FR2847974A1 - Heat exchanger tubes with flow perturbation, uses rectangular section tubes with raised spoiler surfaces on opposite internal faces of the tubes - Google Patents

Abstract

The heat exchanger tubes are of rectangular section with a long side (8) defining width (d) of the tube and short sides (10) defining height (hi). Flow spoilers (12) are formed inside the long sides of the tube, with one spoiler on each inner face and the spoilers offset relative to the center of the tube and each other. The hydraulic diameter of the tubes is between 1.4 mm and 3.3 mm.

Description

VCL1421.FRD

  The invention relates to heat exchangers, in particular evaporators for the air conditioning circuit, radiators for heating the passenger compartment and radiators for cooling the engine of motor vehicles.

  More specifically, the invention relates to a heat exchanger tube formed by two large faces defining the width of the tube and two small faces defining the height of the tube, the two large faces and the two small faces delimiting a circulation duct for a fluid, disturbers being formed at least on one of the large faces of the tube. The inner section of the tube can be separated by a central partition, formed of one or more thicknesses of material.

  It also relates to a heat exchanger of this type.

  EP 0 710 811 already discloses a type of heat exchanger comprising such a tube.

  The thickness of the material used to form the tube is determined by two main constraints. On the one hand it is advantageous to reduce the thickness, in order to reduce the mass involved in the manufacture of the tube and to obtain the lightest possible exchanger. This tends to reduce fuel consumption when using the vehicle incorporating this exchanger. On the other hand, the wall of the tube must be thick enough to be able to form the disruptors which are the subject of the invention, while satisfying the mechanical and chemical constraints linked to the use of the exchanger incorporating such a tube, in the vehicle heating circuit.

  The thickness of this exchanger formed from tubes corresponds approximately to the width of the tubes. It is advantageous to form an exchanger having a small thickness, in order to limit the size of the air conditioning unit where this exchanger is incorporated. This promotes compactness of the vehicle, which in turn promotes a reduction in the fuel consumption of this vehicle.

  The purpose of the disturbers is to disturb the flow of the fluid so as to increase the turbulence and thus allow a better coefficient of heat transfer.

  However, in the currently known exchangers, the distribution of the disturbers is not optimal. The turbulence caused by the disturbers results in a significant drop in pressure of the heat transfer fluid so that the improvement in the heat transfer coefficient is small in comparison with the increase in the pressure drop. In addition, if the disturbers are not well distributed, high speeds of the fluid can be reached around the latter, which can lead to phenomena of tube erosion.

  The object of the present invention is to provide heat exchange tubes for heat exchangers in which the distribution of the disturbers is optimized so as to improve the performance of heat exchanges in single-phase flow thanks to an increase in the heat transfer coefficient. .

  These objects are achieved, in accordance with the invention, by the fact that the mean hydraulic diameter of the circulation duct is between 1.4 mm and 3.3 mm. The mean hydraulic diameter of a tube is defined as equal to the average, over the whole of the tube, of four times the passage section of the tube divided by the perimeter of the section circumventing the flow of the fluid. When the tube is a straight tube, the hydraulic diameter is preferably between 1.4 mm and 2.6 mm; in the case of a U-shaped tube, the hydraulic diameter is preferably between 1.5 mm and 3.3 mm.

  In the case of a U-shaped tube, this can include a return formed by a manifold.

  In the invention, the tube preferably comprises two channels and then constitutes a so-called "bi-channel" tube.

  The width of the tube is chosen and it is preferably less than or equal to 31 mm. In addition, the tube is advantageously formed from a material having a thickness of between 0.15 and 0.35 mm.

  Thanks to these characteristics, an improvement in the heat transfer coefficient of the tubes is obtained, which allows better performance without increasing the surface area. This saves material and saves the number of tubes per exchanger.

  The shape, dimensions and distribution of the disturbers are optimized to ensure optimization of the overall performance. This optimization is made by considering the maximization of the transmitted power and a minimization of the pressure drop.

  To this end, the ratio between the area of the disturbers projected onto the large face, on which these disturbers are formed, and the total area of this large face is between 0.05 and 0.25.

  The ratio of the depth of the disturbers to the interior height of the circulation duct is between 0.25 and 0.5.

  The characteristic length of the disturbers is between 2 and 6 mm. If the disturbers are circular, their characteristic length is equal to their diameter. If they have an oval shape, their characteristic length corresponds to their width.

  Advantageously, the ratio between the pitch of the disturbers and their characteristic length is between 2 and 4.

  The ratio between the offset of the disturbers and their steps is between - 0.125 and + 0.125.

  The measurements carried out show that with identical characteristics, a gain in heat transfer of up to 15% is obtained with the tubes according to the present invention with respect to smooth tubes.

  The invention also relates to heat exchangers which comprise at least one tube as defined above. These exchangers may in particular constitute a radiator for heating a passenger compartment of a motor vehicle or a radiator for cooling a motor vehicle engine.

  When the vehicle is idling, its engine runs at a speed of between 700 and 1000 revolutions per minute. This regime corresponds to a laminar flow in the tubes. The Reynolds number is between 500 and 1000 in the tubes of the exchanger. In laminar mode, the gain in thermal power transmitted by the tubes according to the invention is approximately 1 to 4% depending on the operating conditions such as the temperature and the air flow rate.

  When the vehicle is traveling at a moderate speed, for example at around 90 km per hour, its engine speed is between 2,500 and 3,500 rpm. This regime corresponds to Reynolds numbers in the tubes from around 1000 to 2000. Under these conditions, the gain in thermal power transmitted by the tubes is around 5 to 10%, depending on operating conditions such as temperature. and the air flow.

  Finally, when the vehicle is used in intensive conditions, its engine running at a speed greater than 4000 revolutions per minute, the Reynolds number in the tubes is greater than 3000. Under these conditions, the gain in thermal power transmitted by the tubes is about 11 to 15%.

  Other characteristics and advantages of the present invention will appear on reading the following description, of exemplary embodiments given by way of illustration with reference to the appended figures. In these figures: Figure 1 is a partial perspective view of a flat tube heat exchanger; Figure 2 is an end view of a tube of the exchanger shown in Figure 1; Figure 3 is a detail view showing the profile of a disturbance; Figure 4 is a front view of a large wall of the tube of Figure 2; FIG. 5a is a view of an exemplary embodiment of a half-tube in which there is no offset between the disturbers on the two faces and FIG. 5b and a sectional view of the half-tube of FIG. 5a; FIG. 6a represents an embodiment of a half-tube in which the offset between the disturbers of the two faces is equal to more or less fifty% and FIG. 6b is a section view of the half-tube shown in FIG. 6a; FIG. 7 is a detailed view which illustrates the reduction in contact between the tubes and the fins caused by the presence of the disturbers; FIG. 8 represents the reduced thermal powers, transmitted by tubes with and without disturbers as a function of the hydraulic diameter for a flow tube in I; FIG. 9 represents the reduced thermal powers, transmitted by tubes with and without disturbers as a function of the hydraulic diameter for a U-shaped flow tube; FIG. 10 represents the variation of the heat exchange efficiency as a function of the surface converged by the disturbers; FIG. 11 illustrates the variations in the power exchanged and in the pressure drop; FIG. 12 illustrates the influence of the shift of the disturbers with respect to the step of the disturbers, on the thermal power transferred; and Figures 13, 14 and 15 are perspective views of three heat exchangers comprising tubes according to the invention and having respectively an I-flow (Figure 13) or a U-flow (Figures 14 and 15).

  FIG. 1 shows a partial perspective view of a heat exchanger, for example a radiator. It is produced from 1 flattened bent and 25 brazed tubes. The sheet is preferably made of aluminum or an aluminum-based alloy. Each tube 1 consists of a metal band folded back so as to form the envelope of two parallel channels 2 and 3. It is then a so-called "two-channel" tube. Channels 2 and 3 are separated by a spacer 4 30 produced by folding the sheet metal strip 90 inwards towards the inside of the tube. The tubes 1 have their ends engaged in slots 5 of at least one header plate 6 (a single plate being shown) on which is mounted an end header box of the heat exchanger. During assembly, cooling fins 7, made of corrugated sheet, are interposed between the tubes 1. FIG. 2 shows an end view of a tube 1 of the heat exchanger shown in FIG. 1. As explained above, the interior volume of the tube 1 is divided into two parallel channels 2 and 3 by a central spacer 5 formed by the union of two walls 4a and 4b which respectively constitute the two edges of the sheet of which is from the tube. Each of the two channels 2 and 3 has a generally rectangular shape consisting of two large rectilinear walls 8 parallel to one another and two small 10 rectilinear walls 10 also parallel to each other.

  For each of the channels 2 and 3, one of the small walls merges with the walls 4a and 4b. Thus, the total width of the tube 1 is equal to twice the width d of a large wall 8 (in this exemplary embodiment the widths of the channels 2 and 3 are equal). By way of example, the width of the tube may be less than or equal to 31 mm and the thickness of its constituent material, that is to say of sheet metal, between 0.15 and 0.35 mm.

  In an alternative embodiment, the tube 1 could have no internal spacer. The interior volume of the tube would thus be constituted by a single circulation channel. In this case, the width of the tube 1 would be equal to the width of the circulation channel, that is to say the width of each of the two large parallel faces 8. In yet another variant, the tube 1 could comprise more than two traffic channels, for example three or more. In this case, the total width of the tube would be three times or more than three times the width of each of the channels. The small faces 10 delimit, for each channel 2 and 3, a circulation duct of interior height hi.

  Disturbers 12 are formed on each of the large faces 8 of the channels 2 and 3. However, the disturbers 12 could only be provided on one of the faces 8 of the channels 2 and 3 or else, the disturbers 12 could not be provided in only one of the channels 2 and 3. The disturbers 12 are produced by an inward deformation of the wall of the tube 1, for example by means of rollers. FIG. 3 shows an enlarged detail view on a scale 5 of a disturbance device 12. The letter p denotes the depth of the disturbance, that is to say the distance separating the internal face from the wall 8, on which this disruptor is formed, from the top of the disruptor. The ratio between the depth p of the disturbers 12 and the internal height 10 hi of the circulation duct is advantageously between 0.25 and 0.5.

  FIG. 4 shows a front view of the tube 1 represented in FIGS. 2 and 3. The channels 2 and 3 can define a U-shaped circulation of the heat transfer fluid, as shown diagrammatically by the arrows 13, 14 and 15. In this embodiment, the fluid circulates in the opposite direction in channels 2 and 3. On the contrary, in another embodiment, the fluid can circulate in the same direction in channels 2 and 3 (co-current circulation) as shown schematically by the arrow 13 and by arrow 16 shown in broken lines.

  As can be seen, the disturbers 12 are regularly distributed over the surface of each of the large faces 8 of the circulation channels 2 and 3. They are staggered (in order to simplify the representation, only two rows of disturbers have been shown by channel). The spacing step of the disturbers is regular. It is designated by the reference e.

  FIGS. 5a, 5b, 6a and 6b illustrate the relationship between the disturbers formed on one of the large faces 8 of each of the channels 2 and 3 and the disturbers formed on the other large face 8 of each of these channels. Figures 5a and 6a are schematic front views of a half-tube 1, while Figures 5b and 6b are cross-sectional views of the half-tubes of Figures 5a and 6a respectively. In these figures, the references 12 denote the disturbers formed on the large upper face 8 (according to FIGS. 5b and 6b) and the reference 122 the disturbers formed on the large lower face 8 (according to FIGS. 5b and 6b). As explained above, the spacing step of the disturbers 121 on the upper face 8 is designated by the reference e and, likewise, the spacing step of the disturbers 122 located on the lower face 8 is also designated by this same reference, the spacing step of the disturbers being equal on each of the two two faces 8.

  In FIGS. 5a and 5b, the disturbers 12 are located at the same level, in a plane of cross section of the tube 1, as the disturbers 122 of the lower face 8. In other words, the offset between a disturber 12, of the upper face 8 and the disturber 122 located opposite on the inner face, is equal to 0. On the contrary, according to FIGS. 6a and 6b, the disturbers 122 formed on the lower face 8 are located between the disturbers 121 formed on the upper face 8. The letter f denotes the offset 20 separating the disturbers 12 and the disturbers 122. The offset f can be expressed as a percentage of the spacing step e of the disturbers. It can be between 0% of this spacing step (Figures 5a and 5b) and more or less 50% of the spacing step e (Figures 6a and 6b). FIG. 7 is a detail view which shows the contact of the cooling fins 7 (corrugated spacers) with the external wall of the tubes 1. As can be seen, the presence of the disturbers 12 has the effect of reducing the contact surface between the generatrices of the corrugated inserts and the tube 1. In fact, the disturbers 12 constituting a deformation towards the inside of the wall of the tube, thermal contact is not established between the wall of the tube and the fin 7 at the level disruptors. In FIG. 7, reference 7a has designated a particular undulation of the corrugated interlayer which is not in contact with the wall of the tube 1 due to the presence of the disturber 12a.

  Consequently, the density of the disturbers 12 must be chosen judiciously so as not to reduce too much the thermal contact between the cooling fins and the tubes.

  FIG. 8 shows the evolution of the power 5 transmitted by the radiator as a function of the hydraulic diameter (see the definition given above) for single-pass tubes (I-flow tubes according to FIG. 13) equipped with disruptive and disruptive. Ri, called reduced thermal power, corresponds to the ratio of the power exchanged at a given hydraulic diameter Dh to the power exchanged for a hydraulic diameter Dh = 1.4 mm, this value being the hydraulic diameter optimized for smooth tubes (without disturbers ), as shown by the solid line curve.

  R2 corresponds to the ratio of the thermal power transmitted as a function of the hydraulic diameter Dh to the thermal power transmitted for a hydraulic diameter Dh = 1.8 mm, this value corresponding to the optimum of the hydraulic diameter20 only for tubes comprising disturbers, such as represented by the dashed line curve.

  As can be seen in this figure, the power transmitted by tubes comprising disturbers is greater than that transmitted by smooth tubes from a hydraulic diameter substantially equal to 1.6 mm. In general, it is also advantageous for the mean hydraulic diameter to be substantially equal to twice the internal height hi of the circulation duct. FIG. 9 shows the evolution of performance as a function of the hydraulic diameter for U-shaped tubes or for tubes against the current of a radiator, for example a radiator for heating the passenger compartment of a motor vehicle for smooth tubes (without disturbers) and for tubes comprising disturbers. RI corresponds to the ratio of the thermal power transmitted as a function of the hydraulic diameter Dh to the thermal power transmitted for a hydraulic diameter equal to 1.8 mm, this value it being the value of the hydraulic diameter optimized for smooth tubes. R2 corresponds to the ratio of the thermal power transmitted as a function of the hydraulic diameter Dh to the power transmitted for a hydraulic diameter equal to 5 2.4 mm, this value corresponding to the hydraulic diameter optimized for tubes comprising disturbers. As can be seen, the power transmitted by the tubes fitted with disturbers is greater than the power transmitted by the smooth tubes from a hydraulic diameter substantially equal to 2 mm.

  FIG. 10 represents the variation in the efficiency of the heat exchange as a function of the ratio of the surface of the disturbers (SP) relative to the total surface (ST) of the wall on which these disturbers are formed. More precisely, this is the ratio (SP / ST) between the surface of the disturbers 12 projected onto the large face 8, on which these disturbers 12 are formed, and the total surface of this large face. The projected surface of a disturbance 20 corresponds to its base connecting it to the internal face of the tube.

  As can be seen, this yield goes through a maximum for an SP / ST ratio of between 5 and 25%. Below 5%, the surface has a too low disturbance density; above 25%, the density of disruptors adversely affects the contact surface between the wall of the tube and the interlayer. In other words, for a surface of disturbers of between 0.05 and 0.25 times the total surface of the face 8 on which the disturbers 12 have been provided, the efficiency is maximum.

  FIG. 11 illustrates the influence of the ratio between the spacing pitch e of the disturbers 12 and the characteristic length L of these disturbers on the pressure drop 35 (curve in solid lines) and on the transmitted power (curve in broken lines) . For circular shaped disturbers the characteristic length L is equal to the diameter. For disturbers of elongated shape, for example oval, the characteristic length L is equal to the width (considered in the direction of the spacing), as can be seen in FIG. 4. This characteristic length is advantageously between 2 and 6 mm. If the ratio e / L is less than 2, the distance between two disturbers 5 is too small and the flow, after the passage of a disturber is still very turbulent when it meets the next disturber. This results in the fact that the pressure drop is high while the efficiency of the heat transfer is not particularly increased. If, on the contrary, the ratio is greater than 4, the distance between two disturbers 12 is too great and the flow of the liquid is laminar when it meets the next disturber. As a result, the pressure drop of the liquid is low but the efficiency of the heat transfer is not high enough. This is why this ratio should preferably be between 2 and 4.

  FIG. 12 illustrates the effect of the ratio between the spacing step e of the disturbers 12 and the offset f of these 20 disturbers on the power exchanged. For an offset f equal to 0, the heat transfer coefficient of the tubes is maximum. The distribution of turbulence in the flow caused by the disturbers is more homogeneous along the tubes. For an offset f equal to plus or minus 0.5 times the spacing step e of the disturbers 12, the turbulence of the flow caused by the disturbers is not homogeneous. The flow is turbulent only in certain regions of tube 1. The overall effect is a minimum heat transfer coefficient. This is why the ratio must preferably be chosen between −12.5% and + 12.5% as illustrated in FIG. 12.

  The tubes of the invention can be used to produce different types of heat exchangers, in particular in the field of the automobile industry, for example to constitute a radiator for heating a passenger compartment of a vehicle or even a radiator for cooling a 'a motor vehicle engine. Several cases arise depending on the type of fluid circulation in the exchanger. In the case illustrated in FIG. 13, the exchanger comprises a bundle of tubes 1 and fins 7 mounted between an upper manifold box 17 and a lower manifold box 18. The entire cross section of the set of tubes is used for a passage of the fluid and the flow is said "at I". In the example, the fluid flows from the lower manifold 18 to the upper manifold 17 as shown by the arrows.

  In the case illustrated in Figure 14, the entire cross section of half of the tubes 1 is used to circulate the fluid in one direction, while the other half of the tubes is used for a passage in the direction of the previous , as shown by the arrows. One then constitutes a flow known as "in U". In the example, the fluid enters a compartment 19 of the upper manifold 17, to reach the lower manifold 18 and 20 then join another compartment 20 of the upper manifold.

  In the case illustrated in FIG. 15, half of the cross section of the set of tubes 1 is used for a passage of the fluid in one direction, while the other half of the cross section is used for a passage in opposite of the previous one, as shown by the arrows. Here again, a so-called "U-shaped" flow is constituted. In the example, the fluid enters a longitudinal compartment 21 30 of the upper manifold 17, to gain the lower manifold 18 by using each time a first channel of each of the tubes. Then, the fluid borrows a second channel from each tube to join another longitudinal compartment 22 of the upper manifold 35. The return of the U is thus effected by the lower manifold 18.

Claims (11)

claims   1 - Heat exchanger tube formed by two large faces (8) defining the width (d) of the tube (1) and two small 5 faces (10) defining the internal height (hi) of the tube (1), the large faces (8) and the two small faces (10) delimiting a circulation conduit for a fluid, disturbers (12) being formed on at least one of the large faces (8) of the tube (1), characterized in that that the mean hydraulic diameter (Dh) of the circulation duct is between 1.4 mm and 3.3 mm.   2 - Tube according to claim 1 characterized in that the tube (1) is a straight tube and in that the hydraulic diameter as (Dh) is between 1.4 mm and 2.6 mm.   3 - Tube according to claim 1, characterized in that the tube (1) is a U-shaped tube and in that the hydraulic diameter (Dh) is between 1.5 mm and 3.3 mm. 4 - Tube according to claim 3, characterized in that the U-shaped tube (1) has a return formed by a manifold (18).   5 - Tube according to one of claims 1 to 4, characterized in that the tube (1) has two channels (2, 3).   6 - Tube according to one of claims 1 to 5, characterized in that the width (d) of the tube (1) is less than or equal to 30 31 mm.   7 - Tube according to one of claims 1 to 6, characterized in that the tube (1) is formed from a material having a thickness between 0, 15 and 0.35 mm. 8 - Tube according to one of claims 1 to 7, characterized in that the ratio between the surface (SP) of the disturbers (12) projected onto the large face (8), on which these disturbers (12) are formed, and the total area (ST) of this large face is between 0.05 and 0.25.   9 - Tube according to one of claims 1 to 8, characterized in 5 that the ratio between the depth (p) of the disturbers (12) and the internal height (hi) of the circulation duct is between 0.25 and 0 5.   - Tube according to one of claims 1 to 9, characterized 10 in that the characteristic length (L) of the disturbers (12) is between 2 and 6 mm.   11 - Tube according to one of claims 1 to 10, characterized in that the ratio between the spacing pitch (e) of the disturbers (12) and their characteristic length (L) is between 2 and 4.   12 - Tube according to one of claims 1 to 11, characterized in that the ratio between the offset (f) of the disturbers 20 (12) and their spacing pitch (e) is between -0.125 and + 0.125.   13 - Heat exchanger characterized in that it comprises at least one tube (1) according to one of the preceding claims. 25 14 - Heat exchanger according to claim 13, characterized in that the exchanger is a radiator for heating a passenger compartment of a motor vehicle.   15 - Heat exchanger according to claim 13, characterized in that the exchanger is a radiator for cooling a motor vehicle engine.