A Surface Barrier Discharge System for EHD Flow Acceleration

The atmospheric pressure dielectric barrier discharge in air (GDBD) has been argued to be very efficient in boundary layer modification: barrier discharge is uniform, has got a high ionization efficiency and can be produced at a lower voltage rather than a corona discharge. In particular, a poly-phase design of its power supplying system in conjunction of an asymmetrical electrodes set-up allows to accelerate or decelerate significantly the boundary layer, permitting its use as an aerodynamic actuator. Unfortunately, it has been proven in literature that the overall efficiency of the acceleration process is compromised when an external air field is present. Schlieren imaging techniques permitted to verify that the plasma thickness is contracting when a consistent external airfield is present. More than this, spectroscopic measuremets permitted to state that there is a change in plasma properties, with a variation of the vibrational and rotational temperatures as a function of the external fluid velocity. A specific plasma panel has been designed, machined and placed into the wind tunnel test section to gain information on the different plasma properties at different regimes. The discharge occurs on the surface of the insulating material panel, between several electrode strips. An asymmetric design between the upper electrode surface and the lower electrode surface of the panel allows the confinement of the plasma in the gaps between the asymmetric electrodes. This leads to a paraelectric acceleration of the air on the surface of the panel. Subsequent electrodes pair are connected to the various phases of the power supplying system, in order to achieve also a phase to phase electric field effect. The choice of the insulating material is crucial to obtain a reliable plasma between electrodes. The insulating material must have a good dielectric strength, must be chemically inert, must sustain thermal stresses and must have a small dielectric constant in order to minimize the dielectric losses between electrodes. The material that better matches those requirement is Teflon (Polytetrafluoroethylene , PTFE): it has a dielectric strength of 100 kV mm-1 and a dielectric constant that lies between 2.0 and 2.1 depending on the frequency. On the Teflon panel, nine electrodes pair have been realized on both sides: this leads to three electrodes pair for each phase.