Abstract:
A vaporizer, a fuel cell system including the vaporizer, and a method of vaporizing fuel in a fuel cell system are disclosed. The fuel cell system includes a fuel reservoir ( 24 ) for storing a liquid fuel and a fuel cell ( 10 ) for consuming a fuel and generating electricity therefrom. A fuel vaporizer ( 28 ) is interposed between the fuel reservoir ( 24 ) and the fuel cell ( 10 ) for receiving liquid fuel and vaporizing it and delivering it ultimately to the fuel cell ( 10 ). The fuel vaporizer ( 28 ) includes a heat exchanger which includes a hot fluid inlet ( 65 ), a hot fluid outlet ( 67 ) and a core ( 50 ) interconnecting the inlet ( 65 ) and the outlet ( 68 ). The core ( 50 ) has alternating fuel flow structures ( 68 ) and hot fluid structures ( 69 ) with the fuel flow structures ( 68,69 ) having an inlet ( 56 ) and an outlet ( 58 ).

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to fuel cell systems of the type including a reformer that creates a hydrogen rich gas for use in the fuel cell from a liquid fuel whose composition includes hydrogen. More specifically, the invention relates to the vaporization of the fuel prior to its admission to the reformer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Recent years have seen a marked increase in interest in fuel cells for the generation of electric power. One area where interest is high is in the design of propulsion systems for vehicles. As is well known, a typical fuel cell combines hydrogen and oxygen to generate electricity which may then be used to power an electric motor which can be used to provide propulsion for a vehicle.  
           [0003]    More recently, there have been a variety of proposals of fuel cell systems employing a so-called reformer. Reformers are chemical processors which take an incoming stream of a hydrocarbon containing or hydrocarbon based material and react it with water to provide an effluent that is rich in hydrogen gas. This gas, after being further treated to rid it of fuel cell poisoning constituents, most notably carbon monoxide, is then provided to the anode side of a fuel cell. Ambient air is provided to the cathode side of the fuel cell. The oxygen in the air and the hydrogen in the anode gas are reacted to provide water and generate electricity that may be used to power a load such as an electric motor.  
           [0004]    The reformer must receive the fuel and water in vapor form. Consequently, if the disadvantage of high pressure vessels associated with some pure hydrogen fuel cells is to be avoided, some means of carrying the fuel in a liquid form in a tank comparable to gasoline or diesel fuel tanks must be provided along with a means for vaporizing the water and the fuel prior to its admission to the reformer. While for many non-vehicular applications, the matter of vaporizing the water and the fuel may be handled relatively simply, the problem is much more difficult where the production of electricity by the fuel cell is expected to respond rapidly to a change in electrical load. In the vehicular context, this means that the fuel cell must respond rapidly to changes commanded by the driver of the vehicle through changes in the position of the fuel cell equivalent of a conventional gas pedal.  
           [0005]    It has been determined that the rapidity of response of the fuel cell to a commanded change depends on the mass of water and fuel in the vaporizer that feeds vaporized water and fuel to the reformer. The greater the mass of fuel and water in the vaporizer, the longer the response time. Consequently, it has been determined that to be effective in fuel cell systems powering loads which require rapid response to a change in conditions, the mass of fuel and water in the vaporizer be held to an absolute minimum. To meet this requirement, it is highly desirable that the fuel and water side of the vaporizer have as small a volume as possible.  
           [0006]    In vehicular applications, it is also highly desirable that the overall vaporizer be as small in size as possible in terms of volume and in weight. Bulk and weight are highly disadvantageous in that weight reduces the overall fuel efficiency of the vehicle and bulk reduces the load carrying capacity of the vehicle to the point that it is impractical to provide a vehicle that can compete with conventionally powered vehicles in use today. It is also desirable to achieve a very short system start-up time.  
           [0007]    The present invention is directed to overcoming one or more of the above problems.  
         SUMMARY OF THE INVENTION  
         [0008]    It is the principal object of the invention to provide a new and improved fuel cell system of the reformer type and more particularly, an improved fuel vaporizer for use in a reformer containing fuel cell system.  
           [0009]    It is also a principal object of the invention to provide a new and improved method of vaporizing liquid fuel and/or water (collectively referred to hereinafter as liquid fuel) for use in a fuel cell system.  
           [0010]    According to one facet of the invention involving a fuel cell system including a fuel cell having a vaporized fuel inlet, a source of liquid fuel to be vaporized and a fuel vaporizer interconnecting the fuel cell and the source, the invention includes a method of vaporizing fuel in the fuel vaporizer which comprises the steps of: (a) introducing liquid fuel to be vaporized into a fuel inlet for a fuel flow path of the fuel vaporizer; (b) passing a heated fluid through at least one hot fluid flow path in heat exchange relation to the fuel flow path to heat and vaporize fuel in the fuel flow path; and (c) controlling the pressure drop of the fuel as it passes from the inlet to an outlet whereat the fuel emerges as a vapor such that the majority of the pressure drop occurs near the inlet before virtually any of the liquid fuel is vaporized.  
           [0011]    In a preferred embodiment, the majority of the pressure drop is at least about 70% and even more preferably is about 95%.  
           [0012]    According to another facet of the invention, a fuel cell system is provided. The fuel cell system includes a fuel reservoir for storing a liquid fuel for a fuel cell and a fuel cell for consuming the fuel and generating electricity therefrom. A fuel reformer for receiving fuel in a vaporized state is connected to the fuel cell for providing a fuel thereto for consumption therein and the system further includes a fuel vaporizer interposed between the fuel reservoir and the fuel reformer for receiving liquid fuel from the fuel reservoir and vaporizing the liquid fuel to a vaporized state for delivery to the fuel reformer. The fuel vaporizer includes a heat exchanger having a hot fluid inlet, a hot fluid outlet and a core interconnecting the inlet and outlet. The core has alternating hot fluid passages extending between the hot fluid inlet and the hot fluid outlet and in heat exchange relation with liquid/vaporized fuel passages. The hot fluid passages each include a fin or fins extending the length thereof and two separator plates are bonded to and sandwich the fin(s). The heat exchanger further includes a liquid fuel inlet and a vaporized fuel outlet. The liquid/vaporized fuel passages extend between the liquid fuel inlet and the vaporized fuel outlet and include two abutting plates, each having elongated slots therein. The slots extend diagonally to the mean direction of fuel flow in the liquid/vaporized fuel passages with slots in one plate criss-crossing slots in the other plate to be in fluid communication therewith. Separator plates are bonded to and sandwich the abutting plates. The slots, in one embodiment, have a progressively decreasing angle with the mean direction of fuel flow through the fuel vaporizer from the liquid fuel to the vaporized fuel outlet.  
           [0013]    In a preferred embodiment, the liquid/vaporized fuel passages include a maze capable of providing a high pressure drop adjacent the inlet and a low pressure drop section extending between the maze and the vaporized fuel outlet.  
           [0014]    In one embodiment, the maze includes a plurality of intersecting relatively short and narrow slots connected in hydraulic series and in fluid communication with the liquid fuel inlet and a relatively long manifold generally transverse to the mean direction of the fuel flow and which in turn includes a plurality of orifice slots extending, at uniformly spaced intervals, to the remainder of the criss-crossing elongated slots.  
           [0015]    A preferred embodiment also contemplates that each liquid fuel passage include a plurality of hydraulically isolated channels extending from the liquid fuel inlet to the vaporized fuel outlet with each channel having substantially equal flow resistance.  
           [0016]    The invention, in another facet, includes a fuel reservoir, a fuel cell, a fuel reformer and a fuel vaporizer as before. In this embodiment, the fuel vaporizer includes a hot fluid inlet, a hot fluid outlet, and a core interconnecting the inlet and the outlet. The core has alternating hot fluid passages extending between the hot fluid inlet and the hot fluid outlet which are in heat exchange relation with liquid/vaporized fuel passages. The hot fluid passages each include a fin or fins extending the length thereof and two separator plates are bonded to and sandwich the fin(s). The heat exchanger further includes a liquid fuel inlet and a vaporized fuel outlet with the liquid/vaporized fuel passages extending between the two and which include two abutting plates, each having elongated slots therein which extend diagonally to the mean direction of fuel flow in the liquid/vaporized fuel passages with the slots in one plate criss-crossing the slots in the other plate to be in fluid communication therewith. Separator plates are bonded to and sandwich the abutting plates and the slots have a progressively increasing width from the liquid fuel inlet to the vaporized fuel outlet.  
           [0017]    In each of the foregoing, the separator plates for the hot fluid passages and the separator plates for the abutting plates are the same, being common to both.  
           [0018]    According to still another facet of the invention, there is a fuel system generally as described above and which includes a fuel vaporizer which has a fuel passage structure in heat exchange relation with a passage for a heated heat exchange medium defined by at least one fuel passage sheet having a plurality of fuel flow areas therein, an inlet at or near one end of the sheet to deliver liquid fuel to the fuel flow areas and an outlet at or near an opposite end of the sheet. The outlet includes an enlarged opening in the sheet connected to the fuel flow areas and serving as a collection manifold for vaporized fuel exiting the fuel flow areas. A pair of separator plates sandwich the at least one plate to close the fuel flow areas and the enlarged opening. At least one of the separator sheets includes a series of raised, spaced dimples in the enlarged opening and contacting and bonded to the other of the separator sheets.  
           [0019]    In a preferred embodiment, the dimples are elongated and even more preferably, the outlet further includes a fuel outlet passage in fluid communication with the enlarged opening and the dimples are elongated and oriented within the enlarged opening to direct the flow of vaporized fuel from the fuel flow areas to the fuel outlet passage.  
           [0020]    According to still another facet of the invention, a fuel cell system as described previously is provided. A fuel vaporizer includes a stack of fuel passage structures alternating with heated heat exchange medium structures to define heated medium passages. Each fuel passage structure includes at least one fuel passage sheet having a fuel flow area therein sandwiched between two separated plates and each heated medium structure includes a fin between two second separator plates. Second separator plates extend beyond opposite ends of the fuel passage plate and have aligned, enlarged openings beyond the opposite ends which serve as inlet and outlet manifolds in fluid communication with the heated medium passages. The second separator plates are sealed to each other about the enlarged openings except at a location establishing fluid communication with the heated medium passages.  
           [0021]    According to still another facet of the invention, a vaporizer is provided which includes a plurality of fuel flow structures, each comprising two abutting fuel flow sheets sandwiched between first separator sheets. The fuel flow sheets have elongated slots extending in a progressively decreasing angle to a mean direction of fuel flow through the fuel flow structures with increasing widths from one end of the fuel flow sheet to the other. The slots in one fuel flow sheet are in criss-cross relation with slots in the other fuel flow sheet. The vaporizer further includes a plurality of heated medium flow structures comprising a fin sandwiched between two second separator plates with each second separator sheet extending beyond the ends of the fuel flow sheets and having first aligned openings therein in alignment with one another at locations beyond the fuel flow sheet ends. The fuel flow structures and the heated medium structures are located in a stack in alternating relation with the fuel flow structures in the stack being aligned with one another and the heated medium structures in the stack being aligned with one another. A common fuel inlet to the fuel flow structures is located on a side of the fuel flow sheets near one of the ends. Also provided is a common fuel outlet from the fuel flow structures on a side of the fuel flow sheets near the other of the ends. Second aligned, enlarged openings are located in the fuel flow sheets at the other end and intersect some of the slots and are connected to the common fuel outlet. A highly flow resistant maze is located in each of the fuel flow sheets at the one end and intersects others of the slots and is connected to the common fuel inlet. A common heated medium inlet is located at the other end of the stack and is in fluid communication with the first enlarged openings thereat. A common heated medium outlet at said one end is in fluid communication with the first enlarged openings thereat.  
           [0022]    Numerous other objects and advantages of the invention will become apparent from the following description of the intention and the accompanying drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a schematic illustrating a typical fuel cell system of the type employing a reformer with which the fuel vaporizer of the present invention may be employed;  
         [0024]    [0024]FIG. 2 is a perspective view of a fuel vaporizer made according to the invention;  
         [0025]    [0025]FIG. 3 is an exploded view of one embodiment of one fuel flow structure and one hot gas structure employed in the invention;  
         [0026]    [0026]FIG. 4 is a plan view of one of the fuel flow plates illustrated in FIG. 3;  
         [0027]    [0027]FIG. 5 is a plan view of the other fuel flow plate illustrated in FIG. 3;  
         [0028]    [0028]FIG. 6 is a plan view of the fuel flow plates of FIGS. 4 and 5 superimposed upon one another;  
         [0029]    [0029]FIG. 7 is a view similar to FIG. 4 but showing a modified embodiment of one fuel flow plate;  
         [0030]    [0030]FIG. 8 shows a second fuel flow plate of the embodiment of FIG. 7;  
         [0031]    [0031]FIG. 9 shows the fuel flow plates of FIGS. 7 and 8 superimposed upon one another;  
         [0032]    [0032]FIG. 10 shows still another embodiment of one fuel flow plate;  
         [0033]    [0033]FIG. 11 shows a second fuel flow plate according to the embodiment of FIG. 10;  
         [0034]    [0034]FIG. 12 shows the fuel flow plates of FIGS. 10 and 11 superimposed upon one another;  
         [0035]    [0035]FIG. 13 illustrates still another embodiment of one fuel flow plate;  
         [0036]    [0036]FIG. 14 shows a second fuel flow plate to be used in the embodiment of FIG. 13;  
         [0037]    [0037]FIG. 15 shows the fuel flow plates of FIGS. 13 and 14 superimposed upon one another;  
         [0038]    [0038]FIG. 16 shows still another embodiment of a fuel flow plate;  
         [0039]    [0039]FIG. 17 shows another fuel flow plate according to the embodiment of FIG. 16;  
         [0040]    [0040]FIG. 18 shows the fuel flow plates of FIGS. 16 and 17 superimposed upon one another;  
         [0041]    [0041]FIG. 19 is a fragmentary plan view of the end of the separator plate  108 , and specifically the end thereof including the openings  134  to be connected to the outlet manifold  57 ; and  
         [0042]    [0042]FIG. 20 is a fragmentary, enlarged, sectional view taken approximately along the line  20 - 20  in FIG. 19.  
         [0043]    FIGS.  4 - 18 , inclusive, are scale drawings. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Exemplary embodiments of the invention will be described herein in the environment of intended use in a vehicle and one which utilizes methanol as a hydrogen containing liquid that is combined with water to produce a hydrogen rich gas for use in a fuel cell. Methanol is a preferred fuel because it is easy to reform into the anode gas. However, it is to be understood that the invention may be employed with efficacy in non-vehicular applications, particularly where rapid response to a load change is required. The vaporizer may also be employed with efficacy in other reformer type fuel cell systems that employ a liquid fuel other than methanol as, for example, ethanol, gasoline, diesel fuel, etc. Consequently, the invention should not be regarded as limited to vehicular systems or methanol type systems except insofar as expressly so stated in the appended claims.  
         [0045]    Turning now to FIG. 1, one type of fuel cell system embodying a reformer with which the invention may be used is illustrated in FIG. 1. This system is specifically intended to be employed in a vehicle but may be used to advantage in other environments.  
         [0046]    The system includes a fuel cell  10  with provision for an anode gas inlet stream on a line  12 . The anode gas typically will be hydrogen, carbon dioxide, and water vapor.  
         [0047]    The fuel cell also includes an inlet line  14  leading to the cathode side of the fuel cell and through which an oxygen rich stream is received. In the usual case, the stream will be air.  
         [0048]    The fuel cell also includes a cooling loop, generally designated  16 , as is well known.  
         [0049]    The cathode exhaust is discharged on a line  18  which eventually leads to a water tank or reservoir  20 . That is to say, water, the product of the chemical reaction within the fuel cell  10 , is provided to the water tank  20  for later reuse in the reforming process.  
         [0050]    In addition to the water tank  20 , the system includes a fuel tank  24  which, in the system shown, contains methanol. Pumps  26  that are electrically driven by battery power during start-up or by the electricity produced by the fuel cell  10  during operation, meter water and methanol in a desired ratio to a common inlet or separate inlets of a fuel vaporizer  28  made according to the invention. (A common inlet is disclosed herein and is preferred but the invention contemplates the use of separable inlets as well.) The water/methanol mixture is vaporized and discharged on a line  30  to the inlet of a reformer and catalytic burner  32 . The reformer and catalytic burner  32  in turn discharges reformate (hydrogen, water, carbon monoxide and carbon dioxide) on a line  34  to a gas purification reactor  36  where the carbon monoxide content of the gas is reduced to the point where it will not poison the fuel cell  10 . The gas purification reactor  36  discharges to the inlet line  12  to the anode of the fuel cell  10 .  
         [0051]    Hot so-called tail gas generated in the reformer and catalytic burner  32  is discharged on a line  37  to the vaporizer  28  to serve as a source of heat to vaporize the methanol and water therein.  
         [0052]    The system also includes an exhaust line  38  on which exhaust gas is discharged. The exhaust gas is expanded through a compressor/expander  44  and discharged as exhaust. A recirculation line  46  for hot gas may also be provided.  
         [0053]    Electric power generated by the fuel cell  10  is employed, during operation, to drive pumps, motors, etc. within the system as well as to provide electric power for the load to be driven by the system. For start up, battery power may be used. In the case of a vehicular propulsion system, the load will typically be an electric motor coupled to the vehicle traction system.  
         [0054]    Turning now to FIG. 2, a preferred form of a fuel vaporizer  28  made according to the invention is illustrated. The same includes a core  50  made up of a series of plates, bars and/or flanges on the peripheries of the plates, spacers and fins to be described in greater detail hereinafter. These components define a fuel/water flow path and a heated heat exchange medium flow path through the vaporizer. The heated heat exchange media typically will be a hot gas but a heated liquid could also be employed. A liquid fuel inlet to the fuel/water flow path is provided by a header  54  and a relatively small diameter tube  56  connected thereto. A similar header  57  supports a large diameter tube  58  which serves as a vaporized fuel outlet. The difference in size of the tubes  56  and  58  is due to the fact that the fuel and water mix enters the tube  56  as a liquid and thus is at a relatively greater density than the fuel exiting through the outlet tube  58  which is in vapor form. Consequently, to avoid a large pressure drop, because of the greater volumetric flow rate at the outlet tube  58 , the outlet tube  58  has a larger cross-sectional area.  
         [0055]    The core  50  has opposed ends  60  and  62 . The end  60  is an inlet end for the heated media and includes an inlet header  64 . A hot gas inlet tube  65  extends to the header  64 . The end  62  is an outlet end for the heated media and includes an outlet header  66  from which a hot gas outlet tube  67  extends. The header  64  is connected to receive hot gas from the reformer and catalytic burner  32  (FIG. 1) and deliver it through the hot gas fluid flow passages that are in heat exchange relation with the hot gas flow path which is in the form of a plurality of passages as well.  
         [0056]    The core  50  is a stack of the previously mentioned components that define alternating fuel/water flow path structures  68  and hot gas flow path structures  69 . It is to be noted that the inlet and outlet headers  64 , 66  for the hot gas optionally could be pyramid shaped housings (not shown)having a round opening (not shown) at their apexes and an opposite, open base (not shown) which is in fluid communication with the hot gas fluid flow paths (not shown) within the core  50 .  
         [0057]    Turning to FIG. 3, both a typical fuel side subassembly constituting the methanol/water flow path defining structure  68  and a typical hot gas side subassembly defining the hot gas flow path defining the structure  69  are shown in an exploded view. The fuel side subassembly includes two plates  70 , 72  that are superimposed upon each other. The plates  70  include an upstream end  74  and a downstream end  76 .  
         [0058]    The plate  70  includes a plurality of angled elongated slots  78  intermediate the ends  74  and  76 . Adjacent the end  74  is a series of slots which define a part  80  of a maze, generally designated  82 , for purposes to be seen. Adjacent the opposite end  76 , an enlarged opening  84  is present. Further, additional enlarged openings  86 , 88  are disposed just beyond the ends  74 , 76  for purposes to be seen. The plate  70  also includes a solid section or boundary  90  about its entire periphery as well as a solid section  92  isolating the enlarged opening  86  from the maze  82  and a solid section  94  isolating the enlarged opening  88  from the enlarged opening  84 .  
         [0059]    The plate  72  also includes angled elongated slots  96 , a series of slots  98  also defining part of the maze  82 , an enlarged opening  100  and additional enlarged openings  102 , 104  which are located past the ends of the plate  72  defined by the enlarged opening  100  and the maze  82 . The plate  72  also includes the solid border  90  and the solid sections  92  and  94 . The arrangement is such that the plates  70  and  72  can be superimposed upon one another with their borders  90  and solid sections  92 , 94  aligned with and in contact with one another thereby providing alignment of the enlarged opening  88  with the enlarged opening  104 , and alignment of the enlarged opening  86  with the enlarged opening  102 . Further, the angled slots  78 , 96  will then criss-cross each other. The other enlarged openings  84 , 100  are also aligned with one another as are the slots  80 , 98  making up the maze.  
         [0060]    The invention contemplates that the slots could be formed as through slots as shown or merely as grooves in imperforate plates and having facing open sides which do not extend fully through the associated plates. The grooves can be etched, machined or stamped in the plates, as desired. In such a case, the plates would be imperforate and consequently separator sheets (to be described hereinafter) between the plates may often be omitted.  
         [0061]    Also shown in FIG. 3 are two separator plates  106 , 108 . The plate  106  includes a down turned, peripheral flange  110  while the separator plate  108  includes an upturned peripheral flange  112  which is adapted to abut the flange  110  and be sealed thereto as, for example, by suitable bonding such as brazing. However, other metallurgical, fluid tight bonds such as welds or soldering could be employed if desired.  
         [0062]    The plates  106  and  110  also include enlarged openings  116  at their opposite ends. The enlarged openings  114  and  116  align with the enlarged openings  86 , 102 , and  88 , 104 , respectively. The border  118  of the separator plate  106  opposite the flange  110  is sealed and bonded to the boundary  90  of the plate  72  as by any metallurgical bond as mentioned previously while the border of the separator plate  108  (not shown) would be sealed and bonded to the boundary  90  of a plate  70  (not shown), that is the next lowermost plate in the stack.  
         [0063]    Because of the presence of the engaging flanges  110 , 112  on the separator plates  106  and  108 , there will be a space that extends between the openings  114 , 116  which serves as a hot gas or heated fluid medium passageway. To promote excellent heat exchange, a turbulator or fin  120  is located therein and is disposed between the openings  114 , 116 . The fin  120  is preferably a conventional lanced and offset fin and if desired, can be made in one or more sections which may or may not include small spaces between adjacent sections. That is to say, the fin  120  can be made as generally described in the commonly assigned, copending application of Reinke et al, filed ______, Ser. No. ______, (attorney&#39;s docket number 655.00936) and entitled “Method and Apparatus for Vaporizing Fuel for a Reformer Fuel Cell System”, the entire disclosure of which is herein incorporated by reference.  
         [0064]    It will be noted that the plates  70 , 72  include two apertured tabs  122  which may be employed in the manufacturing process to achieve exact alignment between the plates  70  and  72  when they are superimposed upon one another. In addition, one side  124  of each of the plates  70 , 72 , 106 , 108  includes a small tab  126  adjacent the location of the maze  82 . The tabs  126  include apertures  128  which are aligned with one another and, when the plates are assembled to one another, define a common manifold or inlet to the maze  82 .  
         [0065]    Adjacent one side of the enlarged openings  84 , 100 , is a similar, but larger tab  130  which defines a sideways extension of the enlarged openings  84 . The tabs  130  in the plates  106 , 108  includes circular openings  134  which align with the extensions of the enlarged openings  84 , 100  in the plates  70 , 72  and, when the plates are assembled to one another, form a sealed outlet manifold. Specifically, the manifolds thus defined are the fuel inlet manifold  54  and the fuel outlet manifold  57  and the same are common to all the fuel flow passages  68  and are connected to the tubes  56  and  58 , respectively, as previously described in connection with the description of FIG. 2.  
         [0066]    It will thus be appreciated that each of the fuel flow structures  68  is made up of two of the plates  70 , 72  superimposed upon one another and bonded together and sandwiched between one of the separator plates  106  and one of the separator plates  108 . It will also be appreciated that each of the hot gas structures  69  is made of a fin  120  sandwiched between two of the separator plates  106  and  108 . While the separator plates  106 , 108  for the fuel flow structure  68  may be totally separate from those used for the hot gas structure  69 , it is preferable that they be shared or common to both as illustrated to both minimize volume and to minimize the amount of material employed. By making the separator plates  106 , 108  common to both the fuel flow structures and the hot gas structures, volume, weight and the cost of materials required to form the fuel vaporizer are all minimized.  
         [0067]    It is also to be noted that the hot gas flow path structure  69  including the separator plates  106 , 108  and the fin  120  as described herein may be employed in all of the embodiments involving different types of sheets or plates used in the fuel flow structure  68  also described herein.  
         [0068]    Turning now to FIGS.  4 - 6 , the plates  70 , 72  will be described in greater detail. The mean direction of fuel flow through the fuel flow structures  68  is indicated by an arrow  136  and will typically be generally parallel to the direction of elongation of the plates  70 , 72 . By “mean” direction of fuel flow, it is meant that the fuel flow at any given point in a fuel flow structure  68  will be going in a particular direction dependent upon the direction of the slots  78 . Nonetheless, the mean fuel flow or overall fuel flow direction is from the inlet manifold  56  to the outlet manifold  57 . As can be seen in FIG. 5, lines  138 ,  140  and  142  show that the slots  78  are in various rows centered on the lines  138 ,  140  and  142 . These rows have a decreasing angle with respect to the mean direction of fuel flow represented by the arrow  136  as one progressively moves from the inlet manifold  56  toward the outlet manifold  57 .  
         [0069]    It will be further seen that the slots  78  are progressively wider as one moves from the inlet manifold  56  toward the outlet manifold  57 . For example, the slots  144  adjacent the inlet manifold  56  and just downstream of the maze  82  are significantly more narrow than the slots shown at  146  adjacent the outlet manifold  57 . The purpose of this structure is to accommodate the increasing volume of fuel flow through the vaporizer as it changes from the liquid state in which it is introduced into the inlet manifold  56  to the vaporized state at which it exits the outlet manifold  57 . Furthermore, it will be seen that the slots  78  are tapered in the sense that at their upstream ends  148 , they are narrower than at their downstream ends  150 . The purpose of this construction is similarly to accommodate the expansion of the fuel as it changes from the liquid to the vapor or gaseous state.  
         [0070]    In the embodiment illustrated in FIGS.  3 - 6 , the slots  78  in each of the rows  138 , 140 , 142  have different lengths. Generally, but not always, relatively short elongated slots  150  alternate with relatively long elongated slots  152 . Of course, as one approaches the periphery of each of the plates  70 , 72 , the slot length in the sequence in each row will, to some extent, be dictated by the need to preserve the boundary  90  in each plate.  
         [0071]    It will be observed in FIGS. 4 and 5 that the downstream most slots  78  in the plate  72  terminate in a somewhat arcuate boundary section  154  that defines one side of the enlarged opening  104  whereas, in the plate  70 , a boundary section  156  between the enlarged opening  84  and the slots  78  is serrated. Further, it will be appreciated that the downstream most slots  146  in the plate  70  (FIG. 5) extend beyond the boundary section  154  (FIG. 4) in the plate  72  and thus establish fluid communication between the slots  78  and the aligned enlarged openings  84 , 100 , and thus to the outlet manifold  57 . It is desirable to provide the boundary sections  154 , 156  to eliminate loose ends of solid sections of each of the plates as the slots  78  merge into enlarged openings  84 , 100 .  
         [0072]    The maze  82  is intended to be a highly flow resistant maze such that the majority, at least 50%, of the pressure drop from the inlet manifold  56  to the outlet manifold  57  occurs immediately adjacent the inlet manifold  56 . It is preferred that the construction be such that at least 70% of the overall pressure drop occurs here and even more preferably, that 80-95% of the overall pressure drop occurs at this location. It has been determined that by causing the vast majority of the pressure drop to occur immediately adjacent the inlet manifold  56 , distribution of the incoming liquid fuel/water mixture between the various rows of slots is significantly more uniform, thereby avoiding fuel starvation on one side of the fuel flow structure coupled with an excess of the fuel at another part of the fuel flow structure. This promotes maximum efficiency of the vaporizing process.  
         [0073]    As seen in FIGS. 4 and 5, the maze  82  is made up of a plurality of elongated slots  160  of various lengths and in a plurality of rows, four such rows being illustrated. The slots  160  are generally transverse to the mean direction of fuel flow  136 . Near the sides of each of the plates  70  and  72  are additional elongated slots  162  having the relative lengths illustrated in the drawings and which are generally parallel to the mean direction of fuel flow  136 . As seen in FIG. 6, the slots  160  and  162  overlap one another and thus establish fluid communication with one another.  
         [0074]    In the embodiment illustrated in FIGS.  4 - 6 , first and second relatively elongated slots  164  form part of the group of slots  160  and open to relatively narrow orifice slots  166  which extend in the mean direction of fuel flow to the upstream most ones of the slots  78 . It will be seen that there are two groups of the orifice slots  166 , with one set of four orifice slots  166  being downstream of another set of orifice slots  166 , the former being four in number and the latter being two in number. The orifice slots  166  are uniformly spaced across the width of each of the plates  70  and  72  and are highly effective in creating the relatively high pressure drop desired in the maze  82 .  
         [0075]    In addition to providing uniform flow of fuel through each of the fuel flow structure  68 , the maze  82  as just described, because of its high flow resistance, and the accompanying relatively high pressure drop is mentioned previously, promotes uniformity of flow from one fuel flow section  68  to the next throughout the stack  50 .  
         [0076]    The various components have dimensions in millimeters shown in FIG. 4 which are representative of corresponding positions in the fuel flow structure for the plate  70  shown in FIG. 5.  
         [0077]    FIGS.  7 - 9  inclusive show a modified embodiment of a fuel flow structure  68 . In this embodiment, the fuel flow plates taken on the same general configuration as described previously with the only difference from the structure shown in FIGS.  4 - 6  being that slots  78  extend from side to side of each of the plates  70 , 72 . That is to say, a single long slot  78  replaces the alternating long and short slots  150 , 152  shown in the embodiment of FIGS.  4 - 6 . Again, the angle of the mean direction of fuel flow  136  decreases as one progressively moves from the inlet manifold  56  to the outlet manifold  57 . And again, the slots  78  are tapered, being narrower at their upstream ends than at their downstream ends. Further, again, the upstream slots  78  are narrower than the downstream slots  78 .  
         [0078]    Turning now to FIGS.  10 - 12 , inclusive, still another embodiment of the plates  70 , 72  is illustrated. In this embodiment, which is generally similar to that illustrated in FIGS.  7 - 9 , inclusive, to achieve better uniformity of distribution of the fuel to be vaporized, the slots  78  are angled, tapered and of increasing width as before and are separated into three channels  170 ,  172  and  174  by solid separator sections  176  which comprise imperforate parts of the respective plates  70  and  72 . The channels  170 ,  172  and  174  in this embodiment, as in other embodiments employing channels, preferably are of the same length and same hydraulic configuration to promote uniform flow through all channels. Further, the boundary section  154  is straight rather than arcuate. In addition, the tabs  126  and  130  defining the inlet and outlet manifolds  56  and  57 , respectively, are on opposite sides of the plates.  
         [0079]    As an additional difference, the maze  82  is formed as three identical but separate mazes  177 , 178 , 179 , one for each channel  170 , 172 , 174 . Each maze  177 , 178 , 179  includes several rows of relatively short but nonetheless elongated slots  180 . The slots  180  in the plates  70  are diagonally oriented away from the outlet tab  130  while the slots  180  in the plate  72  are diagonally formed at an opposite angle, generally in the direction of the outlet tab  130 . A single manifold slot  182  communicates with the inlet manifold  126  via a short row of slots  184  in each of the plates  70 , 72  and is generally transverse to the mean direction of fuel flow  136 . The manifold slot  182  also is in fluid communication with the upstream end of the mazes  177 , 178 , 179  and serves as a common manifold for each. The downstream ends of the slots  180  terminate in small groups of transverse slots  186  which discharge into the center of each of the channels  170 ,  172  and  174 . Again, the mazes  177 ,  178  and  179  are highly pressure resistant and provide the vast majority of pressure drop from the inlet manifold  56  to the outlet manifold  57  in the range mentioned previously.  
         [0080]    The embodiment of FIGS.  10 - 12  is highly preferred in that the channelization provided by the channels  170 , 172 , 174  coupled with the maze  82  and its particular construction provide excellent distribution of the incoming fuel/water mixture to each of the channels in any given fuel flow structure  68  and to all of them in the stack  50 . This excellent distribution is further enhanced by the use of three separate mazes  177 , 178 , 179 , one for each channel  170 , 172 , 174 , fed by a common manifold slot  180 . Further, the isolation between the channels serves to minimize the effect on outside forces on fuel distribution within the fuel flow structures  68 . For example, if the fuel cell system is employed in a vehicle, acceleration, deceleration, or cornering forces, which would ordinarily tend to cause the fuel, particularly the fuel that is in the liquid state, to move to one side or the other of the fuel flow passages, do not have as great an effect because of the presence of the isolated channels  170 , 172 , 174  provided by the solid sections  176 .  
         [0081]    Again, the slots  78  have a decreasing angle from the inlet manifold  56  to the outlet manifold  57  in the mean direction of fuel flow. As with the previously described embodiments, the slots are straight and progressively widen from the inlet manifold  56  to the outlet manifold  57 . In addition, the slots  78  are tapered so as to be narrower at their upstream ends than at their downstream ends.  
         [0082]    FIGS.  13 - 15  illustrate still another embodiment of the invention wherein the fuel flow plates  70 , 72  are also provided with a plurality of slots  78 . In this embodiment, the slots are at a constant angle from the inlet manifold  56  to the outlet manifold  57 . Those sections of each plate containing the enlarged openings  86 , 88 ,  102 , 104 , have been omitted for clarity. The slots  78  progressively widen from the inlet manifold  56  to the outlet manifold  57  even though the slots  78  are at a constant angle to the mean direction of fuel flow  136  and are not tapered.  
         [0083]    The maze  82  is of a simplified construction as well. Each of the plates  70 , 72  includes an elongated, upstream slot  187  which align with one another and extend transversely to the mean direction of fuel flow  136  from the inlet manifold  56  across substantially the entire width of each plate  70 , 72 . A series of orifice slots  188  extend in the mean direction of fuel flow from the slot  187  in the sheet  70  at uniform intervals to be in fluid communication with the upstream ends of the upstream slots  78  and the plates  72  when the two are superimposed. Thus, unlike prior embodiments, there is only one row of the orifice slots  88  in the embodiment illustrated in FIGS.  13 - 15 . And again, the slots  78  in the embodiment of FIGS.  13 - 15  are straight.  
         [0084]    FIGS.  16 - 18  illustrate still another embodiment of the plates  70 , 72 . And again, those parts of the plates including the enlarged openings  86 , 88 , 114 , 116  have been omitted for clarity. This embodiment, like the prior one, can be made without such openings if the alternative form of conical headers is employed. Alternatively, the enlarged openings  86 , 88 , 102 , 104  may be included though not shown and the assembly headered as previously described.  
         [0085]    In the embodiment illustrated in FIGS.  16 - 18 , the solid sections  176  are provided to separate the fuel flow passages into three channels  170 ,  172  and  174  in the same manner mentioned in connection with the description of FIGS.  10 - 12 , inclusive. However, the slots  78  are not straight in the embodiment illustrated in FIGS.  16 - 18 . Rather, they are chevron or V-shaped. However, the embodiment of FIGS.  16 - 18  retains many of the characteristics of previously described embodiments. For example, the angle of each leg of each V-shaped slot  78  progressively decreases as one moves from the inlet manifold  56  to the outlet manifold  57 . Similarly, the width of each slot  78  also progressively increases as one moves form the inlet manifold  56  to the outlet manifold  57 . If desired, each leg of each of the chevron-shaped or V-shaped slots  78  could be narrower on its downstream end than at its upstream end.  
         [0086]    The maze  82  is again a simplified maze although it is somewhat more complex than that described in connection with the embodiment of FIGS.  13 - 15  inclusive. The plate  70  includes a plurality of elongated slots  190  which are oriented in the mean direction of fuel flow  136  on the upstream side thereof and an elongated slot  192  that is traverse to the mean direction of fuel flow  136  on its downstream side. The plate  72  includes two rows of elongated slots  194  which are oriented to intersect the slots  190  in the plate  70  when the two are assembled. Also included is an elongated slot  196  which aligns with the slot  192  and together therewith serves a distribution manifold. The slot  196  is, of course, transverse to the mean direction of fuel flow  136 .  
         [0087]    The plate  70  also includes one orifice slot  198  which is centrally oriented to serve channel  172  while the slot  196  in the plate  72  includes two side orifice slots  200  which are oriented to serve channels  170 , 174 . Again, the maze  82  is configured to provide the vast majority of pressure drop from the inlet manifold  56  to the outlet manifold  57  and in the range mentioned previously.  
         [0088]    The embodiment of FIGS.  16 - 18  may also be fitted with the enlarged openings  86 , 88 , 102 , 104  if a headering scheme such as illustrated in FIG. 2 is desired. Alternatively, such enlarged openings may be omitted and conical headers as alluded to previously employed.  
         [0089]    A desirable feature of the invention that is usable with any of the disclosed embodiments is illustrated in FIGS. 19 and 20. Specifically, adjacent the enlarged opening  116  in the separator plate  106 , and located so as to be receivable in the enlarged openings  84 , 100  in the fuel flow plates  70 , 72 , is a series of dimples  202  which are elongated. Adjacent the opening  134 , a pair of spaced dimples  204  may be located and a single dimple  206  may be located at the opposite end of the array of dimples  202 . All of these dimples are intended to be located so as to be receivable in the enlarged openings  84 , 100  of the plates  70 , 72 . The dimples extend from the plate  106  a distance sufficient to equal the combined thickness of the plates  70  and  72 . Consequently, as illustrated in FIG. 20, they will be in abutment with the plate  108  and will braze thereto during the assembly operation if brazing is employed. Alternatively, the dimples  202 ,  204  or  206  or all of them can be formed in both separator plates  106 , 108  and aligned with one another so as to abut and braze together during the assembly operation. Where brazing is employed, of course, the various plates will be provided with braze clad material or a braze foil may be used as an alternative to braze cladding of the various components. In general, the use of braze foil will be desired because the braze foil may be configured with slots, openings, etc. corresponding to those found in the various plates, thus minimizing the amount of braze material required for cost purposes as well as reducing the chances of clogging resulting from flow of excessive braze metal.  
         [0090]    Thus, the plates  106 , 108  are bonded together along the entire length of the elongated openings  84 , 100  to provide pressure resistance in this area.  
         [0091]    Importantly, the elongated dimples  202  serve a second function. They may all have their direction of elongation placed at different angles as illustrated by lines  208 ,  210 ,  212 ,  214  and  216  as illustrated in FIG. 19. These angles are at a selected set of angles that intersect, at substantial acute angles, the mean direction of fluid flow at fuel flow  136  through each of the fuel flow structures  68 . The angles are directed generally toward the outlet provided by the opening  134  which is connected to the outlet header  57 . Thus, the dimples  202  act additionally as flow directors to direct vaporized fuel toward the fuel outlet manifold  57 .  
         [0092]    Various alternatives will occur to those skilled in the art. For example, the flanges  110 , 112  on the separator plates may be omitted in favor of bars at the same location as the flanges  110 , 112 . Alternatively, additional plates having one large elongated central opening and the total thickness equal to the combined height of the flanges  110 , 112 , could be employed if desired. The use of flanged separator plates is, however, preferred in that it reduces the number of individual parts than must be handled during assembly as well as inventory storage requirements.  
         [0093]    In the invention, there is countercurrent flow between the hot gas and the fuel. Other flow regimes such as concurrent or combined concurrent countercurrent flow could be used. However, countercurrent flow is preferred since it minimizes thermal stress by minimizing the temperature differential between the fluid in the fuel flow structures  68  and the hot gas structures  69 .  
         [0094]    The use of the decreasing angle of the slots from the inlet manifold  56  to the outlet manifold decreases flow resistance in the sections of the fuel flow structures employing slots with a progressively decreasing angle to the mean flow direction. This serves as a means of assuring that the vast majority of the pressure drop occurs in the maze  82  to achieve the desired uniformity of flow through each fuel flow structure  68  and from one fuel flow structure  68  to another within the stack  50 .  
         [0095]    It is highly desirable that the area occupied by the slots  78  be as large as possible. Such exposes more of the separator sheets  106 , 108  that is in direct heat transfer with the fuel and thus provides more efficient heat transfer than the heat transfer that occurs through the ribs separating one slot from another.  
         [0096]    The criss-cross pattern of the slots  78  provides a great deal of turbulence in all directions of flow within each fuel flow structure  68 . Such turbulence minimizes boundary layer formation and thus substantially increases heat transfer efficiency.  
         [0097]    Those embodiments employing multiple channels in each fuel flow structure minimize fuel starvation problems that might be caused by outside forces as mentioned previously. A vaporizer made according to the invention including such channels operates with excellent efficiency in all orientations, thus providing a great deal of flexibility and installation. Multiple channels are also thought to improve stability of operation of the vaporizer that otherwise might be upset due to those outside forces.  
         [0098]    The embodiment utilizing alternating long and short slots, or interrupted slots of any sort assures more changes of direction of the fuel flow from the inlet  56  to outlet  57  and thus minimizes boundary layer formation to improve efficiency. The plates  70 , 72  may be formed of sheets or plates of minimum thickness that are fractions of a millimeter. In this way, the mass of fuel within the vaporizer at any given point of time is minimized and so the response to a change in load is increased. That is to say, the response time is substantially decreased.  
         [0099]    Ideally, the maze  82  provides the requisite pressure drop while the entering liquid fuel/water mixture is still in a liquid form. It is not desirable to have the maze have sufficient length that vaporization occurs in the maze because such might increase the overall pressure drop from the inlet manifold  56  to the outlet manifold to undesirably high levels. Thus, the particular shape of the maze is not particularly important so long as it is capable of providing the necessary high flow resistance to achieve the necessary pressure drop and is of short enough length that vaporization of the fuel/water mixture within the maze will not occur or is minimal.  
         [0100]    The hot gas manifolds employing the enlarged openings  86 , 88 , 102 , 104 , 114 , 116  provide a cost advantage over cone-shaped manifolds mentioned previously. And the use of dimples in the fuel outlet manifold defined by the enlarged openings  84 , 100  not only maintain spacing between the separator plates  106 , 108  during assembly such as during a brazing cycle, they also prevent bulging or oil canning effects during operation of the vaporizer in response to the pressure differential between the hot gas and the fuel.  
         [0101]    The thickness of the sheets or plates is selected as a function of the desired pressure drop. Thinner sheets, of course, will be cheaper than thicker sheets. Thinner fuel flow sheets also reduce the total fuel charge or capacity of the vaporizer to improve the transient response rate, i.e., the response to a change in load. Further, using thinner sheets allows the overall area of each particular slot to be larger than would otherwise be the case while still retaining the desired pressure drop characteristics. Larger slots are less sensitive to clogging, both during assembly and during operation.  
         [0102]    As alluded to generally previously, FIGS.  4 - 18 , inclusive, are scale drawings with the various components bearing the measurements and/or angles illustrated. Further, the thickness of the fuel flow plates  70 , 72 , may be in the range of about 0.1 mm to about 0.5 mm and more preferably, about 0.2 mm in one embodiment of the invention. Of course, depending upon fuel flow requirements and response requirements, the thickness may be changed as needed.