Abstract:
A rotary ramjet engine. A rotary ramjet engine is provided operating with a very low axial flow component. The engine has a closely housed rotor and shaft mounted for rotary motion with respect to an engine case. A plurality of ramjet combustors are provided at the periphery of the rotor, and a set of spaced apart helical strakes are provided extending outward from the surface portion of the rotor toward the interior wall of the engine case, less a running clearance therefrom. A centerbody is provided for each ramjet inlet. The centerbody is disposed along a helical axis parallel to the strakes, and includes a leading edge structure, opposing sidewalls, and a shaped cavity, and a rear end wall. Each set of strakes cooperate to define, rearward of the rear end wall of each inlet centerbody, a combustion chamber for mixing therewithin and inlet fluid and burning fuel therein to form hot combustion gases therefrom. A ramjet outlet nozzle structure, including a converging ramjet nozzle throat, and diverging ramjet nozzle are provided for receiving the hot combustion gases and discharging, at a preselected helical angle to the plane of rotation of the rotor, a jet of hot combustion exhaust gases. The hot combustion exhaust gases can be further utilized in an impulse turbine for extraction of kinetic energy, or in heat exchange equipment for recovery of thermal energy therefrom.

Description:
RELATED PATENT APPLICATIONS  
       [0001]    This invention is related to my U.S. Provisional Patent Application Express Mail No. EUEU230416515US (petition to convert to provisional patent application filed Jul. 17, 2002), converted from prior U.S. patent application Ser. No. 09/912,265, filed on Jul. 23, 2001, entitled Rotary Ramjet Engine With Flameholder Extending to Running Clearance At Engine Casing Interior Wall, the disclosure of which is incorporated herein in its entirety by this reference. 
     
    
     
         [0002]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The patent owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.  
         TECHNICAL FIELD  
         [0003]    This invention relates to the field of combustion technology. More particularly, the invention relates to the design of a ramjet inlet and combustion chamber for a rotary ramjet engine, as well as to a method of operating such engines.  
         BACKGROUND  
         [0004]    Over the years, aero-derivitive gas turbine engines have been successfully adapted for use in stationary power generation devices. However, improvements in overall cycle efficiency, necessary in order to reduce fuel cost, and reductions in complexity of generation devices, necessary in order to reduce maintenance costs, would still be desirable. Historically, various unconventional gas turbine designs have also been attempted. One such design was suggested by Campbell, in U.S. Pat. No. 3,557,551, issued Jan. 26, 1971, showing a gas turbine engine with rotating combustion chambers and high nozzle velocities. However, that device, like many others, did not adequately address the aerodynamic features necessary to reduce parasitic “∫ PdA” drag (i.e., aerodynamic pressure P acting over an exposed area A), or the friction drag of high speed rotating elements, to within tolerable limits in order to economically deploy such technology at higher (e.g., supersonic) inlet velocities.  
           [0005]    Further, although there have been various attempts at developing an apparatus that incorporates the use of ramjet engines for the production of stationary power, most of such designs as taught by others have been practically incapable of operation at supersonic speeds, or were potentially capable of such operation only at low Mach numbers and with considerable aerodynamic drag losses. Even where the use of ramjets operating at supersonic speeds and employing the use of oblique shock wave compression were envisioned, such as in Price, U.S. Pat. No. 2,579,049, such devices were inherently inefficient for stationary power production, since such designs were based on axial flow devices, where the bulk of the flow field occurs along the shaft axis, rather than on a tangential flow device, where the bulk of the flow field and therefore the thrust is oriented tangential to the rim of a rotor.  
           [0006]    To provide a ramjet engine realistically adapted to stationary power production, it is desirable, particularly in locales with strict environmental regulations, to provide a device in which undesirable combustion products are reduced. Thus, it would be desirable to provide a ramjet inlet, and more particularly, a supersonic ramjet inlet and the accompanying combustion chamber structure, that enables the engine to maintain high efficiency power output while reducing the generation of undesirable products of combustion (such as nitrogen oxides) or incomplete products of combustion (such as carbon monoxide).  
           [0007]    Moreover, it is beneficial in such devices that parasitic drag be decreased, to increase overall efficiency, and thus decrease specific fuel consumption. Thus, it would be desirable that both the axial and the tangential fluid flow fields, at both the engine inlet and at the engine exhaust, as well as with respect to engine components located in the fluid flow path therebetween, be substantially matched such that, particularly at full load design conditions, the engine operates with high efficiency.  
           [0008]    Depending upon the specific operating needs of a particular implementation, certain subsets of (or even all) of the foregoing can be implemented using various combinations of exemplary embodiments and aspects thereof described in the sections following.  
         SUMMARY  
         [0009]    One embodiment of a novel rotary ramjet engine design disclosed herein has a combustor configuration in which a flameholder is provided that extends toward the running clearance at a stationary, preferably substantially cylindrical tubular peripheral wall. This design preferably utilizes an inlet centerbody in which ramjet compression is achieved at supersonic inlet velocities, by exploiting an oblique shock extending from a leading edge structure laterally outwardly to, at the design velocity, confining inlet and outlet strakes. Preferably, the combustor and accompanying strakes are affixed to the rotor in a preselected, substantially matched helical angle orientation, so as to smoothly and continuously acquire clean inlet air and discharge the resulting products of combustion. The combustion chamber is simplified in that a rear wall of the inlet centerbody serves as a forward wall of a combustion chamber, providing for flame holding. By virtue of the rear wall of the inlet centerbody extending from the rim of the rotor outward to the cylindrical interior peripheral sidewall (less running clearance), a combustor cavity is defined to provide for thorough mixing of fuel and air, and to provide sufficient residence time for reaction of fuel with oxidant in order to minimize the escape of incomplete combustion products from the combustor.  
           [0010]    The foregoing combustion chamber configuration provides for efficient mixing of fuel and air at supersonic inlet inflow velocities. As stated previously, this combustor flameholder extends outward from the rim of the rotor toward the stationary, substantially cylindrical tubular interior peripheral wall (less running clearance). In this manner, by the utilization of a rear wall of an inlet body, multiple shear layers are created, i.e., on both sides of the inlet centerbody, so that fuel/air mixing is improved. The shear layers lead to the creation of mixing vortices behind the rear wall of the inlet centerbody (i.e., within the flameholder), which provides for a more compact primary zone and for stable flameholding that is desirable at the design operational velocity. A still further feature is provided by an embodiment incorporating multiple inlet centerbodies, wherein multiple flameholders are utilized.  
           [0011]    Further, it is to be understood that although a combustor cavity having roughly a segmented annulus shape and having a substantially rectangular cross-section at any selected station along the flow path is depicted, other designs utilizing an inlet body rear wall flameholder shape other than that just described are also possible (e.g, non-rectangular cross-sectional shape). However, by optimizing combustor volume, the “hot section” components of the ramjet engine are reduced.  
           [0012]    In another embodiment, a fuel/air mixture may be supplied at high velocity via inlet fluid compression ducts adjacent to the inlet centerbody, so that flashback from the combustor may be reliably avoided even in the case of fuels that have a very high flame velocity. Such a the high velocity inlet can also acoustically decouple the upstream fuel system acoustics from the combustion chamber acoustic perturbations. Thus, combustion may be more easily confined to the mixing zones behind the flameholder, i.e., the rear wall of the inlet centerbody.  
           [0013]    Yet another aspect may involve matching the axial and tangential flows at the inlet inflow plane and at the exhaust outlet nozzle, providing a primarily tangential flow engine with reduced energy loss due to unmatched flow rates, or due to excess or unnecessary axial flow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    In order to enable the reader to attain a more complete appreciation of the invention, and of the novel features and the advantages thereof, attention is directed to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 shows a partially sectioned perspective view of a rotary ramjet engine having an inlet centerbody for lateral compression of inlet fluid, and a combustion chamber with a rear wall flameholder extending to the stationary peripheral circular wall (less running clearance).  
         [0016]    [0016]FIG. 2 shows a detailed perspective view of a set of rim segments, detached from a rotor, illustrating one inlet centerbody design, here using a pentagon shape, and showing in broken lines the shock pattern at supersonic design conditions, illustrating how the shock structure is contained between an inlet strake and an exhaust strake, after reflection from the leading edge structure of the inlet centerbody.  
         [0017]    [0017]FIG. 3 is a detailed perspective view of a set of rim segments, detached from a rotor, illustrating the inlet centerbody design similar to that of FIG. 2, but now showing the construction of the rear wall of the inlet centerbody and the combustion chamber extending rearwardly therefrom.  
         [0018]    [0018]FIGS. 4, 5, and  6  illustrate a method of operating the rotary ramjet engine at design conditions, and the resultant shock wave patterns, using a substantially polygonal shaped centerbody, such as a seven sided centerbody design.  
         [0019]    [0019]FIG. 4 illustrates the operation of an inlet centerbody at design conditions, which, for one preferred embodiment, is Mach 2.75.  
         [0020]    [0020]FIG. 5 illustrates the operation of an inlet centerbody at moderate speed off-design conditions, e.g., at about Mach 2.0.  
         [0021]    [0021]FIG. 6 illustrates the operation of an inlet centerbody at low speed off-design conditions, e.g., at about Mach 1.5.  
         [0022]    [0022]FIG. 7 shows a circumferential view, looking down on a set of rim segments which incorporate therein an inlet strake, and outlet strake, and an inlet centerbody design having a rear wall flame stabilizing structure, and further showing the combustion zone in the shaded area, and recirculation and fuel/air mixing zone with various reference arrows.  
         [0023]    [0023]FIG. 8 shows a sectional view of a rim segment, taken along line  8 - 8  of FIG. 7, which illustrates the inlet and outlet strakes, and a pair of sidewalls of an inlet centerbody.  
         [0024]    [0024]FIG. 9 shows a sectional view of a rim segment, taken along line  9 - 9  of FIG. 7, which illustrates the rear wall of the inlet centerbody (i.e, the combustion chamber front late), extending outward to the stationary circular peripheral wall (less running clearance).  
         [0025]    [0025]FIG. 10 illustrates the inlet and exhaust velocity vector triangles for the rotary ramjet employed in the ramjet engine design disclosed herein.  
         [0026]    [0026]FIG. 11 illustrates a side view of a ramjet hot section rotor cartridge, illustrating the full circumferential length of the combustor chamber, and also showing the ramjet outlet nozzle.  
         [0027]    [0027]FIG. 11A illustrates the detail of a ramjet outlet nozzle region.  
         [0028]    [0028]FIG. 12 is a graphical depiction, on a logarithmic scale, of the nitrogen oxides (NOx) and carbon monoxide (CO) emissions from a proposed 800 kW rotary ramjet engine, showing the emissions at full rotor speed as a function of percent of rated output power.  
         [0029]    [0029]FIG. 13 is a cross-sectional view of a prior art ramjet burner design for a stationary, rotary ramjet engine, illustrating the use of a single, long flame front.  
         [0030]    [0030]FIG. 14 is view taken along the circumference of a rotor, looking down on the novel combustor taught herein, showing the use of at least two short length flame fronts.  
         [0031]    [0031]FIGS. 15, 16,  17 , and  18  illustrate various enhanced embodiments of a novel combustor design for a stationary rotary ramjet engine. FIGS. 15A, 16A,  17 A, and  18 A illustrate the same combustor designs as in FIGS. 15, 16,  17  and  18 , but enhanced with the use of a pilot flame. In each of the just mentioned figures, use of both impingement and effusion cooling of the rear wall are shown.  
         [0032]    In FIG. 15, the combustor design utilizing a simple bluff body, as earlier shown in FIG. 7, is now shown enhanced with respect to required cooling load, with the combination of impingement and effusion cooling on the flameholder.  
         [0033]    In FIG. 16, the combustor design is further enhanced via use of side ramps to increase the flame holding surface area and to encourage higher combustion intensity in the primary zone.  
         [0034]    In FIG. 17, the combustor design is further enhanced via use of flow splitting devices to enhance mixing through the impingement of multiple transverse jets via the action of side skirts and side ramps.  
         [0035]    In FIG. 18, the combustor efficiency is further enhanced via use of a trapped vortex design, which locks stationary vortices between the fore and aft bodies, and enhances primary air entrainment through the pumping of secondary airflow into the primary zone.  
         [0036]    [0036]FIG. 19 provides a perspective of the design first set forth in FIG. 17A above, and further illustrates the use of both side skirts and side ramps to change direction of an inlet air stream to achieve thorough mixing in the combustion chamber,  
         [0037]    [0037]FIG. 20 illustrates the use of multiple inlet centerbodies in a ramjet inlet, where in two or more centerbodies are located between an inlet strake and outlet strake.  
         [0038]    [0038]FIG. 21 illustrates a trapped vortex design that provides a forebody and an aft body to provide a combustor.  
         [0039]    [0039]FIG. 22 illustrates a trapped vortex design that provides an enhanced trapped vortex embodiment, which may improve flame stability and increase combustor performance, by the addition of a forebody, a first aft body, and a second aft body.  
         [0040]    [0040]FIG. 23 further illustrates the design first set forth in FIG. 16A, and further illustrates the use of side ramps. 
     
    
       [0041]    The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual implementations depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of the rotary ramjet engine are also shown and briefly described to enable the reader to understand how various optional features may be utilized in order to provide an efficient, reliable engine.  
       DETAILED DESCRIPTION  
       [0042]    A detailed view of an exemplary embodiment of a rotary ramjet engine E is provided in FIG. 1. Inlet air as indicated by reference letter A is supplied via inlet duct  14  to a circumferential inlet air supply plenum  16  and thence through radial air inlet  18  for supply to a pre-swirl compressor inlet  20 . From inlet  20 , a pre-swirl compressor  22  provides compression of the inlet air A from inlet pressure to a desired superatmospheric pressure sufficient to create the desired relative inlet velocity between the rotating ramjet inlet and the inlet air or fuel/air premix stream. In a preferred embodiment, the compressed inlet air (or fuel/air premix) is allowed to decelerate in a diffuser portion  24  of pre-swirl compressor outlet duct  26 , to build a reservoir of low velocity pressurized inlet air. Subsequently, portion  28  of outlet duct  26  supplies pressurized air to the inflow plane of the air inlet. Primary fuel is supplied at injectors  30 . Then, the resultant fuel air mixture is deflected and expanded by inlet guide vanes  32  (of which only one guide vane  32  in the guide-vane row is shown in FIG. 1) to provide both axial and tangential ramjet inlet velocities as required to produce, at design conditions, a negligible inflow angle of attack at the leading edge  34  of the ramjet inlet centerbody  36 . This is illustrated in additional detail in FIG. 10, where a desirable design aspect of substantially matching the resultant vectors of the inlet fluid V I  flow and the exhaust gas flow V E  is further depicted. Fundamentally, this rotary ramjet engine E acts on an air mass having two basic flow field components—namely an axial flow component (along the longitudinal shaft axis), and a tangential flow component (along the plane of, and tangential to, the rim  38  of the rotor). For example, the inlet fluid stream V I  has a tangential velocity V I(T) , i.e., the rim speed of the rotor  40  in front of the leading edge  34  of ramjet inlet centerbody  36 , and an axial velocity V I(A) , along the longitudinal axis of shaft  42 , from the rotor inlet edge  44  to the rotor exhaust edge  46 . The velocity vector triangles illustrated in FIG. 10 show the relationship between the axial and tangential fluid flow fields. At the inlet, the angle alpha (∝) at which the inlet fluid proceeds at a velocity of V I  is substantially equal to the inverse tangent of the ratio of the axial velocity of the inlet fluid V I(A)  to the tangential velocity of the inlet fluid V I(T) :  
         tan        (   α   )       =       V     1        (   A   )             V     1        (   T   )         :                             
 
         [0043]    Likewise, at the outlet, the angle theta (I) which the exhaust gas stream proceeds at a velocity of V E  is substantially equal to the inverse tangent of the ratio of the axial velocity of the exhaust gas V E(A)  to the tangential velocity of the exhaust gas V E(T). :  
         tan        (   φ   )       =       V     E        (   A   )             V     E        (   T   )         :                             
 
         [0044]    Further, in a preferred embodiment, the angle alpha (∝) of the inlet fluid matches the angle theta (φ) of the exhaust gas (in the moving reference plane) at design conditions. In such a case, the inlet and exhaust strakes  50  and  52 , respectively, are preferably offset at an angle from the plane of rotation R of the rotor  40  at the same matching angle, i.e, they are parallel, as noted in FIG. 11. This is further depicted in FIG. 10 as angle delta (Δ)S I  for the inlet strake  50 , and as angle delta (Δ)S E  for the exhaust strake  52 . Likewise, the inlet centerbody structure  36  is preferably offset at the same matching angle (i.e., the centerline CL of the centerbody structure is offset at the same matching angle), and thus the ramjet has a negligible angle of attack. This is depicted as angle delta (Δ)C for the inlet centerbody structure in FIG. 10. Although a variety of designs can be developed utilizing the teachings hereof, it has been found that substantially conforming angles alpha, theta, and delta S I , delta S E , and delta C is advantageous. For the exemplary design conditions depicted herein, those angles are preferably conformed in the range from more than zero up to about 15 degrees; more preferably from more than zero up to about 10 degrees, is advantageous. For an exemplary design point of about Mach 2.75 with respect to the relative velocity of inlet air to the leading edge  34 , these angles are in one embodiment in the 1 degree to 5 degree range, and may, in such embodiment be in the 1 degree to 2 degree range, and in one exemplary embodiment, may be established at about 1.5 degrees, and may be more precisely established at about 1.554 degrees. More specifically, in producing a negligible inflow angle of attack for a rotary ramjet engine operating around such Mach number, the inlet axial velocity would be about 82 feet per second, and inlet tangential velocity would be about 3076 feet per second. At the outlet, the exhaust axial velocity would be about 131 feet per second, and the exhaust tangential velocity would be about 5005 feet per second (in the rotating frame of reference). In this way, both the inlet flow vector angle alpha and the exhaust flow vector angle theta are about 1.5 degrees. Accordingly, the inlet strake  50  angle delta  SI  and the outlet strake  52  angle delta SE  are set at about 1.5 degrees. More specifically, in one embodiment of our exemplary 800 kW rotary ramjet engine design, the inlet strake angle delta S I  is 1.554 degrees. The matched velocity vector technique enables the inlet fluid to be supplied with minimal pressure loss, viscous fluid flow complications, or parasitic power losses. Moreover, for a given ramjet rotor speed, increasing the amount of positive pressure provided by the pre-swirl impeller results in an increased tangential velocity component entering the plane of rotation of the ramjet rotor, and an increase in the inlet inflow Mach number, and thus an increased ramjet cycle compression ratio.  
         [0045]    The supersonic ramjet inlet utilizes the kinetic energy inherent in the air mass due to the relative velocity between the ramjet inlet and the supplied air stream, depicted as V I  in FIG. 10, to compress the inlet air (or, alternately, the inlet fuel/air mixture), preferably via an oblique shock wave structure, as depicted in FIGS. 2, 3, and  4 . As illustrated herein, in order to carry out reliable, thorough combustion in the combustion chamber  56 , the inlet stream is compressed utilizing a flow pattern operating with compression primarily laterally with respect to the plane of rotation of the rotor, via an oblique shock compression fan  58  (see FIGS. 2, 4,  5 , and  6 ), to compress the inlet fuel/air mix between the inlet centerbody  36  and adjacent inlet strake  50  and outlet strake  52  structures. Of course, as can be appreciated from FIGS. 1, 8, and  9 , the compressed inlet fuel/air mix is also contained by the substantially cylindrical tubular interior sidewall portion  60  of the engine casing  62 . After compression, the compressed inlet fluid stream is preferably maintained at a very high velocity through a constant area supersonic diffuser D, as shown in FIGS. 7 and 10. Optionally, as indicated in FIGS. 4, 5, and  6 , an at least partially subsonic diffuser Dl having increasing diffuser cross-sectional area may be provided rearward of a leading edge  34 ′ and more precisely, rearward flow-wise of a constant area diffuser DC, as shown in the alternate inlet center body structure  36 ′. This alternate inlet centerbody structure  36 ′ design as illustrated in FIGS. 4, 5, and  6  provides a seven sided structure, having an inlet leading edge  34 ′, a pair of compression sidewalls  70  and  72  that project shock waves  74  and  76  laterally outward, a pair of constant area diffuser DC walls  78  and  80 , a pair of increasing area diffuser DI walls  82  and  84 , and a rear wall  86  which operates as a flameholder. Note that the use of the increasing area diffuser DI as provided by the ramjet inlet centerbody  36 ′ design still further reduces the velocity of the inlet fluid stream, indicated by reference arrows F in FIGS.  4 ,  5 ,and  6 , before it reaches the combustion chamber  56  itself. In the ramjet inlet centerbody  36  design shown in FIGS. 1, 2,  3 ,  7  and  10 , the centerbody  36  is of a pentagon design, with leading edge  34 , a pair of compression sidewalls  92  and  94  that project shock waves  96  and  98  laterally outward, a pair of opposing sidewalls  100  and  102  that also operate with inlet  50  and exhaust  52  strakes to define a constant area diffuser D, and a rear wall  104  that operates as a flameholder for the combustion chamber  56 . As noted in FIG. 9, a running clearance C 104  is provided between the top  105  of rear wall  104  and the interior sidewall portion  60  of engine casing  62 . For the particular operating conditions depicted herein, the value of this running clearance is about 0.025 inches, although those skilled in the art will appreciate that the value will differ for other operating conditions. Similarly, at these exemplary operating conditions:  
         [0046]    a running clearance C50 of about 0.005 inches to about 0.010 inches is provided between the top  106  of outlet strake  50  and the interior sidewall portion  60  of engine casing  62 ;  
         [0047]    a running clearance C 52  of about 0.005 inches to about 0.010 inches is provided between top  108  of outlet strake  52  and the interior sidewall portion  60  of engine casing  62 .; and  
         [0048]    the sidewalls  100  and  102  of inlet centerbody  36  have height H 100  and H 102 , respectively, and a running clearance of C 100  and of C 102 , respectively, of about 0.025 inches each.  
         [0049]    In the exemplary rotary ramjet engine implementation described herein, although the ramjet configuration superficially resembles those previously attempted in supersonic gas turbine systems that attempted to utilize ramjet engine techniques, in the rotary ramjet engine shown herein, the compression and combustion is achieved utilizing only a small number of ramjets, (preferably expected to be in the range from 2 to 5 total, with accompanying inlet and outlet strakes for each ramjet), and within an aerodynamic duct formed by the spirally disposed, or more specifically, helically disposed inlet and outlet strakes, as opposed to a traditional gas turbine or other axial flow compressor using many rotor and stator blades.  
         [0050]    In an exemplary method of construction illustrated herein, a method of constructing the inlet strakes  50 , exhaust strakes  52 , inlet centerbody  36 , and related rotating parts including rim portions of the rotor, are shown manufactured as individual rim segments RS, a plurality N (where N is a positive integer, preferably of more than 100, depending on the size of the rotor  40 ) of which taken in a group comprising the series of rim segments from RS 1  to RS N , together provide the various structures discussed above. As indicated in FIGS. 2 and 3, one method of attachment of each of the rim segments RS to the rotor  40  is via use of dove tail shaped features  110  on each rim segment RS; these features interlock with complementary features on the rotor  40 . Any single rim segment RS X  (where X is any number from 1 to N) may comprise an inlet strake  50  portion, an outlet strake  52  portion, or both, and may further comprise, alternately, one or more inlet centerbody  36  portions, a combustor floor  120  portion, a converging portion  122  of a ramjet outlet nozzle, a ramjet-nozzle/throat  124  portion, or the diverging portion  126  of a ramjet outlet nozzle. Various elements of such construction can be easily appreciated from a review of FIGS. 2, 3, and  7  through  10 . As also seen in FIGS. 2 and 3, it is not necessary to provide structure for an interior of inlet centerbody  36 , as a quiescent zone  130  can be provided interior to the above described components which comprise the centerbody  36  structure.  
         [0051]    In situations where environmental concerns are significant, in order to obtain the proper conditions for combustion while producing low-pollution products of combustion, the fuel and combustion air may be premixed prior to feed to the ramjet inlet. As illustrated in FIG. 1, fuel injectors  30  add the fuel to an inlet fluid (which may be either be a fuel free oxidant containing stream, or which may contain some high value fuel such as hydrogen, or some low value fuel, such as coal bed methane, coal mine purge gas, landfill methane, biomass produced fuel gas, sub-quality natural gas, or other low grade fuels) provided through diffuser  24 . In order to carry out the actual combustion step in an operationally reliable manner, the velocity of the compressed inlet fuel/air mixture should preferably be high at the intermixing point between the combustion chamber and the delivery point of the combustible fuel/air mixture, so that flashback of the flame front from the combustor toward the inlet is reduced or avoided. In the exemplary rotary ramjet engine described herein, when operated at the exemplary conditions described herein, the residence time in the diffuser is too short to initiate an auto-ignition process in the prescribed residence time. Further, the aerodynamics of the diffuser design and of the inlet section are not conducive to flame holding.  
         [0052]    In order to stabilize the combustion process downstream of the rear wall  104  of ramjet inlet centerbody  36 , may be stabilized by substantially reducing the velocity through the combustion chamber  56  by providing a combustion chamber  56  having substantially larger cross-sectional flow area than provided by the inlet ducts thereto (for example, ducts D &amp; D, as illustrated in FIGS. 7, 8, and  9 . Localized recirculation zones may also be provided in order to have an adequate residence time to substantially minimize creation of carbon monoxide in the combustor, in order to bring the remaining CO in the exiting combustion gases to an environmentally acceptable low residual level. Overall, this configuration reduces the size of the primary zone by introducing a short flame front at the entrance of the combustor flow path. In the base design illustrated herein, a combustion chamber with a constant duct height and a predetermined overall length L c  is provided (see FIG. 3). Preferably, this overall length L C  is determined by providing a combustor residence time of about 5 ms to about 10 ms for the oxidation of CO to CO 2 , based on equilibrium flame temperature calculations and current gas turbine industry practice. Of course, changes in industry practice or environmental regulations may require shorter or longer residence times. A ramjet exhaust nozzle is provided at the outlet of the combustion chamber  56 , so that exhaust gas outlet velocity will propel the rotor  40  at the desired rim speed under design load conditions. Accordingly, in the ramjet configuration illustrated, the acceleration and deceleration of the inlet fluid, and the acceleration and deceleration of the outlet combustion gases, is accomplished efficiently.  
         [0053]    Emissions calculations for the combustor described herein, utilizing rear wall  104  of ramjet inlet centerbody as a flameholder extending to the running clearance at a cylindrical tubular engine casing interior peripheral wall  60  have been developed utilizing computational fluid dynamics to visualize the flowfield within the combustor. The calculated flowfield was then utilized to predict the emissions; such methods have been widely accepted in the gas turbine industry. Accordingly, three major flowfield zones were modeled: the recirculation zone  150  behind the rear wall  104  of the inlet centerbody  36 , the shear layer zone  152  (seen at the downstream edges of the gray zone in FIG. 7), where the inflowing premixed fuel and air mixes with the burning gases from the recirculation zone, and the burnout zone  154  (see FIG. 11), which is located downstream from the recirculation zone  150  and shear layer  152  zones. For evaluation of a combustor in a proposed 800 kW engine size, the recirculation zone  150  was modeled as a perfectly stirred reactor. In this region, inlet fluid is decelerated to a suitable velocity level, and longitudinal (flow stream wise) vortices (see reference arrows  151  in FIG. 7) are generated at rearward of the rear wall  104  of the inlet body  36 . Then, the shear layer  152  was modeled as a series of perfectly stirred reactors, to simulate the mixing of the inflowing air-fuel stream with the recirculation zone gases. The burnout zone  154  was then modeled as a plug flow reactor. The interaction or flow exchange among the various components was based on estimates predicted by the computational flow dynamics results. Finally, air-fuel combustion chemistry was simulated utilizing accepted methodology developed by the Gas Research Institute for modeling the combustion of natural gas. In FIG. 12, the projected emissions of an exemplary 800 kW rotary ramjet engine are shown, assuming operation at full rotor speed (as in synchronous electrical generator service) from idle to full load conditions. The full load condition emissions are projected at less than 5 ppm of NOx, and are actually expected at only 4 ppm of NOx and 36 ppm of CO, corrected to an O 2  concentration dry basis of 15 percent. Therefore, the rotary ramjet engine E with inlet centerbody  36  and combustor  56  design configuration illustrated herein provides emission results which are well within the range of measured NOx and CO data from various lean pre-mixed, gas turbine and laboratory combustors that were operating under conditions similar to that of the proposed 800 kW engine at full load. Overall, for this exemplary combination of engine size and operating conditions, NOx is maintained below 100 ppm, and more preferably below 50 ppm, and yet still preferably at less than 25 ppm, and ideally below 10 ppm, and more ideally below 9 ppm. Projections of NOx below 5 ppm, and even at 4 ppm, when achieved in practice, will provide outstanding benefits to the operator of the equipment.  
         [0054]    Returning now to FIGS. 1 and 11, the hot exhaust gases  156  (products of combustion), directly after discharge from the combustion chamber, flow through the ramjet outlet nozzle 124 , and thence along the outlet strake  52 , and are directed, preferably at low pressure but still containing axial and tangential swirl kinetic energy to exhaust gas blades  157  in an impulse turbine  158 , for extraction of the kinetic energy based on the overall swirl energy inherent in such hot exhaust gases  156 . Finally, in one embodiment (for enhanced efficiency), the hot exhaust gases  156  may be further utilized by being directed to an exhaust heat exchanger  160  to heat condensate  161  and produce steam  162 . The steam  162  can optionally be directed through high-pressure steam supply nozzles  164  and thence through inlet vanes  166 , preferably fixed in orientation, and thence into the steam blades  168  in the impulse turbine, for added energy recovery. Subsequently, low pressure steam  170  is exhausted from the impulse turbine  158  via steam discharge nozzles  172  and is directed to a condenser (not shown) and pumped to the heat exchanger  160  for replenishment of the supply of high pressure steam  162  to be sent to the high pressure steam supply nozzles  164  mentioned above.  
         [0055]    Returning now to FIG. 1, note that in order to match optimum tangential speed of rotor  40  and the desired rotational speed of impulse turbine  158 , a planetary gear system  200  is used for transmission of power from the impulse turbine  158  to a geared spline  202  on output shaft portion  204 . The impulse turbine  158  is not directly affixed to, and turns at a different speed and direction than rotor  40 . Additionally, it should be noted that in order to minimize aerodynamic drag and efficiently operate the outer portions of the rotor  40  at supersonic tangential velocities, means can be provided to reduce drag of the rotor. This can take the form of a fixed housing with a small interior gap G between the rotor surface  210  and an interior housing  212 , or, alternately, take the form of a vacuum means to remove air from adjacent the rotor. Some rotor drag minimizing techniques are taught in U.S. Pat. No. 5,372,005, issued Dec. 14, 1994 to Lawlor, the disclosure of which is incorporated herein by this reference.  
         [0056]    [0056]FIGS. 4, 5, and  6  schematically illustrate how the inlet fluid spill in a proposed 800 kW engine design transitions from a high spillage condition at low inflow Mach number (FIG. 6) to lower spill levels at increasing inflow (FIG. 5), and finally to no spill when the shock  58  on inlet strake  50  lip  222  design condition is achieved (FIG. 4). Spillage occurs in front of the intake strake  50 , and through a plurality of slots  220  in the exhaust strake  52 . This inlet spillage mechanism enables the inlet to smoothly and continuously transition from operating in a fully un-started, normal shock mode to a fully started condition with full inlet shock capture. Of course, when the inlet is operating at inflow Mach numbers below the design Mach number, and the inlet is spilling, aerodynamic drag occurs due to the spillage process, which drag is accounted for in the system starting characteristics, and startup power is provided accordingly.  
         [0057]    Attention is now directed to FIGS. 13 through 19, where yet further improved combustor designs are illustrated. For comparison purposes, in FIG. 13, a cross-sectional view of a prior art ramjet burner design for a stationary, rotary ramjet engine is illustrated, showing a rotor  300  having at the outer reaches thereof a ramjet burner section  302  having a flameholder  304 , rearward of which, flow-wise, is a single, long flame front  306 , behind which hot combustion products  308  circulate. The flame front  306  spreads outward until contacting the adjacent stationary peripheral wall  310 . In that prior art design, a relatively long primary mixing zone “L 1 P z ” results.  
         [0058]    We have now determined that the length of a primary mixing zone “L 2 P z ” in a ramjet combustor can be shortened considerably by adopting the configuration illustrated in FIG. 14. Then a burnout zone L z B z  can be provided as appropriate to achieve emissions objectives. This figure is a view taken looking radially inward along the circumference of a rotor, looking down on the exemplary combustor  320  taught herein. Combustor  320  is situated between an inlet strake  322  and an outlet strake  324 , behind rear wall  326  of centerbody  328 . In one embodiment, rear wall  326  of the centerbody  328  includes provision by way of a perforated wall  330  operably communicating with cooling gas source, for impingement cooling of rear wall  326  by the addition of perforations for coolant passageways  348  in wall  326 . Alternately, or in addition, effusion cooling is provided for rear wall  326 . As shown in FIG. 14, at least two short length flame fronts  332   A  and  332   B  are provided. This results in a primary mixing zone of length “L 2 P z ” which is roughly half as long as when a single flame front is utilized as in the prior art design such as shown in FIG. 13. For a combustor of given length, the burnout zone, L 2 B z , is lengthened when such a combustor  320  arrangement is utilized.  
         [0059]    [0059]FIGS. 15, 15A,  16 ,  17 , and  18  illustrate yet further enhanced embodiments of an exemplary combustor for a stationary rotary ramjet engine. In FIG. 15, the combustor design utilizes a simple bluff body  340 , as earlier shown in FIG. 7. As further illustrated in FIG. 15A, this embodiment is now shown enhanced with the combination of impingement and effusion cooling on the rear wall/flameholder  342  of centerbody  344 , utilizing an interior wall  346  with coolant passageways  348  therethrough, similar to the configuration just indicated in FIG. 14. Although this simple, robust bluff body combustor design allows easy sizing for the required inlet velocity, the combustion efficiency is less than optimum since the length of primary zone is not as compact as it could be, and perhaps would encounter unsteady vortex shedding, and thus combustion efficiency might be less than optimal. Therefore reduced combustion intensity would be expected, compared to even further improved combustors as explained below. Also, this design may be expected to be somewhat susceptible to vortex shedding, and thus, would not be an optimum design solution for many commercially important rotary ramjet combustor designs.  
         [0060]    Turning now to FIG. 16, an exemplary combustor  350  has been developed that is further enhanced via use of side ramps  352  and  354  which convert some tangential fluid momentum to transverse fluid momentum. Side ramps  352  and  354  cooperate to deflect combustion gases away from the inlet strake  322  and outlet strake  324  and towards a centerline C L  of the rear wall  342  of the inlet centerbody  344 , by each extending transversely into the combustor  350  a distance L b . Preferably, this length L b  is the same for each vortex generator. This combustor  350  configuration increases the flame holding, because in this modified bluff body design, increased flame holding surface area is provided due to side ramps  352  and  354 . This combustor  350  also affords a short length of primary combustion zone “Lp z ”. However, for certain applications, this design may also have additional (potentially) undesirable pressure drop, flow-wise through the combustor  350 , as compared to the configuration just illustrated in FIG. 15 above. A perspective view of this embodiment is provided in FIG. 23.  
         [0061]    In FIGS. 17 and 17A, another exemplary combustor  370  is further enhanced via use of flow splitting side skirts  372  and  374  to enhance mixing with multiple jets, i.e., the side skirts  372  and  374  moves entering fuel air mixtures in a radially inward or outward direction, and another set of side ramps  376  and  378  move the burning gases in transverse, axial and opposing direction. The combustor  370  configuration has a strong recirculation zone due to the jet impingement in the swirller space  380 , defined by a distance d s  between the tails  390  and  392  of the side skirts  372  and  374 , and the trailing edges  394  and  396  of the side ramp vortex generators  376  and  378 , respectively. The combustor  370  has a high efficiency, i.e., heat release per unit volume, due to the increased turbulence activity resulting from jet impingement. This design is better seen in FIG. 19, where a first side skirt  372  is provided on the inlet side of a first extended centerbody wall  382 , and a second side skirt  374  is are provided on the inlet side of a second extended centerbody wall  384 .  
         [0062]    In yet a further enhancement, in FIG. 18, an exemplary combustor  400  is provided using an aft body  402  for provision of trapped vortices  404  and  406 . This combustor  400  configuration has lower pressure drop through the combustor  400  relative to the simple bluff body configuration illustrated in FIG. 15. Additionally, flame stability is improved through the locking of downstream vortices between the fore body  342  and the aft body  402 . And, by providing for sufficient space rearward (flow-wise) of the aft body  402 , the large burnout zone B z  is provided. Thus, a compact primary zone Pz having a relatively high combustor efficiency is provided. Note that this or the other just provided designs are shown utilizing the impingement and effusion cooling methods first discussed in connection with FIG. 15 above.  
         [0063]    In FIGS. 17 and 18, the efficient combustion and high heat release per unit of combustor volume is made possible with a highly turbulent primary zone. Importantly, the jet impingement, and pumping action, as is further illustrated in FIG. 19, may increase combustor efficiency to at least 99% or more, and more preferably, to at least 99.5% or more.  
         [0064]    [0064]FIG. 20 illustrates the use of multiple inlet centerbodies  500  and  502  on a rotor  504  for use in a rotary ramjet engine inlet, where two or more centerbodies are located between an inlet strake  506  and an outlet strake  508 . As illustrated, aft bodies  510  and  512  are provided rearward of flameholding rear walls  514  and  516  of centerbodies  500  and  502 , in order to provide for trapping of a vortex between the rear walls  514  or  516  and the aft bodies  510  or  512 , respectively.  
         [0065]    In FIG. 21, an exemplary enhanced double wall flameholding rear wall  600  with both impingement cooling passageways  602  in a first wall  603  and effusion cooling passageways  604  in a second wall  605  is provided for inlet centerbody  610 . Also, pilot fuel ports  612  and  614  are provided. Additionally, an aft body  620  with pilot fuel ports  622  and  624  provide for trapped vortex operation of combustor  630 .  
         [0066]    In FIG. 22, an exemplary double bluff body configuration is shown. Here both a first aft bluff body  620  and a second aft bluff body  700  are illustrated. This exemplary embodiment may utilize the double wall flameholding rear wall  600  configuration as just illustrated in FIG. 21 above. However, a second bluff body  700  having a rear wall  704  is provided for creating both a second recirculation zone  730 , in addition to the first recirculation zone  630  as illustrated in FIG. 21 above. This configuration allows further enhancement of combustion efficiency.  
         [0067]    It is to be appreciated that the various aspects and embodiments of the inlet and combustion chamber designs described herein are an important improvement in the state of the art of rotary ramjet engines. Although only a few exemplary embodiments have been described in detail, various details are sufficiently set forth in the drawings and in the specification provided herein to enable one of ordinary skill in the art to make and use the invention(s), which need not be further described by additional writing in this detailed description. Importantly, the aspects and embodiments described and claimed herein may be modified from those shown without materially departing from the novel teachings and advantages provided by this invention, and may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Therefore, the embodiments presented herein are to be considered in all respects as illustrative and not restrictive. As such, this disclosure is intended to cover the structures described herein and not only structural equivalents thereof, but also equivalent structures. Numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention(s) may be practiced otherwise than as specifically described herein. Thus, the scope of the invention(s), as set forth in the appended claims, and as indicated by the drawing and by the foregoing description, is intended to include variations from the embodiments provided which are nevertheless described by the broad interpretation and range properly afforded to the plain meaning of the claims set forth below.