DIFFUSOR

Abstract: A diffusor or nozzle comprises a cross section-wise endless wall (5), which widens from an inlet (6) for arriving fluid to an outlet (9) for outgoing fluid. At least a distance upstream the inlet (6), the wall (7) which is passed by the arriving fluid has a relief-like surface structure (10) constructed with projections and alternating countersinks, having the purpose of bringing about small turbulent movements of the component parts of the fluid which pass in the immediate vicinity of said wall (7) so as to counteract separation of fluid from the widening diffusor wall (5).

Description: DI FFUSOR Technical Field of the Invention This invention relates to a diffusor including a cross-section-wise endless wall, which widens from an inlet for arriving fluid to an outlet for outgoing fluid. Background of the Invention A fundamental purpose for diffusors of the kind gen- erally described above is to bring about pressure recovery of a fluid flowing through, more precisely by reducing the speed of an arriving fluid having a comparatively low pressure, while obtaining an outgoing fluid having a lower speed and a higher pressure. In one version, the diffusor exists as a sim- pie tube, which diverges from a thinner pipe for the arriving fluid to a thicker tube for the outgoing fluid. In other versions the diffusor constitutes a part of a nozzle which on the inlet side thereof has an arched, most often trumpet-like, surface converging to a neck having a minimal cross-section area. In these two versions, the diffusor may be of a two- dimensional, tapered or ring-shaped type. Common for all diffusors is that they are included in some kind of flow-carrying structure, the fluid being present in an arbitrary form, e.g. as water, water steam or the like. In practice, diffusors are found in a large number of nuclear power plants, in particular in steam generators or in the dome of reactor containments, and then in the shape of nozzles. The present invention originates from steam generator nozzles for nuclear power plants. For this reason, the inven- tion will be described below with reference to such nozzles, although the general idea according to the invention is also applicable in other arbitrary diffusors included in flow-car ¬ rying structure. A problem, which is difficult to master in the steam generators of nuclear power plants, is that unstable flows during certain circumstances may arise in the nozzle together with the subsequent, outgoing pipe included in the roof of the generator building. This risk for unstable flows is especially marked when said pipe is angular; something which usually is the case, because of the fact that the building-in space above the generator is limited. In the technology area in question, there are different theories about the reasons for the emergence of the unstable flows as well as how the problems should be fought (see, among others, Cutler, A.D. and Johnston J.P.: "The effects of inlet conditions on the performance of straight-walled diffusers at low subsonic Mach number". A review Report PD-26, Stanford University, Stanford, CA, USA, October 1981. Johnston, J.P.: "Internal Flows Chapter 3 in Turbulence 2nd Ed., Editor P. Bradshaw, Springer-Verlag, Berlin-Heidelberg-New York 1978) . These theories, supported by experiments and calculations, show that at least one factor which contributes to the emergence of unstable flows is constituted by the so called separation in association with the diverging wall in the diffusor. Such separation manifests itself in that individual particles included in the fluid suddenly comes off from the wall and are thrown into the flow, where they give rise to vortex formations and/or turbulence phenomena. The unstable flows, which arise every now and then, give rise to vibrations in large parts of the flow-carrying structure and in particular in the pipe being positioned upstream the diffusor. These vibrations in turn expose the structure to mechanical stresses that may be risky. In particular there is a risk that existing welds are fatigued. Different solutions have been proposed to the above- mentioned problem. The idea of one proposal is to powerfully reinforce the fastening of at least those parts of the flow- carrying structure, which are housed within the reactor containment by mechanical means. However, this is an extremely costly alternative that is complicated and time-consuming to carry out in existing nuclear power plants. The idea of another proposal is to mount a rotatable (rotating in the same alternatively the opposite direction) vortex producer upstream the nozzle. Such vortex producers have, however, proven to have a moderate effect in respect of the ability to counteract separation in the diffusor. Objects and Features of the Invention The present invention aims at obviating the shortcomings of the above-mentioned proposals to solutions of the problem and at creating an improved diffusor. Thus, a primary object of the invention is to create a diffusor which, with simple and inexpensive means, is able to effectively counteract the emergence of separation phenomena within the same and which thereby reduces the risk for unstable flows arising in the diffusor together with the subsequent outlet pipe. Another object is to enable achievement of the desired result, i.e. to avoid unstable flows, by revising existing diffusors afterwards . According to the invention, at least the primary object is attained by the features defined in the characteriz- ing clause of claim 1. Preferred embodiments of the invention are furthermore defined in the dependent claims. Brief Description of the Appended Drawings In the drawings: Fig 1 is a vertical section through the upper part of a steam generator together with a diffusor in the shape of a nozzle included therein as well as a pipe exist ¬ ing downstream the nozzle, Fig 2 is an enlarged section through a nozzle of principally the same type as the nozzle according to fig 1, Fig 3 is a section through a simple diffusor between two different thick tubes, Fig 4-8 are extremely enlarged cross-sections through surface portions of the inlet side of a nozzle made according to the invention, and Fig 9-13 are schematic views illustrating different geometric designs of bead-shaped projections included in said surface portions. Detailed Description of Preferred Embodiments of the Invention In fig 1 numeral 1 designates a roof-forming upper part to a steam generator, in other respects not closer shown, a diffusor in the shape of a nozzle generally designated 2 being included in said roof part. An outgoing pipe 3, which is angularly shaped, is connected to the nozzle 2. Fluid in the form of water steam is fed with high speed (usually more than 100 m/s) from the interior of the generator by the nozzle 2 and further on through the outgoing pipe 3, as is outlined by the arrows. In a conventional way, the nozzle 2 includes a diverging cavity 4, which in the example is supposed to have a curtailed tapered basic shape, i.e. the cavity is delimited by a conically shaped wall surface 5. In the area of the bottom part of the nozzle, there is an inlet 6, most often denomi- nated neck, having a smallest cross-section area or diameter. The cavity 4 widens on the downstream side of this neck. On the upstream side of the neck 6 a rotary symmetric, arched guiding surface 7 is found which widens in a trumpet-like way from the neck in the direction towards the interior of the steam generator. It should also be noted that the pipe 3 is connected to the outlet end of the nozzle 2 by a weld 8. In the shown example, the pipe 3 has a larger diameter than the outlet mouth 9 of the cavity 4. Only by way of an example, it should be mentioned that the neck 6 may have a diameter in the magnitude of 400 mm at the same time as the outlet mouth 9 has a diameter of approx. 550 mm. For a given length of about 560 mm, the angle θ (see fig 2) will then attain 7,5°. In other words, the cone angle 2θ is in such cases 15,0°. For such dimensions, the pipe 3 may have an inner diameter of 750 mm and the thickness of the tube wall may be within the range of 50-80 mm. Concerning the cone angle 2θ it should be pointed out that the tendency to separation in the water steam flowing through, generally seen, increases with an increasing cone angle. As far as the shown nozzle 2 has been described hitherto, the same is in all essentials previously known. Characteristic for the present invention is that the fluid guiding surface 7 which defines the cavity 4 of the noz- zle has a relief-like surface structure 10 constructed with projections and alternating countersinks, which extends at least a distance upstream the neck 6. This relief-like, i.e. non-smooth, surface structure has the purpose of bringing about small turbulent movements of the component parts of the steam which passes in the immediate vicinity of the wall surface upstream the neck. Therefore, when the steam, after having passed the neck, enters into the cone-shaped cavity 4, turbulence movements, which successively die away, are formed in the interface to the wall surface 5, but which counteract the tendency of individual component parts to suddenly and abruptly come off from the wall surface. This turbulence improves the possibility to exchanges (admixture) transverse to the direction of flow and decreases the skewness or peak- ness in the speed profile. As will be described closer with reference to figures 4 to 13, the relief-like surface structure 10 may be realized in various ways. However, independent of the nature of the surface structure, it should extend across a considerable area on the upstream side of the neck. Thus, it is outlined in fig 1 and 2 with thick lines how the surface structure extends from a periphery 11 (in the example circular) all the way to the neck 6. In practice, the outer diameter of the surface structure is 2-4, suitably 2,5-3 times larger than the diame- ter of the neck 6. In fig 3 is illustrated schematically how a relieflike surface structure 10 may also be provided upstream the inlet or the neck of a simple diffusor in the shape of a cone- shaped tube 2' between two tubes having differently large cross-sections. Also in this case, the surface structure 10 extends a considerable distance upstream the thin inlet to the diffusor, i.e. inside the thin tube, which joins the diffusor. Reference is now made to fig 4-8, which illustrate different, alternative embodiments of surface structures. Thus, in fig 4 is shown how the relief-forming projections may be in the shape of long narrow beads or ridges 12, between which countersinks 13 are delimited. In this case the beads are in the main as wide as the countersinks 13. In fig 5 is shown how the beads 12 are considerably thinner than the coun- tersinks 13. In practice, the width of the individual bead may amount to 10-40 % of the width of the countersink. The depth or height of the individual bead is according to calculations made dependent upon the diameter of the neck 6. Thus, the height of the individual bead should be within the range of 0,05-0,5, suitably 0,20-0,30 % of the of the diameter of the neck. Most preferably, the height of the bead is selected to approx. 0,25 % of the diameter of the neck. In other words, the bead should be of a height of approx. 1 mm in case the diameter of the neck amounts to 400 mm. More generally expressed, the height of the bead may also be said to be connected to the state of flow expressed by the flow parameter Reynold's number R e = UD/υ. Proceeding with the information about the turbulent boundary layer flow and the so-called Taylor scale, the height (λ) of the bead could be expressed by the equation: λ = k • (Re) "0,5 where k is of the magnitude of 1 to 10. In fig 6 is shown how the irregular surface structure may be in the shape of a saw tooth pattern. In this case, the individual bead is provided by a projecting surface 12 extending substantially radially in relation to the arch of the guiding surface 7, and a subsequent countersink being delimi- ted by an oblique surface 13. In fig 7 projections are shown in the shape of a plurality of beads having cross-section-wise triangular shape, which are separated by means of smooth intermediate surfaces. In all embodiments according to fig 4 to 7, the pro- jections or beads are intended to be provided by machining the material in the inlet part of the nozzle (e.g. milling, spark machining or the like) . In fig 8 an alternative embodiment is shown according to which the individual beads 12 consist of separate bodies that in a suitable way are applied in grooves in the arched guiding surface of the nozzle. In analogy with the embodiments according to fig 6 and 7, the outside of these bead bodies may be oblique so that the individual body slenders cross-section-wise in the direction downstream. In other words, the front surface of the individual bead body will form an abrupt flow obstacle, which effectively brings about limited or low-intense turbulence formation in the interface between the fluid and the guiding surface 7. The same type of surface structure or pattern which is shown in connection with a convexly arched surface in fig 4 to 8 may of course also be used in the interior surface structure in the slender, cylindric tube upstream the diffusor 2' according to fig 3. In fig 9, which shows the surface structure 10 accor- ding to fig 1 in the direction from below, is illustrated how the beads 12 may be in the shape of circular, concentric rings having differently large diameters. In fig 10 an embodiment is shown having one single continuous bead 12 which is helical. This helical shape implies that individual sections of the bead extend somewhat obliquely in relation to the axial direction through the nozzle. In that manner, the bead will apply to the arriving fluid or water steam a certain tendency to rotate in relation to the geometric centre axis of the nozzle. This means that the fluid coming in to the cavity 4 will not only involve turbulence movements in the interface to the wall surface 5, but also a certain rotary movement. In fig 11 an embodiment is shown according to which the surface structure is formed of beads 12 which extend from the periphery in the direction towards the central neck 6. More precisely, the individual bead is straight viewed in the plane projection according to fig 9, but extends in an oblique angle to the centre axis of the nozzle. By this obliquity of the beads, a rotary movement will also in this case be applied to the fluid. In other words, the different beads jointly serve as a rotation generator for the arriving fluid. The embodiment according to fig 12 differs from the embodiment according to fig 11 only in that the individual bead 12 is at least partly curved instead of straight. In fig 13, finally, an embodiment is visualised according to which beads are oriented at an angle to each other and across each other. Preferably, the beads extend obliquely in relation to the axial direction of the diffusors. In the example shown, the beads crossing each other define countersunk areas with a romboid shape. In the above-described embodiments, the relief-like surface structure is provided as an integrated part of the nozzle. However, it is also feasible to form the desired sur ¬ face structure in a separate supplementary part, which may be mounted in association with the inlet of a nozzle, which in a conventional way has smooth surfaces. Although the above diffusor according to the invention has been exemplified in the shape of a nozzle included in a steam generator, it is also applicable in other contexts, e.g. in connection with reactor vessels or other pressure vessels. Although the irregular surface structure according to the invention advantageously is provided by means of long narrow beads, it is also possible per se to bring about the desired turbulence formation by means of individual small projections in the shape of warts, tumblings or the like. It should also be mentioned that the relief-like surface structure does not necessarily need to end exactly in the neck, i.e. the portion of the passage, which has a minimal cross- section. Thus, the surface structure may end a distance upstream as well as a distance downstream the neck.

Claims: Claims 1. Diffusor including a cross-section-wise endless wall (5) which widens from an inlet (6) for arriving fluid to an outlet (9) for outgoing fluid, c h a r a c t e r i z e d in that at least a distance upstream the inlet (6), the wall (7), which is passed by the arriving fluid, has a relief-like surface structure (10) constructed with projections (12) and alternat- ing countersinks (13), said surface structure having the purpose of bringing about small turbulent movements of the component parts of the fluid which pass in the immediate vicinity of said wall (7) so as to counteract separation of the fluid from the widening diffusor wall (5) . 2. Diffusor according to claim 1, c h a r a c t e r i z e d in that the projections are in the shape of long narrow beads (12) . 3. Diffusor according to claim 2, c h a r a c t e r i z e d in that at least two beads (12) cross each other. 4. Diffusor according to claim 2 or 3, c h a r a c t e r i z e d in that at least some beads (12) extend obliquely in relation to the axial direction of the diffusor with the purpose of generating also rotary movements of the passing fluid. 5. Diffusor according to any one of the preceding claims, c h a r a c t e r i z e d in that the individual projection (12) has a height within the range of 0,05-0,50 % of the diameter of the inlet (6). 6. Diffusor according to any one of the preceding claims, c h a r a c t e r i z e d in that the relief-forming surface structure (10) constitutes an integrated part of the diffusor and/or a tube connecting thereto. 7. Diffusor according to any one of claims 1-5, c h a r a ct e r i z e d in that the relief-forming surface structure constitutes a part of a separate supplement which in turn is mounted in the diffusor and/or a tube connecting thereto. 8. Diffusor according to any one of the preceding claims, the same constituting a part of a nozzle having an inlet in the shape of a neck (6) with a smallest width or diameter from which the diffusor wall extends downstream and on the upstream side of which there is a rotary symmetric, trumpet-like widening arched wall surface (7) , c h a r a c t e r i z e d in that the relief-like surface structure (10) extends from the neck (6) and along the entire arched wall surface (7) .

Inventors: HENRIKSSON MATS, HENRIKSSON, MATS
Applicants: HENRIKSSON MATS, VATTENFALL AB PUBL, HENRIKSSON, MATS, VATTENFALL AB (PUBL)
Application year: 1998
Main classification: F15D1
IPC codes: F15D1