Abstract:
An apparatus for generating high-speed pulsed fluid jets. A valve assembly has a valve body with an inlet and an outlet. A valve shuttle is slidably or movably mounted with respect to the valve body. The valve shuttle is positioned within a cavity of the valve body and divides the cavity into an upper or inlet cavity and a lower or outlet cavity. The valve shuttle has a passage in communication with the upper cavity and the lower cavity. In an open condition of the valve assembly, fluid communication is formed between the inlet, the inlet cavity, the passage, the outlet cavity and the outlet.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The term “waterjet” denotes high-speed water jets generated at high static pressures with special pumps and nozzles. Such waterjets perform a wide range of useful work such as cleaning tanks, ship hulls and various structures and also cutting alloys and composite materials with computer-controlled nozzle movement. Static pressures of water as high as 80,000 pounds per square inch (psi) are generated with special motor-driven or engine-driven piston pumps and special fluid-powered pressure intensifiers, and with nozzles equipped with gem orifices. The term “waterjet technology” describes the various processes and applications of waterjets. The term “abrasive waterjet” describes a particular waterjet technology in which selected industrial abrasive particulates are added into the jet stream with special nozzles to further enhance the capability of waterjets. Very hard and difficult materials are cut or removed with such abrasive waterjets. In fact, it is the only method that can now be used to cut carbon-fiber laminates that are widely used in modern aircrafts. 
         [0002]    The pumps and pressure intensifiers known for generating waterjets are positive-displacement piston pumps which have multiple pistons and check valves to build up the potential energy of a fluid. The energy transfer from the piston to the fluid is usually not smooth, due to factors such as fluid compressibility, the finite number of pistons in the pump, and the phase limitations. As a result, there are pressure pulsations in the output fluid. For example, a triplex crankshaft pump has only three cylinders and pistons operating at about 600 rotations per minute (rpm) and a double-acting hydraulic pressure intensifier has only two cylinders and pistons operating at about one stroke per second. These pumps are used to push or build water pressures from atmospheric to 55,000 psi or higher. The output pressure of water at the outlet of each cylinder is not phased properly with the output pressure of other cylinders to cover the entire cycle and to provide smooth pressure output. The rough power output is similar to automobile engines where the power output of a 3-cylinder engine is rougher or not as smooth as the power output of an 8-cylinder engine. Thus, if a waterjet nozzle is placed at the outlet of a triplex pump or a double-acting intensifier, the waterjet will not form a smooth stream. Instead, the waterjet will form a pulsed jet with a stream of water slugs. The water slugs are phased according to the piston motion of the pump. For example, a triplex pump operating at 600 rpm would generate a pulsed waterjet of 3×600=1800 pulses per minute. A double-acting intensifier operating at one stroke per second would produce a pulsed waterjet of 60 pulses per minute. 
         [0003]    However, in waterjet applications, nozzles are not positioned next to the pump. Tubes or hoses are used to transport the pressurized water from the pump to a remote or distant nozzle. Inside the tubes or hoses the pressure pulsations in the water is damped and only a portion remain at the nozzle. In many applications, the residue pressure pulsations present no problem but in double-acting intensifiers there may be a problem. Due to the very low stroke rate and the extreme pressures involved, water at the nozzle of an intensifier pump system may have pressure pulsations too high for applications such as abrasive waterjet cutting of composites. An additional pressure attenuator may be required to further damp out the pressure variations. In such applications, the smoothness of cut surface may be related to or a function of the pressure pulsation of the waterjet. 
         [0004]    In many waterjet applications, a pulsed waterjet can be more effective than a continuous waterjet when each is at an identical pump power level. One reason is the mitigation of waterjet interference when a waterjet impacts a flat surface. When a continuous waterjet impacts a hard surface, the waterjet rebounds from the surface and collides with the incident waterjet. As a result, a significant portion of the waterjet energy is wasted. In a pulsed waterjet, the water slugs impact the surface individually and the energy of each slug of water has time to dissipate. If the waterjet slugs are phased properly, waterjet interference can be completely avoided. With a pulsed waterjet, the impact pressure on a surface can be greater if the mass of each water slug is greater. Reducing waterjet interference is one reason why waterjetting is widely applied today in industrial cleaning processes, such as by spinning a nozzle assembly at a high speed. Many waterjets generated at known pump pressures are supersonic, and it is difficult to avoid waterjet interference. Rotating a nozzle assembly at a high speed requires a rotating joint with good seals. The durability of such high-pressure seals is a maintenance issue in industrial processes. An impacting power of a waterjet is also reduced when the nozzle is rotating at a high speed. 
         [0005]    There are many known investigations using pulsed waterjets for a wide range of jobs. One benefit of a pulsed waterjet is to remove materials, such as concrete, that have significant granular structures of materials. The waterjet pulses can better penetrate into pores of the porous structures, to rupture the structure and wash away the debris. Similar benefits of pulsed waterjet have been reported with coating removal. There are other benefits of using pulsed waterjets. 
         [0006]    Even with the benefits of pulsed waterjets, the method is not applied widely today because the pulsed waterjet processes reported in several publications have not been commercialized. One highly publicized known pulsejet technology is not now commercialized, presumably because components involved in that particular pulsejet technology are not matured or there were technical difficulties not overcome. It is difficult to design an on-off valve for use with high-pressure water as the working fluid. To produce a pulsed waterjet at a nozzle is extremely difficult due to many factors. It is difficult to interrupt the flow of water at very high pressures. 
         [0007]    Only some known pulsed waterjet processes are applied commercially, including one that uses an ultrasonic transducer placed at the tip of a waterjet nozzle to generate forced pulses at 20,000 cycles per second. Electrical energy is introduced into the nozzle assembly to generate the axial vibrations and forced waterjet pulses. Up to 1 kilowatt of electrical energy may be required to overcome the static water pressure at the nozzle. With this pulsed waterjet process it is possible to remove coatings at static pressures considerably lower than those associated with a conventional continuous waterjet. This 20 kHz pulsed waterjet process is not widely applied because of shortcomings and also the required electricity to power its nozzle. Mixing electricity and water in a handheld piece of field equipment is not a safe practice. 
         [0008]    Pulsed waterjets are normally generated with available pumps. Once the pressure pulsations are dampened with tubes and hoses it can be difficult to recreate pressure pulsations at a waterjet nozzle. It is also difficult to interrupt the water flow at very high pressures. Problems, such as water hammer effect and metal fatigue, can occur if the flow interruption is not handled properly. 
         [0009]    A process that allows a pulsed waterjet to be generated at a nozzle at a wide range of water pressures is valuable to the entire waterjet technology and would have applications in shipyards and concrete structure repairs and in everyday cleaning applications. It is particularly valuable if the process requires no energy from external or outside sources and requires no use of a heavy component with uncertain durability. This invention can be used to provide a waterjet process that produces a genuine pulsed waterjet by tapping a very small amount of water energy to produce waterjet pulses at a controllable frequency and at a wide range of static pressures. The apparatus and process of this invention will be valuable to waterjet technology and its use in industry. 
       SUMMARY OF THE INVENTION 
       [0010]    This invention provides a method for generating a genuine pulsed fluid jet at a wide range of fluid pressures and flowrates without the need for an external power source or input and without the need for bulky, heavy, or unreliable equipment. 
         [0011]    This invention can be used to generate a genuine pulsed fluid jet near or at a nozzle, to minimize the chance of pulsation dampening and to put the pulsejet to work. 
         [0012]    This invention can incorporate the pulsejet technology into other mechanical and hydraulic systems to do useful work. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    This invention is explained in greater detail below in view of exemplary embodiments shown in the drawings, wherein: 
           [0014]      FIG. 1  is a cross-sectional view of a pulsejet valve/nozzle, in a closed position, according to one embodiment of this invention; 
           [0015]      FIG. 2  is a cross-sectional view of the pulsejet valve/nozzle, as shown in  FIG. 1 , but in an open condition; 
           [0016]      FIG. 3  is a cross-sectional view of a pulsing valve/nozzle assembly, in a closed condition, according to one embodiment of this invention; 
           [0017]      FIG. 4  is a cross-sectional view of a pulsing valve/nozzle assembly, in an open condition, according to another embodiment of this invention; 
           [0018]      FIG. 5  is a cross-sectional view of a pulsejet valve/nozzle assembly, in a closed condition, according to another embodiment of this invention; 
           [0019]      FIG. 6  is a cross-sectional view, with a valve shuttle rotated 90 degrees, of the valve/nozzle assembly as shown in  FIG. 5 , but in an open condition; 
           [0020]      FIG. 7  is a cross-sectional view of a valve/nozzle assembly, in a closed condition, according to one embodiment of this invention; 
           [0021]      FIG. 8  is a cross-sectional view of a pulsejet generator, in a closed condition, according to one embodiment of this invention; and 
           [0022]      FIG. 9  is a cross-sectional view of a pulsejet generator, in an open condition, according to still another embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    This invention provides a method for generating pulsed fluid flow without using an external power source. The energy consumed in the process is derived from the potential energy contained in a pressurized fluid from a pressurized source. It is known that a pressurized fluid such as compressed air and pressurized water contains an enormous amount of energy introduced into the fluid during the pumping process. In this invention, a very small amount of fluid energy is taken from the pressurized fluid to generate flow discontinuities in a suitable valve so that the flow discontinuities become fluid jet pulses, particularly if a nozzle is placed downstream from the valve. The amount of energy consumed in generating the flow discontinuities is so relatively small that the fluid jet usefulness is not affected. Also, flow discontinuities do not normally cause a water hammer effect in the fluid system because the flow of fluid is not cut off completely. 
         [0024]    In one embodiment of a pulsed fluid jet generator of this invention, such as shown in  FIG. 1 , the pulsejet valve/nozzle  100  of this invention comprises a nozzle body  101  having a fluid inlet  102 , a fluid outlet  103 , and a cylindrical cavity  104  in communication with the inlet  102  and the outlet  103 . Inside the cavity  104 , a generally cylindrical valve poppet  105  has a tapered end  106  in contact with an outlet port  107  of the fluid outlet  103  and an other cylindrical end  108  accommodates a compression spring  109  that abuts the valve poppet  105  in one end and abuts a valve plug  110  on the other end. The valve poppet  105  has a central fluid passage  111 . The valve poppet  105  divides the valve cavity  104  into two parts, an upper cavity  112  and a lower cavity  113 . A poppet seal  114  can prevent fluid leakage across the valve poppet  105  although the valve poppet  105  is sized to fit the valve cavity  104  snugly, but is also free to slide up and down. 
         [0025]    Still referring to  FIG. 1 , in some embodiments, the valve/nozzle assembly  100  of this invention is assembled with the valve poppet  105  in an upright position, relative to the direction shown in  FIG. 1 , and the spring  109  is compressed to exert a force on the valve poppet  105  urging it to butt against or abut the outlet port  107 , thus closing the valve/nozzle  100 . If fluid flows into this valve assembly, it will fill the lower cavity  113  but will be stopped by the valve poppet  105 , from being discharged through the outlet  103 . In some embodiments of this invention, the valve poppet  105  has a diameter D 1  and a cross-sectional area A 1 . The tapered end  106  contacts the outlet port  107  to form a contact or a seal circle, or a ring, of a diameter D 2  and of a cross-sectional area A 2 . Thus, in some embodiments, the valve poppet  105  has a donut-shaped cross-sectional area A 1 −A 2 =ΔA exposed to the fluid in the lower cavity  113 . If the fluid is pressurized to a value of Pf psi, then the fluid exerts a fluid-induced force Ff of P×ΔA pounds of force against the valve poppet  105  in lifting it. At the same time, the spring  109  exerts a spring force Fs on the valve poppet  105  to keep it down. If Fs is greater than Ff, then the valve poppet  105  will stay in place and the valve remains closed. On the other hand, if Ff is greater than Fs, then the valve poppet  105  is pushed up by the pressurized fluid, thus opening the outlet port  107 . The fluid will then flow from the inlet  102  through the lower cavity  113  to the outlet  103 . At the same time, the fluid will also flow through the fluid passage  111  of the valve poppet  105  into the upper cavity  112 . As a result, the pressurized fluid will be on both ends of the valve poppet  105  and the poppet lifting force Ff is eliminated or goes to zero. Here, the valve poppet  105  feels only the force from the spring  109  and thus moves down to close the outlet port  107 , thus returning the valve assembly  100  back to its earlier state and completing one cycle of its pulsing action. This cyclic motion can continue automatically as long as the pressurized fluid supply continues. The fluid flow out of the valve assembly  100  will be chopped and if a nozzle  115  is placed at the outlet  103 , a pulsed fluid jet will be formed, such as shown in  FIG. 2 . 
         [0026]    One example can be used to further explain the valve assembly  100  of this invention. If the valve poppet  105  has a diameter of 0.5 inches, then its cross-sectional area inside the cavity  104  is 0.196 square inches. If the tapered end  106  of the valve poppet  105  contacts the outlet port  107  with a seal ring of 0.312 inches, a cross-sectional area of 0.076 square inches, then the cross-sectional area of the valve poppet  105  exposed to the fluid inside the lower cavity  113  when the valve is closed is ΔA=0.196−0.076=0.120 square inches. If the spring  109  exerts a force of 20 pounds on the valve poppet  105 , then the outlet port  107  will be closed by this force. If a fluid such as water enters into the valve assembly  100 , for example at 100 psi, then the valve will not open because the fluid induced force Ff=100×0.120=12 pounds force, which is smaller than the spring force of 20 pounds. However, if the fluid pressure is increased to 200 psi, the fluid force on the valve poppet  105  will be increased to 24 pounds, which is greater than the spring force 20 pounds, and the valve poppet  105  will move up to open the outlet port  107 . This 200-psi pressurized water will then flow out of the valve assembly  100  but will also flow into the upper cavity  112  to balance the pressure across the valve poppet  105 . The 4 pound force differential is eliminated or goes to zero, and the valve poppet  105  then moves down to close the outlet port  107 . This cyclic motion can continue automatically as long as the force differential is significant and there is no appreciable fluid leakage across the valve poppet  105  with the valve in a closed condition. A pulsed waterjet can be generated at the nozzle  115 . The frequency of this cyclic fluid motion is a function of the flow rate of the fluid and the size of the valve cavity. The fluid pressure determines if the valve will function but will not affect the cyclic frequency. The opening of the nozzle is one parameter that determines the flow rate at a given pressure. Because the spring  109  is compressed by the fluid during each cycle of valve operation, energy is consumed and lost in the form of heat. 
         [0027]    The use of the compression spring  109  in the valve assembly  100  of this invention has limitations. Because a spring or bias element can fatigue and fail, the spring can supply only a relatively limited force. A spring of 20 pound compression force is considered to be a relatively strong spring and is classified commonly as a die spring but can only handle fluid of relatively low pressures. At relatively high fluid pressures, the fluid pressure inside the lower cavity  113  usually does not diminish much and the spring  109  may not return the valve poppet  105  to its closed position to complete a clean cycle or a complete cycle. Thus, the valve poppet  105  may get hung up to create a leak or a leaking valve. In some embodiments, eliminating the spring  109  results in a suitable force from the fluid. 
         [0028]    An improved pulsing valve/nozzle assembly  200  of this invention is shown in  FIG. 3 . The valve assembly  200  comprises a valve body  201  having a fluid inlet  202 , a fluid outlet  203 , an upper cavity  212  and a lower cavity  213  connected by a passage  210 . A valve poppet  205  has a shoulder  206  and a central fluid passage  211 . The valve poppet  205  straddles across the upper cavity  212  and the lower cavity  213  through the passage  210 . The valve poppet  205  has a tapered end  208  situated or positioned in the lower cavity  213  and the shoulder  206  in the upper cavity  212 . There is a seal/bushing  214  around the valve poppet  205  in the upper cavity  212  that fits snugly against a cavity wall and around the valve poppet  205  to prevent fluid from leaking across the shoulder  206 . The seal/bushing  214  and the shoulder  206  divide the cavity to an upper cavity  212  and a lower cavity  216 . The lower cavity  216  has a small bleed hole  217  in communication with the outside environment. The valve poppet  205  is free to slide across the passage  210  for a short distance. The valve poppet  205  has a diameter D 1  and a cross-sectional area A 1  in the lower cavity  213  and a seal ring of diameter D 2  and a cross-sectional area A 2  when the valve poppet  205  is in contact with the outlet port  207 . The valve poppet  205  and the seal/bushing  214  in the upper cavity  212  define a diameter D 3  and a cross-sectional area A 3 . A spacer spring  209  can be inserted into the upper cavity  212  to keep the seal/bushing  214  in place and to urge the valve poppet  205  down, relative to the orientation shown in  FIG. 3  when there is no fluid inside the valve/nozzle assembly  200 . In some embodiments of this invention, D 3  is greater than D 2  and D 1 , and is much greater than D 1 −D 2 . In some embodiments of this invention, there can be a seal/bushing  218  and the spring spacer  219  in the lower cavity  213  serving a purpose similar to that of the seal/bushing  214  and the spacer spring  209  in the upper cavity  212 . Any suitable nozzle  215  in the outlet  203  can be used to generate fluid jets. 
         [0029]    As shown in  FIG. 3 , when a fluid of pressure P enters into the lower cavity  213 , it encounters the surface A 1 −A 2  and quickly exerts a force of Ff=P(A 1 −A 2 ) to lift the valve poppet  205  up from the valve port  207 . Once lifted, the entire cross-sectional area of the valve poppet  205  is exposed to the fluid. Thus a force of Ff=PA 1  is exerted on the valve poppet  205  and pushes it to an uppermost position. Thus, the valve port  207  is wide open and the fluid flows through the outlet  203  and the nozzle  215 . At the same time, the fluid flows into the upper cavity  212  through the fluid passage  211  and encounters the cross-sectional area A 3  and exerts a force of Ff=P·A 3  to push the valve poppet  205  down. Because the lower cavity  216  below the shoulder  206  is exposed to an atmosphere, there is a net downward force of P(A 3 −A 1 ) to push the valve poppet  205  down. This force is very significant if D 1  and D 3  are relatively far apart. Because of this downward force, the valve poppet  205  will move down to close the outlet port  207  and thus complete one cycle of its up-and-down motion. This motion will continue as long as pressurized fluid continues to flow. A pulsed fluid jet can be generated at the nozzle  215 . 
         [0030]    Another embodiment of a pulsejet valve/nozzle of this invention is shown in  FIG. 4 . In this embodiment, the seal/bushing assemblies are eliminated. The valve poppet  305  sits inside the upper cavity  312  and the passage  310  with a snug fit to minimize fluid leakage. A small fluid leakage rate may not affect the function of this valve/nozzle assembly and can actually lubricate and thus assist the motion of the valve poppet  305 . One advantage of the valve/nozzle assembly  300  is its simple design. In some embodiments, one design requirement is that D 3  be greater than D 1  by a certain margin, which can be a function of the fluid pressure P and the sizing of the outlet port  307 . 
         [0031]    Another embodiment of a pulsejet valve/nozzle assembly of this invention is shown in  FIG. 5 . The valve/nozzle assembly  400  has an inline arrangement wherein a fluid flows into the valve body  401  from an upper inlet  402  into the upper cavity  412 , through the fluid passage  411 , and into the lower cavity  413 . The valve poppet  405  straddles the upper cavity  412  and the lower cavity  413  through the passage  410 . The valve poppet  405  has a tapered inlet end  409  and a tapered outlet end  408 . The valve poppet  405  has a side inlet port  420  situated or positioned in the upper cavity  412  and the side outlet port  419  situated or positioned in the lower cavity  413 . The inlet port  420  and the outlet port  419  are connected by the passage  411 . The tapered inlet end  409  mates with valve inlet port  414  and the tapered outlet end  408  mates with the valve outlet port  407 . The valve poppet  405  has a shoulder  406  that fits sealably or snugly inside the lower cavity  413 . The valve poppet  405  is free to slide up and down between the inlet port  414  and the outlet port  407 . 
         [0032]    Referring to  FIG. 6 , when a pressurized fluid enters into the valve/nozzle assembly  400  through the inlet  402 , it pushes down the valve poppet  405  and enters into the upper cavity  412  and into the side ports  420 . The fluid then flows through the passage  411  and enters the lower cavity  413  through the side port  419 . At this moment, the valve poppet  405  is down and the tapered outlet end  408  seals the outlet port  407  with a fluid induced force Ff=PA 1 , where A 1  is a cross-sectional area of the valve poppet  405  in the upper cavity  412 . The fluid of pressure P in the lower cavity  413  quickly sees the cross-sectional area of the poppet shoulder  406  and exerts a lifting force of a magnitude of P(A 3 −A 1 ), where A 1  is the cross-sectional area of the valve poppet  405  inside the lower cavity  413 . This lifting force cancels the downward force P·A 1  in the upper cavity  412 . As a result, the valve poppet  405  moves up and opens the outlet port  407  and closes the inlet port  414 . Simultaneously, the fluid inside the lower cavity  413  flows out of the nozzle  415 . As the fluid pressure inside the lower cavity  413  diminishes, the lifting force on the valve poppet  405  is reduced to a level of less than the downward force inside the upper cavity  412 , and the valve poppet  405  moves down to close the outlet port  407  and thus completes one cycle of the poppet movement. As long as the pressurized fluid flow continues, a pulsed fluid jet will be generated at the nozzle  415 . Fluid flow may be interrupted inside the valve/nozzle assembly  400  but will not be blocked completely. Thus, there will be no water hammer effect in the fluid system. This inline pulsejet valve/nozzle assembly  400  of this invention has one advantage of a relatively slim construction and a simple or logical flow pattern ideally, which is suited for use with handheld tools. 
         [0033]    Another embodiment of a pulsejet valve/nozzle assembly  500  is shown in  FIG. 7 , and comprises a valve body  501  having an inlet  502 , a cylindrical cavity  504  containing a valve cartridge  510 , and an outlet  503  with a nozzle  514 . The valve cartridge  510  connects the inlet  502  to the outlet  503  in a fluid tight manner. The valve cartridge  510  can have a cylindrical shape and can contain a flow modulating mechanism, such as discussed in this specification. The valve cartridge  510  has an inlet  521 , an inlet cavity  512 , a poppet  505 , an outlet cavity  513 , and an outlet  522 . The valve poppet  505  has an inlet side port  519 , a central fluid passage  511 , an outlet side port  520 , and tapered ends to mate with the inlet  502  and outlet  503  of the valve cartridge  510 . The valve cartridge  510  has a side bleed hole  517  connecting the cavity  516  inside the valve cartridge  510  to an outer atmosphere or the outside. When a pressurized fluid enters into the valve/nozzle assembly  500  of this invention, it flows into the valve cartridge  510  in which its flow is modulated by movement of the valve poppet  505  and the fluid can flow out of the nozzle  514  in the form of a pulsed jet. This cartridge arrangement can simplify the maintenance as the valve poppet  505  and its contact surfaces are subject to wear and the fluid leakage becomes too excessive. It is then the time for maintenance to replace the valve cartridge  510 . This cartridge arrangement can also provide a cartridge having one of various lengths to be used inside the same nozzle body so that various flow modulation frequencies can be used. 
         [0034]    In some fluid jet applications, a mass of each fluid jet pulse needs to be substantial so that the pulse frequency can be reduced, which relates to the so-called water cannon technology, particularly when the fluid is water. The water cannon technology is known and characterized by the high power of the fluid pulses that can cause significant damage when impacting a surface. This capability can be useful in many geotechnical applications. This invention can provide the necessary technology to meet the needs of water cannons. 
         [0035]    Referring to  FIG. 8 , a pulsejet generator  600  of this invention comprises a gas accumulator cylinder  621  connected to one end of a valve inlet head  627 . The other end of the valve inlet head  627  is connected to a valve cylinder  601 . The valve inlet head  627  has an inlet cavity  611  with a tapered inlet port  613  in communication with a valve inlet  602 . The inlet cavity  611  has a tapered inlet port  613  connected to the valve inlet  602  and a central hole  615  that accommodates a cylindrical valve shuttle  605 . The valve shuttle  605  has a tapered inlet end  606  that is mateable with the inlet port  613 . The inlet cavity  611  has a seal  616  around the valve shuttle  605  to minimize fluid leakage. The valve cylinder  601  has a floating piston  617  that straddles around the valve shuttle  605  through a center hole  618 . The piston  617  has an outside diameter seal  619  and an inside diameter seal  620  to isolate or separate the fluids. The valve shuttle  605  has a side inlet port  608  inside the inlet cavity  611 , a central fluid passage  610 , and an outlet side port  609  inside the outlet cavity  612 . The valve shuttle  605  has an upper catch  623  in a gas cavity  604  on top of a piston  617  and a lower catch  624  in the outlet cavity  612  and below the piston  617 . The two catches  623  and  624  on the valve shuttle  605  define a distance that the valve shuttle  605  can travel. The gas cylinder  621  has a gas cavity  622  connected to the gas cavity  604  by the passage  625  drilled through the valve inlet head  627 . When the gas cylinder  621  is filled with a gas such as nitrogen or air to a pressure Pg, the gas will flow into the gas cavity  604  and will push the piston  617  down against the valve shuttle catch  624  and will move the shuttle  605  down to close the outlet port  614 . The outlet port  614  is tapered to mate with the tapered outlet end  607  of the valve shuttle  605 . As a result, the outlet port  614  can be closed by the valve shuttle  605  under a downward force exerted on the valve shuttle  605  in the cavity  611 . The gas pressure Pg can be selected based on characteristics of the system fluid and the intended application. In different embodiments of this invention, Pg is smaller than the pressure of the system fluid entering into the pulsejet generator  600 . 
         [0036]    As a system fluid of pressure Pf flows into the inlet cavity  611  through the inlet  602 , the fluid can follow the side inlet port  608 , the passage  610  and the side outlet port  609  of the valve shuttle  605  and can enter into the cavity  612 . Once in the cavity  612 , the fluid encounters the closed outlet port  614  which it cannot open because of the fluid seating force in the cavity  611 . The fluid also encounters the piston  617  and pushes it upward. By design, the gas pressure in the cavity  604  is lower than the fluid pressure in the cavity  612 . Thus, the piston  617  can rise and eventually engage the catch  623  on the valve shuttle  605 . Now, the valve shuttle  605  can rise if the gas pressure in the cavity  604  is lower than the fluid pressure in the cavity  612 . The outlet port  614  can thus open and allow the system fluid to flow out or discharge. Now, the system fluid encounters the entire cross-sectional area of the outlet end  607  and pushes it up to keep the inlet port  613  closed until the fluid loses pressure. The piston  617  can move down with the fluid and engage the lower catch  624  to move the valve shuttle  605  down to the closed outlet port  614 . Thus, the valve shuttle  605  and the piston  617  complete one cycle of their movement. When the flow of pressurized system fluid continues, a pulsed fluid jet can be generated at the nozzle  626 . The cyclic movement of the piston  617  determines the frequency of the pulsejet and the volume of system fluid swept by the piston  617  determines the mass of each pulse. The gas pressure inside the gas cavity  604  can vary during each cycle because the gas is compressing and expanding but remains below that of the system fluid, otherwise the cyclic movement cannot continue. As a result, the pulsejet generated at the nozzle  626  varies in energy content in each slug of fluid, higher at the start of slug and lower at the end. The presence of a gas accumulator allows the use of a large nozzle to generate a pulsejet of high impact energy. If the gas accumulator is replaced with a strong spring, the ability to store energy can be limited and the operation may not be smooth. 
         [0037]    In known waterjet operations, the water pressure often exceeds 10,000 psi, which is substantially higher than the gas pressure commonly employed in gas accumulator practices because gas at such high pressure becomes very dangerous and difficult to handle. To accommodate water at very high pressures, the gas accumulator used in the pulsejet generator  600  of this invention can be replaced with a gas pressure intensifier by incorporating a piston-plunger setup into the pulsejet valve/nozzle assembly of this invention. As a result, there is another embodiment of a pulsejet generator  700  of this invention, capable of handling system fluid of very high pressures. With this gas intensifier, a gas can be used to store energy at manageable pressures to accommodate water at pressures above 40,000 psi. Water, due to its non-compressible nature, is easier to handle than a gas at 4,000 psi. 
         [0038]    Referring to  FIG. 9 , the pulsejet generator  700  of this invention comprises a gas cylinder  726  with a gas chamber  731 , a gas piston  727  housed in the gas chamber  731  with an associated piston seal  728 , a hollow valve cylinder  701  attached to the gas cylinder  726  on one end, a hollow plunger  722  attached to the gas piston  727  on one end which has an end cap  718  at the other end, a valve inlet head  715  situated inside the valve plunger  722 , a fluid supply tube  725  in the center of the gas chamber  732  connecting an outside valve inlet  702  to the valve inlet head  715  through a center hole  729  on the gas piston  727 , a cylindrical valve shuttle  705  straddling across the end cap  718 , and a valve outlet  703  attached to the other end of the valve cylinder  701 . The valve inlet head  715  has an inlet cavity  711  with a tapered inlet port  713  connected to the valve inlet  702 . The inlet cavity  711  has a central hole  716  to accommodate the inlet end  706  of the valve shuttle  705  and a seal  717  around the valve shuttle  705  to prevent fluid leakage. The inlet end  706  is tapered to mate with the inlet port  713 . The plunger end cap  718  has a center hole  719  to accommodate the valve shuttle  705  and has an outside diameter seal  720  and an inside diameter seal  721  to prevent fluid leakage. The plunger end cap  718  defines the outlet cavity  712  and the plunger cavity  730 , which is connected to the atmosphere through a bleed  734  on the hollow plunger  722  and on the gas cylinder  726 . The valve shuttle  705  has a tapered outlet end  707  situated or positioned in the outlet cavity  712 . The inlet end  706  has a side inlet port  708  situated or positioned in inlet cavity  711 , an outlet side port  709  on the outlet end  707  in the outlet cavity  712 , and an internal fluid passage  710  connecting the two side ports. The valve shuttle  705  comprises an upper catch  723  situated or positioned in the plunger cavity  730  and a lower catch  724  situated or positioned in the outlet cavity  712 . The plunger end cap  718  can slide along the valve shuttle  705  between the two catches  723  and  724 . A cushion spring may be placed between the plunger end cap  718  and the lower catch  724  to soften the contact. 
         [0039]    Still referring to  FIG. 9 , the pulsejet generator  700  can be filled with a gas such as nitrogen or air in the gas chamber  731  to a pressure Pg, which can be determined by the pressure of the system fluid involved. The gas can push down the gas piston  727  and the plunger  722 , and the plunger end cap  718  will then push down the valve shuttle  705  to close the outlet port  714 . The pulsejet generator  700  can now be used to generate pulsed fluid jets. 
         [0040]    When a pressurized system fluid, such as water, enters in the pulsejet generator  700  through the inlet  702  at a pressure Pw, it flows into the inlet cavity  711  through the supply tube  725 . In the cavity  711 , it exerts a force on the inlet end  706  of the valve shuttle  705  to push it down while the fluid flows through the valve shuttle  705  into the outlet cavity  712 . In the outlet cavity  712 , water sees or encounters the closed outlet port  714  and cannot open it. Instead, the water pushes the end cap  718  of the plunger  722  against the gas force acting on the piston  727 . If the water force is greater than the gas force, then the plunger end cap  718  rises along the seated valve shuttle  705 . Eventually, the plunger end cap  718  engages the upper catch  723 . At this point, if the water force pushing up the plunger end cap  718  is still greater than the gas force acting-on the piston  727 , then the valve shuttle  705  can be moved or dislodged from the outlet port  714  and water can flow into the valve outlet  703  and discharge at the nozzle  736 . At this time, water in the outlet cavity  712  sees the entire cross-sectional area of the outlet end  709  of the valve shuttle  705  and thus exerts a force pushing it upward to close the inlet port  713  of the valve inlet head  715  until water pressure inside the cavity  712  is reduced to a lower level. Once the outlet port  714  is open, the plunger end cap  718  can move down with the water and eventually engage the lower catch  724  and push down the valve shuttle  705  to close the outlet port  714 . Thus, the valve shuttle  705  and the plunger  722  complete one cycle of their up-and-down movement. If the water supply is continued, the pulsed waterjet can be produced at the nozzle  736 . A time period required to complete this cycle determines a frequency of the pulsed waterjet. The water pressure and the intensification ratio of the intensifier determine the energy content of the waterjet pulses. The intensification ratio is determined by the effective cross-sectional area of the gas piston  727  and the effective cross-sectional area of the plunger end cap  719 . If this ratio is 20 and the gas pressure inside the gas chamber  731  is 2000 psi, the pulsejet generator  700  can handle water at pressures above 40,000 psi. The total volume of the gas chamber  731  can affect the amount of water energy that can be stored during each pulse. Thus, the energy content of each waterjet pulse can also be affected by the gas volume. The larger the gas chamber  731 , the flatter can be the energy profile of a waterjet pulse. Greater energy in waterjet pulses often relates to greater power in doing work. 
       Example 1 
       [0041]    To better illustrate this invention, a pulse valve/nozzle  200  was constructed according to the embodiment shown in  FIG. 3 . The valve/nozzle  200  had a rectangular body  201  made of stainless steel with a side fluid inlet  202  of 0.156 inches in diameter, a cylindrical cavity  212  and  213  of 0.500 inches in diameter, and a bottom fluid outlet  203  of 0.156 inches in diameter. Attached to fluid outlet  203  was a nozzle  215  having a replaceable orifice. A valve shuttle  205  with the shoulder  206  was constructed of stainless steel and placed inside the upper cavity  212  with the seal/bushing  214  and  218 . The valve shuttle  205  had a diameter of 0.312 inches and the shoulder  206  had a diameter of 0.498 inches. The seal/bushing  214  and  218  were made of brass disks and polymer packed in a sandwich form and fit the valve shuttle  205  and the cavities  212  and  213  snugly but otherwise the valve shuttle  205  was free to slide. A side bleed hole 0.047 inches in diameter was drilled on the side of valve/nozzle body  201 , as shown in  FIG. 3 . The valve/nozzle body  201  was 2 inches wide, 3.7 inches long, and 1 inch thick. The valve shuttle  205  was 2 inches long with the shoulder  206  of 0.1 inches thick and the tapered outlet end  208  of 60 degrees, and had a central fluid passage  211  of 0.125 inches in diameter. The outlet port  207  had a taper of 59 degrees and a contact ring of 0.250 inches in diameter was formed when the valve shuttle  205  made contact with the valve port  207 . Thus, a differential cross-sectional area of the valve shuttle  205  and the contact ring was 0.0764−0.0591=0.0273 square inches, which is the surface that fluid inside cavity  213  encountered while exerting an upward lifting force on the valve shuttle  205 . When 70 psi tap water was introduced into the lower cavity  213 , for example, a lifting force of about 2 pounds was produced. When constructed, the pulsejet valve/nozzle  200  was closed because of the compression spring  209  inside the upper cavity  212 . The spring  209  was relatively light, exerting an estimated force of less than 0.1 pound on the valve shuttle  205 . 
         [0042]    The valve/nozzle  200  was tested with 70-psi tap water. When the water was introduced into the inlet  202 , a pulsed waterjet was issued or discharged at the nozzle  215 , immediately. The nozzle  215  was inserted with a sapphire orifice of 0.052 inches in diameter. The oscillation of the valve shuttle  205  inside the valve body  201  could be felt and heard but the waterjet pulses were not clearly visible with naked eyes. The pulses were bunched too closely due to the high pulsating frequency, which was estimated at 100 cycles per second. However, photographing this pulsejet with a digital camera clearly revealed the water pulses. 
       Example 2 
       [0043]    A pulsejet generator was constructed according to the embodiment shown in  FIG. 8 . The pulsejet generator  600  was constructed with 1¼-inch Schedule-40 PVC pipe rated for pressures up to 370 psi, and with pipe components such as a tee, an elbow and end caps. A PVC tee was used as the centerpiece of the pulsejet generator  600 . On one end of the tee was the gas accumulator  621  which was made of a 5-inch long section of PVC pipe and a cap and the other end was the valve cylinder  601  made of a 6-inch-long PVC pipe, an end plug, and a cap. The overall length of the assembled accumulator/valve cylinder combination was about 15 inches. A fluid inlet head  627  made of stainless steel was positioned in the center of the tee and had a fluid passage connected to the fluid inlet  602 . The inlet head  627  had a fluid inlet cavity  611  and a tapered inlet port  613  that mated with the tapered inlet end  606  of the valve shuttle  605 . The valve shuttle  605  was made of stainless steel and was 0.500 inches in diameter, 5 inches in length, and was machined to have the upper catch  623  and the lower catch  624  of 0.063 inches in height and 0.010 inches in thickness. The valve shuttle  605  had ends with a 60-degree taper and had the inlet side port  608  and the outlet side port  609  connected by an internal fluid passage  610 . The side ports were 0.125 inches in diameter and the fluid passage  610  was 0.250 inches in diameter. Generator  600  had a gas piston  617  straddling around the valve shuttle  605  between the catch  623  and the catch  624 . The gas piston  617  had an outside diameter of 1.312 inches and a center hole of 0.500 inches in diameter and was fitted with an outside diameter seal  619  and an inside diameter seal  620  around the valve shuttle  605 , and could travel a maximum distance of 3.0 inches between the catch  623  and the catch  624 . The volume of space swept by the gas piston  617  during its maximum travel was 3.3 cubic inches. The gas piston  617  divided the valve cylinder interior space into two parts, an upper gas cavity  604  and a lower outlet cavity  612 . The gas in the accumulator  622  could flow into the gas cavity  604  by the passage  625  drilled through the inlet head  627 . The valve shuttle  605  straddled across three cavities, the inlet cavity  611 , the gas cavity  604  and the outlet cavity  612 . The valve shuttle catch  623  was situated or positioned in the cavity  604  and the catch  624  situated or positioned in the cavity  612 . 
         [0044]    Still referring to  FIG. 8 , when the accumulator  622  was filled with compressed air to 60 psi, the gas piston  617  was pushed down with the valve shuttle  605  to close the outlet port  614 . The generator  600  was then ready for generating a pulsed fluid jet of choice. In this case, compressed air of 90 psi was selected as the system fluid in order to generate a pulsed air jet for a special application. When the 90-psi compressed air entered into the upper cavity  612 , it saw but could not open the closed outlet port  614 . Instead, the 90-psi air started to push the gas piston  617  upward with a total force of about 100 pounds, which was greater than the downward force of about 69 pounds on the gas piston from the 60-psi air in the accumulator  621 . As a result, the gas piston  617  started to move up while the outlet port  614  remained closed. After traveling for 3 inches, the gas piston  617  made contact with the upper catch  623  and exerted a lifting force on the valve shuttle  605  to open the outlet port  614  and to close the inlet port  613 . At this moment, 90-psi air in the cavity  612  saw the entire cross-sectional area of the valve shuttle  605 , thus exerting a force to keep the inlet port  613  closed. The 90-psi air in the cavity  612  started to flow out of the nozzle under the pushing force of the gas piston  617 . Quickly, the gas piston  617  caught up with the lower catch  624  and the valve shuttle  605  moved down to close the outlet port  614 , thus completing one cycle of the valve operation. This up-and-down movement of the gas piston  617  continued and the pulsed air jet was generated at the nozzle, which had an opening of 0.75 inches. The pulsed air jet was very unique due to the substantial amount of energy it packs. When generated in water, the air jet could propel a small boat such as a kayak or canoe. On the other hand, a continuous stream of compressed air would not be suitable for such use. Likewise, the pulsed air jet or other fluid jet from the generator  600  of this invention will find many other applications. 
       Example 3 
       [0045]    A pulsejet generator  700  was constructed for water applications according to the embodiment shown in  FIG. 9 . The generator  700  was made of two attached cylinders, an upper gas cylinder  726  made of carbon steel and a lower water cylinder  701  made of hardened stainless steel. The gas cylinder  726  was 9 inches long and 3.5 inches in diameter and the water cylinder  701  was 5.25 inches long and 2.5 inches in diameter for an assembled overall length of 14.5 inches. The gas cylinder  726  had a gas chamber  731  of 2.5 inches in diameter and housed a gas piston  727  made of aluminum alloy and was fitted with a polymeric outside diameter seal  728 . A hollow plunger  722  made of hardened stainless steel was attached to the gas piston  727  on one end and was fitted with an end cap  718  on the other end. The plunger  722  was housed inside the water cylinder  701  and was free to slide. The plunger end cap  718  was made of hardened stainless steel and was fitted with a polymeric outside diameter seal  720  and a polymeric inside diameter seal  721  around a cylindrical valve shuttle  705 . The valve shuttle  705  was made of hardened stainless steel and was 0.250 inches in diameter, 3.25 inches long, and had tapered ends  706  and  707  of 60 degrees. The valve shuttle  705  also had side ports  708  and  709  of 0.094 inches diameter and an inside fluid passage  710  of 0.125 inches in diameter connecting the two side ports  708  and  709 . The valve shuttle  705  also had machined catches  723  and  724  of 0.063 inches high and 0.010 inches thick. 
         [0046]    Still referring to  FIG. 9 , the constructed pulsejet generator  700  had a water supply tube  725  placed in the center of gas cylinder  726  connecting the outside water inlet  702  to a valve inlet head  715  situated or positioned inside the hollow plunger  722 . The water tube  725 , made of stainless steel, was 0.250 inches in outside diameter, was 0.094 inches in inside diameter, and was 6.5 inches in length. The valve inlet head  715 , made of stainless steel, was 0.560 inches in outside diameter and 1.0 inch in length, and had a tapered inlet port  713  of 60 degrees, an inlet cavity  711  of 0.312 inches in diameter, and a shuttle opening of 0.250 inches in diameter fitted with a polymeric seal  717 . The valve shuttle  705  straddled across cavities  711 ,  730 , and  712  with its inlet side port  708  situated or positioned in the cavity  711  and its outlet port  709  in the cavity  712 . Seals  717 ,  720  and  721  kept fluid leakage to a minimum. The cross-sectional area of the gas piston  727  was 4.91 square inches and the cross-sectional area of water tube was 0.049 square inches. Thus, the effective gas surface area on the gas piston  727  was 4.91−0.049=4.857 square inches. The cross-sectional area of plunger end cap  718  was 0.52 square inches. Thus, the intensification ratio of the pressure intensifier was 4.857÷ 0.52=9.34. This intensification ratio indicates that the maximum water pressure the pulsejet generator  700  could accommodate is 9.34×Pg, with Pg being the gas pressure inside the gas chamber  731 . 
         [0047]    The pulsejet generator  700  was filled with compressed air to 300 psi. The gas piston  727  was pushed down by the compressed air and the outlet port  714  was closed. Tap water pressurized to 2000 psi from a motorized jet washer was introduced into the pulsejet generator  700 , and a pulsed waterjet issued or discharged immediately at the nozzle  736 , which had a sapphire orifice of 0.052 inches in diameter. The waterjet pulses could be seen with the naked eye and the modulating motion of the valve shuttle inside the generator was felt by hand. The frequency was estimated to be less than 20 cycles per second and the volume of water per pulse was estimated to be less than 0.5 cubic inches. The resultant pulsed waterjet appeared to be quite powerful and compared very favorably against a conventional straight waterjet issued or discharged by the same nozzle in impacting against a concrete block. 
         [0048]    While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that this invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.