GENERATION OF PIN-HOLEGENERATION OF PIN-HOLE DISCHARGEDISCHARGESS IN LIQUIDS IN LIQUIDS
František Krčma, Zdenka Kozáková, Michal Vašíček, Lucie Hlavatá, Lenka Hlochová
Faculty of Chemistry, Brno University of Technology Czech Republic
Patrick Vanraes
Department of Applied Physics, Ghent University Belgium
EElectrical discharges in water
Highly polar liquid with large relative permittivity (εr=81) and high dielectric strength E > 1 MV/cm compared to ~ 30 kV/cm of air
Relatively dense environment with high concentration of ions (H+, OH- etc.) that determine electrolytic conductivity of water (i.e. resistance)
Low mobility of ions compared to electrons => ions alter propagation of discharge channel in water by compensation of space charge electrical field on the streamer head
Solution conductivity strongly affects electrical breakdown of water => high requirements on power supply and reactor design
To ensure electrical breakdown of water under moderate (reasonable) conditions is necessary to use non-uniform electrode configurations
point-point, point-plane, wire-cylinder, pin-hole
Physical properties of water
Experimental devices and parametersExperimental devices and parameters
Experimental parameters:
High voltage: DC 1-3 kV
Discharge current: 90-250 mA
Input power: 90-300 W
Electrodes: planar, stainless steel, Pt
Dielectric diaphragm: PET, 0.25 mm, shapal ceramics 0.3 - 2 mm
Pin-hole: centred, initial diameter 0.2 - 1.0 mm
supporting electrolyte: NaCl, NaBr, NaNO3, Na2HPO4∙12H2O, Na2SO4, etc.
Optimal solution conductivity: 150-1300 μS∙cm-1
Devices: Batch discharge reactor HV source: DC, constant voltage
Pin-hole configurationsPin-hole configurations
d
l
d is typically 0.1 – 2 mm
l ≈ d diaphragm dischargel » d capillary discharge
Pin-hole configurationsPin-hole configurations
d
l
d is typically 0.1 – 2 mm
l ≈ d diaphragm dischargel » d capillary discharge
symmetric asymmetric
Diagnostics Diagnostics
• electrical measurements of voltage and current
• sound generation
• light generation – PMT, high speed camera, iCCD
• optical emission spectroscopy – non-time resolved
Principle of pin-hole discharge formationPrinciple of pin-hole discharge formation
Theories of electrical discharge creation in liquids:
thermal (bubble) theory – bubble formation due to Joule heating
electron theory – analogy to Townsend´s theory in gases
Principle of pin-hole discharge formationPrinciple of pin-hole discharge formation
Theories of electrical discharge creation in liquids:
thermal (bubble) theory – bubble formation due to Joule heating
electron theory – analogy to Townsend´s theory in gases
+̶.
positive streamers negative streamers
Principle of diaphragm discharge formation in liquidsPrinciple of diaphragm discharge formation in liquids
Cathode space – positive plasma streamers
P = 75 W P = 90 W P = 120 W P = 160 W
Anode space – negative plasma streamersP = 200 W
positive streamers negative streamers
+̶.
Mean current-voltage characteristicsMean current-voltage characteristics
V-A characteristic for gas (red line) and NaCl solution (blue crosses)
Mean current-voltage characteristicsMean current-voltage characteristics
V-A characteristic for gas (red line) and NaCl solution (blue crosses)
U
Rd
U
Rs Rd Rs
Rd « Rs
liquid
gas
Time resolved current-voltage characteristics - electrolysisTime resolved current-voltage characteristics - electrolysisso
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Time resolved current-voltage characteristics - bubblesTime resolved current-voltage characteristics - bubblesso
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Time resolved current-voltage characteristics - bubblesTime resolved current-voltage characteristics - bubblesso
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Time resolved current-voltage characteristics - breakdownTime resolved current-voltage characteristics - breakdownso
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Time resolved current-voltage characteristics - breakdownTime resolved current-voltage characteristics - breakdownso
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Time resolved current-voltage characteristics - breakdownTime resolved current-voltage characteristics - breakdownso
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Time resolved current-voltage characteristics - breakdownTime resolved current-voltage characteristics - breakdownso
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Time resolved current-voltage characteristics - dischargeTime resolved current-voltage characteristics - dischargeso
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Time resolved current-voltage characteristics - dischargeTime resolved current-voltage characteristics - dischargeso
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Mean current-voltage characteristicsMean current-voltage characteristics
electrolyzisbubbles generation
discharge
electrolyzisbreakdown
discharge
bubbles
Mean current-voltage characteristicsMean current-voltage characteristics
Effect of solution kind – breakdown voltageEffect of solution kind – breakdown voltage
Diaphragm/capillary – bubbling voltageDiaphragm/capillary – bubbling voltage
d = 0.3 mm
Diaphragm/capillary – breakdown voltageDiaphragm/capillary – breakdown voltage
d = 0.3 mm
Diaphragm/capillary – breakdown voltageDiaphragm/capillary – breakdown voltage
l = 0.25 mm
Diaphragm/capillary – breakdown currentDiaphragm/capillary – breakdown current
d = 0.3 mm
Discharge running in bubblesDischarge running in bubbles
plasma streamers bubbles
Discharge running in bubbles Discharge running in bubbles
discharge inside bubbles but plasma streamers propagate into the liquid
diaphragm 0.3/0.3 mm bubble at pin, diameter 2 mm
ConclusionsConclusions
• Pin-hole discharge is generated in bubbles (microbubbles) if non fast pulsing voltage is applied
• Voltage of bubbles generation as well as the breakdown voltage increase with the increase l/d parameter but there are some limits
• The breakdown current is more or less independent on the pin-hole length except very thin barriers
• Breakdown voltage decreases with the solution conductivity increase but it is independent of the water solution kind
• Bubbles can generate significant sound even without discharge
• Streamers (streamer like channels) propagates from the bubble into the liquid even at low voltage
Thank you for your attention!!!Thank you for your attention!!!
This work was supported by Czech Ministry of Culture project No. DF11P01OVV004
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