Abstract:
Apparatus for reducing the total pressure of a compressible fluid fuel. The apparatus includes at least two closely spaced apart constant enthalpy expansion sections, each section having at least one orifice, the orifices in adjacent sections being noncoaxial. The pressure reduction lowers flow velocity when mixed with the air to below the flame speed to promote ignition and stable combustion.

Description:
The present application claims priority from U.S. Provisional Application No. 60/131,048, filed Apr. 26, 1999, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the combustion of fuels and more particularly to the efficient combustion of compressible fluid fuel. 
     BACKGROUND OF THE INVENTION 
     Efficient combustion of liquid fuels usually involves creating fine liquid droplets or vaporizing the liquid to the gaseous state and thereafter mixing the fuel with air or oxygen in order to initiate and sustain combustion. As disclosed and claimed in our co-pending United States Patent Application Serial No. 08/992,983, now U.S. Pat. No. 6,010,544, the contents of which are incorporated herein by reference, an alternative approach involves forming a supercritical water/hydrocarbon fuel mixture. As taught in that patent, a water/hydrocarbon fuel mixture is heated and pressurized to a level at or above the critical point of the mixture. At critical conditions, the mixture is a homogeneous single phase that can be combusted in a more efficient manner and with considerably reduced undesirable emissions. As further taught therein, the critical temperature of the mixture is at or above approximately 363° C. and the critical pressure is at or above 3000 psi. Optimal conditions are considered to be 390° C. and 4000 psi. 
     As disclosed and claimed in our co-pending U.S. patent application Ser. No. 09/359,509, now U.S. Pat. No. 6,240,893, the contents of which are incorporated herein by reference, another alternative approach involves forming a sub-critical water/hydrocarbon fuel mixture which is at a pressure that is below the critical pressure charactistic of the mixture and is at a temperature that is at least the greater of about 250° C. and the boiling point of water at the mixture pressure. As taught in that application, a water/hydrocarbon fuel mixture is pressurized to a level below the critical point of the mixture and heated to a temperature that is at least the greater of about 250° C. and the boiling point of water at the mixture pressure. The specified sub-critical mixture provides a local environment of water molecules, tending to limit hydrocarbon polymerization and other undesrable side reactions and keeping the hydrocarbon from precipitating from the mixture. At these specified sub-critical conditions, combustion of the mixture provides a “cleaner” combustion with considerably reduced particulate matter and oxides of nitrogen emissions compared with that which results from the combustion of otherwise comparable water-hydrocarbon mixtures at temperatures below the boiling point of water at the sub-critical pressure employed. 
     In some applications, the high pressure and temperature of the supercritical mixture or the above-specified subcritical mixtures are directly suitable for injection into a combustion chamber. In other applications, the high pressure of these water/hydrocarbon fuel mixtures result in subsequent fuel/air flow velocities higher than the flame speed of the combustible mixture. Flow velocities which far exceed the flame speed lead to difficulties with ignition and flame stabilization. Thus, for applications such as nearly atmospheric pressure oil burners the fuel/air mixture flow velocity must be reduced by lowering both the pressure and velocity. 
     A valve could be used to lower the flow velocity since it provides for a variable size orifice, but some of total pressure ahead of the valve is recovered in the velocity downstream of the valve and a valve also introduces increased thermal mass and dwell time. Furthermore, after the fluid passes through the valve its temperature and pressure change dramatically and the fluid&#39;s metastable condition can be adversely affected by these changes in the length between the valve and the combustion process. Other solutions to for lowering flow velocity are thus needed. 
     SUMMARY OF THE INVENTION 
     In one aspect, the fuel system of the invention includes structure containing a mixture of water and hydrocarbon fuel in which the mixture is at or above its critical temperature and pressure such that the mixture is a homogeneous single phase. Apparatus is provided for rapidly reducing the total pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combusting a hydrocarbon/water mixture including the steps of producing a mixture of water and hydrocarbon fuel at or above its critical point such that the mixture is a homogeneous single phase and rapidly reducing the pressure of the mixture without an excessive change in the velocity. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that the mixture be maintained at a temperature above 363° C. and at a pressure above 3000 psi before reduction. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for atmospheric combustion. The temperature of the supercritical mixture may be in the range of 363° C.-450° C. and in a pressure range of 3000 psi 4500 psi. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the total pressure be reduced to a range of 2 to 10 times the pressure in the combustion chamber. 
     In another aspect, the fuel system of the invention includes structure containing a sub-critical water/hydrocarbon fuel mixture which is at a pressure that is below the critical pressure characteristic of the mixture and is at a temperature that is at least the greater of about 250° C. and the boiling point of water at the mixture pressure. Apparatus is provided for rapidly reducing the pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combustion of a sub-critical hydrocarbon/water mixture of the above-specified characteristics and rapidly reducing the pressure of the mixture. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that the mixture be maintained at a temperature that is between about 25° C. and about 100° C. greater than the boiling point of water at the mixture pressure. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for combustion at up to 10 atmospheres of pressure. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the pressure be reduced to a range of 200-500 psi. for combustion at 10 to 20 atmospheres 
     Apparatus is provided for rapidly reducing the total pressure of the mixture prior to delivery to a combustion chamber. In another aspect, the invention is a process for combusting a hydrocarbon/water mixture including the steps of producing a supercritical or specified sub-critical mixture of water and hydrocarbon fuel and rapidly reducing the pressure of the mixture. Thereafter, the reduced pressure mixture is delivered into a combustion chamber. In this aspect, it is preferred that a supercritical mixture be maintained at a temperature above 363° C. and at a pressure above 3000 psi. It is preferred that the pressure reduction occur within a time period less than 1 millisecond prior to being delivered to the combustion chamber and that the pressure be reduced to below 200 psi for combustion at 1 to 10 atmospheres. The temperature of the supercritical mixture may be in the range of 363° C.-450° C. and in a pressure range of 3000 psi-4500 psi. It is also preferred that the pressure reduction take place within a time period in the range of 0.1-2 milliseconds prior to being delivered to a combustion chamber and that the pressure be reduced to a range of 200-500 psi for combustion at 10 to 20 atmospheres. 
     In another aspect, the apparatus for reducing the total pressure of the supercritical or specified sub-critical mixture comprises at least two closely spaced apart constant enthalpy expansion sections, each section having at least one orifice with orifices in adjacent sections being non-coaxial. Each section defines an enclosed volume and the size of the orifices increases in a flow direction in adjacent sections. It is preferred that the orifice size at each section be selected to provide sonic or choked flow at each section. In a preferred embodiment, the first section includes a single orifice and subsequent sections include at least two orifices and preferably three orifices. Also in a preferred embodiment the pressure downstream of the orifice is reduced to approximately one-third of the upstream pressure at each section. In this embodiment, the volumes between orifices serve to dissipate velocity of expansion by local shock waves, thus reducing total as well as static pressures. 
     In yet another aspect, the apparatus for reducing the pressure of the supercritical water/hydrocarbon fuel mixture includes three closely spaced apart constant enthalpy expansion sections for receiving the mixture. A first section includes a single orifice located on a central axis of the first section and a second section includes three orifices located off the central axis. A third section includes six orifices arranged on a substantially spherical surface. This device provides the mixture at pressures near 1 atmosphere and velocity suitable for mixing with air and combusting. 
     This invention relates to the combustion of compressible fluid fuels, including but not limited to the above-described supercritical or sub-critical water/hydrocarbon fuel mixtures. Compressible fluid fuels (e. g., methane or “natural gas”) are at least in part gas like (i. e., compressible) at ambient conditions and typically are used by being injected into the cylinder of a reciprocating engine at near top dead center. The space available for this injection is so limited that the gaseous fuel must be delivered under high pressure. Injection of this gaseous fuel under high pressure directly into a cylinder can result in a gas jet, which transits the cylinder too rapidly to entrain the appropriate amount of air. Delivery of the gaseous fuel to the cylinder from the injector through apparatus of this invention with more than one set of orifices in series can eliminate or minimize this problem. 
     The multisection cascade orifice nozzle provides for relieving the momentum of the high-pressure jet while maintaining the intimate mixing of water and oil under the supercritical conditions. The invention produces the correct fluid velocity distribution by interacting the flow from the final orifice it with the combustion air mass. The momentum exchange to the air also includes mixing so that the proper mixture of air and fuel is obtained locally. 
     In contrast, the mixture issuing from a single orifice is moving so fast that it entrains too much air and is too lean for stable combustion. Pressure reductions from the 4000-psi range down to the 100-psi range can be achieved within a length of approximately 1-inch and the time spent in this volume can be reduced to the millisecond range for high flow rates. Metastable conditions at reduced pressures can be sustained long enough to begin and maintain stable combustion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described with reference to the several figures of the drawing, in which, 
     FIGS. 1 a  and  1   b  are cross-sectional and end views of a first section of a 4-stage pressure reducing apparatus of the invention; 
     FIGS. 2 a  and  2   b  are cross-sectional and end views of a second section of the 4-stage pressure reducing apparatus; 
     FIGS. 3 a  and  3   b  are cross-sectional and end views of a third section of the 4-stage pressure reducing apparatus; 
     FIGS. 4 a  and  4   b  are cross-sectional and end views of a fourth section of the 4-stage pressure reducing apparatus; 
     FIG. 5 is a cross-sectional view of the assembled sections forming the 4-stage pressure reducing apparatus; 
     FIGS. 6 a  and  6   b  are cross-sectional and end views of a first section of a 3-stage pressure reducing apparatus of the invention; 
     FIGS. 7 a  and  7   b  are cross-sectional and end views of a second section of the 3-stage pressure reducing apparatus; 
     FIGS. 8 a  and  8   b  are cross-sectional and end views of a third section of the 3-stage pressure reducing apparatus; 
     FIG. 9 is a cross-sectional view of the assembled sections forming the 3-stage pressure reducing apparats; and 
     FIG. 10 is a schematic representation of a fuel delivery system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     As discussed above, the flow velocity of compressible fluid fuels must often be reduced to achieve a stable flame. However, the pressure must be reduced rapidly and over a short distance so that the combustion advantages of the supercritical or specified sub-critical mixtures are not sacrificed and so that excessive velocity is not generated. As will be appreciated by those skilled in the art, as pressure is reduced, thermodynamic driving forces tend to cause the mixture to revert to liquid water and liquid fuel droplets. These thermodynamic driving forces are in competition with kinetic conditions. That is to say, there is a finite amount of time required to accomplish a phase change and the reversion to liquid water and liquid fuel droplets is limited by diffusion of each of the species. Thus, in order to preserve the combustion advantages provided by the supercritical state, pressure, and hence flow velocity, must be reduced rapidly so that combustion can occur before the homogeneous single phase of the supercritical mixture reverts to separate phases for water and fuel or the specified sub-critical conditions are no longer maintained. 
     Accordingly, it is important to lower the flow velocity of the supercritical and specified sub-critical fuel/water mixture from its reservoir to a combustion chamber by rapidly reducing its total pressure. The total pressure is defined as the sum of the static pressure, P, and the velocity head, ½ρ×V 2 /(144 g c ), where: 
     P=static pressure, psi 
     ρ=density, lb m /ft 3    
     V=velocity, ft/sec, and 
     g c  a conversion constant, ft lb m /lb f  sec 2    
     The total pressure of a mixture will essentially be the static pressure if the contribution of the velocity term is negligible, as will be the case if, at the point of measurement, the fluid is essentially quiescent. However, the total pressure and static pressure would be significantly different, if in the mixture at the point of measurement, the fluid is significantly in motion so that the velocity term is relatively large. 
     The methods and structures of the present invention allow rapid reduction of flow velocity of compressible fluid fuels without loss of their beneficial combustion properties. 
     With reference to FIGS. 1 a  and  1   b  a first section  10  includes a single orifice  12  and defines a small volume  14 . A suitable orifice  12  diameter is  0 . 016  inches. A suitable diameter for the section  10  is 0.375 inches. A suitable depth for the recess defining the volume  14  is 0.060 inches and the length of the section  10  is approximately ⅛ of an inch. A suitable material for section  10  is 316 stainless steel. 
     A second section  20  is shown in FIGS. 2 a  and  2   b.  As shown in FIG. 2 b,  the section  20  includes three orifices  22 ,  24  and  26 . A suitable diameter for the orifices  22 ,  24  and  26  is 0.01732 inch. It is also suitable that the orifices  22 , 24  and  26  be arranged on a 0.080 inch bolt circle. Other suitable dimensions are as set forth with respect to section in FIG. 1 a.    
     A third section  30  is shown in FIGS. 3 a  and  3   b.  Section  30  includes three orifices  32 ,  34  and  36  with diameters of 0.030 inches. These orifices are also located on a 0.080 inch bolt circle. The dimensions of section  30  are comparable to those of sections  10  and  20 . 
     A fourth section  40  is shown in FIGS. 4 a  and  4   b.  The section  40  has a generally spherical configuration and includes three orifices  42 ,  44  and  46 . The orifices  42 ,  44  and  46  may have a diameter of 0.052 inches. These orifices may be located approximately 30° from an axis of the section  40  as shown in FIG. 4 a.  A suitable dimension for an outer radius  48  is 0.188 inches and a suitable dimension for an inner radius  50  is 0.158 inches. The length of the section  40  is approximately ¼ of an inch. 
     FIG. 5 shows pressure-reducing apparatus  60  of the invention with the various sections assembled to form a unitary structure. It is preferred that the sections  10 ,  20 ,  30  and  40  be welded together to form the unitary structure  60 . It is important that the sections be assembled to preserve the arrangement of the orifices as shown in the previous figures so that the orifices are not coaxial. For example, the section  20  is assembled with the orifice  22  adjacent to the orifices  32  and  34  of section  30  thereby resulting in a non-coaxial cascade of nozzle orifices. In a preferred embodiment, a 0.005 screen  62  is positioned before section  10  to protect the orifice  12  from becoming blocked by dirt in the system. 
     Orifice sizes are selected to assure a pressure ratio from section to section sufficient to provide choking of the flow at each stage. In this manner, an upstream orifice will be the flow limiting device and the number of orifices in the series cascade is chosen to obtain a desired total pressure ratio. The area ratio between successive orifices should be the inverse of the overall desired pressure ratio. That is, if the pressure ratio after an orifice is one-third the pressure before the orifice, the area of the successive orifice should be three times that of the first orifice. 
     For mixtures for which the thermodynamic properties are well known, the pressure ratios and orifice areas can be more accurately calculated. All orifices have sonic or smaller pressure ratios and the orifice aggregate area must pass the same mass flow at each stage. As an example, in order to drop from approximately 4000 psi to approximately 49 psi, four sections are suitable in the cascade. If the orifice  12  in the section  10  is 0.016 inches, the pressure will drop from 4000 psia to 1333 psi after passing through the orifice  12 . With the orifices  22 ,  24  and  26  in section  20  having diameters of 0.01732 inches the pressure will drop from 1333 psia to 444 psi. If the orifices  32 ,  34  and  36  in section  30  have diameters of 0.030 inches the pressure will drop from 444 psi to 148 psia. Finally, if the orifices  42 ,  44  and  46  of the section  40  have diameters of 0.052 inches, the pressure will drop from 148 psi to 49 psia. At this pressure level mixing with air in a burner tube is much easier. The cascade nozzle shown in FIG. 5 accomplishes this pressure drop in less than ¾ inch. The small volumes such as the volume  14  separating the orifices can be cylindrical volumes having a diameter of four individual hole diameters and a length of at least two hole diameters. The final pressure can be measured by a sufficiently small pitot tube placed axially in the jet from one of the orifices  42 ,  44  and  46 . The total pressure will be significantly different from the 4000 psia measured in a single stage device. At this static pressure level mixing with air in a burner tube is much easier and the total pressure of the mixture delivered to the combustion chamber will be significantly reduce to about 49 psia. 
     In operation, a supercritical mixture of a hydrocarbon fuel and water is introduced into the left side of the structure  60  shown in FIG. 5. A suitable mixture is 5-70% water with a hydrocarbon fuel. Supercritical conditions for the mixture are achieved at temperatures in the range of approximately 363° C.-450° C. and at a pressure range of approximately 3000 psia 4500 psia. As the supercritical mixture passes through the successive stages of orifices, pressure will be reduced so that when the mixture exits through the orifices  42 ,  44  and  46  the mixture is at a pressure less than 500 psia and preferably less than 200 psia. In addition, the lower pressure will result in flow velocities when mixed with air of less than a resulting flame velocity to assure ignition and stable burning. As will be appreciated by those skilled in the art, the section  40  will be positioned to introduce the mixture into a combustion chamber or other burner. 
     The cascade nozzle shown in FIG. 5 accomplishes this pressure drop in less than ¾ inch. The small volumes such as the volume  14  separating the orifices can be cylindrical volumes having a diameter of four individual hole diameters and a length of at least two hole diameters. In practice it has been found that 4-stage nozzles do not provide as good mixing or burning as 3-stage nozzles (shown in FIGS. 6 through 9) which are therefore preferred. A 4-stage nozzle could, however, be useful at high altitudes. The three-stage cascade nozzle in the following embodiment has worked well in an Allison T-63 combustor operated at atmospheric pressure in static burn tests. 
     With reference to FIGS. 6 a  and  6   b  a first section  80  includes a single orifice  82  and defines a small volume  84 . A suitable orifice  82  diameter is 0.016 inches for a flow of 200 g/min of fuel and 170 g/min of water. A suitable diameter for the section  80  is 0.375 inches. A suitable depth for the recess defining the volume  84  is 0.060 inches and the length of the section  80  is approximately ⅛ of an inch. A suitable material for section  80  is  304  stainless steel. 
     A second section  90  is shown in FIGS. 7 a  and  7   b.  As shown in FIG. 7 b,  the section  40  includes three orifices  92 ,  94  and  96 . A suitable diameter for the orifices  92 ,  94  and  96  is 0.01732 inch. It is also suitable that the orifices  92 ,  94  and  96  be arranged on a 0.080-inch bolt circle. Other suitable dimensions are as set forth with respect to section  80  in FIG. 6 a.    
     A third section  110  is shown in FIGS. 8 a  and  8   b.  The section  110  has a generally spherical configuration and includes three orifices  112 ,  114  and  116 . The orifices  112 ,  114  and  116  may have a diameter of 0.030 inches. These orifices may be located approximately 30° from an axis of the section  110  as shown in FIG. 8 a.  A suitable dimension for an outer radius  118  is 0.188 inches and a suitable dimension for an inner radius  120  is 0.158 inches. The length of the section  110  is approximately ¼ of an inch. Alternatively  6  holes of 0.021 inches diameter could be used for the last stage. 
     FIG. 9 shows pressure-reducing apparatus  130  of the invention with the various sections assembled to form a unitary structure. It is preferred that the sections  80 ,  90 ,  110  and  130  be welded together to form the unitary structure  60 . It is important that the sections be assembled to preserve the arrangement of the orifices as shown in the previous figures so that the orifices are not coaxial. For example, the section  90  is assembled with the orifice  92  adjacent to the orifices  92 ,  94  and  96  of section  110  thereby resulting in a non-coaxial cascade of nozzle orifices. Should the orifices be coaxial, the kinetic energy of the first orifice jet will not be completely dissipated and more total pressure will be recovered. In a preferred embodiment, a 0.005″ screen  112  is positioned before section  80  to protect the orifice  82  from becoming blocked by dirt in the system. 
     Orifice sizes are selected to assure a pressure ratio from section to section sufficient to provide choking of the flow at each stage. In this manner, an upstream rim orifice will be the flow limiting device and the number of orifices in the series cascade is chosen to obtain a desired total pressure ratio. The area ratio between successive orifices should be the inverse of the overall desired static pressure ratio. That is, if the static pressure ratio after an orifice is one-third the static pressure before the orifice, the area of the successive orifice should be three times that of the first orifice. As an example, in order to drop the total pressure of a supercritical fuel/water mixture of from approximately 4000 psia to approximately 49 psia, four sections are preferred in the cascade. Because the velocity drops to zero on a macroscopic scale in each interorifice volume, the static pressure equals the total pressure in areas away from the jet. 
     To calculate the pressures in the 3-stage cascade nozzle the pressure ratio. P downstream /P upstream , across each orifice should be smaller than that required to produce sonic flow for air, which is a nearly ideal gas. This ratio for air is 0.528. Because of the non-ideal nature of the complex steam-oil mixture, a ratio of 0.333 to guarantee sonic flow is selected in this non-ideal situation. To the extent density is proportional to pressure, 3 times the area is then required to pass the same flow at ⅓ the pressure. The orifice size for the first embodiment (a three-stage nozzle) described below is selected in this manner. 
     Cascade nozzles were constructed having two and three stages of expansion respectively and were tested in an Allison T-63 jet turbine combustor exhausting to atmospheric pressure. Both operated with clear, steady flames over a wide range of equivalence ratios. The 3-stage nozzle was intended for operation at 1 atmosphere and the 2-stage nozzle for operation at higher pressures. At the one atmosphere test condition the three stage nozzle had a somewhat wider range of stable operation than the two stage nozzle. 
     FIG. 10 shows a schematic representation of a fuel delivery system  140  of this invention. The system includes a source of a mixture of water and hydrocarbon fuel  142  (the mixture is either (i) at or above the mixture critical point such that the mixture is a homogeneous single phase or (ii) characterized by a critical pressure, the mixture being at a pressure that is below the critical pressure of the mixture and being at a temperature that is at least the greater of 250° C. and the boiling point temperature of water at the mixture pressure). The mixture may be pressurized and heated to these temperatures by conventional means (e.g., a dual piston metering pump and transferred heat from an exhaust manifold of an engine or auxiliary electric heaters) as disclosed in U.S. Pat. No. 6,010,544 noted above, or pressurized by any of the gear or piston pumps and heated by any the electrical, process fluid or other heating sources, each as taught in U.S. Pat. No. 6,240,883, also noted above.) The system also comprises an apparatus  144  for rapidly reducing the pressure of the mixture and a delivery apparatus  146  for introducing the reduced pressure mixture into combustion chamber  148 . 
     The embodiments described above are entirely exemplary and are not limiting as to their effect on the appended claims. For example, each of the sections may have a single orifice as long as they are not coaxially oriented. Structure may also be provided to provide a swirling air mass within the structure  60 , such as by drilling holes in sections illustrated in FIGS. 2 a,    3   a  and  4   a  at 45° angles to the plane of the sectional view instead of the 90° angles shown to create a tangential swirling velocity in the volume chamber. The angular direction may then be reversed in successive plates. To provide better mixing over a wider area of the flow the number and angular orientation of the holes in the last stage nozzle can be changed as long as the total area is maintained. Physical dimensions given are specific only to particular embodiments of the invention. 
     It is intended that all modifications and variations of the embodiments disclosed herein be included within the scope of the appended claims.