Stability of large-scale coal-fired MHD channels

Stability of large-scale coal-fired MHD channels is studied by (1) linearized stability analysis, and (2) time-dependent 1-D analysis. The channel length is 15 m with 600 electrode pairs, and the output power ranges from 220.6 MW through 258.7 MW. Linearized stability analyses show that the Faraday channels operated with fixed loading resistance are stable, whereas the two waves of u and u–a (u, a: gas and sound velocity) become unstable in the Faraday channel with fixed loading factor. Two waves of u and u–a are unstable in the diagonal channel with fixed loading current and the u + a wave becomes unstable in the diagonal channel with fixed electrode current. Time-dependent one-dimensional analyses indicate that the Faraday channels with fixed load resistance are smooth without growth of fluctuation. The diagonal channels with fixed electrode current are smooth with no fluctuation, though the linear theory indicates that the u + a wave is unstable. The diagonal channel with fixed load current suffers large disturbance along the latter half of the channel, being consistent with the linearized analysis which indicates that the u - a and u waves are unstable.     

New power consolidation-inversion-control system for faraday MHD generator

The authors propose a new power consolidation-inversion-control system for the Faraday MHD generator using the voltage source PWM inverters. The dc output power for each electrodes pair of the MHD generator is at first inverted into three-phase ac power by a voltage source PWM inverter, and then the ac powers are consolidated by transformers. The proposed system does not need such expensive equipment as an ac filter or phase modifier and can independently and simultaneously control the active and reactive powers provided to the electric power system. Numerical simulations of the whole system, including the Faraday MHD generator, the proposed power consolidation-inversion-control system, and the electric power system, show that the proposed system can stably and steadily transmit and control the electric power from the MHD generator to the electric power system. It is also confirmed that the proposed system can independently and simultaneously control the active and reactive powers and can be used as a fast power controller.     

Performance characteristics of open-cycle linearly-diverging diagonal type single- and multiple-load generators

First, a new quasi-two-dimensional theory is proposed in order to analyse the whole electrical and gasdynamical performances of a diagonal type generator. Next, the performances of a large scale linearly-diverging diagonal type generator with thermal input of about 2000 MW driven by combustion gas of heavy oil fuel are evaluated. From numerical calculation results of single-load generators, it is made clear that the distributions of electrical and gasdynamical quantities in the inlet and exit regions of the generator channel obtained by the quasi-two-dimensional theory fairly differ from the ones obtained by the conventional quasi-one-dimensional theory. The generator output power obtained from the quasi-one-dimensional calculation shows 10–20% larger values compared to the results of the quasi-two-dimensional calculation. Choking phenomenon is apt to occur in the channel of small diverging angle. The diagonal angle and load current have great influence on the generator performances. The best angle is about 30° and the best value of load current 4 × 10⁴ A. A multiple-load generator can give considerably larger output power than the single-load one by suitably sharing the load current with several loads.     

Fracture toughness in the transition region

In order to study the origin of the scatter in the fracture toughness in the transition region and to specify the lower bound of the scatter, approximately 100 pieces 0.5T-CT NiCrMoV steel specimens were tested in the transition region, and their fracture surfaces were investigated. Major portion of the scatter was caused by the scatter in the length of preceding dimple crack which was generated at the fatigue precrack before conversion to final cleavage fracture. A method to predict the lower bound of the scatter in the small-specimen fracture toughness was proposed. It employs Weibull plot of new parameter J₁ which represents the scatter in the cleavage strength of material. The cleavage fracture origin may be associated with micro-stress-concentration in the microstructure which may not be caused by the micromechanism in terms of grain size unit.     

Study of a coal-fired transonic disk MHD generator for a power system with CO2 recovery

A conceptual design of a transonic disk MHD channel is carried out for a power generation system with liquefaction recovery of CO₂. A previous study has shown that the subsonic disk MHD channel has rather poor performance and the supersonic disk channel yields sufficiently high power output, although its stability should be improved. The present paper proposes a transonic disk channel which can be stably operated with high power output. It is assumed that the transition between supersonic flow and subsonic flow is accompanied by a cylindrical shock wave in the channel. The transonic channel yields enthalpy extraction ratios of 20.2 and 22.9%, respectively, for thermal inputs of 1100 and 2000 MW, and is nearly equal to the performance of the supersonic channel. The stability of the transonic disk channel is examined by r-0 two-dimensional time-dependent calculations. The two-dimensional analysis shows that the transonic disk channel works stably with fewer load sections than the supersonic channel even when inlet perturbations are added. © 1998 Scripta Technica. Electr Eng Jpn, 122(2): 21–29, 1998