21-03-2011, 04:47 PM
presented by:
vivek upadhyay
MHD REPORT.doc (Size: 523.5 KB / Downloads: 336)
MAGNETOHYDRODYNAMICS (MHD)
It is the academic discipline which studies the dynamics of electrically conducting fluids.
The idea of MHD is that magnetic fields can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself. The set of equations which describe MHD are a combination of the Navier-Stokes equations of fluid dynamics and Maxwell's equations of electromagnetism. These differential equations have to be solved simultaneously, either analytically or numerically. Because MHD is a fluid theory, it cannot treat kinetic phenomena, i.e., those in which the existence of discrete particles, or of a non-thermal distribution of their velocities, is important.
INTRODUCTION TO MAGNETOHYDRODYNAMICS GENERATORS
The MHD (magnetohydrodynamic) generator or dynamo transforms thermal energy or kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they can operate at high temperatures without moving parts. MHD was eagerly developed because the exhaust of a plasma MHD generator is a flame, still able to heat the boilers of a steam power plant. So high-temperature MHD was developed as a topping cycle to increase the efficiency of electric generation, especially when burning coal or natural gas. It has also been applied to pump liquid metals and for quiet submarine engines.
The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or plasma to generate the currents which generate the electrical energy. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. The functional difference between an MHD generator and an MHD dynamo is the path the charged particles follow.
MHD generators are now practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as combined cycles in which a gas turbine's or molten carbonate fuel cell's exhaust heats steam for steam turbine. The unique value of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded to high efficiency.
Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities. From their perspective the earth is a global MHD dynamo and with the aid of the particles on the solar wind produces the aurora borealis. The differently charged electromagnetic layers produced by the dynamo effect on the earth's geomagnetic field enable the appearance of the aurora borealis. As power is extracted from the plasma of the solar wind, the particles slow and are drawn down along the field lines in a brilliant display over the poles.
This type of thruster generates magnetic fields by passing an electric current through a liquid conductor, such as sea water. Using another magnetic field, the liquid can be pushed in a chosen direction, therefore generating thrust. You can easily make one of these devices from household materials and a couple of neodymium magnets. In the diagram below the small arrows represent the intersecting electric and magnetic fields. the large blue arrow represents the flow of water.
PRINCIPLE OF MHD GENERATOR
The Lorentz Force Law describes the effects of a charged particle moving in a constant magnetic field. The simplest form of this law is given by the vector equation.
• F is the force acting on the particle (vector),
• Q is charge of particle (scalar),
• v is velocity of particle (vector),
• x is the cross product,
• B is magnetic field (vector).
The vector F is perpendicular to both v and B according to the Right hand rule
A VIEW OF MHD GENERATOR
MHD GENERATOR DESIGN
Faraday generator
The Faraday generator is named after the man who first looked for the effect in the Thames river (see History, below). A simple Faraday generator would consist of a wedge-shaped pipe or tube of some non-conductive material. When an electrically conductive fluid flows through the tube, in the presence of a significant perpendicular magnetic field, a charge is induced in the field, which can be drawn off as electrical power by placing the electrodes on the sides at 90 degree angles to the magnetic field.
There are limitations on the density and type of field used. The amount of power that can be extracted is proportional to the cross sectional area of the tube and the speed of the conductive flow. The conductive substance is also cooled and slowed by this process. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C.
The main practical problem of a Faraday generator is that differential voltages and currents in the fluid short through the electrodes on the sides of the duct. The most powerful waste is from the Hall effect current. This makes the Faraday duct very inefficient. Most further refinements of MHD generators have tried to solve this problem. The optimal magnetic field on duct-shaped MHD generators is a sort of saddle shape. To get this field, a large generator requires an extremely powerful magnet. Many research groups have tried to adapt superconducting magnets to this purpose, with varying success.
Hall generator
The most common answer is to use the Hall effect to create a current that flows with the fluid. The normal scheme is to place arrays of short, vertical electrodes on the sides of the duct. The first and last electrodes in the duct power the load. Each other electrode is shorted to an electrode on the opposite side of the duct. These shorts of the Faraday current induce a powerful magnetic field within the fluid, but in a chord of a circle at right angles to the Faraday current. This secondary, induced field makes current flow in a rainbow shape between the first and last electrodes.
Losses are less than a Faraday generator, and voltages are higher because there is less shorting of the final induced current. However, this design has problems because the speed of the material flow requires the middle electrodes to be offset to "catch" the Faraday currents. As the load varies, the fluid flow speed varies, misaligning the Faraday current with its intended electrodes, and making the generator's efficiency very sensitive to its load.
Disc generator
The third, currently most efficient answer is the Hall effect disc generator. This design currently holds the efficiency and energy density records for MHD generation. A disc generator has fluid flowing between the center of a disc, and a duct wrapped around the edge. The magnetic excitation field is made by a pair of circular Helmholtz coils above and below the disk. The Faraday currents flow in a perfect dead short around the periphery of the disk. The Hall effect currents flow between ring electrodes near the center and ring electrodes near the periphery.
Another significant advantage of this design is that the magnet is more efficient. First, it has simple parallel field lines. Second, because the fluid is processed in a disk, the magnet can be closer to the fluid, and magnetic field strengths increase as the 7th power of distance. Finally, the generator is compact for its power, so the magnet is also smaller. The resulting magnet uses a much smaller percentage of the generated power
WORKING OF MAGNETOHYDRODYNAMIC GENERATOR
It is basically relates to the conversion of thermal energy to electrical energy using the magneto hydrodynamic principle which eliminates the turbine or engine used in conventional conversion systems.
In MHD generators a conducting fluid is caused to flow through a channel placed between the poles of an electro magnet. An electric current is induced in the fluid at right angles to both the direction of fluid flow and the magnetic flux and is utilized by an external load connected across electrodes placed in contact with the fluid.
A magneto-hydrodynamic (MHD) electric generator, works on the principle that any conductor of electricity that is moved through a magnetic field will generate in itself a current of electricity. This applies not only to copper wires (as in conventional generators), but to gases, which become conductors when they are made so hot that some of their atoms separate (ionize) into electrically charged particles. If forced through a magnetic field, a stream of ionized gas causes an electrical current to flow across it. The gas must be so hot (at least 4,000° F.) that it destroys many structural materials. Another problem is the poor conductivity of most gases big enough to supply commercial power. A major advantage: an MHD generator has no primary moving parts, with the exception of those in the relatively simple compressor.
Coal is burn in a stream of compressed, preheated air. While passing through the flame, the air gets hotter, expands .
A small amount of potassium chloride fed into it increases its ionization and makes it a better electrical conductor. Then the stream shoots into a hollow cone made of a heat-resisting, nonconducting material . Electrical coils outside the cone create a strong magnetic field. As the gas speeds through, a powerful current of electricity flows across it and is collected by two electrodes inside the cone.
From Nose Cones:-. Most of the electricity generated by the system comes from the electrodes, but waste heat can be harnessed to drive a conventional turbogenerator, adding importantly to the system's efficiency
Most of the electricity generated by the system comes from the electrodes, but waste heat can be harnessed to drive a conventional turbogenerator, adding importantly to the system's efficiency. The estimation is that a 450,000-kw. coal-fired MHD generator will produce electricity with the sensational thermal efficiency of 55%. The best that conventional plants can do is 40%.
Even more exciting is the possibility of using the MHD system with a nuclear reactor. In this case the gas will probably be argon or helium, laced with cesium to make it more conductive. It will circulate through the reactor, then through the generator and back to the reactor again. This system will have to wait for the development of high-temperature reactor cores, but Project Rover, the Atomic Energy Commission's nuclear-rocket program, has shown that the prospects of this are promising.
vivek upadhyay
MHD REPORT.doc (Size: 523.5 KB / Downloads: 336)
MAGNETOHYDRODYNAMICS (MHD)
It is the academic discipline which studies the dynamics of electrically conducting fluids.
The idea of MHD is that magnetic fields can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself. The set of equations which describe MHD are a combination of the Navier-Stokes equations of fluid dynamics and Maxwell's equations of electromagnetism. These differential equations have to be solved simultaneously, either analytically or numerically. Because MHD is a fluid theory, it cannot treat kinetic phenomena, i.e., those in which the existence of discrete particles, or of a non-thermal distribution of their velocities, is important.
INTRODUCTION TO MAGNETOHYDRODYNAMICS GENERATORS
The MHD (magnetohydrodynamic) generator or dynamo transforms thermal energy or kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they can operate at high temperatures without moving parts. MHD was eagerly developed because the exhaust of a plasma MHD generator is a flame, still able to heat the boilers of a steam power plant. So high-temperature MHD was developed as a topping cycle to increase the efficiency of electric generation, especially when burning coal or natural gas. It has also been applied to pump liquid metals and for quiet submarine engines.
The basic concept underlying the mechanical and fluid dynamos is the same. The fluid dynamo, however, uses the motion of fluid or plasma to generate the currents which generate the electrical energy. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. The functional difference between an MHD generator and an MHD dynamo is the path the charged particles follow.
MHD generators are now practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as combined cycles in which a gas turbine's or molten carbonate fuel cell's exhaust heats steam for steam turbine. The unique value of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded to high efficiency.
Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities. From their perspective the earth is a global MHD dynamo and with the aid of the particles on the solar wind produces the aurora borealis. The differently charged electromagnetic layers produced by the dynamo effect on the earth's geomagnetic field enable the appearance of the aurora borealis. As power is extracted from the plasma of the solar wind, the particles slow and are drawn down along the field lines in a brilliant display over the poles.
This type of thruster generates magnetic fields by passing an electric current through a liquid conductor, such as sea water. Using another magnetic field, the liquid can be pushed in a chosen direction, therefore generating thrust. You can easily make one of these devices from household materials and a couple of neodymium magnets. In the diagram below the small arrows represent the intersecting electric and magnetic fields. the large blue arrow represents the flow of water.
PRINCIPLE OF MHD GENERATOR
The Lorentz Force Law describes the effects of a charged particle moving in a constant magnetic field. The simplest form of this law is given by the vector equation.
• F is the force acting on the particle (vector),
• Q is charge of particle (scalar),
• v is velocity of particle (vector),
• x is the cross product,
• B is magnetic field (vector).
The vector F is perpendicular to both v and B according to the Right hand rule
A VIEW OF MHD GENERATOR
MHD GENERATOR DESIGN
Faraday generator
The Faraday generator is named after the man who first looked for the effect in the Thames river (see History, below). A simple Faraday generator would consist of a wedge-shaped pipe or tube of some non-conductive material. When an electrically conductive fluid flows through the tube, in the presence of a significant perpendicular magnetic field, a charge is induced in the field, which can be drawn off as electrical power by placing the electrodes on the sides at 90 degree angles to the magnetic field.
There are limitations on the density and type of field used. The amount of power that can be extracted is proportional to the cross sectional area of the tube and the speed of the conductive flow. The conductive substance is also cooled and slowed by this process. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C.
The main practical problem of a Faraday generator is that differential voltages and currents in the fluid short through the electrodes on the sides of the duct. The most powerful waste is from the Hall effect current. This makes the Faraday duct very inefficient. Most further refinements of MHD generators have tried to solve this problem. The optimal magnetic field on duct-shaped MHD generators is a sort of saddle shape. To get this field, a large generator requires an extremely powerful magnet. Many research groups have tried to adapt superconducting magnets to this purpose, with varying success.
Hall generator
The most common answer is to use the Hall effect to create a current that flows with the fluid. The normal scheme is to place arrays of short, vertical electrodes on the sides of the duct. The first and last electrodes in the duct power the load. Each other electrode is shorted to an electrode on the opposite side of the duct. These shorts of the Faraday current induce a powerful magnetic field within the fluid, but in a chord of a circle at right angles to the Faraday current. This secondary, induced field makes current flow in a rainbow shape between the first and last electrodes.
Losses are less than a Faraday generator, and voltages are higher because there is less shorting of the final induced current. However, this design has problems because the speed of the material flow requires the middle electrodes to be offset to "catch" the Faraday currents. As the load varies, the fluid flow speed varies, misaligning the Faraday current with its intended electrodes, and making the generator's efficiency very sensitive to its load.
Disc generator
The third, currently most efficient answer is the Hall effect disc generator. This design currently holds the efficiency and energy density records for MHD generation. A disc generator has fluid flowing between the center of a disc, and a duct wrapped around the edge. The magnetic excitation field is made by a pair of circular Helmholtz coils above and below the disk. The Faraday currents flow in a perfect dead short around the periphery of the disk. The Hall effect currents flow between ring electrodes near the center and ring electrodes near the periphery.
Another significant advantage of this design is that the magnet is more efficient. First, it has simple parallel field lines. Second, because the fluid is processed in a disk, the magnet can be closer to the fluid, and magnetic field strengths increase as the 7th power of distance. Finally, the generator is compact for its power, so the magnet is also smaller. The resulting magnet uses a much smaller percentage of the generated power
WORKING OF MAGNETOHYDRODYNAMIC GENERATOR
It is basically relates to the conversion of thermal energy to electrical energy using the magneto hydrodynamic principle which eliminates the turbine or engine used in conventional conversion systems.
In MHD generators a conducting fluid is caused to flow through a channel placed between the poles of an electro magnet. An electric current is induced in the fluid at right angles to both the direction of fluid flow and the magnetic flux and is utilized by an external load connected across electrodes placed in contact with the fluid.
A magneto-hydrodynamic (MHD) electric generator, works on the principle that any conductor of electricity that is moved through a magnetic field will generate in itself a current of electricity. This applies not only to copper wires (as in conventional generators), but to gases, which become conductors when they are made so hot that some of their atoms separate (ionize) into electrically charged particles. If forced through a magnetic field, a stream of ionized gas causes an electrical current to flow across it. The gas must be so hot (at least 4,000° F.) that it destroys many structural materials. Another problem is the poor conductivity of most gases big enough to supply commercial power. A major advantage: an MHD generator has no primary moving parts, with the exception of those in the relatively simple compressor.
Coal is burn in a stream of compressed, preheated air. While passing through the flame, the air gets hotter, expands .
A small amount of potassium chloride fed into it increases its ionization and makes it a better electrical conductor. Then the stream shoots into a hollow cone made of a heat-resisting, nonconducting material . Electrical coils outside the cone create a strong magnetic field. As the gas speeds through, a powerful current of electricity flows across it and is collected by two electrodes inside the cone.
From Nose Cones:-. Most of the electricity generated by the system comes from the electrodes, but waste heat can be harnessed to drive a conventional turbogenerator, adding importantly to the system's efficiency
Most of the electricity generated by the system comes from the electrodes, but waste heat can be harnessed to drive a conventional turbogenerator, adding importantly to the system's efficiency. The estimation is that a 450,000-kw. coal-fired MHD generator will produce electricity with the sensational thermal efficiency of 55%. The best that conventional plants can do is 40%.
Even more exciting is the possibility of using the MHD system with a nuclear reactor. In this case the gas will probably be argon or helium, laced with cesium to make it more conductive. It will circulate through the reactor, then through the generator and back to the reactor again. This system will have to wait for the development of high-temperature reactor cores, but Project Rover, the Atomic Energy Commission's nuclear-rocket program, has shown that the prospects of this are promising.