Abstract:
A plate fin heat exchanger includes a plurality of finned cold layers configured to conduct a first fluid, and a plurality of finned warm layers configured to conduct a second fluid. The finned warm layers include an inlet side and an outlet side. A first portion of fins of at least one finned warm layer of the plurality of finned warm layers includes a plurality of aligned peaks and valleys defining a wave configuration for each fin of the first portion of fins. An upstream leading edge of the first portion of fins begins at a point of the wave configuration that is at least one of the peaks and valleys.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein generally relates to heat exchangers, and more specifically, to wave fin structure for heat exchangers. 
         [0002]    A typical air-to-air plate fin heat exchanger consists of a stack of brazed, thermally interconductive air flow sections or layers. Hot air and cold air are forced through alternate layers in order to exchange heat. In a gas turbine air conditioning system, the hot air comes from the engine bleed and flows through bleed layers. The cold air is outside air and flows through ram layers. These alternately stacked ram and bleed layers are joined together along a thermally conductive medium called the parting sheet, and heat from the bleed layers is transmitted through the parting sheets to the ram air flow. 
         [0003]    The ram and bleed layers are similar and each includes an array of cooling fins and frames or closure bars which are positioned on the parting sheets to define each layer. Frames or closure bars are placed along the edges of the layers to support the ends of the parting sheets. In addition to supporting the ends of the parting sheets, theses bars close off each layer, except where there is an air inlet or an air outlet. At the air inlets and outlets the fins provide support for the parting sheets. 
         [0004]    To fabricate the heat exchanger, the ram and bleed layers are stacked alternately one on top of another and then placed in a vacuum furnace for brazing. During the brazing process the stack is squeezed so as to force the layers together. The brazing is complete when the fins are brazed to the parting sheets and the edges of the sheets are uniformly brazed along the closure bars. The bleed and ram air flows are supplied from corresponding manifolds that are subsequently welded to the closure bars. 
         [0005]    Due to their size, such heat exchangers may be subjected to significant thermal stresses when they warm up and cool down. These stresses may occur when the bleed air flow is started and stopped. During these heating and cooling cycles of the exchanger, the core expands and contracts. Over time, the high thermal stresses may degrade the fins thereby causing fractures that may lead to the deterioration of sections of the core. This may compromise the structural integrity of the heat exchanger and its ability to provide the required cooling performance. 
       BRIEF SUMMARY 
       [0006]    In one aspect, a plate fin heat exchanger is provided. The plate fin heat exchanger includes a plurality of finned cold layers configured to conduct a first fluid, and a plurality of finned warm layers configured to conduct a second fluid. The finned warm layers include an inlet side and an outlet side. A first portion of fins of at least one finned warm layer of the plurality of finned warm layers includes a plurality of aligned peaks and valleys defining a wave configuration for each fin of the first portion of fins. An upstream leading edge of the first portion of fins begins at a point of the wave configuration that is at least one of the peaks and valleys. 
         [0007]    In addition to one or more of the features described above, or as an alternative, further embodiments may include: wherein the plurality of finned warm layers further comprises a second portion of fins disposed at the inlet side adjacent the upstream leading edge of the first portion of fins, wherein the fins of the second portion of fins have a thickness greater than a thickness of the fins of the first portion of fins; a slot formed in a leading edge of at least one fin of the second portion of fins; and/or wherein the fins of the second portion of fins are between two and four times thicker than the fins of the first portion of fins. 
         [0008]    In another aspect, a dual core heat exchanger is provided. The dual core heat exchanger includes a first core and a second core fluidly separate from the first core. The first core includes a first plurality of finned cold layers configured to conduct a first fluid, a first plurality of finned warm layers configured to conduct a second fluid, the first plurality of finned warm layers having an inlet side and an outlet side. At least a first portion of the fins of each finned warm layer of the first plurality of finned warm layers includes a plurality of aligned peaks and valleys defining a wave configuration for each fin of the first portion of fins. An upstream leading edge of the first portion of fins begins at a point of the wave configuration that is at least one of the peaks and valleys. The second core includes a second plurality of finned cold layers configured to conduct the first fluid and a second plurality of finned warm layers configured to conduct a third fluid. The second plurality of finned warm layers include an inlet side and an outlet side. 
         [0009]    In addition to one or more of the features described above, or as an alternative, further embodiments may include: wherein the first core further comprises a guard fin positioned at the inlet side of each of the finned warm layers of the first plurality of finned warm layers, wherein the guard fin has a fin thickness greater than a thickness of the fins of the first finned warm layers; a second guard fin positioned at the inlet side of each of the finned warm layers of the second plurality of finned warm layers, wherein the second guard fin has a fin thickness greater than a thickness of the fins of the second finned warm layers, wherein at least a second portion of the fins of each finned warm layer of the second plurality of finned warm layers includes a plurality of aligned peaks and valleys defining a wave configuration for each fin of the second portion of fins, wherein an upstream leading edge of the second portion of fins begins at a point of the wave configuration that is at least one of the peaks and valleys; a slot formed in a leading edge of the guard fin; wherein the guard fin is between two and four times thicker than the fins of the first plurality of finned warm layers; and/or a first inlet header fluidly coupled to the inlet side of the first plurality of finned warm layers, the first inlet header configured to supply bleed air from an engine to the first plurality of finned warm layers, a ram air manifold coupled to an inlet of the first plurality of finned cold layers, the ram air manifold configured to supply ram air to the first plurality of finned cold layers, and a second inlet header fluidly coupled to the inlet side of the second plurality of finned warm layers, the second inlet header configured to supply compressed air from a compressor to the second plurality of finned warm layers. 
         [0010]    In yet another aspect, a method of fabricating a heat exchanger is provided. The method includes providing a plurality of finned cold layers and providing a plurality of finned warm layers having an inlet side and an outlet side, wherein a first portion of fins of each finned warm layer includes a plurality of aligned peaks and valleys defining a wave configuration for each fin of the first portion of fins. The method further includes cutting along the aligned peaks and valleys of adjacent fins of the first portion of fins to form an upstream leading edge of the first portion of fins that begins at a point of the wave configuration that is at least one of the peaks and valleys, and coupling the plurality of finned cold layers and the plurality of finned warm layers. 
         [0011]    In addition to one or more of the features described above, or as an alternative, further embodiments may include: providing a plurality of guard fins having a fin thickness greater than a fin thickness of the fins of the finned warm layers, orienting guard fins of the plurality of guard fins at the inlet side of the finned warm layers of the plurality of finned warm layers, and wherein the step of coupling comprises coupling the plurality of finned cold layers, the plurality of finned warm layers, and the plurality of guard fins; forming a slot in a leading edge of at least one guard fin of the plurality of guard fins; wherein the slot is formed using an electrical discharge machining process; and/or wherein the step of cutting along the aligned peaks and valleys of the wave configuration is performed using an electrical discharge machining process. 
         [0012]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0014]      FIG. 1  is a perspective view of an exemplary heat exchanger; 
           [0015]      FIG. 2  is a perspective view of the heat exchanger shown in  FIG. 1  with exemplary headers; 
           [0016]      FIG. 3  is a cross-sectional view of the heat exchanger shown in  FIG. 1  taken along line  3 - 3 ; 
           [0017]      FIG. 4  is a cross-sectional view of an exemplary bleed guard fin of the heat exchanger shown in  FIG. 3  and taken along line  4 - 4   
           [0018]      FIG. 5  is a cross-sectional view of an alternative embodiment of the heat exchanger shown in  FIG. 3 ; 
           [0019]      FIG. 6  is an enlarged view of the heat exchanger shown in  FIG. 5  and taken along section  6 ; and 
           [0020]      FIG. 7  is an enlarged view of heat exchanger fins shown in  FIGS. 5 and 6 . 
       
    
    
       [0021]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION 
       [0022]      FIGS. 1 and 2  illustrate an exemplary air-to-air heat exchanger  10 . In the exemplary embodiment, heat exchanger  10  is a high temperature aluminum dual heat exchanger for an air generation unit of an aircraft. However, the features described herein may be used with any suitable heat exchanger structure. 
         [0023]    Heat exchanger  10  generally includes a primary core  12  and a secondary core  14 . Each core  12 ,  14  includes a bleed air inlet side  16 , a bleed air outlet side  18 , a ram air inlet side  20 , and a ram air outlet side  22 . With reference to  FIG. 2 , hot bleed air from an engine (not shown) enters core  12  from a primary inlet header  24  and exits through a primary outlet header  26 . Similarly, hot air from a compressor outlet enters secondary core  14  from a secondary inlet header  28  and exits through a secondary outlet header  30 . Ram air passes from inlet  20  to outlet  22  through both primary core  12  and secondary core  14  to cool both the hot bleed air and the hot air from the compressor. The ram air may be supplied to inlet side  20  by a manifold or flow guidance device (not shown) and removed from outlet side  22  by a manifold or flow guidance device (not shown). 
         [0024]    Heat exchanger  10  includes a plurality of layers defined by parting sheets  32  and cooling fins  34  that are located between parting sheets  32 . Cold or ram air is forced through inlet  20  in the direction of arrow  36  and flows through a plurality of ram air layers  38 . The ram air layers  38  are located between hot air or bleed layers  40 , which receive hot air through headers  24 ,  28 . The hot air flows through the inlets defined between bleed closure bars  42 , which seal off bleed layer  40  with respect to the ram flow direction  36 . Similarly, ram closure bars  44  seal off ram air layers with respect to the bleed flow. 
         [0025]    With reference to  FIG. 3 , cooling fins  34  have a varied thickness throughout the layers  38  and/or  40 . For example, cooling fins  34  may include bleed guard fins  46 , upstream fins  48 , and downstream fins  50 . In the exemplary embodiment, a thickness of guard fins  46  is greater than a thickness of upstream fins  48 , which is greater than a thickness of downstream fins  50 . Guard fins  46  are fabricated with a thickness greater than those of fins  48 ,  50  due in part to the bleed air being at its hottest temperature at bleed air inlet  16 . Because of the increased thickness, guard fins  46  are better able to withstand the high thermal stresses such as expansion and contraction of adjacent closure bars  42 , as well as expansion and contraction of individual guard fins  46 . Accordingly, the thermal fatigue life of guard fins  46  is greatly increased. 
         [0026]    Similarly, in the exemplary embodiment, upstream fins  48  are fabricated with a thickness greater than downstream fins  50  due in part because the bleed air is reduced in temperature as it travels from inlet  16  to outlet  18 . Thus, the thermal stress on fins  34  decreases as the fins extend from inlet  16  to outlet  18 , so the thickness of fins  34  may be reduced further downstream. Alternatively, upstream and downstream fins  48 ,  50  may have the same thickness while guard fins  46  are thicker. Guard fins  46  may be used in primary core  12  and/or secondary core  14 . Moreover, in the exemplary embodiment illustrated in  FIG. 3 , guard fins  46  are straight or planar and fins  48 ,  50  are wavy, serrated, or offset. Alternatively, guard fins  46  may be wavy and fins  48 ,  50  may be straight. 
         [0027]    In one embodiment, guard fins  46  are between 40% and 60% thicker than upstream fins  48  and between two and four times thicker than downstream fins  50 . In another embodiment, guard fins  46  are between approximately 40% and approximately 60% thicker than upstream fins and between approximately two and approximately four times thicker than downstream fins  50 . In one embodiment, guard fins  46  are 55% thicker than upstream fins  48  and three times thicker than downstream fins  50 . In another embodiment, guard fins  46  are approximately 55% thicker than upstream fins  48  and approximately three times thicker than downstream fins  50 . 
         [0028]    In one embodiment, the thickness of guard fins  46  is between 0.008 inches and 0.01 inches. In another embodiment, the thickness of guard fins  46  is between approximately 0.008 inches and approximately 0.01 inches. In yet another embodiment, the thickness of guard fins  46  is 0.009 inches or approximately 0.009 inches. In one embodiment, the thickness of upstream fins  48  is between 0.004 inches and 0.006 inches. In another embodiment, the thickness of upstream fins  48  is between approximately 0.004 inches and approximately 0.006 inches. In yet another embodiment, the thickness of upstream fins  48  is 0.005 inches or approximately 0.005 inches. In one embodiment, the thickness of downstream fins  50  is between 0.002 inches and 0.004 inches. In another embodiment, the thickness of downstream fins  50  is between approximately 0.002 inches and approximately 0.004 inches. In yet another embodiment, the thickness of downstream fins  50  is 0.003 inches or approximately 0.003 inches. However, guard fins  46 , upstream fins  48 , and downstream fins  50  may have any thickness that enables the fins to function as described herein. 
         [0029]    As illustrated in  FIG. 4 , guard fins  46  may include a slot  52  formed in a leading edge  54  to facilitate compliance with the thermal growth of adjacent structure (e.g., closure bars  42 ). Slot  52  allows guard fin leading edge  54  to expands and contract during the rapid thermal changes on primary heat exchanger inlet  16  of primary core  12  and/or secondary core  14 . In the exemplary embodiment, slot  52  includes a rounded end  56 . However, slot end  56  may have any shape that enables guard fin  46  to function as described herein. In the exemplary embodiment, slot  52  is formed in leading edge  54  by electrical discharge machining However, slot  52  may be formed using any suitable process. 
         [0030]    In one embodiment, a slot depth  58  is between 20% and 40% of a fin length  60 . In another embodiment, slot depth  58  is between approximately 20% and approximately 40% of fin length  60 . In yet another embodiment, slot depth  58  is 30% or approximately 30% of fin length  60 . In one embodiment, slot depth  58  is between 0.15 inches and 0.35 inches. In another embodiment, slot depth  58  is between approximately 0.15 inches and approximately 0.35 inches. In yet another embodiment, slot depth  58  is 0.25 inches or approximately 0.25 inches. In one embodiment, fin length  60  is between 0.8 inches and 1.0 inch. In another embodiment, fin length  60  is between approximately 0.8 inches and approximately 1.0 inch. In yet another embodiment, fin length  60  is 0.9 inches or approximately 0.9 inches. 
         [0031]    In one embodiment, a slot width  62  is between 20% and 40% of a fin width  64 . In another embodiment, slot width  62  is between approximately 20% and approximately 40% of fin width  64 . In yet another embodiment, slot width is 30% or approximately 30% of fin width  64 . In one embodiment, slot width  62  is between 0.05 inches and 0.07 inches. In another embodiment, slot width  62  is between approximately 0.05 inches and approximately 0.07 inches. In yet another embodiment, slot width 62 is 0.06 inches or approximately 0.06 inches. In one embodiment, fin width 64 is between 0.15 inches and 0.35 inches. In another embodiment, fin width  64  is between approximately 0.15 inches and approximately 0.35 inches. In yet another embodiment, fin width  64  is 0.25 inches or approximately 0.25 inches. 
         [0032]    Heat exchanger  10  may be fabricated by stacking parting sheets  32  with closure bars  42 ,  44  and cooling fins  34  (including guard fins  46 ) in place. Weight is then applied to the layers so as to squeeze them together, and the assembly is then placed in a vacuum furnace where it is heated to a temperature at which parting sheets  32  become brazed to closure bars  42 ,  44  and fins  34 . Slots  52  may then be formed in guard fin leading edges  54 , for example by electric discharge machining. Headers  24 ,  26 ,  28 ,  30  are then attached to heat exchanger  10 . 
         [0033]    With reference to  FIG. 5 , another embodiment of heat exchanger  10  includes cooling fins  134 , and like reference numerals indicate like parts. In the illustrated embodiment, cooling fins  134  include bleed guard fins  46 , wavy upstream fins  148 , and downstream fins  50 . Upstream fins  148  include an upstream leading edge  150  disposed adjacent to and downstream of bleed guard fins  46  to define a gap  149  therebetween. Alternatively, cooling fins  134  may not include guard fins  46  such that upstream leading edge  150  is disposed at a heat exchanger leading edge  152  (i.e., where guard fin leading edge  54  would be positioned). 
         [0034]    With further reference to  FIGS. 6 and 7 , each wave-like upstream fin  148  includes a plurality of peaks/valleys  154  that define a fin cycle  155  ( FIG. 7 ). In one embodiment, wavy fins  148  may have a sinusoidal shape that defines each peak/valley  154 . As shown in  FIG. 7 , the peaks/valleys  154  of adjacent fins  148  are aligned, and fins  148  include upstream leading edge  150  that is formed and begins at a point on fin cycle  155  (of each fin  148 ) that is either a peak or a valley  154 . 
         [0035]    In one embodiment, upstream fins  148  are uniformly cut or trimmed along commonly aligned peaks/valleys  154  using electrical discharge machining (EDM) to form upstream leading edge  150 . Because leading edge  150  is formed at a peak/valley  154  of each fin  148 , upstream fins  1448  are better able to withstand high thermal stresses. Accordingly, the thermal fatigue life of fins  148  is greatly increased. As such, aligning the peaks/valleys  154  of adjacent wavy fins  148  provides unexpected increased fin fatigue life. In one experiment, fatigue testing demonstrated that fin wave control with leading edge  150  beginning at the peaks/valleys  154  of each fin  148  increased the number of fatigue cycles of heat exchanger  10  by a significant amount. Inclusion of guard fins  46  also increased the number of fatigue cycles of heat exchanger  10 . Accordingly, testing confirmed fin-crack initiation is directly related to the position of the fin wave at leading edge  150 . 
         [0036]    In other embodiments, a trailing edge  156  of wavy upstream fins  148  may be trimmed along peaks/valleys  154  to further increase fin fatigue life. Furthermore, downstream fins  50  may also have a wave-like configuration and include an upstream leading edge  158  and/or a trailing edge  160  that are formed or trimmed along the peaks and valleys of wavy downstream fins  50 . 
         [0037]    Cooling fins  134  may be formed with wavy upstream fins  148 . A gage (not shown) may be used to align the individual wavy fins  148 , and wavy fins  148  may be subsequently cut (e.g., by EDM) along aligned peaks/valleys  154  to form leading edge  150 . As such, adjacent wavy fins  148  are aligned along the same point of fin cycle  155  to provide a uniform leading edge  150  (see  FIG. 6 ) that increases the fatigue life of cooling fins  134 . In some embodiments, cooling fins  134  may be provided with bleed guard fins  46  disposed upstream of leading edge  150  to further increase fatigue life of cooling fins  134 . In other embodiments, cooling fins  134  may include edges  156 ,  158 , and/or  160  that are similarly trimmed along the peaks/valleys of adjacent wavy fins. 
         [0038]    Heat exchanger  10  may be fabricated by stacking parting sheets  32  with closure bars  42 ,  44  and cooling fins  134  in place. Weight is then applied to the layers so as to squeeze them together, and the assembly is then placed in a vacuum furnace where it is heated to a temperature at which parting sheets  32  become brazed to closure bars  42 ,  44  and fins  134 . If guard fins  46  are included, slots  52  may then be formed in guard fin leading edges  54 , for example by an EDM process. Headers  24 ,  26 ,  28 ,  30  are then attached to heat exchanger  10 . 
         [0039]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.