| 1. |
- Bao, Yupan, et al.
(författare)
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Effect of a single nanosecond pulsed discharge on a flat methane–air flame
- 2023
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Ingår i: Applications in Energy and Combustion Science. - 2666-352X. ; 16
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Tidskriftsartikel (refereegranskat)abstract
- Successful implementation of plasma-assisted combustion in applied thermal processes heavily relies on how the plasma can be formed as it interacts with the reactive flow and what the effects are of such a plasma on the combustion process. The current study is an experimental investigation of a plasma-assisted lifted flat methane–air flame by a nanosecond pulsed discharge at atmospheric pressure. The nanosecond pulsed discharge, with a pulse duration of 4 ns and an amplitude of 30 kV to 50 kV, is used to stimulate the flame with a repetition rate of 1 Hz. The flame/plasma interactions are investigated with electrical and optical/laser diagnostics to study plasma-formation and its effect on the temperatures and formaldehyde formation. The flame speed seems to be accelerated for tens of milliseconds after the plasma stimulation, without noticeable gas temperature increase at the flame front and in the post-flame region. Formaldehyde is formed in the unburnt region while there is a slight increase in formaldehyde signal in the preheat zone. These results show that a volumetric effect of plasma-assisted combustion can be achieved with a short nanosecond plasma from a single excitation.
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| 2. |
- Bao, Yupan, et al.
(författare)
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Single-shot 3D imaging of hydroxyl radicals in the vicinity of a gliding arc discharge
- 2021
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Ingår i: Plasma Sources Science and Technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:4
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Tidskriftsartikel (refereegranskat)abstract
- Chemical processing by plasma is utilized in many applications. Plasma-related studies, however, are challenging to carry out due to plasmas' transient and unpredictable behavior, excessive luminosity emission, 3D complexity and aggressive chemistry and physiochemical interactions that are easily affected by external probing. Laser-induced fluorescence is a robust technique for non-intrusive investigations of plasma-produced species. The hydroxyl radical (OH) is an interesting molecule to target, as it is easily produced by plasmas in humid air. In this letter, we present 3D distributions of ground state OH radicals in the vicinity of a glow-type gliding arc plasma. Such radical distributions, with minimal plasma emission, are captured instantaneously in one single camera acquisition by combining structured laser illumination and a lock-in based imaging analysis method called FRAME. The orientation of the plasma discharge can be reconstructed from the 3D data matrix, which can then be used to calculate 2D distributions of ground state OH radicals in a plane perpendicular to the orientation of the plasma channel. Our results indicate that OH distributions around a gliding arc are strongly affected by gas dynamics. We believe that the ability to instantaneously capture 3D transient molecular distributions in a plasma discharge, with minimal plasma emission interference, will have a strong impact on the plasma community for in-situ investigations of plasma-induced chemistry and physics.
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| 3. |
- 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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| 4. |
- Kong, Chengdong, et al.
(författare)
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Characteristics of a Gliding Arc Discharge Under the Influence of a Laminar Premixed Flame
- 2019
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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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Kong, Chengdong, et al.
(författare)
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Layered structure around an extended gliding discharge column in a methane-nitrogen mixture at high pressure
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 114:19, s. 194102-194102
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Tidskriftsartikel (refereegranskat)abstract
- The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in N2-CH4 gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH4 in nitrogen, compared to that in pure N2. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic species, surrounded by the relatively cold N2-CH4 mixture. At the mixing layer between the high-temperature zone and the N2-CH4 mixture, some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column.
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| 8. |
- Kong, Chengdong, et al.
(författare)
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Non-thermal gliding arc discharge assisted turbulent combustion (up to 80 kW) at extended conditions : phenomenological analysis
- 2024
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 196:2, s. 161-176
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Tidskriftsartikel (refereegranskat)abstract
- Plasma assisted combustion has been proposed as an efficient technique to enhance combustion, especially under the extreme conditions. For shedding light on the interactions between plasma and turbulent flame at extended conditions, a burner design with integrated electrodes was used to couple a non-thermal gliding arc (GA) discharge to a turbulent flame. The morphology and dynamic behaviors of the GA assisted flame under extended flow rates and gas temperatures were investigated by high-speed video imaging. It is found that two distinct types of flame (named as Flame A and Flame B) can be sustained by the GA discharge depending on the local flow conditions. Flame A was sustained by the GA on stable anchor points, while Flame B moved together with the thin plasma volume of the gliding arc, behaving as an unstable flame. When the fed air gas temperature was increased, Flame A became more stable while Flame B became fragile and extinguished easily. Furthermore, the phenomenological findings under different flow conditions imply typical four flame types for the GA discharge assisted combustion system, including the self-sustained flame at relatively low Reynolds number (Re), the GA sustained stable flame at moderate Re number, the GA sustained unstable flame and the GA assisted auto-ignited and propagating flame at relatively large Re number. In all, the GA discharge seems to provide various effects on combustion depending on the overall turbulence as well as the local equivalence ratio, the gas temperature, etc.
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| 9. |
- 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
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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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| 10. |
- Kong, Chengdong, et al.
(författare)
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Stabilization of a turbulent premixed flame by a plasma filament
- 2019
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 208, s. 79-85
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of stabilizing a turbulent premixed methane-air flame using warm filamentary plasma is investigated by using laser diagnostics. First, stabilization of a turbulent jet flame is demonstrated in a setup using a pin-to-pin plasma discharge. The coupled plasma-flame structures were visualized utilizing planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and methylidyne radicals (CH), as well as laser Rayleigh scattering thermometry imaging. The results show that the plasma channel and the flame front are spatially separated by a layer of hot burning products attributed to the flame propagation from the plasma core. Because of this spatial separation, the impacts of plasma on combustion are primarily thermal since the energetic radical species (such as O, H), produced by the discharge, have short equilibration time and cannot spread far away from the discharge channel before reaching the equilibrium state. From this point of view, turbulence would be beneficial for promoting the transport of plasma-produced radicals and thus bridge the gap between the plasma and the flame front. The plasma is still able to stabilize the flame. Based upon the experimental results, a frequent ignition-flame propagation (FIFP) model is proposed to explain the flame stabilization process. For the contracted plasma filament, the local power density is high enough to initialize the flame kernel that propagates away from the plasma channel until extinction. The propagation process is, however, strongly affected by turbulence. Local extinction is highly probable and thus the flame front has to be close to the ignition source at strong turbulence. At such conditions, the stabilized flame can be regarded as a large number of flame pockets, repeating the three phases of ignition, propagation and extinction, which can be summarized as the FIFP model. It infers that the flame propagation phase is important for sustaining the flame to complete combustion. Hence, this phase should be extended, which is more probable to achieve if the plasma ignition pilot is located in a section of limited turbulence.
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