Abstract:
The present invention is a pulse detonation combustion system, having a plurality of detonation initiation devices coupled to a main combustion chamber, where each of the detonation initiation devices is operating out-of-phase with each other. Each of the detonation initiation devices assists in the initiation of a detonation in the main combustion chamber, out-of-phase from each other such that the operational frequency of the pulse detonation combustion system is related to the number of detonation initiation devices multiplied by the operational frequency of a single detonation initiation device.

Description:
[0001]     This invention was made with government support under Contract No. DABT63-00-C-0001 awarded by DARPA. The government may have certain rights to the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to pulse detonation engines, and more particularly, to multiple detonation initiators for pulse detonation combustors.  
         [0003]     In recent years, efforts to address the need of a combination of combustion systems to obtain a wide range of flight speeds for aircraft have led to the development of pulse detonation combustors, which can be used on aircraft engines (as well as other applications). When used on aircraft engines, pulse detonation engines aid in increasing the available flight speed range of an aircraft engine while reducing the need for a combination of combustion systems.  
         [0004]     Pulse detonation combustors create high pressure and temperature detonation waves by combusting a mixture of gas (typically air) and a hydrocarbon fuel. The detonation waves exit the pulse detonation combustor tube as pulses, thus providing thrust. Because of the nature of the operation of pulse detonation combustors (i.e. a series of discrete detonations), there is a limit to the frequencies at which conventional simple tube pulse detonation combustors can operate. This is especially true of tube pulse detonation combustors which use a mixture of air and a hydrocarbon fuel as the detonable component. One of the limitations of hydrocarbon-air mixtures is the relatively long time for run-up to detonation at ambient conditions, with a weak detonation initiation.  
         [0005]     However, for a number of reasons, it is desirable to operate pulse detonation combustors at as high a frequency as possible. First, the operation of a pulse detonation combustor at a high frequency minimizes the excitation of the mechanical structure of an engine, or other structure surrounding or supporting the pulse detonation combustor. Operation at low frequencies tends to inflict resonant damage to supporting structure or engines. Second, operation at higher frequencies minimizes the pressure and velocity fluctuations flowing to upstream parts of the flow system, including: inlets, compressors and diffusers.  
         [0006]     The present invention addresses the above issues.  
       SUMMARY OF THE INVENTION  
       [0007]     In an embodiment of the invention, a high frequency of detonation is achieved by using a plurality of detonation initiators with a main pulse detonation combustor, where the detonation initiators operate out-of-phase with each other. Each of the initiators operates at a lower frequency than the main pulse detonation combustor, but because they are operating out-of-phase with each other, the operational frequency of the main pulse detonation combustor is increased. In an embodiment, the operational frequency of the main pulse detonation combustor is simply the number of initiators multiplied by the frequency of the initiators.  
         [0008]     In an embodiment of the present invention, the initiators themselves are smaller pulse detonation initiators of the tube type, and are placed within the main pulse detonation combustor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiment of the invention which is schematically set forth in the figures, in which:  
         [0010]      FIG. 1  is a diagrammatical representation of an embodiment of the present invention;  
         [0011]      FIG. 2  is a diagrammatical representation of another embodiment of the present invention;  
         [0012]      FIG. 3  is a diagrammatical representation of an additional embodiment of the present invention; and  
         [0013]      FIG. 4  is a graphical representation of a time line for two initiators in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The present invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way.  
         [0015]      FIGS. 1-3  are diagrammatical representations of various embodiments of the pulse detonation combustor  10  of the present invention. The pulse detonation combustor  10  contains a main combustion chamber  12 , a main combustor resonator surface  14 , a plurality of detonation initiators  16 , at least one inlet port  20  and a main combustor exit  22 .  FIG. 4  is a graphical timeline for the operation of an embodiment of the invention.  
         [0016]     A detailed discussion of the operation and structure of the pulse detonation combustor  10  is set forth below.  
         [0017]     During the operation of the pulse detonation combustor  10  a mixture of a gas, typically air, and a fuel, typically a hydrocarbon fuel, are placed into the main combustion chamber  12 , which is cylindrical in shape. The fuel and gas enter the main combustion chamber  12  through the inlet ports  20 . In one embodiment, the fuel/gas mixture is premixed, prior to passing through the inlet ports  20  and into the chamber  12 . In another embodiment, the fuel and gas enter through specifically designated inlet ports  20  (for example, alternating ports) and the mixing occurs within the chamber  12 .  
         [0018]     In an additional embodiment, the shape of the main combustion chamber  12  has a shape other than cylindrical. Namely, it is contemplated that the shape of the chamber  12  is rectangular, square, hexagonal, octagonal, or the like, depending on the design and operational requirements of the chamber  12 .  
         [0019]     The inlet ports  20  are distributed radially around the circumference of the chamber  20 , and are spaced evenly. Further, the inlet ports  20  are configured such that the flow from the inlet ports  20  exits the inlet ports  20  perpendicular to the surface  26  of the chamber  12  at the point of entry. Additionally, the inlet ports  20  are located within the same plane with respect to the chamber  12 . The number, shape and size of inlet ports  20  are selected based on operational parameters and needs.  
         [0020]     In another embodiment, the spacing and distribution of the inlet ports  20  are configured to maximize mixing and performance of the combustor  10 . For example, a number of the inlet ports  20  are located in a different plane then the remaining inlet ports  20 , and the spacing is asymmetric with respect to the chamber  12 . Further, in another embodiment, the inlet ports  20  are angled such that the flow from inlet ports  20  enters the main combustion chamber  12  at an angle different than perpendicular to the surface  26  of the chamber  12  at the point of entry.  
         [0021]     Further, the pulse detonation combustor  10  contains a main combustor resonator surface  14  and a plurality of detonation initiators  16 . In an embodiment of the present invention, each of the detonation initiators  16  is a tube pulse detonation initiator which operates in a similar fashion as the pulse detonation combustor  10 .  
         [0022]     In  FIGS. 1 through 3 , three exemplary embodiments of the present invention are depicted. In  FIG. 1  the detonation initiators  16  are located upstream of the main combustion chamber  12  such that the exits  24  of the detonation initiators  16  flow downstream into the main combustion chamber  12  through the surface of the resonator  14 . In the  FIG. 2  embodiment the detonation initiators  16  are located internally within the main combustion chamber  12  and direct the initiation from the exit  24  upstream into the chamber  12 . In the  FIG. 3  embodiment the detonation initiators  16  are ducted to the main combustion chamber  12  such that the exit  24  of the initiators  16  are positioned on a surface  26  of the main combustion chamber  12 . In this embodiment, the exits  24  are configured such their centerlines are perpendicular to the surface  26  of the chamber  12  at the point of entry into the chamber  12 , and the exits  24  are co-planar with respect to the chamber  12 . In another embodiment, the initiators  16  and their exits  24  are configured such that the flow exiting the initiators  16  enters the chamber  12  at an angle with respect to the surface  26  of the chamber  12 . Further, in another embodiment, some of the exits  24  of the initiators  16  are not co-planar with each other. For example, in a four initiator  16  configuration, two of the initiators  16  are in a first plane of the chamber  12 , while the remaining two are in a second plane, which can be downstream or upstream of the first plane.  
         [0023]     Further, in each of the exemplary embodiments shown in FIGS.  1  to  3 , the exits  24  of the initiators  16  are distributed symmetrically with respect to the chamber  12 . However, in another embodiment, the exits  24  of the initiators  16  are distributed asymmetrically, depending on the operational needs and characteristics of the combustor  10 .  
         [0024]     Moreover, in the exemplary embodiments shown in  FIGS. 1 and 2 , the initiators  16  are located at an outer radial position with respect to the chamber  12 . However, in another embodiment, the radial position of the initiators  16  is located closer to a centerline of the chamber  12 . In one embodiment, the plurality of the initiators  16  are symmetrically grouped at a centerline of the chamber  12 .  
         [0025]     Further, in the exemplary embodiments shown in  FIGS. 1 through 3  there are a total of four initiators  16  depicted positioned symmetrically with respect to the chamber  12 . However, it is contemplated that the number of initiators  16  varies depending on the desired operation and characteristics of the pulse detonation combustor  10 . For example, if the desired operational frequency of the pulse detonation combustor  10  is 400 Hz, and each initiator  16  can operate at 100 Hz, four initiators  16  are used. Further, if the desired operational frequency is 200 Hz, and each initiator  16  operates at 100 Hz, then two initiators  16  are used.  
         [0026]     In one embodiment the initiators  16  operate in a frequency range of 20 to 100 Hz. In a further embodiment, the initiators operate at approximately 40 Hz.  
         [0027]     The structure and operation of the initiators  16  will now be described. As indicated above, in one embodiment of the present invention, the detonation initiators  16  operate similarly to the pulse detonation combustor  10 . Each detonation initiator  16  contains at least one inlet  28  through which a fuel/gas mixture enters a deflagration-to-detonation chamber  30  having a resonator surface (not shown). In one embodiment, the fuel and gas for the initiator  16  is mixed after entering the deflagration-to-detonation chamber  30 . In a further embodiment, the fuel/gas mixture is the same fuel/gas mixture employed for the detonation within the main combustion chamber  12  within the combustor  10 . Further, each of the initiators  16  contains an initiation source (not shown), which is used to initiate the detonation within the deflagration-to-detonation chamber  30  of the initiators  16 . Examples of an initiation source include, but are not limited to, spark or spontaneous detonation.  
         [0028]     Additionally, each of the initiators  16  contain an initiator duct  32  which delivers the initiation wave to a nozzle  18  and the initiator exit  24 . In one embodiment, the nozzle  18  is a converging nozzle where the area of the exit  24  is less than the area of the initiator duct  32 . Further, in an embodiment of the invention, the cross section of the deflagration-to-detonation chamber  30 , duct  32 , nozzle and exit  24  are circular. However, embodiments are contemplated using alternative cross-sectional geometries, including square, rectangular, oval, hexagonal and the like, depending on the design and operational parameters and requirements. Moreover, in an alternative embodiment, the cross-section of the exit  24  has a different shape than the duct  32  and/or the deflagration-to-detonation chamber  30 .  
         [0029]     The operation of an embodiment of the pulse detonation combustor  10  is set forth below, and as shown in  FIG. 4 .  
         [0030]     The main combustion chamber  12  is filled with the mixture of fuel and gas to a desired level. Upon reaching this level, one of the detonation initiators  16  initiates detonation of the filled mixture within the main combustion chamber  12 . This is accomplished by initiating a detonation within the deflagration-to-detonation chamber  30  of the firing initiator  16 . Upon this detonation, a detonation wave travels through the initiator  16  and is passed into the main combustion chamber  12 . The high pressure and temperature of the exiting initiation wave initiates the detonation in the main detonation chamber  12 . In an alternative embodiment, the detonation in the main chamber  12  is assisted with a secondary initiation device (not shown).  
         [0031]     The detonation in the main chamber  12  creates a high temperature and high-pressure wave, at least some of which reflects off of the main combustor resonator surface  14 . The detonation wave propagates through the main combustion chamber  12  and exits the pulse detonation combustor  10  at a main combustor exit  22 . The shape of the resonator surface  14  is selected for optimal performance of the pulse detonation combustor, and can be of the conical, semicircular, parabolic, flat or rounded shape.  
         [0032]     At approximately the same time (t 1 ) the detonation in the main combustion chamber  12  is initiated by one of the detonation initiators  16 , the main combustion chamber  12  begins to fill again with a fuel/gas mixture from the inlet ports  20 . Thus, at approximately the same time the detonation wave resulting from the first initiation propagates (t 3 ) out of the main combustion chamber  12  the mixture for the second detonation has re-filled the main combustion chamber  12 . At this time, a second detonation initiator  16  has initiated detonation of the second fill. At approximately the same time the second initiation begins (t 3 ) the blow down for the first detonation begins. Further, at approximately the same time (t 4 ) the purge from the first detonation begins, the blow down of the second detonation begins, and at approximately the same time (t 5 ) the cycle of the first detonation ends the purge of the second detonation begins.  
         [0033]     In an embodiment of the present invention, the cycle time for one complete detonation (from begin fill to end of purge, i.e. t to t 5 ) in the pulse detonation combustor  12  is 10 ms. Thus, with four detonation initiators  16 , the pulse detonation combustor  10  operates at 400 Hz.  
         [0034]     During operation of the combustor  10  each of the initiators  16  are operated out-of-phase with each other such that for each detonation, within the main chamber  12 , only one of the initiators  16  has fired. In an embodiment, the subsequent initiator  16  to fire is adjacent to the previous initiator  16  to fire. However, in another embodiment, non-adjacent initiators  16  are fired sequentially.  
         [0035]     In an alternative embodiment, at least two initiators  16  fire at the same time to assist the detonation within the main chamber  12 , and these two initiators  16  operate out-of-phase of other pairs of initiators  16 . For example, the combustor  10  comprises six initiators  16 , and for any given detonation initiation two of the initiators  16  fire.  
         [0036]     In one embodiment of the present invention, the cycle timing of the detonation initiation and the pulse detonation combustor  10  is adjustable. In an embodiment, the frequency of operation of the pulse detonation combustor is in the range of 100 to 400 Hz. In an alternative embodiment, the operational frequency of the pulse detonation combustor  10  is adjustable during operation of the combustor  10 . As the operational parameters and requirements of the combustor  10  changes during its operation, the frequency of the combustor  10  is changed. Further, as the operational frequency changes, the frequency of the firing of the detonation initiators  16  changes to ensure proper frequency detonation. In an alternative embodiment, the changing of the operational frequency of the combustor  10  is performed automatically by constantly monitoring power and operational characteristics and requirements of the combustor  10 , and adjusting the cycling and detonation initiation accordingly.  
         [0037]     In an alternative embodiment, each of the nozzles  18  of the initiators  16  are equipped with a valve device (not shown) which closes or narrows when the respective initiator  16  is not firing. The valve device prevents back flow into the initiator  16  from the detonation within the main chamber  12 . In another embodiment, the geometry of the exit  24  and/or nozzle  18  of each of the initiators  16  is configured to minimize backflow.  
         [0038]     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. Namely, although the present invention has been discussed in the context of aircraft engine applications, it is contemplated that the present invention can be employed in all applications which use gas turbine engines, or the like.