Abstract:
A vapor cooling heat exchanger is provided with: a partition wall for partitioning path for a fluid to be cooled through which a fluid to be cooled flows, and path for a refrigerant through which a refrigerant for cooling the fluid to be cooled flows; and fins which are disposed within path for a fluid to be cooled, and which is thermally connected to the partition wall. The fins constitute a first fin and a second fin, the local heat flux of which on the partition wall is smaller than the first fin. The first fin and the second fin are arranged on the basis of the relationship between the local heat flux on the partition wall and the heat flux limit of the refrigerant. As a consequence, the occurrence of local burn-out on the vapor cooling heat exchanger is suppressed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an vapor cooling heat exchanger. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typically, a heat exchanger for an vapor cooling apparatus includes a cooling target fluid passage, in which cooling target fluid flows, and a refrigerant passage, in which liquid refrigerant for cooling the cooling target fluid flows. Heat exchange is brought about between the cooling target fluid passage and the refrigerant passage through a partition wall. In the heat exchanger, the liquid refrigerant flowing in the refrigerant passage cools the partition wall, which has been heated by the cooling target fluid, and is thus heated. When the temperature of the heat transmitting surface of the partition wall exceeds the saturation temperature of the liquid refrigerant, nucleate boiling, in which generation and departure of bubbles are repeated, is initiated on the wall surface of the partition wall. The cooling target fluid flowing in the cooling target fluid passage is thus cooled by using the boiling vaporization latent heat produced through the nucleate boiling. One such type of heat exchanger for an vapor cooling apparatus is disclosed in, for example, Patent Document 1. 
         [0003]    Patent Document 1 describes a plate fin type exhaust gas heat exchanger. The exhaust gas heat exchanger is configured by a necessary number of layers each including a fluid passage having a corrugated fin for increasing a heat transmission surface area. The layers are arranged between a pair of tube plates, which are partition walls, having a pair of spacer bars that close opposite sides of the tube plates. In the exhaust gas heat exchanger, high-temperature fluid (cooling target fluid) and low-temperature fluid (liquid refrigerant) enter separate fluid passages through adjacent side surfaces. Heat exchange is thus caused between the high-temperature fluid and the low-temperature fluid through the tube plates and the corrugated fins. Meanwhile, vapor cooling is carried out. 
       PRIOR ART DOCUMENT 
     Patent Document 
       [0000]    
       
         Patent Document 1: Japanese Laid-Open Utility Model Publication No. 3-79070 
       
     
       SUMMARY OF THE INVENTION 
       [0005]    However, in the exhaust gas heat exchanger described in Patent Document 1, heat flux in the fluid passage in which the high-temperature fluid flows locally increases in a contact portion of each tube plate that contacts the corresponding corrugated fin. This promotes boiling of the low-temperature fluid at the position corresponding to the contact portion, where the heat flux locally increases, in the fluid passage in which the low-temperature fluid flows. As a result, film boiling may occur and cover each tube plate with a bubble film, thus causing a burnout. This causes dryness in the fluid passage in which the low-temperature fluid flows at the position corresponding to the contact portion and decreases cooling performance of the heat exchanger. 
         [0006]    Accordingly, it is an objective of the present invention to prevent local burnout in an vapor cooling heat exchanger having a fin arranged in a cooling target fluid passage. 
         [0007]    In accordance with one aspect of the present invention, an vapor cooling heat exchanger that includes a partition wall and a fin is provided. The partition wall separates a cooling target fluid passage, in which cooling target fluid flows, from a refrigerant passage, in which refrigerant for cooling the cooling target fluid flows. The fin is arranged in the cooling target fluid passage and thermally connected to the partition wall. The fin includes a first fin and a second fin. The second fin decreases a local heat flux of the partition wall by a greater degree than the first fin does. The first fin and the second fin are arranged based on the relationship between the local heat flux of the partition wall and the critical heat flux of the refrigerant. 
         [0008]    In accordance with one aspect, the thickness of the second fin is greater than the thickness of the first fin. 
         [0009]    In accordance with another aspect, the second fin is arranged in an upstream portion of the cooling target fluid passage in the flow direction of the cooling target fluid where the local heat flux of the partition wall increases. 
         [0010]    In accordance with a further aspect, the second fin is arranged in a zone of the cooling target fluid passage that corresponds to a downstream portion of the refrigerant passage in the flow direction of the refrigerant where the critical heat flux decreases. 
         [0011]    Also, in accordance with another aspect, the cooling target fluid passage and the refrigerant passage are arranged side by side such that the flow direction of the cooling target fluid and the flow direction of the refrigerant cross each other. The second fin is arranged in an upstream portion of the cooling target fluid passage in the flow direction of the cooling target fluid that is a zone in the cooling target fluid passage that corresponds to a downstream portion of the refrigerant passage in the flow direction of the refrigerant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view schematically showing an vapor cooling heat exchanger according to a first embodiment of the present invention; 
           [0013]      FIG. 2  is an exploded perspective view showing a heat exchanging portion of the heat exchanger illustrated in  FIG. 1 ; 
           [0014]      FIG. 3A  is a vertical cross-sectional view showing a second fin; 
           [0015]      FIG. 3B  is a vertical cross-sectional view showing a first fin; 
           [0016]      FIG. 4  is a graph representing the relationship between positions in the flow directions in a cooling target fluid passage and the flow direction in a refrigerant passage and heat flux in the first embodiment; 
           [0017]      FIG. 5  is an exploded perspective view showing a portion of a heat exchanging portion of a second embodiment; 
           [0018]      FIG. 6  is a graph representing the relationship between positions in the flow direction in a cooling target fluid passage and the flow direction in a refrigerant passage and heat flux in the second embodiment; 
           [0019]      FIG. 7  is an exploded perspective view showing a portion of a heat exchanging portion of a third embodiment; 
           [0020]      FIG. 8  is a graph representing the relationship between positions in the flow direction in a cooling target fluid passage and heat flux in the third embodiment; 
           [0021]      FIG. 9  is an exploded perspective view showing a portion of a heat exchanging portion of another embodiment; 
           [0022]      FIG. 10  is a graph representing the relationship between positions in the flow direction in a cooling target fluid passage and the flow direction in a refrigerant passage and heat flux in the embodiment illustrated in  FIG. 9 ; 
           [0023]      FIG. 11  is an exploded perspective view showing a portion of a heat exchanging portion of another embodiment; and 
           [0024]      FIG. 12  is a graph representing the relationship between positions in the flow direction in a cooling target fluid passage and the flow direction in a refrigerant passage and heat flux in the embodiment illustrated in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0025]    An vapor cooling heat exchanger (hereinafter, referred to simply as a heat exchanger) for an EGR (exhaust gas recirculation) gas vapor cooling device (an EGR cooler) used in an EGR apparatus for vehicles will now be described with reference to  FIGS. 1 to 4 . The heat exchanger  11  for the EGR apparatus causes heat exchange between EGR gas serving as cooling target fluid and water (liquid refrigerant) serving as refrigerant, thus boiling some of the water to cool the EGR gas. For the description below, a “forward-rearward direction”, an “upward-downward direction”, and a “leftward-rightward direction” are defined as represented by the corresponding arrows in  FIG. 1 , unless otherwise specified. 
         [0026]    As shown in  FIG. 1 , a heat exchanging portion  12  is received in a substantially rectangular box-like housing  11   a,  which configures an outer casing for the heat exchanger  11 . In the housing  11   a,  a cooling target fluid inlet portion  14  is arranged forward of the heat exchanging portion  12 . A cooling target fluid outlet portion  15  is arranged rearward of the heat exchanging portion  12 . 
         [0027]    An inlet pipe  16  for introducing EGR gas into the cooling target fluid inlet portion  14  is connected to the front surface of the housing  11   a.  An outlet pipe  17  for discharging the EGR gas out from the cooling target fluid outlet portion  15  is connected to the rear surface of the housing  11   a.  The heat exchanger  11  is used with the inlet pipe  16  connected to the inlet of an EGR passage and the outlet pipe  17  connected to the outlet of the EGR passage. 
         [0028]    First ends  22   a  of a refrigerant inlet pipe  22 , which introduces water into the heat exchanging portion  12  of the housing  11   a,  are drawn into the housing  11   a  through the right surface of the housing  11   a,  which is one of the opposite surfaces located between the front and rear surfaces. Also, first ends  23   a  of a refrigerant outlet pipe  23 , which discharge water out from the heat exchanging portion  12 , are drawn into the housing  11   a  through the left surface of the housing  11   a,  which is the other one of the opposite surfaces located between the front and rear surfaces. 
         [0029]    With reference to  FIG. 2 , the heat exchanging portion  12  includes a plurality of (in the first embodiment, three) passage defining bodies  18 . Each of the passage defining bodies  18  is configured by a first fin  31  and a second fin  30 , which are arranged between two flat partition walls  19 . A pair of spacer bars  20  close opposite sides of the partition walls  19 . As illustrated in  FIG. 1 , the heat exchanging portion  12  includes a front wall  13   a  joined to the front opening ends of the passage defining bodies  18  and a rear wall  13   b  joined to the rear opening ends of the passage defining bodies  18 . The passage defining bodies  18  are joined to the front wall  13   a  and the rear wall  13   b  such that the corresponding partition walls  19  of the adjacent pairs of the passage defining bodies  18  are spaced apart at uniform intervals. 
         [0030]    The front opening of each passage defining body  18  is arranged on the side corresponding to the cooling target fluid inlet portion  14  and the rear opening of the passage defining body  18  is located on the side corresponding to the cooling target fluid outlet portion  15 . The front wall  13   a  has elongated holes  13   e,  which are formed at positions corresponding to the passage defining bodies  18  to allow communication between the interior of the cooling target fluid inlet portion  14  and the front openings of the corresponding passage defining bodies  18 . The rear wall  13   b  has elongated holes (not shown) formed at positions corresponding to the passage defining bodies  18  to allow communication between the interior of the cooling target fluid outlet portion  15  and the rear openings of the corresponding passage defining bodies  18 . 
         [0031]    After flowing from the inlet pipe  16  into the cooling target fluid inlet portion  14 , EGR gas enters the passage defining bodies  18  from the front openings through the corresponding elongated holes  13   e  in the front wall  13   a.  The EGR gas flows out from the rear openings into the cooling target fluid outlet portion  15  through the corresponding elongated holes in the rear wall  13   b.  The EGR gas is then directed to the outlet of the EGR passage through the outlet pipe  17 . As a result, with reference to  FIG. 2 , the interior of each passage defining body  18  forms a cooling target fluid passage  21 , in which EGR gas flows. 
         [0032]    The front opening of each one of the passage defining bodies  18  is the inlet for EGR gas into the corresponding cooling target fluid passage  21 . The portion corresponding to the inlet of each passage defining body  18  is an upstream portion  21   a  of each of the corresponding cooling target fluid passage  21  in the flow direction of the EGR gas (as represented by arrow X 1  in  FIG. 2 ). The rear opening of each passage defining body  18  is the outlet for the EGR gas from the corresponding cooling target fluid passage  21 . The portion corresponding to the outlet of each passage defining body  18  is a downstream portion  21   b  of each of the corresponding cooling target fluid passage  21  in the flow direction of the EGR gas. In the first embodiment, the “upstream portion  21   a  of each cooling target fluid passage  21 ” refers to a zone close to the inlet with respect to a middle portion in the flow direction in the cooling target fluid passage  21 . The “downstream portion  21   b  of each cooling target fluid passage  21 ” refers to a zone close to the outlet with respect to the middle portion in the flow direction in the cooling target fluid passage  21 . 
         [0033]    In the heat exchanging portion  12 , a refrigerant passage  24  is defined between the opposing partition walls  19  of each adjacent pair of the passage defining bodies  18 . As shown in  FIG. 1 , the front opening of each refrigerant passage  24  communicates with the corresponding one of the first ends  23   a  of the refrigerant outlet pipe  23  through a hole formed in the front wall  13   a.  The rear opening of the refrigerant passage  24  communicates with the corresponding one of the first ends  22   a  of the refrigerant inlet pipe  22  through a hole  13   c  formed in the rear wall  13   b.  The opposite sides of each refrigerant passage  24  perpendicular to the front wall  13   a  and the rear wall  13   b  are closed by the left and right surfaces of the housing  11   a.    
         [0034]    The first ends  23   a  of the refrigerant outlet pipe  23  face the front openings of the corresponding refrigerant passages  24 . The first ends  22   a  of the refrigerant inlet pipe  22  face the rear openings of the corresponding refrigerant passages  24 . A second end of the refrigerant inlet pipe  22  is connected to a first end of a water circulation pipe (not shown) and a second end of the refrigerant outlet pipe  23  is connected to a second end of the circulation pipe. Water is thus introduced into the refrigerant passages  24  through the corresponding first ends  22   a  of the refrigerant inlet pipe  22 . The water is then passed through the refrigerant passages  24  and discharged through the first ends  23   a  of the refrigerant outlet pipe  23  to return to the circulation pipe. 
         [0035]    The rear opening of each refrigerant passage  24 , which faces the corresponding first end  22   a  of the refrigerant inlet pipe  22 , is the inlet into the refrigerant passage  24 . The inlet portion of the refrigerant passage  24  is an upstream portion  24   a  of the refrigerant passage  24  in the water flow direction (represented by arrow X 2  in  FIG. 2 ). The front opening of each refrigerant passage  24 , which faces the corresponding first end  23   a  of the refrigerant outlet pipe  23 , is the outlet from the refrigerant passage  24 . The outlet portion of the refrigerant passage  24  is a downstream portion  24   b  of the refrigerant passage  24  in the water flow direction. In the first embodiment, the “upstream portion  24   a  of each refrigerant passage  24 ” refers to a zone close to the inlet with respect to a middle portion in the flow direction in the refrigerant passage  24 . The “downstream portion  24   b  of each refrigerant passage  24 ” refers to a zone close to the outlet with respect to the middle portion in the flow direction in the refrigerant passage  24 . 
         [0036]    In the heat exchanging portion  12 , each cooling target fluid passage  21  and the adjacent refrigerant passage  24  are arranged side by side in the manner described below. Specifically, the inlet of the refrigerant passage  24  is arranged at the positions corresponding to the position of the outlet of the cooling target fluid passage  21  in the direction in which the passage defining bodies  18  are laminated. The position of the outlet of the refrigerant passage  24  corresponds to the position of the inlet of the cooling target fluid passage  21  in the lamination direction of the passage defining bodies  18 . As a result, in the heat exchanger  11  of the first embodiment, EGR gas and water form opposite flows, or, in other words, the flow direction of the EGR gas and the flow direction of the water are opposite to each other. Each of the partition walls  19 , which form the corresponding passage defining body  18 , separates the corresponding cooling target fluid passage  21  from the adjacent refrigerant passage  24 . 
         [0037]      FIG. 4  is a graph representing the relationship between the positions in the flow direction in the cooling target fluid passage  21  and the flow direction in the refrigerant passage  24  (the axis of abscissas) and the heat flux (the axis of ordinates). The line formed by a long dash alternating with two short dashes represents the local heat flux in a portion of each partition wall  19  thermally connected to a corresponding conventional fin in a case in which conventional fin having equal thicknesses over the entire zone (from the upstream portion to the downstream portion in the flow direction) of the cooling target fluid passage  21 . The broken line represents the critical heat flux of the water. 
         [0038]    Since only a short time has passed after EGR gas reached the cooling target fluid passage  21 , the temperature of the EGR gas in the upstream portion  21   a  of each cooling target fluid passage  21  is high. The EGR gas is then cooled through heat exchange with the water as the EGR gas proceeds toward the downstream portion  21   b,  and the temperature of the EGR gas decreases. Accordingly, as represented by the line formed by a long dash alternating with two short dashes in  FIG. 4 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding conventional fin becomes greater toward the upstream portion  21   a  of the cooling target fluid passage  21  and smaller toward the downstream portion  21   b  of the cooling target fluid passage  21 . 
         [0039]    In contrast, the temperature of the water in the upstream portion  24   a  of each refrigerant passage  24  is low because only a short time has passed since the water entered the refrigerant passage  24 . As the water proceeds toward the downstream portion  24   b,  some of the water is boiled through heat exchange with the EGR gas and transformed into a form mixed with bubbles. The water thus moves in the refrigerant passage  24  toward the outlet of the refrigerant passage  24  in the form mixed with bubbles. As the bubbles flow toward the downstream portion  24   b  of the refrigerant passage  24 , the bubbles may join one another and grow or increase in number. 
         [0040]    As a result, as represented by the broken line in  FIG. 4 , the critical heat flux of the water becomes smaller from the upstream portion  24   a  toward the downstream portion  24   b  in each refrigerant passage  24 . Accordingly, the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding conventional fin is likely to become greater than or equal to the critical heat flux of the water in the upstream portion  21   a  of the corresponding cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the corresponding refrigerant passage  24 . 
         [0041]    In the first embodiment, each second fin  30  is arranged in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone corresponding to the downstream portion  24   b  of the refrigerant passage  24 . Each second fin  30  extends in a corrugated shape in the direction perpendicular to the flow direction of the EGR gas. With reference to  FIG. 3A , the second fin  30  has flat surfaces  30   a  each contacting a wall surface  19   a  of the partition wall  19 . The height of the second fin  30  is equal to the interval between the two associated partition walls  19 . The second fin  30  has corner portions  30   b,  each of which is formed on one of the opposite sides of the corresponding one of the flat surfaces  30   a  and has a round shape. Brazing filler R 1  is poured into the gap between each of the corner portions  30   b  and the corresponding one of the partition walls  19  and melted so that the second fin  30  is brazed to the partition wall  19 . The second fin  30  and the partition wall  19  are thus thermally connected together and heat is transmissible between the second fin  30  and the partition wall  19 . 
         [0042]    Each first fin  31  is arranged in the downstream portion  21   b  of the associated cooling target fluid passage  21 , which is a zone corresponding to the upstream portion  24   a  of the refrigerant passage  24 . Each first fin  31  extends in a corrugated shape in the direction perpendicular to the flow direction of EGR gas. With reference to  FIG. 3B , the first fin  31  has flat surfaces  31   a  each contacting the corresponding wall surface  19   a  of the partition wall  19 . The height of the first fin  31  is equal to the interval between the two associated partition walls  19 . The first fin  31  has corner portions  31   b,  each of which is formed on one of the opposite sides of the corresponding one of the flat surfaces  31   a  and has a round shape. Brazing filler R 1  is poured into the gap between each corner portion  31   b  and the corresponding partition wall  19  and melted so that the first fin  31  is brazed to the partition wall  19 . The first fin  31  and the partition wall  19  are thus thermally connected together and heat is transmissible between the first fin  31  and the partition wall  19 . 
         [0043]    With reference to  FIGS. 3A and 3B , the thickness L 1  of each second fin  30  is greater than the thickness L 2  of each first fin  31 . The second fin  30  thus has a greater cross-sectional area than the first fin  31 . The second fin  30  and the first fin  31  have substantially equal surface areas. The proportion of the surface area with respect to the cross-sectional area perpendicular to the heat transmitting direction in the second fin  30  is smaller than the corresponding proportion in the first fin  31 . 
         [0044]    Operation of the heat exchanger  11 , which has the above-described configuration, will hereafter be described with reference to the graph of  FIG. 4 . 
         [0045]    When the vehicle operates, EGR gas, which is some of the exhaust gas discharged from the internal combustion engine, flows to the inlet of the EGR passage and is directed to the cooling target fluid passage  21  via the cooling target fluid inlet portion  14  and the corresponding elongated holes  13   e.  The EGR gas then flows from the inlet to the outlet in each of the cooling target fluid passages  21 . 
         [0046]    Meanwhile, a non-illustrated pump arranged in the circulation pipe is actuated to forcibly circulate water in the circulation pipe. The water is thus introduced into the refrigerant passages  24  through the refrigerant inlet pipe  22 . The water then flows from the inlet to the outlet in each of the refrigerant passages  24 . 
         [0047]    In the heat exchanger  11 , heat is transmitted from the EGR gas at a high temperature to the water at a low temperature through the partition walls  19 , the second fins  30 , and the first fins  31 . Each second fin  30  is arranged in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 , where the local heat flux in the portion of each of the partition walls  19  thermally connected to the corresponding one of the conventional fins is likely to become greater than or equal to the critical heat reflux of the water. The cross-sectional area of the second fin  30  is greater than the cross-sectional area of each first fin  31 , which is located in a zone other than the above-described zones. As a result, the heat in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is dispersed in the corresponding cooling target fluid passage  21 , compared to the heat in the portion of the partition wall  19  thermally connected to the corresponding first fin  31 . This prevents an increase in the local heat flux. 
         [0048]    Since there are a great number of bubbles in the downstream portion  24   b  of each refrigerant passage  24 , the critical heat flux of the water decreases in the downstream portion  24   b  of the refrigerant passage  24 . However, each second fin  30  is arranged in a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b.  As a result, even though the critical heat flux of the water is small, the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water. 
         [0049]    In the graph of  FIG. 4 , the solid line represents the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  and the corresponding first fin  31 . 
         [0050]    As is clear from  FIG. 4 , by increasing the cross-sectional area of each second fin  30 , the local heat flux in the portion of the corresponding partition wall  19  thermally connected to the second fin  30  is reduced. Specifically, the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water, thus preventing local burnout. The thickness L 1  of each second fin  30  must be set to such a value that the local heat flux at the position where the second fin  30  and the corresponding partition wall  19  are thermally connected together is smaller than the critical heat flux of the water. 
         [0051]    As heat exchange occurs between the water and the EGR gas through the partition walls  19 , the water boils and evaporates at the wall surfaces  19   a  of the partition walls  19 , thus generating boiling vaporization latent heat. The latent heat is then used to cool the EGR gas flowing in the cooling target fluid passage  21 . After having cooled the EGR gas, the water is directed from the outlet of each refrigerant passage  24  into the circulation pipe through the refrigerant outlet pipe  23 . The water is then condensed by a non-illustrated refrigerant condensing portion arranged in the circulation pipe and returned to the heat exchanger  11 . Meanwhile, the cooled EGR gas is directed from the outlet of each cooling target fluid passage  21  into the outlet of the EGR passage through the outlet pipe  17 . The EGR gas is then returned from the outlet of the EGR passage to the intake system of the internal combustion engine. 
         [0052]    The first embodiment has the advantages described below. 
         [0053]    (1) In the upstream portion  21   a  of each cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is likely to increase and the critical heat flux of the water is likely to decrease. Each second fin  30  is thus arranged in the aforementioned zone. Each first fin  31  is arranged in another zone. The thickness L 1  of each second fin  30  is greater than the thickness L 2  of each first fin  31 . This increases the heat transmission area of the portion of each partition wall  19  thermally connected to the corresponding second fin  30 , thus decreasing the local heat flux in this portion of the partition wall  19 . As a result, the local heat flux in the portion of the partition wall  19  thermally connected to the second fin  30  is prevented from becoming greater or equal to the critical heat flux of the water. Local burnout is thus prevented from occurring. 
         [0054]    (2) Heat exchange performance is high in the upstream portion  21   a  of each cooling target fluid passage  21 . Accordingly, by arranging each second fin  30 , which has a thickness greater than the thickness of a conventional fin in the upstream portion  21   a  of the cooling target fluid passage  21 , the heat transmission area is increased and film boiling is prevented. 
       Second Embodiment 
       [0055]    A heat exchanger for an EGR (exhaust gas recirculation) gas cooling device (an EGR cooler) in an EGR apparatus for vehicles according to a second embodiment of the present invention will now be described with reference to  FIGS. 5 and 6 . The same or like reference numerals are given to components of the second embodiment that are the same as or like corresponding components of the first embodiment. Repeated description of the components is omitted or simplified herein.  FIG. 5  shows only a portion of the heat exchanging portion  12  for illustrative purposes. 
         [0056]    As illustrated in  FIG. 5 , in the heat exchanging portion  12 , the inlets of each cooling target fluid passage  21  is arranged at a position corresponding to the inlet of the refrigerant passage  24  in the lamination direction of the passage defining bodies  18 . The position of the outlet of the cooling target fluid passage  21  corresponds to the position of the outlet of the refrigerant passage  24  in the lamination direction of the passage defining body  18 . As a result, in the heat exchanger  11  of the second embodiment, EGR gas and water form parallel flows, or, in other words, the flow direction of the EGR gas (represented by arrow X 1  in  FIG. 5 ) is parallel to the flow direction of the water (represented by arrow X 2  in the drawing). 
         [0057]    Each second fin  30  is arranged in the downstream portion  21   b  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 . Each first fin  31  is arranged in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the upstream portion  24   a  of the refrigerant passage  24 . 
         [0058]    As represented by the line formed by a long dash alternating with two short dashes in the graph of  FIG. 6 , in a case in which conventional fin is arranged in the entire zone of the cooling target fluid passage  21 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding conventional fin is likely to become greater than or equal to the critical heat flux of the water in the downstream portion  21   b  of the corresponding cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the corresponding refrigerant passage  24 . However, in the second embodiment, each second fin  30  is arranged in the downstream portion  21   b  of the associated cooling target fluid passage  21 , which is a zone corresponding to the downstream portion  24   b  of the refrigerant passage  24 . Each first fin  31  is arranged in a zone other than the aforementioned zone. As a result, as represented by the solid line in the graph of  FIG. 6 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water. 
         [0059]    Accordingly, the second embodiment has the same advantage as the advantage (1) of the first embodiment. 
       Third Embodiment 
       [0060]    A heat exchanger for an EGR (exhaust gas recirculation) gas cooling device (an EGR cooler) in an EGR apparatus for vehicles according to a third embodiment of the present invention will now be described with reference to  FIGS. 7 and 8 .  FIG. 7  shows only a portion of the heat exchanging portion  12  for illustrative purposes. 
         [0061]    As shown in  FIG. 7 , the inlet of the refrigerant passage  24  is arranged on one side in the direction perpendicular to the flow direction of EGR gas (represented by arrow X 1  in  FIG. 7 ). The outlets of the refrigerant passages  24  are located at the other side in the direction perpendicular to the EGR gas flow direction. Accordingly, in the heat exchanger  11  of the third embodiment, EGR gas and water form perpendicular flows, or, in other words, the flow direction of the EGR gas and the flow direction of the water (represented by arrow X 2  in  FIG. 7 ) extend perpendicular to each other. The cooling target fluid passage  21  and the refrigerant passage  24  are thus arranged side by side such that the EGR flow direction and the water flow direction cross each other, or, more specifically, extend perpendicular to each other. 
         [0062]    Each second fin  30  is arranged in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 . As viewed from above, each of the second fins  30  is shaped like a right triangle. Each first fin  31  is arranged in a zone other than the zone corresponding to the second fin  30 . 
         [0063]    The graph in  FIG. 8  represents the relationship between positions in the flow direction in each cooling target fluid passage  21  and heat flux, as observed along the cross section taken along line A-A in  FIG. 7 . 
         [0064]    As represented by the line formed by a long dash alternating with two short dashes in the graph of  FIG. 8 , in a case in which a conventional fin is arranged in the entire zone of the cooling target fluid passage  21 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding conventional fin is likely to become greater or equal to the critical heat flux of the water in the upstream portion  21   a  of the corresponding cooling target fluid passage  21 , which is a zone corresponding to the downstream portion  24   b  of the corresponding refrigerant passage  24 . However, in the third embodiment, each second fin  30  is arranged in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 . Each first fin  31  is arranged in a zone other than the aforementioned zone. As a result, as represented by the solid line in the graph of  FIG. 8 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water. 
         [0065]    Accordingly, the third embodiment has the same advantages as the advantages (1) and (2) of the first embodiment. 
         [0066]    The illustrated embodiments may be modified to the forms described below. 
         [0067]    In the second embodiment, each second fin  30  is arranged in the downstream portion  21   b  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the downstream portion  24   b  of the refrigerant passage  24 . Each first fin  31  is arranged in a zone other than the aforementioned zone. However, the present invention is not restricted to this configuration. For example, as illustrated in  FIG. 9 , each second fin  30  may be located in the upstream portion  21   a  of the associated cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the upstream portion  24   a  of the refrigerant passage  24 . Each first fin  31  is arranged in a zone other than the aforementioned zone. As represented by the line formed by a long dash alternating with two short dashes in the graph of  FIG. 10 , in a case in which a conventional fin is arranged in the entire zone of the cooling target fluid passage  21 , the local heat flux in the portion of each partition walls  19  thermally connected to the corresponding one of the conventional fins is likely to become greater than or equal to the critical heat flux of the water in the upstream portion  21   a  of the corresponding cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the upstream portion  24   a  of the corresponding refrigerant passage  24 . However, in this modified embodiment, the second fin  30  is arranged in the upstream portion  21   a  of the cooling target fluid passage  21 , which is a zone in the cooling target fluid passage  21  corresponding to the upstream portion  24   a  of the refrigerant passage  24 . Each first fin  31  is located in a zone other than the aforementioned zone. As a result, as represented by the solid line in the graph of  FIG. 10 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water. 
         [0068]    In the second embodiment, for example, as illustrated in  FIG. 11 , each second fin  30  may be arranged in a middle portion of the associated cooling target fluid passage  21  and each first fin  31  may be arranged in a zone in the cooling target fluid passage  21  other than the zone corresponding to the middle portion. As represented by the line formed by a long dash alternating with two short dashes in the graph of  FIG. 12 , in a case in which a conventional fin is arranged in the entire zone of each cooling target fluid passage  21 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding the conventional fin is likely to become greater than or equal to the critical heat flux of the water in the middle portion of the cooling target fluid passage  21 . However, in this modified embodiment, the second fin  30  is located in the middle portion of the cooling target fluid passage  21  and the first fin  31  is arranged in a zone other than the zone corresponding to the middle portion. As a result, as represented by the solid line in the graph of  FIG. 12 , the local heat flux in the portion of each partition wall  19  thermally connected to the corresponding second fin  30  is smaller than the critical heat flux of the water. 
         [0069]    In the illustrated embodiments, the thickness L 1  of each second fin  30  is greater than the thickness L 2  of each first fin  31 . However, the present invention is not restricted to this configuration. For example, the second fin  30  and the first fin  31  may have the same thickness. In this case, brazing filler R 1  is supplied into the gap between each corner portion  30   b  of the second fin  30  and the corresponding partition wall  19  by an amount greater than the amount of the brazing filler R 1  used in the gap between each corner portion  31   b  of the first fin  31  and the corresponding partition wall  19 . In this manner, the contact area by which the base portion of each second fin  30  is held in contact with the brazing filler R 1  is increased. 
         [0070]    In the illustrated embodiments, the height of each second fin  30  may be smaller than the height of each first fin  31 . This decreases the surface area of the second fin  30  compared to the surface area of the first fin  31 . As a result, compared to a case in which the second fins  30  and the first fins  31  have equal heights, the exposure area by which each second fin  30  is exposed to EGR gas decreases, thus reducing the local heat flux in the portion of the corresponding partition wall  19  thermally connected to the second fin  30 . 
         [0071]    In the third embodiment, the shape of each second fin  30  is not restricted to the right triangle as viewed from above. The shape of the second fin  30  is not particularly restricted and may be any suitable shape such as a rectangular shape as viewed from above, as long as the second fin  30  is arranged in correspondence with minimum portion of the upstream portion  21   a  of the cooling target fluid passage  21 , which are minimum portions of the zones corresponding to the downstream portions  24   b  of the refrigerant passages  24 . 
         [0072]    In the heat exchanger  11  of the third embodiment, EGR gas and water form the perpendicular flows, or, in other words, the flow direction of the EGR gas and the flow direction of the water extend perpendicular to each other. However, the flow directions of the EGR gas and the water may simply cross each other, without extending perpendicular to each other. 
         [0073]    In the illustrated embodiments, the second fins  30  and the first fins  31  each have a corrugated shape. However, each second fin  30  and each first fin  31  may be shaped in any other suitable shape. 
         [0074]    In the first embodiment, to introduce water into the heat exchanging portion  12  and direct the water out from the heat exchanging portion  12 , the first ends  22   a  of the refrigerant inlet pipe  22  are passed through the right surface of the housing  11   a  and the first ends  23   a  of the refrigerant outlet pipe  23  extend through the left surface of the housing  11   a.  However, the configuration for directing the water into and out from the heat exchanging portion is not restricted to this. For example, as described in “Japanese Laid-Open Patent Publication No. 7-159074”, the refrigerant inlet pipe  22  may be connected to the top surface of the housing  11   a  to introduce water into the heat exchanging portion. In this case, the refrigerant outlet pipe  23  is connected to the bottom surface of the housing  11   a  to direct the water out from the heat exchanging portion. 
         [0075]    In each of the illustrated embodiments, the heat exchanger  11  is embodied as the heat exchanger  11  used in an EGR gas vapor cooling device (an EGR cooler). However, the heat exchanger  11  may be embodied as a heat exchanger employed in a cooler for a device mounted in a vehicle, a refrigerator, or a freezer. 
         [0076]    In each of the illustrated embodiments, the cooling target fluid is EGR gas. However, the cooling target fluid is not restricted to this and may be gas other than the EGR gas or high-temperature liquid. 
         [0077]    The present invention may be employed in a shell-and-tube type vapor cooling heat exchanger.