Patent Application
20050155853
The present invention relates to fuel ignition and, more specifically, to optically initiated chemical reactions to establish combustion in a propulsion engine using storable high-density solid state solid fuels.
Solid state solid fuels are propulsion fuels that are in solid form when stored at ambient temperatures. As with most any material that is in a solid phase, the mass density and energy density of the fuel is much high in the solid state than when in a liquid or gas phase. As a result, the specific impulse and thrust potential from the fuel is much higher in solid state solid fuels, herein also referred to as solid fuels. However, fuels are more difficult to ignite using traditional electric spark or torch-ignition techniques when in a solid state than when in a liquid or gas form.
Therefore, it would be highly desirable to provide an efficient and sufficiently simple method of initiating a combustive reaction in a solid fuel.
In a preferred implementation, the present invention provides a method for initiating and sustaining a combustive reaction in a solid fuel. The method includes generating at least one pulsed optical signal and directing the pulsed optical signal to a plurality of ignition points within at least one combustion chamber containing a solid fuel. The pulsed optical signal is generated by an optical source, e.g. a laser pump, and modulated using an intensity profiler. The intensity profiler modulates the pulsed optical signal to initially have a first peak power sufficient to initiate a combustive reaction in a solid fuel. The intensity profiler further modulates the pulsed optical signal to subsequently have a second peak power sufficient to sustain the combustive reaction until sufficient exothermic energy is released by the combustive reaction to make the reaction self-sustaining.
In another preferred implementation the present invention provides a propulsion system including at least one combustion chamber. The combustion chamber receives a solid fuel and oxidizer mixture used to provide propulsion by igniting the mixture. The propulsion system additionally includes at least one optical source for generating at least one pulsed optical signal used to ignite and sustain a combustive reaction of the solid fuel and oxidizer mixture. An optical fiber coupler connected to the optical source directs the pulsed optical signal to a plurality of ignition points within the combustion chamber. Furthermore, the propulsion system includes an intensity profiler adapted to modulate the pulsed optical signal to have a first peak power sufficient to initiate the combustive reaction. The intensity profiler further modulates the pulsed optical signal to have a second peak power sufficient to sustain the combustive reaction. The pulsed optical signal sustains the combustive reaction until sufficient exothermic energy is released by the combustive reaction to make the reaction self-sustaining.
The features, functions, and advantages of the present invention can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
Corresponding reference numerals indicate corresponding parts throughout the several views of drawings.
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application or uses. Additionally, the advantages provided by the preferred embodiments, as described below, are exemplary in nature and not all preferred embodiments provide the same advantages or the same degree of advantages.
With initial reference to
In a preferred embodiment, the fiber coupler 50 comprises a collection or series of optical fibers in a bundle. The fibers interconnect with multiple ignition positions within a single combustion chamber 70. Having multiple ignition positions with a single combustion chamber 70 increases the ease of igniting the solid fuel and the ease in sustaining the combustive reaction. Alternatively, the optical fibers interconnect with multiple combustion chambers 70 within the vessel 12. The collection of fibers may be designed in several ways. In one form, each optical fiber connects with a separate optical source 20. Each fiber directs the optical energy to a single ignition point. In an alternative form, the fiber coupler 50 includes an optical splitter adapted to receive a single pulsed optical signal from the optical source 20 and divide the signal into a plurality of pulsed optical signals. The optical splitter splits the optical energy and directs the optical energy optical splitter can be any suitable optical splitter, for example, an active coupler in which an optical pulse enters the coupler and is optically switched to one of the output optical fibers. In this manner, the optical energy can be serially directed to each of the output fibers.
The propulsion produced by any engine is the result of an exothermic chemical reaction. In order to ignite the engine, the activation energy of the chemical reaction must be overcome. As with any chemical reaction, the microscopic behavior is dictated by quantum mechanical behaviors. The inherent stochastic nature of the quantum behaviors implies that there is a probability distribution associated with the ignition. In a gas phase ignition, the activation energy is overcome by applying energies well above a threshold value. Typically, for solid fuels, different areas have different threshold energies. Small differences in the chemical constituents will also change the propagation of a flame front, once ignition is achieved. This can lead to local flameouts, whose location cannot be determined ahead of time. These difficulties can be mitigated by increasing the number of ignition points within the solid fuel structure, as described above. In an alternative preferred embodiment, to assure ignition, multiple optical signals can be sent to one or more ignition points.
The characteristics of laser light emitted from the optical source 20 will now be described in greater detail. Characteristics associated with laser light must be optimized for optically initiating combustion. These characteristics can include laser pulse duration, pulse intensity envelope shape, laser energy within the envelope, peak optical power, center wavelength and frequency bandwidth. Optimization of these characteristics involves selecting the characteristics to assure that maximum coupling of optical energy into the molecular bonds of materials in the propulsion mixture. In the case of a solid fuel, additional constraints need to be imposed. For example, the laser light wavelength must be short enough so that absorption via linear or nonlinear mechanisms leads to molecular dissociation of fuel, oxidizer or both. The shape of the intensity envelope can control not only the amount, but also the deposition speed of energy into the internal molecular energy states.
The implication is that the light must be in the ultraviolet range of the spectrum, preferably shorter than 300 nanometers. In most practical applications, a diode-pumped solid state laser will be used as optical source 20 because of its mechanical robustness. The light from these lasers, however, will typically be in the near infrared, requiring nonlinear optical conversion to shorter wavelengths. After the conversion is accomplished, there will be remnants of longer wavelengths in the laser light. Before introduction into the fiber coupler 50, optical wavelength filter 40, or an equivalent filtering medium, removes any residual light at longer wavelengths.
For ignition to occur in a solid fuel 92, a balance must be reached between the light energy absorbed into the fuel/oxidizer mixture 90 and the volume of the mixture that is excited. In other words, the absorbed energy density of the mixture is as important as the absorbed energy itself. If too much energy is deposited in a highly localized volume of solid fuel 92, it will not be sufficient to allow the exothermic chemical reaction to reach a self-initiating condition. In normal gas or liquid phase fuels, nonlinear effects are highly independent of absolute position in the volume because the local density fluctuations do not affect the local optical susceptibility. However, for solid fuels, tailoring the optical intensity is very important. This is because the interaction with the solid fuel/oxidizer mixture 90 will begin with a nonlinear optical absorption. Thus, the light emitted from optical source 20 is preferably in a pulsed format so that high peak laser powers can be generated. Generally, the peak power associated with a laser generated pulse is equal to the energy in the pulse divided by the duration of the pulse. As an example, a laser pulse may only contain 1 millijoule of energy emanated from a one milliwatt laser in one second. This does not represent a large amount of energy. However, if that one millijoule of energy is contained within a pulse that is, for example, one to three nanoseconds in duration, then the peak power is one Gigawatt. Even though the pulse duration is short, the surrounding medium will react to the laser pulse as if it were a one Gigawatt power laser, although the effect will only last the duration of the laser pulse. In this manner, sufficient energy in each pulse generates a peak power that is associated with the onset of nonlinear optical behavior, for example approximately 1-2 Megawatts.
Additionally, the pulse shape and/or format of the optical signal emitted from the optical source 20 is modulated by the intensity profiler 30 for optimized interaction with the high densities associated with the solid fuel 92. Because the initial absorption volume in the solid fuel 92 will be small due to the higher density, it will be advantageous to output an optical pulse from the optical source 20 having a high peak power at the beginning of the pulse and a lower peak power during a later portion of the pulse. Also, the nature of the solid fuel 92 will lead to larger density fluctuations that cause changes in the local absolute value of an electric field associated with the light signal emitted from optical source 20. In any medium, the local electric field is due to both an applied field and a field induced in the medium. The nonlinear optical process is dependent on this local field. Consequently, any nonlinear optical process may begin at slightly different intensity levels at different locations within the solid fuel/oxidize mixture 90. Further yet, because of the high density of the solid fuel 92, the solid fuel 92 will be generally less transparent than gas or liquid materials. Therefore, as a result of the optical opacity of the solid fuel 92, the solid fuel 92 will absorb a high percentage of the laser light emitted from optical source 20, disproportionate to the light absorbed by the surrounding media. More specifically, the lower transparency results in a higher degree of light absorption that aids in coupling, i.e. routing, the optical energy into internal energy and consequently heating of the fuel/oxidizer mixture 90.
The dissociation of the molecules in both the solid fuel 92 and the oxidizer 94 is associated with light wavelengths in the ultraviolet shorter than 300 nm. The association with the light wavelengths is due to the fact that the electronic excitations leading to the dissociation of the molecules characteristically occur with internal energies that exceed 3 electron-volts (ev). The internal heating of molecules, that is, the excitation of energy level corresponding to vibration motion, is associated with light wavelengths in the infrared, longer than 900 nm. Furthermore, the high absorption creates an unusual situation wherein molecular dissociation and molecular heating processes are simultaneously enhanced. More specifically, the molecular dissociation and molecular heating processes proceed more quickly and at higher efficiency levels due to the high absorption. For this reason, the intensity of the laser signal emitted from the optical source 20 is profiled to have a high peak power at the initiation of ignition, when molecular dissociation dominates the physical process, and a lower power level after ignition is established, when internal heating dominates the process. Thus, the internal heating sustains the combustive reaction until sufficient exothermic energy is released to make the reaction self-sustaining.
The intensity profiler 30 will now be described in greater detail. It will be appreciated by those skilled in the art that the location of intensity profiler 30 is merely exemplary and may be positioned subsequent to optical wavelength filter 40. In a preferred embodiment, shown in
In another preferred embodiment, shown in
When used multiple times at multiple points of ignition, a variety of pulse sequences and the ability to switch the pulses to different areas, allows the exact ignition timing sequence can be controlled. Several locations may be ignited simultaneously or specific physical locations can be ignited before other locations. For example, it may be advantageous to ignite the center of the solid fuel/oxidizer mixture 90 first, with the ignition of the outer areas being ignited later. In this manner, the ignition flame front from the first ignition area will reach other areas of the solid fuel/oxidizer mixture 90 and the subsequent ignition pulses will arrive at the same time as the ignition flame front. As a result, the exothermic energy of the flame will coincide with the optical energy, leading to a fuel state that contains more internal molecular energy, increasing the probability for sustained ignition.
In each embodiment, the initial high peak power will quickly generate a micro-plasma that is opaque to most laser wavelengths. The time elapsed between the high and low power excitations is short enough such that all the energy of the lower peak power will be uniformly absorbed without causing other undesirable nonlinear optical processes to interfere with the optical initiation. For example, the time between the high and lower power excitations is preferably less than ten nanoseconds, but possibly as long as 100 nanoseconds.
The ignition of the solid fuel/oxidizer mixture 90 using optical source 20 will now be described in greater detail. The equation governing the optical intensity to drive the optical breakdown is given by:
Icr={mcEI(1+(ωτ)2]/[2TTe2τ]}[g+1/τρ loge(ρcr/ρ0)]
where ρcr is the critical electron number for breakdown, τρ is the laser pulse width; m, e, c are the electron constants; ω is the optical field frequency; EI is the ionization energy of the fuel 92 or the oxidizer 94; τ is the momentum transfer collision time; g is the electron loss rate; and ρ0 is the “initial” electron density. Although this depends on the particular characteristics of the solid fuel/oxidizer mixture 90, the propulsion system 10 is designed to deliver the level of optical intensity into the combustion chamber 70, as dictated by the equation. The optical energy delivered in accordance with the equation is the pulsed optical energy described above that is delivered into the combustion chamber 70 to initiate and sustain the propulsion reaction.
Once a finite number of solid fuel 92 and/or oxidizer 94 molecules have been dissociated, the resulting physical state is an optically opaque medium. The dissociation occurs when sufficient energy is absorbed by the molecular bond such that the electrons associated with that bond can no longer bond the atoms together. This process is very fast, for example, by the end of a one nanosecond pulse, the dissociations have already occurred. All the subsequent energy in the laser pulse is absorbed into this medium. Additionally, the optical spot size of the optical signal is a function of the intensity at which the fuel oxidizer molecules break down. For example, the optical intensity is increased by using a smaller optical spot size, therefore, the spot size will affect the optical intensity and consequently the strength of the nonlinear optical absorption. Thus, the absorption leads to the molecular dissociation necessary for ignition of the solid fuel/oxidizer mixture 90.
The breaking down of solid fuel 92 is generally simple because metal particles in the solid fuel 92 both increase optical absorption and enhance the optical nonlinearity of the media. For example, peak powers of approximately 1-2 Megawatts at ultraviolet wavelengths, preferably less than 300 nanometers, will be sufficient to initiate breakdown, with the breaking down beginning to occur near the densest volumes of the solid fuel 92. Internal energies sufficient to drive the mixture into a self-sustaining condition can then be generated with a lower power portion of the same pulse or with a lower power second laser pulse to complete the initiation of the reaction. The initiation is complete when the exothermic energy of the reaction is sufficient to continue driving the reaction, i.e. the reaction is self-sustaining. This self-sustaining chemical reaction is the combustion reaction that produces the engine propulsion.
Generally, optical delivery systems, such as optical source 20, can generate laser energies on the order of 10 millijoules. Fiber coupler 50 is adapted to transmit pulses that simultaneously have a high peak power and a short wavelength. Preferably, fiber coupler 50 includes one or more non-solarizing optical fibers that support the high peak power and short wavelength requirements and transmit the pulse(s) with substantially no loss of energy or intensity. For example, the absorption volume in the solid fuel 92 can be on the order of approximately 100 to 115 cubic microns. A corresponding energy density of approximately 5 to 15 GJ/cubic meter can then be produced to initiate combustion. Through the use of non-linear absorption, enough free electrons are created within a high intensity focus region of the solid fuel/oxidizer mixture 90 to allow the solid fuel/oxidizer mixture 90 to take on the absorption characteristic of plasma. Generally, plasma ranges from highly absorbing to completely opaque and allows for a finite fraction of the pulse energy to be absorbed by the medium, e.g. the solid fuel/oxidizer mixture 90.
In addition, in the high density of the solid fuel 92 enhances the optical nonlinearity of the medium. The nonlinearity of the solid fuel/oxidizer mixture 90 is used to enhance the absorption process that leads to the initiation of the chemical reaction. The resulting mixture 80 after ignition will be comprised of partially dissociated air and chemically cracked fuel. The mixture includes molecular and atomic oxygen, an array of hydrocarbon fragments, low molecular weight hydrocarbon compounds and some remaining parent carrier fuel.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
This application is related to copending U.S. patent application Ser. No. 10/007,994, titled Apparatus And Method For Initiating A Combustion Reaction With Slurry Fuel, filed on Nov. 8, 2001.
