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Abstract
The OH radical concentration was measured by applying tunable diode laser absorption spectroscopy, which is an in situ optical method. An optical absorption region (P7.5ff transition at 1502.7 nm) of the OH radical was selected in the near-infrared range to measure the OH radicals quantitatively in premixed CH4/air flames. An improved direct absorption spectroscopy (DAS) method based on wavelength division multiplexing was proposed to extract the H2O absorption signal that interfered with the OH light absorption signal, and the integral intensity of OH* chemiluminescence was compared to the measured OH radical concentration based on the improved DAS method.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
18 August 2022: A typographical correction was made to the author affiliations.
1. Introduction
Carbon-neutral policies are being implemented worldwide to reduce global warming and air pollution. Currently, hydrocarbon-based fuels contribute most of the energy produced by combustion systems. However, the use of hydrocarbon-based fuels is gradually declining because of carbon-neutral policies. The use of hydrogen, ammonia, and synthetic fuels produced by hydrocarbon fuel reforming is expected to increase significantly by 2050 [1]. However, because air pollutants and greenhouse gases (GHGs) are also generated in hydrogen- and ammonia-fuel-based combustion systems, the development of clean, high-efficiency, and carbon-neutral combustion technologies is being actively promoted, and it is becoming important to analyze the combustion state and emissions for the practical application of new technologies using hydrogen-based fuels. It is critical to develop real-time combustion environment scanning technology to improve the combustion efficiency of combustion systems that use hydrogen-based fuels and to control air pollutants and GHG emissions. The hydroxyl (OH) radical is a chemical species associated with combustion efficiency and air pollutants in the flame of the combustion system. It is an important chemical species in the combustion of almost all hydrogen-based fuels and is a major carrier in flame propagation and chain-branching reactions [2]. When an oxidant is used as air, the OH radicals are converted into NOX by reacting with N2 components in the air under various conditions, such as high temperature, which is also closely related to the generation of air pollutants [3]. In addition, H2O in the combustion system is a major combustion product and is known to be an indicator of the overall combustion efficiency as a typical product of complete combustion. Because of the harsh environment and rapid flow, measuring both the internal temperature and chemical species in a large combustion system is difficult. In particular, owing to the characteristics of OH radicals, it is difficult to perform quantitative measurement because their formation and extinction occur rapidly. Therefore, research has been actively conducted to diagnose flames directly in combustion environments. To understand flame behavior more fully, researchers have used optical measurement technology, which is gaining popularity as a non-intrusive method. Typical optical methods for measuring OH radicals in flames can be classified as laser-induced fluorescence (LIF), chemiluminescence (CL), and tunable diode laser absorption spectroscopy (TDLAS). In the LIF method, after the OH radicals in the flame are excited to a high-energy state using a laser, spontaneously generated excess energy is emitted as fluorescence. This method is employed to capture images of specific wavelengths of light emitted through a band-pass filter [4–6]. It is mainly used for qualitative image analysis and has the drawback of a large laser system, that it is difficult to perform measurement in harsh environments. Meanwhile, the CL method utilizes the luminescence phenomenon that accompanies the chemical reaction of chemical species in the flame. This method measures the emission of a unique wavelength emitted by OH radicals during multiple combustion reactions using a band-pass filter [7–10]. It is frequently used to confirm the qualitative characteristics of OH radical distributions in flames, and there is an error in the quantitative analysis. Finally, TDLAS, which uses a light source with a narrow linewidth, has been extensively applied and has the advantage of absorbing specific wavelengths for each gas. It is also useful for measuring chemical species in harsh environments based on highly time-resolved and in-situ measurements in flames and gas flow [11–14]. Research on the measurement of OH radicals based on tunable diode laser absorption spectroscopy in the near-infrared (IR) and ultraviolet (UV) wavelength regions can be found in the literature. OH radical measurements using TDLAS in the UV wavelength range have been previously performed in various combustion environments. Meyer et al. measured the OH radical concentrations from ethylene (C2H4)/air diffusion flames at a wavelength of 313.5 nm using the direct absorption spectroscopy (DAS) method of TDLAS [15]. Recently, Wang and Hanson used a 308.6 nm UV laser to obtain the concentration of OH radicals on a shock tube using DAS [16]. In short, it was discovered that the UV absorption region of OH radicals was free of interference from other chemical species and that there was only one absorption of OH radicals. Next, in the near-IR wavelength region, Aizawa et al. used the wavelength modulation spectroscopy (WMS) method of the TDLAS to measure the OH radical concentrations in premixed propane (C3H8)/air flames and premixed methane (CH4)/air flames. Here, the near-IR wavelengths selected to measure the OH radical concentration were 1557.30 nm and 1554.09 nm, and the absorption signal interference caused by H2O generated in the 10%–20% concentration range after the combustion reaction could not be excluded [17]. Upschulte et al. measured the OH radicals in methane/air flames using DAS at a wavelength of 1557.65 nm. However, H2O interference occurred in the flame at the selected wavelength. Therefore, they proposed a method of correcting the H2O absorption signal by simulating the spectrum based on HITRAN-HITEMP data [18]. Wagner and Ebert analyzed the OH radical concentration in methane (CH4)/air diffusion flames by calculating the interference H2O absorption signal from the DAS-multi-line spectral fitting method using a wavelength of 1541.12 nm [19]. Bürkle et al. measured OH radicals at a wavelength of 1526.97 nm based on DAS in a methane (CH4)/O2 (30%)+CO2 (70%) diffusion flame. The absorption signal of the OH radical had a low signal-to-noise ratio (SNR), so the OH radical was analyzed using a Savitzky-Golay filter with a seventh-order polynomial to correct this uncertainty [20]. In 2019, Hayden et al. derived the OH radical concentration in the premixed methane (CH4)/air flame of a ribbon burner at a wavelength of 1491 nm using WMS and dual-frequency comb spectroscopy (DCS). To summarize the research of Hayden et al., the temperature and H2O concentration were firstly derived using two lasers (wavelengths of 1392 nm and 1469 nm) based on the WMS 2f/1f signal. Subsequently, the derived temperature and H2O concentration were used to simulate the absorbance of H2O interfering with the 1491 nm wavelength of OH radicals from HITEMP 2010. The simulated H2O absorbance was integrated with the OH radical absorbance measured at 1491 nm. Finally, the integrated absorbance of OH and H2O was injected into the lock-in amplifier and used in the 2f/1f fitting routine process to measure a single OH radical [21]. Recently, Lu et al. derived the OH radical and H2O concentration using broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) [22]. Summarizing the research on OH radical measurement using tunable diode laser absorption spectroscopy, in the case of UV wavelengths, there is no interference of the H2O light absorption signal in the OH radical light absorption signal. To operate the tunable laser in the UV absorption wavelength region, a light source combined with a 5 W s polarized 532 nm laser, Ti:Al2O2 resonator, and 10 W CW 532 nm pump is irradiated by injecting it into the sum-frequency generator module. This step is difficult in harsh combustion measurement environments owing to the complex optical alignment and laser systems. At near-IR wavelengths, the laser system for measuring chemical species using a fiber coupled-diode laser can be simplified and applied to various combustion systems, but there is an error in measuring the concentration of OH radicals due to interference of the H2O absorption signal. Thus, the interfered H2O absorption signal has been simulated and corrected by employing HITRAN/HITEMP data, or the OH radical concentration has been derived via multi-line spectral fitting and complex iterative calculations. Unlike in previous studies, we used a near-IR DFB laser with an improved DAS method to measure the OH radicals in CH4/air flames. When the absorption signal of H2O in the near-IR region overlaps with the absorption signal of the OH radicals, the concentration of the OH radicals is quantified by removing the interference from H2O using the improved DAS method based on wavelength division multiplexing (WDM), as proposed in this report.
2. Theory
2.1. Concentration equation
The basic theory of TDLAS is governed by the Beer-Lambert law in Eq. (1). When spectrally narrow light at a specific frequency $\nu $ passes through a uniform gas medium, the transmittance τ(ν) can be expressed as the ratio of the transmitted intensity ${(I )_\nu }$ to the initial intensity ${({{I_0}} )_\nu }$ [23]:
The spectral absorption coefficient ${k_\nu } ({c{m^{ - 1}}} )$ is given by Eq. (2). In general, the product of the absorption coefficient and the optical path length L $({cm} )$ is the absorbance:
For a single transition j of a specific gas species i, the absorption coefficient can be expressed as the product of the total pressure P $({atm} )$ in the target gas medium to be measured, line strength ${S_{i,j}}(T ) ({c{m^{ - 2}}at{m^{ - 1}}} )$ of the transition at temperature $T(K )$, mole fraction ${\chi _i}$ of the absorbing species, and lineshape functio $\phi ({\nu - {\nu_0}} ) ({cm} )$:
The line strength ${S_{i,j}}(T )$ in Eq. (3) is a parameter close to the molecular motion characteristics of gas species that occur when narrow light at a specific frequency passes through the gas medium. In Eq. (3), ${T_0}(K )$ is the initial temperature (usually ${T_0}$ is the reference temperature, 296 K), and $T(K )$ is the temperature in the measurement environment. In addition, $h ({Js} )$ is Planck's constant, c $({cm\; {s^{ - 1}}} )$ is the speed of light, k $({J\; {K^{ - 1}}} )$ is Boltzmann’s constant, ${\nu _{0,i}} ({c{m^{ - 1}}} )$ is the center wavelength, ${E^{\prime\prime} } ({c{m^{ - 1}}} )$ is the lower-state energy, and $Q(K )$ is the partition function of the target gas molecule. The partition function $Q(K )$ can be expressed as a polynomial, as shown in Eq. (4), according to the gas species and temperature range in the environment to be measured [24,25]:
The line shape function in Eq. (2) can be quantified as $\int {{\phi _\nu }d\nu = 1}$. As described above, the product of the absorption coefficient and optical path length is the absorbance, and the integrated absorbance (absorbance area) ${A_i}$ $({c{m^{ - 1}}} )$ can be expressed as shown in Eq. (5):
Equation (6) can be derived from Eq. (5) to determine the mole fraction of gas species in the measurement environment:
According to Eq. (6), the mole fraction of the gas species is proportional to the integrated absorbance, ${A_i}$. In addition, the mole fraction is inversely proportional to the pressure in the measurement environment, optical path length, and line strength.
2.2. Temperature equation using two-line thermometry
Temperature measurements using two-line thermometry were performed to obtain the ratio of each integrated absorbance area corresponding to the two absorption wavelengths. If the two absorption wavelengths are sufficiently close, then the last proportion term in Eq. (3) can be approximated as 1 [26]. In addition, two integrated absorbances can be obtained from the same gas partial pressure and optical path length in the measurement environment. The two integrated absorbance ratios R can be expressed as two line strength ratios, as shown in Eq. (7) [27]. The tendencies of the line strength and integrated absorbance become the same, depending on the temperature range of the measurement environment.
The relative sensitivity of the integrated absorbance ratio R according to the temperature can be determined using Eq. (8), which can be derived by differentiating Eq. (7):
Therefore, the temperature $T(K )$ can be expressed as shown in Eq. (9) using two-line thermometry:
2.3. Line broadening
Combustion products in combustion environments interact with light on a molecular basis to form various line-broadening mechanisms. Line broadening can be broadly divided into pressure-dominated collision broadening and temperature-dependent Doppler broadening. Collision broadening is associated with the collision of gas molecules under pressure, which can be expressed by the Lorentzian line-shape function, as in Eq. (10):
2.4. Improved DAS method
The purpose of this study was to measure the OH radicals, H2O concentration, and flame temperature in a premixed CH4/air flame simultaneously by irradiating selected laser lights with blended OH+ H2O(#1) and two partially overlapped H2O(#2) wavelength ranges using fiber-coupled WDM. Table 1 and Fig. 1 show the selected wavelengths and analysis process for the improved DAS method. The theoretical and analytical methods were performed according to the following steps.
- 1. Through the fiber-coupled WDM system, laser light with an A-laser light absorption signal and a B-laser light absorption signal is simultaneously irradiated into the premixed CH4/air flame.
- 2. By using a laser with B-laser absorption signals, the temperature in the premixed CH4/air flame was measured using two thermometry techniques from Eq. (9). It was confirmed that the laser with two partially overlapped B-laser absorption signals has high resolution and high sensitivity for measuring the temperature and H2O concentration in the flame [29].$$\Rightarrow {T_{flame}} = \frac{{\left[ {\frac{{hc}}{k}({E_{2\_H2O(\# 2)}^{\prime\prime} - E_{1\_H2O(\# 2)}^{\prime\prime} } )} \right]}}{{\left[ {\ln \frac{{{A_{1\_H2O(\# 2)}}}}{{{A_{2\_H2O(\# 2)}}}} + \ln \frac{{{S_{2\_H2O(\# 2)}}({{T_0}} )}}{{{S_{1\_H2O(\# 2)}}({{T_0}} )}} + \frac{{hc}}{k}\frac{{({E_{2\_H2O(\# 2)}^{\prime\prime} - E_{1\_H2O(\# 2)}^{\prime\prime} } )}}{{{T_0}}}} \right]}}$$
- 3. After substituting the flame temperature measured in step 2 into Eq. (3), the line strength of the B-laser corresponding to that temperature was calculated, and the B-laser light absorption signal was employed to calculate the H2O concentration in the flame where the light passed. It is reasonable to derive the H2O concentration in the premixed CH4/air flame because the H2O concentration can be calculated using the sum of the line strengths within the selected wavelength, which is equal to the entire absorption area within the B-laser light absorption signal.$$\begin{aligned} &\Rightarrow \int\limits_{ - \infty }^\infty {{\alpha _{H2O(\# 2)}}d\nu = P \cdot {\chi _{H2O(\# 2)}} \cdot \sum\limits_{i,j} {{S_{i,j}}(T) \cdot L = {A_{H2O(\# 2)}}} } \\ &\Rightarrow {\chi _{H2O(\# 2)}} = \frac{{{A_{H2O(\# 2)}}}}{{P \cdot \sum\limits_{i,j} {{S_{i,j}}(T )\cdot L} }} \end{aligned}$$
- 4. Because the laser lights with the A-laser light absorption signal and the B-laser light absorption signal are irradiated on the optical path at the same time and in the same space, the H2O concentrations in the light absorption signals of the A-laser and B-laser are the same. Therefore, after applying the H2O concentration measured from the B-laser light absorption signals to calculate the absorbance area (absorbance) of H2O(#1) using Eq. (5), when the absorbance area of H2O(#1) is removed from the [A-laser] total absorbance area, only the absorption area of the original OH radical is extracted.$$\begin{aligned} &\Rightarrow {\chi _{H2O(\# 2)}} = {\chi _{H2O(\# 1)}}\\ &\Rightarrow {A_{H2O(\# 1)}} = \int\limits_{ - \infty }^\infty {{\alpha _{H2O(\# 1)}}d\nu = P \cdot {\chi _{H2O(\# 2)}} \cdot \sum\limits_{i,j} {{S_{i,j}}(T )\cdot L} } \\ &\Rightarrow \int\limits_{ - \infty }^\infty {{\alpha _{OH}}d\nu = \int\limits_{ - \infty }^\infty {{\alpha _{OH + H2O(\# 1)}}d\nu - } } \int\limits_{ - \infty }^\infty {{\alpha _{H2O(\# 1)}}d\nu } \end{aligned}$$
- 5. The line strength corresponding to the wavelength of the (single) OH (1502.69 nm) radical was calculated after applying the flame temperature derived from the B-laser light absorption signals to Eq. (3). Finally, the OH radical concentration in the premixed CH4/air flame was derived by substituting the calculated line strength of the OH radical and the absorbance area of the extracted OH radical into Eq. (6).
Table 1. Selected laser wavelengths for simultaneously measuring of OH radical, H2O concentration, and temperature in the flame.
3. Experimental method and setup
3.1. Line selection
Before performing the experiments in this study, the light absorption region for measuring the OH radical concentration was selected. Figure 2 shows the line strength of the OH radical wavelength selected for this study and compares the line strength of the OH radical wavelengths of previous studies from the HITRAN2016 database [30]. Figure 2(g) compares the OH and H2O line strengths in the near-IR 1450–1600 nm wavelength range based on a temperature 1473.15 K. It can be confirmed that the line strength of the OH radical is strong and that of H2O is weak in the wavelength region of 1450–1600 nm. However, this relationship cannot guarantee a proper SNR in the combustion environment because the H2O concentration in the combustion product is higher than the OH radical concentration, and the light absorption signal of H2O becomes sufficiently high to interfere with the OH radical measurement. Figures 2(a)–(f) present the line strengths of the OH radical selected from previous research using near-IR wavelengths at intervals of 0.4 nm on the X-axis, as mentioned in regard to the previous research mentioned in Section 1 in which OH radicals were measured. The graph shows the Y-axis on a log-scale to confirm the interference of H2O on the line strength of the OH radical absorbing wavelength. In general, a waveform (current modulation) with a constant frequency and amplitude is applied to the light source of a butterfly-type DFB laser to measure gas species using the TDLAS technique. At this time, the frequency used in the combustion environment based on the near-IR wavelength is 1–10 kHz, and the wavelength scan range is approximately 0.25–0.35 nm (based on 1 kHz) [31]. That is, the line strengths of the OH wavelengths selected in previous studies mostly overlapped with the H2O line strength owing to line broadening.
Fig. 2. (a)–(f) Line strengths of OH radical and H2O from previous research on a log scale in the 1490–1558 nm spectral range and (g) line strengths of OH radical and H2O on a log scale in the 1450–1600 nm spectral range at 1473.15 K from the HITRAN database.
In previous studies, because the H2O line strength partially or entirely overlapped with the line strength of the selected OH wavelength, the H2O light absorption signal was corrected using a multi-line spectral fitting method or a HITRAN database-based simulation method. However, these methods tend to be complex or increase the size of the optical system. Therefore, this paper proposes a new method of removing the H2O light absorption signal that overlaps with the OH radical light absorption signal using a simple system and an improved DAS method. Figure 3 shows the wavelengths selected for measuring the OH radicals, H2O concentration, and temperature in the flame. As demonstrated by Fig. 3(a), the light absorption wavelength range of OH radicals in this study was selected after considering the interference with the light absorption regions of other combustion products. No interference from other combustion products, except H2O, was observed in the selected OH radical wavelength range. Although interference is expected in the received signal due to the line broadening of H2O, a wavelength of 1502.69 nm was selected for the A-laser, where the line strength of the OH radicals was high and that of H2O was low. Because it is expected that the light absorption signals of OH and H2O in the selected wavelength range entirely overlap, only the light absorption signal of H2O should be removed. The laser wavelength range of 1349.23–1349.27 nm [H2O(#2)] is shown in Fig. 3(b), which is an additional selected H2O light absorption signal pair used to remove the H2O light absorption signal from the blended signal. There was no interference from other combustion products, including the OH radicals.
Fig. 3. (a) Simulated absorbance using Voigt simulation of [OH + H2O(#1) of A-laser] in the 1502.60–1502.76 nm spectral range and (b) simulated absorbance using Voigt simulation of [H2O(#2) of B-laser] and combustion products in the 1349.16–1349.34 nm spectral range at 1385 K from the HITRAN database
3.2. Experimental setup
Figure 4(a) and (b) show a schematic diagram of a combustion system designed to form a premixed CH4/air flame and the optical setup. The experiment in this study focuses on whether it is possible to measure the OH radical concentration in the premixed CH4/air flame after separating the [H2O(#1) of the A-laser] light absorption signal blended in the light absorption signal region of the OH radical. Therefore, the optical laser with the [A-laser] and the [B-laser] light absorption signal that measures the temperature and H2O concentration in the premixed CH4/air flame should be simultaneously irradiated. The fiber-coupled optical component WDM is used for this, which combines and separates two lights with different wavelengths. Fiber-coupled WDM can be divided into multiplexers (MUX) and de-multiplexer s(De-MUX). A fiber-coupled MUX is a system that combines laser light with two different wavelengths. The combined two laser lights are transmitted through the measurement space in the premixed CH4/air flame, and then the two laser lights are separated by the difference in the wavelength refractive index of the grating in the fiber-coupled De-MUX. Figure 4(b) is a schematic diagram showing the improvement in the detection limit of the concentration (χi) because the light absorption area (A) increases when the optical path length (L) increases, as shown in Eq. (5). Therefore, a gold mirror with high reflectivity (Thorlabs, PF05-03-M01) was used to increase the optical path length (L). The two laser lights transmitted through the premixed CH4/air flame are separated by the difference in the refractive index of the wavelength by a fiber-coupled De-MUX, and each separated laser light with absorption information is focused on photodetectors (1) and (2) (Thorlabs, PDA20CS-EC). These were obtained using a signal processing data acquisition (DAQ) and integrated analysis system.
Fig. 4. Schematics of the experimental setup for measuring the OH* intensity, OH radical, H2O concentration, and temperature in the combustion system with the metal-fiber burner.
The combustor used in the experiment was a metal fiber burner, as shown in Fig. 4(a), to form a premixed CH4/air flame. The total capacity of the burner is approximately 9,000 kcal/h. Premixed CH4 gas and dry air were formed into a flame through the metal fiber surface. The equivalence ratio (Φ, the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio.), which is set as a variable in the experiment, is adjusted to the fuel-air flow rate using a calibrated mass flow controller (MFC). The equivalence ratio was determined only when the fuel–air ratio in the burner space was premixed. In addition, a collimating lens was installed close to the burner surface to minimize the influence of outside air in the space through which the laser light passes. Table 2 lists the conditions of the combustion experiments. To fix the heat load at approximately 9,000 kcal/h, the fuel supply was fixed at 8.46 L/min and the equivalence ratio was determined using air as the oxidizer. The equivalence ratio was set at 0.15 intervals from the fuel-lean condition of 0.7 to the fuel-rich condition of 1.30.
Table 2. Operating conditions for forming the premixed CH4/air flame according to the equivalence ratio.
3.3. Chemiluminescence setup and condition for measuring the premixed CH4/air flame
Figure 4(b) shows the chemiluminescence system installed in front of the metal fiber burner. The UV emissions (OH* radical) from the premixed CH4/air flame were visualized using a chemiluminescence measurement system, and images of the OH* radical were compared to the measured OH radical concentration using the equivalence ratio of the improved DAS method.
The chemiluminescence measurement system consisted of an intensified charge coupled device (ICCD) camera (Andor iStar model DH334T-18F-03, 16 bit) with a 1024${\times} $1024 pixel array (maximum resolution 1024${\times} $1024 pixels). The intensifier gain was set to 1500 (digital delay generator, delay 0.01 µs and width 0.05 s) for all the chemiluminescence acquisitions. The flame images for the analysis were captured with a resolution of 1024${\times} $1024 (flame view area of 135${\times} $135 mm). For each flame image, according to the equivalence ratio, the accumulation of images was 100 frames with 1${\times} $1 pixel hardware binning. A band-pass filter at 307.1 nm with an FWHM of 25 nm and a diameter of 50 mm was employed. The focal length of the ICCD camera was set to the center of the metal fiber burner surface.
4. Results and discussion
4.1. OH radical concentration measured using improved DAS method
The objective of this study was to measure the OH and H2O concentrations and temperatures in a premixed CH4/air flame using the improved DAS method based on the WDM system, because the H2O light absorption signal within the range of the OH light absorption signal is blended in the near-IR region. An important point is that the selected A-laser and B-laser lights were irradiated simultaneously. After the accurate temperature and H2O concentration in the flame were derived using the B-laser light absorption signals, H2O(#1) of the A-laser light absorption signal was separated from the A-laser light absorption signal by using the measured H2O concentration. Finally, the concentration of a single OH radical was obtained. Even if two different wavelengths of laser light irradiated the measurement space at the same time and in the same space, the measured concentration and temperature should be the same. As described above, the temperature and H2O concentration in the premixed CH4/air flame should be measured using the B-laser light absorption signal.
Figure 5 provides graphs showing the light absorption signals of the A- and B-lasers and measured at an equivalence ratio of 1.00. The current modulation applied to these signals is 1 kHz (sampling rate, 500 kS/s). Also, the transmission loss Tr(t) and background emission E(t) of the signal were eliminated through our DAS algorithm using amplitude modulation based on the laser characteristics [32]. Figure 5(a) shows the B-laser light absorption signals. These signals were used to obtain the exact temperature of the targeted measuring path in the flame by the method mentioned in Ref. [29]. The total integrated area of these signals was also used to calculate the H2O concentration by applying Eqs. (15) and (16). Figure 5(b) shows the blended OH and H2O light absorption signals obtained by using the A-laser. The blended H2O light absorption signal should be removed from the improved DAS method. Figure 5(c) is the absorbance graph analyzed from the light absorption signal in Fig. 5(b) using the Voigt fitting.
Fig. 5. (a) B-laser absorption signals for measuring temperature and H2O concentration, (b) A-laser absorption signals for measuring OH radical concentration under an equivalence ratio of 1.00, and (c) Voigt fitting and residuals of absorbance measured with an A-laser at an equivalence ratio of 1.00.
Figure 6 presents the results of measuring the flame temperature and H2O concentration according to the equivalence ratio using the B-laser light absorption signals. As shown in Fig. 6(a), the temperature measured by the B-laser light absorption signals was compared with the average temperature measured using an R-type T/C corrected for heat loss (radiative, etc.). The temperature acquired by utilizing the R-type T/C was obtained by averaging the measured data at 0.5 cm intervals along the laser path. Figure 6(b) provides a graph comparing the H2O concentration in the premixed CH4/air flame measured using the B-laser in this study and the H2O concentration calculated by the chemical equilibrium simulation (Chemkin). Chemkin simulations used a chemical reaction mechanism based on GRI 3.0 with a premixed laminar burner-stabilized flame code suitable for CH4/air flames. The temperature and H2O concentration measured by the [B-laser] light absorption signals are the average values derived from 10 repeated measurements, depending on the equivalence ratio. Table 3 lists the standard deviations of the measured temperatures and H2O concentrations.
Fig. 6. (a) Comparison of temperatures measured using H2O(#2) absorption signal and R-type thermocouple and (b) comparison of H2O concentrations measured using H2O(#2) absorption signals and simulated H2O concentration obtained with Chemkin (GRI 3.0) according to various equivalence ratios in a combustion environment.
Table 3. Temperatures and H2O concentrations measured using B-laser absorption signals in a premixed CH4/air flame and standard deviations.
For the temperature measured by the B-laser light absorption signals, the temperature was the highest at 1385.8 K at an equivalence ratio of 1.00, and the temperature decreased as the ratio decreased to 0.70, which is the fuel-lean condition. The flame temperature was measured to be 1374.9 K and 1342.3 K at equivalence ratios of 0.85 and 0.70, respectively. Also, the flame temperature was measured to be 1384.4 K and 1337.0 K at equivalence ratios of 1.15 and 1.30, respectively, which are fuel-rich conditions. These findings agree well with the general tendency that the temperature decreased toward the fuel-lean and fuel-rich regions based on the equivalence ratio of 1.00, where the measured flame temperature was the highest. Because the porous burner head is a metal-fiber type, the temperature is approximately 700 K lower than those of general burners due to heat transfer loss and wide flame characteristics, which are different from the characteristics of general burners [33]. According to the measured temperature results, when the amount of air decreases based on the equivalence ratio of 1.00 in the combustion reaction (fuel-rich condition), a large amount of unburned CO concentration is generated [34]. Therefore, it is expected that the temperature will decrease because of incomplete combustion. However, if the equivalence ratio of the combustion reaction is less than 1.00 (fuel-lean condition), the unreacted air cools the combustion heat in the flame and reduces the flame temperature. As the oxygen concentration of the combustion reactants increases, the flame temperature is expected to decrease [35,36].
In the H2O concentration measurements, as mentioned in the experimental setup, because the light is transmitted at a height of approximately 5 mm above the surface of the metal-fiber burner, the H2O concentration simulated by Chemkin was considered, based on a height of approximately 5 mm above the burner surface. The H2O concentration calculated by Chemkin according to the equivalence ratio decreased to 16.30% ($\mathrm{\Phi }$ = 0.85) and 13.70% ($\mathrm{\Phi \;\ }$= 0.70) under the fuel-lean conditions. The H2O concentration was expected to decrease significantly owing to the increase in unreacted air in the fuel-lean condition, and the H2O concentration (18.80%) of the equivalence ratio of 1.15 in the fuel-rich condition was derived to be higher than the equivalence ratio of 1.00. In general, the H2O concentration was maximized when it was slightly fuel-rich ($\mathrm{\Phi \;\ }$= 1.15) but not when it was stoichiometric ($\mathrm{\Phi \;\ }$= 1.00). This phenomenon occurred because the heat of combustion and specific heat decreased simultaneously when the equivalence ratio exceeded 1.00. That is, for equivalence ratios between $\mathrm{\Phi }$ = 1.00 and $\mathrm{\Phi }$ = 1.05, the heat capacity decreased more rapidly than heat combustion, whereas the opposite was true beyond $\mathrm{\Phi }$ = 1.05. The number of product moles formed per mole of fuel burned dominates the decrease in heat capacity, with the decrease in mean specific heat being less significant [37,38].
Consequently, the standard deviations of the measured temperature and concentration were analyzed in the ranges of approximately 11–16 K and 0.30%–0.33%, respectively. The temperature measured by TDLAS and the H2O concentration agreed well with the temperature measured by the corrected R-type T/C and H2O concentration calculated by Chemkin. Therefore, as shown in Table 4, the H2O concentration derived from the B-laser was used to calculate the H2O(#1) absorption area in the total OH + H2O(#1) absorption area of the A-laser. When the calculated H2O(#1) absorption area of the A-laser was removed from the OH + H2O(1) absorption area of the A-laser, the absorption area of the single OH radicals could be extracted. This result corresponds to Steps 1–4 of the improved DAS method.
Table 4. OH integrated area extracted using the improved DAS method (step 1–4) in a premixed CH4/air flame according to various equivalence ratios.
Finally, the temperature derived from the B-laser light absorption signals using Eq. (3) was used to calculate the OH radical line strength corresponding to the wavelength of OH (1502.69 nm).
Table 5 lists the calculated parameters for deriving the OH radical concentrations. The OH radical concentrations according to the equivalence ratio were calculated by substituting the line strength of the OH radical corresponding to the measured flame temperature, optical path length of 94 cm, pressure of 1 atm, and absorbance of the extracted OH radical into Eq. (6). Depending on the equivalence ratio, the OH concentrations measured by the improved DAS method are the average values derived from 10 repeated measurements.
Table 5. OH radical concentrations measured using the improved DAS method (step 5) in the premixed CH4/air flame and standard deviations obtained according to the various equivalence ratios.
Figure 7 compares the OH radical concentration in the premixed CH4/air flame measured using the improved DAS method in this study and that calculated in the chemical equilibrium simulation (Chemkin). Chemkin simulations use a chemical reaction mechanism based on GRI 3.0, with a premixed laminar burner-stabilized flame code suitable for CH4/air flames, such as the Chemkin condition for H2O [39]. Because the laser light is transmitted approximately 5 mm above the burner surface, the calculated OH radical concentration obtained according to the flame height from the burner surface using the Chemkin simulation is shown in Fig. 7(a). The OH radical concentration calculated by the Chemkin simulation was the highest at an equivalence ratio of 1.00 within the flame height range of 1–5 mm. In addition, the concentration of OH radicals decreased towards the fuel-lean and fuel-rich regions. In particular, it was calculated that the OH radical concentration at an equivalence ratio of 0.70 in the fuel-lean condition was slightly higher than the OH radical concentration at an equivalence ratio of 1.15, which is a fuel-rich condition. After a flame height of 32.5 mm, the OH radical concentration with an equivalence ratio of 1.00 remains in equilibrium, and the OH radical concentrations with other equivalence ratios were calculated to be in equilibrium at a flame height of 30 mm.
Fig. 7. (a) Simulated OH radical concentrations according to flame height from the burner surface using Chemkin (GRI 3.0) and (b) comparison of measured OH radical concentration using improved DAS method and simulated OH radical concentration according to various equivalence ratios in a combustion environment.
Figure 7(b) shows a graph comparing the OH radical concentration calculated by Chemkin at the 1–5 mm flame height and the OH radical concentration measured using the improved DAS method according to the equivalence ratio. The OH radical concentration obtained from the improved DAS method was measured to be the highest at 0.2592% at an equivalence ratio of 1.00, and the OH radical concentration at equivalence ratios of 0.85 and 0.70 under the fuel lean condition were measured to be 0.2340% and 0.1529%, respectively. The OH radical concentration decreased rapidly from an equivalence ratio of 1.00 to the fuel-rich condition. The OH radical concentrations were measured at 0.1318% and 0.0791%, respectively, under the fuel-rich conditions, with equivalence ratios of 1.15 and 1.30, respectively. The OH radical concentration measured using the improved DAS method proposed in this study showed a tendency similar to the OH radical concentration simulated by Chemkin (flame height of 3 mm). Because of the round shape of the metal fiber burner surface and the reaction between the air in the atmosphere and the flame, this similarity is expected to be the result of inaccuracy of the flame height (the position of the transmitted laser light).
To analyze the measured OH radical concentration, the chemical kinetics of OH radicals should be examined. OH radicals are produced by a typical R10 reaction in many reactions [40]. In the fuel-lean flame, the loss of H atoms by not including diffusional effects influences the chain branching reaction R10, which is the main source of OH radical generation. In addition, it affects the main OH regeneration reaction in reaction R9 involving H2O, which is a recombination product of OH radicals. In fuel-rich flames, low concentrations of O and O2 render reaction R10 less important, and the corresponding R9 is known to be the reaction most affected by the loss of H atoms [40].
In particular, the OH radical concentration was significantly reduced under the fuel-rich condition compared with the fuel-lean condition. For the stoichiometric ($\mathrm{\Phi }$ = 1.00) and lean fuel based on the OH radical decay rate $\frac{{d[{OH} ]}}{{dt}} ={-} {k_{OH}}{[{OH} ]^2}$, the slope of the decay rate is almost equally low owing to a constant decay constant k. However, the slope of the decay rate increases significantly as the decay constant increases under fuel-rich conditions [33]. Consequently, OH radical concentration obtained from the improved DAS method was comparable to that calculated by Chemkin.
4.2. OH* radical intensity measured using chemiluminescence system
An ICCD camera was used to compare the OH radical concentration measured by the improved DAS method and the OH* radical intensity. Figure 8 shows the OH* radical intensity measured using an ICCD camera with a UV bandpass filter according to the equivalence ratio. Figure 8(a) provides a photograph of an actual flame captured using a digital single-lens reflex (DSLR) camera (Canon EOS5D Mark III) with an EF85 mm lens F1.2L USM. The surface of the metal-fiber burner shines brighter owing to the radiation caused by the flame with an equivalence ratio of 1.00 under the fuel-lean condition. Flames are more clearly observed above the surface of the metal fiber burner as the fuel-rich condition increases. When the flame height increases under fuel-rich conditions, it is expected that the unburned CO reacts with air in the atmosphere to form a diffusion flame. Figure 8(b) shows the OH* radical intensity according to the equivalence ratio. When the entire metal fiber burner is visualized by intensity, as shown in Fig. 8(b), it is difficult to identify the distribution of the OH* radical intensity according to the equivalence ratio. Consequently, Fig. 8(c) shows an image of the surface of the metal fiber burner (red line section) in Fig. 8(b). The OH* radical intensity with an equivalence ratio of 1.00 in Fig. 8(c) confirms the presence of strong OH* radicals near the surface of the metal fiber burner. However, the OH* radical intensity distribution on the surface of the metal fiber burner gradually decreases toward the fuel-lean condition. As can be seen in the actual flame, which is the fuel-rich condition in Fig. 9(a), the OH* radical intensity in the fuel-rich condition (1.15, 1.30) in Fig. 8(c) is strongly distributed because of the diffusion flame regions.
Fig. 8. (a) Photographs of an actual premixed CH4/air flame using DSLR camera, (b) images of OH* emission from the entire metal-fiber burner obtained using an ICCD camera and various equivalence ratios, and (c) images of OH* emission from the surface of the metal-fiber burner acquired using the ICCD camera.
Fig. 9. (a)–(e) Intensities of OH* radical emissions visualized using the ICCD camera, and (f) intensity versus equivalence ratio.
Figure 9 shows the OH* radical intensities visualized using the ICCD camera. As shown in Fig. 8(c), these results correspond to a height of approximately 5 mm through which the laser is transmitted from the surface of the metal fiber burner. As shown in Figs. 9(a)–(c), the average OH* radical intensity at an equivalence ratio of 1.00 is the highest, at 490,885, and those corresponding to equivalence ratios of 0.85 and 0.70 under the fuel lean conditions are 401,811 and 281,938, respectively. In addition, as shown in Figs. 9(d) and (e), the intensity is 287,035 and 235,863 under fuel-rich conditions with equivalence ratios of 1.15 and 1.30, respectively. Under the fuel-lean condition with an equivalence ratio 0.70 and the fuel-rich condition with an equivalence ratio of 1.15, the opposite tendency is observed when comparing the OH radical concentrations measured by the improved DAS method with the OH* radical intensities visualized by the ICCD camera.
In the analysis of these results, as in Fig. 9(a) ($\mathrm{\Phi }$ = 0.70) and Fig. 9(d) ($\mathrm{\Phi }$ = 1.15), the OH* radical intensity under the fuel-lean condition was found to be greater than the OH* radical intensity under the fuel-rich condition in the pixel number range of 200–800. However, the OH* radical intensity due to the diffusion flame is distributed at pixel numbers of 0–200 and 800–1000 under the fuel-rich condition with an equivalence ratio of 1.15. When obtaining OH* radical intensities under fuel-rich conditions due to diffusion flames, the focal length of the ICCD camera was fixed at the center of the surface of the metal fiber burner. This phenomenon is predicted to be caused by the accumulation of unstable flame sections owing to diffusion flames that react with the outside air under fuel-rich conditions. However, the TDLAS method has a high proportion of the laser light transmitted to the flame center of the metal-fiber burner based on the line average measurement, so the OH radical concentration is judged to be higher at an equivalence ratio of 0.70 under the fuel-lean condition, than at an equivalence ratio of 1.15 under the fuel-rich condition. Thus, when comparing the trend of the OH* radical intensity measured using the ICCD camera with the OH radical concentration measured using the improved DAS method, it was confirmed that the improved DAS method for removing the H2O(#1) light absorption signal blended with the OH radical light absorption signal could quantitatively measure the OH radical concentrations in the flame.
5. Conclusion
In this study, an improved DAS method with WDM was proposed to measure the OH radicals, H2O concentration, and temperature in flames simultaneously. An optical system for simultaneously measuring the OH radicals, H2O concentration, and temperature in the flame was installed close to the combustion system designed for premixed CH4/air flame formation. For the optical system, the WDM system was applied to A- and B-laser irradiation based on the improved DAS method. The temperature and H2O concentration in the flame were measured using the H2O(#2) absorption signal from the B-laser. The temperature measured by the TDLAS was compared with that measured by the calibrated R-type T/C. In addition, the H2O concentration in the flame was derived from the entire H2O(#2) absorption signal of the B laser. At this time, the H2O concentration measured by TDLAS according to the equivalence ratio agreed well with the trend of the H2O concentration simulated by Chemkin based on GRI 3.0. The high sensitivity and resolution of the measured temperature and H2O concentration were confirmed in Ref. [29]. Through the improved DAS theory, the blended OH and H2O absorption signals obtained by the A-laser were separated by the temperature and H2O concentration measured by the B-laser, and the single OH radical absorption signal was separated. The OH radical concentration, based on the equivalence ratio, was derived using a single OH radical absorption signal. The OH radical concentration measured by TDLAS according to the equivalence ratio was almost the same as the OH radical concentration tendency simulated by Chemkin using a chemical reaction mechanism based on GRI 3.0 with a premixed laminar burner-stabilized flame code suitable for CH4/air flames. However, due to the reaction between the premixed CH4/air flame and the air in the atmosphere, there was a slight difference between the OH radical concentrations measured by TDLAS and analyzed by Chemkin according to flame height. The reliability of the measured OH radical concentration was determined by comparing the OH radical concentration measured by TDLAS with the OH* radical intensities analyzed by the ICCD camera. Unlike in previous studies, the OH radicals, H2O concentration, and temperature in the flame were simultaneously measured using a simple analysis method. Finally, the improved DAS method for measuring OH radical concentration proposed in this study is considered to have positive impacts on combustion energy efficiency and pollutant reduction in practical combustion.
Funding
Korea Institute of Science and Technology (2E31980-22-P009).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
References
1. International Energy Agency, Net Zero by 2050-A Road Map for the Global Energy Sector (Paris, France, 2021).
2. M. I. Hassan, K. T. Aung, and G. M. Faeth, “Measured and predicted properties of laminar premixed methane/air flames at various pressures,” Combust. Flame 115(4), 539–550 (1998). [CrossRef]
3. D. Zhao, H. Yamashita, K. Kitagawa, N. Arai, and T. Furuhata, “Behavior and effect on NOX formation of OH radical in methane-air diffusion flame with steam addition,” Combust. Flame 130(4), 352–360 (2002). [CrossRef]
4. A. Matynia, M. Idir, J. Molet, C. Roche, S. de Perisis, and L. Pillier, “Absolute OH concentration profiles measurements in high pressure counterflow flames by coupling LIF, PLIF, and absorption techniques,” Appl. Phys. B 108(2), 393–405 (2012). [CrossRef]
5. A. B. Sahu and R. V. Ravikrishna, “Quantitative LIF measurements and kinetics assessment of NO formation in H2/CO syngas-air counterflow diffusion flames,” Combust. Flame 173, 208–228 (2016). [CrossRef]
6. T. Li, P. Farmand, C. Geschwindner, M. Greifenstein, J. Köser, C. Schumann, A. Attili, H. Patsch, A. Dreizler, and B. Böhm, “Homogeneous ignition and volatile combustion of single solid fuel particles in air and oxy-fuel conditions,” Fuel 291, 120101 (2021). [CrossRef]
7. J. Yang, Y. Gong, Q. Gue, H. Zhu, F. Wang, and G. Yu, “Experimental studies of the effects of global equivalence ratio and CO2 dilution level on the OH* and CH* chemiluminescence in CH4/O2 diffusion flames,” Fuel 278, 118307 (2020). [CrossRef]
8. H. Zhu, Y. Gong, L. He, Q. Gue, X. Chen, and G. Yu, “Effects of CO and H2 addition on OH* chemiluminescence characteristics in laminar rich inverse diffusion flames,” Fuel 254, 115554 (2019). [CrossRef]
9. M. G. D. Giorgi, A. Sciolti, S. Campilongo, and A. Ficarella, “Flame structure and chemiluminescence emissions of inverse diffusion flames under sinusoidally driven plasma discharges,” Energies 10(3), 334–348 (2017). [CrossRef]
10. M. Zhao, D. Buttsworth, and R. Choudhury, “Experimental and numerical study of OH* chemiluminescence in hydrogen diffusion flames,” Combust. Flame 197, 369–377 (2018). [CrossRef]
11. E. Thorin and F. M. Schmidt, “TDLAS-based photofragmentation spectroscopy for detection of K and KOH in flames under optically thick conditions,” Opt. Lett. 45(18), 5230–5233 (2020). [CrossRef]
12. Z. Qu, R. Ghorbani, D. Valiev, and F. M. Schmidt, “Calibration-free scanned wavelength modulation spectroscopy – Application to H2O and temperature sensing in flames,” Opt. Express 23(12), 16492–16499 (2015). [CrossRef]
13. O. Witzel, A. Klein, C. Meffert, S. Wagner, S. Kaiser, C. Schulz, and V. Ebert, “VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines,” Opt. Express 21(17), 19951–19965 (2013). [CrossRef]
14. S. Wang, D. F. Davidson, and R. K. Hanson, “Shock tube measurements of OH concentration time-histories in benzene, toluene, ethylbenzene and xylene oxidation,” Proc. Combust. Inst. 37(1), 163–170 (2019). [CrossRef]
15. T. R. Meyer, S. Roy, T. N. Anderson, J. D. Miller, V. R. Katta, R. P. Lucht, and J. R. Gord, “Measurements of OH mole fraction and temperature up to 20 kHz by using a diode-laser-based UV absorption sensor,” Appl. Opt. 44(31), 6729–6740 (2005). [CrossRef]
16. S. Wang and R. K. Hanson, “High-sensitivity 308.6-nm laser absorption diagnostic optimized for OH measurement in shock tube combustion studies,” Appl. Phys. B 124(3), 37–43 (2018). [CrossRef]
17. T. Aizawa, T. Kamimoto, and T. Tamary, “Measurements of OH radical concentration in combustion environments by wavelength-modulation spectroscopy with a 1.55-µm distributed-feedback diode laser,” Appl. Opt. 38(9), 1733–1741 (1999). [CrossRef]
18. B. L. Upschulte, D. M. Sonnenfroh, and M. G. Allen, “Measurements of CO, CO2, OH, and H2O in room-temperature and combustion gases by use of a broadly current-tuned multisection InGaAsP diode laser,” Appl. Opt. 38(9), 1506–1512 (1999). [CrossRef]
19. S. Wagner and V. Ebert, “Spatially resolved in situ TDLAS measurement of absolute H2O and OH mole fraction in a laminar 2D diffusion flame,” AIAA 556, 1–11 (2012). [CrossRef]
20. S. Bürkel, A. Dreizler, V. Ebert, and S. Wagner, “Experimental comparison of a 2D laminar diffusion flame under oxy-fuel and air atmosphere,” Fuel 212, 302–308 (2018). [CrossRef]
21. T. R. S. Hayden, N. Malarich, D. Petrykowski, S. P. Nigam, J. D. Christopher, C. Lapointe, N. T. Wimer, P. E. Hamlington, and G. B. Rieker, “OH radical measurements in combustion environments using wavelength modulation spectroscopy and dual-frequency comb spectroscopy near 1491 nm,” Appl. Phys. B 125(12), 225–238 (2019). [CrossRef]
22. C. Lu, F. S. Vieira, F. M. Schmidt, and A. Foltynowicz, “Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame,” Opt. Express 27(21), 29521–29533 (2019). [CrossRef]
23. V. Ebert, H. Teichert, P. Strauch, T. Kolb, H. Seifert, and J. Wolfrum, “Sensitive in situ detection of CO and O2 in a rotary-kiln-based hazardous waste incinerator using 760 nm and new 2.3-µm diode lasers,” Proc. Combust. Inst. 30(1), 1611–1618 (2005). [CrossRef]
24. R. R. Gamache, S. Kennedy, R. Hawkins, and L. S. Rothman, “Total internal partition sums for molecules in the terrestrial atmosphere,” J. Mol. Struct. 517-518, 407–425 (2000). [CrossRef]
25. R. R. Gamache, C. Roller, E. Lopes, I. E. Gordon, L. S. Rothman, O. L. Polyyansky, N. F. Zobov, A. A. Kyuberis, J. Tennyson, S. N. Yurchenko, A. G. Császár, T. Furtenbacher, X. Huang, D. W. Schwenke, T. J. Lee, B. J. Drouin, S. A. Tashkun, V. I. Perevalov, and R. V. Kochanov, “Total internal partition sums for 166 isotopologues of 51 molecules important in planetary atmospheres: Application to HITRAN2016 and beyond,” J. Quant. Spectrosc. Radiat. Transfer 203, 70–87 (2017). [CrossRef]
26. X. Zhou, X. Liu, J. B. Jeffries, and R. K. Hanson, “Development of a sensor for temperature and water concentration in combustion gases using a single tunable diode laser,” Meas. Sci. Technol. 14(8), 1459–1468 (2003). [CrossRef]
27. R. K. Hanson and P. K. Falcone, “Temperature measurement technique for high-temperature gases using a tunable diode laser,” Appl. Opt. 17(16), 2477–2480 (1978). [CrossRef]
28. X. Liu, J. B. Jeffries, and R. K. Hanson, “Measurement of spectral parameters of water-vapour transitions near 1388 and 1345 for accurate simulation of high-pressure absorption spectra,” Meas. Sci. Technol. 18(5), 1185–1194 (2007). [CrossRef]
29. S. So, N. Jeong, A. Song, J. Hwang, D. Kim, and C. Lee, “Measurement of temperature and H2O concentration in premixed CH4/air flame using two partially overlapped H2O absorption signals in the near infrared region,” Appl. Sci. 11(8), 3701–3716 (2021). [CrossRef]
30. I. E. Gordon, L. S. Rothman, C. Hill, R. V. Kochanov, Y. Tan, P. F. Bernath, M. Birk, V. Boudon, A. Campargue, K. V. Chance, B. J. Drouin, J–M Flaud, R. R. Gamache, J. T. Hodges, D. Jacquemart, V. I. Perevalov, A. Perrin, K. P. Shine, M–AH Smith, J. Tennyson, G. C. Toon, H. Tran, V. G. Tyuterev, A. Barbe, A. G. Császár, V. M. Devi, T. Furtenbacher, J. J. Harrison, J–M Hartmann, A. Jolly, T. J. Johnson, T. Jarman, I. Kleiner, A. A. Kyuberis, J. Loos, O. M. Lyulin, S. T. Massie, S. N. Mikhailenko, N. Moazzen-Ahmadi, H. S. P. Moller, O. V. Naumenko, A. V. Nikitin, O. L. Polyansky, M. Rey, M. Rotger, S. W. Sharpe, K. Sung, E. Starikova, S. A. Tashkun, J. Vander Auwera, G. Wagner, J. Wilzewski, P. Wcisto, S. Yu, and E. J. Zak, “The HITRAN 2016 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 203, 3–69 (2017). [CrossRef]
31. Z. Wang, P. Fu, and X. Chao, “Laser absorption sensing systems: Challenges, modeling, and design optimization,” Appl. Sci. 9(13), 2723 (2019). [CrossRef]
32. O. Witzel, A. Klein, C. Meffert, C. schulz, S. A. Kaiser, and V. Ebert, “Calibration-free, high-speed, in-cylinder laser absorption sensor for cycle-resolved, absolute H2O measurements in a production IC engine,” Proc. Combust. Inst. 35(3), 3653–3661 (2015). [CrossRef]
33. R. J. Cattolica, “OH radical nonequilibrium in methane-air flat flames,” Combust. Flame 44(1-3), 43–59 (1982). [CrossRef]
34. S. So, J. Park, A. Song, J. Hwang, M. Yoo, and C. Lee, “Detection limit of CO concentration measurement in LPG/air flame flue gas using tunable diode laser absorption spectroscopy,” Energies 13(16), 4235–4245 (2020). [CrossRef]
35. F. Wang, Q. Huang, H. Zhang, J. Yan, and K. Cen, “Simultaneous measurement of 2-dimensional H2O concentration and temperature distribution in premixed methane/air flame using TDLAS-based tomography technology,” Opt. Commun. 346, 53–63 (2015). [CrossRef]
36. S. So, D. G. Park, N. Jeong, D. Kim, J. Hwang, and C. Lee, “Study on the simultaneous measurement of O2 and CO concentrations in the exhaust gas of a methane/air flame using tunable diode laser absorption spectroscopy,” Energy Fuels 34(3), 3780–3787 (2020). [CrossRef]
37. R. T. Stephen, An Introduction to Combustion: Concepts and Applications, 2nd ed. McGraw-Hill Education, New York, (2002).
38. C. K. Law, Combustion Physics, 1st ed., Cambridge University, New York, (2006).
39. G. P. Smith, D. M. Golden, M. Frenklach, N. W. Moriarty, B. Eiteneer, M. Goldenberg, C. T. Bowman, R. K. Hanson, S. Song, W. C. Gardiner, V. V. Lissianski, and Z. Qin, “GRI-mech 3.0,” http://combustion.berkeley.edu/gri-mech/.
40. A. Matynia, J. Molet, C. Roche, M. Idir, S. de Persis, and L. Pillier, “Measurement of OH concentration profiles by laser diagnostics and modeling in high-pressure counterflow premixed methane/air and biogas/air flames,” Combust. Flame 159(11), 3300–3311 (2012). [CrossRef]
