| 1. |
- Zhu, Zhifeng, et al.
(författare)
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Spatiotemporal control of femtosecond laser filament-triggered discharge and its application in diagnosing gas flow fields
- 2022
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Ingår i: Plasma Science and Technology. - : IOP Publishing. - 1009-0630. ; 24:2
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Tidskriftsartikel (refereegranskat)abstract
- Precise control of the discharge in space and time is of great significance for better applications of discharge plasma. Here, we used a femtosecond laser filament to trigger and guide a high-voltage DC pulse discharge to achieve spatiotemporal control of the discharge plasma. In space, the discharge plasma is distributed strictly along the channel generated by the femtosecond laser filament. The breakdown voltage threshold is reduced, and the discharge length is extended. In time, the electrical parameters such as the electrode voltage and the electrode gap affect discharge delay time and jitter. By optimizing the parameters, we can achieve sub-nanosecond jitter of the discharge. Based on the spatiotemporal control of the discharge, we applied filament-triggered discharge for one-dimensional composition measurements of the gas flow field. Besides, the technique shows great potential in studying the spatiotemporal evolution of discharge plasma.
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| 2. |
- Cai, Zun, et al.
(författare)
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Ignition processes and modes excited by laser-induced plasma in a cavity-based supersonic combustor
- 2018
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 228, s. 1777-1782
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Tidskriftsartikel (refereegranskat)abstract
- The ignition processes in an ethylene-fueled supersonic combustor excited by laser-induced plasma (LIP) were investigated experimentally under the condition of inflow Ma number of 2.92. The LIP excitation was implemented near the center floor of a flameholding cavity. Optical measurements, including simultaneous CH∗/OH∗ chemiluminescence imaging and Schlieren photography, were used to investigate the ignition processes. It is found that the CH∗ and OH∗ initiated by the LIP are mainly in the region between the cavity front wall and the LIP excitation site. The CH∗ and OH∗ are quenched rapidly, in particular at a low fueling rate. After a short delay, the distribution of CH∗ and OH∗ appears in the region between the rear wall of the cavity and the LIP excitation site, showing the onset of ignition therein. A stable flame was established in the shear layer between the downstream part of the cavity and the outer supersonic flow. It is concluded that the ignition processes excited by the LIP can be divided into a LIP initiation regime and a transient ignition reaction regime. Both the fueling rate and the LIP energy significantly affect the cavity ignition processes. Increasing the fueling rate or the laser energy can shorten the ignition processes in the cavity. A weak ignition mode and an intense ignition mode are postulated to explain the combustion behavior of the ignition processes in the cavity-based supersonic combustor.
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| 3. |
- Ehn, Andreas, et al.
(författare)
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Advanced laser-based techniques for gas-phase diagnostics in combustion and aerospace engineering
- 2017
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 71:3, s. 341-366
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Forskningsöversikt (refereegranskat)abstract
- Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.
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| 4. |
- Ehn, Andreas, et al.
(författare)
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Plasma assisted combustion: Effects of O3 on large scale turbulent combustion studied with laser diagnostics and Large Eddy Simulations
- 2015
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35:3, s. 3487-3495
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract In plasma-assisted combustion, electric energy is added to the flame where the electric energy will be transferred to kinetic energy of the free electrons that, in turn, will modify the combustion chemical kinetics. In order to increase the understanding of this complex process, the influence of one of the products of the altered chemical kinetics, ozone (O3), has been isolated and studied. This paper reports on studies using a low-swirl methane (CH4) air flame at lean conditions with different concentrations of O3 enrichment. The experimental flame diagnostics include Planar Laser Induced Fluorescence (PLIF) imaging of hydroxyl (OH) and formaldehyde (CH2O). The experiments are also modeled using Large Eddy Simulations (LES) with a reaction model based on a skeletal CH4-air reaction mechanism combined with an O3 sub-mechanism to include the presence of O3 in the flame. This reaction mechanism is based on fundamental considerations including reactions between O3 and all other species involved. The experiments reveal an increase in CH2O in the low-swirl flame as small amounts of O3 is supplied to the CH4-air stream upstream of the flame. This increase is well predicted by the LES computations and the relative radical concentration shift is in good agreement with experimental data. Simulations also reveal that the O3 enrichment increase the laminar flame speed, su, with ∼10% and the extinction strain-rate, Ïext, with ∼20%, for 0.57% (by volume) O3. The increase in Ïext enables the O3 seeded flame to burn under more turbulent conditions than would be possible without O3 enrichment. Sensitivity analysis indicates that the increase in Ïext due to O3 enrichment is primarily due to the accelerated chain-branching reactions H 2 + O â OH + H , H 2 O + O â OH + OH and H + O 2 â OH + O . Furthermore, the increase in CH2O observed in both experiments and simulations suggest a significant acceleration of the chain-propagation reaction CH 3 + O â CH 2 O + H .
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| 5. |
- Ehn, Andreas, et al.
(författare)
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Setup for microwave stimulation of a turbulent low-swirl flame
- 2016
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 49:18
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Tidskriftsartikel (refereegranskat)abstract
- An experimental setup for microwave stimulation of a turbulent flame is presented. A low-swirl flame is being exposed to continuous microwave irradiation inside an aluminum cavity. The cavity is designed with inlets for laser beams and a viewport for optical access. The aluminum cavity is operated as a resonator where the microwave mode pattern is matched to the position of the flame. Two metal meshes are working as endplates in the resonator, one at the bottom and the other at the top. The lower mesh is located right above the burner nozzle so that the low-swirl flame is able to freely propagate inside the cylinder cavity geometry whereas the upper metal mesh can be tuned to achieve good overlap between the microwave mode pattern and the flame volume. The flow is characterized for operating conditions without microwave irradiation using particle imaging velocimetry (PIV). Microwave absorption is simultaneously monitored with experimental investigations of the flame in terms of exhaust gas temperature, flame chemiluminescence (CL) analysis as well as simultaneous planar laser-induced fluorescence (PLIF) measurements of formaldehyde (CH2O) and hydroxyl radicals (OH). Results are presented for experiments conducted in two different regimes of microwave power. In the high-energy regime the microwave field is strong enough to cause a breakdown in the flame. The breakdown spark develops into a swirl-stabilized plasma due to the continuous microwave stimulation. In the low-energy regime, which is below plasma formation, the flame becomes larger and more stable and it moves upstream closer to the burner nozzle when microwaves are absorbed by the flame. As a result of a larger flame the exhaust gas temperature, flame CL and OH PLIF signals are increased as microwave energy is absorbed by the flame.
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| 6. |
- Gao, Jinlong, et al.
(författare)
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In-Situ Non-intrusive Diagnostics of Toluene Removal by a Gliding Arc Discharge Using Planar Laser-Induced Fluorescence
- 2017
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Ingår i: Plasma Chemistry and Plasma Processing. - : Springer Science and Business Media LLC. - 0272-4324 .- 1572-8986. ; 37:2, s. 433-450
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Tidskriftsartikel (refereegranskat)abstract
- A non-equilibrium gliding arc discharge anchored on two diverging stainless steel electrodes was extended into open air by a toluene-containing air jet. The removal process of the toluene by the non-equilibrium gliding arc discharge was investigated through in situ and non-intrusive laser-based techniques. Simultaneous planar laser-induced fluorescence (PLIF) of toluene and OH radicals were employed to achieve on-line visualization of the toluene decomposing process by the gliding arc discharge column. Toluene PLIF images with high spatial and temporal resolution showed that the non-equilibrium plasma of the gliding arc discharge is effective in decomposing toluene molecules. Instantaneous toluene removal efficiency was estimated from the toluene PLIF images, showing that the initial toluene concentrations and oxygen concentrations affected the toluene removal efficiency. The toluene removal efficiency decreased with the initial toluene concentration, whereas the efficiency increased with the oxygen concentration. The OH generation in the discharge was found to be enhanced with an increase of the toluene concentration from the OH PLIF results. The relative instantaneous distribution between the OH produced from the discharge channels and the toluene flow was simultaneously visualized. The instantaneous distributions of toluene and OH radicals that were acquired simultaneously by PLIF, were well complementary, suggesting that radicals generated by the gliding arc discharge were responsible for toluene removal in the active volume of the gliding arc discharge. The effective width of the plasma volume for the toluene removal were measured, which gives a new insight into the optimization of industrial design for practical gliding arc reactors.
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| 7. |
- Gao, Jinlong, et al.
(författare)
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Visualization of instantaneous structure and dynamics of large-scale turbulent flames stabilized by a gliding arc discharge
- 2019
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:4, s. 5629-5636
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Tidskriftsartikel (refereegranskat)abstract
- A burner design with integrated electrodes was used to couple a gliding arc (GA) discharge to a high-power and large-scale turbulent flame for flame stabilization. Simultaneous OH and CH2O planar laser-induced fluorescence (PLIF) and CH PLIF measurements were conducted to visualize instantaneous structures of the GA-assisted flame. Six different regions of the GA-assisted flame were resolved by the multi-species PLIF measurements, including the plasma core, the discharge-induced OH region, the post-flame OH region, the flame front, the preheat CH2O region and the fresh gas mixture. Specifically, the OH profile was observed to be ring-shaped around the gliding arc discharge channel. The formaldehyde (CH2O) was found to be widely distributed in the entire measurement volume even at a low equivalence ratio of 0.4, which suggest that long-lived species from the gliding arc discharge have induced low-temperature oxidations of CH4. The CH layer coincides with the interface of the OH and CH2O regions and indicates that the flame front and the discharge channel are spatially separated by a distance of 3-5 mm. These results reveal that the discharge column acts as a movable pilot flame, providing active radicals and thermal energy to sustain the flame. High-speed video photography was also employed to record the dynamics of the GA-assisted flame. This temporally resolved data was used to study the ignition and propagation behaviors of the flame in response to a temporally modulated burst-mode discharge. The results indicate that turbulent flame can be sustained by matching temporal parameters of the high-voltage bursts to the extinction time of flame.
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| 8. |
- He, Yong, et al.
(författare)
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In-situ Measurement of Sodium and Potassium Release during Oxy-Fuel Combustion of Lignite using Laser-Induced Breakdown Spectroscopy: Effects of O-2 and CO2 Concentration
- 2013
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:2, s. 1123-1130
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced breakdown spectroscopy (LIBS) was used in this study to measure quantitatively the sodium (Na) and potassium (K) release from burning coal particles under oxy-fuel combustion environments. A specially designed laminar premixed burner was employed to provide a postflame environment with different O-2 and CO2 concentrations, in which the effects of O-2 and CO2 on the release of Na and K during coal oxy-fuel combustion were studied systematically. For the devolatilization stage, neither O-2 nor CO2 had significant influence on the Na and K release. The release of Na and K during the char stage, however, changed significantly at different O-2 and CO2 concentrations. Under these experimental conditions, when the O-2 concentration increased from 3.9% to 10.6%, the peak concentration of Na at the char stage increased from 15.2 mg/m(3) to 33.7 mg/m(3), and the maximum concentration of K increased from 6.2 mg/m(3) to 11.7 mg/m(3). When the CO2 concentration increased from 35.8% to 69.4%, the release of Na and K was inhibited during the char stage, with the peak concentration decreasing from 8.9 mg/m(3) to 6.9 mg/m(3) for Na and from 3.7 mg/m(3) to 2.4 mg/m(3) for K. During the ash stage, the release of Na and K decreased with the O-2 concentration, whereas it increased with the CO2 concentration.
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| 9. |
- Kong, Chengdong, et al.
(författare)
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Characteristics of a Gliding Arc Discharge Under the Influence of a Laminar Premixed Flame
- 2019
-
Ingår i: IEEE Transactions on Plasma Science. - 0093-3813. ; 47:1, s. 403-409
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Tidskriftsartikel (refereegranskat)abstract
- The effect of combustion on a gliding arc (GA) discharge is investigated using simultaneous measurements of current and voltage waveforms, as well as imaging and spectroscopic analysis of plasma and flame luminescence. Attributed to the existence of flame, the breakdown voltage and current peak are reduced and the bright sparks during breakdown are dampened. The intrinsic reason is largely owing to the thermal effect of flame. Electrical breakdown is mainly determined by the reduced electric field strength (E/N), which is inversely proportional to temperature. Assuming a constant E/N for breakdown, the combustion-induced temperature increment gives rise to a reduction of the breakdown voltage. The gas composition seems to have less impact on the breakdown voltage. However, the addition of CH₄ can induce more radicals (e.g., H atoms) that enhance the intensity of relevant spectral emissions, especially from OH*. Due to the transport of relatively long-lived radicals, the width of the plasma column of the GA discharge is broadened to form a local reaction zone, serving as a flame holder. Interestingly, the plasma channel moves more smoothly as the flame is present. It implies that the flow field is less turbulent owing to combustion.
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| 10. |
- Kong, Chengdong, et al.
(författare)
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Characterization of an AC glow-type gliding arc discharge in atmospheric air with a current-voltage lumped model
- 2017
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 24:9
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative characterization of a high-power glow-mode gliding arc (GM-GA) discharge operated in open air is performed using a current-voltage lumped model that is built from the perspective of energy balance and electron conservation. The GM-GA discharge is powered by a 35 kHz alternating current power supply. Instantaneous images of the discharge volume are recorded using a high-speed camera at a frame rate of 50 kHz, synchronized with the simultaneously recorded current and voltage waveforms. Detailed analyzation indicates that the electrical input power is dissipated mainly through the transport of vibrationally excited nitrogen and other active radicals (such as O). The plasma is quite non-thermal with the ratio of vibrational and translational temperatures (Tv/Tg) larger than 2 due to the intense energy dissipation. The electron number density reaches 3 × 1019 m-3 and is always above the steady value owing to the short cutting events, which can recover the electron density to a relatively large value and limits the maximum length of the gliding arc. The slow decaying rate of electrons is probably attributed to the decomposed state of a hot gaseous mixture and the related associative ionization.
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| 11. |
- Kong, Chengdong, et al.
(författare)
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Effect of turbulent flow on an atmospheric-pressure AC powered gliding arc discharge
- 2018
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:22
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Tidskriftsartikel (refereegranskat)abstract
- A high-power gliding arc (GA) discharge was generated in a turbulent air flow driven by a 35 kHz alternating current electric power supply. The effects of the flow rate on the characteristics of the GA discharge were investigated using combined optical and electrical diagnostics. Phenomenologically, the GA discharge exhibits two types of discharge, i.e., glow type and spark type, depending on the flow rates and input powers. The glow-type discharge, which has peak currents of hundreds of milliamperes, is sustained at low flow rates. The spark-type discharge, which is characterized by a sharp current spike of several amperes with duration of less than 1 μs, occurs more frequently as the flow rate increases. Higher input power can suppress spark-type discharges in moderate turbulence, but this effect becomes weak under high turbulent conditions. Physically, the transition between glow- and spark-type is initiated by the short cutting events and the local re-ignition events. Short cutting events occur owing to the twisting, wrinkling, and stretching of the plasma columns that are governed by the relatively large vortexes in the flow. Local re-ignition events, which are defined as re-ignition along plasma columns, are detected in strong turbulence due to increment of the impedance of the plasma column and consequently the internal electric field strength. It is suggested that the vortexes with length scales smaller than the size of the plasma can penetrate into the plasma column and promote mixing with surroundings to accelerate the energy dissipation. Therefore, the turbulent flow influences the GA discharges by ruling the short cutting events with relatively large vortexes and the local re-ignition events with small vortexes.
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| 12. |
- Kong, Chengdong, et al.
(författare)
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Re-igniting the afterglow plasma column of an AC powered gliding arc discharge in atmospheric-pressure air
- 2018
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 112:26
-
Tidskriftsartikel (refereegranskat)abstract
- The stability and re-ignition characteristics of the plasma column of an alternating current (AC) powered gliding arc discharge operating in atmospheric-pressure air were investigated for better plasma-mode controlling and optimized applications. By modulating the AC power supply and the air flow field, the states of afterglow plasma column were varied. When pulsating the AC power supply sequence, re-ignitions of the afterglow columns were introduced and their characteristics were studied using simultaneous high-speed photography and electrical measurements. Two re-ignition types were observed in the afterglow column with different decay times (the temporal separation of two sequential pulsed AC power trains). For a short decay time (<200 μs at 10 l/min air flow), the afterglow column can be recovered mildly without current spikes, which is called a glow re-ignition event. If the decay time is so long that the electric field strength becomes larger than 120 kV/m, the re-ignition event occurs with current spikes and bright emissions, which is called a spark re-ignition event. A quasi-equilibrium model is proposed to estimate the chemical compositions in the plasma column and to explain the observed phenomena. It infers that the chemical dissociation and ionization processes enhanced by vibrationally excited nitrogen molecules are dominating in the afterglow plasmas and thereby the electrons can survive a long time to keep the conductivity of the afterglow column, forming a glow re-ignition event. Whereas under large electric field strength (>120 kV/m), the electron impact ionization becomes dominant to trigger the spark re-ignition event.
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| 13. |
- Kristensson, Elias, et al.
(författare)
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Stray light suppression in spectroscopy using periodic shadowing
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:7, s. 7711-7721
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that spectroscopic measurements suffer from an interference known as stray light, causing spectral distortion that reduces measurement accuracy. In severe situations, stray light may even obscure the existence of spectral lines. Here a novel general method is presented, named Periodic Shadowing, that enables effective stray light elimination in spectroscopy and experimental results are provided to demonstrate its capabilities and versatility. Besides its efficiency, implementing it in a spectroscopic arrangement comes at virtually no added experimental complexity. (C) 2014 Optical Society of America
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| 14. |
- Kristensson, Elias, et al.
(författare)
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Stray light suppression in spectroscopy using periodic shadowing
- 2014
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Ingår i: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 22:7, s. 7711-7711
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that spectroscopic measurements suffer from an interference known as stray light, causing spectral distortion that reduces measurement accuracy. In severe situations, stray light may even obscure the existence of spectral lines. Here a novel general method is presented, named Periodic Shadowing, that enables effective stray light elimination in spectroscopy and experimental results are provided to demonstrate its capabilities and versatility. Besides its efficiency, implementing it in a spectroscopic arrangement comes at virtually no added experimental complexity.
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| 15. |
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| 16. |
- Kusano, Yukihiro, et al.
(författare)
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Stability of alternating current gliding arcs
- 2014
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Ingår i: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics. - : Springer Science and Business Media LLC. - 1434-6060. ; 68:10, s. 9-319
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 17. |
- Kusano, Yukihiro, et al.
(författare)
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Water-cooled non-thermal gliding arc for adhesion improvement of glass-fibre-reinforced polyester
- 2013
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 46:13
-
Tidskriftsartikel (refereegranskat)abstract
- A non-equilibrium quenched plasma is prepared using a gliding-arc discharge generated between diverging electrodes and extended by a gas flow. It can be operated at atmospheric pressure and applied to plasma surface treatment to improve adhesion properties of material surfaces. In this work, glass-fibre-reinforced polyester plates were treated using an atmospheric pressure gliding-arc discharge with air flow to improve adhesion with a vinylester adhesive. The electrodes were water-cooled so as to operate the gliding arc continually. The treatment improved wettability and increased the density of oxygen-containing polar functional groups on the surfaces. Double cantilever beam specimens were prepared for fracture mechanic characterization of the laminate adhesive interface. It was found that gliding-arc treatment significantly increases the fracture resistance in comparison with a standard peel-ply treatment.
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| 18. |
- Lantz, Andreas, et al.
(författare)
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Investigation of ozone stimulated combustion in the SGT-800 burner at atmospheric conditions
- 2016
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Ingår i: Combustion, Fuels and Emissions. - 9780791849750 ; 4A-2016
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Konferensbidrag (refereegranskat)abstract
- The effect of ozone (O3) in a turbulent, swirl-stabilized natural gas/air flame was experimentally investigated at atmospheric pressure conditions using planar laser-induced fluorescence imaging of formaldehyde (CH2O PLIF) and dynamic pressure monitoring. The experiment was performed using a dry low emission (DLE) gas turbine burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. CH2O PLIF imaging was carried out for four different seeding gas compositions and seeding injection channel configurations. Two seeding injection-channels were located around the burner tip while the other two were located along the center axis of the burner at different distances upstream the burner outlet. Four different seeding gas compositions were used: nitrogen (N2), oxygen (O2) and two ozone/oxygen (O3/O2) mixtures with different O3 concentration. The results show that the O3 clearly affects the combustion chemistry. The natural gas/air mixture is preheated before combustion which is shown to kick-start the cold combustion chemistry where O3 is highly involved. The CH2O PLIF signal increases with O3 seeded into the flame which indicates that the pre-combustion activity increases and that the cold chemistry starts to develop further upstream. The small increase of the pressure drop over the burner shows that the flame moves upstream when O3 is seeded into the flame, which confirms the increase in pre-combustion activity.
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| 19. |
- Sun, Zhiwei, et al.
(författare)
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Optical diagnostics of a gliding arc
- 2013
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Ingår i: Optics Express. - 1094-4087. ; 21:5, s. 6028-6044
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic processes in a gliding arc plasma generated between two diverging electrodes in ambient air driven by 31.25 kHz AC voltage were investigated using spatially and temporally resolved optical techniques. The life cycles of the gliding arc were tracked in fast movies using a high-speed camera with framing rates of tens to hundreds of kHz, showing details of ignition, motion, pulsation, short-cutting, and extinction of the plasma column. The ignition of a new discharge occurs before the extinction of the previous discharge. The developed, moving plasma column often short-cuts its current path triggered by Townsend breakdown between the two legs of the gliding arc. The emission from the plasma column is shown to pulsate at a frequency of 62.5 kHz, i.e., twice the frequency of the AC power supply. Optical emission spectra of the plasma radiation show the presence of excited N-2, NO and OH radicals generated in the plasma and the dependence of their relative intensities on both the distance relative to the electrodes and the phase of the driving AC power. Planar laser-induced fluorescence of the ground-state OH radicals shows high intensity outside the plasma column rather than in the center suggesting that ground-state OH is not formed in the plasma column but in its vicinity. (c) 2013 Optical Society of America
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| 20. |
- Weng, Wubin, et al.
(författare)
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Investigation of formaldehyde enhancement by ozone addition in CH4/air premixed flames
- 2015
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:4, s. 1284-1293
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Tidskriftsartikel (refereegranskat)abstract
- Abstract The ozone (O3) decomposition in the pre-heat zone of flames can initiate and accelerate the chain-branching reactions. In the present study, formaldehyde (CH2O) was investigated by both experiment and simulation methods in methane/air laminar premixed flames under atmospheric conditions. The formaldehyde concentration profiles in the flames were measured with CH2O-PLIF. When 4500 ppm of ozone was added, the formaldehyde concentration in Bunsen type laminar flame was enhanced by 58.5% at fuel-rich condition (Ï = 1.4) and 15.5% at stoichiometric condition. In the simulation work, the most recent ozone sub-mechanism was coupled with GRI-mech 3.0 kinetic mechanism. It showed that with 4500 ppm ozone addition, the formaldehyde concentration was enhanced by about 48.1% at rich condition (Ï = 1.4) and about 14.7% in stoichiometric mixture. The simulation suggested an early production of CH2O with ozone addition, especially in rich conditions. These reactions occurred at relatively low temperature, around 500 K. In order to isolate these reactions from the flame, experiments with preheated unburned mixtures were carried out. A larger amount of formaldehyde was produced in the zone far from the flame as the preheating temperature was increased. It indicated that the combustion enhancement with ozone could be caused by the additional reactions of ozone at relatively low temperature. Simulations showed that methoxy radical (CH3O) is the key specie for production of formaldehyde at lower temperatures. Early in the pre-heat zone of the laminar flame, formaldehyde occurs via decomposition of CH3O while in the pre-heated gas mixture via reaction of CH3O with O2. Furthermore, the O3 effect on turbulent flames was investigated showing a greater enhancement in formaldehyde signal than that in the laminar cases. This difference in formaldehyde signal enhancement could be attributed to the expansion of the preheat zone, due to turbulence.
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| 21. |
- Zhu, Jiajian, et al.
(författare)
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Dynamics, OH distributions and UV emission of a gliding arc at various flow-rates investigated by optical measurements
- 2014
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 47:29
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a plasma discharge which is generated between two diverging electrodes and extended into a gliding arc in non-equilibrium condition by an air flow at atmospheric pressure. Effects of the air flow rates on the dynamics, ground-state OH distributions and spectral characterization of UV emission of the gliding arc were investigated by optical methods. High-speed photography was utilized to reveal flow-rate dependent dynamics such as ignitions, propagation, short-cutting events, extinctions and conversions of the discharge from glowtype to spark-type. Short-cutting events and ignitions occur more frequently at higher flow rates. The anchor points of the gliding arc are mostly steady at the top of the electrodes at lower flow rates whereas at higher flow rates they glide up along the electrodes most of the time. The afterglow of fully developed gliding arcs is observed to decay over hundreds of microseconds after being electronically short-cut by a newly ignited arc. The extinction time decreases with the increase of the flow rate. The frequency of the conversion of a discharge from glow-type to spark-type increases with the flow rate. Additionally, spatial distributions of ground-state OH were investigated using planar laser-induced fluorescence. The results show that the shape, height, intensity and thickness of ground-state OH distribution vary significantly with air flow rates. Finally, UV emission of the gliding arc is measured using optical emission spectroscopy and it is found that the emission intensity of NO gamma (A-X), OH (A-X) and N-2 (C-B) increase with the flow rates showing more characteristics of spark-type arcs. The observed phenomena indicate the significance of the interaction between local turbulence and the gliding arc.
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| 22. |
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| 23. |
- Zhu, Jiajian
(författare)
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Optical Diagnostics of Non-thermal Plasmas and Plasma-assisted Combustion
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-thermal plasma is regarded as a collection of free electrons, ions and neutral particles that are not at local thermodynamic equilibrium. The high-energetic electrons formed in non-thermal plasmas are capable of generating chemically active species and modifying chemical kinetics in practical applications. One promising application of non-thermal plasmas is to achieve plasma-assisted combustion, in which new reaction pathways can be generated to increase chemical reaction rates, as well as to improve combustion efficiency and reduce pollutant emission. Developing and applying optical diagnostic tools help one to understand the underlying mechanisms of non-thermal plasmas and plasma-assisted combustion. A gliding arc discharge is a simple and low-cost technique to provide non-thermal plasmas at atmospheric pressure. The plasma column of gliding arc discharges can be ignited at the narrowest gap between two diverging electrodes by a high-voltage power supply, after which it moves along the electrodes by means of gas flow. In the thesis, optical diagnostics of a gliding arc discharge were carried out with the aim in particular of being able to better understand discharge characteristics involved. High-speed photography at an exposure time of a few microseconds was used to capture the instantaneous structure of the plasma columns, and to record the spatial and temporal evolution of the gliding arc discharge. The plasma column was found to experience short-cutting events and transitions between glow-type and spark-type discharge, as well as a cycle of ignition, extension and extinction. Ground-state OH is an important chemically reactive species that can be generated by gliding arc discharges in humid air. Planar laser-induced fluorescence (PLIF) measurements demonstrated that ground-state OH was distributed around the plasma column and that the thickness of the OH was much greater than that of the plasma column. Turbulent effects played an important role in determining the OH distribution and the dynamics of the gliding arc discharge. Three-dimensional (3D) particle tracking velocimetry (PTV) and 3D reconstructions of the plasma columns were performed, providing a more accurate 3D determination of the slip velocity and the length of the gliding arc discharge. The translational temperature of the gliding arc discharge was measured by planar laser-induced Rayleigh scattering, and the electron temperature was calculated using the measured reduced electric field strength. The rotational and vibrational temperatures were determined by comparing the experimental and simulated spectra. The results are able to contribute to the optimized operation of the gliding arc discharge for practical applications and to a better understanding of the mechanisms governing non-thermal plasmas at atmospheric pressure. The effects of non-thermal plasmas on combustion were investigated experimentally using PLIF. Several plasma sources, such as gliding arc discharges, microwave discharges and products (O3) stemming from dielectric barrier discharges, were employed for stimulating premixed CH4/air flames. It was found that the gliding arc discharge was able to promote the formation of CH2O and OH in the flame. An increase in CH2O was observed by means of PLIF when a high-power-density turbulent low-swirl flame was provided with small amounts of O3. The microwave increased both the chemiluminescence of the high-power-density turbulent flame and the PLIF signals from the OH and CH2O. The distributions of the CH2O signals shifted to be closer to the burner nozzle, indicating an increase in flame speed for the turbulent flame as the O3 and the microwaves were applied to the flame.
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| 24. |
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| 25. |
- Zhu, Jiajian, et al.
(författare)
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Spatiotemporally resolved characteristics of a gliding arc discharge in a turbulent air flow at atmospheric pressure
- 2017
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 24:1
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Tidskriftsartikel (refereegranskat)abstract
- A gliding arc discharge was generated in a turbulent air flow at atmospheric pressure driven by a 35 kHz alternating current (AC) electric power. The spatiotemporally resolved characteristics of the gliding arc discharge, including glow-type discharges, spark-type discharges, short-cutting events and transitions among the different types of discharges, were investigated using simultaneously optical and electrical diagnostics. The glow-type discharge shows sinusoidal-like voltage and current waveforms with a peak current of hundreds of milliamperes. The frequency of the emission intensity variation of the glow-type discharge is the same as that of the electronic power dissipated in the plasma column. The glow-type discharge can transfer into a spark discharge characterized by a sharp peak current of several amperes and a sudden increase of the brightness in the plasma column. Transitions can also be found to take place from spark-type discharges to glow-type discharges. Short-cutting events were often observed as the intermediate states formed during the spark-glow transition. Three different types of short-cutting events have been observed to generate new current paths between two plasma channel segments, and between two electrodes, as well as between the channel segment and the electrodes, respectively. The short-cut upper part of the plasma column that was found to have no current passing through can be detected several hundreds of microseconds after the short-cutting event. The voltage recovery rate, the period of AC voltage-driving signal, the flow rates and the rated input powers were found to play an important role in affecting the transitions among the different types of discharges.
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| 26. |
- Zhu, Jiajian, et al.
(författare)
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Sustained diffusive alternating current gliding arc discharge in atmospheric pressure air
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:23, s. 234102-234102
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Rapid transition from glow discharge to thermal arc has been a common problem in generating stable high-power non-thermal plasmas especially at ambient conditions. A sustained diffusive gliding arc discharge was generated in a large volume in atmospheric pressure air, driven by an alternating current (AC) power source. The plasma column extended beyond the water-cooled stainless steel electrodes and was stabilized by matching the flow speed of the turbulent air jet with the rated output power. Comprehensive investigations were performed using high-speed movies measured over the plasma column, synchronized with simultaneously recorded current and voltage waveforms. Dynamic details of the novel non-equilibrium discharge are revealed, which is characterized by a sinusoidal current waveform with amplitude stabilized at around 200 mA intermediate between thermal arc and glow discharge, shedding light to the governing mechanism of the sustained spark-suppressed AC gliding arc discharge.
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| 27. |
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| 28. |
- Zhu, Jiajian, et al.
(författare)
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Translational, rotational, vibrational and electron temperatures of a gliding arc discharge
- 2017
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Ingår i: Optics Express. - 1094-4087. ; 25:17, s. 20243-20257
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Tidskriftsartikel (refereegranskat)abstract
- Translational, rotational, vibrational and electron temperatures of a gliding arc discharge in atmospheric pressure air were experimentally investigated using in situ, non-intrusive optical diagnostic techniques. The gliding arc discharge was driven by a 35 kHz alternating current (AC) power source and operated in a glow-type regime. The two-dimensional distribution of the translational temperature (Tt) of the gliding arc discharge was determined using planar laser-induced Rayleigh scattering. The rotational and vibrational temperatures were obtained by simulating the experimental spectra. The OH A–X (0, 0) band was used to simulate the rotational temperature (Tr) of the gliding arc discharge whereas the NO A–X (1, 0) and (0, 1) bands were used to determine its vibrational temperature (Tv). The instantaneous reduced electric field strength E/N was obtained by simultaneously measuring the instantaneous length of the plasma column, the discharge voltage and the translational temperature, from which the electron temperature (Te) of the gliding arc discharge was estimated. The uncertainties of the translational, rotational, vibrational and electron temperatures were analyzed. The relations of these four different temperatures (Te>Tv>Tr >Tt) suggest a high-degree non-equilibrium state of the gliding arc discharge.
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