Abstract:
A corrugated fin, in particular for a heat exchanger, having a substantially corrugated design, having a plurality of fin surfaces, wherein adjoining fin surfaces are connected to one another by means of a fin arch in such a way that a medium can flow through an intermediate space between adjoining fin surfaces, wherein the fin surfaces are arranged at an angle with respect to one another perpendicularly with respect to a throughflow direction for the medium, wherein the fin surface has at least one bulge which protrudes out of the plane of the fin surface, wherein the extent of the bulge in a direction perpendicularly with respect to the air throughflow direction is smaller than the extent of the fin surface in said direction.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2013/057587, which was filed on Apr. 11, 2013, and which claims priority to German Patent Application No. DE 10 2012 205 916.4, which was filed in Germany on Apr. 11, 2012, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a corrugated fin and to a method for the production thereof and to a heat exchanger having such a corrugated fin. 
         [0004]    2. Description of the Background Art 
         [0005]    Corrugated fins are known in the conventional art for use in heat exchangers to improve the heat transfer. In this regard, corrugated fins are known, for example, for the air-side heat transfer, which are arranged substantially wave-shaped or zigzag-shaped folded back and forth between lateral sides of tubes, so that a medium can flow in the intermediate spaces of the fin. In this regard, corrugated fins have become known in which adjacent fin surfaces, connected to one another by means of a fin arch, are parallel to one another or are arranged at an acute angle to one another. In this regard, the fin arch can be an arch which is continuous in the flow direction of the medium and lies against a tube side surface or it can be formed as an offset arch, which is divided into intervals and is offset. 
         [0006]    Such corrugated fins have become known, for example, from DE 602 03 721 T2, which corresponds to U.S. Pat. No. 6,546,774. In these corrugated fins with parallel fin surfaces, the possibility of introducing additional turbulence-generating elements in the side surfaces is limited, because the corrugated rollers would again destroy these during the rolling of the parallel side surfaces. 
         [0007]    DE 10 2009 015 849 A1, which corresponds to U.S. Pat. No. 8,516,699, discloses a corrugated fin in which the side surfaces have dimples which extend up to the fin arch and deform it to form a wavy contour. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the present invention to provide a corrugated fin and a heat exchanger having a corrugated fin, both of which are improved compared with the conventional art and are nevertheless more simple to produce. 
         [0009]    DE 10 2008 015 064 A1, which is incorporated herein by reference, discloses a corrugated fin with fin surfaces which are oriented perpendicular to one another and are deformed inwardly by deformations. 
         [0010]    An exemplary embodiment provides a corrugated fin, particularly for a heat exchanger, with a substantially corrugated design, having a plurality of fin surfaces, whereby adjacent fin surfaces are connected to one another by means of a fin arch in such a way that a medium can flow through an intermediate space between adjacent fin surfaces, whereby the fin surfaces are arranged at an angle with respect to one another perpendicular to a throughflow direction for the medium, whereby a fin surface has at least one bulge which protrudes out of the plane of the fin surface, whereby the extent of the bulge in a direction perpendicular to the air throughflow direction is smaller than the extent of the fin surface in this direction. This means that the bulge in the direction perpendicular to the air throughflow direction does not protrude relative to a vertical line, defined by the end regions of the fin surface. The bulge therefore does not enter a spatial region which is defined by the fin arch and the space, defined for this purpose perpendicular to the air throughflow direction, or surface region. 
         [0011]    In an embodiment, the bulge can be spaced apart from an end region of the fin surface, which is adjacent to a fin arch, or also from both end regions of the fin surface, said regions each adjacent to a fin arch. This achieves that the bulge does not enter a spatial region, defined by the fin arch and the space, defined for this purpose perpendicular to the air throughflow direction, or the surface region. 
         [0012]    A plurality of bulges can be formed and arranged per fin surface, whereby the bulges of a fin surface protrude toward one side with respect to the plane of the fin surface. This promotes the transfer of heat between the fin and the flowing medium such as, for example, air. 
         [0013]    A plurality of bulges can be formed and arranged per fin surface, whereby a first portion of bulges of a fin surface protrude toward a first side and a second portion of bulges toward a second side with respect to the plane of the fin surface. This also promotes the transfer of heat between the fin and a flowing medium. 
         [0014]    The first portion of bulges can be formed spaced apart from the second portion of bulges or these touch one another or merge into one another. If the bulges are spaced apart, surface regions of the fin surface that separate the individual bulges from one another, are arranged between bulges. If the bulges merge into each other or touch each other, there are either no surface regions of the fin surface or only a boundary between the individual bulges. 
         [0015]    The bulges can have a round or elongated and/or oval contour. 
         [0016]    Bulges with an elongated and/or oval contour can have a longitudinal axis, which is arranged at an angle to the throughflow direction. 
         [0017]    The angle for all bulges can be the same. This means that the orientation of the bulges in air throughflow direction of at least one fin surface or all fin surfaces is the same. 
         [0018]    In an embodiment, the angle for adjacent bulges can be different. Thus, it can be achieved that alternating bulges have different angles. It can be advantageous in this case that each second bulge has the same angle. 
         [0019]    In an embodiment, the angle for adjacent bulges can be symmetric relative to a vertical line when viewed in regard to the air direction. This means that, for example, an angle of the first bulge is 45° and the angle of the adjacent bulge is 135°. The angles of the two bulges then add up to 180°. 
         [0020]    The bulge can be formed as a corrugated embossing on one or on opposite fin surfaces. 
         [0021]    The corrugated embossings on opposite fin surfaces can be formed projecting in the same direction. 
         [0022]    The corrugated embossing can extend along the fin surface in the flow direction. 
         [0023]    In an embodiment, the corrugated embossing can modulate the fin surface perpendicular to the flow direction. 
         [0024]    Also, an arcuate embossing, which surrounds the corrugated embossing above and/or below, can be provided above and/or below the corrugated embossing. 
         [0025]    The arcuate embossings can form a band running parallel to the corrugated embossing. 
         [0026]    Further, the bulge depth of the corrugated embossings and/or the arcuate embossings can be formed constant or variable over the height of the fin. 
         [0027]    In an embodiment, the bulge out of the trapezoidal fin surface in the region of the trapezoid base can be smaller than a bulge in the region of the trapezoid top. 
         [0028]    In an embodiment, a bulge into the trapezoidal fin surface in the region of the trapezoid base can be greater than a bulge in the region of the trapezoid top. 
         [0029]    The bulge height can be between 60% and 95% of the fin height H, preferably 80%. 
         [0030]    Further, a counter-bulge can be formed in at least one or in each trough and/or peak of the corrugated embossing. 
         [0031]    In an embodiment, an object a heat exchanger is provided having fluid channels with side surfaces of the fluid channels, with spatial regions between adjacent side surfaces, whereby a top corrugated fin is arranged between adjacent side surfaces such that it lies against one of the side surfaces by means of opposite fin arches. 
         [0032]    In an embodiment, a method is provided for producing a corrugated fin, in which bulges proceeding from a band are embossed in the band with a set of rollers and then the flat band is shaped into a corrugated fin by means of a set of rollers. 
         [0033]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0035]      FIG. 1  shows a schematic view of a tube with a corrugated fin; 
           [0036]      FIG. 2  shows a schematic view of a corrugated fin in section; 
           [0037]      FIG. 3  shows a schematic view of a corrugated fin in a fin rolling machine; 
           [0038]      FIG. 4  shows a schematic view of a corrugated fin in a fin rolling machine; 
           [0039]      FIG. 5  shows a schematic view of a corrugated fin in section; 
           [0040]      FIG. 5   a  shows an exploded section of  FIG. 5 ; 
           [0041]      FIG. 6  shows a perspective view of a corrugated fin; 
           [0042]      FIG. 7  shows a perspective view of a corrugated fin; 
           [0043]      FIG. 8  shows a perspective view of a corrugated fin; 
           [0044]      FIG. 8   a  shows a section through a bulge; 
           [0045]      FIG. 9  shows a perspective view of a corrugated fin; 
           [0046]      FIG. 10  shows a perspective view of a corrugated fin; 
           [0047]      FIG. 11  shows a perspective view of a corrugated fin; 
           [0048]      FIG. 12  shows a perspective view of a corrugated fin; 
           [0049]      FIG. 13  shows a perspective view of a corrugated fin; 
           [0050]      FIG. 14  shows a partial view of a corrugated fin; 
           [0051]      FIG. 15  shows a perspective view of a corrugated fin; 
           [0052]      FIG. 16  shows a view of a corrugated fin from above; 
           [0053]      FIG. 17  shows a section through a corrugated fin; 
           [0054]      FIG. 18  shows a corrugated fin in a view of a fin surface; 
           [0055]      FIG. 19  shows a perspective view of a further corrugated fin; 
           [0056]      FIG. 20  shows a view of the corrugated fin according to  FIG. 19  from above; 
           [0057]      FIG. 21  shows a perspective view of a further corrugated fin; 
           [0058]      FIG. 22  shows a view of the corrugated fin according to  FIG. 21  from above; 
           [0059]      FIG. 23  shows a section through a corrugated fin according to  FIG. 21 ; and 
           [0060]      FIG. 24  shows a view of the corrugated fin on a fin surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]      FIG. 1  shows a tube  1 , which is designed as a fluid channel of a heat exchanger, whereby the tube has two wide side surfaces  2 ,  3  and two narrow side surfaces  4 ,  5 , which lie opposite to each other and define an interior space  6 , which is suitable for throughflow for a medium. A corrugated fin  7 , formed by fin surfaces  8 ,  9 , each of which is connected together by fin arches  10 , is arranged in the interior of tube  1 . In this case, fin arches  10  each lie against a side surface  2 ,  3  of the tube. Fin arches  10  can preferably be soldered to the side surfaces. They can also be applied merely mechanically. 
         [0062]    In a further exemplary embodiment, corrugated fin  7  can also be arranged between two tubes, whereby fin arches  10  are each in contact with a side surface of an adjacent tube or connected thereto. 
         [0063]      FIG. 2  shows a section of a corrugated fin  11  in section, whereby the corrugated fin has fin surfaces  12  and fin arches  13  connecting fin surfaces  12 , whereby fin surfaces  12  are inclined in a plane perpendicular to the air flow direction. In this case, the air flow direction in the case of corrugated fin  11  according to  FIG. 2  is substantially perpendicular to the plane of the sheet of  FIG. 2 . Bulges  14 , which bulge out of the plane of fin surface  12 , are provided in fin surfaces  12 . As can be seen, two adjacent fin surfaces  12  together with fin arch  13  form an approximately trapezoidal spatial region, which is available for throughflow of a medium. In this regard, bulges  14  can protrude into this spatial region proceeding from the plane of fin surface  12 . 
         [0064]    In the exemplary embodiment of  FIG. 2 , fin arch  13  has a width B, the height of the fin from a fin arch to an opposite fin arch is H, the height of the projection in the plane of the side surface is designated by A, and the distance of two fin arches corresponds to twice the value of L, whereby in  FIG. 2  only the value of L is shown as the distance of two opposite fin arches, in the plane perpendicular to the air throughflow direction. The values for a fin are, for example, L=2 to 3 mm, preferably 2.5 mm, H=6 to 8 mm, preferably 7 mm, A=4 to 7 mm, preferably 5.4 mm, B=0.8 to 2 mm, preferably 1.3 mm, and the fin thickness is approximately 0.08 to 0.15 mm, preferably 0.12 mm. 
         [0065]      FIG. 3  shows an arrangement of a fin  20  in roller gap  21  of a rolling machine with two rollers  22 ,  23 , whereby it can be seen that fin arch  24  is connected to fin surface  25 , which is provided with a projection  26 . It can also be seen that the substantially corrugated contour of the fin in a plane perpendicular to the throughflow direction is already created by the rolling. 
         [0066]    In  FIG. 4 , a further rolling machine  30  can be seen with rollers  31  and  32 , which have regions  33  and  34  interlocking comb-like, whereby corrugated fin  35  is taken up with its fin surfaces  36  in this roller setup. In this case, fin arches  37  are arranged in the head region of comb-like elements  33 ,  34  in order to form the structure of the corrugated fin, whereby fin surfaces  36  with bulges  38  are arranged between the comb-like elements, whereby the bulges of comb-like elements  33 ,  34  are spaced apart, so that they are not damaged or destroyed while the fin is being formed. 
         [0067]      FIG. 5  shows a corrugated fin  40  with fin arches  41  and fin surfaces  42  and bulges  43  of fin surfaces  42 . It can be seen in this case that spatial region  44  defined by the width of fin arch  41  is kept clear by bulges  43 . The detail in  FIG. 5   a  shows this once again in an enlargement. Here, bulge  43  is created in fin surface  42  such that it does not protrude beyond line  45  in order to enter the spatial region of fin arch  41 , which is defined by width  44 . This is achieved in that bulge  43  is not provided in the lower or upper end region of fin surface  46 , but begins spaced apart from the fin arch from the transitional region or end region of the fin surface. Region  46  thus serves as a gap to achieve that bulge  43  does not enter the region designated by  44 . 
         [0068]      FIG. 6  shows a corrugated fin  50  with fin surfaces  51 , which run substantially along an air flow direction according to arrow P, whereby fin surfaces  51  are arranged in a V-shaped or trapezoidal fashion angled to one another. Fin surfaces  51  are connected together by fin arches  52  in such a way that two fin surfaces  51  are always connected together at an end via a fin arch  52 , whereby fin surfaces  51  are typically connected at both ends via a fin arch  52  to another fin surface  51 . Projections  53 ,  54 , which are curved outward or inward substantially in the manner of a circle or spherical shell, are provided in fin surfaces  51 . In this regard, projections  53  and  54  are curved in an opposite direction, so that the elements are curved alternating relative to one another. It can be seen that projection  53  and projection  55  of two opposite fin surfaces are arranged such that the projection is oriented in the opposite direction, so that they point away from one another. Adjacent projections  54  and  56 , whereby projection  56  cannot be seen in  FIG. 6 , are curved inwardly toward one another, however, so that they enter the spatial region that exists between the two adjacent fin surfaces  51 . 
         [0069]    As can be seen in  FIG. 6 , projections  53 ,  54  and  55 ,  56  alternate in the lengthwise direction of the fin surfaces, whereby the structure of the embossing is the same in every second fin surface. 
         [0070]      FIG. 7  shows a further exemplary embodiment of a fin of the invention, in which the projections of fin  60  along a fin surface  61  are always formed alternatingly, so that projections  62  are embossed toward one side and projections  63  lying therebetween are embossed in the opposite direction. 
         [0071]    In the adjacent fin surface  64 , curvature  65  is embossed in the same direction as curvature  62 , so that the curvatures with the same distance from the front edge always point in the same direction when viewed in the air flow direction. Alternating orientations of the curvatures at the same height always point in the same direction and no curvatures of adjacent fin surfaces are present that point toward or away from one another, but always point in the same direction. 
         [0072]      FIG. 8  shows projections which are formed elongated or oval. To this end, a series of projections  73 , which in the top region  74  are curved outward and in the bottom region  75  are curved inward, is introduced in fin  70  in fin surface  71  or  72 . This means that the projections are formed S-shaped, whereby the S in the area of its center line cuts the plane of the fin surface and in the top region faces outward and in the bottom region faces inward, so that two projections are produced, which are connected by a straight boundary surface. In contrast, the projections of  FIG. 6  or  7  are arranged and formed separately from one another, whereby a planar region of the fin surface surrounds such a curved region and two projections are separated from one another. In the exemplary embodiment of  FIG. 8 , the indicated S-shaped embossings are also separated from one another by a planar region of the fin surface, whereby the S-shaped embossings can also be defined as two embossings with a straight line-like connection.  FIG. 8   a  shows a section through a projection  74 ,  75 , which is formed S-shaped relative to fin surface  71 . 
         [0073]    It can be seen in  FIG. 8  that the projections of adjacent fin surfaces are formed with an opposite curvature; this means that projections  74 , which point away from the adjacent fin surface, find their counterpart in the adjacent fin surface and these projections there also point away from the first fin surface. Projections  75 , those of the first fin surface pointing toward the adjacent fin surface, find their counterpart in the adjacent fin surface such that the related embossing also points to the first fin surface. 
         [0074]    Adjacent projections along the extension of fin surface  71  are oriented parallel to one another and formed with the same orientation of the projections. This means that all projections in the top region are curved outward and the [curvatures] in the bottom region inward. 
         [0075]      FIG. 9  shows a further exemplary embodiment in which projections  81  of fin surfaces  82  of fin  80  in adjacent fin surfaces  82 ,  83  are embossed in a different direction in such a way that projections embossed in first fin surface  82  in the top region point outward and the embossing in adjacent fin surface  81  in the top region point inward, therefore toward adjacent fin surface  81 , whereas in first fin surface  81  the projections are arranged in the top region, which point away from second fin surface  81 . Accordingly, the opposite applies to the bottom regions, so that in the bottom region the projections of first fin surface  81  point toward second fin surface  81 , whereby the projections of second fin surface  81  point away from the first fin surface. Along a fin surface  81 , the adjacent projections are again formed identically in the same direction, whereby the projections are formed oval and elongated and oriented substantially perpendicular to the air flow. 
         [0076]      FIGS. 10 to 13  show exemplary embodiments of corrugated fins with oval embossings, whereby in  FIGS. 10 and 11  the embossings are arranged at an angle, when viewed relative to the air flow direction L, whereby the angle of the orientation of the embossings is the same in each case. 
         [0077]    A corrugated fin  90  with fin surfaces  91 ,  92  can be seen in  FIG. 10 , whereby embossings  93 ,  94  are provided. Embossings  93  of side surface  92  is curved counter to the direction to side surface  91  and projection  94  is curved in the direction to side surface  91 . Projection  95  of side surface  91 , in contrast, is curved in the direction to side surface  92 , and the adjacent projection  96  (which cannot be seen) is curved in the direction so that it points away from fin surface  92 . This means that the projections that are arranged as the next ones from the front edge all point toward the left, the second projections, when viewed in the air flow direction, all point toward the right, and the third projections then again point to the left, etc. 
         [0078]    In  FIG. 11  projections  101  are arranged in fin  100 , which are embossed alternating in relation to projections  102 , and are embossed at an angle of approximately 45° to the air flow direction L. The projections of adjacent fin surfaces are embossed oppositely, which means that adjacent embossings on adjacent fin surfaces are embossed either toward or away from one another. Projection  103  is therefore embossed oppositely to projection  101 . The embossing  104  (not shown) which is adjacent to [projection]  103  is oriented toward embossing  102 , so that both embossings  102  and  104  are embossed toward one another. 
         [0079]      FIGS. 12 and 13 , in contrast to  FIGS. 10 and 11 , show projections in corrugated fins which are oriented relative to one another at a different angle to the air flow direction. In this case, the first projections are embossed which are arranged at an angle of approximately 45° to the air flow direction, whereby adjacent projections are embossed at an angle of approximately 135° to the air flow direction. The direction of the embossing corresponds to the directions in  FIGS. 10 and 11 , which means that in  FIG. 12  fins  110  in side surface  111  have a projection  112  that is oriented away from fin surface  113 , whereby adjacent projection  114  is embossed toward fin surface  113 . Projection  115  of fin surface  113  is embossed toward fin surface  111 , whereby the adjacent projection  116  (not shown) of fin surface  113  is oriented away from fin surface  111 . 
         [0080]    Instead of an angle of 45° or 135°, other complementary angles can also be provided., It is expedient in this case, if the sum of the angles corresponds to 180°, when the angles of two adjacent projections are added. 
         [0081]      FIG. 13  shows a corrugated fin  120 , in which adjacent projections  121 ,  122  are arranged alternating in their curvature direction, whereby adjacent projections of adjacent fin surfaces are also embossed in the opposite direction. This means that projection  123  of fin surface  124  is curved away from fin surface  125 , whereas projection  121  is also curved away from fin surface  124 . Adjacent projections  122  and  125 , which are arranged on fin surface  124  next to projection  123 , in contrast, are curved toward one another. In the illustrations of  FIGS. 10 to 13 , adjacent projections are embossed alternating in a fin surface, whereby in  FIGS. 12 and 13  the angles of the orientation of the longitudinal axis of the projections are shown at a different angle. 
         [0082]      FIG. 14  shows schematically a view of a corrugated fin  200  with two adjacent fin surfaces  201 ,  202 . In this case, projections  203 ,  204  can be seen on both adjacent fin surfaces  201 ,  202 , which are embossed at an angle  205  to one another. Advantageously, projections  203 ,  204  are embossed so that the projections of adjacent fin surfaces are arranged and oriented crosswise. Here, angle  205  can be approximately 90°. However, other angles different therefrom can also be provided such as, for example, 120°. 
         [0083]      FIGS. 15 to 18  show a further exemplary embodiment of a corrugated fin  300  of the invention, in which on opposite fin surfaces  301  a corrugated embossing  302  is provided running along the fin surface. Corrugated embossing  302  modulates the fin surface perpendicular to the air throughflow direction. Above and below the corrugated embossing  302  arcuate embossings  303 ,  304  are provided, which surround the corrugated embossing above and below and form a band that runs parallel to the corrugated embossing. 
         [0084]    Corrugated embossing  302  thereby forms bulges  305 ,  306  that extend transverse to the air throughflow direction in alternating directions. 
         [0085]    The bulges in the same embossing direction are arranged every 7 mm to 20 mm, preferably 10 mm, so that a related periodicity is produced. As a result, the medium, such as air, is conducted in wavy lines through the fin. Thus, the bulges point alternatingly in opposite directions. 
         [0086]    Arcuate embossings  303 ,  304  above and below corrugated embossings  302  are also executed with this periodicity. 
         [0087]    The bulge depth of corrugated embossings  302  and arcuate embossings  303 ,  304  is variable and not constant over the height of the fins, as can be seen in  FIG. 17 . 
         [0088]    Bulge  303 ,  304  out of the trapezoidal surface of the fins is smaller in the region of trapezoid base  304  than in the region of trapezoid top  303 , but at most so large that the free passage h is 0.5 mm to 1.5 mm, preferably 0.8 mm. 
         [0089]    The embossing having a different depth is used for minimizing a bypass in the frustum region. 
         [0090]    A bulging into the trapezoidal surface is greater in the region of trapezoid base  303 ′ than in the region of trapezoid top  304 ′, but at most so large that the free passage h is 0.5 mm to 1.5 mm, preferably 0.8 mm. 
         [0091]    The different depth of the embossing is used for minimizing a bypass in the frustum region. 
         [0092]    The bulge height i is preferably between 60% and 95% of the fin height H, preferably 80%. 
         [0093]      FIGS. 19 and 20  show a further exemplary embodiment of a corrugated fin  400  of the invention, in which on opposite fin surfaces  401  a corrugated embossing  402  is provided, which runs along the fin surface. Corrugated embossing  402  modulates the fin surface perpendicular to the air throughflow direction. Above and below the corrugated embossing  402  arcuate embossings  403 ,  404  are provided, which surround the corrugated embossing above and below and form a band that runs parallel to the corrugated embossing. Corrugated embossing  302  thereby forms bulges  405 ,  406 , which extend transverse to the air throughflow direction in alternating directions. 
         [0094]    The medium is conveyed in a wavy manner in the flow direction by corrugated embossing  402  with bulges  403 ,  404 , whereby in each trough and peak of bulges  405 ,  406  a counter-bulge  407  is formed, which increases the turbulence in the channel and thereby the heat transfer. 
         [0095]    Counter-bulge  407  is embossed into the depth between 10% and 60% of the original bulge  405 ,  406 , preferably approximately 40%. 
         [0096]      FIGS. 21 to 24  show a further exemplary embodiment of a corrugated fin  500  of the invention, in which on opposite fin surfaces  501  oval embossings  502  are provided, which are arranged spaced apart along the fin surface. Oval embossings  502  modulate the fin surface perpendicular to the air throughflow direction. Embossing  502  thereby forms bulges which extend transverse to the air throughflow direction in alternating directions. 
         [0097]    The oval embossings thus form bulges, which project alternatingly into the trapezoid base-shaped cross section of fin  500  and out of the trapezoid base-shaped cross section of fin  500 . 
         [0098]    The oval embossings form elongated oval bulges, whose longitudinal direction is inclined at an angle of 0°&lt;α&lt;90°, preferably 35°&lt;α&lt;70°, to the flow direction. The oval embossings have a narrow end region, which has a more circular shape and a further narrow end region, which is rather acute. 
         [0099]    The bulge depth and width along the bulge shape of the embossing are not constant relative to surface  501 ; see  FIG. 23 . The bulge out of the trapezoidal shape is smaller in the region of trapezoid base  503  than in the region of trapezoid top  504 , but at most so large that the free passage h is 0.5 mm to 1.5 mm, preferably 0.8 mm. The embossing having a different depth is used for minimizing a bypass in the frustum region. 
         [0100]    The bulge into the trapezoidal shape is smaller in the region of trapezoid top  505  than in the region of trapezoid base  506 , but at most so large that the free passage h is 0.5 mm to 1.5 mm, preferably 0.8 mm. The embossing having a different depth is used for minimizing a bypass in the frustum region. 
         [0101]    Bulge height k corresponds to between 60% and 95% of fin height I, preferably 80%. 
         [0102]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.