The invention concerns a heat exchanger according to the preamble of Claim 1.
Such heat exchangers, which have a heat exchanger core, known as a core for short, with flow channels arranged parallel to one another, are known, for example, as coolant/air radiators in motor vehicles. A medium to be cooled, for example, the coolant of a cooling circuit of an internal combustion engine of a motor vehicle, flows through the flow channels. The coolant is preferably cooled by air (ambient air), wherein secondary exchange surfaces in the form of fins can be provided. Various flow patterns are known for such a core, for example, downflow radiators or crossflow radiators with one or two flow filaments. In the latter case, the throughflow of the core is in the shape of a U. In this respect, two collection boxes are provided on the core, wherein the first has an inlet and an outlet chamber and the second is designed as a deflection box. The deflection of the flow thus takes place “in the width,” that is, in the longitudinal direction of the deflection box. The division of the core into a first and a second passage, as a rule, is done 50:50, so that the flow rates in the tubes of the two core halves are the same. The flow direction of the cooling air is perpendicular to the flow direction of the medium to be cooled, so that the heat transfer occurs in crossflow. As a result of cooling, the temperature of the medium in the tubes of the first passage is higher than the temperature of the medium in the second passage. In comparison to a parallel flow radiator (wherein the entire core receives a throughflow in one direction), flow rates which are increased due to the deflection are produced in the flow channels, which lead to an improved heat transfer with small coolant throughputs. Different expansions of the tubes are produced due to the temperature differential between the tubes in the first and second passage; these lead to thermal stresses in the heat exchanger. In particular, with an increase in the coolant inlet temperature as a result of a higher motor load, there is an increased temperature differential between the first and second passage of the core, since with such a transient process a temporary lag appears, with the medium flowing through the two passages successively.
By the applicant's DE 197 22 099 B4, a heat exchanger became known, which has a collection box with an inserted separation wall and inlet and outlet connections. Thus, a U-shaped throughflow of the heat exchanger is made possible, which leads to the aforementioned temperature differences in the first and second flow passages.
A heat exchanger designed for internal combustion engines became known from the applicant's DE 32 12 891 C2; it consists of a fin/tube core, an upper and a lower box, and side parts which are designed as flow channels and receive a coolant throughflow. The medium to be cooled is withdrawn from the boxes and thus cools the side parts, which in this way obtain a lower component temperature. Thus, excessively high temperature differences between the cooling tubes and side parts and increased temperature stresses are avoided.
Proceeding from a heat exchanger which can receive a throughflow in the shape of a U, it is the objective of the present invention to avoid or to reduce thermal stresses produced by temperature differences in the heat exchanger, in particular in its flow channels.
This objective is attained by the features of Claim 1. Advantageous developments of the invention can be deduced from the subclaims.
According to the invention, provision is made first of all for a bypass to be associated with the first passage of the heat exchanger, i.e. the first U leg of the flow path; this means that a proportion of the medium to be cooled is diverted before entry into the first passage of the heat exchanger, conducted through the bypass, and again supplied, uncooled, to the main flow after the first passage or before the second passage. In this way there is the advantage that the temperature in the second passage is raised and thus the temperature difference is reduced. Therefore, the thermal stresses in the flow channels, for example, the tube and tube plate connections, are also reduced.
Advantageously, a first collection box with an inlet and outlet chamber and a second collection box in the form of a deflection box are associated with the core of the heat exchanger. In this case, the bypass channel extends between the inlet chamber and deflection box, wherein the local inlet of the bypass channel into the deflection box can be designed as variable, that is, dependent on the desired temperature increase in the second passage. Preferably, the inlet of the bypass into the deflection box can be at the level of a separation wall that separates the inlet and outlet chambers from one another. In this case, the heat exchanger preferably has horizontal flow channels and collection boxes arranged vertically. The deflection box has an inlet opening. The bypass channel discharges into the deflection box. The bypass channel and/or the deflection box carries only slightly cooled medium. The bypass channel is located in the deflection box. Slightly cooled medium arrives at the deflection box via an inlet opening. The closer the inlet opening of the bypass channel to the inlet to the second passage, the less mixing there will be with the cooled medium of the first passage, with a greater rise in the temperature in the second passage.
The division of the core into a first passage and a second passage can be 1:1, but can also deviate from that. With an equal division, essentially the same flow rates are produced in the two passages. The flow rate in the bypass channel, on the other hand, is higher and can be established by dimensioning its cross section or flow resistance to the desired value. The higher the flow rate in the bypass channel, the more rapidly will the temperature front of the hot medium reach the deflection box or the inlet to the second passage. Thus, sudden temperature increases of the medium to be cooled and the related increased temperature differences between the first and the second passages can be compensated, since the temperature fronts in the first and in the second passages run opposite one another.
The bypass channel can be advantageously designed as a separate bypass line to the heat exchanger or can be integrated into the heat exchanger. The latter can, for example, be effected by integration of the bypass channel into a side part of the heat exchanger. The side part is thereby designed as a flow channel, that is, hollow, and is fluidically connected with the inlet box and the deflection box.
According to a preferred embodiment of the invention, the heat exchanger is designed as a coolant/air radiator in the coolant circulation of an internal combustion engine for a motor vehicle. The radiator core thus consists, as a rule, of tubes and fins which receive a coolant throughflow; they are impinged upon by the ambient air. The fin/tube core can be made mechanically or constructed as a soldered core. The collection boxes can be made of plastic or metal, in particular, aluminum, for example in all-aluminum radiators.
Advantageously, the bypass line has a diameter in the range of 7 to 16 mm. The proportion of the throughput through the bypass, relative to the total throughput through the radiator, is thus between 10 and 25%.
Embodiments of the invention are shown in the drawing and are described in more detail below. The figures show the following:
On the one hand, an increased flow rate of the coolant, as a result of the deflection, and thus an improved heat exchange, are advantageous in this arrangement. On the other hand, with certain installation conditions, the arrangement of coolant inlet and outlet connections on the same side or on the same collection box can be advantageous.
The inlet opening 7 is preferably located in an area b, which deviates by approximately 15% of the width of core 2 on either side of line m. The inlet opening or inlet point 7 is understood to mean the location where the bypass flow (the coolant flow through the bypass channel 6) meets the coolant flow in the deflection box 4 and the two flows mix.
According to a preferred embodiment of the invention, the diameter of the bypass line for a radiator is in the range of 7-16 mm, thus establishing the proportion of the bypass flow relative to the total throughput through radiator 5 between 10% and 25%.
In the drawing, that is, in a preferred embodiment, the inlet opening 7 in the deflection box 4 is located above line m that separates the first passage 2a, which is uppermost in the drawing, from the second passage 2b, which is lowermost in the drawing. Since the first passage 2a and the second passage 2b have the same number of tubes (not shown) with the same flow cross sections, the upper and the lower core halves 2a, 2b are the same. However, design of the flow cross sections of the passages 2a, 2b in a ratio different from 50:50, for example, at 40:60, also lies within the scope of the invention.
By means of the bypass flow, that is, the proportion of the coolant flowing through the bypass line 6 and the reentry of the practically uncooled coolant in the middle area of the deflection box 4, hot or relatively uncooled coolant is supplied to the second passage 2b, so that the temperature of the coolant in the second passage 2b rises. This effect of the bypass flow according to the invention is explained in more detail below.
In the diagram, T1 is the low coolant inlet temperature, whereas T2 represents the increased coolant inlet temperature, which as mentioned above can arise with an increased motor load. The continuous lines, which represent the time dependence of the temperature TE on the time t, show the delay with which a temperature increase of T1 to T2 at the radiator inlet is propagated up to the deflection box. While the coolant inlet temperature increases to T2 in a time period (t2−t1), a time period (t4−t2) also elapses until the temperature T2 has arrived at the deflection box, that is, at the inlet to the second passage.
Based on the flow resistance and the cross section of the bypass line, the delay between the rise in temperature in box 3a and in box 4 at location 7 can be varied and adjusted.
Number | Date | Country | Kind |
---|---|---|---|
10 2006 042 239.2 | Sep 2006 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2007/007782 | 9/6/2007 | WO | 00 | 3/25/2009 |