High Hall Parameter Research Articles

Analyse the disk closed cycle MHD generator performance with the influence of channel characteristics

The enthalpy extraction ratio is one of the most significant parameter of a disk closed cycle MHD generator. There are two methods to improve the enthalpy extraction, those are the increase of channel cross-sectional area ratio and the implementation of inlet swirl. In this study, the mechanism of enthalpy extraction improvement has been confirmed by the two-dimensional numerical calculation. As a result, by increasing the channel cross-sectional area ratio of the disk MHD generator, the increase of static pressure and the velocity deceleration can be suppressed due to the Lorentz force, and it is possible to maintain a high flow velocity inside the channel and a high Hall parameter. The implemention of inlet swirl is possible to maintain a low static pressure inside the channel and the enthalpy extraction ratio rises due to the increase of Hall parameter. In addition, the channel cross-sectional area ratio increases due to the swirl implementation, the static pressure is kept low, and the channel inlet flow velocity increases. This also leads to the increase of enthalpy extraction ratio, that is the increase of output power.

Computational study of strong magnetic field generation in a nonspherical, cone-guided implosion

It is known that a strong magnetic field is generated in laser plasma, especially during an asymmetric implosion such as in a cone-guided implosion used for fast ignition of nuclear fusion. As the first approximation, in this paper, the magnetic field for a nonspherical, cone-guided implosion is simulated using temporal evolution equations of the magnetic field coupled with the simulated result of a two-dimensional radiation hydrodynamic simulation for fast ignition. The results show that the magnetic field is generated by the ∇Te × ∇ne term, and is compressed by the implosion. In addition, we find that the magnetic field reaches 5 MG at maximum compression, which has not been investigated previously. Also, a high Hall parameter region appears between the cone tip and core plasma. This magnetic field is strong enough to affect the implosion dynamics and the hot electron transport, and should therefore be considered in simulations for fast ignition.

Generalized Ohm’s law and potential equation in computational weakly-ionized plasmadynamics

A variant of the generalized Ohm’s law that is suited for a weakly-ionized multicomponent plasma in a magnetic field is here derived. The latter takes into consideration the current due to the non-neutrality of the plasma, the current due to the Hall effect, and the currents due to the ion slip associated with each type of ion. An equation for the electric field potential applicable to a non-neutral multicomponent plasma in the presence of a magnetic field is then presented. Despite some similarities between the potential equation and the Poisson equation, it is argued that the discretization of the potential equation cannot be accomplished in the same manner by using only central differences. It is here proven (and subsequently verified through a test case) that when the plasma exhibits conjunctly a high Hall parameter and a high electrical conductivity gradient, the centered stencils introduce spurious oscillations which can lead to severe numerical error. A novel discretization of the potential equation consisting of a blend of central and upwind differences is then presented. The proposed scheme is consistently monotonic for any value of the Hall parameter and is second-order accurate except in the vicinity of discontinuities.

Three-dimensional numerical analyses on magnetohydrodynamic accelerator

A magnetohydrodynamic (MHD) accelerator is proposed as a next generation propulsion system. The fundamental acceleration performance has been, so far, evaluated using one-dimensional (1-D) numerical analysis. However, the actual phenomena in the accelerator channel are highly coupled with boundary layers and with electrode phenomena and are three-dimensional. In this paper, research for investigating the three-dimensional (3-D) phenomena of an MHD accelerator is described. The validity of the 3-D numerical analysis is assessed through comparisons with the results of 1-D analysis. An existence of a local Hall current not predicted through 1-D analysis enhances the Joule dissipation and induces a secondary flow. It was found that a significant local Hall current influences the growth of the velocity overshoot, and this phenomenon is promoted particularly in a high Hall parameter environment. This study of 3-D analysis offers a more realistic prediction of the accelerator performance of the MHD accelerator than that of the 1-D predictions.

Experimental studies on isentropic efficiency of a nonequilibrium MHD disk generator

Isentropic efficiency of the nonequilibrium MHD power generator was studied by a shock tube driven disk generator. Cesium seeded helium was used as a working gas. From the measurements of Faraday current density distribution, it was possible to estimate the general tendency of Joule dissipation in the generator. The Joule dissipation did not decrease due to the occurrence of nonuniformity of the plasma when external load resistance was low, although it decreased with the decrease in the load resistance when the load resistance was high. The electrical efficiency increased with the increase in applied magnetic flux density. This fact is thought to be caused by high Hall parameter and the stabilization of the plasma due to high degree of seed ionization.

Improvement of enthalpy extraction over 30% using a disk MHD generator with inlet swirl

Performance of a disk-type CCMHD generator has been investigated. In the present power generation experiments, special attention has been devoted to the effect of inlet swirl on generator performance. In order to clarify this effect, one-dimensional numerical simulation has also been performed. The results showed that the enthalpy extraction ratio was successfully improved up to 32.5% by employing inlet swirl with the swirl ratio S(= U 0 U r ) of 1. Static pressure and Hall potential distributions were measured and showed that the inlet swirl reduced the steep increase in static pressure owing to the strong Lorentz force at the inlet of the disk channel. Hence, relatively low static pressure and, thus, high Hall parameter were kept throughout the channel, and therefore, the remarkable increase in output performance was achieved. Numerical simulation can qualitatively confirm these experimental results well. Realization of high Hall parameter and high enthalpy extraction with smaller pressure ratio of inlet to exit for the case with inlet swirl led to improvement of the isentropic efficiency.

High-Enthalpy Extraction Experiments with Disk MHD Generators.

Power generation experiments have been performed for disk MHD generators with different channel shapes. The increase in the enthalpy extraction ratio by enlargement of the area ratio has been shown experimentally, and the highest enthalpy extraction ratio of 18.0% was achieved. The experimental results have shown that for the generator with the large area ratio, the wall static pressure in the MHD channel is kept lower than that for the small-area-ratio generator. These phenomena suggest that a high Hall parameter is obtained by enlargement of the area ratio. Furthermore, the swirl ratio at the channel exit was measured and was characterized by the ratio between Hall current and mass flow rate of the working gas.

High Enthalpy Extraction Experiments with Disk MHD Generators.

MHD power generation experiments have been performed for disk-type generators with different channel shapes. The increase in the enthalpy extraction ratio due to enlargement of the area ratio has been shown experimentally and the highest enthalpy extraction ratio of 18.0% was achieved. The experimental results have shown that for a generator with a large area ratio, the wall static pressure in the MHD channel is kept lower than that for a small area ratio generator. These phenomena suggest that a high Hall parameter is obtained by enlargement of the area ratio. Furthermore, a swirl ratio at the channel exit was measured and was characterized by the ratio of the Hall current and the mass flow rate of the working gas.