FR2906355A1 - Tube for a heat exchanger of an automobile vehicle, comprises an oval form cross-sectional boundary with small and large axes, successive undulations defined by a longitudinal periodic variation, and a wall with constant thickness - Google Patents

Abstract

The tube (12) for a heat exchanger of an automobile vehicle, comprises an oval form cross-sectional boundary (26) with a small axis and a large axis with a constant diameter, successive undulations (30) defined by longitudinal periodic variation in a small axis region, and a wall with constant thickness. The boundary has a double symmetry according to the axes, a diameter shrinkage with a curvature inversion in a small axis region, and defines two wall concavities in a central part of the tube section. The concavities turn one towards the other in a center of the boundary. Independent claims are included for: (1) a heat exchanger; and (2) a process for the fabrication of a tube.

Description

1 tube for heat exchanger, exchanger comprising such a tube and

  The invention relates to tubes for heat exchangers, in particular for heat exchangers of self-moving vehicles, and a heat exchanger comprising such tubes. It also relates to methods of manufacturing such tubes and such exchangers. The invention applies in particular to heat exchangers made from a multiplicity of tubes passing through aligned holes formed through a multiplicity of parallel flat fins, and in which the tubes are secured to the fins of purely mechanical way, without brazing. These exchangers use tubes of circular section, oval or other oblong shape, for example a biconvex lenticular section, or bean-shaped or the like. An exchanger of this type is described in particular in FR-A-2 750 482, which discloses a method for radially expanding the tubes to achieve mechanical and thermal contact between tubes and fins. This document teaches the use for this purpose of an expansion tool comprising a rod of section smaller than that of the tube, extended by an enlarged or "olive" portion having a flared portion terminated by a peripheral edge whose Maximum dimensions are greater than the internal dimensions of the section of the tube, to allow radial expansion or expansion of the latter when the olive is forcibly moved inside the tube over the entire length of the tube. to ensure the crimping of the tubes on the fins. In these exchangers, the tubes are mechanically assembled not only with the fins, but also with at least one header plate or "hole plate" in which the ends of the tube are received sealed, the header plate being further capped by a fluid box, for example a water box. In operation, a heat transfer fluid, for example liquid, circulates in the tubes and receives or transfers heat with the external environment through the fins.

  Fluid flow conditions within the tube significantly affect the thermal performance of the heat exchanger. In particular, the creation of turbulence inside the tube makes it possible to increase the stirring of the fluid circulating therein, thus favoring the heat exchange between the wall of the tube and the mass of fluid moving in the latter. It is known, for example from the publication FR-A-2 839 776, a technique consisting in forming in a tube protruding projections, also called "dimples" (Anglo-Saxon term designating bumps), generally directed to the inside of the tube and whose function is to disrupt the circulation of the heat transfer fluid to improve the heat exchange, further increasing the exchange surface. This technique is implemented by means of rollers for continuously creating the reliefs on a flat metal strip, which is then deformed and folded by means of a profiling machine to give it a flat tube conformation, the contour of the tube being then closed by electrofusion or stapling. This method of forming "dimples" is, however, applicable only to exchangers employing flat tubes obtained by profiling a metal band of defined width.

By "flat tube" is meant a tube having a very flattened cross-section, with two large flat and parallel faces joined together at each edge by two rounded, small and high curvature connecting portions. Heat exchangers made from flat tubes comprise successive layers of these tubes arranged in parallel, each sheet being separated from the adjacent layer by corrugated metal spacers or spacers to ensure heat exchange with the environment. The tubes are soldered to the corrugated inserts and the collector plates at both ends. Such flat-tube brazed heat exchangers, however, employ a manufacturing technique quite different from that of oval-tube heat exchangers passing through aligned holes through a multiplicity of parallel planar fins. In mechanical type heat exchangers, the tubes and fins are assembled purely mechanically by radial expansion of the tubes.

An object of the invention is to provide a tube and a heat exchanger, and corresponding manufacturing processes, where the tube is an oval contour tube and mechanically assembled by expansion, but which comprises in In addition to deformations in a direction transverse to the axis of the tube, these deformations making it possible to disturb the longitudinal flow of the coolant circulating in the tube.

Another object of the invention is to propose a method for producing a heat exchanger comprising such tubes provided with reliefs, which does not require any additional step or additional tools to be used in the production line. manufacture of exchangers.

The tube of the invention is a tube for heat exchanger as disclosed in particular by the aforementioned publication FR-A-2,750,482, that is to say having in cross section an oval-shaped contour with a minor axis. and a major axis. By "contour of oval shape" will generally be designated a non-circular curved closed contour, which may be an ellipse, an approximating shape, or even an oval shape flattened in its central part by a diameter narrowing producing a curvature inversion of the contour, which can give the contour an approximate form of bean (oval of Booth with two concavities, in mathematics). In any case, this oval contour does not define large rectilinear and parallel faces, unlike the "flat tubes" defined above. The contour is preferably - but not necessarily - symmetrical with respect to its minor axis and / or its major axis.

In a manner characteristic of the invention, the tube comprises along its length successive undulations defined by variations of the diameter of the contour in the region of the small axis, the diameter in the region of the major axis being substantially constant. The term diameter is used here with its usual meaning in mathematics to designate the location of the midrange strings parallel to a given direction of any curve. This term is indeed not limited to the particular case of the circle and can therefore be used in the present case where the curve considered is an oval contour.

In other words, the tube is in the longitudinal direction as a non-cylindrical tube, with successive waves or waves.

These corrugations formed transversely to the tube have the property of bending toward the center of the tube the movement of the heat transfer fluid circulating in the vicinity of the wall, with the consequence of creating turbulence and improving the mixing of the fluid in the tube. providing a better heat exchange between the walls and the coolant. The variations which define the corrugations of the tube are advantageously periodic variations in the longitudinal direction.

The wall thickness of the tube is preferably substantially constant. This occurs preferably both in circumferential and longitudinal direction.

Advantageously, the contour of the tube has a double symmetry, with respect to its minor axis and with respect to its major axis. It may be in particular a contour pre-feeling in the region of the small axis a diameter shrinkage with inversion of curvature, defining in the central portion of the tube section two concavities of the wall, turned one towards the other in the direction of the center of the said contour. The invention also relates to a known type of heat exchanger comprising a multiplicity of tubes passing through holes formed in alignment through a multiplicity of fins and in which the tubes are mechanically fastened to the fins by radial expansion, and in the tubes are tubes according to the invention, as described above. In this exchanger, the punctual and repetitive increase of the contacts between the tubes and the fins in the zones of the corrugations makes it possible to further improve the thermal exchanges, thanks to a better conduction between the tubes and the fins. In another aspect, the invention relates to a method of manufacturing a tube as set forth above, comprising the steps of: a) preparing a tube having a cross-sectional contour of oval shape with a small axis (d) and a major axis; b) introducing at one end of the tube an expanation tool having in cross section an associated external shape not homothetic with that of said contour, this tool having a diametral dimension higher than the corresponding dimension of the tube in the region of the large axis thereof, and a smaller diameter dimension in the region of the minor axis; and c) controllably displacing the expansion tool within the tube along the length thereof, so as to produce a non-homogeneous radial expansion of the longitudinally extending tube wall resulting in shrinkage of the tube within the tube. the region of the major axis, this shrinkage being accompanied, correlatively, a buckling in the region of the minor axis, this buckling being suitable for creating in this region successive undulations along the length of the tube.

It may be advantageous to apply to the tube an axial load force simultaneously with the execution of step c). The invention also relates to a method for manufacturing a heat exchanger of the above type, in which process a multiplicity of tubes according to the invention is prepared, each with implementation of an expansion tool such as as explained above, so that the radial expansion mechanically secures the tubes to the fins.

A major advantage of this method is that the corrugations can be performed during the expansion operation of the tubes, at the time of mechanical assembly of the fin tubes, and thus do not require any additional steps or additional posts. in the production line.

An embodiment of the device of the invention will now be described with reference to the appended drawings in which the same reference numerals designate identical or functionally similar elements from one figure to another.

Figure 1 is a partial schematic perspective view of a heat exchanger comprising a plurality of oval tubes associated with a plurality of parallel planar fins. FIG. 2 is a cross-sectional view of a first example of a tube contour for the exchanger of FIG. 1. FIG. 3 is a cross-sectional view of a second example of a tube contour for the exchanger. of Figure 1. Figure 4 schematically illustrates the step of radial expansion of the tube with simultaneous formation of the corrugations. FIG. 5 is a cross-sectional view of the tube obtained after carrying out the step of FIG. 4. FIG. 6 is a longitudinal sectional view of the tube, respectively according to a-y of FIG. 5 before and after expansion step, and according to bb of Figure 5.

FIG. 7 is an example of a strain characteristic of a ductile material suitable for producing the tube according to the invention.

The heat exchanger shown partially in FIG. 1 comprises a multiplicity of planar fins 10 of generally rectangular shape, arranged parallel to one another. These fins are thin metal plates, for example aluminum, comprising series of holes in which are threaded tubes 12 passing through the aligned holes of the fins. The tubes are tubes in which a coolant circulates, in particular motor coolant, which transfers or receives heat to the external environment via the fins 10. The tubes 12 are secured to the fins purely mechanically, without brazing step, by radial expansion of the tubes in the region 14 of the holes.

The tubes 12 are oval section tubes, for example of approximately elliptical section as illustrated in FIG. 2, with a small axis d and a major axis D.

In an advantageous variant, illustrated in FIG. 3, the section of the tube 12 is a section having a diametral narrowing in the area of the minor axis d, thus defining two concavities of the wall facing each other towards the inside towards the center of the tube. The term "oval contour" will also apply to this type of contour, although it is not exclusively convex.

In either case, the tubes are arranged in the exchanger with their large axes D parallel to each other and parallel to the flow direction of the ex-dull fluid, generally air, passing through the heat exchanger. As a result, the external charge loss experienced by this fluid flow is much lower than if the tube bodies had circular sections, which has the effect of improving the heat exchange. between these tubes and the external environment.

Figure 4 schematically illustrates how the tubes are expanded radially to allow them to be secured to the fins. The technique used is derived from that described in the publication FR-A-2 750 482 mentioned above, and implements an expansion tool 20 comprising an enlarged or "olive" shape 22 mounted at the end of a rod. crossing the tube along its length. The olive has an enlarged portion whose peripheral rim 26 is locally larger than the contour of the tube. The insertion of the olive 22 into the tube and its forced displacement along the entire length thereof (displacement illustrated by the arrow 28) will cause a local increase of the diameter, essentially without any change in the thickness. of the tube wall. This radial expansion operates the mechanical and thermal assembly of the tube with the fins that this tube passes through. The starting point of the invention lies in the observation that, during this expansion operation, the increase in the diameter, and thus the perimeter, of the cross-section of the tube (the thickness of which wall does not change) results in a decrease in the axial length of the tube, because of the longitudinal components of the contrain- 2906355 11 then exerted on the tube (necking phenomenon). In the known methods, where the outer contour 26 of the olive 22 is homothetic to the inner contour of the tube, the stresses are uniformly distributed over the perimeter of the tube, and a homogeneous shrinkage is obtained. The invention proposes, in contrast, to use an olive whose shape is not homothetic of the internal section of the tube, so as to produce a non-homogeneous radial expansion stress on the perimeter of the tube, concentrated mainly in the regions of the major axis (the most distant regions of the oval contour). This differential shrinkage has the remarkable property of generating, in the manner to be discussed hereinafter, transverse periodic corrugations along the length of the tube. More precisely, considering FIGS. 2 and 3, it can be seen that the outline 26 of the olive has a non-homothetic shape of that of the tube 12, this contour 26 having a diameter greater than that of the tube in the regions 34 of the major axis D (the regions of the oval contour farthest from each other) and, conversely, a diameter smaller than that of the tube in the regions 32 of the minor axis d (the regions of the oval contour closest to each other). In the case of Figure 3, where the contour of the tube has in its central region concavities 36 extending relative to the convex end regions 38 on either side of inflection points 40, the contour 26 The olive has diametrical dimensions greater than those of the tube 10 in the convex regions 38, and smaller dimensions in the concave regions 36. In either case, this particular form of olive 5 is causing an expansion of the localized tube to the zones 34 located in the region of the major axis, thus inducing a narrowing (arrows 42 in Figure 4) in these regions. On the other hand, in zones 32 situated in the region of the minor axis, the absence of stress exerted on this point by the olive will induce a buckling (arrows 44 in FIG. 4) of the wall of the tube, due to the reduction in length of the latter due to the shrinkage 42 in the end zones 34.

The differential - positive shrinkage in the regions of the major axis, zero in the regions of the minor axis - will thus generate on the same tube constraints in opposite directions, respectively along narrowing lines and buckling lines. It is following this phenomenon that will be formed along the lines of buckling undulations 30 along the length of the tube after passage of the olive.

Figures 6 (a) and (a ') are longitudinal sections of the tube in the region of the minor axis, i.e. along the necking lines, respectively before and after passage of the olive: It can be seen that the passing of the olive results in a decrease in the length of the tube with, correlatively, the creation of periodic variations in the diameter giving ripples as a result of the buckling of the tube wall locally in the tube. this region. On the other hand, in the region of the major axis, that is to say along the buckling lines, corresponding to the longitudinal section illustrated in 6 (b), the passage of the olive causes a homogeneous diameter increase over the length of the tube. The phenomenon of appearance of the corrugations is more or less marked according to the type of tube, the shape of the olive and the mechanical characteristics of the material of the tube.

The "bean" shape illustrated in FIG. 3, with concavities 36 in the central part, is thus particularly suitable for the formation of undulations. As regards the material, this is for example an aluminum alloy having certain ductility mechanical properties, advantageously with an elongation / stress characteristic such as that illustrated in FIG. 7. This characteristic comprises first of all a zone 46 of elastic deformation with a linear slope, followed beyond the apparent elastic limit Re of a declutch-ment 50, then a zone of ductility 48 between the apparent yield stress Re and the maximum stress This zone 48, where the material deforms plastically without breaking, promotes the different shrinkage and therefore the formation of the corrugations. Furthermore, it is possible to control the amplitude and the period (peak-to-peak distance in the longitudinal direction) of the corrugations by applying to the end of the tube 30 during the expansion an axial force more or less favoring the blazing of the central area. This stress acts on the load subjected to the material: an increase in the force reduces both the amplitude and the period of the corrugations (the undulations are not very marked and very close together), conversely a decrease of the effort increases the amplitude and the period (very marked and widely spaced waves).

The deformation of the tube with differential shrinkage is advantageously carried out at the time of assembly of the tubes with the fins. Thus, it is not necessary to provide either an additional step or additional tooling to make the corrugations on the tubes, which are simply obtained by choosing an olive of appropriate contour, and the application possible controlled axial force to the tube during the expansion operation.

Claims (10)

claims   1. Tube for heat exchanger, having in cross section an oval contour with a small axis (d) and a major axis (D), characterized in that it comprises along its length successive corrugations (30) defined by variations in the diameter of said contour in the region (32) of the minor axis, the diameter in the region (34) of the major axis being substantially constant.   2. Tube according to claim 1, characterized in that said variations are periodic variations in longitudinal di-rection.   3. Tube according to one of claims 1 and 2, characterized in that its wall thickness is substantially cons-tant.   4. Tube according to one of claims 1 to 3, characterized in that said contour has a double symmetry, with respect to its minor axis (d) and with respect to its major axis (D).   5. Tube according to one of claims 1 to 4, characterized in that said contour has in the region (32) of the minor axis a diameter shrinkage with curvature inversion, defining in the central portion of the tube section two concavities (36) of the wall, facing each other towards the center of said contour.   6. A heat exchanger, of the type comprising a multiplici-30 tee tubes (12) passing through holes formed in alignment through a plurality of fins (10), and in which the tubes are joined together mechanically to the fins by radial expansion, characterized in that said tubes (12) are tubes according to one of claims 1 to 5. 5   7. A method of manufacturing a tube according to one of claims 1 to 5, characterized in that it comprises the steps of: a) preparing a tube (12) having in cross section an oval contour with a small axis (d) and a major axis (D); b) introducing at one end of the tube an expansion tool (20) having in cross section an associated external shape (26) not homothetic with that of said contour, this tool having a diametrical dimension greater than the corresponding dimension of the tube in the region (34) of the major axis thereof, and a smaller diametrical dimension in the region (32) of the minor axis; and c) controllably moving the expansion tool within the tube along the length thereof, so as to produce a non-homogeneous radial expansion of the longitudinally extending tubular wall of a tube (42). ) of the tube in the region (34) of the major axis, this shrinkage being accompanied, correlatively, a buckling (44) in the region (32) of the minor axis, this buckling being able to create in this region ( 32) successive corrugations (30) along the length of the tube. 30   8. The method of claim 7, characterized in that it further comprises, simultaneously with the execution of step c), the application to the tube of an axial load force. 2906355 17   9. A method of manufacturing a heat exchanger of the type comprising a plurality of tubes (12) passing through holes formed in alignment through a multiplicity of fins (10), characterized in that it comprises the 5 pes consisting of: a) preparing a multiplicity of tubes (12) having in cross section an oval contour with a minor axis (d) and a major axis (D); b) thread the tubes into the aligned holes of a multiplicity of fins (10); c) introducing at one end of each tube an expansion tool (20) having in cross section an associated outer shape (26) not homothetic to that of said contour, this tool having a diametrical dimension higher than the corresponding dimension of the tube in the region (34) of the major axis thereof, and a smaller diametral dimension in the region (32) of the minor axis; and d) controllably moving the expansion tool within each tube along the length thereof, so as to produce a non-homogeneous radial expansion of the longitudinally extending tubular wall of a shrink (42). ) of the tube in the region (34) of the major axis, this shrinkage being accompanied, correspondingly, a buckling (44) in the region (32) of the minor axis, this buckling being able to create in this region (32) ) successive corrugations (30) along the length of the tube, and said radial expansion being able to mechanically fasten the tubes to the fins. 2906355 18   10. The method of claim 9, characterized in that it further comprises, simultaneously with the execution of step d), the application to the tube of an axial load force.