Apparatus and method of improving mixing of axial injection in thermal spray guns

Abstract

An improved thermal spray apparatus and method of promotes mixing of axially fed particles in a carrier stream with a heated effluent stream without introducing significant turbulence into either the effluent or carrier streams. An axial injection port includes a plurality of chevrons at the distal end of the port. The chevrons are located radially around the circumference of the distal end of the axial injection port to increase the shared area between the two flow streams at the outlet of the port.

Description

STATEMENT REGARDING SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

REFERENCE TO SEQUENCE LISTING

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to improved thermal spray application devices, and particularly to a feedstock injector for injecting feedstock material axially into a downstream flow of heated gas.

2. Description of Related Art

Thermal spraying may generally be described as a coating method in which powder or other feedstock material is fed into a stream of energized gas that is heated, accelerated, or both. The feedstock material is entrapped by the stream of energized gas from which it receives thermal and/or kinetic energy. The energized feedstock is then impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the repeated cladding of subsequent thin layers.

It has been previously recognized that, in the case of some thermal spray applications, injecting feedstock axially into an energized gas stream presents certain advantages over other feedstock injection methods. Typically, feedstock is fed into a stream in a direction generally described as radial injection, in other words in a direction more or less perpendicular to the direction of travel of the stream. Radial injection is commonly used as it provides an effective means of mixing particles into an effluent stream and thus transferring the energy to the particles in a short span. Such is the case with plasma where short spray distances and high thermal loading require rapid mixing and energy transfer for the process to apply coatings properly. Axial injection can provide advantages over radial injection due to the potential to better control the linearity and the direction of feedstock particle trajectory when axially injected. Other advantages include having the particulate in the central region of the effluent stream, where the energy density is likely to be the highest, thus affording the maximum potential for energy gain into the particulate. Lastly axial injection tends to disrupt the effluent stream less than radial injection techniques currently practiced.

Thus, in many thermal spray process guns, axial injection of feedstock particles is preferred to inject the particles, using a carrier gas, into the heated and/or accelerated gas simply referred to in this disclosure as effluent. The effluent can be plasma, electrically heated gas, combustion heated gas, cold spray gas, or combinations thereof. Energy is transferred from the effluent to the particles in the carrier gas stream. Due to the nature of stream flow and two phase flow, this mixing and subsequent transfer of energy is limited in axial flows and requires that the two streams, effluent and particulate bearing carrier, be given sufficient time and travel distance to allow the boundary layer between the two flows to break down and thus permit mixing to occur. During this travel distance, energy is lost to the surroundings through heat transfer and friction resulting in lost efficiency. Many thermal spray process guns that do utilize axial injection are then designed longer than would normally be required to allow for this mixing and subsequent energy transfer to occur.

These limitations to mix the particulate bearing carrier and effluent streams becomes even more pronounced when the particulate-bearing carrier fluid is a liquid, and, in many cases, they have prevented the use of liquid feeding into axial injection thermal spray process guns. For liquid injection techniques the use of gas atomization to produce fine droplet streams aids in getting the liquid to mix with the effluent stream more readily to enable liquid injection to work at all but this method still requires some considerable distance to allow the gas and fine droplet stream and effluent stream to mix and transfer energy. This method also produces a certain amount of turbulence in the stream flows.

Attempts at promoting mixing such as introduction of discontinuities and impingement of the flows also produces turbulence. Radial injection, commonly used with thermal spray processes such as plasma to ensure mixing in a short distance also produces turbulence as the two streams intersect at right angles. In fact, most acceptable methods of injection that promote rapid mixing currently use methods that deliberately introduce turbulence as the means to promote the mixing. The turbulence serves to break down the boundary layer between the flows and once this is accomplished mixing can occur.

The additional turbulence often results in unpredictable energy transfer between the effluent and particulate bearing carrier stream as the flow field is constantly in flux, producing variations within the flow field that affect the transfer of energy. Turbulence represents a chaotic process and causes the formation of eddies of different length scales. Most of the kinetic energy of the turbulent motions is contained in the large scale structures. The energy “cascades” from the large scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is the Kolmogorov length scale. Thus the turbulence results in conversion of some of the kinetic energy to thermal energy. The result is a process that produces more thermal energy rather than kinetic for transfer to the particles, limiting the performance of such devices. Complicate the process by having more than one turbulent stream and the results are unpredictable as stated.

Turbulence also increases energy loss to the surroundings as the turbulence results in loss of at least some of the boundary layer in the effluent flow field and thus promotes the transfer of energy to the surroundings as well as frictional affects within the flow when flows are contained within walls. For flow in a tube the pressure drop for a laminar flow is proportional to the velocity of the flow while for turbulent flow the pressure drop is proportional to the square of the velocity. This gives a good indication of the scale of the energy loss to the surroundings and internal friction.

Thus there remains a need in the art for an improved method and apparatus to promote rapid mixing of axially injected matter into thermal spray process guns and also limits the generation of turbulence in the flow streams as a result.

SUMMARY OF THE INVENTION

The invention as described provides an improved apparatus and method for promoting mixing of axially fed particles in a carrier stream with a heated and/or accelerated effluent stream without introducing significant turbulence into either the effluent or carrier streams. Embodiments of the invention utilize a thermal spray apparatus having an axial injection port with a chevron nozzle. For purposes of this application, the term ‘chevron nozzle’ may include any circumferentially non-uniform type of nozzle.

One embodiment of the invention provides a method for performing a thermal spray process (where, for purposes of the invention, the term ‘thermal spray process’ may also include cold spray processes). The method includes the steps of heating and/or accelerating an effluent gas to form a high velocity effluent gas stream; feeding a particulate-bearing stream through an axial injection port into said effluent gas stream to form a mixed stream, wherein said axial injection port has a plurality of chevrons located at a distal end of said axial injection port; and impacting the mixed stream on a substrate to form a coating.

In another embodiment, the invention provides a thermal spray apparatus that includes a means for heating and/or accelerating an effluent gas stream; an injection port configured to axially feed a particulate-bearing stream into said effluent gas stream, said axial injection port having a plurality of chevrons located at a distal end of said axial injection port; and a nozzle in fluid connection with said accelerating means and said injection port.

In yet another embodiment of the invention a thermal spray apparatus is provided. The apparatus includes an effluent gas acceleration component configured to produce an effluent gas stream; an axial injection port with a plurality of chevrons, said axial injection port configured to axially feed a fluid stream into said effluent gas stream; and a nozzle in fluid connection with said effluent gas acceleration component and said injection port.

In yet another embodiment an axial injection port for a thermal spray gun is provided. The injection port includes a cylindrical tube having an inlet and an outlet, said inlet configured to receive fluid flow through said cylindrical tube and said outlet comprising a plurality of chevrons located radially about the circumference of said outlet.

Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF FIGURES

The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1 provides a schematic of a thermal spray gun suitable for use in an embodiment of the invention;

FIG. 2 provides a cut-away schematic of the combustion chamber and exit nozzle regions of a thermal spray gun in accordance with an embodiment of the invention;

FIG. 3 provides a schematic of the distal end of a conventional axial injection port;

FIG. 4 provides a detailed schematic of the distal end of an axial injection port that includes chevrons according to an embodiment of the invention;

FIG. 5 provides a detailed schematic of the distal end of an axial injection port that includes chevrons according to another embodiment of the invention;

FIG. 6 provides boundary area change between two flows over a traveled distance emitted from a nozzle according to an embodiment of the invention;

FIG. 7 provides a schematic of an axial injection velocity particle stream without use of chevrons;

FIG. 8 provides a schematic of an axial injection velocity particle stream with use of non-inclined chevrons according to an embodiment of the present invention; and

FIG. 9 provide a schematic of an axial injection velocity particle stream with use of 20 degree outward inclined chevrons according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

FIG. 1 provides a schematic of a typical thermal spray gun 100 that may be used in accordance with the present invention. The gun includes a housing 102 that includes a fuel gas feed line 104 and an oxygen (or other gas) feed line 106. The fuel gas feed line 104 and an oxygen feed line 106 empty in to a mixing chamber 108 where fuel and oxygen are combined and fed into a combustion chamber 110 through a plurality of ports 112 that are typically located radially around a feedstock and carrier fluid axial injection port 114. The gun housing 102 also includes a feed line for feedstock and carrier fluid 116. The feedstock and carrier fluid feed line empties into the combustion chamber 110, with the axial injection port 114 generally aligned axially with the exit nozzle 118 of the thermal spray gun 100.

In operation, the oxygen/fuel mixture enters the combustion chamber through the ports 112, and feedstock and carrier fluid exit the axial injection port 114 simultaneously. The oxygen/fuel mixture is ignited in the combustion chamber and accelerates feedstock toward the exit nozzle 118. Proper mixing of the two flow streams—the ignited gas effluent from the radial ports 112 shown as F₁ and the carrier gas/feedstock stream from axial injection port 114 shown as F₂—impacts efficiency of the thermal spray process. The mixing of the feedstock and heated gas stream and subsequent transfer of energy may be optimized by use of a notched chevron nozzle on the axial injection port 114.

In the embodiment of FIG. 1, the fuel gas feed line 104, the oxygen feed line 106, the mixing chamber 108, the combustion chamber 110, and the plurality of ports 112 may generally be referred to as components or means necessary to accelerate an effluent gas stream. Other thermal spray processes may use different effluent acceleration components and gasses that are equally applicable to the present invention. Embodiments of the present invention are applicable to a wide variety of thermal spray processes using or potentially can use axial injection. Examples of processes that may be used with embodiments of the present invention include, but are not limited to, cold spraying, flame spraying, high velocity oxy fuel (HVOF) spraying, high velocity liquid fuel (HVLF) spraying, high velocity air fuel (HVAF) spraying, arc spraying, plasma spraying, detonation gun spraying, and spraying utilizing hybrid processes that combine one or more thermal spray processes. Carrier fluids are typically the carrier gasses used in thermal spray guns, including but not limited to argon and nitrogen, that contain the typical thermal spray particulate of various size ranges from about 1 um to larger than 100 um according to each process. One benefit of the invention that may result from the improved mixing is the ability to process higher mass flow rates of particulate as the mixing promotes better energy transfer with less wasted energy. Liquid based carrier fluids containing particulates, or dissolved feed stock in solution, or as a precursor, will also benefit from enhanced mixing, especially in the form of a gas atomized stream generated just prior to the axial injection port exit.

FIG. 2 provides a schematic view of the convergent chamber 110 and divergent exit nozzle 118 regions of a cold spray gun. Axial injection port 114 is shown with a plurality of chevrons 120 at the distal end of the port defining an outlet. Each of the chevrons is generally triangular in configuration. The chevrons 120 are located radially—and in some embodiments equally spaced—around the circumference of the distal end of the axial injection port 114. Introducing the chevrons 120 to the axial injection port 114 increases mixing between the two flow streams F₁ and F₂ as they meet. The energy of the effluent stream passing through the chamber 110 and accelerated in the nozzle 118 more readily transfers the thermal and kinetic characteristics of the effluent flow to the carrier flow and particulate with the use of these chevrons.

FIG. 3 provides a schematic of the distal end of a conventional axial injection port. In contrast, FIG. 4 provides a schematic of the distal end of axial injection port 114 including four chevrons 120 according to an embodiment of the present invention. In some embodiments, each chevron 120 includes a generally triangular shaped extension of the axial injection port 114. In the embodiment of FIG. 4, each chevron 120 is generally parallel to the wall of the axial injection port 114 to which the chevron is joined. Another embodiment, shown in FIG. 5, incorporates chevrons 130 that are flared, curved bent, or otherwise directed radially outward relative to the plane defining the distal end of the axial injection port 114. In another embodiment, the chevrons may be flared, curved, bent, or otherwise directed radially inward relative to the plane defining the distal end of the axial injection port. Angles of inclination for the chevrons up to 90 degrees inward or outward will provide enhanced mixing, while preferred inclination angles may be between 0 and about 20 degrees. Inclination angles higher than about 20 degrees, although providing enhanced mixing, may also tend to produce undesirable eddy currents and the possibility of turbulence depending upon the relative flow velocities and densities.

While FIG. 5 shows the chevrons 130 equally flared, other contemplated embodiments may have non-symmetrical flared chevrons that can correspond with non-symmetrical gun geometries, compensate for swirling affects often present in thermal spray guns, or other desired asymmetrical needs. In other embodiments different shape and/or arrangement may be used in place of a chevron shapes shown in FIGS. 4 and 5. For purposes of the present application, the term ‘chevron nozzle’ may include any circumferentially non-uniform type of nozzle. Non-limiting examples of alternative chevron shapes include radially spaced rectangles, curved-tipped chevrons, semi-circular shapes, and the like. For purposes of the present application such alternate shapes are included under the general term chevrons. In another embodiment the wall thickness of each chevron may be tapered toward the chevron point.

Almost any number of chevrons can be used to aid in mixing. Four chevrons 120, 130 are shown in the embodiment of FIGS. 4 and 5, respectively. In some embodiments, 4 to as many as 6 chevrons may be ideal for most applications. However, other embodiments may use more or fewer chevrons without departing from the scope of the present invention. For the thermal spray gun depicted in FIG. 2 the number of chevrons on distal end of axial injection port 114 may coincide with the number of radial injection ports 112 to allow for symmetry in the flow pattern to produce uniform and predictable mixing in the combustion chamber 110.

In some embodiments, the chevrons shown in the various figures are generally a uniform extension of the axial injection port. In other embodiments, chevrons may be retrofit onto existing conventional axial injection ports by, for example, mechanical attachment. Retrofit applications may include use of clamps, bands, welds, rivets, screws or other mechanical attachments known in the art. While the chevrons would typically be made from the same material as the axial injection port, it is not required that the materials be the same. The chevrons may be made from a variety of materials known in the art that are suitable for the flows, temperatures and pressures of the axial feed port environment.

FIG. 6 provides a schematic of various computer-modeled cross-sections of a modeled flow spray path for a thermal spray gun in an embodiment of the present invention. The bottom of the figure shows a side view of the nozzle 118 and axial injection port 114, and above are shown cross-sections 204a, 204b, 204c, 204d of the effluent and carrier flow paths at various points. Referring to FIG. 6, as the particulate bearing carrier flow F₂ and heated and/or accelerated effluent F₁ reach the chevrons 120, the physical differences, such as pressure, density, etc. between the flows causes the boundary between the flows to change from the initial interface shape, shown in cross-section 202—which is typically cylindrical, as dictated by the shape of the axial injection port 114—to a flower-like or asterisk-like shape shown in the cross-section 204a, increasing the shared boundary area between flows F₁ and F₂. The pressure differential that exists between the flows F₁ and F₂ will cause the higher pressure flow—either the effluent F₁ or carrier F₂—to accelerate radially in response to the pressure differential (potential flow) as the flows F₁ and F₂ progress down the length of the chevrons 120 to equalize the pressure. This radial acceleration will also be distorted to drive the flow around the chevron to equalize the pressure under the chevron as well. As shown in the subsequent shape cross-sections 204b, 204c, and 204d this asterisk-like shape continues to propagate as the flows F₁ and F₂ travel together, further increasing the shared boundary area between flows F₁ and F₂. Since the mixing of the streams is a function of the boundary area, the increase in boundary area increases the mixing rate as exemplified in FIG. 6. The use of inward or outwardly inclined chevrons increases the mixing affect by increasing the pressure differential between the flows thus causing a more rapid formation and extent to the shaping of the boundary area. The inclination can be either inwardly or outwardly directed depending upon the relative properties of the two streams and the desired affects.

Spray paths exiting nozzle shapes depicted in FIGS. 3, 4, and 5 were modeled in the cold spray gun similar to that depicted in FIG. 2. FIG. 7 provides the results of a computational fluid dynamic (CFD) model run of an axially injected particle velocity stream for a cold spray process as modeled in FIG. 2 without the use of chevrons as depicted in FIG. 3. FIG. 8 provides the results of a CFD model run of an axially injected particle velocity stream for a cold spray process as modeled in FIG. 2 with use of chevrons as depicted in FIG. 4 according to an embodiment of the present invention. Applying CFD modeling to an axial injection cold spray gun has shown measurable improvement in mixing of the particulate bearing carrier stream F₂ and heated and/or accelerated effluent stream F₁ and in the transfer of energy from the effluent gas directly to the feedstock particles. In FIG. 7, the resulting particle velocities and spray width is smaller than the particle velocities and spray width shown in FIG. 8 as a result of the improved mixing afforded by the addition of the chevrons. Furthermore, FIG. 9 provides the results of a CFD model run of an axially injected particle velocity stream for a cold spray process as modeled in FIG. 2 with use of outwardly inclined chevrons as depicted in FIG. 5 according to an embodiment of the present invention. As shown in FIG. 9, the particle velocities have increased even higher than with straight chevrons (FIG. 8), indicting an even better transfer of energy from the effluent gas to the particles occurred when using the outwardly inclined chevrons. Thus, the introduction of the chevrons, and even more so the inclined chevrons, has increased the overall velocity of the particles and expanded the particle field well into the effluent stream.

The inclusion of chevrons on axial injection ports can benefit any thermal spray process using axial injection. Thus, embodiments of the present invention are well-suited for axially-fed liquid particulate-bearing streams, as well as gas particulate-bearing streams. In another embodiment, two particulate-bearing streams may be mixed. In still another embodiment two or more gas streams may be mixed by sequentially staging axial injection ports along with an additional stage to mix in a particulate bearing carrier stream. In yet another embodiment, the chevrons can be applied to a port entering an effluent flow at an oblique angle by incorporating one or more chevrons at the leading edge of the port as is enters the effluent stream chamber.

In another embodiment, stream mixing in accordance with the present invention may be conducted in ambient air, in a low-pressure environment, in a vacuum, or in a controlled atmospheric environment. Also, stream mixing in accordance with the present invention may be conducted in any temperature suitable for conventional thermal spray processes.

Anyone skilled in the art can envision further enhancements to the apparatus as well as the use of shapes other than triangular for the chevrons. This apparatus will work on any thermal spray gun using axial injection to introduce particulate bearing carrier gas as well as liquids, additional effluent streams, and reactive gases.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general invention concept as defined by the appended claims and their equivalents.

Claims

1. A thermal spray apparatus, comprising: a device configured for at least one of heating or accelerating an effluent gas stream;an injection port configured to axially feed a particulate-bearing stream into said effluent gas stream, said axial injection port comprising a plurality of chevrons located at a distal end of said axial injection port; wherein said device surrounds and is coaxial with said injection port; anda nozzle, co-axial with said injection port, that surrounds and is in fluid connection with said device and said injection port.

2. The thermal spray apparatus of claim 1, wherein said chevrons are positioned at an angle up to 90 degrees inward or outward relative to a plane defining the distal end of said axial injection port.

3. A thermal spray apparatus, comprising: a component configured to produce at least one of a heated or an accelerated effluent gas stream;an axial injection port comprising a plurality of chevrons located at a distal end of said axial injection port, said injection port configured to axially feed a particulate-bearing stream into said effluent gas stream; anda nozzle, co-axial with said injection port, that surrounds and is in fluid connection with said component and said injection port.

4. An axial injection port for a thermal spray gun comprising a cylindrical tube having an inlet and an outlet that is positionable as part of the thermal spray gun, said inlet configured to receive fluid flow through said cylindrical tube and said outlet comprising a plurality of chevrons located radially about the circumference of said outlet.

5. The axial injection port of claim 4, wherein said plurality of chevrons are inclined outward to a larger diameter than the outlet of said injection port.

6. The axial injection port of claim 4, wherein said plurality of chevrons are inclined inward to a larger diameter than the outlet of said injection port.