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Yong Tang - One of the best experts on this subject based on the ideXlab platform.
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flammability enhancement of swirling ammonia air combustion using ac powered gliding Arc Discharges
Fuel, 2021Co-Authors: Yong Tang, Baolu Shi, Dingjiang Xie, Ningfei WangAbstract:Abstract Aiming at the potential development of ammonia as a carbon-free renewable fuel, this work investigates the flammability and NOx emission of swirling ammonia/air flames and utilizes plasma to enhance ammonia combustion. First, the well-designed rapidly-mixed swirl burner can anchor compact ammonia/air flames under a wide range of flow conditions, with the current maximum heat release of approximately 4.7 kW. The flame is progressively detached from the quartz confinement tube and goes blow-off when the equivalence ratio drops below approximately 0.7–0.8. Then, to alleviate the problem of low flammability, gliding Arc Discharges driven by a 12.5 kHz alternating current (AC) power supply are facilitated to extend the lean blow-off margin to approximately 0.3–0.4. The localized flame kernels surrounding the discharge column are detected by high-speed photography. The planar laser-induced fluorescence (PLIF) imaging of OH radicals, optical emission spectroscopy, and NH2* chemiluminescence measurement are performed to interpret the intermediate chemistry. Finally, the NOx emission of the swirling ammonia/air flame is measured by a flue gas analyzer. Results show that although the AC-powered gliding Arc exhibits weak global effects on the NOx reduction of burner-stabilized ammonia/air flames prevailing at higher equivalence ratios (e.g., φ > 0.75), leaner flames stabilized by Discharges can achieve NOx emissions below 100 ppm due to the thermal DeNOx mechanism.
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extension of flammability and stability limits of swirling premixed flames by ac powered gliding Arc Discharges
Combustion and Flame, 2021Co-Authors: Jinguo Sun, Yong Tang, Baolu Shi, Qiang YaoAbstract:Abstract This work seeks to achieve stable combustion of swirling premixed flows using gliding Arc Discharges and understand how plasma alters flame dynamics near blow-off. Gliding Arc Discharges powered by 7 kHz and 25 kHz alternating current (AC) supplies are demonstrated to significantly extend the flammability and stability limits of swirling premixed flames either confined or unconfined within a quartz tube, with time-averaged discharge power close to 1% of the power typically released by a stable flame. Side visualization of OH profiles using the planar laser-induced fluorescence (PLIF) technique illustrates the unsteady evolution of flames approaching lean blow-off, confirming the positive effect of gliding Arcs on the ultra-lean residual flames. Contrary to the unconfined flame anchored near the nozzle exit or the bluff body, the confined flame detached from the wall progressively presents different unstable morphologies, e.g., oscillating flames and lifted helicon flames. The activation of gliding Arcs provides the additional anchoring mechanism for the lean swirling flame and particularly suppresses the natural flame oscillation with heat-release rate fluctuations of above 30%. The mechanism controlling flame responses to gliding Arc includes a nonlocal effect through the externally applied AC electric field displacing flame and a local effect of spark discharge triggering flame kernels by producing heat and active species. The external sub-breakdown electric field can enlarge the spreading angle of OH-PLIF layers in the unconfined flame, but demonstrates few positive effects on the blow-off limit. Regarding the local effects within or near the Arc column, top infrared imaging provides visible evidence for flame kernels initiated upstream of the inner recirculation zone, yielding repetitive ignition in the fresh reactants.
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plasma assisted stabilization of premixed swirl flames by gliding Arc Discharges
Proceedings of the Combustion Institute, 2021Co-Authors: Jinguo Sun, Yong TangAbstract:Abstract This paper presents key effects of gliding Arc Discharges on the stability limits of externally perturbed methane/air swirl flames. The low-frequency flow disturbance mimicking the transient conditions in practical combustor was generated by a flow pulsation system with repetition of ∼5 Hz. The synchronized OH planar laser-induced fluorescence and particle imaging velocimetry (OH-PLIF/PIV) measurements were utilized to visualize the instantaneous flow and flame structures. We find that, with gliding Arc Discharges, the lean blowout limits in perturbed flames are dramatically extended by up to 60%, which is more pronounced than in non-perturbed flames. OH-PLIF/PIV results show that, the stabilization effect can be explained by the expansion of the inner recirculation zone, wherein the opening angle increases from 52° to 62° due to the plasma enhancement. Time-resolved OH-PLIF images illustrate that, in the presence of gliding Arc Discharges, the flame response to the flow disturbance consists of three stages, including enhancement, extinction, and re-ignition processes. Furthermore, proper orthogonal decomposition (POD) analysis of OH-PLIF images reveals a considerable increase of mean energy contents and suppression of turbulent fluctuations. It suggests that the enhancement of gliding Arc Discharges is not a simple superposition of the energy deposit, but a strong non-linear coupling between the plasma and the turbulent flame with closed-loop feedbacks.
M S Benilov - One of the best experts on this subject based on the ideXlab platform.
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numerical modelling of high pressure Arc Discharges matching the lte Arc core with the electrodes
Journal of Physics D, 2017Co-Authors: Marina Lisnyak, M D Cunha, M S Benilov, Jeanmarc BauchireAbstract:A widely used approach to simulation of high-pressure Arc Discharges is based on the system of magneto-hydrodynamic equations written in the approximation of local thermodynamic equilibrium (LTE). In this work, boundary conditions on the surface of the electrodes are formulated with the use of equations of balance of energy in the non-equilibrium near-electrode layers that separate the LTE bulk plasma and the electrodes. As an example, numerical simulations of a free-burning Arc in atmospheric-pressure argon plasma in the current range from 20 to 200 A are reported. Simulation results are in reasonably good agreement with those given by more sophisticated models and with the experiment. Simulations performed for cathodes of slightly different geometries have predicted a strong effect produced by details of the cathode geometry over the distribution of the current density along the cathode surface and therefore over the plasma temperature, an interesting and potentially important result that is worth further numerical investigation and experimental verification.
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account of near cathode sheath in numerical models of high pressure Arc Discharges
Journal of Physics D, 2016Co-Authors: M S Benilov, M D Cunha, Larissa G Benilova, N A Almeida, M Baeva, D UhrlandtAbstract:Three approaches to describing the separation of charges in near-cathode regions of high-pressure Arc Discharges are compared. The first approach employs a single set of equations, including the Poisson equation, in the whole interelectrode gap. The second approach employs a fully non-equilibrium description of the quasi-neutral bulk plasma, complemented with a newly developed description of the space-charge sheaths. The third, and the simplest, approach exploits the fact that significant power is deposited by the Arc power supply into the near-cathode plasma layer, which allows one to simulate the plasma–cathode interaction to the first approximation independently of processes in the bulk plasma. It is found that results given by the different models are generally in good agreement, and in some cases the agreement is even surprisingly good. It follows that the predicted integral characteristics of the plasma–cathode interaction are not strongly affected by details of the model provided that the basic physics is right.
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multiple solutions in the theory of dc glow Discharges and cathodic part of Arc Discharges application of these solutions to the modeling of cathode spots and patterns a review
Plasma Sources Science and Technology, 2014Co-Authors: M S BenilovAbstract:A new class of stationary solutions in the theory of glow Discharges and plasma–cathode interaction in ambient-gas Arc Discharges has been found over the past 15 years. These solutions exist simultaneously with the solution given in textbooks, which describes a discharge mode with a uniform or smooth distribution of current over the cathode surface, and describes modes with various configurations of cathode spots: normal spots on glow cathodes, patterns of multiple spots recently observed on cathodes of glow microDischarges and spots on Arc cathodes. In particular, these solutions show that cathode spots represent a manifestation of self-organization caused by basic mechanisms of the near-cathode space-charge sheath; another illustration of the richness of the gas discharge science. As far as Arc cathodes are concerned, the new solutions have proved relevant for industrial applications. This work is dedicated to reviewing the multiple solutions obtained to date, their systematization, and analysis of their properties and physical meaning. The treatment is performed in the context of general trends of self-organization in bistable nonlinear dissipative systems, which allows one to consider glow Discharges or Arc–cathode interaction within a single physically transparent framework without going into mathematical details and offers a possibility of systematic computation of the multiple solutions. Relevant computational aspects and experimental data are discussed.
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sheath and Arc column voltages in high pressure Arc Discharges
Journal of Physics D, 2012Co-Authors: M S Benilov, Larissa G BenilovaAbstract:Electrical characteristics of a 1 cm-long free-burning atmospheric-pressure argon Arc are calculated by means of a model taking into account the existence of a near-cathode space-charge sheath and the discrepancy between the electron and heavy-particle temperatures in the Arc column. The computed Arc voltage exhibits a variation with the Arc current I similar to the one revealed by the experiment and exceeds experimental values by no more than approximately 2 V in the current range 20–175 A. The sheath contributes about two-thirds or more of the Arc voltage. The LTE model predicts a different variation of the Arc voltage with I and underestimates the experimental values appreciably for low currents but by no more than approximately 2 V for I ≳ 120 A. However, the latter can hardly be considered as a proof of unimportance of the space-charge sheath at high currents: the LTE model overestimates both the resistance of the bulk of the Arc column and the resistance of the part of the column that is adjacent to the cathode, and this overestimation to a certain extent compensates for the neglect of the voltage drop in the sheath. Furthermore, if the latter resistance were evaluated in the framework of the LTE model in an accurate way, then the overestimation would be still much stronger and the obtained voltage would significantly exceed those observed in the experiment.
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sheath vs quasi neutral plasma voltages in high pressure Arc Discharges
International Conference on Plasma Science, 2012Co-Authors: M S Benilov, Larissa G BenilovaAbstract:Summary form only given. Two very different approaches to theoretical description of interaction of high-pressure Arc plasmas with refractory cathodes exist in the literature. One group of reseArchers believes that space-charge effects on refractory cathodes are small and contribute to the Arc voltage no more than one or two volts. Accordingly, these reseArchers use simulation models which are based on the assumption of quasi-neutrality, or even stronger assumption of local thermodynamic equilibrium (LTE), in the whole Arc volume. Other reseArchers believe that the voltage drop in the near-cathode space-charge sheath is no less than 10V and rely on models where the near-cathode sheath plays a central role.
G. Rego - One of the best experts on this subject based on the ideXlab platform.
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Fibre optic devices produced by Arc Discharges
Journal of Optics, 2010Co-Authors: G. RegoAbstract:We present an overview of the applications of the electric Arc technique related to optical fibre technology. The use of Arc Discharges ranges from the well-known fibre splicing, going through the fabrication of basic devices such as fibre tapers and microspheres, to tailoring the spectra of UV-induced gratings such as in the apodization of fibre Bragg gratings and also in the fabrication of phase-shifted and sampled fibre Bragg gratings. However, in the past decade a topic more intensively investigated was probably long-period fibre gratings. Therefore, some devices based on Arc-induced gratings, namely, phase-shifted and step-changed gratings and bandpass filters are discussed. We also present an electrically insulated thermocouple assembled in situ using Arc Discharges. This sensor is very useful in the determination of the temperature attained by the fibre during an Arc discharge, this property being fundamental for the discussion of the mechanisms of formation and for the understanding of the thermal properties of Arc-induced devices. © 2010 IOP Publishing Ltd.
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refractometric sensor based on a phase shifted long period fiber grating
Applied Optics, 2006Co-Authors: Rosane Falate, G. Rego, O Frazao, Jose Luis Fabris, J. L. SantosAbstract:A refractometric sensor based on a phase-shifted long-period fiber grating written by electric-Arc Discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water.
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Arc induced long period gratings in aluminosilicate glass fibers
Optics Letters, 2005Co-Authors: G. Rego, J. L. Santos, Rosane Falate, Jose Luis Fabris, H M Salgado, Sergei L Semjonov, E M DianovAbstract:Permanent long-period gratings were written using Arc Discharges in two aluminosilicate fibers, one of which was doped with erbium. Reversible gratings were also mechanically induced in both fibers. The thermal behavior of the Arc-induced gratings was investigated at up to 1100°C. It was found that the shift of the resonant wavelengths exhibited a well-defined linear dependence on temperature up to 700°C.
Qiang Yao - One of the best experts on this subject based on the ideXlab platform.
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extension of flammability and stability limits of swirling premixed flames by ac powered gliding Arc Discharges
Combustion and Flame, 2021Co-Authors: Jinguo Sun, Yong Tang, Baolu Shi, Qiang YaoAbstract:Abstract This work seeks to achieve stable combustion of swirling premixed flows using gliding Arc Discharges and understand how plasma alters flame dynamics near blow-off. Gliding Arc Discharges powered by 7 kHz and 25 kHz alternating current (AC) supplies are demonstrated to significantly extend the flammability and stability limits of swirling premixed flames either confined or unconfined within a quartz tube, with time-averaged discharge power close to 1% of the power typically released by a stable flame. Side visualization of OH profiles using the planar laser-induced fluorescence (PLIF) technique illustrates the unsteady evolution of flames approaching lean blow-off, confirming the positive effect of gliding Arcs on the ultra-lean residual flames. Contrary to the unconfined flame anchored near the nozzle exit or the bluff body, the confined flame detached from the wall progressively presents different unstable morphologies, e.g., oscillating flames and lifted helicon flames. The activation of gliding Arcs provides the additional anchoring mechanism for the lean swirling flame and particularly suppresses the natural flame oscillation with heat-release rate fluctuations of above 30%. The mechanism controlling flame responses to gliding Arc includes a nonlocal effect through the externally applied AC electric field displacing flame and a local effect of spark discharge triggering flame kernels by producing heat and active species. The external sub-breakdown electric field can enlarge the spreading angle of OH-PLIF layers in the unconfined flame, but demonstrates few positive effects on the blow-off limit. Regarding the local effects within or near the Arc column, top infrared imaging provides visible evidence for flame kernels initiated upstream of the inner recirculation zone, yielding repetitive ignition in the fresh reactants.
J. L. Santos - One of the best experts on this subject based on the ideXlab platform.
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refractometric sensor based on a phase shifted long period fiber grating
Applied Optics, 2006Co-Authors: Rosane Falate, G. Rego, O Frazao, Jose Luis Fabris, J. L. SantosAbstract:A refractometric sensor based on a phase-shifted long-period fiber grating written by electric-Arc Discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water.
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Arc induced long period gratings in aluminosilicate glass fibers
Optics Letters, 2005Co-Authors: G. Rego, J. L. Santos, Rosane Falate, Jose Luis Fabris, H M Salgado, Sergei L Semjonov, E M DianovAbstract:Permanent long-period gratings were written using Arc Discharges in two aluminosilicate fibers, one of which was doped with erbium. Reversible gratings were also mechanically induced in both fibers. The thermal behavior of the Arc-induced gratings was investigated at up to 1100°C. It was found that the shift of the resonant wavelengths exhibited a well-defined linear dependence on temperature up to 700°C.
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In situ temperature measurement of an optical fiber submitted to electric Arc Discharges
IEEE Photonics Technology Letters, 2004Co-Authors: Gaspar Rego, Luís M. N. B. F. Santos, Bernd Schröder, Paulo Marques, J. L. Santos, Henrique M. SalgadoAbstract:Type S thermocouples were assembled in situ by applying high intensity electric Arc Discharges to the contact junction of two platinum (Pt) and Pt-10% rhodium (Pt-10% Rh) wires, inserted on a silica capillary. The electrically insulated thermocouples built in this way were afterwards employed to estimate the temperature of an optical fiber subjected to Arc Discharges. For typical values of the Arc discharge parameters used to Arc-induce long-period fiber gratings (electric current I=9 mA and Arc duration t=1 s), a capillary peak temperature value of 1420/spl deg/C/spl plusmn/40/spl deg/C was obtained by extrapolation of the experimental data for the limit situation of having a thermocouple with negligible diameter. The temperature profiles in the capillary and in an optical fiber were calculated based on a heat transfer model implemented by a finite element algorithm and fitted to the experimental temperature distribution in the Pt and Pt-10% Rh wires. The correspondent peak temperatures computed for the capillary and for the fiber were 1450/spl deg/C and 1320/spl deg/C, respectively. A good agreement between the capillary temperature values determined graphically and numerically was obtained.