BACKGROUND OF THE INVENTION

This invention relates generally to pulse detonation engines and more particularly to pulse detonation engines utilizing magnetohydrodynamic flow control.

Most internal combustion engines currently used for propulsion rely on deflagration combustion whereby the combustion effects occur at relatively slow rates (i.e., less than the speed of sound within the combustible mixture) and at constant pressure. Detonation combustion, however, occurs at rates well in excess of the speed of sound and simultaneously provides a significant pressure rise. Because of the advantageous thermodynamic cycle, there is a high degree of interest in developing propulsive devices that rely on detonation combustion rather than deflagration combustion.

One such device is a pulse detonation engine that uses an intermittent combustion process to create a temperature and pressure rise by detonating a flammable mixture. The conditions for detonation are governed by the environment of the mixture (pressure, temperature, equivalence ratio, etc.) such that when enough energy is released to start ignition, the chemical kinetics occur at supersonic speeds. A pulse detonation engine is typically a tube of a specified length that is open at the aft end and includes some sort of valve device at the front end to keep the detonation process from traveling forward. In operation, a charge of air and fuel is fed into the tube through the valve, and the valve is then closed. Detonation of the fuel-air mixture is initiated by an igniter located in the tube, and the resulting detonation shock waves travel down the tube, raising both the temperature and the pressure of the products. The combustion products are expelled out of the open aft end, creating a pulse of forward thrust. When the shock waves have reflected within the tube to the appropriate conditions, a new charge is fed into the tube through the valve and the cycle repeats. It is generally desirable to generate pulses at a high frequency to produce smooth, nearly steady state propulsion.

Upon ignition, the resulting pressure waves and detonation flame front will tend to travel in both longitudinal directions. In current pulse detonation devices, however, ignition is initiated at the forward end of the tube so that the waves will generally travel downstream toward the open exhaust end. The valve is provided at the forward end of the tube to prevent pressure waves from escaping out the front of the device and, more importantly, to prohibit the detonation flame front from traveling into the fuel-air inlet system. The pulse detonation cycle requires that the valve operate at extremely high temperatures and pressures and must also operate at exceedingly high frequencies to produce smooth propulsion. These conditions significantly reduce the high cycle fatigue (HCF) reliability of conventional valve systems, such as poppet or flapper-type valves.

Accordingly, it would be desirable to have a high frequency valving or flow control system for pulse detonation engines that is lightweight, reliable, easily controlled and offers minimal performance loss.

BRIEF SUMMARY OF THE INVENTION

The above-mentioned need is met by the present invention, which provides a pulse detonation engine that includes a tube having an open forward end and an open aft end and a fuel-air inlet formed in the tube at the forward end. An igniter is disposed in the tube at a location intermediate the forward end and the aft end. A magnetohydrodynamic flow control system is located between the igniter and the fuel-air inlet for controlling detonation in the tube forward of the igniter. The magnetohydrodynamic flow control system creates a magnetic field forward of the igniter to dissipate the forward traveling detonation flame front.

The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIGS. 1 and 2

show a schematic cross-section of a pulse detonation engine having a first embodiment of a magnetohydrodynamic flow control system.

FIG. 3

is a perspective view showing an alternative configuration for the embodiment of

FIGS. 1 and 2

.

FIGS. 4 and 5

show a schematic cross-section of a pulse detonation engine having a second embodiment of a magnetohydrodynamic flow control system.

FIGS. 6 and 7

show a schematic cross-section of a pulse detonation engine having a third embodiment of a magnetohydrodynamic flow control system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,

FIGS. 1 and 2

show a first embodiment of a pulsed detonation engine

10

capable of generating forward thrust and useful in many propulsive applications such as a turbofan augmentor, a replacement for the high pressure turbomachinery of a conventional gas turbine engine, and a rocket engine. The pulse detonation engine

10

includes a tube

12

having a prescribed length and defining an internal combustion chamber

14

. The tube

12

has an open forward end

16

and an open aft end

18

. The open forward end

16

functions as a fuel-air inlet

20

to the tube

12

, while the open aft end

18

provides an exhaust to the ambient. A fuel-air mixture from a source

22

enters the combustion chamber via the inlet

20

. The source

22

can be any means of providing a mixture of fuel and air, many of which are known in the combustion art.

An igniter

24

is provided in the tube

12

at a location intermediate the forward and aft ends

16

,

18

, and preferably closer to the forward end

16

than the aft end

18

. The igniter

24

produces sufficient energy to detonate the fuel-air mixture in the combustion chamber

14

. The region of the combustion chamber

14

in the immediate vicinity of the igniter

24

is referred to herein as the detonation zone. Detonation combustion depends on the pressure, temperature and equivalence ratio of the fuel-air mixture, as well as the amount of energy released to start ignition. By locating the detonation zone closer to the forward end

16

, a larger portion of the tube length is devoted to generating thrust. The overall length of the tube

12

will depend on the desired operating frequency of the pulse detonation engine

10

.

A magnetohydrodynamic (MHD) flow control system

26

is located between the detonation zone and the fuel-air inlet

20

for controlling the detonation process in the forward portion of the tube

12

. The MHD flow control system

26

comprises an electric field coil

28

wrapped around the exterior of the tube

12

at an axial location that is between the igniter

24

and the fuel-air inlet

20

. A pair of magnets

30

are arranged in proximity to the electric field coil

28

and on opposite sides of the tube

12

so that a magnetic field is created in the tube

12

in a direction perpendicular to the longitudinal axis of the tube

12

, as indicated by the arrows B. The magnets can be either permanent magnets or electromagnets. However, the use of permanent magnets would result in a passive system not requiring an additional energy input.

Upon detonation of the fuel-air mixture in the combustion chamber

14

, detonation wave groups (pressure wave and flame front) will propagate in both the forward and aft directions. As depicted in

FIGS. 1 and 2

, the forward detonation wave group comprises a forward pressure wave

32

and a forward flame front

34

. Likewise, the aft detonation wave group comprises an aft pressure wave

36

and an aft flame front

38

. As a result of the combustion, the combustion products become weakly ionized and are thus electrically conductive. As the electrically conductive flow of the forward wave group

32

,

34

passes perpendicularly through the magnetic field created by the magnets

30

, an electrical current is generated in the electric field coil

28

by electromagnetic induction. The energy extracted from the forward wave group

32

,

34

dissipates the forward flame front

34

. Thus, the MHD flow control system

26

controls the forward detonation process, particularly prohibiting the forward flame front

34

from migrating to the fuel-air source

22

, by extracting power to dissipate the forward wave group. The electric field coil

28

is accordingly designed such that an adequate amount of energy is extracted from the forward wave group

32

,

34

.

The electrical power generated by the electric field coil

28

can be used to charge the igniter

24

. In this case, the electric field coil

28

is connected to a power conditioning control system

40

that is provided for directing the electrical power at the appropriate times to the igniter

24

. In addition, the electricity could be used for other purposes, such as powering onboard devices in a vehicle being propelled by the pulse detonation engine

10

.

The MHD flow control system

26

includes a supplemental ionization source

42

for boosting and/or maintaining the ionization of the post combustion products passing through the magnetic field. While the combustion process produces ionized combustion products, the charged particles tend to recombine quickly such that the combustion products lose their ionization. The ionization source

42

is thus provided to boost ionization and maintain the electron density of the combustion products passing through the magnetic field. Typically, an electron density of 10

13

electrons per cubic centimeter is desired to achieve sufficient influence from the magnetic field. The ionization source

42

can be any device capable of supplementing ionization, such as an electron gun that bombards the combustion products with extra electrons or an RF generator that further heats the combustion products, thereby inhibiting recombination of charged particles. The ionization source

42

is preferably located at the aft end of the electric field coil

28

, although it could also be located along the length of the coil

28

. In addition, the inlet flow of the fuel-air mixture could be seeded with a catalyst to enhance ionization of the combustion products. Examples of suitable catalysts include potassium carbonate (powder) and cesium hydroxide (spray).

Operation of the pulse detonation engine

10

begins by filling the combustion chamber

14

with a charge of the fuel-air mixture introduced through the inlet

20

. The igniter

24

is then activated to detonate the fuel-air mixture and generate the forward and aft detonation wave groups as shown in FIG.

1

. The forward wave group

32

,

34

travels forward in the tube

12

and is dissipated by the MHD flow control system

26

in the manner described above. The aft wave group

36

,

38

travels downstream from the detonation zone through the generally longer aft portion of the tube

12

, as shown in

FIG. 2

, consuming the fuel-air mixture along the way. As the aft pressure wave

36

, which is a compression wave, accelerates through the combustion chamber

14

, it raises both temperature and pressure. When the aft wave group

36

,

38

reaches the aft end

18

of the tube

12

, the hot, high pressure combustion products are expelled out of the open aft end, creating a pulse of forward thrust. The aft pressure wave

36

is then reflected at the aft end

18

as an expansion wave that propagates forward back through the tube

12

. The expansion wave lowers pressure in the combustion chamber

14

and further evacuates the tube

12

so that a fresh charge of fuel-air mixture from the inlet

20

is drawn into the combustion chamber

14

, thereby readying the pulse detonation engine

10

for the next cycle.

FIG. 3

shows an alternative configuration for the embodiment of

FIGS. 1 and 2

. In this arrangement, the electric field coil is replaced with a pair of electrodes. Specifically, the alternative MHD flow control system

26

′ is located between the detonation zone and the fuel-air inlet (not shown in

FIG. 3

) of the tube

12

′, which is substantially rectangular in cross-section. The MHD flow control system

26

′ comprises a pair of magnets

30

′ arranged on opposite sides of the tube

12

′ so that a magnetic field is created in the tube

12

′ in a direction perpendicular to the longitudinal axis of the tube

12

′, as indicated by the lines B. The magnets

30

′ can be either permanent magnets or electromagnets. A pair of electrodes

28

′ are located on opposite sides of the tube

12

′ and perpendicular to the magnets

30

′. As before, detonation of the fuel-air mixture in the combustion chamber

14

will cause detonation wave groups to propagate in both the forward and aft directions. The ionized flow passing perpendicularly through the magnetic field created by the magnets

30

′ induces an electrical current between the electrodes

28

′. The energy extracted to induce the electrical current dissipates the forward flame front. This arrangement ca n also be employed with a tube of axisymmetric crosssection as long as the ionized flow, magnetic field and the induced current flow perpendicular to one another.

Referring now to

FIGS. 4 and 5

, a pulsed detonation engine

44

employing a second embodiment of MHD flow control is shown. The pulse detonation engine

44

includes a tube

12

having a prescribed length and defining an internal combustion chamber

14

. The tube

12

has an open forward end

16

and an open aft end

18

. The open forward end

16

functions as a fuel-air inlet

20

to the tube

12

, while the open aft end

18

provides an exhaust to the ambient. A fuel-air mixture from a source

22

enters the combustion chamber via the inlet

20

. One or more sources of ionization

46

are located in the inlet

20

so that just the fuel vapor of the fuel-air mixture becomes ionized upon entering the combustion chamber

14

. Suitable sources of ionization include electrostatic grids across which a voltage potential is applied, fuel reactive, high electron density electrodes, and the like. As in the first embodiment, seeding the fuel-air mixture with a suitable catalyst can enhance ionization. Being sufficiently ionized, the fuel-air mixture within the combustion chamber

14

is affected by magnetic fields.

An igniter

24

is provided in the tube

12

at a location intermediate the forward and aft ends

16

,

18

, and preferably closer to the forward end

16

than the aft end

18

. The igniter

24

produces sufficient energy to detonate the fuel-air mixture in the combustion chamber

14

. The region of the combustion chamber

14

in the immediate vicinity of the igniter

24

is referred to herein as the detonation zone. Detonation combustion depends on the pressure, temperature and equivalence ratio of the fuel-air mixture, as well as the amount of energy released to start ignition. By locating the detonation zone closer to the forward end

16

, a larger portion of the tube length is devoted to generating thrust. The overall length of the tube

12

will depend on the desired operating frequency of the pulse detonation engine

44

.

An MHD flow control system

48

is located between the detonation zone and the fuel-air inlet

20

for controlling the detonation process in the forward portion of the tube

12

. The MHD flow control system

48

comprises a magnetic field coil or coils

50

connected to a real time controller

52

capable of engaging an energy source (not shown) such that an electric current flows through the coil

50

. The controller

52

also controls the igniter

24

and the ionization sources

46

. The magnetic field coil

50

is wrapped around the exterior of the tube

12

at an axial location that is between the igniter

24

and the fuel-air inlet

20

. Thus, when the controller

52

causes an electric current to flow though the coil

50

, a magnetic field is created in the portion of the tube

12

enclosed by the coil

50

. Due to the ionization of the fuel in the fuel-air mixture, the charged fuel particles would be directionally influenced by the magnetic field coil

50

when it becomes energized. Thus, activation of the magnetic field coil

50

would tend to separate the fuel-air mixture in the portion of the combustor chamber

14

encircled by the coil

50

. As shown in

FIGS. 4 and 5

, this would result in a rich fuel zone in the center of the combustion chamber

14

surrounded by a lean air zone.

Operation of the pulse detonation engine

44

begins by filling the combustion chamber

14

with a charge of the fuel-air mixture introduced through the inlet

20

. As mentioned above, the ionization sources

46

ionize the fuel-air mixture as it enters the combustion chamber

14

. The magnetic field coil

50

is not activated while the combustion chamber

14

is being filled to ensure that the fuel-air mixture remains properly mixed throughout the combustion chamber

14

. The controller

52

then activates the igniter

24

to detonate the fuel-air mixture. At the same time, the controller

52

also activates the magnetic field coil

50

causing the fuel-air mixture in the region of the combustion chamber

14

that is encircled by the coil

50

to become separated. Upon detonation of the fuel-air mixture, as shown in

FIG. 4

, forward and aft detonation wave groups will be generated. The forward detonation wave group comprises a forward pressure wave

32

and a forward flame front

34

, and the aft detonation wave group comprises an aft pressure wave

36

and an aft flame front

38

. The forward wave group

32

,

34

travels forward in the tube

12

to the MHD flow control system

48

. At this point, the forward flame front

34

encounters the separated fuel and air zones. The fuel and air separation starves the combustion process forward of the detonation zone, thereby dissipating the forward flame front

34

as it passes through the MHD flow control system

48

, as shown in FIG.

5

. Thus, the MHD flow control system

48

controls the forward detonation process by dissipating the forward flame front

34

, thereby prohibiting it from migrating to the fuel-air source

22

. Once the forward flame front

34

is dissipated, the coil

50

is deactivated.

Meanwhile, the aft wave group

36

,

38

travels downstream from the detonation zone through the generally longer aft portion of the tube

12

, consuming the fuel-air mixture along the way. As the aft pressure wave

36

, which is a compression wave, accelerates through the combustion chamber

14

, it raises both temperature and pressure. When the aft wave group

36

,

38

reaches the aft end

18

of the tube

12

, the hot, high pressure combustion products are expelled out of the open aft end, creating a pulse of forward thrust. The aft pressure wave

36

is reflected at the aft end

18

as an expansion wave that propagates forward back through the tube

12

. The expansion wave lowers pressure in the combustion chamber

14

so that a fresh charge of fuel-air mixture from the inlet

20

is drawn into the combustion chamber

14

, thereby readying the pulse detonation engine

44

for the next cycle. The controller

52

is set up to fire the igniter

24

and activate the magnetic field coil

50

at the desired frequency, which is coordinated with the timing of the pressure wave reflections.

Turning to

FIGS. 6 and 7

, a third embodiment of a pulsed detonation engine

54

is shown. The pulse detonation engine

54

utilizes a hybrid MHD flow control approach that combines the energy extraction and fuel-air separation techniques of the embodiments discussed above. Specifically, the pulse detonation engine

54

includes a tube

12

having a prescribed length and defining an internal combustion chamber

14

. The tube

12

has an open forward end

16

and an open aft end

18

. The open forward end

16

functions as a fuel-air inlet

20

to the tube

12

, while the open aft end

18

provides an exhaust to the ambient. A fuel-air mixture from a source

22

enters the combustion chamber via the inlet

20

. One or more sources of ionization

46

are located in the inlet

20

so that the fuel-air mixture will be ionized upon entering the combustion chamber

14

. Again, seeding the fuel-air mixture with a suitable catalyst can enhance ionization of the fuel-air mixture.

An igniter

24

is provided in the tube

12

at a location intermediate the forward and aft ends

16

,

18

, and preferably closer to the forward end

16

than the aft end

18

. The igniter

24

produces sufficient energy to detonate the fuel-air mixture in the combustion chamber

14

. The region of the combustion chamber

14

in the immediate vicinity of the igniter

24

is referred to herein as the detonation zone. Detonation combustion depends on the pressure, temperature and equivalence ratio of the fuel-air mixture, as well as the amount of energy released to start ignition. By locating the detonation zone closer to the forward end

16

, a larger portion of the tube length is devoted to generating thrust. The overall length of the tube

12

will depend on the desired operating frequency of the pulse detonation engine

54

.

An MHD flow control system

56

is located between the detonation zone and the fuel-air inlet

20

for controlling the detonation process in the forward portion of the tube

12

. The MHD flow control system

56

comprises a passive electric field coil

28

wrapped around the exterior of the tube

12

at an axial location that is between the igniter

24

and the fuel-air inlet

20

. A pair of electrodes disposed on opposite sides of the tube

12

could be used as an alternative to the coil

28

, as discussed above in connection with FIG.

3

. An active magnetic field coil

50

is also wrapped around the exterior of the tube

12

at an axial location just forward of the electric field coil

28

and aft of the fuel-air inlet

20

. A pair of magnets

30

are arranged in proximity to the electric field coil

28

and on opposite sides of the tube

12

so that a magnetic field is created in the tube

12

in a direction perpendicular to the longitudinal axis of the tube

12

, as indicated by the arrows B. The magnets can be either permanent magnets or electromagnets.

The MHD flow control system

56

includes a supplemental ionization source

42

for boosting and/or maintaining the ionization of the post combustion products passing through the magnetic field. While the combustion process produces ionized combustion products, the charged particles tend to recombine quickly such that the combustion products lose their ionization. The ionization source

42

is thus provided to boost ionization and maintain the electron density of the combustion products passing through the magnetic field. The ionization source

42

is preferably located at the aft end of the electric field coil

28

, although it could also be located along the length of the coil

28

.

Ionized, electrically conductive material passing perpendicularly through the magnetic field created by the magnets

30

will result in an electrical current being generated in the electric field coil

28

by electromagnetic induction. As shown in

FIGS. 6 and 7

, the electric field coil

28

is connected to a power conditioning control system

40

that is provided for directing the electrical power at the appropriate times to the igniter

24

so that the electric power generated by the electric field coil

28

can be used to charge the igniter

24

. In addition, the electricity could be used for other purposes, such as powering the ionization sources

46

, the supplemental ionization source

42

or onboard devices in a vehicle being propelled by the pulse detonation engine

10

.

The power conditioning control system

40

also functions as a real time controller that selectively engages an energy source (not shown) such that an electric current flows through the magnetic field coil

50

. When the power conditioning control system

40

causes an electric current to flow though the coil

50

, a magnetic field is created in the portion of the tube

12

enclosed by the coil

50

. Due to the ionization of the fuel-air mixture, the charged fuel particles would be directionally influenced by the coil

50

when it is energized. Thus, activation of the magnetic field coil

50

would tend to separate the fuel-air mixture in the portion of the combustor chamber

14

encircled by the coil

50

. As shown in

FIGS. 6 and 7

, this would result in a rich fuel zone in the center of the combustion chamber

14

surrounded by a lean air zone. The power conditioning control system

40

also controls the ionization sources

46

and the supplemental ionization source

42

.

Operation of the pulse detonation engine

54

begins by filling the combustion chamber

14

with a charge of the fuel-air mixture introduced through the inlet

20

. As mentioned above, the ionization sources

46

ionize the fuel-air mixture as it enters the combustion chamber

14

. The magnetic field coil

50

is not activated while the combustion chamber

14

is being filled to ensure that the fuel-air mixture remains properly mixed throughout the combustion chamber

14

. The power conditioning control system

40

then activates the igniter

24

to detonate the fuel-air mixture. At the same time, the power conditioning control system

40

activates the magnetic field coil

50

causing the fuel-air mixture in the region of the combustion chamber

14

that is encircled by the coil

50

to become separated.

Upon detonation of the fuel-air mixture, as shown in

FIG. 6

, forward and aft detonation wave groups will be generated. The forward detonation wave group comprises a forward pressure wave

32

and a forward flame front

34

, and the aft detonation wave group comprises an aft pressure wave

36

and an aft flame front

38

. The forward wave group

32

,

34

travels forward in the tube

12

to the MHD flow control system

56

. As the electrically conductive flow of the forward wave group

32

,

34

passes perpendicularly through the magnetic field created by the magnets

30

, an electrical current is generated in the electric field coil

28

by electromagnetic induction. The energy extracted from the forward wave group

32

,

34

at least partially dissipates the forward flame front

34

, as shown in FIG.

7

. As the dissipated forward wave group

32

,

34

continues forward, it encounters the separated fuel and air zones. The fuel and air separation starves the combustion process forward of the detonation zone, thereby completely dissipating the forward flame front

34

. Thus, the MHD flow control system

56

controls the forward detonation process by extracting power from the forward wave group

32

,

34

and then quenching the forward flame front

34

. This prohibits the forward flame front

34

from migrating to the fuel-air source

22

. Once the forward flame front

34

is dissipated, the coil

50

is deactivated.

Meanwhile, the aft wave group

36

,

38

travels downstream from the detonation zone through the generally longer aft portion of the tube

12

, consuming the fuel-air mixture along the way. As the aft pressure wave

36

, which is a compression wave, accelerates through the combustion chamber

14

, it raises both temperature and pressure. When the aft wave group

36

,

38

reaches the aft end

18

of the tube

12

, the hot, high pressure combustion products are expelled out of the open aft end, creating a pulse of forward thrust. The aft pressure wave

36

is reflected at the aft end

18

as an expansion wave that propagates forward back through the tube

12

. The expansion wave lowers pressure in the combustion chamber

14

so that a fresh charge of fuel-air mixture from the inlet

20

is drawn into the combustion chamber

14

, thereby readying the pulse detonation engine

54

for the next cycle. The controller

52

is set up to fire the igniter

24

and activate the magnetic field coil

50

at the desired frequency, which is coordinated with the timing of the pressure wave reflections.

The foregoing has described various MHD flow control systems for pulse detonation engines. The flow control systems do not require moving parts and are thus highly reliable and capable of operating at extremely high frequencies. They are also easily controlled by electronic means. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.