DE19511970A1 - Forming double=walled tubing for air ducting or car industry - Google Patents

Abstract

Double walled tubing is produced by making one tube shorter than the other, and forming an initial gap between them by bending the longer one away from the shorter tube. The tubes ends are sealed with piston-shaped stoppers, and appropriately shaped inner core (7) and outer die (8) are located in place before a pressurised liquid (9) is forced between the tubes to plastically deform them. After shaping the tubes, the core, die, stoppers and liquid (if required) are removed, and the initial gap is closed.

Description

The invention relates to a method for producing deformed multi-walled tubes with voids between the walls using multi-walled straight pipes, if necessary by using malleable fillers, especially under pressure standing liquids, brought into the desired shape will.

Such a method is particularly there needed where for reasons of thermal insulation, sound and / or corrosion protection or heat resistance deformed tubular components with thermally insulated Wall design and possibly with several Different quality pipe walls required are. The main areas of application of the invention are therefore the internal combustion engine and automotive technology that Materials handling technology as well as air conditioning and ventilation technology.

When funding fluid media, the Further development of technology with increasingly higher ones Requirements for the use value of the required Conveyor systems.

While providing stretched multi-walled Pipes with cavities are no significant problems poses, leads to the production of deformed multi-wall pipes to considerable complications.

In pipe conveyor systems for gases and / or liquids it can be very significant the tubular Design components so that with their help at the same time tasks of jacket heating, jacket cooling, the reduction or improvement of Heat radiation, the reduced sound emission, the  Heat or corrosion resistance, the mechanical Stabilization and / or if necessary the Change in component natural frequency at least from Parts of the multilayer pipe wall can be met can.

It is essential that the desired Properties of the tubular conveyor system are continuously available and due to the manufacturing process not to the straight lines Conveyor routes must remain limited.

Such requirements are for example Exhaust systems of high-performance internal combustion engines posed.

There has been no shortage of attempts to find solutions for Creation of rational processes for the production of deformed multi-walled pipes with cavities between to develop the walls, the individual Walls of the tubular components if necessary have significantly different material qualities can.

A more recent development that has become known looks to Solution to the problem before, stretched double-walled pipes to deform with voids between the tube walls, after the delivery cross section of the inner tube in itself known manner with moldable fillers and Cavity between the tube walls with a crystallizable liquid was filled. The Pipe deformation takes place after the Result of thermal stress on the liquid between the tube walls to form a moldable filler has converted, essentially the cross sections of the inner tube and the cavity between the Pipe walls also preserved in the deformed areas stay. After completing the deformation of the tubular The component is again subject to thermal stress required to fill the moldable filler between the To liquefy pipe walls again and then from the remove tubular component.

This in terms of energy, manufacturing technology and time commitment is relatively expensive  Procedure appears especially for mass production of multi-wall deformed tubular components sufficiently suitable.

In addition, the known technical solution from Principle her differentiations of the cross-sectional shape and the cross-sectional dimensions of the inner and outer contour as well as the cavity between the tube walls compared to the prefabricated stretched double-walled Pipe not allowed.

The object of the invention is therefore that Disadvantages of the known prior art overcome. In addition, a further developed technical solution the manufacture of multi-walled deformed tubular components from industrial ready-made or individually prefabricated allow multi-wall pipes.

According to the invention, the object is achieved by characterizing features of claim 1 solved. After that, the proposed method of manufacture of deformed multi-walled tubes with cavities between the walls characterized in that initially industrially or individually prefabricated multi-walled pipes with no or very little Distance between the pipe walls using moldable fillers or other known manner be deformed. Then, if necessary, from a multi-walled tube at least on one side or several pipe walls cut to the end.

After that, the straight or curved multi-walled ones Pipes at least at least at one end Manufacture of one or more starting columns expanded.  

It is possible to attach the pipe walls that have not been cut to length the ends of the tubular component at least partially to be provided with conical widenings.

Then the pipe ends with suitable means closed, for example in such a way that the closure the ends of the tubular component with at least one Sealing shoulder and if necessary axial and radial Holes for the delivery of hydraulic fluid having pressure piston is made.

Furthermore, the tubular component to be treated enclosed with a matrix and / or with a profiled one or multi-part stabilization capable flexible mold core.

By using the start columns and / or by Pressure equalization openings in the inner tube wall the introduction of hydraulic fluid between individual Walls of the multi-wall pipe. It is under the Effect of the hydraulic fluid the plastic Deformation of at least one pipe wall up to shaping inner surface of the die and / or up to shaping outer surface of the stabilizable flexible mold core reached.

After completing the deformation of the individual pipe wall or several pipe walls, the hydraulic fluid, the die and / or the stabilizable flexible Mold core removed from the tubular component. After that if necessary, at least the starting columns locked.

This proposed procedure can be used deformed multi-wall tubular components in large Variety with both in shape and size equip definable cavities.

It is possible to compare the inner tube cross section with that stretched prefabricated multi-wall pipe unchanged to let and the desired cavity through hydraulic expansion of the outer tube or tubes to reach.

On the other hand, the proposed solution allows the maintain the outer dimension of the pipe cross section and the desired cavity in the pipe wall through the hydraulic deformation of the inner tube or tubes effect.  

Finally, it leaves the process engineering developed also to pass through the cavity between the tube walls the simultaneous deformation of both external and of inner tube walls.

The variability of the profiles of The inner contour of the die and the outer contour of the mandrel are included set only limited limits.

The application of the method is simplified especially in cases where the inner tube of the multi-wall deformed tubular component in the Cross section should remain practically unchanged and this state is more external during the deformation process Pipe walls already through the parallel use of the Hydraulic fluid as a support fluid for the inside of the pipe is achieved.

It is advantageous as an industrial multi-wall pipe prefabricated or individually made pipes at least two uncoated and / or metallic or non-metallic coated metallic pipe walls to use the same or different quality. This possibility allows to use the case to choose related material combinations without the The availability of corresponding semi-finished products depends on his.

An existing minor manufacturing condition ter or design distance between the Pipe walls can advantageously be used to manufacture of cavities between the tube walls in a defined Size can be used.

To produce a sealing press connection between Pressure piston and die, it is advantageous that a or several outer tube walls compared to the rest Length of the tubular component shortened by 5 to 100 mm will.

The same procedure can be followed by one or more inner tube walls opposite the remaining length of the tubular component by 5 to 100 mm be shortened.  

After completing the creation of the cavities between the Pipe walls are the non-cut parts of the Pipe wall and for the production of welding, socket or Flange connections can be used. In the interest if possible less void-free sections of the multi-walled deformed pipe can the non-cut parts of the tubular components after creating the cavities shortened to the required extent in the pipe walls will.

The starting columns are preferred with chisel and / or thorn-shaped tools between the cut and the non-cut pipe walls, preferably over the Evenly distributed, manufactured.

It is possible to use the tubular component enclosing die as one over the entire length or only at the ends of the tubular component to apply adjacent matrix.

If the shape of the inner tube wall is retained, between the pressure pistons closing the pipe ends introduced a hydraulic fluid, which is more advantageous An axial manner in at least one of the pressure pistons Hole is used.

If the desired deformation of the inner tube wall or the inner tube walls is at least one with one of the Pressure piston connected stabilizable flexible Mold core with one of the desired inner tube deformations appropriate surface contour used. At more complicated forms of deformed multi-walled tubular component may require two each connected to a pressure piston to use stabilizable flexible mold cores.

Between the non-cut part of the pipe wall on End of the tubular component, the sealing shoulder of the The pressure piston and the die become a press connection manufactured that the hermetic seal of the hydraulically loaded tubular component between Die and pressure piston guaranteed.  

As hydraulic fluid for hydraulic deformation are advantageously low-viscosity Liquids, preferably fluids from mixtures of Water and synthetic additives or acid-free oils, applied.

If necessary, this is not just an acceleration of the To ensure the forming process itself, but it minimized residual contents of the Hydraulic fluid after the deformation process is completed secured and unwanted reactions with the Materials of the pipe walls and their possible Coatings excluded.

At the end of the deformed end to be equipped with starting gaps multi-walled tubular component becomes a pressure piston used, which is equipped with longitudinal grooves, wherein these longitudinal grooves one of the cross-sectional shape of the in parts of the inner tube protruding Starting columns has a corresponding cross-sectional shape.

It is also possible to use a radially split die to use. It is between the contact surfaces Submatrices a freely selectable distance maintained. In this area forms under the effect of Hydraulic fluid the pipe wall of the required Circumference with a larger length dimension of the selected outer tube an outward bulge. Thereupon will at least one movably designed partial die axially so moved to the opposite die part, so that between the contact surfaces of the partial matrices Form bead is formed.

With the design of the shaping inner surface of the Matrix and / or the shaping outer surface of the can stabilize flexible mold core if necessary, the deformed inner and / or outer tube with stiffening, optically effective, the liability of further envelopes influencing the heat transfer or correct the flow in the inner tube and / or Profiles that change the natural resonance be equipped.  

The overall advantages of the invention are that even complicated tubular conveyor systems now from individual multi-walled straight, curved, with constant and / or variable cross-sectional shapes industrial along the length of the tubular components can be produced.

This allows diminished in such systems Heat losses, improved sound insulation values, one mechanical stabilization, securing if necessary changed component natural frequencies and in special cases also effective trace heating or cooling enforce.

The invention is intended to be illustrated below with exemplary embodiments are explained in more detail.

Show in the attached drawing

Figure 1 shows the schematic longitudinal section of a multi-walled tube.

Figure 2 shows the schematic longitudinal section of a curved double-walled tube with a cut inner tube.

Figure 3 shows the schematic cross section of a double-walled tube with starting gaps.

Fig. 4 is the schematic longitudinal section of a double walled tube at the end adjacent the plunger and a die;

5 shows the schematic longitudinal section of a pipe bend of double-walled pipe with stabilization enabled flexible mold core and inwardly positioned weiteter pipe wall.

Fig. 6 shows the schematic longitudinal section of a pipe bend of double-walled pipe with supporting liquid and outwardly flared in the profiled die pipe wall;

Fig. 7 shows the schematic longitudinal section of a straight double-walled tube with a radially divided die and formed bead on the deformed outer tube.

Embodiments example 1

To manufacture is a double-walled exhaust manifold for an internal combustion engine characterized by supposed to be a heat-resistant steel for the inner tube and a corrosion-resistant steel for the outer tube is provided.

For this purpose, a prefabricated, straight, multi-walled tube 1 is used without a clearance 13 according to FIG. 1 with a clear diameter of 60 mm, consisting of steel tubes with the desired qualities. The individual steel tubes have tube wall thicknesses 2 of 1 mm each.

This semifinished product is first deformed into a 90-degree bend using known bending processes in such a way that the round cross section of the double pipe is largely preserved over the entire pipe length. Referring to FIG. 2, the inner tubes previously cut mm by 40 on both pipe ends 3.

Subsequently, with a special tool at both ends of the pipe bend according to FIG. 3, starting gaps 4 are produced between the inner and the outer pipe, which are arranged offset by approximately 90 degrees around the pipe axis.

The pipe walls 5 of the outer pipe which have not been cut to length have a conical widening 6 on both sides. These parts of the outer tube are shown in FIG. 4 in the immediate manufacturing process of the multi-walled deformed tube with a defined cavity between the walls 2 both in contact with the split hollow die 7 and with the sealing shoulders 14 of the two pressure pistons 17 , which in turn are axially in the working state be clamped to the tubular component.

The correspondingly prepared component is then inserted into an axially divided hollow die 7 which, according to FIG. 6, has a clear diameter of 84 mm between the tube end regions.

After locking the two halves of the hollow die 7 , pressure pistons 17 are inserted into the open ends of the hollow die 7 .

The pressure pistons 17 have on their cylindrical piston outer surface longitudinal grooves 18 , centrally arranged axial bores 15 and outgoing from these central bores 15 each have 4 radial bores 16 evenly distributed over the circumference, each of which ends in the longitudinal grooves 18 . In the interior of the system formed from the hollow die 7 and the pressure piston 17 there is a space enclosed on all sides.

The central axial bores 15 of the pressure pistons 17 are then connected to a hydraulic unit, with which pressures of up to 200 bar can be generated if necessary.

Via the central axial bores 15 of the pressure pistons 17 , the fluid first arrives as a support liquid 9 in the region which is formed between the end faces of the pressure pistons 17 and the wall of the inner tube 2 .

Likewise, the fluid passes through the radial bores 16 into the area of the previously created starting gaps 4 , through which the fluid is pumped between the inner and outer tubes.

At a pressure of initially less than 400 bar, the pressure fluid 9 causes the outer tube to be plastically deformed until it abuts the inner surface 10 of the hollow die 7 on all sides.

The hydraulic system is then relaxed, the two pressure pistons 17 are removed and, after unlocking the divided hollow die 7, the double-walled pipe bend is removed from the hollow die 7 .

Between the inner and the outer tube there is now a cavity 12 with an average distance between the tube walls 2 of approximately 10 mm, from which the pressure fluid 9 is removed via the total of 8 starting gaps 4 .

One end 3 of the outer tube is finally formed into a connecting flange, while the other end 3 of the outer tube is prepared for the welding connection with another double-walled tube.

Example 2

Fig according to manufacture. 5 shows a pipe connection for the exhaust system of a turbine of preformed doppenwandigem tube 1 with an internal diameter of 100 mm, the inner wall 2 of heat-resistant and corrosion-resistant steel and the outer wall 2 consists of structural steel.

The tube wall 2 of the inner tube is 0.75 mm thick, while a wall thickness of 2 mm was chosen for the outer tube. There is a production-related distance 13 of an average of 0.3 mm between the inner and outer tubes. In the prefabrication of the double pipe, the inner pipe was cut to length on both sides by 70 mm.

At the beginning of the deformation process, the projecting tube ends 3 of the non-cut tube wall 5 of the outer tube each have an outwardly shaped cone 6 for producing the tight press fit between the die 7 , the outer tube and the sealing shoulders 14 of the two parts of the stabilizable flexible mandrel 8 . The stabilizable flexible mandrel 8 consists of an inflexible area with a sealing shoulder 14 and axial and radial bores 15 , 16 and a stabilizable flexible part which, if necessary, allows the deformation of the inner tube according to a selected contour.

The die 7 is designed as a divided hollow die with a clear distance between the inner surfaces 10 of the die 7 of 120 mm.

The outer surface 11 of the stabilizable flexible mandrel 8 , which in the present example is divided in the middle in the longitudinal direction, has a diameter of 90 mm outside the inflexible press-fit area.

After inserting the curved double-walled pipe intermediate piece into the divided hollow die 7 , the latter is closed. Then the two halves of the stabilizable flexible mandrel 8 are inserted in such a way that, after appropriate axial loading, these press fit between the sealing shoulders 14 of the inflexible press fit area, the conically shaped ends 3 of the outer tube and the corresponding contact surface of the die 7 , without causing it direct contact of the two end faces of the stabilizable flexible mold cores 8 comes. The inflexible press fit areas have axial and radial bores 15 , 16 . While the axial bores 15 extend from the outer end face of the mandrel 8 into the plane in front of the sealing shoulder 14 , the eight radial bores 16, each equally distributed over the circumference of the mandrel 8 , are connected to the centrally arranged axial bore 15 .

They each end on the outer surface 11 of the mandrel 8 directly between the sealing shoulder 14 and the beginning of the tapering of the diameter of the mandrel 8 .

Subsequently, pressure fluid 9 is introduced in the form of a synthetic fluid via the two axial bores 15 of the braced sealing shoulder areas of the stabilizable flexible mandrel halves 8 .

The pressure fluid 9 passes through the radial bores 16 connected to the central axial bores 15 into the area in front of the respective sealing shoulders 14 , the pressure fluid 9 penetrating into the manufacturing-related gap 13 between the inner tube and the outer tube at a selected initial pressure of 80 bar, the outer tube to plastically deformed so that it bears firmly against the inner surface of the die 10 and the inner tube until it bears firmly against the outer surface of the mandrel 11 .

After removal of the stabilizable flexible mold cores 8 and opening of the die 7 , a double-walled tube intermediate piece can be removed from the die 7 , which has a cavity 12 between the inner and the outer tube, which is characterized by a mean distance of 12 mm between the inner and outer tubes .

After the pressure fluid 9 has been removed from the cavity 12 between the two pipes, the remaining joints between the inner and outer pipes are welded tightly and the protruding flared ends 6 of the outer pipe are formed into cylindrical sleeves.

Example 3

A double-walled, curved pipe piece with a clear diameter of 60 mm and a cavity 12 between the inner and outer pipe is required for a special exhaust system.

In this case, the outer tube should have a shaped bead 19 on a straight end 3 of the tube piece according to FIG. 7, with the aid of which the frictional fastening of the tube piece in the supporting structure can be ensured while avoiding thermal bridges.

For this purpose, a die 7 is used, which is not only divided in the axial position but also in the radial position.

The radial separation point of the die 7 is located outside the bent area of the pipe section. First, the inner tube of the preformed pipe section is cut to length of 60 mm at one end and cut to length of 120 mm at the other end. The divided die 7 has a diameter of the inner surface of 80 mm and has a radial separation point which is located at a distance of 250 mm, measured from the end of the pipe wall 5 not cut to length, on that side of the pipe section with the more shortened inner pipe.

At the beginning of the deformation process, the distance between the radial contact surfaces 20 of the sub-dies is 40 mm.

The tube ends 3 of the outer tube with their conical widening 6 are clamped during the deformation process between the sealing shoulders 14 of the two pressure pistons 17 used and the split die 7 by means of a press fit. The pipe section to be deformed is pressurized with support and pressure fluid 9 via the bores 15 , 16 of the pressure piston 17 according to FIG. 6. In addition, the promotion of pressurized liquid 9 is accelerated between the pipe walls 2 via pressure compensation openings 21 in the inner tube wall. 2 The outer tube is initially deformed until the outer tube lies largely against the inner surface of the die 10 . At the same time, a radial bulge of the deformed outer tube is recorded in the area which exists between the contact surfaces 20 of the partial matrices arranged at a distance of 40 mm.

The pressurization continues until the diameter of the bulge between the contact surfaces 20 of the submatrices has reached a value of 110 mm.

Then the fluid used as support and pressure fluid 9 is released and the press fit connection between the movable part 7 a of the radially divided die, the conically widened tube end 6 of the outer tube and the sealing shoulder 14 of the pressure piston 17 arranged there is released.

Then there is the mechanical merging of the axially movable sub-die 7 a of the radially divided die with the axially fixed sub-die 7 b of the radially divided die including the tubular bend, the area of the outer tube bulging out between the contact surfaces 20 of the sub-dies to form a bead 19 with an outer one Diameter of about 115 mm is formed. The pipe end 3 with the conical widening 6 detaches from the sealing shoulder 14 of the pressure piston 17 and is displaced with the axially movable partial die 7 a of the radially divided die.

After this procedure, both the pressure piston 17 and the individual elements of the split die 7 , 7 a, 7 b are removed from the deformed tube section.

The created cavity 12 between the inner and outer tube is then emptied of the pressure fluid 9 used . The double-walled tube piece with a cavity 12 between the inner and outer tube now has an approximately 15 mm high bead 19 on the outer tube, which can be used as a fastening or positioning element if necessary.

Reference list

1 multi-wall pipe
2 pipe wall
3 pipe end
4 starting columns
5 pipe wall not cut to length
6 conical expansion
7 die
7 a axially movable sub-die
7 b axially fixed partial die
8 flexible mold core that can be stabilized
9 hydraulic fluid
10 inner surface of the die
11 outer surface of the mandrel
12 cavities
13 production-related distance
14 sealing shoulder
15 axial bore
16 radial bore
17 pressure pistons
18 longitudinal grooves
19 shaped bead
20 contact surfaces of the submatrices
21 pressure equalization opening
22 Pipe profiling

Claims (15)

1. A method for producing deformed multi-walled tubes with cavities between the walls using multi-walled straight tubes, which are brought into the desired shape if necessary by using moldable fillers, in particular pressurized liquids, characterized in that

• a) if necessary, one or more tube walls ( 2 ) are cut to length on the end face of a multi-wall tube ( 1 ) at least on one side;
• b) straight or curved multi-wall tubes ( 1 ) are at least partially expanded at least at one end ( 3 ) to produce one or more starting gaps ( 4 );
• c) if necessary, the pipe walls ( 5 ) which have not been cut to length are at least partially provided with conical widenings ( 6 ) at the ends ( 3 ) of the tubular component;
• d) the pipe ends ( 3 ) are closed with suitable means;
• e) the tubular component to be treated is enclosed with a die ( 7 ) and / or equipped with a profiled single or multi-part mold core ( 8 );
• f) by using the starting gaps ( 4 ) and / or pressure compensation openings ( 21 ) in the inner tube wall ( 2 ), pressure fluid ( 9 ) is introduced between individual tube walls ( 2 );
• g) under the action of the pressure fluid ( 9 ), the plastic deformation of at least one tube wall ( 2 ) up to the shaping inner surface ( 10 ) of the die ( 7 ) and / or up to the shaping outer surface ( 11 ) of a stabilizable flexible mold core ( 8 ) is achieved ;
• h) after completion of the deformation of the tube wall ( 2 ) or the tube walls ( 2 ) the pressure liquid ( 9 ) if necessary, the die ( 7 ) and / or the stabilizable flexible mandrel ( 8 ) removed from the tubular component and then at least if necessary Start gaps ( 4 ) are closed.

2. The method according to claim 1, characterized in that industrially prefabricated or individually manufactured tubes made of at least two uncoated and / or metallic or non-metallic coated metallic tube walls ( 2 ) of the same or different quality are used as a multi-wall tube ( 1 ). 3. The method according to claims 1 and 2, characterized in that between the different tube walls ( 2 ) before the production of defined cavities ( 12 ) between the tube walls ( 2 ) already a slight manufacturing or design distance ( 13 ) used becomes. 4. Process according to claims 1 to 3, characterized in that one or more outer tube walls ( 2 ) are shortened by 5 to 100 mm compared to the remaining length of the tubular component. 5. Process according to claims 1 to 3, characterized in that one or more inner tube walls ( 2 ) are shortened by 5 to 100 mm compared to the remaining length of the tubular component. 6. The method according to claims 1 to 5, characterized in that the starting gaps ( 4 ) with chisel and / or mandrel-shaped tools between the cut and the non-cut pipe walls ( 2 ), preferably evenly distributed over the circumference, are made. 7. The method according to claims 1 to 6, characterized in that the closure of the ends of the tubular component with at least one sealing shoulder ( 14 ) and, if necessary, axial and radial bores ( 15 , 16 ) for the delivery of the pressure fluid ( 9 ) having pressure pistons ( 17 ) is made. 8. The method according to claims 1 to 7, characterized in that as the tubular component enclosing die ( 7 ) over the entire length or only at the ends ( 3 ) of the tubular component tightly applied die ( 7 ) is used. 9. The method according to claims 1 to 8, characterized in that a pressure fluid ( 9 ) is introduced if the shape of the inner tube wall ( 2 ) is desired between the pressure pistons ( 17 ) closing the tube ends ( 3 ). 10. The method according to claims 1 to 8, characterized in that at the desired deformation of the inner tube wall ( 2 ) or the inner tube walls ( 2 ) at least one with one of the pressure pistons ( 17 ) connected stabilizable flexible mold core ( 8 ) with one of the desired inner tube deformation corresponding surface contour is used. 11. The method according to claims 1 to 10, characterized in that between the non-cut part ( 5 ) of the tube wall ( 2 ) at the end ( 3 ) of the tubular component, the sealing shoulder ( 14 ) of the pressure piston ( 17 ) and the die ( 7 ) a press connection is made. 12. The method according to claims 1 to 11, characterized in that low-viscosity liquids, preferably fluids from mixtures of water and synthetic additives or acid-free oils, are used as the pressure fluid ( 9 ). 13. The method according to claims 1 to 12, characterized in that at least one of the pressure pistons ( 17 ) is used as a tool equipped with the cross-sectional areas of the starting gaps ( 4 ) corresponding longitudinal grooves ( 18 ). 14. The method according to claims 1 to 13, characterized in that a radially divided die ( 7 ) is used, wherein a freely selectable distance is maintained between the contact surfaces ( 20 ) of the submatrices ( 7 a, 7 b) in which under the action of the pressure fluid ( 9 ), the tube wall ( 2 ) bulges the outer tube selected to a greater extent to the outside, whereupon at least one movably designed partial die ( 7 a) is axially displaced to the opposite partial die ( 7 b) in such a way that a shaped bead ( 19 ) is formed between the contact surfaces ( 20 ) of the partial matrices ( 7 a, 7 b). 15. The method according to claims 1 to 14, characterized in that with the design of the shaping inner surface ( 10 ) of the die ( 7 ) and / or the shaping outer surface ( 11 ) of the stabilizable flexible mandrel ( 8 ) the deformed inner and / or the outer tube is provided with stiffening, optically effective profiles which influence the adhesion of further coverings, correct the heat transfer or the flow in the inner tube and / or change the natural resonance.