What is a Magnetohydrodynamic Drive?

Magnetohydrodynamics is a branch of physics that studies the interaction of plasma conductive fluids and electromagnetic fields. Magnetic fluid mechanics treats plasma as a continuous medium, and its characteristic scale is much larger than the average free path of particles, and the characteristic time is much larger than the average collision time of particles. It is not necessary to consider the motion of a single particle. Because magnetohydrodynamics only cares about the average effect of fluid elements, it is an approximate description method that can explain most of the phenomena in plasma and is widely used in the research of plasma physics. A more accurate description method is to consider the kinetic theory of particle velocity distribution function. The basic equations of magnetohydrodynamics are the Navier-Stokes equations in fluid mechanics and the Maxwell equations in electrodynamics. Magnetohydrodynamics was founded by Swedish physicist Hannibal Alvin, who won the 1970 Nobel Prize in Physics.

Semiconductor magnetohydrodynamic model is a type of macro-hydrodynamic equations that appears in the science of semiconductor devices. It describes electrons and ions under the influence of self-compatible electromagnetic fields. It describes semiconductor devices that operate under high-frequency conditions. Internally charged transport process. The model equations are composed of the conservation law equations of the mass and velocity of the electrons and the Maxwell equations of the electromagnetic field.

There have been many studies on semiconductor magnetohydrodynamic models. As far as the types of semiconductor magnetohydrodynamic model equations are concerned, it is a class of symmetric quasi-linear hyperbolic equations. In general, even under smooth small initial conditions, the classical solution of quasi-linear hyperbolic equations will still rupture within a finite time and generate shock waves. ^[1]

In magnetohydrodynamics, a plasma can be considered good

The science that studies the properties of magnetic fluid motion, also known as hydromagnetics or magnetogas dynamics. A branch of dynamics, an interdisciplinary subject of electrodynamics and fluid dynamics. The object of study is fluid motion that interacts with magnetic fields.

The main research contents of magnetohydrodynamics are: the motion properties of liquid metal; the theoretical and applied research of the mobility of ionized gas or plasma. Including controlled thermonuclear reactions, simulation of ultrasonic flight conditions, ion force for external space propulsion, braking of space vehicles returning to the atmosphere, high-energy particle accelerators, microwave generators, thermionic energy conversion devices, applications of thin metal coatings, Studies of the universe and upper atmospheric phenomena.

The basic system of magnetohydrodynamics has 16 scalar equations, including 16 unknown scalars, so it is complete. Combining boundary conditions can solve this system of equations. In magnetohydrodynamics, the plasma can be regarded as a good conductor, and the characteristic time of the electromagnetic field change is much longer than the time of particle collision.

Magnetohydrodynamics is mainly used in three areas: astrophysics, controlled thermonuclear reactions, and industry.

Magnetohydrodynamics

Stars and interstellar gases in the universe are both plasma and have magnetic fields. Therefore, magnetohydrodynamics was first developed and applied in astrophysics, solar physics, and geophysics. At present, research topics on the sun include: the nature and origin of the solar magnetic field, and the influence of the magnetic field on the corona, sunspots, and flares. In addition, there is the possibility that no force field exists in interstellar space, bow shocks generated by the interaction of the solar wind and the Earth's magnetic field, the eruption of nova and supernova, the origin of the Earth's magnetic field, and so on.

Aspects of magnetohydrodynamics controlled thermonuclear reactions

The application of this aspect of the controlled thermonuclear party is likely to enable humans to obtain huge energy from deuterium in seawater. The purpose of a controlled thermonuclear reaction is to heat a gas composed of light elements to a temperature high enough for nuclear fusion to occur and constrain it for a sufficient time so that the energy generated by the nuclear reaction is greater than the energy consumed. For deuterium and tritium mixtures, the temperature is required to reach 50 million to 100 million Kelvin and the product of particle density and constraint time is not less than 10 seconds / cm (Rausson condition). Tokamak (annular magnetic restraint device) has shown superiority in the study of controlled thermonuclear reactions. The United States, the Soviet Union, and some western European countries have made progress in their research on Tokamak, but only plasmas that meet the Launson condition with a single index are obtained. Plasmas that meet the Launson condition in temperature, density, and constraint time are not obtained. The operating range of magnetic mirrors, tokamak, and other magnetic restraints are all limited by stability, that is, the greater the current or particle density, the worse the stability, so the prediction of equilibrium and large-scale instability in the plasma must be carried out In order to obtain a stable and fully utilized magnetic field tokamak magnetic restraint device.

Magnetic fluid dynamics

In addition to being related to the development and utilization of nuclear fusion energy, magnetohydrodynamics is also closely related to the generation of magnetic fluids. The principle of magnetic fluid power generation is to replace the generator rotor with plasma, eliminating the need for rotating components. This can increase the efficiency of ordinary thermal power stations or nuclear power stations by 15-20% or even higher, which can save energy and reduce pollution. In order to improve the thermal efficiency of magnetic fluid power generation equipment, it is necessary to use magnetic fluid mechanics to analyze the flow laws, heat transfer, mass transfer laws, and electrical characteristics in the power generation channel. Research on the use of pulverized coal as fuel for magnetic fluid power generation is of great significance to coal-producing countries. This research is currently developing in the industrial power generation stage. The Soviet Union has realized natural gas magnetic fluid power generation.

Equipment that replaces the rotor of a motor with a conductive fluid, that is, a device that uses magnetic force to drive a conductive fluid includes an electromagnetic pump and a magnetohydrodynamic space thruster (see Electromagnetic Propulsion). Electromagnetic pumps have been used for the transfer of liquid metals in the heat transfer circuits of nuclear power plants, the automatic quantitative pouring and stirring of molten metals in the metallurgical and foundry industries, and the transportation of hazardous and dangerous fluids such as mercury, potassium and sodium in the chemical industry. Electromagnetic propulsion research uses magnetic field forces to accelerate the plasma in order to obtain a much larger specific impulse than chemical rockets.

When the aircraft reenters the atmosphere, the shock waves and air friction on the aircraft cause the surface air of the aircraft to be heated and ionize into plasma. Therefore, the magnetic field can be used to control the heat transfer and resistance to the aircraft. But because the magnetic field device is too heavy, this kind of assumption has not been realized.