Aero Engine Flow Control by Surface Discharge Plasma

Experimental investigations and numerical modeling with the objective to study the effects that surface discharge plasmas can provoke on aerodynamic flow situations, including high speed flows present in aero-engines, are undertaken at EPFL. The actuator’s potential to influence the flow under realistic flight and engine operating conditions, largely depends on its geometric configuration and the power supply. Initially, a long-life (more than 12 hours) surface discharge actuator was designed at EPFL and proven to survive transonic flow speeds on typical compressor blades. This was the first time that such a long-term, continuously operating actuator was applied successfully. However, with AC voltage applied to the electrodes, the effects on the shock waves did not appear to be significant [1, 2]. Recent experiments conducted at EPFL focused on fast rise voltage pulse driven dielectric barrier discharge (DBD) actuators and the development of a power supply that could provide the required voltage pulses. A generic DBD actuator with two electrodes in asymmetric configuration, separated by a dielectric was chosen. High speed imaging was applied to investigate the spatial and temporal discharge development. In order to study the influence of voltage and current on the plasma, several power supply configurations were examined by varying voltage rise and fall time, maximum voltage, peak current and pulse width. Two discharge periods were observed on the positive and the negative edge of the voltage pulse. It was observed that the characteristics of the plasma differ considerably between these two periods. Furthermore, the light emission intensity and the propagation speed of the plasma along the surface of the dielectric increased with the maximum voltage and peak current. A similar effect was observed when the voltage rise time was decreased. In a next step, a technique similar to synthetic schlieren was applied to examine the interaction of the DBD actuator with air under atmospheric pressure. A micro shock wave that propagates from the electrode edge normal to the surface was observed, which agrees well with numerical results gained during a parallel investigation. In addition, the impact on the flow structure of maximum voltage and frequency is currently examined. First results suggest that the intensity of the micro shock wave increases with the voltage whereas the frequency primarily provokes electrode heating. With the objective to investigate the modelling of the fast rise pulse driven DBD discharge, a numerical tool was developed that applies emission spectroscopy to determine the electron energy distribution function (EEDF) by comparing the relative intensity of bands, as described by several authors [3-5]. The EEDF is then used to infer populations present in the gas through electron impact ionization, excitation or dissociation cross sections data [6]. Contrary to previous studies, this approach employs two naturally occurring transitions in atmospheric plasmas in conjunction with recent data on collisional quenching [7], which makes the method more reliable and does not require the adjunction of other species such as Helium. References: 1. Pavon S., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Effects of high-speed airflows on a surface dielectric barrier discharge”, J. Phys. D: Appl. Phys. 40. No 6.2 1733-1741, 2007. 2. Pavon S., Sublet A., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Long Lifetime system for the generation of Surface Plasmas”, International Patent no: P1883PC00/13-71, PCT/IB2009/050489, 2008. 3. Bibinov N. K., Kokh D.B., Kolokolov N.B., Kostenko V.A., Meyer D., Vinogradov I.P., Wiesemann K., “A comparative study of the electron dis-tribution function in the positive columns in N2 and N2/ He dc glow dis-charges by optical spectroscopy and probes”, Plasma Sources Science and Technology, 298-309,1998. 4. Behringer K., Fantz U., “Spectroscopic diagnostics of glow discharge plasmas with non-maxwellian electron energy distributions”, Journal of Applied Physics D, 2128-2135, 1994. 5. Isola L.M., Gómez B.J., Guerra V., “Determination of the electron tem-perature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy”, Journal of Applied Physics D, 015202, 2010. 6. Itikawa Y., “Cross sections for electron collisions with nitrogen mole-cules”, Journal of Physical and Chemical Reference Data, 31-53, 2006. 7. Dilecce G., Ambrico P.F., Benedictis S., “On the collision quenching of N2plus by N2 and O2 and its influence on the measurement of E over N by intensity ratio of nitrogen spectral bands”, Journal of Applied Physics D, 195201, 2010.