To achieve high efficiency of closed cycle MHD generator it is necessary to reduce plasma and gas fluctuations and nonuniformities as much as possible.Three different physical phenomena evolved together are considered and evaluated in an attempt to find out most significant physical and technical problems needed to be clarified for further improvement of the efficiency of closed cycle MHD powerconversion. These phenomena are plasma nonuniformities associated with ionization instability, flow turbulence and seed fraction fluctuations caused by imperfection of seed injection system.First, spatial and temporal characteristics of the flow turbulence in the MHD channel are analyzed. It has been concluded from the estimation of the dissipation energy that the velocity pulsation of the turbulent vortex having the maximum space scaleabout twice lower than the height of the MHD channel has the order of magnitude about one tenth of the average flow velocity. The life span of such a vortex is about hundred microseconds.Second, the effect of gasdynamic turbulent fluctuations on the ionization instability is analyzed. It has been concluded from the estimation of critical Hall parameter pulsation that the flow velocity fluctuations are high enough to violate ionizationstability condition. Characteristic time and space scales of ionization instability are much lower compare with spatial-temporal characteristics of the flow turbulence. Therefore it has been revealed that flow turbulence causes large deviations from fullseed ionization and as a result the development of ionization instability.Finally, the effect of seed fluctuations on the ionization instability and performances of closed cycle MHD generator is considered and evaluated. Space seed nonuniformities caused by imperfection of seed injection system and local space nonuniformitiescaused by seed injection in the form of liquid droplets are considered. It is concluded that most significant influences on the performances of MHD generator exert the radial nonuniformity of seed fraction in the hot duct and local space nonuniformity.On the basis of obtained results it seems to be quite reasonable that the concept of ionization instability suppression by fully ionized seed should be revised because ionization instability can not be suppressed completely. For such a purpose it isproposed to investigate the effective plasma parameters under flow turbulence and local seed fluctuations.On the basis of the conducted research, it seems to be most reasonable that at present stage of experimental study the seed fraction fluctuations exert the most significant influence on the performances of existent experimental closed cycle MHDgenerators. It is proposed to investigate seed nonuniformities and improve seed injection system to reduce seed fraction fluctuations.
展开▼
