Performance of a Seed-Free Disk Magnetohydrodynamic Generator With Self-Excited Joule Heating in the Nozzle

In a seed-free inert-gas disk-shaped magnetohydrodynamic (MHD) generator, the effect of the self-excited Joule heating in the nozzle region on the generator performance has been examined with 2-D r-z numerical simulations. The working gas (pure inert gas) can be ionized efficiently with the assistance of the self-excited Joule heating in the nozzle located upstream of the generator channel, even for a small, externally applied preionization power (PP). The increase in the ionization degree in the nozzle and the decrease in the optimum electrical conductivity in the generator channel with increase in the magnetic flux density reduce the optimum PP that maximizes the net enthalpy extraction (EE) ratio. For a typical argon disk MHD generator, a maximum net EE ratio of 25% and an isentropic efficiency of 65% can be obtained at a small PP of 0.3% of the inflow enthalpy under a magnetic flux density of 4 T. The enhancement of the ionization degree in the nozzle and the inhomogeneity of the plasma in the vertical direction because of the current concentration near the anode edge are more marked for argon and xenon, with their higher ionization rates, than for helium.