Analysis of the seeded combustion gas boundary layer near a cold electrode

Abstract

The boundary layer of potassium-see ded combustion gas over a cold electrode under the condition of diffusive current mode without arcs is studied theoretically and experimentally. The theoretical analysis features the introduction of charge separation between electrons and ions, electron thermal nonequilibrium, finite ionization rate, and no assumption of thin sheath layer adjoining the electrode surface. To simplify the problem, the case when the current is produced by an applied external electric field without a magnetic field is studied. In this paper, because of the reliability and suitability of the electric boundary condition adopted in the analysis, the boundary layer around the anode is specifically taken up. For the laminar boundary layer, numerical results for the colder electrode and smaller seed fraction show that the region of the electron-ion charge separation extends much wider than the Debye length from the surface and that both electron thermal and ionizational nonequilibria are considerably remarkable near the electrode even in the seeded combustion gas, and the calculated performance of the electrode voltage drop and current density agrees fairly well with experimental results. On the other hand, for the turbulent boundary layer under the condition of practical MHD generator operation, it is shown from numerical calculations that the charge separation region is limited to the extent of several Debye lengths from the surface and that the electron number density is very close to that obtained by Saha equation for the electron temperature.