investigation of pulsed electrical discharges at atmospheric pressure in porous media and alveolar...
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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
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