Abstract:
A nozzle assembly for a kinetic spray system includes a convergent portion, a throat portion, and a divergent portion, each cooperating together to define a passage therethrough for passing a mixture of powder particles suspended in a flow of a high pressure heated gas. The nozzle assembly further includes an extension portion attached to the divergent portion and extending to a distal end a pre-determined length from the divergent portion of the nozzle assembly. The extension portion permits a dragging force exerted on the powder particles by the flow of high pressure heated gas to act upon the powder particles for a longer duration of time, thereby permitting the powder particles to accelerate to a greater velocity than has been previously achievable.

Description:
[0001]     This application is a continuation-in-part of U.S. Ser. No. 10/924270 filed Aug. 23, 2004 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The subject invention generally relates to a nozzle assembly for a kinetic spray system.  
         [0004]     2. Description of the Related Art  
         [0005]     A nozzle assembly for a kinetic spray system typically comprises a mixing chamber for mixing a stream of powder particles under positive pressure with a flow of a heated gas. The mixing chamber is connected to a converging diverging deLaval type supersonic nozzle. The heated gas is also introduced into the mixing chamber under a positive pressure, which is set lower than the positive pressure of the stream of powder particles. In the mixing chamber, the flow of heated gas and the stream of powder particles mix together to form a gas/powder mixture. The gas powder mixture flows from the mixing chamber into the supersonic nozzle, where the powder particles are accelerated to a velocity between the range of 200 to 1,300 meters per second.  
         [0006]     U.S. patent application Ser. No. 2005/0214474 A1 (the &#39;474 application) discloses a deLaval type nozzle assembly for a kinetic spray system. The nozzle assembly includes a convergent portion defining an inlet and an outlet. The outlet is in spaced relationship relative to the inlet. A divergent portion defines an entrance and an exit, with the exit in spaced relationship relative to the entrance. A throat portion interconnects the outlet of the convergent portion and the entrance of the divergent portion. The convergent portion, the throat portion, and the divergent portion define a passage therethrough having a perimeter narrowing between the inlet and the outlet of the convergent portion, and expanding between the entrance and the exit of the divergent portion.  
         [0007]     During operation of the nozzle assembly, such as the nozzle assembly disclosed in the &#39;474 application, the particles exit the nozzle and adhere to a substrate placed opposite the nozzle assembly, provided that a critical velocity has been exceeded. The critical velocity of the powder particles is dependent upon its material composition and its size. Higher density particles generally need a higher velocity to adhere to the substrate. Additionally, it is more difficult to accelerate larger powder particles. Accordingly, the coating density and deposition efficiency of the particles can be very low with harder to spray powder particles. The velocity of the powder particles, upon exiting the nozzle assembly, varies inversely to the size and the density of the powder particles. Increasing the velocity of the flow of heated gas increases the velocity of the powder particles upon exiting the nozzle assembly. However, there is a limit to the achievable velocity of the flow of heated gas within the kinetic spray system. Thus, there is a need to improve the nozzle assembly to increase the velocity of the powder particles to improve adherence to the substrate of hard to spray powder particles having a high density and a larger size.  
       SUMMARY OF THE INVENTION AND ADVANTAGES  
       [0008]     The subject invention provides a nozzle assembly for a kinetic spray system. The nozzle assembly comprises a convergent portion defining an inlet and an outlet. The outlet is in spaced relationship relative to the inlet. A divergent portion defines an entrance and an exit, with the exit in spaced relationship relative to the entrance. A throat portion interconnects the outlet of the convergent portion and the entrance of the divergent portion. The convergent portion, the throat portion, and the divergent portion define a passage therethrough. The passage includes a perimeter narrowing between the inlet and the outlet of the convergent portion, and expanding between the entrance and the exit of the divergent portion. An extension portion further defines the passage and extends from the exit of the divergent portion to a distal end spaced a pre-determined length from the exit. The perimeter of the passage defined by the extension portion is at least equal to or greater than the perimeter of the passage defined by the exit of the divergent portion.  
         [0009]     The subject invention also provides a method of coating a substrate with a powder applied by the kinetic spray system. The method comprises the steps of mixing the powder with a flow of heated gas; directing the flow of heated gas through the convergent portion, the throat portion, and the divergent portion of the nozzle assembly to accelerate the flow of heated gas and provide a drag force to act upon the powder to accelerate the powder; and passing the accelerated flow of heated gas and the powder through the extension portion of the nozzle assembly to provide additional time for the drag force of the flow of heated gas to act upon the powder to further accelerate the powder to a critical velocity.  
         [0010]     Accordingly, the subject invention increases the overall length of the nozzle assembly while limiting an expansion ratio of the passage over the pre-determined length of the extension portion to avoid any negative effects that occur by merely extending the divergent portion. This increases the amount of time a stream of powder particles is exposed to a dragging force created by a flow of a heated gas through the nozzle assembly. This increased exposure of the stream of powder particles to the dragging force provides more time for the dragging force to accelerate the powder particles to an increased velocity not previously achievable. The increased velocity of the powder particles improves the ability of the kinetic spray system to adhere hard to spray materials such as high density and larger sized powder particles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
         [0012]      FIG. 1  is a schematic layout illustrating a kinetic spray system;  
         [0013]      FIG. 2  is a cross sectional view of a nozzle for use in the kinetic spray system;  
         [0014]      FIG. 3  is an enlarged cross sectional view of an extension portion of the nozzle;  
         [0015]      FIG. 4  is an end view of the extension portion of the nozzle shown in  FIG. 3 ;  
         [0016]      FIG. 5  is an enlarged cross sectional view of an alternative embodiment of the extension portion of the nozzle;  
         [0017]      FIG. 6  is an end view of the alternative embodiment of the extension portion of the nozzle shown in  FIG. 5 ;  
         [0018]      FIG. 7  is a cross sectional view of an alternative embodiment of a conditioning chamber for the nozzle;  
         [0019]      FIG. 8  is a cross sectional view of an alternative embodiment of the nozzle showing an alternative method of injecting a powder into a high pressure gas flowing through the nozzle; and  
         [0020]      FIG. 9  is an end view an alternative embodiment of the extension portion of the nozzle showing a circular cross section. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention comprises an improvement to the kinetic spray system and nozzle assembly  20  as generally described in U.S. patent application Ser. No. 2005/0214474 A1; U.S. Pat. Nos. 6,139,913 and 6,283,386; and the article by Van Steenkiste, et al. entitled “Kinetic Spray Coatings” published in Surface and Coatings Technology Volume III, Pages 62-72, Jan. 10, 1999. The disclosures of which are all herein incorporated by reference.  
         [0022]     Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a kinetic spray system is generally shown at  20 . Referring to  FIG. 1 , the kinetic spray system  20  applies a coating of powder particles  22  to a substrate material  24 . A flow of heated gas suspends the powder particles  22 , which are then sprayed onto the substrate  24  at high velocities. As disclosed in U.S. Pat. No. 6,139,913 the substrate material  24  may be comprised of any of a wide variety of materials including a metal, an alloy, a plastic, a polymer, a ceramic, a wood, a semiconductor, or any combination and mixture of these materials. The powder particles  22  used in the kinetic spray system  20  may comprise any of the materials disclosed in U.S. Pat. Nos. 6,139,913 and 6,283,386 in addition to other known powder particles  22 . These powder particles  22  generally comprise a metal, an alloy, a ceramic, a polymer, a diamond, a metal coated ceramic, a semiconductor, or any combination and mixture of these materials. Preferably, the particles have an average nominal diameter between the ranges of 1 micron to 250 microns.  
         [0023]     The kinetic spray system  20  includes an enclosure  26  in which a support table  28  or other support device is located. A mounting panel  30  is fixed to the support table  28 , and supports a work holder  32 . The work holder  32  is capable of movement in three dimensions and is able to support a suitable work piece. The work piece is formed from the substrate material  24  that is to be coated. The enclosure  26  includes surrounding walls defining at least one air inlet (not shown) and at least one air outlet  34  connected by a suitable exhaust conduit  36  to a dust collector (not shown). During operation of the kinetic spray system  20  the dust collector continually draws air from within the enclosure  26 , and collects any dust or particles contained in the air for subsequent disposal before exhausting the air.  
         [0024]     The kinetic spray system  20  further includes a gas compressor  38  capable of supplying a flow of a gas at a pressure up to 3.4 MPa (500 psi) to a ballast tank  40 . Many different gases may be utilized in the kinetic spray system  20  including air, helium, argon, nitrogen, or some other noble gas. The ballast tank  40  is in fluid communication with a powder feeder  42  and a gas heater  44  through a system  20  of lines  46 . The gas heater  44  supplies a flow of heated gas, the heated main gas described below, to a nozzle assembly  48 . The powder feeder  42  mixes the powder particles  22  to be sprayed into a stream of unheated gas and supplies the mixture of unheated gas and powder particles  22  to a supplemental inlet line  50  to supply the nozzle assembly  48  with the powder particles  22 . A computer  52  controls the pressure of the gas supplied to the gas heater  44  and to the powder feeder  42 , and the temperature of the heated main gas exiting the gas heater  44 .  
         [0025]     Referring to  FIG. 2 , a main gas passage  54  connects the gas heater  44  to the nozzle assembly  48 . A premix chamber  56  is connected to the main gas passage  54  and directs the heated main gas through a flow straightener  58  and into a mixing chamber  60 . The mixing chamber  60  mixes the powder particles  22  into the flow of heated main gas to suspend the powder particles  22  in the heated main gas. Preferably, the mixing chamber  60  is disposed upstream of a conditioning chamber  62  (described below). A temperature of the heated main gas is monitored by a temperature thermocouple  64  in the main gas passage  54 , and a pressure sensor  68  connected to the mixing chamber  60  monitors a pressure of the heated main gas.  
         [0026]     A powder injector tube  70  is in fluid communication with the supplemental inlet line  50  and directs the mixture of the gas and the powder particles  22  to the mixing chamber  60  to supply the mixing chamber  60  with the powder particles  22 . The powder injection tube extends through the premix chamber  56  and the flow straightener  58  into the mixing chamber  60 . Preferably, the injector tube has an inner diameter between the ranges of 0.3 millimeters to 3.0 millimeters, and is aligned collinear with a central axis C of the nozzle assembly  48 .  
         [0027]     The conditioning chamber  62  is positioned between the powder-gas mixing chamber  60  and a convergent portion  72  (described below) of the nozzle assembly  48 . The conditioning chamber  62  increases the temperature of the powder particles  22  prior to mixing the powder particles  22  with the heated main gas flowing through the nozzle assembly  48 . Preferably, as shown in  FIG. 2 , the conditioning chamber  62  is disposed upstream of the convergent portion  72 . The conditioning chamber  62  includes a length along a longitudinal axis B, preferably collinear with the central axis C of the nozzle assembly  48 . The interior of the conditioning chamber  62  has a cylindrical shape having an interior diameter equal to the inlet  77  of the convergent portion  72  of the nozzle assembly  48 . The conditioning chamber  62  releasably engages the convergent portion  72  of the nozzle assembly  48  and the powder-gas mixing chamber  60 . Preferably, the releasable engagement is by correspondingly engaging threads (not shown) between the exchange chamber, the convergent portion  72 , and the conditioning chamber  62  respectively. It should be understood, however, that the releasable engagement may be through other devices such as a snap fit connection, a bayonet-type connection, or some other suitable type of connection. The length along the longitudinal axis B is preferably at least 20 millimeters or longer. The optimal length of the conditioning chamber  62  depends on the particles that are being sprayed and the substrate material  24 . The optimal length can be determined experimentally, but is preferably between the ranges of 20 millimeters to 1000 millimeters.  
         [0028]     As best shown in  FIG. 3 , the nozzle assembly  48  includes the convergent portion  72 , which defines an inlet  77  and an outlet  74 . The outlet  74  is in spaced relationship relative to the inlet  77 . A divergent portion  76  defines an entrance  78  and an exit  80 , with the exit  80  being in spaced relationship relative to the entrance  78 . A throat portion  82  interconnects the outlet  74  of the convergent portion  72  and the entrance  78  of the divergent portion  76 . The convergent portion  72 , the throat portion  82 , and the divergent portion  76  form a de Laval type converging diverging nozzle as is known in the art, and cooperate together to define a passage  66  therethrough. The passage  66  includes a perimeter  84 , which narrows between the inlet  77  and the outlet  74  of the convergent portion  72  and expands between the entrance  78  and the exit  80  of the divergent portion  76 . An extension portion  86  further defines the passage  66  and extends from the exit  80  of the divergent portion  76  to a distal end  88  spaced a pre-determined length L from the exit  80 . The pre-determined length L of the extension portion  86  is between the ranges of 20 millimeters and 1,000 millimeters. Accordingly, the nozzle assembly  48  includes an overall length spanning the convergent portion  72 , the throat portion  82 , the divergent portion  76 , and the extension portion  86  between the ranges of 100 millimeters and 1,500 millimeters.  
         [0029]     Based on aerodynamics, a drag force is applied to the powder particles  22  by the flow of heated main gas. The drag force may be expressed by the equation:  
             D   =       1   2     ·     C   p     ·     ρ   g     ·       (       V   g     -     V   p       )     2     ·       A   p     .             1           
 
         [0030]     Wherein C p  is a drag coefficient, ρ g  is a density of the heated main gas, V g  is a velocity of the heated main gas, V p  is a velocity of the powder particles  22 , and A p  is an average cross sectional area of the powder particles  22 . The drag force accelerates the powder particles  22  to a critical velocity. It has been discovered that there is a wasted potential in the drag force because the powder particles  22  are not exposed to the drag force for a long enough period of time, i.e., the powder particles  22  may achieve a higher velocity if the powder particles  22  are exposed to the drag force for a longer period of time. Accordingly, by adding the extension portion  86  onto the divergent portion  76  of the nozzle assembly  48 , the powder particles  22  are exposed to the drag force for a longer period of time, thereby minimizing the wasted potential, and thereby maximizing the drag force applied to the powder particles  22 .  
         [0031]     The heated main gas flows through the convergent portion  72 , throat portion  82 , and then into the divergent portion  76 , where the heated main gas accelerates to high velocities. As the velocity of the heated main gas increases, the density of the heated main gas decreases. This is evident with reference to the conservation of mass within the nozzle assembly  48  expressed by the equation: 
 
 f=A·V   g ·ρ g    2. 
 
         [0032]     Wherein f is a mass flow rate of the heated main gas, A is a cross sectional area of the perimeter  84  of the nozzle assembly  48  at any given location within the passage  66 , V g  is the velocity of the heated main gas, and ρ g  is the density of the heated main gas. The decrease in the density of the heated main gas negatively affects the drag force. Additionally, an expansion ratio defined as a rate of change of the perimeter  84  of the passage  66  over a distance along the central axis C extending through the passage  66  limits the increase in the velocity achievable in the divergent portion  76 . As the heated main gas flows through the divergent portion  76 , a boundary layer near an outer wall of the nozzle assembly  48  develops, and tends to separate, creating a shock wave in the flow of heated main gas. The shock wave significantly decreases the velocity of the heated main gas. Accordingly, it is not effective to merely extend the divergent portion  76  of the nozzle assembly  48  outward. Therefore, the perimeter  84  of the passage  66  defined by the extension portion  86  is at least equal to or greater than the perimeter  84  of the passage  66  defined by the exit  80  of the divergent portion  76 . It should be understood that the perimeter  84  of the passage  66  defines a cross sectional shape. Referring to  FIGS. 3 and 4 , the cross sectional shape defined by the perimeter  84  may be uniform throughout the pre-determined length L of the extension portion  86 . It should be understood that the uniform cross sectional shape of the extension portion  86  includes an expansion ratio equal to zero or negligibly small. Alternatively, referring to  FIGS. 5 and 6 , the cross sectional shape of the perimeter  84  defined by the extension portion  86  may slightly increase in area relative to the exit  80  of the divergent portion  76  as the extension portion  86  extends from the exit  80  of the divergent portion  76  to the distal end  88  of the extension portion  86 . Nevertheless, the slightly increasing cross sectional shape defined by the extension portion  86  includes a significantly smaller expansion ratio relative to the expansion ratio of the divergent portion  76 . The uniform cross sectional shape and the alternative slightly increasing cross sectional shape defined by the perimeter  84  of the extension portion  86  permit the drag force to act on the powder particles  22  for a longer period of time without significantly decreasing the density of the heated gas, and also without creating the shock wave within the flow of heated gas.  
         [0033]     As described above, the expansion ratio of the passage  66  defined by the divergent portion  76  is greater than the expansion ratio of the passage  66  defined by the extension portion  86 . This permits the heated main gas to flow through the extension portion  86  without continuing to decrease the density of the heated main gas and to avoid shock waves in the heated main gas. While it is contemplated that the divergent portion  76  may include a constant expansion ratio as shown in  FIGS. 3 and 5 , the expansion ratio of the divergent portion  76  preferably continuously decreases from the entrance  78  to the exit  80  of the divergent portion  76  as shown in  FIG. 7 . This may further be described as having a parabolic or curved shape that continuously diverges from the central axis C at a continuously decreasing rate as the distance from the entrance  78  of the divergent portion  76  increases in a direction toward the exit  80  of the divergent portion  76 . The parabolic or curved shaped divergent portion  76  provides the greatest possible expansion ratio immediately downstream of the throat portion  82 , thereby rapidly increasing the velocity of the heated main gas near the throat portion  82  than near the extension portion  86  to maximize the velocity difference between the heated main gas and the powder particles  22  and to increase the drag force applied on the powder particles  22 . Accordingly, the divergent portion  76  has the largest expansion ratio nearest the throat portion  82 , and the smallest expansion ratio at the exit  80  of the divergent portion  76 . As a result, the gas pressure at the divergent portion  76  drops rapidly due to a high expansion ratio. This allows the powder particles  22  to be injected by a low pressure powder feeder  42  through the powder injector tube  70  as shown in  FIG. 7 .  
         [0034]     The cross section of the perimeter  84  defined by the divergent portion  76  and the extension portion  86  may include a variety of shapes, but preferably includes a rectangular shape. The rectangular shaped cross section of the perimeter  84  defined by the extension portion  86  at the distal end  88  includes a long dimension between the range of 6.0 millimeters and 24.0 millimeters and a short dimension between the range of 1.0 millimeters and 6.0 millimeters. Alternatively, as shown in  FIG. 9 , the perimeter  84  of the passage  66  defined by the divergent portion  76  and the extension portion  86  may define a cross section having a circular shape.  
         [0035]     Preferably, as indicated in  FIG. 5 , the extension portion  86  is releasably attached to the divergent portion  76 . The releasable attachment may be by correspondingly engaging threads between the divergent portion  76  and the extension portion  86 , a snap fit connection, a bayonet type connection, or some other suitable connection. However, as shown in  FIG. 3 , it is contemplated that the extension portion  86  may be integrally formed with the divergent portion  76  as a single unit.  
         [0036]     The perimeter  84  of the passage  66  defined by the throat portion  82  defines a cross section. As shown in  FIG. 9 , the cross section may include a circular shape. The circular shaped cross section of the throat may include a diameter between the ranges of 1.0 millimeters and 5.0 millimeters. However, it should be understood that the cross section of the throat portion  82  may include other shapes. Preferably, referring to  FIGS. 4 and 6 , the cross section of the throat portion  82  includes an elliptical shape. Excessive wear in the rectangular shaped cross section of the divergent portion  76  adjacent the throat portion  82  has been noticed. The excessive wear negatively affects the performance of the nozzle assembly  48 . The excessive wear has been attributed to rapid radial expansion of the heated main gas and powder particles  22  exiting the circular shaped cross section of the throat portion  82 . This excessive wear is reduced by elongating the cross section of the throat portion  82 . Accordingly, the elliptically shaped cross section of the throat portion  82  helps minimize the excessive wear noticed in the rectangular shaped cross section of the divergent portion  76 .  
         [0037]     Referring to  FIGS. 7 and 8 , an alternative embodiment of the nozzle assembly  48  is shown. In the alternative embodiment, the particle injector tube interconnects the conditioning chamber  62  and the divergent portion  76  of the nozzle assembly  48  to supply the powder particles  22  to the divergent portion  76  of the nozzle assembly  48 . The mixing chamber  60  is disposed within the divergent portion  76 , adjacent the throat portion  82 , for mixing the powder particles  22  with the flow of heated main gas in the divergent portion  76  of the nozzle assembly  48  as the heated main gas enters the divergent portion  76  from the throat portion  82 . In the alternative embodiment, the longitudinal axis B of the conditioning chamber  62  is not collinear with the central axis C, and in fact, the conditioning chamber  62  is separated form the nozzle assembly  48 . The particle injector tube interconnects in fluid communication the conditioning chamber  62  and the mixing chamber  60  within the divergent portion  76 . Powder buildup and clogging of the throat portion  82  is thereby minimized by providing the powder particles  22  directly into the divergent portion  76  of the nozzle assembly  48  instead of directing the powder particles  22  through the throat portion  82 . In the alternative embodiment, the gas pressure in the divergent portion  76  drops rapidly due to the high expansion ratio. This enables the powder particles  22  to be injected at a lower pressure (less than 100 psi), compared to the preferred embodiment shown in  FIG. 2 , which injects the powder particles  22  at a higher pressure (typically greater than 300 psi). Furthermore, a detached conditioning chamber  62  may be included that uses external heating to heat the powder particles  22  to an elevated temperature (up to 80% of the melting temperature of the powder particles  22 ). The detached conditioning chamber  62  is in fluid communication with the divergent portion  76  through the powder injector tube  70 , as shown in  FIG. 7 . Alternatively, the detached conditioning chamber  62  may also be in fluid communication with the premix chamber  56  through the powder injector tube  70 , as shown in  FIG. 2 .  
         [0038]     The foregoing invention has been described in accordance with the relevant legal standards; thus, the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiments may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.