GENERATION OF PIN-HOLE DISCHARGES IN LIQUIDS František Krčma, Zdenka Kozáková, Michal...

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)


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

und

[a.u

.]

li

ght [

a.u.

]

cur

rent

[m

A]

v

olta

ge [

V]

Time resolved current-voltage characteristics - bubblesTime resolved current-voltage characteristics - bubblesso

und

[a.u

.]

li

ght [

a.u.

]

cur

rent

[m

A]

v

olta

ge [

V]

Time resolved current-voltage characteristics - breakdownTime resolved current-voltage characteristics - breakdownso

und

[a.u

.]

li

ght [

a.u.

]

cur

rent

[m

A]

v

olta

ge [

V]

Time resolved current-voltage characteristics - dischargeTime resolved current-voltage characteristics - dischargeso

und

[a.u

.]

li

ght [

a.u.

]

cur

rent

[m

A]

v

olta

ge [

V]


electrolyzisbubbles generation

discharge

electrolyzisbreakdown

discharge

bubbles

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

GENERATION OF PIN-HOLE DISCHARGES IN LIQUIDS František Krčma, Zdenka Kozáková, Michal...

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