Patent Application
20140048615
The invention relates to a two-substance nozzle with a nozzle housing, said nozzle housing comprising at least a first fluid inlet for fluid that is to be atomized, a second fluid inlet for gaseous fluid, a mixing chamber, a nozzle outlet opening and an annular gap opening surrounding the nozzle outlet opening, whereby, within the nozzle housing, means are provided for generating a film of fluid that is to be atomized on a wall in the mixing chamber, and inlet openings are provided for injecting gaseous fluid into the mixing chamber. The invention also relates to a cluster nozzle comprising at least two two-substance nozzles in accordance with the invention, as well as a method for the atomization of fluids by means of a two-substance nozzle.
In many process engineering facilities, fluids are injected into a gaseous fluid, e.g., into flue gas that is to be cleaned or cooled. In so doing, it is often highly important that the fluid be atomized in the smallest possible droplets. The smaller the droplets, the larger the specific droplet surface. In process engineering, this can result in significant advantages. For example, the size of the reaction container and the costs for its manufacture depend decisively on the average droplet size. However, in many cases it is by no means sufficient if the average particle size drops below a specified limiting value. Only a few substantially larger droplets can result in considerable disruptions of operation. This is the case, in particular, when the droplets—due to their size—do not evaporate rapidly enough, so that the droplets or even pasty particles are deposited in subsequent components, e.g., on fabric filter tubing or on blower blades, thus leading to malfunctions due to incrustations, corrosion or imbalance.
When fluids are to be atomized to form the finest-possible droplet spray, so-called pressurized gas assisted two-substance nozzles are frequently used in addition to high-pressure single-substance nozzles that are loaded only with the fluid that is to be atomized. In these nozzles, the fluid is sprayed with the assistance of a pressurized gas, e.g., pressurized air or pressurized steam, said pressurized gas forming the first gaseous fluid, into a second gaseous fluid, e.g. flue gas.
In order to simplify the language used, the first gaseous fluid is frequently referred to as the “pressurized air”, even though—in generalized terms—it would be possible to refer to pressurized gas or pressurized steam. Further, as a rule, the second gaseous fluid is referred to as the flue gas.
Depending on prior art that exists concerning the respective applications, a multitude of different two-substance nozzles are available. An important criterion considering the field of use is the composition of the fluid that is to be atomized.
1. Nozzles for the Atomization of Fluids that are Free of Solids.
Relatively easy constraints exist only when the fluid does not contain any suspended matter and when the fluid does not form any solid evaporation residues. This applies, e.g., to nozzles for the atomization of ammonia water in systems for the reduction of nitrogen oxide in flue gas, or to nozzles for the atomization of kerosene in turbine jet engines. In particular, for the last-mentioned case of use, so-called prefilming nozzles were developed, such as are shown in
2. Nozzles for the Atomization of Fluids Containing Solids.
In many cases the fluid is loaded with suspended matter, e.g., with large or small particles. The small particles may be suspended substances that are carried along—corresponding to the mesh size of a filter—as a residual solids load in the fluid to be atomized. Larger particles, usually having the shape of platelets, are formed by fragments detaching from wall deposits in the feed lines to the nozzle. The wall deposits may be fine particle deposits as well as deposits formed by substances that are initially still dissolved in the fluid. In the case of these applications, narrow channels or bores are avoided because they would become rapidly clogged by suspended matter and/or detached coarse particles carried along in the fluid. Furthermore, care is taken that the fluid does not already evaporate inside the nozzle to such an extent that a rapid buildup of deposits of the exhaust steam residue will occur there.
If the cross-sections for the fluid feed-line in the nozzle are too large, great difficulty exists in separating the massive fluid jet into fine droplets. This requires disproportionately much pressurized air, and the energy consumption of such nozzles is accordingly high.
The invention is to provide a two-substance nozzle, a cluster nozzle and a method for atomizing fluids, which can be used to achieve a uniform droplet size and which are characterized by low energy consumption.
To achieve this, the invention provides a two-substance nozzle with a nozzle housing, said nozzle housing comprising at least a first fluid inlet for fluid that is to be atomized, a second fluid inlet for gaseous fluid, a mixing chamber, a nozzle outlet opening and an annular gap opening surrounding the nozzle outlet opening, whereby, within the nozzle housing, means are provided for generating a film of fluid that is to be atomized on a wall in the mixing chamber, and inlet openings are provided for injecting gaseous fluid into the mixing chamber, whereby the inlet opening and the mixing chamber are aligned and designed in a manner so as to inject the gaseous fluid essentially in parallel alignment with the wall into the mixing chamber and to conduct the gaseous fluid inside the mixing chamber essentially in a parallel manner past the wall.
In the nozzle in accordance with the invention, a film of atomized fluid is generated on a wall in the mixing chamber, whereby the mixing chamber extends from the inlet openings for the fluid that is to be atomized to the nozzle outlet opening. By aligning and configuring the inlet openings and the mixing chamber in order to inject the gaseous fluid in essentially parallel alignment with the wall, it is possible to minimize the pressure losses in the gaseous fluid. Then, the gaseous fluid—advantageously, in the form of a high-velocity gas stream—is conducted, inside the mixing chamber, past the wall in essentially parallel alignment with said wall, thereby also resulting in a very low energy demand by the nozzle. For example, the two-substance nozzle in accordance with the invention can be operated at a very low pressurized air pressure of less than 1 bar overpressure, and still an extremely small and, at the same time, uniformly distributed droplet size is achieved. The gas stream of the gaseous fluid drives the film of fluid to be atomized, said film being on the wall in the mixing chamber, up to the nozzle outlet opening. There, this fluid film is drawn out into individual lamellae that are then arranged between the gas stream exiting from the nozzle opening and the annular gap stream exiting from the annular gap opening, and are thus atomized into fine droplets. It is also possible for fine droplets to be already generated inside the mixing chamber itself, in that the fluid film driven by the gas stream in the direction of the nozzle outlet becomes instable, and in that, at this point, a partial atomization occurs before the nozzle outlet opening is reached. The two-substance nozzle in accordance with the invention is characterized by an extremely good part-load behavior. With small water streams that are to be atomized, it is possible to work with air at low pressure, for example 0.2 bar overpressure, in particular when an extremely fine atomization is not desired. The flow rates within the nozzle may be relatively low, resulting, for example, in velocities amounting to 50 m/s at the entry of the mixing chamber and no more than approximately 100 m/s at the nozzle orifice. If small fluid streams are to be atomized in an extremely fine manner or if large fluid streams are to be atomized in a fine manner, higher flow rates are required. This also applies to vapor-assisted atomization. In this case, the velocity of sound is approximately reached—in two-phase flow—at the nozzle orifice of the inventive two-substance nozzle. However, the mixing chamber can also be configured as a Laval nozzle, where the velocity of sound is reached at a smallest cross-section and where the flow cross-section then widens again in order to maintain the flow rate above the velocity of sound. Overall, the two-substance nozzle in accordance with the invention has usefully achieved very low energy consumption with small drop size and uniform drop spectrum.
Advantageously, there are at least three inlet openings for injecting gaseous fluid into the mixing chamber. The inlet openings can be implemented as bores in a ring, for example. The pressurized air jets exiting from the bores run so as to be essentially tangential to the mixing chamber wall and are additionally inclined toward the nozzle axis.
In a further development of the invention, the inlet openings for gaseous fluid in the mixing chamber are aligned at an angle between 0° and 30° relative to the wall in the first third of the length of the mixing chamber.
If the gaseous fluid is injected into the mixing chamber at an angle between 0° and 30° relative to the wall, only a minimal loss of pressure occurs, and the fluid film on the wall of the mixing chamber can still be reliably driven in the direction of the nozzle outlet opening. For example, the mixing chamber can be configured in such a manner that the air is injected in the mixing chamber parallel to the wall, and then, in a second section of the mixing chamber, impinges on the wall at a small angle of less than 30°. As a result of this, the shearing stress effect on the fluid film increases so as to drive said fluid film farther in the direction of the nozzle outlet.
In a further development of the invention, the central axes of the inlet openings for the gaseous fluid are inclined relative to a central longitudinal axis of the mixing chamber in such a manner that the central axes of the inlet openings converge toward the central longitudinal axis of the mixing chamber—viewed in the direction of flow.
In this manner, the formation of zones displaying low gas velocity, i.e., displaying a comparatively slower core air flow, can be prevented and uniform droplet sizes can be ensured. The central axes can be inclined by an angle within the range of 10° to 30° relative to the central longitudinal axis.
In a further development of the invention, the central axes of the inlet openings for the gaseous fluid do not intersect the central longitudinal axis of the mixing chamber.
When the central axes of the inlet openings are arranged skewed relative to the central longitudinal axis of the mixing chamber, said central axes can run toward the central longitudinal axis of the mixing chamber, without, however, intersecting the central longitudinal axis or each other. As a result of this, pressure losses caused by vortical zones are prevented. In the skewed arrangement, the central axes of the inlet openings are inclined by the angle Υ relative to the central longitudinal axis and by the angle δ in circumferential direction, whereby angle δ is preferably within a range of 5° to 15°.
In a further development of the invention, the central axes of the inlet openings are located on the lateral surface of an imagined rotation hyperboloid.
In this manner, the gaseous fluid inside the mixing chamber can be imparted with a twist that promotes the atomization into fine droplets. The central axes of the inlet openings can then be the generatrices of a single-shell hyperboloid.
In a further development of the invention, the mixing chamber also contains droplet loading means for loading the high-velocity gas stream with fluid droplets at least in the regions remote from the wall with the fluid film, said regions not being decelerated by the friction between the fluid film and the high-velocity gas stream.
In this manner, it can be ensured that the injected gaseous fluid is decelerated in each region and thus performs its function, be it to tear the fluid to be atomized into individual droplets, be it to drive the fluid film on the wall of the mixing chamber in the direction of the nozzle exit. Specifically, the formation of a core air flow is prevented, which core air flow—compared with the air stream flowing along the wall in the mixing chamber—is not decelerated or only slightly decelerated and thus leaves the nozzle again, without performing any work.
In a further development of the invention, the droplet loading means comprise a central pin, whereby one inlet opening for the fluid to be atomized is directed at a tip of the central pin and the central pin—starting at the tip and ending at a point of maximum diameter—widens in a cone-like manner, whereby the gaseous fluid is conducted inside the mixing chamber past the point of maximum diameter of the central pin.
With the use of such a central pin, it is possible to separate the fluid to be atomized into a thin fluid film or into individual fluid jets, for example, by means of furrows or channels in the central pin, in which case the energy required therefor is provided by the kinetic energy of the fluid that is to be atomized. The fluid to be atomized leaves the central pin at a point of maximum diameter, where the fluid to be atomized is caught by the gaseous fluid, partially separated into individual droplets and then carried along in the direction of the nozzle outlet and partially impinges on the wall of the mixing chamber in order to form a fluid film. With the use of such a central pin, it is also possible to load the regions of the air flow that are remote from the wall of the mixing chamber with droplets, whereby said air flow can be decelerated and thus contribute to the atomization. The central pin with its suspension device, and/or the nozzle housing defining the mixing chamber, can be made of hard metal or of silicon carbide.
In a further development, the means for generating a film of the fluid that is to be atomized comprise at least one obstacle in the flow path of the fluid to be atomized in order to divide the fluid to be atomized by means of its flow energy into partial streams. Advantageously, the means for generating a film include a twist insert upstream of the fluid inlet into the mixing chamber.
With the use of a twist insert in the flow path of the fluid to be atomized, it is possible to cause the fluid to be atomized to rotate, so that most of said fluid will move along the wall of a flow channel and then can also generate the desired fluid film on the wall of the mixing chamber. An obstacle in the fluid path of the fluid supply can also have the form of at least three channels or furrows that are open toward the central longitudinal axis of the nozzle, said channel or furrows extending like the lands and grooves in a gun barrel.
In a further development of the invention, the means for generating a film of fluid to be atomized comprise a central pin, whereby the inlet opening for the fluid to be atomized is directed at a tip of the central pin, and the central pin initially widens in a cone-like manner starting at the tip.
Consequently, a central pin can perform two functions, namely, on the one hand it can load a core air stream with droplets and, on the other hand, it can generate a film of fluid to be atomized on the wall of the mixing chamber. The fluid that is to be atomized and has been separated leaves the central pin at the point of maximum diameter, is then partially torn into droplets by the core air flow and carried along, and part of said fluid reaches the wall of the mixing chamber at the point of maximum diameter located approximately opposite said wall, and forms the desired fluid film on said wall.
In a further development of the invention, the central pin—viewed in the direction of flow—has a tapering trailing body downstream of a region of maximum diameter.
By means of such a trailing body, for example, in the manner of a tadpole tail, it is possible to prevent a vortical zone and dead zone downstream of the central pin where larger droplets could form. In addition, the tapering trailing body can also ensure that the flow rate of the gaseous fluid in the mixing chamber is maintained at a high level.
In a further development of the invention, the central pin has the shape of a double cone.
In a further development of the invention, the wall in the mixing chamber is arranged so as to be essentially parallel to the tapering trailing body of the central pin.
For example, the central pin has the shape of a circular cone and has the shape of a double cone and is surrounded by the wall of the mixing chamber at a constant distance. As a result of this, the annular gap width can be kept constant; due to the taper of the central pin and the wall of the mixing chamber, the free cross-section of the flow is reduced.
By reducing the free cross-section of the flow of the mixing chamber—viewed in the direction of flow along the trailing body of the central pin—the velocity of the gas flow in the mixing chamber can be maintained at a high level, and a fluid film on the trailing body as well as on the wall of the mixing chamber is subject to a high shearing stress.
In a further development of the invention, the central axes of the inlet openings for the gaseous fluid into the mixing chamber are arranged so as to be essentially parallel to the outside wall of the trailing body of the central pin.
In this manner, the gaseous fluid can be injected into the mixing chamber with an extremely small loss of pressure, and it is possible to achieve a high velocity of the gaseous fluid in the mixing chamber even at low input pressures of the gaseous medium.
In a further development of the invention, a central pin has the shape of a double cone, whereby the region with minimum cross-section of the mixing chamber is arranged on the level of the down-stream tip of the double cone.
In a further development of the invention, a cross-section of the mixing chamber tapers initially, and maintains or enlarges said cross-section in an adjoining region of minimal cross-section.
In this manner, a high-velocity gas flow can be maintained or even accelerated when the velocity of sound is reached in the region of minimal cross-section.
In a further development of the invention, the mixing chamber tapers in the form of a hollow truncated cone and widens again, starting from a point of minimum cross-section in the form of another hollow truncated cone, whereby the central axes of the inlet openings for the gaseous fluid are aligned in the mixing chamber parallel to the inside wall of the mixing chamber in the tapering hollow truncated cone.
In this manner, the gaseous fluid is injected in the tapering region parallel to the wall of the mixing chamber, on which the fluid film is driven along. In the widening region, the gaseous fluid is then also conducted parallel to the wall of the mixing chamber or at a small angle relative to the wall of the mixing chamber. In this arrangement, a small angle may be advantageous in order to increase the action of a shearing stress on the fluid film and to drive said fluid film in the direction of the nozzle outlet.
In a further development of the invention, the means for generating a film of fluid to be atomized comprise a central pin, whereby an inlet opening for the fluid to be atomized is directed against the tip of the central pin, and the central pin is provided, in the region of its flow approach side for the fluid to be atomized, with at least two channels or furrows that extend from one tip of the central pin to a point of largest diameter of the central pin.
By means of such channels or furrows, it is possible to divide the fluid to be atomized and impinging on the tip of the central pin, at least partially, into individual jets using only the kinetic energy of the impinging fluid. These jets then leave the central pin at the point of greatest diameter, are caught by the gaseous fluid injected into the mixing chamber and are partially torn into droplets. The fluid jets leaving the central pin ensure, on the one hand, that a core air flow is loaded with droplets, decelerated and will not tunnel through the nozzle without performing its atomizing function. In addition, the fluid jets impinge on the wall of the mixing chamber, said wall being approximately opposite the point of maximum diameter of the central pin and ensure there the formation of a fluid film on said wall, said fluid film then being driven—by the gaseous fluid injected into the mixing chamber—in the direction of the nozzle outlet. The channels or grooves may extend on the generatrices of the central pin or be inclined with respect to them.
In a further development of the invention, the means for generating a film of fluid to be atomized comprise a central pin, whereby the inlet opening for the fluid to be atomized is directed against a tip of the central pin, and the central pin is connected—via at least two radially extending strips—with the nozzle housing defining an inside wall of the mixing chamber.
Such an arrangement of the central pin is simple from the viewpoint of construction, promotes flow, and, as a result, the central pin is also exchangeable. Such an exchange of the central pin may be necessary in case of wear or also if a nozzle is to be adapted to a different fluid to be atomized or to different pressure conditions.
In a further development of the invention, the annular gap opening surrounding the nozzle outlet opening is provided between the nozzle housing defining an inside wall of the mixing chamber and an annular gap pipe, whereby—upstream of the annular gap opening—a twist body is arranged between the nozzle housing and the annular gap pipe.
By means of such a twist body, it is possible, on one hand, to impart the annular gap air with a rotation that promotes the most thorough possible atomization at the annular gap opening. In addition, this twist body can ensure an extremely precise annular gap width. This applies, in particular, when the twist body is arranged, close to the annular gap opening, between the annular gap pipe and the nozzle housing. Such a twist body can be designed in a simple manner, for example, in that a disk is provided with several circumferential notches.
In a further development of the invention, a veil-of-air nozzle is provided, said nozzle at least partially surrounding the annular gap opening.
By providing a veil-of-air nozzle, it is possible to prevent a coating from forming on the outside skin of the spray lance and, in particular, also in the region of the nozzle orifice. Such deposits can come from the process environment into which material is sprayed. The veil of air can be heated to such a degree that the temperature will not fall below the dew point at the outside skin of the lance.
The object to be achieved by the invention is also achieved by a cluster nozzle for the atomization of fluids, said cluster nozzle comprising at least two two-substance nozzles in accordance with the invention.
The combination of several inventive two-substance nozzles to form a cluster nozzle provides the possibility of atomizing even large fluid amounts into small droplets, at the same time requiring only a minimum of energy.
The object to be achieved with the invention is also achieved with a method for the atomization of fluids by means of one two-substance nozzle having at least one fluid inlet for gaseous fluid and at least one fluid inlet for the fluid to be atomized, as well as a mixing chamber, said method comprising the following steps:
With the method in accordance with the invention, it is possible to atomize a fluid and, in so doing, achieve not only very tiny droplet sizes but also a highly uniform distribution of droplet sizes. Specifically, it can also be ensured with the method in accordance with the invention that there are no individual large drops in the generated spectrum of droplets that could create problems due to fluid deposition during subsequent process steps. The film of fluid to be atomized on a wall of the mixing chamber is driven in the direction of a nozzle outlet opening by the gaseous stream that is being moved parallel to the wall. At the same time, however, the fluid film can already be partially divided into individual droplets. At the nozzle outlet opening, the fluid film is then drawn out into individual fluid lamellae that are received between the annular gap flow and the air flow from the nozzle outlet opening and are thus reliably atomized into very fine droplets. With the use of the method in accordance with the invention, it is also possible to atomize fluid in a very energy-saving manner, because the film of fluid to be atomized can be generated by means of the kinetic energy of the fluid that is to be atomized and is supplied to the nozzle. The gaseous fluid is moved past the fluid film in the mixing chamber in an essentially parallel manner and, as a result of this, experiences only a minimal loss of pressure. This makes it possible even to work at air pressures of less than one bar overpressure and still achieve small droplets and a uniform droplet size distribution.
In a further development of the invention, the additional step is provided of loading the stream of gaseous fluid with droplets of the fluid to be atomized inside the mixing chamber at least in regions that are remote from the wall with the film of the fluid to be atomized.
In this manner, it can be prevented that part of the gaseous fluid flows through the nozzle without performing any work. Instead, the gaseous fluid is also decelerated in regions that are remote from the wall and thus, at the same time, already performs part of the atomizing function.
In a further development of the invention, a stream of fluid to be atomized is divided into partial streams by means of the flow energy of the stream of fluid that is to be atomized.
In this manner, it is possible, for example, to generate fluid jets only by means of the kinetic energy of the fluid to be atomized, said jets then being partially divided by the gaseous air into droplets and partially forming the fluid film on the wall of the mixing chamber. Consequently, energy demand in the nozzle can be kept very low.
In a further development of the invention, the method in accordance with the invention is provided for generating a veil-of-air stream of gaseous fluid, said veil-of-air stream partially surrounding the annular gap air stream at least directly downstream of the annular gap opening. The veil-of-air stream can be heated.
By generating a veil-of-air stream, it is possible to prevent deposits on the outside skin of the nozzle lance and, in particular, in the region of the nozzle orifice.
Additional features and advantages of the invention are obvious from the claims and the description of preferred embodiments of the invention hereinafter in conjunction with the drawings. Individual features of different described embodiments can be combined with each other in any manner without surpassing the frame and scope of the invention.
First, an essential aspect of the invention is that the fluid is divided into the partial jets 17 by means of the central pin 11 by using the kinetic energy of the fluid to be atomized and that then, by means of the jets 17 impinging on the wall 40 of the mixing chamber 7, a fluid film 29 is formed on the walls of the mixing chamber 7. This fluid film 29, however, is naturally formed on the entire inside wall of the mixing chamber 7 that surrounds the central pin 11.
A gaseous fluid, usually pressurized air, enters the mixing chamber 7 through inlet openings 100 that are defined between the central fluid outlet 102 and the inside wall of the mixing chamber 7. The mixing chamber 7 extends from the inlet openings 100 to a nozzle outlet opening 48. The mixing chamber 7 is arranged inside a nozzle housing 104. The inlet openings 100 are aligned and arranged in such a manner that they move the gaseous fluid into the mixing chamber 7 parallel to the wall 40. The mixing chamber 7 comprises a first section having the length L1 in which it tapers in the form of a hollow cone. In a second section having the length L2, first a point having the smallest diameter N3 is passed, whereby, subsequently at this point, the mixing chamber 7 again widens in the form of a hollow truncated cone until the mixing chamber 7 ends at the nozzle mount or the nozzle outlet opening 22. Still outside the nozzle, further mixing takes place downstream of the nozzle orifice; however, this section is no longer referred to as the mixing chamber of the nozzle. The central axes of the inlet openings 100 are thus aligned parallel to the wall 40 in section L1 of the mixing chamber and are aligned at a small angle of less than 30° with respect to the wall in section L2 of the mixing chamber, this corresponding to the unequal opening angles of the hollow double cone in the sections L1 and L2. Due to frictional forces, the gaseous fluid entering into the mixing chamber 7 drives the fluid film 29 that has formed on the wall of the mixing chamber in the direction of the nozzle orifice 48. A part of the fluid film 29 is already atomized into droplets by the gaseous fluid that flows in the form of a high-velocity gaseous stream past the fluid film 29 in section L1, as is indicated in
Referring to the first embodiment of the inventive two-substance nozzle as shown in
Inasmuch as the central pin 11 does not have a plane end surface but is provided with a trailing body in the form of a tadpole tail 15 having the length LP, water deposits and a backflow region are prevented from forming downstream of the widening section of the central pin 11, whereby the water deposits could detach again in the form of large drops. Thus, the reverse side of the central pin 11 in accordance with the invention is configured as a trailing body in the form of a slim tadpole tail 15 and, as a result, has the shape of a double cone, whereby the length of the widening first cone that is provided with the furrows 14 is substantially shorter and amounts to only approximately one fourth of the length of the trailing body. Furthermore, the form of the flow cross-section in section L1 into the mixing chamber is configured so as to be strongly convergent overall, so that also the tadpole tail 15 is subject to a high shearing stress due to the air flow. Thus, the already small amounts of fluid that are able to reach this section on the tadpole tail 15 are also drawn into thin fluid films that subsequently disintegrate into small droplets.
The central pin 11 may display very different shapes. Instead of a pointed cone as shown in
One important aspect of the invention is that, whenever the entire fluid stream 39 is transferred to the region 51 of the inside wall 40 in the mixing chamber 7, there again is no optimal fluid distribution over the nozzle cross-section in the embodiment of an inventive two-substance nozzle as shown in
Therefore, the invention provides that the furrows 14 on the surface of the central pin 11 be dimensioned in such a manner that not the total fluid stream 39 is converted into discrete fluid jets 17. Rather, between the massive fluid jets 17, thin fluid lamellae 18 are formed, said fluid lamellae opposing the atomizing air with only minimal flow resistance and disintegrating into droplets that are carried along by the pressurized air before they are able to reach the wall 40 in the mixing chamber. As a result of the fact that the pressurized air must accelerate these droplets, it cannot break through into the mixing chamber near the axis, without being hindered. Consequently, the droplet jet 31 forming downstream of the nozzle orifice 48 rather represents a solid conical jet. Without the measure described here, a hollow conical jet would be formed, at least at low fluid throughput of the nozzle.
The film surface becomes instable with high fluid throughputs and with a correspondingly high fluid flow in the fluid film 29 on the wall 40 in the mixing chamber. When the inventor investigated the stability limits of the fluid film, it was found that the instability of a fluid film surface is linked with the occurrence of rolling waves under the influence of a high-velocity air flow. These rolling waves have air inclusions as are also known from rolling waves on the ocean surface. If the air inclusions reach the surface of the film surface, the water-shrouded air bubbles burst. In this way, relatively small droplets form. Furthermore, the droplets rise relatively steeply to the film surface. Consequently, the fluid droplets are transported toward the central axis 50 in the mixing chamber. Up to a certain degree, this is desired for two reasons:
In addition to the form of the furrows 14 on the surface of the central pin, the shape of the region 51 of the wall 40 in the region of impingement of the discrete fluid jets 17 also strongly influences the fluid part that is transported in the fluid film 29 on the wall or by the collection of exposed droplets. With a very flat impingement angle a of the fluid jets 17, said angle is almost fully reflected. Then, again, a high droplet number density close to the central axis 50 of the nozzle is attained and, thus, an inadequate droplet disintegration. At a very steep impingement angle a, the impinging fluid jet 17 bursts and the fluid transfer into the fluid film 29 on the wall is inadequate in this case as well. The optimal angle ranges are a function not only of the flow conditions but also of the substance conditions of the fluid. Therefore, a limiting of advantageous angular ranges is hardly possible. With regard to the angle a between the wall tangent in the impingement region of the fluid jets 17 in the region 51 of the wall 40 and the wall tangent at the central pin 11, a range of approximately 20° to 70° is provided.
Also, the advantageous angles p of the central pin in the first, expanding region and in the region of maximum diameter DP of the central pin 11 vary within a wide range, depending on the boundary conditions. A range of approximately 30° to 90° is advantageous for β. The pin diameter DP must be viewed in relation to the diameter of the fluid entry DLN1 (“L” for liquid and “N” for narrow). The ratio DP/DLN1 should be within a range of two to five.
Also, the cross-sections N2 (N for “narrow” on the annular gap 20 between the pin edge 44 and the mixing chamber wall 51) and N3 (narrow point in the mixing chamber downstream of the tail end of the central pin 11) cannot be freely selected. In order to attain a particularly fine droplet spectrum, the objective will be in many cases to achieve the velocity of sound for the two-phase flow at the narrow point N3. The flow rate of the air should not be too high at the narrow point N2 at the maximum diameter of the central pin 11, because then the fluid leaving the pin edge 44 cannot break through in the region 51 of the wall 40 in the mixing chamber 7, so that there will be no film formation. Also in this instance, the dimensioning rules are highly complex. According to experimental investigations, the ratio of the cross-sections N2/N3 may be within a range of 1 to 5.
Likewise, the ratio of the cross-sections N4/N3 (N3: narrow point of the Laval nozzle; N4: cross-section of nozzle outlet) cannot be freely selected. One must understand that the pressurized air experiences a high loss of pressure in the course of acceleration and atomization of the droplets. Consequently, the density of the pressurized air is reduced on the way through the nozzle. And, with an expanded cross-section in the direction of flow, it is thus possible—even at flows below the velocity of sound—for an acceleration of the gaseous phase to occur. Also in this instance, only guide values can be stated. Depending on the basic concept of the nozzle (overly critical pressure conditions or low-pressure atomization), a cross-section ratio within the range of N4/N3=1 to 3 is advantageous.
Regarding data from cross-section measurements, dimensioning rules for the degree of slimness of the essential nozzle sections are difficult. The curvature of the mixing chamber wall at the narrow point N3 must not be severe, because the fluid film 29 should not—beyond a reasonable measure—detach here from the wall 40 due to inertial forces. Also, a certain moving length is required in order to atomize droplets in free flight. The following dimensioning ranges are to provide guide values:
One very important aspect is also the constructive embodiment of the central pin 11. The pin must be installed precisely centered in relation to the entering fluid jet 39. Also, it must be possible to manufacture the pin of a wear-resistant material such as, e.g., hard metal or silicon carbide.
Specifically, this international patent application states that the annular gap nozzle consists of several secondary air jets that are arranged in the shape of a ring, said secondary air nozzles not only being inclined toward a central longitudinal axis of the nozzle, but, in addition, also being inclined in the same circumferential direction. The central axes of these secondary air nozzles then form the generatrix of a single-shell hyperboloid, and the out-flowing annular gap air is imparted with a twist. The individual secondary air nozzles can be configured as bores; however, it is also advantageous to configure these secondary air nozzles as recesses between components. For example, a conically sloped end of the nozzle housing is provided with recesses in the manner of a conical gear with helical toothing, said recesses then being located at a small distance from the inside wall of an annular gap nozzle.
The mixing chamber has a total length L, because an admixing of droplets detaching from the film surface occurs in the air flow not only in the convergent section L1 but also in the divergent section L2. This section L2, which occasionally is referred to as the discharge section of the nozzle, thus also belongs to the mixing chamber of the nozzle. The droplets are also still mixed and generated downstream and outside the mixing chamber when the fluid lamellae are drawn out at the nozzle orifice and atomized. A mixing region of the nozzles in accordance with the invention thus also comprises the mixing chamber and, in addition, a region downstream of the nozzle orifice.
The sectional view of
In the embodiments in accordance with
A schematic view AB of
As a result of this, three features are achieved:
The illustration as in
In a manner known per se, the annular gap air 34 can be supplied to the annular gap via a separate annular space. This is advisable, in particular considering the aspect of energy consumption, when the pressure of the annular gap air is significantly lower than the pressure of the pressurized air for main atomizing which is injected into the bores 5 having the inlet openings 110. In the embodiment of the inventive two-substance nozzle shown in
The two-substance nozzles in accordance with the invention are suitable for the atomization of solids-containing fluids; of course, they can also be used for the atomization of solids-free fluids.
Within the scope of the invention, different furrow structures, for example corresponding to a three-leaf clover, may be provided on the wall of the fluid nozzle 10. In particular, there is also the option of configuring the furrows not in a manner coaxial with respect to the nozzle axis but to provide a peripheral component. In this case, a twisting of the fluid entering into the mixing chamber is also achieved, so that the fluid nozzle 10 may take over the function of a twist generator at the same time.
Despite the skewed arrangement of the central axes of the inlet openings 110 relative to the central longitudinal axis 50 of the nozzle, it is obvious from
In order to ensure the most precise adjustment of the annular gap air width possible between the inside of the annular gap air pipe 124 and the outside of the nozzle housing 122 and to impart the annular gap air with a twist at the same time, a twist body 128 is interposed—at half the distance between the narrow point 116 and the nozzle outlet opening 120—between the nozzle housing 122 and the annular gap air pipe 124. On the one hand, the twist body 128 abuts against the nozzle housing 122 and, on the other hand, against the annular gap air pipe 124, and thus ensures a highly precise adjustment of the annular gap width. In addition, as already mentioned, a twist is applied by the twist body 128 to the annular gap air in the annular gap air pipe 124. The annular gap width can be adjusted more precisely by means of the twist body 128, the closer said twist body is located near the annular gap opening 126. For example, the twist body 128 may be configured as a disk that is provided with obliquely cut grooves extending from the outside circumference.
The nozzle housing 122 is made of two parts and comprises an upstream section 130, as well as a downstream section 132. The upstream section 130 has the inlet opening 134 for the fluid to be atomized and is provided—upstream of the inlet opening 134—with a connecting flange for a supply pipe 136 for the fluid that is to be atomized. Upstream of the inlet opening 134 is a convergent region; downstream of the inlet opening 134 is a divergent region that then extends up to the wall 114 of the mixing chamber. In addition, the upstream section 130 has several inlet openings 110, whereof, for example, four to eight are distributed over the circumference of the nozzle housing 122. The upstream section 130 ends at a holding strip 138 that projects into the mixing chamber and is fastened to a central pin 140 having the shape of a double cone. On at least two sides, the holding strip 138 connects the central pin 140 to the nozzle housing 122 and is specifically connected to the nozzle housing 122 at the dividing point between the upstream section 130 and the downstream section 132. The upstream section 130 and the downstream section 312 of the nozzle housing 122 are held together by means of a union nut 142. After removing the union nut, the sections 130, 132 of the nozzle housing 122 may be separated from each other, and the central pin 140 may be removed together with the strip 138 and be replaced in case of wear, for example.
By means of a differently configured central pin 140, it is possible to adapt the nozzle to different fluids that are to be atomized. For example, the central pin 140 may also consist of hard metal or ceramic.
Basically, the function of the two-substance nozzle shown in
The trailing body of the central pin 140 is configured and arranged in such a manner that its outside wall extends parallel to the wall 114 of the first section of the mixing chamber. An annular gap width between the wall 114 and the central pin 140 in the first section of the mixing chamber, i.e., up to the narrow point 116, thus remains constant whereas a free cross-section of the mixing chamber is tapering.
During operation of the nozzle, the fluid to be atomized passes the inlet opening 134 and impinges on the tip of the central pin 140. As a result of this, the fluid to be atomized is divided—by means of the fluid's own kinetic energy—into a film flowing along the tip of the central pin 140. This film then leaves the central pin 140 at the widest point 144 of said pin, and the largest part of said film reaches the wall 114 of the mixing chamber. As a result of this, a fluid film is formed on this wall 114, said film subsequently being driven in the direction of the nozzle outlet opening 120 by the gaseous fluid that enters through the inlet openings 110. The gaseous fluid is injected through the inlet openings 110 parallel to the wall 114 and also flows parallel to the outside wall of the trailing body of the central pin 140. In the downstream section of the mixing chamber, i.e., downstream of the narrow point 116, the gaseous fluid impinges at a flat angle of approximately 10° to 15° against the wall 118 in the mixing chamber. This flat impinging angle increases the shearing stress between the gaseous fluid and the fluid film on the wall 118 and, in so doing, ensures that the fluid film is rapidly driven in the direction of the nozzle outlet opening 120.
With an appropriate velocity difference between the fluid film on the walls 114, 118 of the mixing chamber and the gaseous fluid, the fluid film displaying sufficient film thickness will be torn into droplets during its movement through the mixing chamber, as has already been previously explained with reference to the formation of rolling waves. The gas velocity, the shearing stress on the fluid film and the film thickness are decisive for this partial tearing.
Also, after leaving the central pin 140 at its widest point 144, a part of the fluid to be atomized is already disintegrated into droplets because the fluid flowing into the inlet openings 110 must pass the fluid film. In the regions that are at a greater distance from the wall 114, the gaseous fluid is also loaded with droplets, and atomization work must be performed and a deceleration occurs. The atomization work is thus viewed as the sum of the work for generating new fluid surfaces, i.e., the generation of droplets, for example of a solid jet, and/or the disintegration of large drops into small droplets, the work required for the acceleration of the droplets, as well as the work for overcoming the frictional forces between gas and fluid and between fluid and wall. Consequently, it is thus avoided that, in the second part of the mixing chamber downstream of the narrow point 116, a faster core air flow is formed, said core air flow not performing any atomizing work and being loaded or only unessentially loaded with droplets, and leaving the nozzle outlet opening 120 essentially without being utilized. Rather, with the nozzle in accordance with the invention it is possible that also the core regions of the stream are loaded with droplets in the section of the mixing chamber downstream of the narrow point 116 and do not flow faster or not substantially faster than the regions flowing close to the wall 118.
After passing the nozzle outlet opening 120, the fluid film on the wall 118 is then drawn into thin fluid lamellae that then are atomized into fine droplets by means of the gaseous fluid exiting from the mixing chamber and by means of the annular gap air.
As has already been explained, the central pin may also be provided with channels or furrows in order to generate discrete fluid jets that then impinge against the wall 114 of the mixing chamber.
It must be added that a partial disintegration of this fluid film on the walls 114, 118 does not necessarily need to begin already inside the nozzle. In the region of low fluid throughputs, the film is so thin that it could not be atomized even by a supersonic air flow inside the mixing chamber. In such a case, the entire atomization occurs only at the nozzle outlet opening 120 when the fluid film is drawn into lamellae and packed between the central atomizing air exiting from the nozzle outlet opening 120 and the annular gap air stream. The film flow is, in fact, instable only at high fluid flow rates in the fluid film on the walls 114, 118, and a partial atomization occurs already inside the mixing chamber, i.e., long before the nozzle outlet opening 120 is reached.
The nozzle outlet opening 120 is represented by the downstream end of the nozzle housing 122. In order to prevent the adhesion of fluid droplets at the face of the nozzle housing 122, this face that surrounds the nozzle outlet opening 120—the so-called front bank—is made as small as possible. In an embodiment of the nozzle housing 122 in stainless steel, the width of this annular face may be between 0.1 mm and 0.4 mm, in the case of a hard metal embodiment between 0.2 mm and 0.5 mm. Due to the minimal width of this face, the nozzle housing 122 is shock-sensitive in the region of the nozzle outlet opening 120. In order to protect the shock-sensitive front bank of the nozzle housing 122, the annular gap air pipe 124 projects slightly beyond the front bank of the nozzle housing 122 in the direction of flow. With the annular gap nozzle, the width of the face or the width of the front bank is comparatively uncritical because no fluid exits through the annular gap opening 126 and thus it is not possible for fluid droplets to deposit on the front bank of the annular gap air pipe 124. Because the annular gap air pipe projects minimally farther in the direction of flow than the nozzle housing 122, an optimal function of the two-substance nozzle can be combined with insensitivity to shock.
The twist body 154 may be connected to the nozzle housing 158 or even consist of one piece with the nozzle housing 158. In the embodiment of
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 056 784.1 | Nov 2008 | DE | national |
| 10 2009 037 828.6 | Aug 2009 | DE | national |
This is a continuation of prior U.S. application Ser. No. 12/590,527, filed Nov. 10, 2009, the disclosure of which is hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12590527 | Nov 2009 | US |
| Child | 14061300 | US |
