Patent Publication Number: US-2020292252-A1

Title: Heat exchanger

Description:
TECHNICAL FIELD 
     The present invention relates to a heat exchanger. 
     BACKGROUND ART 
     As shown in  FIG. 14 , a heat exchanger  40  of Patent Document 1 includes an outer wall  41  and partition walls  44 . The outer wall  41  has the form of a rectangular tube. The partition walls  44  partition the inner side of the outer wall  41  into a plurality of first cells  42  and a plurality of second cells  43  extending in an axial direction of the outer wall  41 . In a cross section orthogonal to the axial direction of the outer wall  41 , the first cells  42  and the second cells  43  are arranged in lines in a vertical direction. Specifically, from the left side of the plane of  FIG. 14 , the first cells  42  are located in the first, third, fifth, and seventh lines, and the second cells  43  are located in the second, fourth, sixth, and eighth lines. In the heat exchanger  40 , heat is exchanged between a first fluid flowing through the first cells  42  and a second fluid flowing through the second cells  43 . 
     The heat exchanger  40  of Patent Document 1 is set so that each second cell  43  has a cross-sectional flow area that is larger than that of each first cell  42 . When heat is exchanged between fluids having different thermal capacities, the second fluid having a smaller thermal capacity flows through the second cells  43  having a larger cross-sectional flow area to increase the amount of the second fluid in the heat exchanger  40 . This matches the thermal capacity of the first fluid as a whole with the thermal capacity of the second fluid as a whole in the heat exchanger  40  and increases the heat exchange efficiency. 
     PRIOR ART LITERATURE 
     Patent Literature 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2015-140960 
     SUMMARY OF THE INVENTION 
     Problems that the Invention is to Solve 
     A heat exchanger such as that shown in  FIG. 14  may be used to exchange heat between a gas, such as exhaust gas, and a liquid heat medium, such as a coolant. In this case, the heat of the gas is transferred through the partition walls of the heat exchanger to the liquid heat medium. However, it is difficult to increase the heat exchange efficiency of the heat exchanger because the heat of the gas transferred to the partition walls is limited. Accordingly, one object of the present invention is to provide a heat exchanger that increases the heat exchange efficiency. 
     Means for Solving the Problems 
     A heat exchanger in accordance with the present invention that solves the above problem includes a tubular outer wall and partition walls that partition an inner side of the outer wall into heat medium passage cells and gas passage cells extending in an axial direction of the outer wall. The heat exchanger exchanges heat between a liquid heat medium flowing through the heat medium passage cells and a gas flowing through the gas passage cells. The ratio of the number of the heat medium passage cells to the number of the gas passage cells is 1:3 to 1:6. 
     With this structure, the number of gas passage cells is three times or greater than the number of heat medium passage cells thereby increasing the total cross-sectional flow area of the gas passage cells and decreasing the velocity of the gas flowing through the gas passage cells. This increases the amount of time of contact between the gas and the partition walls and also increases the area of contact between the gas and the partition walls. Thus, the heat of the gas is readily transferred to the partition walls. Also, the number of the gas passage cells are less than or equal to six times of the number of the heat medium passage cells. This allows the liquid heat medium flowing through the heat medium passage cells to completely cool the partition walls. When the partition walls are cooled completely, the heat of the gas will be quickly transferred. As a result, the heat exchange efficiency of the heat exchanger is increased. 
     With the heat exchanger in accordance with the present invention, it is preferred that the outer wall has the form of a rectangular tube that includes two opposing first side walls and two opposing second side walls. Further, it is preferred that the heat medium passage cells and the gas passage cells are arranged in heat medium passage cell lines and gas passage cell lines that are parallel to the first side walls in a cross section orthogonal to the axial direction of the outer wall. Preferably, three to six of the gas passage cell lines are arranged between two adjacent ones of the heat medium passage cell lines in a direction extending along the second side walls. With this structure, the concentrated arrangement of the heat medium passage cells and the arrangement of most of the heat medium passage cells in a certain range of the gas passage cells facilitate the partition walls to be completely cooled and reduces pressure loss. 
     With the heat exchanger in accordance with the present invention, it is preferred that the outer wall has one side including an inlet and an outlet for a heat medium that are connected with the heat medium passage cells. With this structure, the arrangement of the inlet and the outlet for the heat medium in one side of the heat exchanger decreases the total volume when connecting, for example, pipes through which the heat medium flows. 
     With the heat exchanger of the present invention, it is preferred that the heat medium passage cells each have the same cross-sectional shape and the gas passage cells each have the same cross-sectional shape in a cross section orthogonal to the axial direction of the outer wall. This structure eliminates differences in the heat exchange efficiency between the gas passage cells and differences in the heat exchange efficiency between the heat medium passage cells, which would have otherwise been caused by different cross-sectional shapes. This also reduces pressure loss in the gas passage cells. 
     With the heat exchanger of the present invention, it is preferred that each of the heat medium passage cells has a cross-sectional shape that is larger in size than that of each of the gas passage cells in a cross section orthogonal to the axial direction of the outer wall. The heat medium flowing through the heat medium passage cells is a liquid. Thus, the heat medium has a larger passage resistance than the gas when flowing through the cells. This structure facilitates the flow of the heat medium having a higher flow resistance. 
     With the heat exchanger of the present invention, it is preferred that the partition walls include silicon carbide as a main component. The silicon carbide has a relatively high thermal conductivity among ceramic materials. Thus, this structure increases the thermal conductivity of the partition walls and increases the heat exchange efficiency of the heat exchanger. 
     Effect of the Invention 
     The present invention succeeds in increasing heat exchange efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a heat exchanger. 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  in  FIG. 2 . 
         FIG. 5  is a diagram illustrating a molding step. 
         FIG. 6  is a diagram illustrating a processing step (a diagram illustrating a state in which a processing jig for a first process is stuck in a molded body). 
         FIG. 7  is a diagram illustrating the processing step (a diagram illustrating a state in which the processing jig for the first process is stuck in and then pulled out of the molded body). 
         FIG. 8  is a diagram illustrating the processing step (a diagram illustrating a second process). 
         FIG. 9  is a diagram illustrating a degreasing step. 
         FIG. 10  is a diagram illustrating an impregnation step. 
         FIG. 11  is a front view of a heat exchanger of a modified example. 
         FIG. 12  is a schematic diagram showing a dimension measurement point in a simulation. 
         FIG. 13  is a temperature distribution chart obtained from the simulation. 
         FIG. 14  is a cross-sectional view of a prior art heat exchanger. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     One embodiment of a heat exchanger will now be described. 
     As shown in  FIGS. 1 and 2 , a heat exchanger  10  of the present embodiment includes an outer wall  11  and partition walls  12 . The outer wall  11  has the form of a rectangular tube. The partition walls  12  partition the inner side of the outer wall  11  into a plurality of heat medium passage cells  13   a  and a plurality of gas passage cells  13   b  extending in an axial direction of the outer wall  11 . The outer wall  11 , which has the form of a rectangular tube, includes two opposing vertical side walls  11   a  (first side walls) and two opposing lateral side walls  11   b  (second side walls). The outer wall  11  is configured so that its cross section orthogonal to the axial direction is rectangular and laterally elongated. 
     As shown in  FIG. 2 , in a cross section orthogonal to the axial direction of the outer wall  11 , the partition walls  12  form a grid-like cell structure and include partition walls  12  parallel to the vertical side walls  11   a  and partition walls  12  parallel to the lateral side walls  11   b . The cell structure of the partition walls  12  is not particularly limited. For example, the cell structure may be configured so that the partition walls  12  have a thickness of 0.1 to 0.5 mm and a cell density of 15 to 93 cells per 1 cm 2  in a cross section orthogonal to the axial direction of the outer wall  11 . 
     As shown in  FIG. 3 , the heat medium passage cells  13   a , through which a heat medium flows, each include two ends that are sealed by a sealed portion  22 . As shown in  FIG. 4 , each gas passage cell  13   b , through which a gas subject to processing flows, includes two open ends. The heat medium is not particularly limited and a known liquid heat medium may be used. Examples of known heat medium include a coolant (long life coolant (LLC)) and an organic solvent, such as ethylene glycol. The gas subject to processing may be, for example, exhaust gas of an internal combustion engine. 
     As shown in  FIG. 2 , in a cross section orthogonal to the axial direction of the outer wall  11 , each heat medium passage cell  13   a  has the same cross-sectional shape as the gas passage cells  13   b.    
     As shown in  FIG. 2 , the heat exchanger  10  includes a plurality of heat medium passage cell lines  14   a  and a plurality of gas passage cell lines  14   b . The heat medium passage cell lines  14   a  include only the heat medium passage cells  13   a  arranged parallel to the vertical side walls  11   a  of the outer wall  11 , and the gas passage cell lines  14   b  include only the gas passage cells  13   b  arranged parallel to the vertical side walls  11   a.    
     The heat exchanger  10  is set so that the ratio of the number of the heat medium passage cells  13   a  to the number of the gas passage cells  13   b  is in a certain range. The ratio (heat medium passage cells  13   a :gas passage cells  13   b ) is 1:3 to 1:6, and preferably, 1:4 to 1:5. 
     In the present embodiment, the ratio is adjusted by the arrangement of the heat medium passage cell lines  14   a  and the gas passage cell lines  14   b . Specifically, in a direction extending along the lateral side walls  11   b  of the outer wall  11 , a plurality of gas passage cell lines  14   b  are arranged between two adjacent heat medium passage cell lines  14   a . This arrangement is repeated in the direction of the lateral side walls  11   b  of the outer wall  11  to form an arrangement pattern. When the number of the gas passage cell lines  14   b  arranged between two adjacent heat medium passage cell lines  14   a  is three to six, the ratio is 1:3 to 1:6. Preferably, the number of the gas passage cell lines  14   b  arranged between two adjacent heat medium passage cell lines  14   a  is four to five. 
     As shown in  FIGS. 1 and 3 , in the heat exchanger  10 , the heat medium passage cell lines  14   a  each include a connection portion  15  extending in the vertical direction. The connection portion  15  extends through the partition walls  12  between adjacent heat medium passage cells  13   a  in the vertical direction and connects the cells of heat medium passage cell lines  14   a . The connection portion  15  has an end at one side in the vertical direction (upper side in  FIG. 3 ) that opens in the outer walls  11  (lateral side wall  11   b ) and an end at the other side in the vertical direction (lower side in  FIG. 3 ) reaching the heat medium passage cell  13   a  that is the farthest from the opening of the connection portion  15 . In other words, each connection portion  15  opens in one side of the outer wall  11  and extends to the heat medium passage cell  13   a  that is the farthest from the opening of the connection portion  15 . The connection portion  15  of the heat exchanger  10  includes a first connection portion  15   a  and a second connection portion  15   b . The first connection portion  15   a  is arranged closer to a first end  10   a , which is located at one side in the axial direction of the heat exchanger  10 , and the second connection portion  15   b  is arranged closer to a second end  10   b , which is located at the other side in the axial direction of the heat exchanger  10 . 
     As shown in  FIG. 3 , a heat medium flow passage  16  is formed inside the heat exchanger  10  by the heat medium passage cells  13   a , the first connection portion  15   a , and the second connection portion  15   b . The opening of the first connection portion  15   a  and the opening of the second connection portion  15   b  in the outer wall  11  of the heat exchanger function as an inlet or an outlet of the heat medium flow passage  16 . Further, as shown in  FIG. 4 , a gas flow passage  17  is formed inside the heat exchanger  10  by each gas passage cell  13   b , with its axial ends  10   a  and  10   b  functioning as an inlet or an outlet of the gas flow passage  17 . The heat exchanger  10  exchanges heat through the partition walls  12  between the heat medium flowing through the heat medium flow passages  16  and the gas flowing through the gas flow passages  17 . 
     The material of the outer wall  11 , which has the form of a rectangular tube, and the partition walls  12  of the heat exchanger  10  is not particularly limited. The material of a known heat exchanger may be used. The material is, for example, a carbide, such as silicon carbide, tantalum carbide, and tungsten carbide, or a nitride, such as silicon nitride and boron nitride. Among these substances, a material including silicon carbide as a main component is preferred since it has a higher thermal conductivity than other ceramic materials and increases the heat exchange efficiency. Here, “main component” refers to a component that is greater than or equal to 50% by mass. An example of a material including silicon carbide as a main component is a material including silicon carbide particles and metal silicon. 
     A method for manufacturing the heat exchanger of the present embodiment will now be described with reference to  FIGS. 5 to 10 . The heat exchanger is manufactured by sequentially performing a molding step, a processing step, a degreasing step, and an impregnation step as described below. 
     Molding Step 
     As a raw material for molding the heat exchanger, silicon carbide particles, an organic binder, and a dispersion medium are mixed to prepare a clay-like mixture. A molded body  20  shown in  FIG. 5  is molded from the clay-like mixture. The molded body  20  includes the outer wall  11 , which has the form of a rectangular tube, and the partition walls  12 , which partition the inner side of the outer wall  11  into a plurality of cells  13  extending in the axial direction of the outer wall  11 . The cells  13  in the molded body  20  each have two open ends. The molded body  20  can be molded, for example, by extrusion molding. A drying process is performed on the obtained molded body  20  to dry the molded body  20 . 
     Processing Step 
     In the processing step, a first process and a second process are performed. The first process is performed to form first connection portions and second connection portions in the molded body. The second process is performed to seal the two ends in some of the cells of the molded body. 
     As shown in  FIG. 6 , in the first process, for example, the first connection portions  15   a  and the second connection portions  15   b  are formed by a heated processing tool  21  that contacts the molded body and removes parts of the outer wall  11  and the partition walls  12  of the molded body  20 . 
     Specifically, as shown in  FIG. 6 , a blade having a contour that corresponds to the first connection portion  15   a  and the second connection portion  15   b  is prepared as the processing tool  21 . The blade is formed from a heat resistant metal (e.g., stainless steel) and has a thickness that is set so as not to exceed the width of the heat medium passage cell  13   a . Subsequently, the blade is heated to a temperature at which the organic binder included in the molded body  20  is burned and removed. For example, when the organic binder is methyl cellulose, the blade is heated to 400° C. or higher. 
     As shown in  FIG. 7 , the heated blade is stuck into the molded body  20  from an outer side and then pulled out to form the first connection portions  15   a  and the second connection portions  15   b . In this case, when the heated blade contacts the molded body  20 , the organic binder included in the molded body  20  is burned and removed at the contact portion. Thus, the insertion resistance of the molded body  20  against the blade is extremely small. This limits deformation and breakage around the portion where the blade is stuck. Further, the burned and removed organic binder reduces the amount of processing waste. 
     As shown in  FIG. 8 , in the second process, among the cells  13  of the molded body  20 , two ends of each cell  13  defining one heat medium passage cell  13   a  are sealed with the clay-like mixture used in the molding step. This forms the sealed portions  22  that seal the two ends of the cell  13 . Then, a drying process is performed on the molded body  20  to dry the sealed portions  22 . 
     A processed molded body is obtained by performing the processing step including the first process and the second process. The order in which the first process and the second process are performed is not particularly limited. The first process may be performed after the second process. 
     Degreasing Step 
     In the degreasing step, the processed molded body is heated to burn and remove the organic binder included in the processed molded body. This removes the organic binder from the processed molded body and obtains a degreased body. As shown in  FIG. 9 , a degreased body  30 , in which the organic binder is removed from the processed molded body in the degreasing step, has a frame portion arranged in a state in which silicon carbide particles are in contact with one another. 
     Impregnating Step 
     In the impregnation step, the inside of each wall forming the degreased body is impregnated with metal silicon. In the impregnation step, the degreased body is heated in a state contacting a cluster of metal silicon to a melting point of the metal silicon or higher (for example, 1450° C. or higher). As shown in  FIG. 10 , molten metal silicon enters the voids between particles, which form the frame portion of the degreased body, through capillary action and impregnates the voids. 
     The heating process in the impregnation step may be performed successively with the heating process of the degreasing step. For example, in a state contacting a cluster of metal silicon, the processed molded body may be heated at a temperature lower than the melting point of metal silicon to remove the organic binder and obtain the degreased body. Then, the heating temperature may be raised to the melting point of the metal silicon or higher to impregnate the degreased body with the molten metal silicon. 
     The heat exchanger is obtained by performing the impregnation step. 
     In the present embodiment, special temperature management is performed in the steps from the degreasing step. Specifically, the steps from the degreasing step are performed at a lower temperature than a sintering temperature of the silicon carbide included in the mixture used in the molding step so that the processed molded body and the degreased body are not exposed to a temperature higher than or equal to the sintering temperature. Therefore, in the degreasing step, heating is performed at a temperature that is higher than or equal to a temperature that burns and removes the organic binder and lower than the sintering temperature. In the same manner, in the impregnation step, heating is performed at a temperature higher than or equal to the melting point of metal silicon and lower than the sintering temperature. 
     The operation and advantages of the present embodiment will now be described. 
     (1) The ratio of the number of the heat medium passage cells to the gas passage cells of the heat exchanger is 1:3 to 1:6. The number of the gas passage cells is three times or greater than the number of the heat medium passage cells. Thus, a total cross-sectional flow area of the gas passage cells is increased, and the velocity of the gas flowing through the gas passage cells is decreased. This increases the amount of time of contact between the gas and the partition walls and the area of contact between the gas and the partition walls thereby allowing heat to be readily transferred from the gas to the partition walls. Also, the number of the gas passage cells is less than or equal to six times of the number of the heat medium passage cells. This allows the liquid heat medium flowing through the heat medium passage cells to completely cool the partition walls. When the partition walls are completely cooled, the heat of the gas will be quickly transferred. As a result, the heat exchange efficiency of the heat exchanger is increased. 
     (2) Three to six gas passage cell lines are arranged between two adjacent heat medium passage cell lines. The concentrated arrangement of the heat medium passage cells and the arrangement of most of the heat medium passage cells in a certain range of the gas passage cells facilitate the partition walls to be completely cooled. Further, pressure loss is reduced. 
     (3) The inlets and the outlets for the heat medium, which are connected with the heat medium passage cells, are located in the same side of the outer wall. The arrangement of the inlets and the outlets of the heat medium on one side of the heat exchanger allows the total volume to be decreased when connecting, for example. pipes through which the heat medium flows. 
     (4) In a cross section orthogonal to the axial direction of the outer wall, the heat medium passage cells each have the same cross-sectional shape, and the gas passage cells each have the same cross-sectional shape. This eliminates differences in the heat exchange efficiency between the gas passage cells and differences in the heat exchange efficiency between the heat medium passage cells that would result from different cross-sectional shapes. 
     (5) The partition walls include silicon carbide as a main component. Among ceramic materials, silicon carbide has a relatively high thermal conductivity. Thus, the partition walls, which include silicon carbide as a main component, have high thermal conductivity. This increases the heat exchange efficiency of the heat exchanger. 
     (6) The heat exchanger of the present embodiment is manufactured by performing temperature management as described above. The frame portion is formed in a state in which the silicon carbide particles are in contact with one another, and the shape of the frame portion is held with the voids filled with the silicon carbide. In other words, the silicon carbide particles do not include connected portions (necks), which result from sintering. This prevents cracking of necks between the silicon carbide particles even when internal temperature differences cause distortion in the partition walls during use of the heat exchanger. This further prevents cracks from spreading through necks. 
     The present embodiment may be modified as described below. Also, the configuration of the above embodiment and following modifications may be combined.
         In the present embodiment, the cells are arranged in the vertical direction of the outer wall, which has the form of a rectangular tube. However, the cells do not have to be arranged in the vertical direction. The heat exchanger may be used sideways and the cells may be arranged in a lateral direction.   The heat medium passage cell lines are not limited to a configuration that only includes the heat medium passage cells. The heat medium passage cell lines may be configured so that 80% or more of the cells are the heat medium passage cells. Further, the gas passage cell lines are not limited to a configuration that only includes the gas passage cells. The heat gas passage cell lines may be configured so that 80% or more of the cells are the gas passage cells. That is, 20% or less of the heat medium passage cell lines may be the gas passage cells. Further, 20% or less of the heat medium passage cell lines may be the heat medium passage cells.   The outer wall does not need to have the form of a rectangular tube. The outer wall may have the form of a round tube or a tube having an elliptic cross section. Also, the partition walls do not have to be grid-like in which the partition walls intersect each other at approximately 90°. The partition walls may be configured so that the cells have cross sections other than rectangular cross sections, such as rhombic or polygonal cross sections. For example, the partition walls may be configured to have hexagonal cross sections.       

     When the outer wall does not form a rectangular tube or when the partition walls are not grid-like and the partition walls do not intersect each other at approximately 90°, the outer wall may form cells with the partition walls that are shaped differently from the other cells. For example, in a configuration in which the partition walls form cells having hexagonal cross sections, the outer wall may form cells with the partition walls that have pentagonal or rectangular cross sections.
         The heat medium passage cells may have different cross-sectional shapes. The gas passage cells may have different cross-sectional shapes.   In the present embodiment, the outer wall and the partition walls are formed of a material including silicon carbide as a main component. Instead, only the partition walls may be formed of a material including silicon carbide as a main component. Alternatively, the outer wall and the partition walls may be formed of a material other than one including silicon carbide as a main component.   The cross-sectional shapes of the heat medium passage cells and the gas passage cells may differ in size in a cross section orthogonal to the axial direction of the outer wall. For example, as shown in  FIG. 11 , the heat medium passage cell  13   a  may be configured to have a larger widthwise dimension than the gas passage cells  13   b  so that the cross-sectional shape of each heat medium passage cell is increased in size. The liquid heat medium flowing through the heat medium passage cells has a greater passage resistance than a gas when flowing through the cells. Thus, when the heat medium passage cells each have a larger cross-sectional shape than the gas passage cells, the heat medium flows more smoothly. For example, in the configuration shown in  FIG. 11 , the widthwise dimension of the heat medium passage cell may be 1.0 to 5.0 mm, and the widthwise dimension of the gas passage cell may be 0.9 to 2.5 mm. Alternatively, the heat medium passage cells may each have a smaller widthwise dimension than the gas passage cells.   In a configuration in which three to six gas passage cell lines are arranged between two adjacent heat medium passage cell lines, the number of the gas passage cell lines, which is three to six, does not have to be fixed. That is, the number of the gas passage cell lines may vary from three to six.   As long as the ratio of the heat medium passage cells to the gas passage cells is 1:3 to 1:6, the arrangement of the heat medium passage cells and the gas passage cells is not limited to the configuration in which three to six gas passage cell lines are arranged between two adjacent heat medium passage cell lines. The ratio of the number of the heat medium passage cells to the gas passage cells being 1:3 to 1:6 means that, for example, in any group of four vertical cells×seven lateral cells, there is four to seven heat medium passage cells.       

     EXAMPLES 
     Specific examples of the above described embodiment will now be described. 
     Example 1 
     First, a mixture having the composition described below was prepared. 
     Particles of silicon carbide with average particle size of 15 μm (large particles): 52.5 parts by mass 
     Particles of silicon carbide with average particle size of 0.5 μm (small particles): 23.6 parts by mass 
     Methyl cellulose (organic binder): 5.4 parts by mass 
     Glycerol (lubricant): 1.1 parts by mass 
     Polyoxyalkylene compound (plasticizer): 3.2 parts by mass 
     Water (dispersion medium): 11.5 parts by mass 
     With this mixture, a molded body was molded to have a honeycomb structure in which the height was 50 mm, the width was 100 mm, the length was 100 mm, the thickness of the outer wall was 0.3 mm, the thickness of the partition walls was 0.25 mm, and the cell width was 1.2 mm. 
     Next, a plate-like jig heated to 400° C. was stuck into the outer wall of the molded body to form the first connection portions and the second connection portions. Then, predetermined cells were sealed with a clay-like mixture having the same composition as the above-described mixture to form the processed molded body in which four gas passage cell lines were arranged between two adjacent heat medium passage cell lines. In other words, in the processed molded body, the ratio of the number of the heat medium passage cells to the number of the gas passage cells was 1:4. Subsequently, the processed molded body was heated at 450° C. for five hours to remove the organic binder and obtain the degreased body. Then, the degreased body was heated at 1550° C. for seven hours in a vacuum in a state in which a 20 gram-metal silicon plate is placed on the degreased body to impregnate the degreased body with metal silicon and obtain the heat exchanger of example 1. 
     Evaluation Tests 
     The heat exchanger of example 1 was evaluated for temperature distribution in the heat medium passage cells and the gas passage cells by a simulation. Further, heat exchangers of examples 2 to 4 were evaluated for temperature distribution under the same condition as example 1 except in that the number of the gas passage cell lines between two adjacent heat medium passage cell lines was set to three, five, and six, that is, the ratio of the numbers of the heat medium passage cells to the gas passage cells was set to 1:3, 1:5, and 1:6. Also, heat exchangers of comparative example 1 and 2 were evaluated for temperature distribution under the same condition as example 1 except in that the number of the gas passage cell lines between two adjacent heat medium passage cell lines was set to two and eight, that is, the ratio of the numbers of the heat medium passage cells to the gas passage cells was set to 1:2 and 1:8. 
     Simulation Condition 
     A simulation condition is described as below.  FIG. 12  shows where measurements were taken with regard to the dimensions of a cell.
         Cell height T: 1.2 mm, cell width H: 1.2 mm, length of heat medium passage cell: 100 mm, length of gas passage cell: 100 mm   Partition wall thickness W: 0.25 mm, thermal conductivity of partition wall: 190 W/m*K   Temperature of heat medium: 40° C., flow rate of heat medium: 10 L/min   Temperature of gas: 400° C., flow rate of gas: 10 g/sec   Name of simulation software: Fluent (registered trademark, manufactured by ANSYS)       

       FIG. 13  shows the results of the simulation. 
     The left column in  FIG. 13  shows the temperature distribution at a central portion in the axial direction of the heat exchanger (10 mm from axial end), and the right column in  FIG. 13  shows the temperature distribution at an outlet side of the heat exchanger (90 mm from axial end). The temperature distribution in the cells is shown in different colors. 
     First, the temperature distribution of example 1 will be described. Halves of the heat medium passage cells (½ of each cell) were arranged at the left side, and two lines of the gas passage cells were located at the right side of the heat medium passage cells to set the ratio of the cells to 1:4. Then, the heat medium and the gas were distributed under a predetermined condition to measure the temperature distribution in the heat medium passage cells, the partition walls, and the gas passage cells. 
     As shown in  FIG. 13 , in examples 1 to 4, the heat medium passage cells and the partition walls were each less than or equal to 50° C. This indicates that the partition walls were completely cooled. At the central portion in the axial direction of the heat exchanger, the maximum temperature in the gas passage cells was lower than or equal to 120° C. At the outlet side of the heat exchanger, the maximum temperature in the gas passage cells was lower than or equal to 58° C. In particular, in examples 1 and 3, the region in which the temperature was close to 58° C. was limited at the outlet side of the heat exchanger. Thus, it was confirmed that the gas in the gas passage cells was cooled in a preferred manner and the heat exchange efficiency was high. 
     In contrast, in comparative examples 1 and 2, at the central portion in the axial direction of the heat exchanger, the maximum temperature in the gas passage cells was higher than or equal to 120° C. At the outlet side of the heat exchanger, the maximum temperature in the gas passage cells was higher than or equal to 58° C. Also, in comparative example 2, a region in which the temperature of the partition walls was 50° C. or higher was present at the central portion in the axial direction of the heat exchanger and thus the partition walls were not completely cooled. Thus, it was confirmed that the heat exchange efficiency was low. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
       10 ) heat exchanger,  11 ) outer wall,  12 ) partition wall,  13   a ) heat medium passage cell,  13   b ) gas passage cell.