Investigation of pulsed electrical discharges at atmospheric pressure in

porous media and alveolar structure

ANR/CNRS program LE DELLIOU Pierre

Laboratoire de Physique des Gaz et des Plasmas (bât 210)

DIREBIO GroupExperimental Thesis directed by

P.Tardiveau

Background

• DIREBIO Group

Molecule

conversionActive species and photon production and

transport

Non equilibrium discharges at high pressure

New discharge processes

CombustionSynthesis

Pollution control Bacteriological decontamination

DNA lesionsNew applications

What does it means?– Pulsed electrical discharges =

« Cold plasma » generated thanks to an high voltage pulse (U=8-18kV, t=20ns)

– Cold Plasma = Non equilibrium ionized gasElectrical energy => e- are accelerated by E. nn >> ne no gas warming cold plasma. e- create active species, needed for flue gas treatment.

– Alveolar and Porous Media : Typical Materials used for air treatment by catalytic process.

Honeycomb Cordierite

HV tip electrode

Grounded Plane

AIR

Discharge in a 9mm air gap in false colors scale

Main IssuePolluting control and air treatment :

Actual Solutions : Catalysts such as Pt, Rh, or Pd are deposited inside porous media where pollutants will be trapped.

Particles Filters which burn pollutants by post-combustion process.

Problems : Low and limited efficiency

Media saturation

Inefficiency at low temperature

Investigations• Catalysis assisted by cold plasma

Electrical discharges are generated inside porous or alveolar media (monoliths, foams). Better selectivity, better efficiency,

better life time of the process

• Aim of my thesis : To understand and to predict the

development and the propagation of this kind of electrical discharges generated in such a two-phase media.

Key-parameters : geometry, dimensions, permittivity, conductivity, deposited energy, wall thickness, surface charges…

Modus Operandi Capillaries are used to simulate a pore or a

cavity of the material

Voltage Range

7kV < U < 18kV

Time gate = 0.2 up to 1ns

500ps or 1 ns between each camera

Single shot experiment

Tip diameter Ø=50µm

Results

The vicinity of the dielectric walls enhance the propagation of the discharge

Plane

Velocities obtained are derived from the discharge propagation in the gap

The propagation in these capillaries is more than one order faster than usual velocities obtained for discharge in the same conditions without capillary

• Propagation velocities

Results

The capillary inner radius is a key parameter both for the velocity propagation and for the behaviour of the

discharge

R = 300µm

Tubular Structure

Homogeneous structure

R = 100µm

R =1mm

Filamentary structure

• Radius effect

Balance between ionization in the volume and recombination on the inner surfaces

Results• Wall thickness effect

If the cavity walls are thin enough, i.e around 50µm, discharges can be triggered outside the cavity and are able to propagate in another pore of the media.

A pore-to-pore propagation permits to decrease the energy needed in air treatment reactors.

Outside reignition

Outside reignition of filamentsCapillary to Capillary Propagation

Future works• Effect of the porosity

• Effect of the dielectric geometry (tube/square/rectangular)

• Effect of the dielectric properties (permittivity, conductivity…)

• Investigation of the interactions between electrical discharges and porous membranes.

Thank you for your kind attention