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Optical Diagnostics of Non-thermal Plasmas and Plasma-assisted Combustion

Zhu, Jiajian LU (2015) LRCP-188.
Abstract (Swedish)
Popular Abstract in Swedish

Plasma består av fria elektroner, joner och neutrala partiklar och utgör det fjärde tillståndet av materia där de andra tre är fast-, flytande- och gasfas. Plasma är den vanligast förekommande tillståndet av materia i universum och uppskattas utgöra mer än 99% av universums totala massa. Naturligt förekommande plasma återfinner vi i blixtar, sol vinden, jonosfären och i solen men människan har även utnyttjat plasmats egenskaper i ett flertal uppfinningar såsom i lysrör och i TV-skärmar. Människans nyfikenheten och innovationsförmåga har därför lett till omfattande forskning kring plasmarelaterade fenomen och tillämpningar vilket resulterat i en rad viktiga uppfinningar samt en grundläggande... (More)
Popular Abstract in Swedish

Plasma består av fria elektroner, joner och neutrala partiklar och utgör det fjärde tillståndet av materia där de andra tre är fast-, flytande- och gasfas. Plasma är den vanligast förekommande tillståndet av materia i universum och uppskattas utgöra mer än 99% av universums totala massa. Naturligt förekommande plasma återfinner vi i blixtar, sol vinden, jonosfären och i solen men människan har även utnyttjat plasmats egenskaper i ett flertal uppfinningar såsom i lysrör och i TV-skärmar. Människans nyfikenheten och innovationsförmåga har därför lett till omfattande forskning kring plasmarelaterade fenomen och tillämpningar vilket resulterat i en rad viktiga uppfinningar samt en grundläggande förståelse för jordelivets existens.

Ett icke-termiskt plasma är en typ av plasma som karakteriseras av hög elektrontemperatur i relation till gasens temperatur. Dessa heta/snabba fria elektroner kollidera med omgivande molekyler och kan därmed skapa nya molekyler och atomer som har stor benägenhet att reagera med omgivande molekyler och på så sätt påverka miljösammansättningens kemi. Denna kemiska effekt vill man använda sig av i till exempel plasma-asisterad förbränning för att kunna styra och effektivisera förbränningen, vilket resulterar i minskade utsläpp av såväl växthusgaser som giftiga ämnen, ökad användningsflexibiliteten av existerande förbränningsanläggningar, samt minskning av förslitningsskador och driftsstop av förbränningsanläggningar. Det finns dock en rad oklarheter i forskningsområdet av plasma-assisterad förbränning, exempelvis vilken typ av plasmakälla som är bäst lämpad för plasma-assisterad förbränning samt vilka kemiska processer som kan och bör påverkas. I undersökningar av förlopp med höga spänningar, höga temperaturer samt aggressiv kemi är optisk diagnostik en kraftfull metod då man kan mäta temperaturfält, flödesdynamik samt ämnespecifika fördelningar på avstånd samt med minimal inverkan på mätmiljön.

I detta avhandlingsarbete tillämpas och utvecklas optisk diagnostik i syfte att förstå plasmarelaterade fenomen och tillämpningar. Den strörsta delen av arbetet har fokuserat på optiska studier av glidande urladdningsplasman. En glidande urladdning skapas i luften i det smalaste gapet mellan två divergerande elektroder om en tillräckligt hög spänning läggs över elektroderna. Genom att applicera ett gasflöde kan urladdningskanalen förlängas vilket också medför att plasmakolumnens kontaktpunkterna glider längs med elektroderna (och därav namnet glidande urladdning). Förlängningen av plasmakanalen pågår tills resistansen i plasmakanalen blir för stor och en ny kolumn av plasma antänds igen vid det kortaste elektrodavståndet och den gamla kanalen slutar därmed leda ström. Dessa dynamiska förlopp har studerats med höghastighetsfotografering för att förstå samspelet mellan gasflödet och plasmat. Vidare har ämnesspecifika studier gjorts med avbildande laserinducerade fluorescencemätningar av hydroxylradikaler och kväveoxid, som är två viktiga ämnen i plasmarelaterade tillämpningar. Laserbaserade temperaturmätningar har även visat att uppställningen för glidande urladdningar i Lund opererar i icketermisk jämvikt med sin omgivning, vilket gör uppställningen ideal för tillämpningar för påverkan av reaktiva gasflöden.

I studier av plasmats påverkan på förbränning har ett flera olika typer av plasmarelaterade verktyg använts såsom barriärurladdningar, glidande urladdningar, mikrovågor samt dielektriska barriärurladdningar och koronaurladdningar för produktion av ozon. Man har funnit att glidande urladdningar kan interagera med flamman och främja bildningen av formaldehyd och hydroxylradikaler i flamman. Dessutom har ökad formaldehydproduktion observerats då små mängder av ozon seedats in i flamman samt att mikrovågsstimulering av förbränningen ökar den kemiska aktiviteten i turbulenta flammor (Less)
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... (More)
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. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Ju, Yiguang, Princeton University, Princeton, NJ, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
High-speed photography, Laser diagnostics, Planar laser-induced fluorescence, Plasma-assisted combustion, Non-thermal plasmas, Gliding arc discharge, Optical diagnostics, Particle tracking velocimetry, Fysicumarkivet A:2015:Zhu
volume
LRCP-188
pages
107 pages
publisher
Lund Report on Combustion Physics
defense location
Lecture hall Rydbergssalen, Department of Physics, Sölvegatan 14, Lund University Faculty of Engineering, LTH.
defense date
2015-09-18 10:15
ISSN
1102-8718
ISBN
978-91-7623-463-1
language
English
LU publication?
yes
id
58c37660-5529-4789-aabb-385fe4616681 (old id 7766773)
date added to LUP
2015-08-25 13:18:56
date last changed
2016-09-19 08:44:45
@phdthesis{58c37660-5529-4789-aabb-385fe4616681,
  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.<br/><br>
 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. <br/><br>
 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.},
  author       = {Zhu, Jiajian},
  isbn         = {978-91-7623-463-1},
  issn         = {1102-8718},
  keyword      = {High-speed photography,Laser diagnostics,Planar laser-induced fluorescence,Plasma-assisted combustion,Non-thermal plasmas,Gliding arc discharge,Optical diagnostics,Particle tracking velocimetry,Fysicumarkivet A:2015:Zhu},
  language     = {eng},
  pages        = {107},
  publisher    = {Lund Report on Combustion Physics},
  school       = {Lund University},
  title        = {Optical Diagnostics of Non-thermal Plasmas and Plasma-assisted Combustion},
  volume       = {LRCP-188},
  year         = {2015},
}