Optimization of Generator Parameters and of Overall Thermal Efficiencies for an MHD-Reactor System

The basic parameters for a direct cycle gas-cooled reactor MHD generating plant are considered. The reactor coolant temperature is limited to the range 1600 to 2000°K. Increased gas pressure normally implies reduced reactor plant size. In an MHD plant, however, increased pressure will normally result in reduced specific power, for which a lower limit of 5 to 10 MW/m3 is assumed. Extrathermal ionization due to enhanced electron temperature is assumed. The gases helium and argon with potassium and caesium seeding are considered. Computed values of power density for the generator are presented for various gas temperatures betweeenlOlO and 2000°K, magnetic field strengths up to 8 T and a generator coefficient of 0.8. The effect of varying Mach number, for mainly subsonic flow, is illustrated in plots of power density versus seed pressure. Optimum seed fraction values result in increase in conductivity of a few orders of magnitude when compared to results computed at seed fractions appropriate to optimized thermal conditions. For argon-potassium the optimum seed for non-equilibrium conditions is in general an order of magnitude less than that for thermal conditions. Plots of computed values of electric field for varying degrees of ionization, at fixed current density, illustrate the effect of reduced conductivity on optimum seed fraction. Comparison is made with similar results obtained by static gas discharge experiments. It is shown that helium is worse than argon by a factor 10 on pressure, for the same MHD power density. For helium at a temperature of 1000°K, ''ion-slip'' starts to become dominant and subatmospheric operation is necessary for attaining power densities as low even as 2 MW/m3. The ionic mobility of helium-potassium is, in fact, much higher than for argon-potassium and for the former combination especially, and in particular for a segmented geometry generator, the effect on Hall field is of considerable importance. Using the values of pressure and temperature suggested by this study the thermal efficiency of a combined reactor-MHD-steam plant is established for the various values of reactor outlet temperature, MHD duct inlet and outlet temperatures and steam generator inlet and outlet temperatures. The gas side was taken to operate on a Brayton cycle with heat exchange and the steam plant on a Rankine cycle with superheat, reheat as necessary both with and without regenerative heating. An expression for the thermodynamic efficiency of the combined gas-steam cycle is derived taking account of the heat lost in the cooler. The procedure for establishing temperatures at inlet and outlet to the steam generator to ensure maximum overall efficiency is described and the variation of efficiency with recuperator ratio is illustrated. These results show that a recuperator ratio of 1 gives almost the same overall efficiency as a ratio of 0 and intermediate values result in a lower efficiency. The remainder of the paper compares the size of reactor, MHD duct and steam generator for the conditions implied by the duct and cycle analysis. (author)