�1�+�2�2+�3�3 and employing data within the non-absorption region. The curve of ln(I/I0)ln(�/�0) vs. ν, as the key relation for temperature calculation deduced in Equations (4) and (5), could be further illustrated in Figure 5b,c.
Figure 5. (a) Absorption frequency-domain diagrams of the transmitted light intensity and (b) the absorption ratio with (c) main peak positions and strengths.
It is necessary to take into account that the line-pair selection had to meet certain conditions, meaning that there was no interference by other spectral lines and these lines were positioned near the central wavelength of the laser. Furthermore, the lines had to be separated by a suitable distance to avoid overlap. Therefore, we chose ν = 7181.15578 cm−1 and ν = 7185.59731 cm−1 under overall consideration. Other important parameters related to the HITRAN database [17] are provided in Table 3. Combining Equations (4) and (5) provide T = 302 ± 1.4 K, and this value—compared with the average experimental value of 301.14 ± 0.8 K by the thermocouples—provides a measurement error of less than 0.3%.
The pressure effects on measurement accuracy were investigated subsequently. The results showed that, with the enlarged initial test pressure, pressure broadening occurred generally and became dominant due to the increasing frequency of molecular collisions. In addition, spectral interference resulting from line overlap became evident at pressures exceeding 0.8 MPa. It should be noted that the measurement errors related to temperature and concentration could be larger than 10% at pressures above 1 MPa.

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