| 1. |
- Gao, Qiang, et al.
(author)
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Femtosecond-laser electronic-excitation tagging velocimetry using a 267 nm laser
- 2019
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In: Sensors and Actuators, A: Physical. - : Elsevier BV. - 0924-4247. ; 287, s. 138-142
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Journal article (peer-reviewed)abstract
- Femtosecond (fs)-laser electronic-excitation tagging velocimetry (FLEET) in a nitrogen flow field using a 267 nm laser was performed under the condition of fs-laser filamentation. The filamentous properties and their effects on velocity measurements were investigated and were compared with those of an 800 nm fs-laser. The results show that the required energy of the 267 nm laser pulse is as low as hundreds of μJ, and this is beneficial for reducing the potential perturbations to the flow flied. The filaments induced by the 267 nm laser are longer and thinner than are those induced by the 800 nm laser, which enlarges the velocity measurements region, and a precision of 1.3% was achieved.
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| 2. |
- Han, Lei, et al.
(author)
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Deep Neural Network-Based Generation of Planar CH Distribution through Flame Chemiluminescence in Premixed Turbulent Flame
- 2023
-
In: Energy and AI. - : Elsevier BV. - 2666-5468. ; 12
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Journal article (peer-reviewed)abstract
- Flame front structure is one of the most fundamental characteristics and, hence, vital for understanding combustion processes. Measuring flame front structure in turbulent flames usually needs laser-based diagnostic techniques, mostly planar laser-induced fluorescence (PLIF). The equipment of PLIF, burdened with lasers, is often too sophisticated to be configured in harsh environments. Here, to shed the burden, we propose a deep neural network-based method to generate the structures of flame fronts using line-of-sight CH* chemiluminescence that can be obtained without the use of lasers. A conditional generative adversarial network (C-GAN) was trained by simultaneously recording CH-PLIF and chemiluminescence images of turbulent premixed methane/air flames. Two distinct generators of the C-GAN, namely Resnet and U-net, were evaluated. The former net performs better in this study in terms of both generating snap-shot images and statistics over multiple images. For chemiluminescence imaging, the selection of the camera's gate width produces a trade-off between the signal-to-noise (SNR) ratio and the temporal resolution. The trained C-GAN model can generate CH-PLIF images from the chemiluminescence images with an accuracy of over 91% at a Reynolds number of 5000, and the flame surface density at a higher Reynolds number of 10,000 can also be effectively estimated by the model. This new method has the potential to achieve the flame characteristics without the use of laser and significantly simplify the diagnosing system, also with the potential for high-speed flame diagnostics.
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| 3. |
- Han, Lei, et al.
(author)
-
Simultaneous measurements of velocity and concentration of gas flow using femtosecond laser-induced chemiluminescence
- 2022
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In: Optics and Lasers in Engineering. - : Elsevier BV. - 0143-8166. ; 155
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Journal article (peer-reviewed)abstract
- The mixing process in a gas flow plays a crucial role in chemical reactions, and simultaneous measurements of both velocity and mixture fraction are desired to fathom the process. Here, we report a scheme for simultaneously measuring both velocity and concentration by femtosecond laser-induced chemiluminescence. The femtosecond laser would induce chemical reactions that generate CN radicals in the B state. The transition of CN (X-B) would emit fluorescence with both strong intensity and long duration, and the decay of the fluorescence versus time showed prominent benefits for the simultaneous measurement. This measurement was accomplished by an ICCD camera worked in the on-chip multi-exposure mode, i.e., the camera had two exposures in succession to capture two luminescent lines on one image. The first line was used to measure the methane concentration and hence, the mixture fraction through a calibration procedure. The second line was the first line displaced by the flow in a known time interval and we demonstrate an algorithm to derive the one dimensional-two components velocity fields from the line shapes of the luminescent lines. The detection limit of the concentration is estimated to be 152 ppm and the minimum measurable velocity is estimated to be 5 m/s.
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| 4. |
- Li, Bo, et al.
(author)
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A review of femtosecond laser-induced emission techniques for combustion and flow field diagnostics
- 2019
-
In: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 9:9
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Research review (peer-reviewed)abstract
- The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers.
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| 5. |
- Li, Bo, et al.
(author)
-
Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence
- 2017
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In: Optics Express. - 1094-4087. ; 25:21, s. 25809-25818
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Journal article (peer-reviewed)abstract
- We demonstrate a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for sensitive CO detection, using a 230 nm pulse of 9 µJ and 45 fs. The advantages of fs-TPLIF in excitation of molecular species were analyzed. Spectra of CO fs-TPLIF were recorded in stable laminar flames spatially resolved across the flame front. A hot band (1, n) together with the conventional band (0, n) of the B→A transitions were observed in the burned zone and attributed to the broadband nature of the fs excitation. The CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite of the steep laser intensity variation, which is beneficial for CO imaging in contrast to nanosecond and picosecond TPLIF. This phenomenon can be explained by photoionization, which over the short pulse duration dominates the population depletion of the excited B state due to the high peak power, but only contributes in total a negligible X state depletion due to the low pulse energy. Single-shot CO fs-TPLIF images in methane/air flames were recorded by imaging the broadband fluorescence. The results indicate that fs-TPLIF is a promising tool for CO imaging in flames.
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| 6. |
- Li, Bo, et al.
(author)
-
Femtosecond laser-induced cyano chemiluminescence in methane-seeded nitrogen gas flows for near-wall velocimetry
- 2018
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In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:29
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Journal article (peer-reviewed)abstract
- We demonstrate a velocimetric technique based on femtosecond laser-induced cyano (CN) chemiluminescence (FLICC). High intensity emission originated from CN(B-X) fluorescence was observed in filaments generated by focusing a femtosecond laser in methane-seeded nitrogen gas flows. The emission is strong and can last for hundreds of microseconds with a proper methane concentration. FLICC was adopted for velocity measurements, and promising results were obtained for near-wall measurements.
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| 7. |
- Li, Bo, et al.
(author)
-
Filamentary anemometry using femtosecond laser-extended electric discharge - FALED
- 2018
-
In: Optics Express. - 1094-4087. ; 26:16, s. 21132-21140
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Journal article (peer-reviewed)abstract
- We demonstrate a non-contact spatiotemporally resolved comprehensive method for gas flow velocity field measurement: Filamentary Anemometry using femtosecond Laser-extended Electric Discharge (FALED). A faint thin plasma channel was generated in ambient air by focusing an 800-nm laser beam of 45 fs, which was used to ignite a pulsed electric discharge between two electrodes separated over 10 mm. The power supplier provided a maximum voltage up to 5 kV and was operated at a burst mode with a current duration of less than 20 ns and a pulse-to-pulse separation of 40 μs. The laser-guided thin filamentary discharge plasma column was blowing up perpendicularly by an air jet placed beneath in-between the two electrodes. Although the discharge pulse was short, the conductivity of the plasma channel was observed to sustain much longer, so that a sequence of discharge filaments was generated as the plasma channel being blown up by the jet flow. The sequential bright thin discharge filaments can be photographed using a household camera to calculate the flow velocity distribution of the jet flow. For a direct comparison, a flow field measurement using FLEET [Michael, Appl. Opt. 50, 5158 (2011)] was also performed. The results indicate that the FALED technique can provide instantaneous nonintrusive flow field velocity measurement with good accuracy.
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| 8. |
- Li, Bo, et al.
(author)
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One-dimensional full-range mixture fraction measurements with femtosecond laser-induced plasma spectroscopy
- 2020
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In: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 61:2
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Journal article (peer-reviewed)abstract
- Abstract: Femtosecond laser-induced plasma spectroscopy (FLIPS) was performed to achieve full-range mixture fraction measurements in non-reacting CH4/air flow fields. A femtosecond laser at 800 nm was used to generate a plasma channel with a uniform intensity distribution. Through measuring spatially resolved spectra and calibration, we found that the spectral intensity ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), C2 (516.5 nm)/N2 (337 nm), C2 (516.5 nm)/N2 (357 nm) and CH (431 nm)/O (777 nm) could be used to realize mixture fraction measurements, and the first four intensity ratios can achieve full-range mixture fraction measurements. Furthermore, through quantitative analysis of the distribution along the plasma channel, we analyzed the one-dimensional measurement capability of FLIPS. The main advantages of FLIPS for mixture fraction measurements are one-dimensional quantitative measurement, full-range measurement, high spatial resolution and no Bremsstrahlung interference. Graphic abstract: [Figure not available: see fulltext.].
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| 9. |
- Li, Bo, et al.
(author)
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Strategy of interference-free atomic hydrogen detection in flames using femtosecond multi-photon laser-induced fluorescence
- 2017
-
In: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 42:6, s. 3876-3880
-
Journal article (peer-reviewed)abstract
- Hydrogen atoms are key species in combustion of hydrogen/hydrocarbon fuels. Interference-free detection of hydrogen atoms natively generated in flames using femtosecond laser-induced fluorescence (LIF) was investigated employing two colors, i.e., 243 nm and 486 nm, as excitation source: two-photon excitation followed by a relay one-photon excitation. This strategy was compared with another commonly adopted two-photon LIF strategy using 205 nm for excitation. The potential interferences were investigated, and a direct verification method was proposed to prove this strategy be interference-free, and imaging of hydrogen atoms natively generated in methane/air flames was achieved.
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| 10. |
- Li, Bo, et al.
(author)
-
Temperature measurements in heated gases and flames using carbon monoxide femtosecond two-photon laser-induced fluorescence
- 2023
-
In: Sensors and Actuators A: Physical. - : Elsevier BV. - 0924-4247. ; 353
-
Journal article (peer-reviewed)abstract
- Nonintrusive temperature measurement is crucial in combustion research. Here, we propose a thermometric technique based on femtosecond two-photon laser-induced fluorescence of carbon monoxide (CO-fs-TPLIF). A femtosecond laser with a wavelength of 230 nm was used as an excitation source. Owing to its intrinsic broad bandwidth, dual vibrational bands of the B1Σ+ ← X1Σ+ transition of CO can be simultaneously excited. As a result, the fluorescence from the conventional bands (0,n) and the hot vibrational bands (1,n) of the B1Σ+ → A1Πu transition of CO can be simultaneously detected. Hence, the temperature-dependent Boltzmann distribution can be assessed from the relative fluorescence intensity related to different ro-vibrational states, and the temperature can be extracted from the analysis of the recorded fluorescence spectra. Two temperature calibration methods were developed, for the low-temperature range (298–1173 K), the rotational-state-associated bandwidths of the spectra were used; for the flame temperature range, the spectral intensity ratios between the hot vibrational bands (1-n) and the conventional bands (0-n) were used. The CO-fs-TPLIF thermometric technique features the advantages of a simple optical setup and the ability of one-dimensional measurements with high spatial resolution.
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