Abstract:
A burner for exhaust gas purification devices, comprising a base, a first pipe section, and a second pipe section. The first pipe section has a base end section, a tip section, a combustion chamber wherein combustion air and fuel are combusted, and a discharge port from which combusted gas is discharged. The base end section is fixed to the base. An air flowpath through which combustion air passes is provided between the first pipe section and the second pipe section. The burner for exhaust gas purification devices also comprises a compressable blocking section fixed to the first pipe section or the second pipe section, and interposed between the tip section of the first pipe section and the second pipe section. The entire perimeter of the tip section of the first pipe section is supported so as to be slidable relative to the second pipe section, via the blocking section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. 371 and claims the benefit of PCT Application No. PCT/JP2013/071431 filed 7 Aug. 2013, which designated the United States, which PCT Application claimed the benefit of Japanese Patent Application No. 2012-174930 filed 7 Aug. 2012, and Japanese Patent Application No. 2012-245098 filed on 7 Nov. 2012, the disclosure of each of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a burner for an exhaust purification device, which is used in an exhaust purification device for purifying an exhaust gas from an internal-combustion engine (hereinafter, referred to as an engine) and raises the temperature of the exhaust gas. 
     BACKGROUND OF THE INVENTION 
     Conventional diesel engines include an exhaust gas purification device in the exhaust passage, and the exhaust gas purification device includes a diesel particulate filter (DPF), which captures particulates contained in an exhaust gas, and an oxidation catalyst. Such an exhaust gas purification device treats an exhaust gas to raise the temperature in order to maintain the function of purifying an exhaust gas. The treatment regenerates the DPF by burning the particulates captured by the DPF and activates the oxidation catalyst. A burner that performs the treatment for raising the temperature of the exhaust gas is arranged upstream of the DPF and the oxidation catalyst. 
     One example of the structure of the burner is a multilayer tube structure. In the multilayer tube structure, a plurality of tubular members is overlapped to be coaxial. A burner having the multilayer tube structure is advantageous for saving space and raising the temperature of air for combustion. 
     For example, Patent Document 1 discloses a combustor that includes a combustion tube including an outer tube and an inner tube, a short auxiliary combustion tube arranged radially inside of the inner tube, and a vaporization tube arranged radially inside of the auxiliary combustion tube. The bottom of each tube is fixed to a base. When the combustor is activated, fuel is injected in the auxiliary combustion tube and is vaporized in a premixing region arranged in the auxiliary combustion tube. The vaporized fuel is mixed with air for combustion supplied from the vaporization tube. A flame occurs in a combustion chamber by igniting a premixed air-fuel mixture, in which the fuel and the air for combustion are mixed. In this way, the premixed air-fuel mixture is combusted. An air flow path, through which air for combustion passes, is provided between the inner and outer tubes. The air for combustion supplied to the flame promotes combustion. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 58-160726 
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     Since a flame occurs in a head portion of the inner tube, the inner tube is heated to a high temperature and thermally expands mostly in the direction parallel to the central axis (in the axial direction). There is a space between the outer and inner tubes, and air passes through the space. Therefore, the outer tube has a lower temperature than the inner tube. As described above, because of the premixing region arranged inside of the auxiliary combustion tube, the auxiliary combustion tube has a lower temperature than the inner tube. For this reason, the expansion amount of the inner tube is greater than those of the auxiliary combustion tube and the outer tube during combustion. 
     In the burner with the multilayer tube structure, a difference of expansion amounts generally occurs between a tube exposed to a high temperature and a tube kept at a relatively low temperature. When the tubes are partially joined to each other by welding and the like, the difference between the expansion amounts of the tubes causes a large stress on the joining portion between the tubes. Even if the tubes are not joined to each other, there is a case, for example, in which the distal end of the inner tube is in a direct and close contact with the distal end of the outer tube as the aforementioned combustor. In this case, due to the radial expansion of the inner tube, the distal portion of the inner tube presses against the inner circumferential surface of the outer tube, and a force acts on the inner tube to hinder expansion in the axial direction. When ignition and extinction of the burner are repeated, each ignition and extinction causes stress on the joining portions, the contacting surfaces, and the like. As a result, depending on the usage conditions, damages such as fatigue and cracks may be occurred. This is not limited to the aforementioned burner, and burners with the multilayer tube structure generally have this kind of problem. 
     It is an objective of the present invention to provide a burner for an exhaust gas purification device that prevents tubes from being damaged due to a difference between the expansion amounts of the tubes in a multilayer tube structure. 
     In accordance with one aspect of the present disclosure, a burner for an exhaust gas purification device comprises a base, a first tube, and a second tube. The first tube includes a basal portion and a distal portion, a combustion chamber for combusting air for combustion and fuel, and a discharge port for discharging post-combustion gas. The basal portion is fixed to the base. An air flow path through which air for combustion passes is arranged between the first tube and the second tube. The burner for an exhaust gas purification device further comprises a compressible closing part, which is fixed to the first tube or the second tube and is arranged between the distal portion of the first tube and the second tube. The distal portion of the first tube has a circumference that is entirely, slidably supported by the second tube via the closing part. 
     According to the present aspect, the air flow path is provided between the second and first tubes. Because of the combustion chamber arranged inside of the first tube, the first tube has a higher temperature than the second tube. Therefore, when combustion starts, the expansion amount of the first tube is greater than that of the second tube. The distal end of the first tube is slidably supported by the second tube via the closing part. The closing part absorbs the radially-outward expansion of the first tube, while allowing the first tube to expand toward the distal end. Moreover, the closing part closes the distal end of an air passage. This suppresses leakage of air for combustion and suppresses damage caused by a difference between the thermal expansion amounts of the first and second tubes. 
     In another embodiment, the first tube is arranged radially inside of the second tube. The second tube includes a radially-narrowed portion. The closing part is held between the radially-narrowed portion of the second tube and the first tube. 
     In this case, since the closing part is held between the radially-narrowed portion of the second tube and the first tube, the thickness of the closing part is decreased compared to when the first and second tubes have constant diameters. Further, this decreases the diameter of the closing part compared to when the first tube has an enlarged diameter to reduce a space between the first and second tubes. 
     In another embodiment, the first tube is arranged radially inside of the second tube. The first tube includes a radially-enlarged portion. The closing part is held between the radially-enlarged portion of the first tube and the second tube. 
     In this case, since the annular closing part is held between the radially-enlarged portion of the first tube and the second tube, the thickness of the closing part is decreased compared to when the first and second tubes have constant diameters. 
     In another embodiment, the first tube includes a flange extending from a portion between the distal portion of the first tube and the closing part toward an inner circumferential surface of the second tube. 
     In this case, the flange arranged in a portion between the distal end portion of the first tube and the closing part closes the distal end of an air flow path at least in part, thereby suppressing leakage of air for combustion. 
     In another embodiment, the closing part is of a wire mesh. The first tube includes a hook for hooking the wire mesh. The hook projects from the outer circumferential surface of the first tube. 
     In this case, the closing part of a wire mesh absorbs the radial expansion of the first tube. The first tube also includes the hook for hooking the wire mesh. The hook projects from the outer circumferential surface of the first tube. The hook prevents the wire mesh from falling off and partially blocks an air flow, thereby suppressing leakage of air for combustion from an air flow path. 
     In another embodiment, the second tube is arranged radially outside of the first tube, so that a flow path of air for combustion is formed between the first tube and the second tube. The burner further comprises a first connecting tube portion and a second connecting tube portion. The first connecting tube portion is connected to the inner surface of the first tube and includes an opening at an end closer to the discharge port. The second connecting tube portion has a lid portion and partitions the combustion chamber from a premixing chamber. The second connecting tube includes a supply hole connected to the combustion chamber. The first connecting tube portion is inserted into the second connecting tube portion while being spaced from the second connecting tube portion. 
     In this case, the first and second connecting tube portions are overlapped, and a premixed air-fuel mixture has a longer flow path. This promotes mixture of the fuel and the air for combustion. The first tube expands in the axial direction without interference and extends toward the distal end. This suppresses change in the width of the flow path arranged between the first and second connecting tube portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a burner for an exhaust gas purification device according to a first embodiment of the present invention; 
         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. 1 ; 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  in  FIG. 1 ; 
         FIG. 5  is a schematic view of a burner for an exhaust gas purification device according to a second embodiment of the present invention; 
         FIG. 6  is a schematic view of a burner for an exhaust gas purification device according to a third embodiment of the present invention; 
         FIG. 7  is a schematic view of a burner for an exhaust gas purification device according to a fourth embodiment of the present invention; 
         FIG. 8  is a cross-sectional view of a principal part of a burner for an exhaust gas purification device according to a modification of the present invention; and 
         FIG. 9  is a cross-sectional view of a principal part of a burner for an exhaust gas purification device according to another modification of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of a burner for an exhaust gas purification device according to the present invention will now be described with reference to  FIG. 1  to  FIG. 4 . 
     As shown in  FIG. 1 , a diesel engine  1  includes, in an exhaust passage  2 , a DPF  3 , which captures particulates contained in an exhaust gas. The DPF  3  has a honeycomb structure made of a porous silicon carbide, for example, and captures particulates in the exhaust gas. A burner for an exhaust gas purification device (hereinafter, simply referred to as a burner)  10  is arranged upstream of the DPF  3 . The burner  10  carries out a regeneration process of the DPF  3  by raising the temperature of an exhaust gas flowing into the DPF  3 . 
     The burner  10  is connected to a compressor  7  via an air supply passage  4  and an intake passage  5  of the diesel engine  1 . The compressor  7  rotates with a turbine  6  arranged in the exhaust passage  2 . 
     An air valve  8  is arranged on the air supply passage  4 . The air valve  8  is capable of changing a flow path cross-sectional area of the air supply passage  4 . Opening and closing of the air valve  8  is controlled by a control unit, not shown. When the air valve  8  is in an open state, some intake air flowing through the intake passage  5  is introduced into the burner  10  from the air supply passage  4  as air for combustion. 
     The burner  10  will now be described in detail. The burner  10  has a dual tube structure, in which an inner tube  11  as a first tube and an outer tube  12  as a second tube are overlapped to be coaxial. The inner tube  11  made from metal and shaped substantially cylindrical has openings at both ends in the direction parallel to the central axis. The inner tube  11  includes a basal portion as a first end portion in the axial direction or a bottom portion, and a distal portion as a second end portion in the axial direction or a head portion. The opening of the bottom portion is fixed to and closed by a disk-shaped base  13 . The opening of the head portion of the inner tube  11  is open, and a flange  11 F projects radially outward from the entire circumferential rim at the distal edge. 
     Blades  15  are arranged in the basal portion of the inner tube  11 . As shown in  FIG. 2 , the blades  15  are formed by cutting and raising parts of the circumferential wall radially inward in the basal portion of the inner tube  11 . The blades  15  are arranged at equal intervals in the circumferential direction of the basal portion. Forming the blades  15  forms first introduction holes  16 , through which the exterior of the inner tube  11  is connected to the interior. 
     As shown in  FIG. 1 , a plurality of second introduction holes  17  extends through the sidewall of the inner tube  11  in the substantially axial center. The second introduction holes  17  are shaped circular and arranged at equal intervals in the circumferential direction of the inner tube  11 . 
     An orifice plate  18  is arranged radially inside of the basal portion of the inner tube  11 . The rim of the orifice plate  18  is joined to the inner circumferential surface of the inner tube  11 . An orifice  18 A is arranged in the center of the orifice plate  18 . The basal portion of the inner tube  11 , the base  13 , and the orifice plate  18  define a first mixing chamber  19  for mixing fuel with air for combustion. 
     A fuel supply port  13 A for fixing the injection port of a fuel supply unit  24  is arranged in the substantially radially-central location of the base  13 . The fuel supply unit  24  is connected to a fuel pump and a fuel valve, neither shown. Opening the fuel valve delivers fuel from the fuel tank to the fuel supply unit  24 . The delivered fuel is vaporized in the fuel supply unit  24  and injected into the first mixing chamber  19 . At this time, the injection direction of fuel is adjusted so that the orifice  18 A is on the line extending in the injection direction. 
     A disk-shaped burner head  20  is arranged closer to the head portion than the orifice plate  18  in the inner tube  11 . The rim of the burner head  20  is joined to the inner circumferential surface of the inner tube  11 . A large number of air holes  20 A extend through the burner head  20 . The burner head  20 , the inner tube  11 , and the orifice plate  18  define a second mixing chamber  21 . The first mixing chamber  19  and the second mixing chamber  21 , described above, form a premixing chamber  22  for mixing fuel with air for combustion. 
     A metal mesh  23  for avoiding backfire is arranged at the burner head  20  at a position close to the opening of the head portion. In the present embodiment, the metal mesh  23  is arranged on the upstream face of the burner head  20 , but may be arranged on the opposite face or on the both. 
     The burner head  20  and the inner tube  11  define a combustion chamber  25  for generating a flame F. An insertion hole is formed in the combustion chamber  25 . The insertion hole is closer to the burner head  20  than the location where the second introduction holes  17  are formed. The insertion hole extends through the inner tube  11 . The ignition portion  27  of a spark plug  26  is inserted into the insertion hole. 
     The outer tube  12  is made from metal and shaped substantially cylindrical. The outer tube  12  has openings at both ends in the direction parallel to the central axis. The outer tube  12  includes a basal portion as a first end portion in the axial direction or a bottom portion, and a distal portion as a second end portion in the axial direction or a head portion. The opening of the bottom portion of the outer tube  12  is closed by the base  13 . A lid portion  30  is arranged on the opening of the head portion of the outer tube  12 . A discharge port  31  is arranged in the center of the lid portion  30 . The discharge port  31  is connected to the exhaust passage  2 , and supplies a post-combustion gas delivered from the combustion chamber  25  to the exhaust passage  2 . 
     An air supply port  12 B for fixing the inlet of the air supply passage  4  is arranged in the outer tube  12  at a position close to the opening of the head portion. As shown in  FIG. 3 , a guide plate  32  is arranged on the inner circumferential surface of the outer tube  12  at a position near the opening of the air supply port  12 B. The guide plate  32  is fixed to the outer tube  12  in a cantilever-like manner in a state that the lateral face of the guide plate  32  is inclined in the direction along the inner circumferential surface of the outer tube  12 . The guide plate  32  is inclined in the same direction as the blades  15  on the inner tube  11 . 
     A distribution chamber  35  is arranged between the inner circumferential surface of the outer tube  12  and the outer circumferential surface of the inner tube  11 . The distribution chamber  35  distributes air for combustion to the first mixing chamber  19  and the combustion chamber  25 . As shown in  FIG. 2 , the distribution chamber  35  is shaped annular to surround the inner tube  11 . As shown in  FIG. 1 , the distribution chamber  35  is connected to the first mixing chamber  19  through the first introduction holes  16  arranged in the basal portion of the inner tube  11 . The distribution chamber  35  is also connected to the combustion chamber  25  through the second introduction holes  17  formed in the substantially center of the inner tube  11 . 
     As shown in  FIG. 1 , the head portion of the outer tube  12  includes a radially-narrowed portion  12 A formed by decreasing the outer and inner diameters. The flow path cross-sectional area is decreased with the radially-narrowed portion  12 A in the distal portion of the distribution chamber  35 . A small gap, which corresponds to a thermal expansion amount, is provided between the radially-narrowed portion  12 A and the flange  11 F of the inner tube  11 . 
     As shown in  FIG. 4 , the wire mesh  33  as a closing part is shaped annular by compression and is supported between the radially-narrowed portion  12 A and the inner tube  11 . The wire mesh  33  is fixed to the outer circumferential surface of the inner tube  11  by spot welding and the like, and comes in contact with the inner circumferential surface of the outer tube  12 . As shown in  FIG. 1 , the distal face of the wire mesh  33  is closed by the flange  11 F. 
     The wire mesh  33  is formed from a metal mesh shaped annular by compression. The metal mesh has a mesh size, which is the distance between the wires, of a few millimeters. When the inner tube  11  radially expands, the wire mesh  33  is compressed to absorb the expansion of the inner tube  11 . 
     Operation of the burner  10  of the first embodiment will now be described. 
     When a regeneration process of the DPF  3  starts, the air valve  8  is controlled to be in the open state, and the fuel supply unit  24  and the spark plug  26  are activated. When the air valve  8  is in the open state, some intake air flowing through the intake passage  5  is introduced to the distribution chamber  35  as air for combustion from the air supply passage  4  through the air supply port  12 B. At this time, as shown in  FIG. 3 , the guide plate  32  guides the air for combustion, thereby suppressing a flow of the air for combustion in the direction against the inclined direction of the guide plate  32 . As shown by the arrows in  FIG. 3 , the air for combustion keeps swirling in a predetermined direction and flows in the direction opposite toward the discharge port  31 . 
     As shown in  FIG. 1 , the wire mesh  33  and the flange  11 F close a gap between the distal portion of the inner tube  11  and the opening of the head portion of the outer tube  12 . This interferes with a flow of air from the air supply port  12 B toward the discharge port  31  and suppresses leakage of air for combustion from the opening of the outer tube  12 . 
     Some of the air for combustion introduced to the distribution chamber  35  is introduced to the combustion chamber  25  through the second introduction holes  17 . As shown in  FIG. 2 , the remaining portion of the air for combustion is introduced to the first mixing chamber  19  through the first introduction holes  16 . As described above, since the guide plate  32  and the blades  15  are inclined in the same direction, the air for combustion does not lose the momentum of swirling. Rather, the air for combustion gains momentum of swirling and is introduced to the first mixing chamber  19 . 
     The swirling flow generated by the blades  15  flows toward the orifice  18 A while converging to the radially-central region of the inner tube  11 , which is a region to which the fuel supply unit  24  supplies fuel. As described above, since the location of the orifice  18 A corresponds to the fuel injection direction, the center of swirling of the air for combustion overlaps with the fuel injection direction by the fuel supply unit  24 . The fuel is caught in the swirling flow and spreads outward from the center of the swirling flow. A large part of injected fuel passes through the orifice  18 A. 
     The premixed air-fuel mixture, in which air for combustion and fuel are mixed, keeps a swirling flow in a predetermined direction and is discharged to the second mixing chamber  21  through the outlet of the orifice  18 A. Since the downstream pressure of the orifice  18 A is more reduced than the upstream pressure, the mixed air-fuel mixture spreads throughout the second mixing chamber  21 . 
     In this way, the premixed air-fuel mixture mixed in the second mixing chamber  21  is introduced to the combustion chamber  25  through the air holes  20 A of the burner head  20 . When the ignition portion  27  ignites the premixed air-fuel mixture flowing into the combustion chamber  25 , a flame F occurs in the combustion chamber  25 . The premixed air-fuel mixture is combusted, and a post-combustion gas is generated. At this time, as shown in  FIG. 1 , air for combustion is supplied to near and downstream of the ignition portion  27  from the distribution chamber  35  through the second introduction holes  17 . As a result, the air for combustion and the post-combustion gas are exchanged, and combustion is promoted. 
     A post-combustion gas generated in the combustion chamber  25  is supplied to the exhaust passage  2  through the discharge port  31 . The temperature of an exhaust gas flowing into the DPF  3  is raised by the post-combustion gas mixed with an exhaust gas in the exhaust passage  2 . In the DPF  3  drawing such an exhaust gas, the temperature rises to the target temperature to burn the captured particles. 
     When a premixed air-fuel mixture is combusted in the combustion chamber  25 , the post-combustion gas at a high temperature heats the inner tube  11 . For this reason, after combustion starts, heat propagated via the inner tube  11  raises the temperature of air for combustion flowing in the distribution chamber  35 . The air for combustion at the raised temperature is introduced to the first mixing chamber  19  through the first introduction holes  16 . This suppresses liquidation of already vaporized fuel after combustion starts and promotes vaporization of liquidized fuel at that time. Moreover, the air for combustion in the distribution chamber  35  swirls around the inner tube  11 , and has a longer flow path in the distribution chamber  35  than when air for combustion linearly flows toward the first introduction holes  16  in the distribution chamber  35 . Thus, the air for combustion at a higher temperature is introduced to the first mixing chamber  19 . 
     In this way, while the inner tube  11  is heated by a post-combustion gas and the like, the outer tube  12  is exposed to the air for combustion passing through the distribution chamber  35 . For this reason, after combustion starts, the expansion amount of the inner tube  11  is greater than the expansion amount of the outer tube  12 , which is small. The inner tube  11  expands radially outward, but the radial expansion amount is small compared to the axial expansion amount. For this reason, the radial expansion amount of the inner tube  11  is absorbed by compression of the wire mesh  33 . The inner tube  11  expands toward the discharge port  31  while the wire mesh  33  and the distal end of the flange  11 F contact and slide on the inner circumferential surface of the outer tube  12 . 
     In contrast, when the outer circumferential surface of the inner tube  11  closely contacts the inner circumferential surface of the outer tube  12  to close the distal portion of the distribution chamber  35 , radially outward expansion of the inner tube  11  presses the distal portion of the inner tube  11  against the inner circumferential surface of the outer tube  12 . Since a force acts on the inner tube  11  to interfere with the axial expansion, the inner tube  11  is not easily extended in the axial direction. However, in the present embodiment, the contact area between the wire mesh  33  and the inner circumferential surface of the outer tube  12  is smaller than, for example, the contact area when the outer circumferential surface of the inner tube  11  closely contacts the inner circumferential surface of the outer tube  12 , and the frictional force is small when sliding. For this reason, the friction between the inner and outer tubes  11 ,  12  does not prevent the inner tube  11  from being axially extended by the expansion. 
     The wire mesh  33  only has to have the substantially same diameter as the inner diameter at the radially-narrowed portion  12 A of the outer tube  12 . If the outer tube  12  has a constant outer diameter from the basal to central portion, the diameter of the wire mesh  33  is smaller than that in a case in which the inner tube  11  has an enlarged diameter to narrow the distal portion of the distribution chamber  35 . Thus, the contact area between the wire mesh  33  and the outer tube  12  can be reduced, and the friction required to slide the inner tube  11  can be further decreased. Moreover, the downsized wire mesh  33  suppresses leakage of air for combustion from the wire mesh  33 , a space between the wire mesh  33  and the outer tube  12 , or the like. 
     As described above, the following advantages are provided according to the first embodiment. 
     (1) The distribution chamber  35  is arranged between the outer and inner tubes  12 ,  11 . Because of the combustion chamber  25  arranged in the inner tube  11 , the inner tube  11  has a higher temperature than the outer tube  12 . Therefore, when combustion starts, the expansion amount of the inner tube  11  is greater than that of the outer tube  12 . The distal portion of the inner tube  11  is slidably supported relative to the outer tube  12  via the wire mesh  33 . For this reason, while the wire mesh  33  absorbs the radially-outward expansion of the inner tube  11 , the inner tube  11  can expand toward the distal end. Further, the wire mesh  33  closes the distal portion of the distribution chamber  35 . This suppresses damage caused by a difference in the thermal expansion between the inner and outer tubes  11 ,  12 , while suppressing leakage of air for combustion. 
     (2) Since the annular wire mesh  33  is held between the radially-narrowed portion  12 A of the outer tube  12  and the inner tube  11 , the thickness of the wire mesh  33  is decreased compared to when the diameters of the outer and inner tubes  12 ,  11  are fixed. Moreover, the diameter of the wire mesh  33  is decreased compared to when the inner tube  11  has an enlarged diameter. 
     (3) The flange  11 F extends from the distal portion of the inner tube  11  toward the inner circumferential surface of the outer tube  12 . For this reason, the flange  11 F closes at least part of the distal portion of the distribution chamber  35 , and this suppresses leakage of air for combustion. 
     Second Embodiment 
     A second embodiment according to the present invention will now be described with reference to  FIG. 5 . A burner  10  of the second embodiment only differs from the first embodiment in a part of the inner tube and a part of the outer tube. Like reference characters designate like or corresponding parts and the parts will not be described in detail. 
     As shown in  FIG. 5 , the outer and inner diameters of the outer tube  12  are uniform from the basal to distal end. A radially-enlarged portion  11 B with increased outer and inner diameters is arranged at the distal end of the inner tube  11 . The flow path cross-sectional area in the distal portion of the distribution chamber  35  is decreased with the radially-enlarged portion  11 B. The flange  11 F is formed at the distal end of the radially-enlarged portion  11 B. The annular wire mesh  33  is supported between the radially-enlarged portion  11 B and the outer tube  12 . 
     Operation of the burner  10  of the second embodiment will now be described. 
     Similar to the first embodiment, when a premixed air-fuel mixture is combusted in the combustion chamber  25 , the inner tube  11  is heated by the post-combustion gas at a high temperature. The heat propagated by the inner tube  11  raises the temperature of air for combustion flowing through the distribution chamber  35 . The air for combustion at the raised temperature is introduced to the first mixing chamber  19  through the first introduction holes  16 . This suppresses liquidation of already-vaporized fuel after combustion starts, as well as promoting vaporization of fuel liquidized at that time. 
     Similar to the first embodiment, after combustion starts, the expansion amount of the inner tube  11  is greater than the expansion amount of the outer tube  12 , which is small. At this time, the radial expansion of the inner tube  11  is absorbed by compression of the wire mesh  33 . The inner tube  11  expands in the axial direction toward the discharge port  31  while the wire mesh  33  and the distal end of the flange  11 F contact and slide on the inner circumferential surface of the outer tube  12 . 
     According to the second embodiment, the following advantage is provided in addition to the advantages (1) and (3) in the first embodiment. 
     (4) Since the annular wire mesh  33  is held between the outer tube  12  and the radially-enlarged portion  11 B of the inner tube  11  in the second embodiment, the thickness of the wire mesh  33  is deceased compared to when the diameters of the outer and inner tubes  12 ,  11  are fixed. 
     Third Embodiment 
     A third embodiment of a burner for an exhaust gas purification device according to the present invention will now be described with reference to  FIG. 6 . The burner  10  of the third embodiment only differs from the first embodiment in the premixing chamber. Like or corresponding parts will not be described in detail. 
     The inner tube  11  and the outer tube  12  are fixed to the base  13  of the burner  10 . The lid portion  30  having the discharge port  31  is arranged in the distal portion of the outer tube  12 . The opening of the head portion of the inner tube  11  is open, and the flange  11 F projects radially outward from the entire circumferential rim. The radially-narrowed portion  12 A is arranged in the head portion of the outer tube  12 . 
     The wire mesh  33  shaped annular by compression is supported between the radially-narrowed portion  12 A and the inner tube  11 . The flange  11 F closes the distal face of the wire mesh  33 . 
     The premixing chamber will now be described. A connecting wall  60  and a burner head  61  are fixed to the inner surface of the inner tube  11 . The connecting wall  60  is arranged to include a portion between the blades  15  and the burner head  61  in the axial direction of the inner tube  11 . The connecting wall  60 , the base  13 , and the inner tube  11  define a first mixing chamber  71 . 
     The connecting wall  60  has an end portion in the axial direction, which projects toward the discharge port  31 . An insertion opening is formed at the end portion. A first connecting tube  62  is inserted in the insertion opening. The first connecting tube  62  extends in the axial direction from the connecting wall  60 , and opens toward the discharge port  31 . The inner space of the first connecting tube  62  is a second mixing chamber  72 . The connecting wall  60  and the first connecting tube  62  form a first connecting tube portion. 
     A connecting hole is formed in the center of the burner head  61 , and a second connecting tube  63  fits into the connecting hole. The burner head  61  and the second connecting tube  63  form a second connecting tube portion. The second connecting tube  63  extends in the axial direction from the burner head  61  toward the discharge port  31 , and the distal end is closed by a closing plate  64 . The second connecting tube  63 , the closing plate  64 , and the opening end of the first connecting tube  62  define a third mixing chamber  73 . The inner circumferential surface of the second connecting tube  63  and the outer circumferential surface of the first connecting tube  62  define a fourth mixing chamber  74 . The connecting wall  60 , the inner tube  11 , and the burner head  61  define a fifth mixing chamber  75 . 
     The mixing chambers  71 - 75  form a premixing chamber  70 . The second to fifth mixing chambers  72 - 75  have flow path cross-sectional areas different from each other. The inner tube  11 , the second connecting tube  63 , the burner head  61 , and the closing plate  64  define a combustion chamber  77 . 
     Operation of the aforementioned burner  10  will now be described. 
     When a regeneration process of the DPF  3  is started, air for combustion flows into the distribution chamber  35 . The air for combustion introduced by the guide plate  32  swirls around the inner tube  11 . 
     Some of the air for combustion flowing in the distribution chamber  35  is introduced to the combustion chamber  77  through the second introduction holes  17 . The remaining portion of the air for combustion is introduced to the first mixing chamber  71  through the first introduction holes  16 . Similar to the first embodiment, a swirling flow is generated in the first mixing chamber  71 . 
     In the first mixing chamber  71 , the fuel supply unit  24  supplies fuel toward the swirling flow to produce a premixed air-fuel mixture, in which the air for combustion and the fuel are mixed. The premixed air-fuel mixture flows into the second mixing chamber  72  while swirling. 
     After passing through the second mixing chamber  72 , the premixed air-fuel mixture turns around in the third mixing chamber  73 , and flows into the fourth mixing chamber  74 . Then, the premixed air-fuel mixture turns around again in the fifth mixing chamber  75 , and flows into the combustion chamber  77  through the supply holes  66  of the burner head  61 . 
     In the premixing chamber  70 , a flow path is lengthened by the length of the mixing chambers  71 - 75 , and this promotes mixing of air and fuel. Since the mixing chambers  71 - 75  have flow path cross-sectional areas different from each other, abrupt changes in the flow path cross-sectional area further promote mixing of air and fuel. 
     Ignition of the air-fuel mixture flowing into the combustion chamber  77  generates a flame F, which is an air-fuel mixture in combustion, in the combustion chamber  77 . The flame F generates a combustion gas. Air for combustion is supplied to the flame F through the second introduction holes  17  formed in the inner tube  11 . 
     The combustion gas generated in the combustion chamber  77  is supplied to the exhaust passage  2  through the discharge port  31 . The combustion gas heats the premixed air-fuel mixture in the fourth mixing chamber  74  via the second connecting tube  63 . This suppresses liquidation of already vaporized fuel and promotes vaporization of non-vaporized fuel. 
     In this way, while the inner tube  11  is heated by a post-combustion gas and the like, the outer tube  12  is exposed to air for combustion passing through the distribution chamber  35 . The radial expansion of the inner tube  11  is absorbed by compression of the wire mesh  33 . The inner tube  11  expands toward the discharge port  31  while the wire mesh  33  and the distal end of the flange  11 F contact and slide on the inner circumferential surface of the outer tube  12 . As described above, although the inner circumferential surface of the inner tube  11  is connected to the connecting wall  60  and the burner head  61 , the inner tube  11  is expandable toward the discharge port  31 . This suppresses change in the flow path cross-sectional areas of the third mixing chamber  73  and the fifth mixing chamber  75 . 
     As described above, according to the burner  10  of the third embodiment, the following advantage is provided in addition to the advantages (1) to (3) in the first embodiment. 
     (5) The premixing chamber  70  of the burner  10  has a portion at which the flow path of the premixed air-fuel mixture is turned around. For this reason, the burner  10  has a longer flow path of the premixed air-fuel mixture than a burner including a premixing chamber not having such a turned-around portion. This promotes mixture of air for combustion and fuel and improves combustion quality of the premixed air-fuel mixture. Thus, the combustion gas contains a less amount of non-combusted fuel. In the outer tube  12  and the inner tube  11  as above, a thermal expansion difference occurs between the tubes. However, since the distal portion of the inner tube  11  is slidably supported by the outer tube  12  via the wire mesh  33 , the inner tube  11  can expand toward the distal end while the wire mesh  33  absorbs the radially-outward expansion of the inner tube  11 . This suppresses change in the flow path cross-sectional area of the premixing chamber  70 . 
     Fourth Embodiment 
     A fourth embodiment of a burner for an exhaust gas purification device according to the present invention will now be described with reference to  FIG. 7 . The burner  10  of the fourth embodiment only differs from the second embodiment in the premixing chamber. In the fourth embodiment, the premixing chamber of the burner  10  of the second embodiment is modified to the premixing chamber of the third embodiment. Therefore, like or corresponding parts will not be described in detail. 
     The radially-enlarged portion  11 B with increased outer and inner diameters is arranged at the distal end of the inner tube  11 . The flange  11 F is formed at the distal end of the radially-enlarged portion  11 B. The annular wire mesh  33  is supported between the radially-enlarged portion  11 B and the outer tube  12 . 
     Operation of the burner  10  of the fourth embodiment will now be described. 
     Flows of combustion air, fuel, and a premixed air-fuel mixture are the same as those in the third embodiment. The radial expansion of the inner tube  11  is absorbed by compression of the wire mesh  33 . The inner tube  11  expands in the axial direction toward the discharge port  31  while the wire mesh  33  and the distal end of the flange  11 F contact and slide on the inner circumferential surface of the outer tube  12 . 
     As described above, according to the burner  10  of the fourth embodiment, the following advantage is provided in addition to the advantages (1) and (3) in the first embodiment and the advantage (4) in the second embodiment. 
     (6) In the distal portion of the inner tube  11 , the annular wire mesh  33  is held between the outer tube  12  and the radially-enlarged portion  11 B of the inner tube  11  having the first to fifth mixing chambers  71 - 75 . Compared to when the diameters of the outer and inner tubes  12 ,  11  are fixed, the thickness of the wire mesh  33  is decreased. The inner tube  11  expands toward the discharge port  31  while maintaining a space between the connecting wall  60  and the burner head  61 . 
     The above embodiments may be modified in the forms described below.
         In the above embodiments, the flange  11 F is arranged at the distal end of the inner tube  11 , but the flange  11 F may be omitted. In the initial state before combustion starts, the distal end of the flange  11 F may contact the inner circumferential surface of the outer tube  12 .   As shown in  FIG. 8 , a hook  11 E for hooking the wire mesh  33  may project from the outer circumferential surface of the inner tube  11 . The hook  11 E extends substantially perpendicular to the outer circumferential surface of the inner tube  11 . A plurality of hooks  11 E is arranged in the axial direction of the inner tube  11 . The hooks  11 E may be successively formed in the circumferential direction of the inner tube  11  to have an annular shape. Alternately, the hooks  11 E may be intermittently formed on the outer circumferential surface to be shaped like sectors of a circle. This fixes the wire mesh  33  more firmly. The hooks  11 E interfere with a flow of air from the air supply port  12 B toward the discharge port  31 , and this suppresses leakage of air from the distribution chamber  35 .   As shown in  FIG. 9 , as a substitute for the wire mesh  33 , a bellows tube  50  may be arranged between the inner and outer tubes  11 ,  12 . The bellows tube  50  is formed substantially cylindrical to entirely surround the inner tube  11 , and the wall has a waved cross-sectional shape. When the inner tube  11  expands radially outward, the bellows tube  50  extends between the inner and outer tubes  11 ,  12  to absorb the radial expansion of the inner tube  11 .   In the above embodiments, the orifice plate  18  is used to diffuse non-combusted fuel, but a funnel-shaped conduit with the inner diameter continuously decreasing from the inlet to the outlet, a Venturi tube, or the like may be used.   The air supply port  12 B may be formed in a portion not close to the head portion such as the central portion of the outer tube  12 . A plurality of air supply ports  12 B may be provided.   In the above embodiments, the swirling flow generating portion includes the blades  15 , which is cut and raised radially inward. However, the swirling flow generating portion may include something shaped different such as swirlers arranged on the outer circumference of the inner tube  11 .   In the above embodiments, the inner tube  11  as the first tube is arranged radially inside of the outer tube  12  as the second tube. However, the first tube may be arranged radially outside of the second tube. For example, when the first tube overlaps with a second tube, which is shorter than the first tube, the length difference between the tubes forms a space in the head portion of the first tube. A combustion chamber may be arranged in the space.   In the above embodiments, the burner includes a premixing chamber, but the burner may be a diffusion combustion type burner.   In the above embodiments, the fuel supply unit  24  is a type of device that vaporizes fuel inside, but may be a type of device that sprays liquid fuel into the inner tube  11 .   The ignition portion  27  may include a glow plug, a laser spark device, and a plasma spark device in addition to the spark plug as necessary. If the ignition portion  27  can generates a flame F, the ignition portion  27  may include only one of the glow plug, the laser spark device, and the plasma spark device.   Not limited to intake air flowing through the intake passage  5 , air for combustion may be air flowing through a pipe connected to a brake air tank or air supplied by a blower for the burner of an exhaust gas purification device.   Not limited to the DPF  3 , the exhaust gas purification device may be a device including a catalyst for purifying an exhaust gas. In this case, the burner  10  raises the temperature of the catalyst and therefore, the temperature promptly rises to the activation temperature.   The engine including the burner for an exhaust gas purification device may be a gasoline engine.       

     DESCRIPTION OF THE REFERENCE NUMERALS 
       10 : burner;  11 : inner tube as a first tube;  11 B: radially-enlarged portion;  11 E: hooks;  11 F: flange;  12 : outer tube as a second tube;  12 A: radially-narrowed portion;  25 : combustion chamber;  31 : discharge port;  33 : wire mesh as a closing part;  35 : distribution chamber as an air flow path;  60 : connecting wall included in a first connecting tube portion;  61 : burner head included in a second connecting tube portion;  62 : first connecting tube included in the first connecting tube portion; and  63 : second connecting tube included in the second connecting tube portion.