EP3118468A1

EP3118468A1 - Re-laminarization of a turbulent flow in a duct

Info

Publication number: EP3118468A1
Application number: EP15176645.8A
Authority: EP
Inventors: Björn HOF; Jakob KÜHNEN; Markus Schaner
Original assignee: Institute Of Science And Technology Austria
Current assignee: Institute Of Science And Technology Austria
Priority date: 2015-07-14
Filing date: 2015-07-14
Publication date: 2017-01-18
Anticipated expiration: 2035-07-14
Also published as: EP3118468B1

Abstract

For re-laminarizing a turbulent flow of a fluid flowing through a duct (4) in a main flow direction (6), a wall (5) of the duct (4) bounding the flow, additional fluid (9) is injected into the duct (4) in the main flow direction (6) as a sheet (11) of fluid covering the wall (5) in a circumferential direction around the main flow direction (6). Alternatively, a cylinder barrel structure is coaxially arranged within the pipe (2). The cylinder barrel structure comprises at least one cylinder barrel which is partly closed at its upstream end in the main flow direction, and the cylinder barrel structure - in the main flow direction - extends over at least twice a diameter (3) of the duct.

Description

The present invention relates to a method of re-laminarizing a turbulent flow of a fluid flowing through a duct. Further, the present invention relates to apparatus for transporting a fluid in a main flow direction, the apparatus comprising a duct, and a re-laminarization station for relaminarizing a turbulent flow of the fluid flowing through the duct.
In a wall-bounded flow, i.e. in a flow of a fluid over a wall, the wall exerts shear forces onto the fluid, and, as a result, a boundary layer of the flow is formed at the flow-bounding wall in which the flow is affected by the wall.
In such a boundary layer, depending on the actual conditions, the flow may be laminar or turbulent, the drag in a boundary layer being much higher with a turbulent flow than with a laminar flow. Thus, a laminar flow often has big advantages over a turbulent flow in that it saves energy, like for example in pumping a liquid through a pipe or channel. Particularly, the present invention relates to flows through pipes.
Particularly, the present invention relates to re-laminarizing turbulent flows, i.e. to eliminating turbulence in a wall-bounded flow of a fluid.
Even more particularly, the present invention relates to re-laminarizing turbulent flows at Reynolds-numbers above 2700 at which the turbulences in the flows do normally form large continuous regions that do not decay so that the flow normally stays turbulent over its entire downstream extension.
The duct through which the fluid flows may be defined by a pipe. In a pipe, the Reynolds-number as used here is defined as Re = UD/ν, where U is the mean flow speed or average flow velocity, D is the pipe diameter and ν is the kinematic viscosity. With a flow through another duct than one defined by a pipe, a corresponding definition of Re is to be applied, like, for example, a definition of Re for a flow through a channel or over a flow-bounding wall.

BACKGROUND OF THE INVENTION

Björn Hof et al.: Eliminating turbulence in spatially intermittent flows, Science 19, March 2010: Vol. 327, No. 5972, pp. 1491-1494, disclose a method of eliminating turbulence in a spatially intermittent flow through a pipe in that the parabolic velocity profile of a laminar flow is distorted to a plug like velocity profile upstream of a turbulent puff. The distortion of the velocity profile reduces the sudden change of the axial velocity across the rear of the turbulent puff. In numerical simulations, this proposal is reported to be successful in eliminating turbulence. Once having eliminated the turbulent puff, a forcing needed to distort the parabolic velocity profile may even be switched off, and the flow continues to be laminar. However, Hof et al. point out, that a distortion of the velocity profile at the turbulent laminar interface cannot be as readily implemented in practice as in simulations. Thus, they proposed to use a second turbulent puff upstream of the original one to distort the velocity profile at the rear end of the original puff. When the second turbulent puff is induced at a short distance upstream of the original puff, the short distance between the two puffs is insufficient to allow a parabolic velocity profile to fully develop, despite the fact that the flow is not turbulent between the two puffs. Hof et al. could show that introducing the additional puff allows for keeping the flow in a pipe laminar downstream of the additional puff, even in the area of the original puff. However, they pointed out that their simple strategy only works well for sufficiently small Reynolds-numbers of Re < 2000 in pipes, and that it becomes less efficient as Re increases, and once the regime of spatially expanding turbulence is reached (Re > 2500 in pipes) it fails. On the other hand, in their numerical simulations, the basic concept of distorting the velocity profile to re-laminarize a turbulence proved successful even with larger Reynolds-numbers and reduced the drag more than by a factor of two.
For actually implementing their concept, Björn Hof et al. continuously inject and simultaneously withdraw water through two small holes in the wall bounding the flow. However, they give no details with regard to an implementation of the injecting and withdrawing of the water or a formation of the holes.
WO 2012/069472 A1 discloses a method and an apparatus for eliminating turbulence in a wall-bounded flow by moving a section of the flow-bounding wall in the direction of the flow. The fluid in the boundary layer of the flow which is located close to the moved section of the flow-bounding wall is accelerated as compared to its velocity of zero with a fixed flow-bounding wall. With a constant average velocity of the flow, this results in a distortion of the velocity profile in that the maximum difference in velocity between the fluid in the boundary layer directly adjacent to the flow-bounding wall and the fluid in the centre of the flow or even outside the boundary layer is reduced. As a direct consequence, the shearing forces in the boundary layer feeding turbulence are reduced. The known method is not only able to avoid the occurrence of turbulence but also to re-laminarize an already turbulent flow. If the flow is not disturbed again downstream of the point at which the known method is executed, it may stay laminar indefinitely. Thus, a local application of the known method may reduce the drag of a flow over a long distance, like for example an entire pipe or channel. Thus, the known method may be used for strongly decreasing the energy spent for pumping fluids like gases and liquids. The suitable length of the flow over which the moved section should include the full flow-bounding wall depends on the velocity at which the section of the flow-bounding wall is moved. Generally, this length of the flow should be at least about 20 boundary layer thicknesses long. In this context the boundary thickness layer is defined as the thickness over which the flow-bounding wall affects the flow. If the flow-bounding wall encloses a lumen through which the flow flows, like in case of a pipe or a channel, the moved section of the flow-bounding wall generally is at least about 20 diameters of this lumen long. The velocity at which the section of the flow-bounding wall which is moved in the direction of the flow according to the known method is preferably at least about 40 % of an average flow velocity of the flow over the unmoved flow-bounding wall.
Although, the method and the apparatus WO 2012/069472 A1 proof to be successful in eliminating turbulence in a wall bounded, their application is quite complicated as continuously moving a section of a flow-bounding wall is not implemented easily.
WO 2014/140373 A2 discloses methods of and apparatus for eliminating turbulence in a wall-bounded turbulent flow by distorting a flow velocity distribution of the flow in a direction perpendicular to the wall. In one variant, the flow velocity distribution is distorted by locally generating additional vortices in the turbulent flow close to the flow-bounding wall. The additional vortices are distributed over a section of the flow-bounding wall extending in a main flow direction of the turbulent flow. Axes of the additional vortices predominantly extend parallel to the flow-bounding wall. The additional vortices may particularly be generated by injecting fluid through the flow-bounding wall and into the turbulent flow. The fluid may be taken from the flow. In another variant, the flow distribution is distorted by increasing the flow velocity close to the flow-bounding wall by locally immersing a flow deviating structure in the turbulent flow. The flow deviating structure is aligned with the main flow direction of the turbulent flow and comprises coaxial rings whose radial distances increase towards the flow-bounding wall. In a further variant, the flow velocity distribution is distorted by equalizing the flow velocities by locally immersing a flow dividing structure in the turbulent flow. The flow dividing structure comprises a plurality of densely packed through-holes of constant cross section extending in the main flow direction of the flow. The diameter of each partial flow through each through-hole is much smaller than the diameter of the entire turbulent flow. As a result, the Reynolds-number of each partial flow is much smaller than the Reynolds-number of the entire turbulent flow. Even with a Reynolds-number of several thousand of the entire turbulent flow, for example, the Reynolds-number of the partial flows may be as low as a few hundred. With these low Reynolds-numbers the turbulence cannot survive in the partial flows. When the partial flows get out of the flow dividing structure, the flow is quite disordered again and not yet necessary laminar. The flow velocity profile, however, is very flat, and hence all disturbances decay within about ten diameters of the flow resulting in a laminar flow further downstream. The flow dividing structure, however, considerably increases the total drag exerted on the flow.

PROBLEM

A need remains for a method of re-laminarizing a turbulent flow of a fluid and for apparatus for transporting a fluid comprising re-laminarization station for re-laminarizing a turbulent flow of the fluid which are effectively applied i.e. both sufficiently easily and with a high level of efficiency.

SOLUTION

The present invention relates to a method comprising the features of independent claim 1, to an apparatus comprising the features of independent claim 5, and to an apparatus comprising the features of independent claim 9.
Preferred embodiments of the method and the apparatus according to the invention are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

According to the present invention, the method of re-laminarizing a turbulent flow of a fluid flowing through a duct in a main flow direction, a wall of the duct bounding the flow, comprises injecting additional fluid into the duct in the main flow direction. Particularly, the fluid is injected as a sheet of fluid covering the wall in a circumferential direction around the main flow direction. This sheet of fluid essentially covers the entire flow-bounding wall over a section of the duct extending in the main flow direction. The sheet of fluid separates the turbulent flow of the fluid from the flow-bounding wall. Consequently, shearing forces exerted by the flow-bounding wall on the turbulent flow are reduced. Further, the velocity profile of the turbulent flow is flattened in that the low flow velocities close to the wall are increased by the injected additional fluid. As a result, turbulence decays quickly.
The additional fluid may particularly be injected through a slotted nozzle extending along the wall in the circumferential direction around the main flow direction. If the duct is completely closed by its wall in the circumferential direction around the main flow direction, the slotted nozzle will be a ring nozzle.
The volume flow rate of the additional fluid may be added to the volume flow rate of the fluid flowing through the duct. Here, the additional fluid may be of the same composition as or of a different composition than the primary fluid. Particularly, the additional fluid may be injected at a point of confluence of two flows, the primary fluid making up the one flow, and the additional fluid making up the other flow.
Alternatively, the additional fluid may be withdrawn from the duct for being re-injected into the duct. Preferably, the additional fluid is withdrawn from the duct through a slotted orifice which may be of a similar design as the nozzle through which the fluid is re-injected into the duct. The point of withdrawing the additional fluid may be upstream or downstream of the nozzle with regard to the main flow direction. The slotted orifice will then face away from the nozzle or face the nozzle in the main flow direction. With an upstream point of withdrawing the additional fluid, the additional fluid bypasses the duct between the slotted orifice and the slotted nozzle; and with a downstream point of withdrawing the additional fluid, the additional fluid only flows through the duct between the slotted nozzle and the slotted orifice. In the first case, the fluid flow is not affected by the slotted orifice after injecting the additional fluid through the slotted nozzle. In the latter case, the section of the duct extending between the slotted nozzle and the slotted orifice may be compared to a section of the flow-bounding wall moving the main flow direction according to WO 2012/069472 . Thus, the details given in WO 2012/069472 with regard to the velocity and the length of the moving section of the flow-bounding wall may also be applied to the velocity of the injected additional fluid and the distance between the slotted nozzle and the slotted orifice. For example, the distance between the slotted nozzle and the slotted orifice is preferably at least 20-times a thicknesses of a boundary layer of the flow upstream of the slotted nozzle in the main flow direction. In so far, the content of WO 2012/069472 A1 is incorporated herein by reference.
The velocity of the injected additional fluid may be at least 50 % of the average velocity of the fluid in the main flow direction. Preferably it is at least 80 % and not more than 150 % of the average velocity, i.e. close to the average velocity of the fluid in the main flow direction.
The thickness of the sheet of the injected additional fluid may be 1 to 10 % of the diameter of the duct. The thickness of the fluid should ensure that the sheet of fluid encloses the turbulent flow of the fluid over a sufficient distance in the main flow direction to enable a decay of the turbulence in the flow. Often, a thickness of the sheet of the injected additional fluid of about 1 mm will be suitable.
The volume flow rate of the injected additional fluid will typically be at least 3 % of the volume flow rate of the fluid not including the additional fluid. Often it will be at least 5 % or about a tenth or even about an eighth of the volume flow rate of the fluid not including the additional fluid. Rarely, the volume flow rate of the injected additional fluid will never be more than 20 % of the volume flow rate of the fluid not including the additional fluid. Most often it will not be more than 15 % of the volume flow rate of the fluid not including the additional fluid.
The additional fluid will often be of a same composition as the fluid of the turbulent flow. The additional fluid may, however, also be a diluent to the fluid of the turbulent flow reducing the viscosity of the fluid of the turbulent flow and thus any shearing forces spreading in the turbulent flow and feeding its turbulence.
The apparatus for transporting a fluid in a main flow direction applying the method according to the present invention comprises a duct extending in the main flow direction and including a wall bounding a flow of the fluid through the duct; and further comprises a re-laminarization station configured to re-laminarize a turbulent flow of the fluid flowing through the duct and including a nozzle configured to inject additional fluid into the duct. The nozzle is a slotted nozzle extending along the flow-bounding wall in the circumferential direction around the main flow direction and configured to inject the additional fluid in the main flow direction as a sheet of fluid covering the wall in a circumferential direction around the main flow direction.
If the duct is closed by its wall in the circumferential direction around the main flow direction, i.e. if the duct is defined by a pipe, the slotted nozzle will be a ring nozzle having a ring-shaped slot extending around the turbulent flow.
The re-laminarization station may further include a pump configured to supply the additional fluid to the slotted nozzle. The pump may further be configured to withdraw the additional fluid from the duct. Particularly, the re-laminarization station may further include a slotted orifice located upstream or downstream of the slotted nozzle in the main flow direction and facing away from or facing the slotted nozzle in the main flow direction. The pump may then be configured to bypass or circulate the additional fluid between the slotted orifice and the slotted nozzle. The bypassed or circulated fluid emerges out of the slotted nozzle into the duct, passes through the duct as the sheet of fluid enclosing the turbulent flow until the turbulence decays, and then enters into the slotted orifice to be pumped back to and through the slotted nozzle again.
The width of an opening of the slotted nozzle may be 1 to 10 % of the diameter of the duct.
The re-laminarization station may further include a metering unit configured to meter the volume flow rate of the injected additional fluid to a value required to achieve the desired velocity of the injected additional fluid of 80 to 150 % of the average velocity of the fluid flowing through the duct.
The other apparatus for transporting a fluid in a main flow direction according to the present invention comprises a pipe of circular free cross section, the pipe extending in the main flow direction, and a re-laminarization station configured to re-laminarize a turbulent flow of the fluid flowing through the pipe. The re-laminarization station comprises a cylinder barrel structure. The cylinder barrel structure includes at least one cylinder barrel which is partly closed at its upstream end in the main flow direction and which has an outer diameter which is smaller than the diameter of the circular free cross section of the pipe. Each cylinder barrel of the cylinder barrel structure is coaxially arranged within the pipe. The entire cylinder barrel structure - in the main flow direction - extends over at least twice the diameter of the circular free cross section of the pipe.
At its upstream end, the at least one cylinder barrel is partially closed by at least 50 % and by up to 95 %. Preferably it is closed there by between 75 and 90 %. At its downstream end, the at least one cylinder barrel is open. any further cylinder barrel of the cylinder barrel structure is open at both of its ends.
The cylinder barrel structure of the re-laminarization station is an effective means of increasing the flow velocity close to the boundary of the circular free cross section of the pipe and, thus, of flattening the flow velocity profile across the circular free cross section of the pipe.
The partial closure of the at least one cylinder barrel and its upstream end is preferably provided by means of an orifice plate whose directions of main extensions are perpendicular to the main flow direction. Most preferably the orifice plate only has one central orifice.
The wall thickness of each cylinder barrel of the cylinder barrel structure should be small as compared to the diameter of the circular free cross section of the pipe. It may be between less than 0,1 % and 5 % of the diameter of the circular free cross section of the pipe. Thus, the circular free cross section of the pipe ist not strongly reduced by the wall of each cylinder barrel. The cylinder barrel structure - in the main flow direction - may extend over between 2,5-times and 30-times the diameter of the circular free cross section of the pipe. This means that the cylinder barrel structure is quite long as compared to the diameter of the circular free cross section of the pipe. Even if the cylinder barrel structure only comprises one cylinder barrel partly closed at its upstream end and fully open at its downstream end, the cylinder barrel structure may extend over about 10-times the diameter of the circular free cross section of the pipe.
If the cylinder barrel structure comprises a plurality of cylinder barrels, these cylinder barrels will be of different diameter such that all the cylinder barrels could be arranged one within the other. In the cylinder barrel structure of the apparatus according to the present invention, however, the cylinder barrels follow to each other with increasing diameter in the main flow direction. The cylinder barrels following to each other in the main flow direction at the outmost partially overlap with each other in the main flow direction, i.e. they either directly follow to each other without a gap in the main flow direction, or they overlap only partially. Further, a free gap extending in a direction perpendicular to the main flow direction remains between any two of the cylinder barrels following to each other in the main flow direction. In this embodiment of the second apparatus according to the present invention, only the cylinder barrel located uppermost upstream in the main flow direction and having the smallest diameter is partially closed at its upstream end, whereas it is open at its downstream end. All other cylinder barrels are both open at their upstream and their downstream ends.
The total number of the cylinder barrels in this embodiment of the apparatus may be between 3 and 10.
The cylinder barrels following to each other in the main flow direction may at the outmost overlap with each other by 20 % of their respective length in the main flow direction. More preferably they at the outmost overlap with each other by 10 % of their respective length in the main flow direction.
The free gaps between the cylinder barrels following to each other in the main flow direction and a difference between the maximum diameter of the cylinder barrels and the diameter of the circular free cross section of the pipe may each be between about 1 % and about 10 % of the diameter of the circular free cross section of the pipe.
Advantageous developments of the invention result from the claims, the description and the drawings. The advantages of features and of combinations of a plurality of features mentioned at the beginning of the description only serve as examples and may be used alternatively or cumulatively without the necessity of embodiments according to the invention having to obtain these advantages. Without changing the scope of protection as defined by the enclosed claims, the following applies with respect to the disclosure of the original application and the patent: further features may be taken from the drawings, in particular from the illustrated designs and the dimensions of a plurality of components with respect to one another as well as from their relative arrangement and their operative connection. The combination of features of different embodiments of the invention or of features of different claims independent of the chosen references of the claims is also possible, and it is motivated herewith. This also relates to features which are illustrated in separate drawings, or which are mentioned when describing them. These features may also be combined with features of different claims. Furthermore, it is possible that further embodiments of the invention do not have the features mentioned in the claims.
The number of the features mentioned in the claims and in the description is to be understood to cover this exact number and a greater number than the mentioned number without having to explicitly use the adverb "at least". For example, if a cylinder barrel is mentioned, this is to be understood such that there is exactly one cylinder barrel or there are two cylinder barrels or more cylinder barrels. Additional features may be added to these features, or these features may be the only features of the respective product.
The reference signs contained in the claims are not limiting the extent of the matter protected by the claims. Their sole function is to make the claims easier to understand.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is further explained and described with respect to preferred exemplary embodiments illustrated in the drawings.

Fig. 1: is an axial section through a first embodiment of a re-laminarization station of a first apparatus according to the present invention.
Fig. 2: is an axial section through a second embodiment of the re-laminarization station of the first apparatus according to the present invention.
Fig. 3: is an axial section through a third embodiment of the re-laminarization station of the first apparatus according to the present invention.
Fig. 4: is an axial section through a first embodiment of the re-laminarization station of a second apparatus according to the present invention.
Fig. 5: is an radial section through the embodiment of the re-laminarization station according to Fig. 4.
Fig. 6: is an axial section through a second embodiment of the re-laminarization station of the second apparatus according to the present invention.
Fig. 7: shows experimental results obtained with the embodiment of the re-laminarization station according to Fig. 2; and
Fig. 8: depicts the evolution of the time-averaged stream wise velocity field in front of and behind the embodiment of the re-laminarization station according to Fig. 5.

DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts an apparatus 1 for transporting a fluid. The apparatus 1 comprises a pipe 2. The pipe 2 has a circular free cross section having a diameter 3 and defining a duct 4 delimited by a wall 5. The fluid transported by the apparatus 1 fills up the entire free cross section of the pipe 2 and flows through the duct 4 in a main flow direction 6 along a pipe axis 7 of the pipe 2. Due to obstacles in the pipe 2 or bendings of the pipe 2 (not depicted here), for example, a flow of the fluid may become turbulent although the flow could still be laminar due to its Reynolds-number. Due to the flow becoming turbulent, the drag exerted on the fluid flowing through the pipe 2 and thus the energy needed to pump a certain volume of the fluid through the pipe 2 increases. A re-laminarization station 8 of the apparatus 1 is provided for re-laminarize the turbulent flow of the fluid through the pipe 2. In the laminarization station 8, an additional fluid 9 is injected into the duct 4 through a ring nozzle 10 extending along the wall 5 in a circumferential direction around the main flow direction 6 and the pipe axis 7. The additional fluid 9 is injected into the duct 4 as a sheet of fluid 11 covering the wall 5 within the re-laminarization station 8 and enclosing the turbulent flow. At the end of the re-laminarization station 8 the additional fluid 9 is withdrawn from the duct 4 through a slotted orifice 12 having the same ring shape as the ring nozzle 10. In the re-laminarization station 8 the turbulence of the turbulent flow is no longer fed by shearing forces due to high velocity differences between the turbulent flow close to the wall 5 and the resting wall 5, as the turbulent flow is enclosed by the sheet of fluid 11 having an increased velocity in the main flow direction 6 as compared to the resting wall 5. Further, the velocity profile perpendicular to the main flow direction 6 is flattened by means of the injected additional fluid 9. Thus, the turbulence of the turbulent flow decays. The distance between the ring nozzle 10 and the slotted orifice 12 in the main flow direction 6 is selected such that the decay of the turbulence sufficiently proceeds to have a laminar flow in the pipe 2 at some point downstream of the re-laminarization station 8. In the re-laminarization station 8 according to Fig. 1 the additional fluid 9 is circulated between the slotted orifice 12 where it is withdrawn from the duct 4 and the ring nozzle 10 where it is injected into the duct 4 in the main flow direction 6 by means of a pump 13.
The additional fluid 9 may alternatively be injected into the duct 4 and then transported with the fluid through the pipe 2. Fig. 2 shows another embodiment of the re-laminarization station 8 of the apparatus 1 in which the additional fluid 9 is only injected through the ring nozzle 10 but not withdrawn from the duct 4 again. In an actual experimental setup of this embodiment of the re-laminarization station 8, the width of the ring nozzle 10 between a reduced diameter 3' of the circular free cross section delimited by the wall 5 directly upstream of the ring nozzle 10 and the full diameter 3 of the circular cross section directly downstream of the ring nozzle 10 is 1 mm. The diameter 3 is 30 mm.
Fig. 3 shows an embodiment of the re-laminarization station 8 of the apparatus 1 which differs from the embodiment of Fig. 1 in that, with regard to the main flow direction 6, the orifice 12 through which the additional fluid 9 is withdrawn from the duct 4 is located upstream of the nozzle 10 through which the additional fluid is injected into the duct 4. Thus, the additional fluid 9 is bypassed to the duct 4 between the orifice 12 and the nozzle 10, and the fluid flow through the duct 4 is not disturbed by any orifice downstream of the nozzle 10 and the formation of the sheet of fluid 11 covering the wall 5.
Fig. 4 and Fig. 5 show another apparatus 1 for transporting a fluid through a pipe 2 whose wall 5 delimits a duct 4 with a circular free cross section of a diameter 3. Within the re-laminarization station 8 of this apparatus 1 a cylinder barrel structure 14 is coaxially arranged within the pipe 2, i. e. aligned with the main flow direction 6 and the pipe axis 7. The cylinder barrel structure 14 comprises a single cylinder barrel 15 here. At its upstream end 16, the cylinder barrel 15 is partially closed by an orifice plate 17 having one central orifice 24 only. At its downstream end 18, the cylinder barrel 15 is open. In an actual experimental setup of this embodiment of the re-laminarization station 8, the diameter 3 of the circular free cross section is 30 mm, the total length 19 of the cylinder barrel structure 14 is 285 mm, the outer diameter 20 of the cylinder barrel 15 is 27,4 mm, the diameter 21 of the central orifice 24 of the orifice plate 17 is 10 mm and the thickness 22 of the orifice plate 17 is 6 mm. The cylinder barrel 15 is held in its coaxial position within the pipe 2 by three small surface stream wise rips 23 depicted in Fig. 5.
In the apparatus 1 depicted in Fig. 6 the re-laminarization station 8 comprises a cylinder barrel structure 14 including further cylinder barrels 25 besides the cylinder barrel 15 that is partially closed at its upstream end by the orifice plate 17. The further cylinder barrels 15 are open at both of their ends. All cylinder barrels 15 and 25 are coaxially arranged with regard to the pipe axis 7. The cylinder barrels 15 and 25 follow each other in the main flow direction 6, and every two cylinder barrels 15 and 25 following each other in the main flow direction 16 in the main flow direction 6 overlap each other by 50 %, here. Perpendicular to the main flow direction 6, a free gap 26 remains between every two cylinder barrels 15 and 25 following each other in the main flow direction 16. All of these gaps 26 have a same width as a distance 27 remaining between the outer diameter of the cylinder barrel 25 having the biggest diameter and the diameter 3 of the circular free cross section of the duct 4, here. However, the widths of the gaps 26 may be varied from the center towards the wall 5 of the duct 4. The cylinder barrel structure 14 according to Fig. 6 - like that one according to Figs. 4 and 5 - increases the flow velocity of the turbulent flow through the pipe 2 close to the wall 5 and thus flattens the flow velocity profile across the circular free cross section of the pipe 2. Consequently, the turbulence of the flow is no longer fed but decays.
Fig. 7 is a time trace of the pressure drop Δp between two measurement points following each other at a distance of 39,5 times the diameter 3 downstream of the actual experimental setup of the re-laminarization station 8 according to Fig. 2 with a turbulent flow at a Reynolds-number of Re = 5,200 entering the re-laminarization station 8. With injecting the additional fluid 9 the pressure drop Δp drops to about the third of its starting value of the turbulent flow. The reduced pressure drop Δp corresponds to a laminar flow for this Reynolds-number.
Fig. 8 shows the evolution of the time-averaged stream wise velocity field in the pipe 2 of the actual experimental setup of the re-laminarization station 8 according to Figs. 4 and 5 starting with a turbulent flow of a Reynolds-number Re = 3,500. The first profile at the left at z = -10 represents the uncontrolled turbulent flow upstream the re-laminarization station 8. The further profiles represent the flow downstream of the cylinder barrel structure 14. The profiles at z = 2 to 6 show increased velocities at the wall 5 of the pipe 2 resulting in a flattened velocity profile at z = 6 to 20. From this flattened velocity profile the typical profile of a laminar flow develops at z = 90.

LIST OF REFERENCE NUMERALS

1: apparatus
2: pipe
3: diameter
4: duct
5: wall
6: main flow direction
7: pipe axis
8: re-laminarization station
9: additional fluid
10: ring nozzle
11: sheet
12: slotted orifice
13: pump
14: cylinder barrel structure
15: cylinder barrel
16: upstream
17: orifice plate
18: downstream
19: length
20: outer diameter
21: diameter
22: thickness
23: rib
24: orifice
25: cylinder barrel
26: gap
27: distance

Claims

A method of re-laminarizing a turbulent flow of a fluid flowing through a duct (4) in a main flow direction (6), a wall (5) of the duct (4) bounding the flow, the method comprising
- injecting additional fluid (9) into the duct (4),
characterized in that the additional fluid (9) is injected in the main flow direction (6) as a sheet (11) of fluid covering the wall (5) in a circumferential direction around the main flow direction (6).
The method of claim 1, characterized in that the additional fluid (9) is injected through a slotted nozzle extending along the wall (5) in the circumferential direction around the main flow direction (6), wherein, optionally, the additional fluid (9) is injected through a ring nozzle (10).
The method of claim 2, characterized in that the additional fluid (9) is withdrawn from the duct (4), wherein, optionally, the additional fluid (9) is withdrawn from the duct (4) downstream of the slotted nozzle in the main flow direction (6) through a slotted orifice (12) facing the slotted nozzle in the main flow direction (6).
The method of any of the preceding claims, characterized in that the velocity of the injected additional fluid (9) is at least 80 % and not more than 150 % of the average velocity of the fluid in the main flow direction (6).
An apparatus for transporting a fluid in a main flow direction (6), the apparatus (1) comprising:
- a duct (4) extending in the main flow direction (6) and including a wall (5) bounding a flow of the fluid through the duct (4); and

- a re-laminarization station (8) configured to re-laminarize a turbulent flow of the fluid flowing through the duct (4) and including a nozzle configured to inject additional fluid (9) into the duct (4),
characterized in that the nozzle is a slotted nozzle extending along the wall (5) in a circumferential direction around the main flow direction (6) and configured to inject the additional fluid (9) in the main flow direction (6) as a sheet (11) of fluid covering the wall (5) in the circumferential direction around the main flow direction (6).
The apparatus (1) of claim 5, characterized in that the duct (4), in the circumferential direction around the main flow direction (6), is closed by its wall (5) and that the slotted nozzle is a ring nozzle (10).
The apparatus (1) of claim 5 or 6, characterized in that the re-laminarization station (8) further includes a pump (13) configured to supply the additional fluid (9) to the slotted nozzle.
The apparatus (1) of claim 7, characterized in that the pump (13) is configured to withdraw the additional fluid (9) from the duct (4), wherein, optionally, the re-laminarization station (8) further includes a slotted orifice (12) located downstream of the slotted nozzle in the main flow direction (6) and facing the slotted nozzle in the main flow direction (6), the pump (13) being configured to circulate the additional fluid (9) between the slotted orifice (12) and the slotted nozzle.
An apparatus (1) for transporting a fluid in a main flow direction (6), the apparatus (1) comprising:
- a pipe (2) of circular free cross section, the pipe (2) extending in the main flow direction (6); and

- a re-laminarization station (8) configured to re-laminarize a turbulent flow of the fluid flowing through the pipe (2), the re-laminarization station (8) comprising a cylinder barrel structure (14), the cylinder barrel structure (14) including at least one cylinder barrel (15) having an outer diameter (20) which is smaller than a diameter (3) of the circular free cross section of the pipe (2), and each cylinder barrel (15, 25) of the cylinder barrel structure (14) being coaxially arranged within the pipe (2),
characterized in

- that the at least one cylinder barrel (15) is partly closed by between 50 and 95 % at its upstream end (16) in the main flow direction (6) and

- that the cylinder barrel structure (14) - in the main flow direction (6) - extends over at least twice the diameter (3) of the circular free cross section of the pipe (2).
The apparatus (1) of claim 9, characterized in that the at least one cylinder barrel (15) is partly closed by between 75 and 90 % at its upstream end (16) in the main flow direction (6).
The apparatus (1) of claim 9 or 10, characterized in that the at least one cylinder barrel (15) is partly closed by an orifice plate (17) at its upstream end (16) in the main flow direction (6), wherein, optionally, the orifice plate (17) has one central orifice (24) only.
The apparatus (1) of any of the claims 9 to 11, characterized in that the cylinder barrel structure (14) - in the main flow direction (6) - extends over between 2.5-times and 30-times the diameter (3) of the circular free cross section of the pipe (2).
The apparatus (1) of any of the claims 9 to 12, characterized in that the cylinder barrel structure (14) comprises a plurality of cylinder barrels (15, 25) of different diameters, the cylinder barrels following to each other with increasing diameter in the main flow direction (6), wherein the cylinder barrels (15, 25) following to each other in the main flow direction (6) at the outmost partially overlap with each other in the main flow direction (6), and wherein a free gap (26) extending in a direction perpendicular to the main flow direction (6) remains between every two of the cylinder barrels (15, 25) following to each other in the main flow direction (6), wherein, optionally, the number of the cylinder barrels (15, 25) is between 3 and 10.
The apparatus (1) of claim 9 or 13, characterized in that the cylinder barrels (15, 25) following to each other in the main flow direction (6) at the outmost overlap with each other by 20 % of their respective length in the main flow direction (6).
The apparatus (1) of any of the claims 9 to 14, characterized in that the free gaps (26) between the cylinder barrels (15, 25) following to each other in the main flow direction (6) and a difference (27) between the maximum diameter of the cylinder barrels (25) and the diameter (3) of the circular free cross section of the pipe (2) are each between 1 % and 10 % of the diameter (3) of the circular free cross section of the pipe (2).