Modeling of an EHD corona flow in nitrogen gas using an asymmetric capacitor for propulsion
Alexandre A. Martins Institute for Plasmas and Nuclear Fusion & Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Mario J. Pinheiro Department of Physics and Institute for Plasmas and Nuclear Fusion, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
AbstractThe present work intends to identify the nature of the propulsive force that occurs during a positive corona discharge in nitrogen gas using an asymmetric capacitor geometry. We are going to apply the known theory of electrohydrodynamics (EHD) and electrostatics in order to compute all hydrodynamic and electrostatic forces that act on the considered geometry in an attempt to provide a physicalinsight on the force mechanism that acts on an asymmetrical capacitor.
In this work we investigate the physical origin of the propulsion observed in an asymmetric capacitor that generates an electrohydrodynamic (EHD) flow through a corona discharge in nitrogen gas, at atmospheric pressure. The structure to be studied, an asymmetric capacitor, is represented in figure 1. The notationused for the electrodes is related to their respective radius of curvature. The positive corona wire has a much smaller radius of curvature r than the facing ground electrode R.
Positive Electrode R r Nitrogen G L Ground Electrode
Figure 1. Asymmetric capacitor with an air gap G, corona wire r and ground electrode R of length L.
Zhao and Adamiak (2005a, 2006) have studied such geometrywith preliminary simulations performed using Fluent and their own code. They attribute the force mechanism mainly to electrostatic forces in the corona wire, directed away from the ground electrode, and state that there are no electrostatic forces on the ground electrode. We disagree with their results since, according to our study, the force on the corona wire has the opposite direction (is directedtowards the ground electrode), and the main electrostatic force is precisely on the ground electrode. In the ensuing discussion we are going to clarify these concepts and we are going to apply the known theory of EHD and electrostatics in order to provide a physical insight on the force mechanism that acts on an asymmetrical capacitor. 1
In our numerical model we will considerelectrostatic force interactions between the electrodes and the ion space cloud, the moment exchange between the mechanical setup and the induced opposite direction nitrogen flow (EHD flow), nitrogen pressure forces on the structure and viscous drag forces. EHD flow is the flow of neutral particles caused by the drifting of ions in an electric field. In our case these ions are generated by apositive high voltage corona discharge in the high curvature (the higher the radius of a sphere, the less is its curvature) portions of the electrodes. The corona wire has a uniform high curvature and therefore will generate ions uniformly (Chen and Davidson, 2002). On the contrary, the body electrode (ground) has a non uniform curvature having the lowest curvature facing the positive corona wire. Thepositively ionized gas molecules will travel from the corona wire ion source towards the collector (ground) colliding with neutral molecules in the process. These collisions will impart momentum to the neutral atoms that will move towards the collector as a result. The momentum gained by the neutral gas is exactly equal to the momentum gained by the positive ions accelerated through the electricfield between the electrodes, and lost in inelastic collisions to the neutral molecules. Corona discharges are non-equilibrium plasmas with an extremely low degree of ionization (roughly 10-8 %). There exist two zones with different properties, the ionization zone and the drift zone. The energy from the electric field is mainly absorbed by electrons in the ionization zone, immediately close to...
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