US20070068663A1

US20070068663A1 - Heat exchanger

Info

Publication number: US20070068663A1
Application number: US11/534,294
Authority: US
Inventors: Oliver Thomer; Uwe Rothuysen; Dieter Thonnessen; Gunter Thiel
Original assignee: Pierburg GmbH
Current assignee: Pierburg GmbH
Priority date: 2005-09-23
Filing date: 2006-09-22
Publication date: 2007-03-29
Also published as: DE202006009464U1

Abstract

The invention relates to a heat exchanger with a channel (4) through which cooling agent flows and a channel (3) through which fluid to be cooled flows, whereby ribs (6) project into at least one of the channels (3, 4). According to the invention, these ribs (6) feature a linear approach edge (11) and a linear flow-off edge (15), whereby the side walls (12) run continuously between the approach edge (11) and the flow-off edge (15). By these means it is achieved that a turbulent boundary layer forms at the ribs, which boundary layer ends in a turbulent eddy in the area of the flow-off edge (15). This leads to an increased efficiency of the heat exchanger and simultaneously to a good homogenization of the fluid. Moreover, sooting is reliably avoided.

Description

This application claims priority from International Patent Application No. PCT/EP2005/010303 filed Sep. 23, 2005, and from German Patent Application No. 20 2006 009 464.4, filed Jun. 16, 2006. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a heat exchanger with a channel through which cooling agent flows and a channel through which fluid to be cooled flows, which channels are separated from one another by at least one wall from which issue ribs extending into at least one of the two channels.

BACKGROUND OF THE INVENTION

Such heat exchangers are generally known and are described in a number of Applications. There exist heat exchangers in which the ribs project only into the channel conducting cooling agent as well as heat exchangers whose ribs project into the channel through which the fluid to be cooled flows and heat exchangers with ribs pointing in both directions. These ribs distinctly improve the heat transfer between the two fluids. In particular, the ribs increase the residence time and the dynamic pressure in the corresponding channel in comparison with embodiments without ribs. In a heat exchanger used as an exhaust gas heat exchanger in internal combustion engines, such ribs can also be used in order to prevent to the greatest extent possible, a sooting or carbon fouling of the channel through which the exhaust gas flows.
Thus in DE 10 2004 045 923 A1 heat exchangers are described whose ribs are shaped in different ways. They project from two inner walls bordering the channel into the channel conducting the fluid to be cooled. All these ribs feature an axially symmetrical shape and are installed at an angle to the main flow direction, at least over a section. Both the approach area and the flow-off area of these ribs are embodied with a radius.
The disadvantage of the above-mentioned embodiments is that the manufacturing cost is relatively high, since both inner walls must be embodied with ribs and secondly a high pressure drop is present due to the relatively large dynamic pressure zone as the rib is approached.
An improved efficiency is achieved through the plate heat exchanger known from U.S. Pat. No. 2,892,618, whose ribs feature side walls arranged concave to one another, each with an approach edge and a flow-off edge.
From GB 892 534 a heat exchanger is known that features ribs with a straight side wall and a concave side wall. As a result of the linear approach edge, the dynamic pressure zone of the flow as it approaches the rib, in which the speed is reduced to zero, is minimized, so that a lower pressure drop is achieved. Moreover the continuously running side walls cause the formation of a boundary layer that is adjacent in the area of the rib, so that heat can be exchanged over a lengthened cooling zone.
In all the above-mentioned embodiments, however, a relatively high susceptibility of the heat exchanger to sooting arises, in particular during use as an exhaust gas heat exchanger. The efficiency is also limited by a lack of mixing of the fluid to be cooled.
The object of the invention is therefore to develop a heat exchanger whose ribs are optimized with respect to the flow, so that the efficiency of the heat exchanger is increased by raising the heat transfer at the ribs, whereby at the same time the pressure drop in the heat exchanger is to remain as low as possible. Moreover it is desirable to achieve the lowest possible sooting of the ribs, and homogeneity of the fluid to be cooled.

SUMMARY OF THE INVENTION

This object is achieved in that each rib features one linear approach edge and two linear flow-off edges, whereby the approach edge and the two flow-off edges delimit two continuously running side walls of the rib. Thus the pressure drop is minimized by means of the single approach edge and boundary layer flows are created along the entire length of the rib due to the continuous course of the side walls, and a separation of the boundary layer flows is prevented, so that the heat transfer is improved. Due to the two flow-off edges, compared with known embodiments a distinctly improved intensive mixing transverse to the flow direction is achieved, so that the homogeneity of the fluid stream is increased, which in turn results in a temperature exchange and temperature equilibrium of the entire mass flow. All this increases the efficiency of the heat exchanger.
In a preferred embodiment, the ribs extend along the main flow direction, as a result of which the pressure drop is minimized and it is ensured that the boundary layer will be adjacent on both sides of the rib. A low pressure drop is particularly advantageous when the heat exchanger is used as an exhaust gas heat exchanger in the low-pressure zone of an internal combustion engine, since in such a use the pressure drop present is very low.
In a further form of embodiment of the invention, the side walls of each rib adjacent to the approach edge and the flow-off edge enclose an angle to one another that is less than or equal to 90°. This ensures that the pressure drop is sufficiently small and undesired turbulence and separation along the cooling zone of each rib are avoided.
In order to ensure that a boundary layer flow first forms behind the dynamic pressure point, i.e. behind the approach edge, in a front area the side walls extending from the approach edge of each rib are arranged with respect to one another essentially wedge-shaped.
In an advantageous alternative embodiment, in a front area the angle between tangents to the two side walls decreases continuously in the main flow direction until the side walls run parallel to one another in a back area. This, too, leads to an increase in the efficiency, since a separation of the boundary layers over the course of the rib is avoided in this manner and a sufficiently long cooling zone is available at the rib.
In a further form of embodiment of the heat exchanger, the ribs are arranged in rows adjacent to one another perpendicular to the main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row. This prolongs the residence time of the fluid flowing through the channel and thus in turn raises the efficiency of the heat exchanger, since a smooth flow-through of the heat exchanger is avoided to the greatest possible extent. Moreover the flow-through speed is raised due to the small cross-sections available for the flow such that a turbulent flow around the ribs is ensured, as a result of which a high wall shearing stress and thus a higher heat transfer factor a is achieved, so that an increase in the cooling performance is ensured by raising the heat convection.
Advantageously a heat exchanger of this type is used as an exhaust gas heat exchanger whose ribs project into the channel conducting exhaust gas. This is particularly advantageous, since a carbon fouling due to the flow speeds and turbulence arising is reliably avoided, whereby at the same time a high efficiency and thus a low necessary size are achieved, which is particularly important in automobile manufacture based on the small space available.
Thus in comparison with the known prior art, a heat exchanger is created that requires a smaller space due to an increase in the efficiency, and is not susceptible to sooting. At the same time it can be produced cost-effectively using the die-casting process. The fluid to be cooled leaves the heat exchanger in a well-homogenized state.
An embodiment of the heat exchanger according to the invention is shown in the Figures and is described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a heat exchanger according to the invention in sectional view.
FIG. 2 shows a head-on view of the heat exchanger from FIG. 1 in sectional view.
FIG. 3 shows a section of the heat exchanger from FIG. 1 in an enlarged view.

DETAILED DESCRIPTION OF THE INVENTION

The heat exchanger shown in the drawings, which is preferably used as an exhaust gas heat exchanger in motor vehicles, is composed of an outer housing 1 in which an inner housing 2, which can be produced using the die-casting process, is arranged. After assembly, a channel through which fluid to be cooled flows, is formed between the inner housing 2 and the outer housing 1. In the interior of the inner housing 2, a channel 4 through which cooling agent flows is arranged whose inflow and outflow connection pieces are not shown in the drawings and that can be arranged as desired, depending on the application. The channel 4 through which cooling agent flows is bounded by walls 5 from which ribs 6 extend into the channel 3 through which fluid to be cooled flows. The channel 3 through which fluid to be cooled flows is embodied in such a way that its entry 7 is arranged at the same head side as the exit 8, so that the fluid to be cooled is diverted by 180° in a back area 9 of the heat exchanger. Accordingly the ribs 6 are arranged in this area following the main flow direction.
The central rib 10 extends from the entry 7 or exit 8 to a back area 9 in which the deflection is embodied and whose height is embodied such that it extends as far as the outer housing 1, by means of which a crossflow and an overflow is prevented via a short path from the entry 7 to the exit 8.
As can be seen in particular in FIG. 1, the ribs 6 seen in the main flow direction, are arranged respectively in rows adjacent to one another, whereby as a first row finishes a second row follows respectively, whose ribs 6 are arranged staggered with respect to the ribs 6 of the first row. Such an arrangement of the ribs 6 increases the residence time of the exhaust gas in the heat exchanger and thus its efficiency, since a straight, obstruction-free flow-through is no longer possible for the fluid to be cooled.
In FIG. 3 a cross-section shape according to the invention of the ribs 6 can be seen. It features an approach edge 11, which extends to the end of each rib 6 in the channel 3 linearly from the wall 5 of the inner housing 2 and can be seen in the Figure only as a dynamic pressure point. The side walls 12 of the ribs 6 adjacent to the approach edge 11 are embodied such that the angle between the two tangents to each side wall 12 of the ribs 6 continuously decreases in a front area 13 until the enclosed angle is 0° and thus the two side walls 12 run parallel to one another in a back area 14. At the end of each rib 6, viewed in the direction of flow, both side walls 12 end at a flow-off edge 15 respectively, so that between a back wall 16 of each rib 6 and the side walls 12, a right angle exists.
Moreover it would be conceivable to allow the ribs 6 to run in a wedge shape from the approach edge 11 and subsequently to allow this wedge shape to change continuously into the parallel guiding of the side walls 12.
The heat exchanger is designed so that turbulent boundary layer flows result at the side walls 12, in which the wall shearing stress is greater than in laminar flows, so that the heat transfer factor α and thus the resulting heat transfer between rib 6 and the fluid to be cooled increases. Accordingly non-continuous embodiments of the side walls 12 in front of the flow-off edge 15 are to be avoided, since these lead to a separation that would prevent good heat transfer in the boundary layer. Correspondingly, the length of the ribs 6 must also be embodied so that a separation is avoided. Instead, the length is embodied in a defined manner at the two flow-off edges 15, which in comparison with known embodiments improve the heat transfer distinctly. This occurs due to the fact that first the good heat exchange in the boundary layer at the ribs 15 is utilized and then due to the linear flow-off edges a distinctly improved fluid exchange is achieved transverse to the main flow direction. The latter is explained by Kelvin-Helmholtz instabilities arising behind the flow-off edges 15, i.e. during separation at approximately stepped profiles. These instabilities occur due to a rolling-up of the shear layers arising at the flow-off edges. The chief trigger of this rolling-up is a strong speed gradient at the shear layer. These Kelvin-Helmholtz instabilities continue and become macroscopically visible as a broad turbulent eddy. Due to the arrangement according to the invention of the two flow-off edges at a distance H from one another, this effect, which occurs at both flow-off edges, which should feature approximately a right angle to their adjacent walls, is again distinctly intensified, since a pairing of the two eddies occurs at the upper and the lower flow-off edge. Due to this pairing, a turbulent area arises behind the ribs whose extent transverse to the flow direction is distinctly greater than the thickness H of the ribs. This turbulent flow produced by the flow-off edge 15 leads to an excellent homogenization of the exhaust gas in the heat exchanger.
In comparison with ribs with only one flow-off edge, the width of the turbulent area behind each rib 6 is distinctly greater and thus a distinctly improved thorough mixing is achieved transverse to the flow direction, since with single flow-off edges a dead water area forms with small eddies and low thickness. This is particularly important because the boundary layer flows forming at the continuous side walls 12 do indeed lengthen the cooling zone and thus improve the heat transfer, but to a great extent prevent a fluid exchange due to transverse flows.
Moreover a high kinetic energy is desired in the rib boundary layer, as a result of which a separation of the boundary layer is delayed. The boundary layer flow thus lies against the cooling rib longer, so that the cooling zone is lengthened. Through these measures a sooting or carbon fouling of the ribs is reliably avoided, so that over a long service life, the heat exchanger features a better efficiency in comparison with other known heat exchangers.
It is clear that the design of the rest of the construction of the heat exchanger can be changed. Both the position of the channel conducting the cooling agent and the position of the channel through which fluid to be cooled flows can be modified. Furthermore ribs embodied in this manner can extend as far as both or either of the two channels through which fluid flows. The above-mentioned advantages are achieved both for a liquid and for a gas.

Claims

1. A heat exchanger comprising:

a channel through which cooling agent flows; and

a channel through which fluid to be cooled flows, wherein channels are separated from one another by a wall from which issue ribs extending into at least one of the two channels, wherein each rib includes one linear approach edge and two linear flow-off edges, whereby the approach edge and the two flow-off edges delimit two continuously running side walls of the ribs.

2. A heat exchanger according to claim 1, wherein the ribs extend along a main flow direction.

3. A heat exchanger according to claim 1, wherein the side walls of each rib adjacent to the approach edge and the flow-off edges enclose an angle to one another that is less than or equal to 90°.

4. A heat exchanger according to claim 3, wherein in a front area, the side walls extending from the approach edge of each rib are arranged essentially wedge-shaped with respect to one another.

5. A heat exchanger according to claim 1, wherein in a front area, an angle between tangents to the two side walls decreases continuously in a main flow direction until the side walls run parallel to one another in a back area.

6. A heat exchanger according to claim 1, wherein the ribs are arranged in rows adjacent to one another perpendicular to a main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row.

7. A heat exchanger according to claim 1, wherein the heat exchanger is an exhaust gas heat exchanger whose ribs project into the channel through which fluid to be cooled flows conducting exhaust gas.

8. A heat exchanger according to claim 2, wherein the side walls of each rib adjacent to the approach edge and the flow-off edges enclose an angle to one another that is less than or equal to 90°.

9. A heat exchanger according to claim 2, wherein in a front area, an angle between tangents to the two side walls decreases continuously in the main flow direction until the side walls run parallel to one another in a back area.

10. A heat exchanger according to claim 2, wherein the ribs are arranged in rows adjacent to one another perpendicular to the main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row.

11. A heat exchanger according to claim 3, wherein the ribs are arranged in rows adjacent to one another perpendicular to a main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row.

12. A heat exchanger according to claim 4, wherein the ribs are arranged in rows adjacent to one another perpendicular to a main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row.

13. A heat exchanger according to claim 5, wherein the ribs are arranged in rows adjacent to one another perpendicular to a main flow direction, whereby the ribs of each row are arranged staggered with respect to the following row.