US3319090A

US3319090A - Magnetogasdynamic generator

Info

Publication number: US3319090A
Application number: US264237A
Authority: US
Inventors: Dzung Lang Shuen
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1962-03-13
Filing date: 1963-03-11
Publication date: 1967-05-09
Anticipated expiration: 1984-05-09
Also published as: CH404787A

Description

QLU'LL F I P85 0 2 May 9, 1967 LANG SHUEN D ZUNG MAGNETOGASDYNAMIC GENERATOR Filed March 11, 1963 INVENTOR.

Lang 5/1uen Dzun PM PM United States Patent 3,319,090 MAGNETOGASDYNAMIC GENERATOR Lang Shuen Dzung, Wettingen, Switzerland, assignor to Aktiengesellschaft Brown Boveri & Cie, Baden, Switzerland, a joint-stock company Filed Mar. 11, 1963, er. No. 264,237 Claims priority, application Switzerland, Mar. 13, 1962, 3,00tl/ 62 1 Claim. (Cl. 310-11) It is a well known fact that magnetogasdynamic generators contain a duct through which flows an ionized gas, for example, a hot gas. When a magnetic field is generated perpendicularly to the direction of gas flow, a voltage is induced in the direction perpendicular to the direction of gas fiow and perpendicular to the direction of the magnetic field. By means of appropriate electrodes, an electric current can be drawn oil and conducted to an external load resistance in the form of electrical power.

Since the gases used as flow media are ionized only to a limited degree because their temperature must be kept within tolerance by the components of the generator, it becomes necessary to seed the gas with an additional material in order to increase its conductivity. This seed material has atoms which ionize easily, for example, caesium, potassium and calcium, and by supplying electrons causes an increase in the electrical conductivity of the gas. The gases of combustion of any fuel can be utilized as carrier gases; the air of combustion can be enriched by oxygen. After flowing through the generator duct, the time gases are piped through a battery of heatexchange devices to utilize the available heat, and thereupon they escape into the atmosphere.

If a closed cycle is employed, the costly seed material such as caesium can be re-used as often as desired by the return of the hot carrier gas which is mixed with the seed material to the cycle after it has flown through the generator duct and the heat-exchange devices, in other words it is re-heated and piped into the generator duct at the speed desired.

The proposal has been made to use rare gases such as argon, neon and helium as carrier gases for a magnetogasdynamic generator with closed cycle because these gases have a low collision profile. The collision profile of a gas stream is defined as a value that is proportional to the probability of collisions of gas atoms with an electron. Since these collisions of gas atoms with the electrons reduce the mobility of the electrons Within an electrical field, a high collision profile is synonymous to a low electrical conductivity. The collision profiles of the above mentioned rare gases are: argon 0.6 1() cm. neon l.l l0- crn. helium 5.7 1O- cm.

The invention relates to a magntogasdynamic generator comprising a duct through which ionized gas flows, means to generate a magnetic field perpendicularly to the direction of gas flow, and two or more pairs of electrodes arranged perpendicularly to the magnetic field, where a hot carrier gas, seeded with substances to increase its conductivity, is circulated in a closed cycle, with a characteristic K-value assigned to said gas, said K-value comprising the molecular weight, the collision profile, the ratio of the specific heat to the gas constant, and a temperature coefficient. It is the invention that a carrier gas is utilized with a K-value which exceeds by at least 10% the corresponding K-value of helium.

The invention is the realization of the idea that the selection of the most appropriate carrier gas must not be based solely on its collision profile but on an over-all value which comprises, in addition to the collision profile, the molecular weight, and the specific heat of the gas as related to one mol.

Design and cost of a magnetogasdynamic generator depend basically on the attainable power density which in turn is determined by the allowable ohmic losses. Maximum power density will be attained if the order of losses allowed is identical with the order of the power generated. However, in this case the magnitude of losses becomes intolerable and lower power densities must be accepted with the losses being limited to approximately 10 to 20% of the power generated. At a specified ratio of the ohmic losses to the power generated and a given magnetic field the power density is proportional to the electrical conductivity, and proportional to the second power of the velocity of flow of the gas.

The conductivity of a carrier gas, seeded with a substance in relatively low concentration, is approximately proportional to the following values:

The a-th power of the temperature where a is a func tion of seed material and temperature, its magnitude being close to the value 10;

The square root of the seed material concentration;

The reciprocal value of the square root of the pressure;

The reciprocal Value of the collision profile of the carrier gas.

However, it is not possible to raise the electrical conductivity by increasing the concentration of the seed material at will because the collision profiles of the seeded molecules are usually much greater than the collision profiles of the carrier gases, and an increase in the concentration of the seed material would lead to an increase of the mean collision profile also. The most elTective concentration of the seed material for maximum conductivity is approximately proportional to the collision profile of the carrier gas. If this most efiective concentration of the seed material would be employed, the

conductivity ceases to be proportional to the reciprocal value of the collision profile of the carrier gas but will be proportional to the reciprocal value of the square root of the collision profile.

The velocity of flow within the duct is produced in a nozzle by expansion of the gas from its state in the heating chamber. At the conversion from thermal to kinetic energy the velocity is increased while temperature and pressure are decreased. The conductivity of the gas will decrease also because the influence of the lower temperature is a preponderant factor. Therefore, there exists an optimum velocity for any given gas for maximum power density. This optimum velocity will depend on the specific heat of the gas as related to one mol. In case of carrier gases with low specific molar heat it is below sonic velocity, and in case of carrier gases with high specific molar heat above the sonic velocity.

It was discovered that the power density of a magnetogasdynamic generator with seed material added in at least approximate optimum concentration and at least approximate optimum gas velocity becomes solely the function of characteristics of the carrier gas it specific values are established for the maximum value of the gas temperature at the entry into the generator duct and the exit from this duct as well as for the maximum value of the pressure. In this case the power density is inversely proportional to the molecular weight of the gas, inversely proportional to the square root of the collision profile of the gas, and increases monotonously with the ratio of specific heat to gas constant. A characteristic K-value can be given for the carrier gas, with the power density of the generator being proportional to this K-value, and the interrelation found is expressed by the formula where:

m represents the molecular weight of the gas,

Q the collision profile,

A the ratio of the specific heat to the gas constant, both in like units,

a the exponent which expresses the conductivity of the gas mixed with the seed material in relation to the temperature.

Therefore, in order to attain a high power density it will be advantageous to select a carrier gas with a low molecular weight because the collision profile of such gas, although being higher per se, aiIect-s the characteristic K-val-ue only by the reciprocal value of its square root but not by the reciprocal value as in case of the molecular weight.

However, the K-value depends also on A, the ratio of the specific heat to the gas constant. For monatomic gases A is constant, its value being 2.5. In case of diatomic gases the A value is 3.5 at room temperature and becomes larger at higher temperatures. Polyatomic gases have a greater specific heat which increases with an increase in temperature, first because the energy of rotation and oscillation contributes to the specific heat, and secondly, because the specific heat is increased greatly due to the dissociation into molecules of lower atomic number. In case of the applicable gas temperature near 2,300 K. the functional relation of A, for values of A up to :12, to the characteristic K-value is represented by the ascending branch of a parabola. Therefore, when a carrier gas is to be selected, next to the molecular weight it is the influence of the specific heat and not the collision profile which is greatly more important for the power density. The invention makes it possible to find a carrier gas which is more advantageous than the gases proposed heretofore in that the characteristic K- value of the carrier gas exceeds by at least the corresponding K-value for helium.

Suitable is, for example, a light and dissociating gas where at the start of the dissociation the ratio of the specific heat to the gas constant is increased greatly and where at the same time the molecular weight In becomes smaller, whereby the magnitude of the characteristic K-value is increased.

A suitable light polyatomic gas is, for example, hydrogen. If hydrogen is utilized as carrier gas, the power density of a magnetogasdynamic generator in which the carrier gas, mixed with seed material and circulated in a closed cycle, has a temperature of 2,250 K. will be proportional to a K-value of 4.51 1O if Q=10- cm. is used, while the corresponding figures for argon and helium being only O.59 lO and l.93 1O respectively. Therefore, it is the surprising result that the rare gases, heretofore proposed as carrier gases, are less advantageous than hydrogen even though their collision profile amounts only to 0.6 10 cm. and 5.7 1O cm? compared to 14 1O cm. for hydrogen. This is due to the fact that at the temperature given, the ratio of the specific heat to the gas constant is 4.2 in case of hydrogen but 2.5 in case of argon and helium, and that the molecular weight of the hydrogen is only 2 compared with 40 for argon and 4 for helium.

A favorable power density can be obtained even if a gas mixture containing hydrogen is used as carrier gas, for example, a gas mixture of helium and hydrogen. In this case the various factors of the characteristic K-value: in (molecular weight), Q (collision profile) and A (ratio of the specific heat to gas constant) must be averaged individually and substituted in the formula for K. It must be established in the form 2 x m E x Q and E x A and substituted for m, Q and A, where x repre sents the molar components of the individual gases in the mixture.

One suitable embodiment of a closed cycle type of magnetogasdynamic generator is shown in schematic form in the attached drawing. Here it will be seen that the generator system includes a combustion chamber 1 which receives fuel such as, for example, coal from a container 2. The fuel burned in the combustion chamber serves to heat compressed, seeded carrier gas admitted to chamber 1 and to deliver the same at high velocity through the duct 3 of the generator. Associated with duct 3 is a magnetic field having a direction perpendicular to the direction of flow of the gas through the duct, this magnetic field being produced by a coil 5 wound upon a magnetic core located outside of the duct. Also associated with duct 3 is at least one pair of oppositely disposed electrodes 4 which are located within the duct. As is Well known, fiow of the hot ionized carrier gas through duct 3 in the direction indicated by the arrow causes a direct voltage to be generated between the electrodes 4, and this voltage is applied to terminals 6 and can then be supplied in this form from these terminals to a load, or it can be converted into an alternating voltage if the load is of the latter type. The magnet coil 5 can also be supplied with its current from the terminals 5, as indicated in the drawing. The carrier gas after passing through the generator duct 3 is delivered to a heat exchanger 10 in which some of the heat energy remaining is extracted, and is then passed in heat exchange relationship through a steam boiler 59 which produces steam for driving a steam turbine 8 which is connected by shafting to drive two axial fiow type compressors 7 arranged in two stages. These compressors serve to re-compress the seeded carrier gas which is then delivered to and fiows through the heat exchanger 10 picking up some heat, and is then admitted once again to the combustion chamber 1 whereupon the cycle is then repeated. The operating cycle is thus a closed one since the seeded carrier gas is re-cycled continuously in a closed path from the combustion chamber 1 through duct 3, heat exchanger 18, steam boiler 9, axial flow compressors 7 and back through heat exchanger 10 to the combustion chamber 1 for re-heating.

In accordance with the invention, the carrier gas utilized in the closed cycle has a K-value which exceeds, by at least 10%, the corresponding K-value for helium.

I claim:

In a magnetogasdynamic generator which includes a duct through which a hot ionized carrier gas seeded with a substance to increase its conductivity is caused to fiow in a closed cycle, means establishing a magnetic field perpendicularly to the path of gas flow in said duct, and at least one pair of spaced electrodes arranged in said duct perpendicularly to said magnetic field and also perpendicularly to the path of gas flow in said duct, the improvement wherein said carrier gas is constituted by a mixture of gases including hydrogen and has a K-value which exceeds by at least 10% the corresponding K-value for helium, said K-value being defined as where m represents the molecular weight of the gas, Q the collision profile, A the ratio of the specific heat to the gas constant, both in like units, a the exponent which expresses the conductivity of the gas mixed with the seed material in relation to the temperature.

and determined by the molecular weight of the carrier gas, its collision profile, the ratio of the specific heat to the gas constant, and the temperature coefiicient.

References Cited by the Examiner UNITED STATES PATENTS 3,099,131 7/1963 Rosa 310-11 X 3,161,789 12/1964 Nagamatsu 31011 3,210,576 10/1965 Brogan 31011 MILTON O. HIRSHFIELD, Primary Examiner.

DAVID X. SLINEY, Examiner.