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 Group Experimental Thesis directed by P.Tardiveau

Background DIREBIO Group Molecule conversion Active species and photon production and transport Non equilibrium discharges at high pressure New discharge processes Combustion Synthesis Pollution control Bacteriological decontamination DNA lesions New 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 gas Electrical energy => e - are accelerated by E. n n >> n e  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 Issue Polluting 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. Outsi de reignit ion Outside reignition of filaments Capillary 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

Opaque Media Modus Operandi : Electrical Diagnostics are performed to calculate the discharge velocity.