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Abstract
Equivalence ratio is one of the most significant parameters in combustion flow fields. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique for instantaneous one-dimensional local equivalence ratio measurements were performed. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurements. Among them, the CH peak at ~431 nm and the O peak at ~777 nm are strong enough to be used to achieve single-shot measurements, which is important to turbulent flow fields. Furthermore, systematic experiments were performed by using FLIPS in both laminar and turbulent flow fields. The FLIPS technique features the abilities of instantaneous one-dimensional quantitative measurements, high spatial resolution, and no Bremsstrahlung interference.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Equivalence ratio (φ) is one of the most critical parameters in combustion flow fields, which represents the proportion of the amount of fuel and air in the mixture. In situ φ measurements are of great significance. Firstly, φ can provide supporting data to simplify combustion models in CFD simulations [1]. Secondly, in some practical combustors, such as gas turbines, the fuel and the oxidizer are usually burned in a partially premixed manner [2,3]. In such a system, local φ often has spatial and temporal non-uniformity. Therefore, the real-time local φ measurements and its precise control can reduce combustion instability and avoid misfires. Thirdly, the impact of φ on engine performance is also well known [4,5]. In particular, lean combustion strategies are commonly used to reduce NOx in engines. However, overly lean conditions can increase the chance of misfires and lead to a significant increase of unburned hydrocarbon emissions [6]. Consequently, it is necessary to operate each cycle or even each cylinder at its optimum φ, which is based on accurate measurements of φ. Fourthly, for scramjet engines, the mixing time between the supersonic air stream and the fuel is largely shorter than that of conventional combustors, and φ fluctuates over a wide range. Therefore, accurate local φ measurements own a great significance for studying the scramjet engines [7].
Various techniques have been developed for equivalence ratio measurements, for instance, spark-induced breakdown spectroscopy (SIBS) [8–10], chemiluminescence spectroscopy [11–13] and laser-based techniques. SIBS [8–10] could interfere with the flow fields due to the intrusion of the electrodes. The chemiluminescence spectroscopy [11–13], although simple, is only available in combustion regions. Laser-based techniques, however, do not possess the above-mentioned limitations, and attract more attention. Traditional laser-based techniques use nanosecond laser as light source, including infrared laser absorption spectroscopy [14,15], laser-induced fluorescence (LIF) [16–19], spontaneous Raman scattering [20–23] and laser-induced breakdown spectroscopy (LIBS) [7,24–35].
For laser absorption spectroscopy [14,15], it is easy to achieve quantitative equivalence ratio measurements, but this kind of technique can only provide line-of-sight information. Compared with absorption spectroscopy, LIF [16–19] has the imaging ability with a much higher spatial resolution. Normally, LIF is a resonant technique which requires a tunable laser. This limits its versatility to some extent. The spontaneous Raman scattering technique [20–23] can achieve quantitative and one-dimensional [22,23] φ measurements. However, the signal of spontaneous Raman scattering is weak and is thus susceptible to be interfered. Alternatively, LIBS [7,24–35] is one of the most commonly used techniques for φ measurements.
Nanosecond lasers are commonly used in LIBS. The laser is focused in a premixed fuel/air mixture and generates a breakdown, which acts as a virtual measurement probe. The intensities of the spectral emission lines emitted from the plasma are sensitive to the concentrations of the corresponding species, which can be used to determine φ quantitatively. The LIBS technique has been performed in various flow fields under different conditions. In most of the previous studies, the ratio of certain spectral lines of the plasma can be used to correlate and quantify φ with single-point measurement, including H/O [28,34], H/N [24,25], C/O [7,33], C/N [7,30,33], C/(N + O) [27] and C2/CN [29]. Furthermore, Wu et al. [32,35] expanded the application of ns-LIBS into one-dimensional measurement. The advantages of LIBS include high emission intensity and less system complexity. However, the typical volume of the plasma induced by nanosecond laser is about 1 mm3 [36], which limits the spatial resolution of LIBS. Meanwhile, the ns-LIBS technique is susceptible to Bremsstrahlung interference. Therefore, additional effort is needed to reduce this interference, such as increasing delay time [29] or using shorter gate width [26,31].
In recent years, femtosecond laser spectropic techniques have expanded its horizon to measure flow field parameters. Richardson et al. [37] demonstrated the femtosecond two-photon-absorption laser-induced fluorescence (fs-TALIF) imaging measurements of krypton (Kr) to study mixing in gaseous flows. Kotzagianni et al. [38,39] examined the femtosecond laser-induced breakdown (fs-LIBS) as a tool for in situ determination of the equivalence ratio. Halls et al. [40] performed femtosecond-laser electronic-excitation tagging (FLEET) technique for mixture fraction measurements in a jet. Li et al. [41] applied femtosecond laser-induced nonlinear spectroscopy (FINS) for remote sensing of methane.
Although the above-mentioned studies have demonstrated the feasibility of femtosecond laser spectroscopy in φ measurements, the multi-dimensional measurements have not been presented nor discussed.
When a femtosecond laser propagates through gas media, laser-induced filament phenomenon will occur [42], and the plasma channel (or called filament) induced by the femtosecond laser can be uniformly distributed [43] over a range of a few centimeters to ~200 cm [44]. This phenomenon was first observed by Braun et al. [45] in 1995, which is due to the dynamic balance between the optical Kerr-effect-induced self-focusing and the defocusing effect of the self-generated plasma. The plasma channel induced by femtosecond laser is fairly long, and the emission along the channel is uniform to a certain extent. Therefore, femtosecond laser-induced plasma is expected to achieve one-dimensional measurements. In addition, the clamped intensity inside the plasma is approximately 1013-1014 W/cm2 [42], which is high enough to generate excited molecules/atoms through femtosecond laser-induced photochemical reactions. Subsequently, the excited molecules/atoms will release fluorescence. By analyzing the fluorescence spectra, quantitative equivalence ratio measurements can be obtained. For these reasons, we expect that the femtosecond laser-induced plasma spectroscopy can be implemented to achieve one-dimensional equivalence ratio measurements. Hsu et al. [46] have developed femtosecond laser-induced plasma spectroscopy for gas sensing at elevated pressures, but its capability for one-dimensional measurements has not been mentioned.
In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique was performed for one-dimensional equivalence ratio measurements. Through a calibration process, we established the relationship between the ratios of different spectral peak area pairs of FLIPS and φ. In addition, we proved that FLIPS technique has the capability for instantaneous measurements, and it can be applied to turbulent flow fields.
2. Experiment
In this work, three experiments were carried out, including spectral measurements, a calibration process and instantaneous one-dimensional equivalence ratio measurements. The spectra measurements and the calibration were performed in a homogeneous room-temperature flow field generated by a standard McKenna burner [47] (Fig. 1(a)). It was supplied with premixed CH4/air mixtures of known equivalence ratios ranging from 0.6 to 1.6, and the gas supply rate was kept constant at 0.15 m/s using gas flow controllers. It should be noted that the burner is only used to generate room-temperature flow fields and there was no combustion involved. The instantaneous one-dimensional φ measurements were performed in inhomogeneous room-temperature flow fields generated by a co-flow piloted jet (Fig. 1(b)). It consists of two coaxial tubes, 2 mm ID for the central tube and 60 mm OD for the outer tube, and they were both supplied with premixed CH4/air mixtures. The equivalence ratio was 1.6 for the jet, and 0.6 for the co-flow. Under laminar conditions, the gas supply rate of the jet was 2.5 m/s, and hence, its Reynolds number of 1460 was obtained. The measurements were performed at the position of y/d = 1. Under turbulent conditions, the gas supply rate was 50 m/s with its Reynolds number of ~6500 and the measurement position of y/d = 5.
Fig. 1 (a) A standard McKenna burner was used to generate homogeneous laminar flow fields where the spectra measurements and the calibration were performed in room-temperature. The burner was supplied with premixed CH4/air mixtures of known equivalence ratio ranging from 0.6 to 1.6. (b) A co-flow piloted jet was used to generate inhomogeneous room-temperature flow fields where instantaneous one-dimensional equivalence ratio measurements were performed. The jet consists of two coaxial tubes that were supplied with premixed CH4/air mixtures of different equivalence ratios. Under laminar conditions, the gas supply rate of the jet was 2.5 m/s with its Reynolds number of 1460. When under turbulent conditions, the gas supply rate was 50 m/s with its Reynolds number of ~6500.
The experimental setups are presented in Fig. 2. A femtosecond Ti:sapphire laser system (Spitfire Ace, Spectra-Physics) was used as light source, with its fundamental output being adopted. The wavelength of the laser centers at 800 nm with a pulse energy of 4 mJ, a pulse duration of ~45 fs and a repetition rate of 1 kHz. The laser was focused by a spherical lens (f = 1000 mm) and plasma channel was generated in the flow field.
Fig. 2 Schematic of the experimental setups. (a) The setup used for spectra measurements. (b) The setup used for calibration and instantaneous one-dimensional equivalence ratio measurements. A femtosecond laser was used as light source with a pulse energy of 4 mJ, a pulse duration of ~45 fs and a repetition rate of 1 kHz.
Figure 2(a) is the setup for spectral measurements. The optical emission from the plasma channel was collected using a spectrometer (Acton SP-2300i, Princeton Instruments) with a spherical lens (f = 100 mm). The input slit, 250 μm in width, of the spectrometer was horizontally orientated (parallel to the laser propagation direction) to record the spatially resolved spectra of the plasma channel. The signal dispersed by a grating (300 grooves/mm blazed at 300 nm) was captured at the exit port by an ICCD camera (PI-MAX3:1024i, Princeton Instruments). Before measurements, the wavelength of the spectrometer was calibrated with a mercury lamp. In addition, the intensity was calibrated by an integrating sphere (PP-02097-000, Labsphere) with a standard halogen light source in it.
For calibration process and instantaneous one-dimensional equivalence ratio measurements, the plasma channel was imaged simultaneously by two ICCD cameras, as can be seen in Fig. 2(b). One camera equipped with an interference filter (center wavelength at 431 nm, FWHM of 8 nm, transmittance of 71.4% [48]) for CH image. The other camera equipped with an interference filter (center wavelength at 777 nm, FWHM of 9.4 nm, transmittance of 65.8%) for O image. Prior to the measurements, a transparent target was placed at the position where the plasma was generated. The pair images of the target recorded by the two cameras were used to correct one image relative to another to make a precise spatial match. In addition, in order to ensure that the two cameras capture the two species at the same moment, a DG535 was used to regulate the delay time. The delay time of the two cameras relative to the laser was both 0 ns.
3. Results and discussion
3.1 Spectral measurements
Figure 3 is an emission spectrum of the femtosecond laser-induced plasma in a CH4/air mixture with the equivalence ratio of 1.0. The spectrum was the average of 20000 laser shots. For the camera that recorded the spectra, the delay time of its gate opening relative to the laser was 0 ns and its gate width was 50 ns. The top part of Fig. 3 is the spatially resolved spectral graph, whose abscissa and ordinate represent the wavelength and spatial position, respectively. In the spectral graph, the broadband emission around 800 nm comes from laser stray light. In quantitative analysis of the spectra, we firstly evaluate the contribution at 777 nm from the broadband emission (by Gaussian fitting of the broadband), and then eliminated this background. The bottom part of Fig. 3 is the spectral curve obtained from the accumulation of the spectral graph.
Fig. 3 FLIPS spectrum in non-reacting premixed CH4/air mixture with equivalence ratio of 1.0. The top part is the spatially resolved spectral graph whose abscissa and ordinate represent the wavelength and spatial position, respectively. The bottom part is the spectral curve obtained from the accumulation of the spectral graph.
From the spectral graph in Fig. 3, we found that the length of the plasma channel is ~15 mm under these experimental conditions. In the plasma channel, the spatial distribution of each spectral component is very uniform, which shows that the FLIPS technique has the potential for one-dimensional measurements. As can be seen from the spectrum, the O I line at ~777 nm stands out. Besides, another strong atomic spectral line is the H I at 656 nm. There are also molecular bands in the spectrum, such as N2 at 337 nm and 357 nm, CN at 388 nm, CH at 314 nm, 390 nm, 431 nm and C2 at 516.5 nm and 563.5 nm.
It is noticeable that there is no Bremsstrahlung interference in FLIPS. Bremsstrahlung phenomenon is a main interference in ns-LIBS [26]. In FLIPS (Fig. 3), however, it is not an issue anymore. This is due to the extremely short pulse duration of femtosecond laser, which is largely shorter than the electron-molecule collision timescale (on the order of picoseconds) [49–51]. This indicates that FLIPS technique has an advantage over ns-LIBS in terms of equivalence ratio measurements.
To further explore the feasibility of FLIPS for one-dimensional quantitative equivalence ratio measurements, systematic experiments were carried out in CH4/air mixtures with equivalence ratio ranging from 0.6 to 1.6. In addition, the quantitative analysis of the atomics/molecules spectral lines that represent air (O, N2) and fuel (CH) was performed, and the results at different positions of the plasma channel were plotted, as shown in Fig. 4. The scatters in Fig. 4 are the experimental data of the spectral peak area ratios (CH/N2-337 nm, CH/N2-357 nm, CH/O) at different positions of the plasma channel (as indicated by P1-P4 in the inset) with different equivalence ratios. The solid lines in Figs. 4(a)-4(c) are the linear fittings of the relevant experimental data, and the dashed lines in Figs. 4(e)-4(g) are the linear fittings of all experimental data regardless of their positions using least-squares method.
Fig. 4 The relationship between the spectral peak area ratios of CH/N2 (337 nm), CH/N2 (357 nm) and CH/O and the equivalence ratio at four positions (labelled as P1-P4 in the inset) of the plasma. The scatters are the experimental data P1 (square), P2 (circle), P3 (triangle), P4 (diamond). The solid lines in (a)-(c) are the linear fittings of the relevant experimental data and the dashed lines in (d)-(f) are the linear fitting of all experimental data regardless of their positions using the least-squares method.
In Fig. 4, it can be seen that the peak area ratio (CH/N2-337 nm, CH/N2-357 nm or CH/O) and the equivalence ratio shows an excellent linear relationship, which indicates that by measuring one of these peak area ratios, its equivalence ratio could be figured out. As can be seen from Figs. 4(a)-4(c), the results at four different positions overlap very well, indicating that FLIPS can be used to achieve one-dimensional measurements. By comparing the calibration curves in Figs. 4(d)-4(f), we found that the slope of CH/N2-357 nm is the highest (6.84). Since a higher slope indicates a higher sensitivity, measuring with CH/N2 is more sensitive. In addition, the R2 of CH/N2-357 nm is also the best (0.993). This demonstrates that the accuracy of the equivalence ratio measurements based on CH/N2-357 nm is the best among others.
Although the slope and R2 of CH/O are not the best, we found that the signal intensities of CH and O are the strongest, which is crucial for instantaneous one-dimensional measurements in turbulent flow fields. To visualize equivalence ratio along the plasma channel, we would hereafter only focus on CH/O calibration.
3.2. Calibration
In order to achieve one-dimensional measurements, the spectrometer was replaced by two ICCD cameras to imaging the CH emission and the O I emission from the plasma channel, respectively. The calibration process was performed based on the images taken by the two cameras. For this part, the broadband emission around 800 nm was not filtered out, but its impact has been taken into consideration during the calibration process. The results are show in Fig. 5.
Fig. 5 Calibration curve for the ratio of CH/O as a function of equivalence ratio obtained by two ICCD cameras. The scatters are the experimental data at four different positions of the plasma channel (as indicated in the inset): P1 (square), P2 (circle), P3 (triangle), P4 (diamond). The solid lines in (a) are the linear fittings of the relevant experimental data, and the dashed line in (b) is the linear fitting of all experimental data regardless of their positions using least-squares method.
The scatters in Fig. 5 are the experimental data at different positions along the plasma channel and with different equivalence ratios, and every point was based on an average of 20 single-shots. The four solid lines in Fig. 5(a) correspond to the calibration curves at four positions, respectively. It is found that the calibration curves obtained at different positions overlap with each other very well, which is consistent with the results obtained by the spectrometer. The dashed line in Fig. 5(b) is the overall calibration curve using all data points regardless of their positions. The slope and R2 are 4.21 and 0.983, respectively. The horizontal error bar represents the error of the equivalence ratio due to the factors such as the uncertainty of the gas flow controllers. This uncertainty is in the range of 0.8% to 3.9%. The vertical error bar represents the standard deviation of 20 single-shots, ranging from 2.5% to 9.6%. Figure 5(b) indicates that based on the CH/O ratio, FLIPS technique can realize the instantaneous one-dimensional quantitative equivalence ratio measurements.
3.3. Instantaneous one-dimensional equivalence ratio measurements
At last, FLIPS technique was performed in both laminar and turbulent flow fields for instantaneous one-dimensional equivalence ratio measurements based on the calibration curve shown in Fig. 5. The measurement results are shown in Fig. 6. The images in Fig. 6(a) are the single-shots of the CH emission and the O I emission in the plasma channel under laminar conditions. The curve of Fig. 6(a) is the one-dimensional distribution of equivalence ratio calculated based on the two images above. The signal-noise-ratios (SNRs) of the CH image and the O image are 13 and 10, respectively. The spatial resolution of the image is ~35 μm, which is also the horizontal spatial resolution of FLIPS. The diameter of the plasma channel is ~150 μm, which is the vertical spatial resolution of FLIPS. The uncertainty in the equivalence ratio, ranging from 4.1% to 9.5%, mainly comes from the calibration curve (Fig. 5(b)), including the uncertainty in gas flow controllers and that the calibration curves at different positions of the plasma channel (Fig. 5(a)) are not perfectly overlapped. From Fig. 6(a), under laminar conditions, the equivalence ratio at the exit of the jet shows an axisymmetric distribution (increases first and then decreases).
Fig. 6 One-dimensional equivalence ratio distribution in laminar (a) and turbulent (b) flow field. The images are the single shots of CH emission and O emission from the plasma channel.
Figure 6(b) displays the results under turbulent conditions, where the distribution of equivalence ratio in the flow field is fluctuant wildly, and Fig. 6(b) is a typical result. The SNRs of the CH image and the O image are 8 and 6, respectively. The quantitative equivalence ratio distribution is shown as the curve in Fig. 6(b) with its measurement uncertainty ranging from 4.3% to 7.0%. The results in Fig. 6 demonstrate that FLIPS technique has the capability of instantaneous one-dimensional measurements and is suitable for turbulent conditions.
4. Conclusion
In conclusion, femtosecond laser-induced plasma spectroscopy (FLIPS) technique was performed to realize instantaneous one-dimensional equivalence ratio measurements in a non-reacting premixed CH4/air free jet. When the femtosecond laser was focused in the room-temperature CH4/air mixtures, a plasma channel with length of ~15 mm was formed. The spectral emission from the plasma channel was observed, and we found that several spectral peak area ratios (CH/N2-337 nm, CH/N2-357 nm and CH/O) are well correlated with the equivalence ratio and could be used to measure it. Furthermore, we found that CH emission and O emission from the plasma channel are the strongest among other pairs, and hence, they could be visualized on a single-shot basis with two ICCD cameras. Based on the single-shot images of the CH emission and the O I emission, we demonstrated the instantaneous one-dimensional equivalence ratio measurements in both laminar and turbulent flow fields.
The main advantages of FLIPS are as follows. Firstly, FLIPS can achieve instantaneous one-dimensional measurement with simple experimental setup. Secondly, the spatial resolution of FLIPS is ~150 μm, which is roughly one order of magnitude lower than that of ns-LIBS. The last but not the least, compared with traditional ns-LIBS, the FLIPS technique does not suffer from Bremsstrahlung interference. Future work is to apply FLIPS in combustion and hot flows, where FLIPS might be a promising tool for mixture fraction measurements.
Funding
National Natural Science Foundation of China (NSFC) (91741205, 51776137).
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