Angewandte Magnetohydrodynamik 7 : Über elektrische Verluste im MHD Generator

By seperation of the flow system in an MHD-generator into boundary layer and bulk a simple model of the electrical conditions in the bulk flow of the generator is set up. With this model the influence of the reductions of the Hallfield on the other parameters in the generator is described by use of a generalized characteristic. At increasing magnetic field a saturation of the Faradayvoltage appears which is also found experimentally by other authors. The calculation shows that this saturation appears in the lower region of the magnetic field where no electrothermal instabilities appear. At higher magnetic fields with electrothermal instabilities the Faradayvoltage increases again with increasing magnetic field. Moreover there exists a lower limit of the velocity below which no nonequilibrium ionization appears if the resistance in the diffusor due to which the Hallfield is reduced is to low. On the other hand there exists an upper limit of the velocity in the subsonic case in a generator with constant cross-section above which the Machnumber increases to the value 1 due to the energy-conversion in the generator. This leads to the existence of a critical length of the subsonicconstant cross-section generator above which it is notpossible to get nonequilibrium conditions without increasing the Machnumber to the value 1 in the channel. The nonlinearity of the current-voltage-characteristic is not always caused by nonequilibrium ionization. Due to electrothermal instabilities the Faraday-komponent of the current is increased also in the case of higher electrontemperaturewithout increased electron density due to relaxationeffects