WO2009087423A1

WO2009087423A1 - Improvements in or relating to jet nozzles

Info

Publication number: WO2009087423A1
Application number: PCT/GB2009/050019
Authority: WO
Inventors: John Redding
Original assignee: John Redding
Priority date: 2008-01-11
Filing date: 2009-01-12
Publication date: 2009-07-16
Also published as: GB0800470D0

Abstract

The present invention relates to jet nozzles. In particular, it relates to a swirling jet nozzle for creating a high- velocity thin-film wall jet. The present invention provides a flυidic nozzle designed to create a long-range submerged mildly swirling round jet with a higher velocity centreline part, which subsequently develops into a high- velocity thin-film radial wall jet on impingement against a surface. The purpose of the high- velocity thin-film wall jet is, variously, suitable for use in surface preparation (such as cleaning, etching and removal and application of coatings and adhering particles), and for surface cooling and heating and drying. In particular we describe a fluid nozzle (1) comprising a circular outlet (2) having an outlet bore and an inlet (3) comprising a plurality of generally triangular-section vanes or fins (4) arranged substantially tangentially about a circle and defining a corresponding plurality of radial inlet openings between vanes and fins.

Description

IMPROVEMENTS IN OR RELATING TO JET NOZZLES

The present invention relates to jet nozzles. In particular, it relates to a swirling jet nozzle for creating a high-velocity thin- film wall jet.

It is well-known that when a non- swirling round jet emerges at high- velocity from a submerged nozzle (for instance, an air-into-air or water-into -water jet) the jet rapidly becomes turbulent and spreads laterally as it penetrates into the ambient fluid. Mixing and entrainment at the margins, associated with instability of the free shear layer, are responsible for linear spreading of the jet and for the attendant exponential decay in centreline axial velocity with distance from the nozzle. Since mixing and entrainment are unavoidable there is a natural limit to which jet spreading can be reduced i.e. by careful nozzle shaping, so unless the nozzle is operated proximate to the impingement surface, an impinging non- swirling round jet will inevitably experience a significant reduction in axial kinetic energy, and a corresponding reduction in applied shear stress where it interacts with the surface. In practice, for many applications involving jet impingement, this means operating the nozzle within a few nozzle diameters of the surface.

This behaviour contrasts with an under-expanded (super-sonic) compressed air jet issuing into an ambient, where the jet tends to remain much more compact (less entrainment and mixing with the ambient) and where the impinging wall jet is thinner and consequently has a much higher radial velocity. However, super-sonic air jets, for obvious reasons, have a far more restricted range of applications than that sought for the jet of the present invention. Similarly, modifying the properties of submerged liquid jets, by changing their viscosity or density or sheathing them in other fluids in order to extend their reach and/or improve their impingement behaviour, naturally imposes limitations on their practical utility.

The present invention seeks to provide a fluidic nozzle designed to create a relatively long-range submerged mildly swirling round jet, the centreline part of which subsequently develops into a high-velocity thin-film radial wall jet on impingement against a surface. The adhering high-velocity thin-film wall jet is, variously, suitable for use in surface preparation (such as cleaning, etching and removal and/or application of coatings and adhering particles), and for surface cooling and heating and drying.

Importantly, the jet from the present nozzle can be used at varying distances from a target surface, with significantly less variation of jet impingement momentum, and the jet fluid (either liquid or gas) can be identical to the ambient. Coupled with these desirable attributes, the jet also impinges over a larger footprint area, while applying high radial shear stresses to the surface.

Accordingly, the present invention provides a fluidic nozzle comprising a nozzle body with a circular outlet and a curved tapering outlet bore; a lateral inlet arrangement comprising a plurality of generally triangular-section vanes or fins arranged substantially tangentially about a circle and thereby defining a corresponding plurality of radial inlet openings; and a top plate, with a curved tapering centrebody, that surmounts the inlet vanes and encloses a central swirl chamber circumscribed by the inner cusp-like edges of the inlet vanes.

Advantageously, the nozzle has no moving parts.

Preferably, the curved tapering sectional profile of the outlet bore is that of a pseudosphere or tractrix curve.

Preferably, each vane or fin has radiussed outer edges to improve fluid flow.

Preferably, the vanes or fins are substantially aerofoil-shaped in cross-section.

Preferably, each radial inlet opening defined by the vanes or fins is tapered inwards. Preferably, the curved conical sectional profile of the centrebody, forming an extension of the top plate, is that of a pseudosphere or tractrix curve.

Preferably, the nozzle further comprises a pressure housing within which the nozzle is mounted.

Preferably the apparatus comprises a nozzle as defined above and a means for generating a fluid flow through the nozzle.

In one embodiment, preferably the top plate is surmounted on the vanes or fins, in such a way that a gap is left between the vanes or fins and the adjacent surface of the top plate, so as to permit a controlled amount of radial inflow with no added swirl. More preferably the width of this gap can be changed in order to permit varying amounts of non-swirling radial inflow.

In a second embodiment, preferably the centrebody is truncated and does not extend beyond the central swirl chamber defined by the inlet vanes or fins. More preferably, the end of the truncated centrebody forms a sloping flat surface.

In a third embodiment, preferably the centrebody is truncated and does not extend beyond the central swirl chamber defined by the inlet vanes or fins. More preferably the centrebody has a hollow stem wherein the upper surface of the top plate is in communication with the centrebody tip by means of a circular channel.

The above and other aspects of the present invention will now be described in further detail with reference to the accompanying drawings, in which:

Figure 1 is a side-sectional view illustrating the principle features of a nozzle in accordance with the present invention;

Figure 2 is a plan-sectional view of the nozzle of Figure 1; Figure 3 is a perspective view of the nozzle of Figure 1;

Figure 4 is a side- sectional view of the nozzle of Figure 1 mounted within a pressure housing;

Figure 5 is a side-sectional view of a first variant of a nozzle in accordance with the present invention;

Figure 6 is a cross-sectional view of Figure 5 just below the top plate;

Figure 7 is a perspective view of the first variant nozzle with the top plate removed to show the alternative gap-defining pegs;

Figure 8 is a side view of the top plate and centrebody of a second variant of a nozzle in accordance with the present invention;

Figure 9 is a side view of the top plate and centrebody of a third variant of a nozzle in accordance with the present invention;

Figure 10 shows schematically the fluid flow from the nozzle and the way the fluid is envisaged to impact against a surface;

As background, certain features of the jet from the present invention can be imagined by considering a thin steady stream of liquid falling vertically from a kitchen tap onto the basin surface below. Despite its everyday occurrence, such a free-falling laminar jet and its impingement behaviour provide a useful starting point for visualising a number of the fundamental flow characteristics of the jet from the present invention.

The reader is thus encouraged to perform this simple kitchen-sink demonstration in order to verify the key features described. The tap must be turned on only sufficiently to create a thin continuous steady round stream of liquid, and the basin must be empty. It will be noticed that in addition to its high length-to-diameter ratio, the liquid stream is widest at the point where it leaves the tap and thins progressively as it descends. This convergence of the fluid stream is symptomatic of an increase in jet velocity as the fluid descends. Increase in velocity (acceleration) is because gravity, acting as a body force, is the primary driver of the flow. The jet imparts no reaction

(thrust) to the tap, since there is no excess pressure driving the jet fluid. The liquid stream reaches its highest velocity at the point where it impacts the basin surface. Within the fluid stream the velocity profile is generally considered to be flat (uniform), because there is no friction to retard the outer part of the flow. In practice, there is often a slight excess of velocity on the symmetry axis, which is a legacy of the no-slip boundary condition (and hence parabolic velocity profile) which pertains inside the antecedent pipe-work.

At the point of impingement, the descending free-fall jet turns sharply to become an outward-radial thin- film wall jet that adheres to the basin surface. The radial velocity of this thin-film wall jet is at first equal to the terminal velocity of the descending stream, so by continuity the proximal thickness of the wall jet must be significantly less than the thickness of the descending stream and the adhering wall jet must thin as it extends radially outwards. At approximately half its radial width, the wall jet reaches its thinnest point and then starts to slowly thicken; although this radial variation in thickness is at a sub-millimetre scale and is, therefore, not readily discernible to the naked eye.

At a radial distance of some 18-20 descending jet diameters from the point of impingement, the wall jet suddenly undergoes a substantial thickening and its outward velocity slows appreciably. This feature is known as a circular hydraulic jump and it marks the transition from supercritical laminar flow to subcritical turbulent flow.

Note that the radial position of the circular hydraulic jump is only indirectly related to the pre-impingement diameter of the jet. It is, however, a direct function of the volume flow rate within the jet. If an erodible coating (such as a thin layer of emulsion paint) is applied e.g. to a flat plate, which is then interposed just above the basin surface, features of the instantaneous erosion capability of the free-fall jet are revealed. It will be noticed that the paint is removed: rapidly and completely from around the impingement area first and then somewhat more slowly (but still completely) with increasing radial distance; but note there is no erosion beyond the circular hydraulic jump position.

Anyone who has tried to remove emulsion paint from a surface simply by washing under water will know that it is quite difficult to do, because the paint adheres to the surface. It is not the volume of water that is important for paint removal, rather the velocity of flow immediately adjacent to the coated surface. This is because the rate of surface erosion varies as a higher power of the near-surface flow velocity (the friction or shear velocity) in excess of the threshold velocity at which erosion is initiated. The thin- film wall jet, as exemplified by the simple kitchen-sink experiment, thus provides clear evidence of the efficacy with which a small fluid flow volume (when correctly applied) can be used for removing surface material. Similar principles also apply to the removal of heat from a surface.

It should additionally be observed that when the plate in the above experiment is tilted, the wall jet (perhaps surprisingly) appears to run uphill and the circular hydraulic jump feature then becomes more elliptical. The plate can be tilted at quite a steep angle with the wall jet continuing to flow uphill, although to a lesser distance. Beyond the hydraulic jump the direction of flow is invariably downhill.

Turning now to the figures and starting with Figures 1 to 4 it will be observed that the invention comprises a nozzle (1), with a circular outlet (2) and a series of vertical slot- like inlets (3). The slot-like inlets, of which eight are shown, are formed by a corresponding number of triangular-section vanes or fins (4) set out in a circle coaxial with the nozzle symmetry axis, and with their cusp-like inner edges (5) projecting inwards at a tangent angle. The two outer edges of each of these vanes are radiused to promote smooth entry flow of fluid into the slots. It will be seen from the cross- sectional view in Figure 2 that the inlet slots are in fact tapered, with adjacent vane faces forming an angle (6) of approximately 10°, and with the narrowest part of each slot being innermost and corresponding in position to the cusp-like edge of each vane.

The vanes are mounted on a lower body component (7), which also incorporates the nozzle outlet. The vanes may be machined as part of the lower body by some appropriate metal-removing and shaping process, or they may be machined separately for subsequent attachment to the lower body. To complete the main components of the nozzle itself, there is a top plate (8), which is jointly secured to the vanes and to the lower body. If the vanes are separate, the top plate, vanes and lower body can conveniently be held together by bolts that extend through vertical holes drilled in the vanes and into the lower body. The positioning of the holes through the vanes can be such as to provide a convenient fulcrum point about which the vanes can be rotated for adjustment purposes.

Machining the vanes directly into the lower body has the merit that the vane orientation and the width of the slots is fixed. Machining the vanes separately, for instance as a continuous bar and then cutting them to length, makes for easier fabrication and also allows for some adjustment of the vane orientation and slot width should this be required. If the vanes are machined directly into the lower body, the top plate may be attached by alternative means than bolting, for example, by projection welding.

The method of securing the main nozzle components together is not important to a basic understanding of how the nozzle works and is not shown in Figures 1 to 4. However, the vanes should maintain, in use, a fixed tangent orientation, with all the inlet slots having the same width. The nozzle fabrication method does, however, become important in relation to a number of variants of the basic nozzle as discussed later. The inner faces and cusp-like edges of the vanes are tangent to and circumscribe a cylindrical inner chamber (so-called swirl chamber (9)). It will be evident from this design that flow entering slots (3) will be forced to rotate within swirl chamber (9) before exiting the nozzle through nozzle outlet (2). The outlet bore (10) formed in the lower body and which connects swirl chamber (9) with nozzle outlet (2) has a smoothly curved tapered shape. The particular curved shape is that of a pseudosphere or tracticoid (tractrix curve-of-revolution). A pseudosphere is a form of hypobolic manifold with a curvature in two planes. Seen in vertical section (in Figure 1), the surface curvature of the bore intersects the upper flat surface (11) of the lower body at a shallow but finite angle. This point of intersection, when seen in plan (see Figure 2), forms a circle (12), which is fractionally smaller in diameter than the cylindrical swirl chamber.

The pseudospherical surface of outlet bore (10) is continuous with the nozzle outlet (2); but note that opposite sides do not attain a parallel condition within the length of the bore. This is because the sides of a pseudosphere, or tractrix curve, only become parallel to the long axis at infinity. Note also that in the case of the nozzle of this invention, if the pseudospherical curvature of the outlet bore were to extend to infinity the diameter so-formed would be only fractionally less than the nozzle outlet diameter.

The pseudospherical curvature and consequent tapering of the outlet bore is designed to both turn the inlet flow smoothly through a near right-angle and at the same time accelerate the flow by causing radial convergence of the flow streamlines. Note that radial convergence is at first rapid and then slows down with distance through the nozzle.

A similarly pseudospherically-curved or tracticoid centrebody (13) forms an extension of top plate (8), which projects downwards into cylindrical swirl chamber (9). This centrebody has a base diameter the same as the lip diameter of the outlet bore. Circle (12) in Figure 2 thus relates to both the centrebody and the outlet bore. However, unlike the outlet bore, the centrebody taper would, if it were extended, come to a point or cusp (i.e. the curvature would meet the long axis) at infinity. However, for practical reasons, the centrebody tip (14) is truncated at a certain distance from the lower surface of the top plate, as indicated generally in Figure 1. It is evident, when seen in vertical sectional profile in Figure 1 , that curved centrebody

(13) also helps to turn the inlet flow smoothly through a near right-angle. However, it has several additional functions, which to some extent depend on its length and the detail of its construction; these will be discussed shortly.

To complete the major components of the nozzle there is a pressure housing (15) into which the nozzle is inserted and to which it is securely attached. The pressure housing is shown in Figure 4. The purpose of pressure housing (15) is to convey pressurised fluid, from any suitable supply source, to the nozzle inlet slots. The means of connection of the nozzle to the pressure housing might be by threaded connection or spigot and bolted flange (as shown in Figure 4). The internal shape of the pressure housing is such as to create a smooth flow-path for the fluid from the supply inlet (16) to the nozzle inlet slots (3).

A key feature of the way in which the nozzle operates is that the pressure of the supply fluid is entirely dissipated within the nozzle itself. Thus fluid emerging from nozzle outlet (2) does so substantially at ambient pressure, there being no residual over-pressure driving the jet. This means that the nozzle will work equally well with incompressible liquids and compressible gases. This is not to say that there are no differences in pressure within the jet flow - there most certainly are - but these are dynamic features of the flow. In order to reinforce the concept of the nozzle as a pure momentum (fluid motion) source it should be noted that the nozzle will work tolerably well if the pressure housing is removed and the nozzle is simply made to rotate about its symmetry axis; anticlockwise in the case of the vane configuration shown in Figure 2. In this case, fluid enters and leaves the nozzle at ambient pressure and is forced to flow through the nozzle purely by rotation. It is important to note also that the various figures showing the geometry of the nozzle are drawn to a true scale in deference to the fact that the nozzle geometry is critical to successful working of the nozzle. In particular the aggregate area of the inlet slots, versus the swirl chamber diameter and the nozzle outlet diameter, determine the ratio of mean swirl velocity to mean axial flow velocity of the issuing jet, otherwise known as the swirl number. The flow swirl number is a dimensionless parameter which has an important bearing on the stability of the exit flow. Swirl number determined solely on the basis of nozzle geometry is known as geometric swirl number (S_g). For the present invention the geometric swirl number, defined as:

S_g = πdD ALns where: d is the nozzle outlet diameter

D is the swirl chamber diameter L is the length of the slots n is the number of slots s is the width of the slots

is 0.23, based on the relative dimensions of the nozzle shown in Figures 1 to 3. Note that the geometric swirl number parameter can be varied in this device simply by changing the length of the vanes or by changing their orientation. Note also that changing the orientation of the vanes (for instance by rotation about the point of fixity) has the effect of changing both the slot width and the swirl chamber diameter, which tend to counteract one another. Sg has been purposely kept below 0.25 with the nozzle design shown in order to reduce the likelihood of vortex breakdown, which is known to occur above this level in similar apparatus with flat top plates (endwalls).

If vortex breakdown were to occur within the nozzle it would completely change the character of the jet and also prevent the beneficial impingement behaviour described. Thus the amount of swirl, as determined by the nozzle geometry, is critical - too much and its effect can be detrimental. With apparatus, including the present device, which are designed to create a vortex jet by swirling flow convergence over an end wall, a centreline jet forms due to the local eruption of viscous boundary layer fluid onto the vortex axis. This centreline jet, with locally higher axial velocity than the rest of the jet, is driven essentially by swirl- induced low pressure on the vortex axis and, specifically, by a favourable pressure gradient along the flow path through the nozzle. Since the centreline jet also convects the axial vorticity responsible for the swirl, it is evident that forced convergence of the flow can create what amounts to a self-feeding process: whereby increased rarefaction drives a higher velocity centreline jet, which increases the swirl, which increases the rarefaction, and so on. With a flat end wall (i.e. no centrebody) the intensity of this self-feeding process is such that the accelerating centreline jet becomes very peaky and the flow rapidly becomes unstable. With the present invention, the internal shape of the nozzle, with it curved outlet bore and matching curved centrebody, is designed to both assist flow convergence, and increase the amount of viscous boundary layer fluid deposited on the flow axis. It achieves the latter by virtue of a larger surface area (compared to a flat end wall surface) for viscous boundary layer flow development. Specifically, the surface area of the pseudospherical centrebody cone (13) in Figure 1 is very nearly twice that of the subtended base circle (circle (12) in Figure 2). This is because a hemi-pseudosphere has the same surface area as a hemisphere. The additional viscous (turbulent) boundary layer flow component helps to offset the effect of flow convergence since increased turbulence equates to increased pressure, which counteracts the swirl-induced low pressure. Although the centreline axial jet that forms with the present invention has a somewhat rounded axial velocity profile it is, nevertheless, susceptible to both cavitation (in water) and shear layer instability. Cavitation along the flow axis is the result of swirl-induced low pressure. Shear layer instability results from the fact that the centreline axial jet has insufficient vorticity to sustain the high velocity gradient at the margins of the jet, and it manifests itself by precession and/or filamentation of the jet. Precession is where the axial jet starts to corkscrew and gyrate around the axis; filamentation is where the axial vortex tubes start to unravel and separate from the flow axis. In addition, it has been found that the basic nozzle tends to emit a high-pitch whistle (vortex whistle). All four of these features (cavitation, filamentation and/or precession and vortex whistle) are considered undesirable for the applications envisaged for the nozzle.

Further modification to the centrebody and/or the inlet flow is, therefore, required in order to create the desired jet, which most closely matches the behaviour of the (non- swirling) free-fall laminar jet described earlier. Note that the purpose of these modifications is to manipulate the flow so that the centreline jet simulates the behaviour of the (non- swirling) laminar jet, rather than the swirling jet as a whole. These modifications are intended to: i) reduce the overall intensity of the swirl, ii) increase the width of the centreline jet and at the same time iii) create an alternative source of centreline jet momentum.

The first modification (shown in Figures 5 to 7) involves creating a small gap (17) between the top of the vanes (4) and the underside of the top plate (8). The purpose of this gap is to allow inlet flow to bypass the tangential inlet slots and so enter the nozzle with depleted angular momentum (compared the bulk of the inlet flow). Since this gap-flow is destined to become the centreline part of the jet, it is evident that the width of gap (17) effectively determines the half- width of the centreline jet, which will in turn become depleted in angular momentum. In practice, a gap of 0.1 to 0.25 times the nozzle diameter has been found to give optimum jet characteristics. The non-swirling gap-flow is effectively separated from the swirling outer flow by a vorticity layer that originates as a system of helical vorticies (tip vortices) shed from the upper trailing edge of each vane.

In the case of a nozzle held together by bolts (18), shown in section in Figures 6, running through holes in the vanes, gaps of any desired width can be created simply by inserting small diameter washers (19) between the vanes and the top plate. However because the diameter of the bolts, and thus of the washers, is necessarily large compared to the width of the gap they tend to have a blocking effect on the flow through the gap. To reduce this blocking effect small pegs (20) can, alternatively, be used, as indicated in Figures 7. Formed into the tops of the vanes, as part of a mono- bloc fabrication of the nozzle body and vanes, pegs (20) provide an alternative means for attachment of the top plate by - projection welding.

The second modification (see Figure 8) involves truncating centrebody (13) so that its length is approximately equivalent to that of the vanes, and the diameter of the tip (14) is approximately 0.3 times the diameter of the outlet (2). Reducing the centrebody length by this amount does not significantly reduce the latter 's surface area, nor the amount of turbulent boundary layer fluid produced. Rather than truncate the centrebody at right angles to the axis of the nozzle, however, the cut is made at an angle (21) of between 30° and 45°. This has the effect of creating a pointed tip (14) that is offset from the nozzle axis, with the result that the vortex is initialised at this offset point and develops as a stationary helix with a diameter approximately equal to the centrebody tip diameter. The turbulent boundary layer fluid, shed from the tip of the centrebody, forms an axial jet surrounded by this helical vortex. Because the helical vortex develops as a result of tilting of the centreline vortex, part of the latter's axial vorticity is converted to azimuthal vorticity. This has the desirable effect of reducing the swirl intensity of the vortex jet as a whole, while at the same time enhancing the axial velocity of the centreline jet.

The third modification (shown in Figure 9) also involves truncating the centrebody so that, like the second modification, its length is approximately equal to that of the vanes. This time the cut is made at right angles to the nozzle axis and a circular central channel (22) is drilled along the centrebody axis to connect the upper surface (23) of the top plate with the centrebody tip (14). This central channel has a radiused inlet (24) leading to an over-sized section (25), which then reduces to a smaller diameter section (26) whose diameter is fractionally smaller than that of the tip. The purpose of the over-sized section is essentially to relieve the length of drilling of the smaller size hole that connects to the centrebody tip. It may also contain a flow straightening device (not shown in Figure 9). The purpose of the resulting hollow- stem centrebody is to inject high velocity non- swirling fluid directly onto the axis of the flow. This has a number of desirable consequences in terms of flow topology and impingement behaviour, which are essentially identical to those achieved with the other two modifications.

An important feature of first and third modifications is that the additional (non- swirling) fluid introduced into the nozzle enters the nozzle at the same pressure as rest of the inlet flow. This is critical from the point of view of dynamic flow equilibrium within the nozzle and the overall stability of the emitted vortex jet. Note that the above-described modifications may also be used in various combinations as a way of 'fine-tuning' the jet for different applications. The third modifications may additionally be used as a means for introducing different fluids (or even suspended particles) onto the jet axis.

Referring to Figure 10, a brief explanation will now be given of the form of jet created by the nozzle of this invention. While the jet may differ in detail between the different modifications, the overall topology, and the way the jet behaves during impingement against a surface, is the same., The main part of Figure 10 shows the overall 'shape' of the jet, as well as the axial velocity profile at two locations within the free part of the jet. The centre and right panels of Figure 10 show the swirl velocity and near-axis vorticity profiles, respectively, at the same two locations. The horizontal lines marked by a 0 represent the zero velocity axes and the vertical lines marked by C/L represent the centreline of the flow.

As the flow emerges from the nozzle outlet (2) its axial velocity is characterised by a uniform part (27) and a centreline jet-like part (28). The uniform part (often referred to as a top-hat velocity profile) results from the acceleration of flow through the nozzle due to flow convergence in the tapering outlet bore (10). The very steep gradient in axial velocity at the free-shear outer margins (29) of the jet, and the associated strength of embedded azimuthal vorticity, initially sustains the columnar shape of the jet. However, with distance from nozzle the jet spreads, as indicated by the line (30) marking the outer extremities of the jet, and the gradient in axial velocity at the margins becomes shallower as mixing takes place. This is indicated by the change in the outer part of the axial velocity profile (31). The centreline jet (28) also has a steep marginal gradient in axial velocity and an overall flat to rounded profile on the centreline. Importantly, the width of this centreline jet does not increase (significantly) with distance from the nozzle and thus the centreline axial velocity is maintained with distance from the nozzle. This is primarily because the flow axis represents neither a sink nor a source for radial flow. The near-axis vorticity profile ((32) see right-hand side of Figure 10) forms twin peaks, compared to a single peak centred on the axis with a normal swirling jet. The vorticity is represented by both axial and azimuthal components in the form of helical vorticity. The form and position of this helical vorticity is represented (diagrammatically) by the helical structure (33) shown in the main part of Figure 10. Since this vorticity is embedded in the free shear layer at the margins of the centreline jet, it effectively forms a streamtube (34) enveloping the centreline jet. It is the strength of the axial component of the helical vorticity, acting as a motion source (rather swirl- induced low pressure), which effectively drives the centreline jet. The effect of offsetting the vorticity peak from the flow axis, to form this helix, is to produce a swirl velocity profile ((35) see centre panel of Figure 10) which crosses the centreline at virtually right angles on a curving trajectory (indicating depleted angular velocity in the centreline area), rather than at an angle with a straight profile (indicating solid-body rotation). Despite spreading of the outer jet margins, there is little change in the near-axis vorticity and swirl velocity profiles with distance from the nozzle and only a slight spreading of the centreline jet.

Provided that the impingement surface is not more than about 10 nozzle diameters from the nozzle, the centreline jet strikes the surface (36) with an axial velocity that is not significantly different from that at the nozzle. Even though the outer part of the jet is forced to spread very rapidly as it approaches the impingement surface, the centreline jet behaves more like the previously mentioned free-falling jet. Following impingement the centreline jet forms a high- velocity thin- film wall jet (37) that adheres to the surface and spreads radially to a point (38) where it becomes unstable and separates from the surface. Note, that in Figure 10, separation from the surface is shown on only one side, although with orthogonal impingement the process would be symmetrically disposed about the point of impingement. Separation is essentially a result of instability of the vortex sheet (39) that separates the thin- film wall jet (37) from the overlying outer wall jet flow (40). This vortex sheet is an extension of the streamtube (34) that envelopes the centreline jet, which becomes re-aligned and stretched in the wall jet region. Separation is associated with a near-surface inward flow (41) of ambient fluid and also the formation of eddies (42), which effectively dissipate the outward radial velocity of the wall jet.

While Figure 10 shows the jet pointing vertically downwards, it should be noted that the jet from this invention can actually be pointed in any direction, unlike the free- falling jet.

Claims

CLAIMS:

1. A fluidic nozzle (1) comprising a circular outlet (2) having an outlet bore (10) and an inlet comprising a plurality of generally triangular-section vanes or fins (4) arranged substantially tangentially about a circle and defining a corresponding plurality of radial inlet openings (3) between vanes or fins (4).

2. A nozzle as claimed in claim 1 wherein each vane or fin (4) has radiussed outer edges to improve fluid flow.

3. A nozzle as claimed in claim 1 or claim 2 wherein the vanes or fins (4) are substantially aerofoil-shaped in cross-section.

4. A nozzle as claimed in Claim 3 wherein each vane or fin is provided with cusp-like inner edges that collectively define an inner swirl chamber (9).

5. A nozzle as claimed in any one of claims 1 to 4 wherein each radial inlet opening (3) is tapered inwards.

6. A nozzle as claimed in any one of claims 1 to 5 wherein the outlet bore (10) has a curved tapering sectional shape.

7. A nozzle as claimed in Claim 6 wherein the curved tapering sectional shape is that of a pseudosphere or tracticoid.

8. A nozzle as claimed in any preceding claim wherein the swirl chamber (9) is further enclosed by a top plate (8) having a curved conical centrebody (13) that projects into swirl chamber (9).

9. A nozzle as claimed in claim 8 wherein the conical centrebody (13) has a pseudospherically-curved or tractricoid shape.

10. A nozzle as claimed in any one of claims 1 to 9 wherein a small gap (17) is formed between the top of each vane or fin (4) and the underside of top plate (8).

11. A nozzle as claimed in claim 10 wherein the width of small gap (17) is between 0.1 and 0.25 times the diameter of outlet (2).

12. A nozzle as claimed in claim 8 or claim 9 wherein the curved centrebody (13) has a truncated tip (14).

13. A nozzle as claimed in claim 12 wherein the truncated tip (14) is truncated such that the tip (14) does not extend into outlet bore (10).

14. A nozzle as claimed in claim 13 wherein the truncated tip (14) is cut at an angle such that the flat face slopes at an angle of between 30° and 45° to the axis of the nozzle.

15. A nozzle as claimed in any one of claims 8, 9, 12 or 13 wherein the top plate (8) includes a hollow-stem centrebody (13) wherein an upper surface (23) of the top plate is in communication with the tip (14) by means of a channel (22).

16. A nozzle as claimed in claim 15 wherein the ho How- stem centrebody is adapted for introduction of chemical reagents and/or suspended solid particles onto the centreline of the primary flow to augment cleaning, scouring and etching applications.

17. A nozzle as claimed in any preceding claim further comprising a pressure housing (15) within which the nozzle is mounted.

18. An apparatus comprising a nozzle as claimed in any one of claims 1 to 17 and further comprising a means for generating a fluid flow through the nozzle.

19. An apparatus as claimed in claim 18 for producing a round, mildly swirling, submerged, fluid jet with a higher velocity centreline part and having long-reach capability in a free-jet condition.

20. An apparatus as claimed in claim 19 wherein the jet creates an adhering high velocity thin- film wall jet upon impact with a surface.

21. An apparatus as claimed in any one of the claims 17 to 19 producing an impinging fluid jet having steady continuous thin- film high- velocity flow over a surface.

22. An apparatus as claimed in Claim 21 for cleaning, scouring, etching, cooling/heating or drying the surface.