Numerical simulation on behavior of subsonic non-equilibrium plasma MHD flow

Transient phenomena in the non-equilibrium subsonic disk MHD generator were examined with timedependent quasi-one-dimensional numerical simulations. Firstly, the transient behavior in the generator at load changes was investigated. When the load resistance is suddenly changed, the fluid-dynamical disturbances are induced near the anode and the cathode. As long as the steady discharge is sustained, the disturbances gradually disappear in the generator channel after propagating and being reflected at the boundaries. Thus, it is confirmed that the generator works stably at load changes, although the over-shooting in Hall potential is induced transiently. Secondly, the response of the generator to the seed fraction change and the influence of the continuous fluctuation of seed fraction on the performance were discussed. When the seed fraction changes to a higher value step-likely, a temporary rise of the enthalpy extraction ratio is caused, and vice versa. For the pulse-like variation, the enthalpy extraction ratio settles after several hundreds of micro seconds, which can be determined by the response time of the MHD flow with reflected waves. For the sinusoidal fluctuation with the period of over ~500μsec, the flow can follow the change of the seed fraction, and the enthalpy extraction ratio fluctuates largely. With the period shorter than ~500μsec. the fluctuation of the enthalpy extraction ratio is reduced since the flow cannot respond. If the seed fraction fluctuates randomly, the time averaged enthalpy extraction ratio remains almost the ideal value, although the peak to peak value can increase. Lastly, the behavior of magneto-acoustic waves in a non-equilibrium subsonic disk MHD generator was studied in detail. The solution of the six-order dispersion relation obtained by linearlizing the set of MHD equations suggested that the magneto-acoustic wave which propagates at the velocity of u_r-a (ur:radial fluid velocity, a:sound velocity), should be damped in the subsonic flow. With time-dependent quasi-onedimensional simulations, it was verified that the pressure disturbance in the subsonic generator is damped at approximately the same rate as the value predicted by the linear theory. From a simplified analytical model, the mechanism of magneto-acoustic instability under the fully ionized seed was discussed, and the damping criterion for the magneto-acoustic wave was clarified.