Recuperator assembly and procedures Number:7,415,764 from the United States Patent and Trademark Office (PTO) owispatent

Title: Recuperator assembly and procedures

Abstract: A construction of recuperator core segments is provided which insures proper assembly of the components of the recuperator core segment, and of a plurality of recuperator core segments. Each recuperator core segment must be constructed so as to prevent nesting of fin folds of the adjacent heat exchanger foils of the recuperator core segment. A plurality of recuperator core segments must be assembled together so as to prevent nesting of adjacent fin folds of adjacent recuperator core segments.

Patent Number: 7,415,764 Issued on 08/26/2008 to Kang,   et al.

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Inventors:	Kang; Yungmo (La Canada Flintridge, CA), McKeirnan, Jr.; Robert D. (Westlake Village, CA)
Assignee:	Capstone Turbine Corporation (Chatsworth, CA)
Appl. No.:	11/336,718
Filed:	January 20, 2006

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	10917118	Aug., 2004	7065873
	60559270	Apr., 2004
	60515080	Oct., 2003

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Current U.S. Class:	29/890.039 ; 165/166; 165/78
Current International Class:	B21D 53/04 (20060101); F28F 3/12 (20060101)
Field of Search:	165/78,166 29/890.039

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References Cited [Referenced By]

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Other References

McDonald "Gas Turbine Recuperator Technology Advancements" Inst. Materials Conf. on Materials Issues in Heat Exchangers and Boilers, Loughborough, UK, Oct. 17, 1995. cited by other . McDonald "Recuperator Technology Evolution for Microturbines" ASME Turbo Expo 2002, Amsterdam, The Netherlands, Jun. 3-6, 2002. cited by other . Ward and Holman "Primary Surface Recuperator for High Performance Prime Movers" SAE Paper No. 920150 (1992). cited by other . Parsons "Development, Fabrication and Application of a Primary Surface Gas Turbine Recuperator" SAE Paper No. 851254 (1985). cited by other.

Primary Examiner: Leo; Leonard R
Attorney, Agent or Firm: Waddey & Patterson, P.C. Beavers; Lucian Wayne

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Government Interests

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This invention was made in conjunction with the US Department of Energy's Advanced Microturbine System Project under contract number DE-FC02-00CH11058. The United States government may have certain rights in this invention.

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Parent Case Text

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This application is a divisional of U.S. patent application Ser. No. 10/917,118 filed Aug. 12, 2004, now U.S. Pat. No. 7,065,873 which claims benefit of U.S. Provisional Patent Application Ser. No. 60/515,080 filed Oct. 28, 2003, entitled "Recuperator Construction for a Gas Turbine Engine", and U.S. Provisional Patent Application Ser. No. 60/559,270, filed Apr. 2, 2004, entitled "Recuperator Construction for a Gas Turbine Engine", both of which are hereby incorporated by reference.

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Claims

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What is claimed is:

1. A method of assembly of a recuperator core, comprising: (a) providing a supply of first heat exchanger foils and a supply of second heat exchanger foils, the first heat exchanger foils having a first fin fold orientation and the second heat exchanger foils having a different second fin fold orientation; (b) forming an indexing indicator on each of said first heat exchanger foils and each of said second heat exchanger foils, such that an improper assembly of two first heat exchanger foils or two second heat exchanger foils is visibly distinguishable from a proper assembly of one first heat exchanger foil and one second heat exchanger foil; (c) assembling a plurality of recuperator core segments, each recuperator core segment including one of said first heat exchanger foils and one of said second heat exchanger foils; (d) providing each of said recuperator core segments with an offset indexing lip alone a radially inner edge of the recuperator core segment, said offset indexing lip being consistently oriented relative to the first heat exchanger foil and the second heat exchanger foil of each recuperator core segment; and (e) forming each recuperator core segment into an involute curve, the curve having a concave side consistently oriented relative to the offset indexing lip, so that when a plurality of said recuperator core segments are stacked together to form a core, the indexing lips of adjacent recuperator core segments nest together and the first heat exchanger foil of each recuperator core segment is adjacent the second heat exchanger foil of the adjacent recuperator core segment, so as to prevent nesting of the heat exchanger foils of adjacent recuperator core segments.

2. The method of claim 1, wherein step (b) comprises forming each of said first and second heat exchanger foils with two corners of different radius; wherein the corners of a first heat exchanger foil are aligned with the corners of the second heat exchanger foil in an assembly of a first and second heat exchanger foil, and wherein an assembly of two first heat exchanger foils or two second heat exchanger foils results in a misalignment of corners, thereby visibly indicating an improper assembly.

3. The method of claim 1, wherein step (a) comprises: providing fin fold material having an undulating array of generally parallel fins on at least one side of said fin fold material, said fins having a generally uniform height, said uniform height being a full height, said fins having at least first and second selectable fin orientation directions relative to at least one dimension reference; cutting said first heat exchanger foils from said fin fold material, said first heat exchanger foils having at least one dimension reference, said first heat exchanger foils cut from said fin fold material so that said fins are oriented in said first fin orientation direction relative to said dimension reference; and cutting said second heat exchanger foils from said fin fold material, said second heat exchanger foils having at least one dimension reference, said second heat exchanger foils cut from said fin fold material so that said fins are oriented in said second fin orientation direction relative to said dimension reference.

4. The method of claim 1, wherein, if a defective recuperator core segment is formed with an improper orientation of its concave side relative to the offset indexing lip, a gap between adjacent offset indexing lips is created when the defective recuperator core segment is assembled with other recuperator core segments, the gap being a visible indication of the presence of a defective recuperator core segment.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to recuperators for gas turbine engines. More particularly, the present invention relates to component construction and assembly procedures designed to provide for foolproof assembly of the recuperator core.

2. Description of the Prior Art

Microturbines are small gas turbines used for small-scale power generation at one point in a distributed network or at a remote location. These power sources typically have rated power outputs of between 25 kW and 500 kW. Relative to other technologies for small-scale power generation, microturbines offer a number of advantages, including: a small number of moving parts, compact size, light weight, greater efficiency, lower emissions, lower electricity costs, potential for low cost mass production, and opportunities to utilize waste fuels.

Recuperator technology allows microturbines to achieve substantial gains in power conversion efficiencies. A conventional microturbine achieves at most 20 percent efficiency without a recuperator. However, with a recuperator, the efficiency of microturbine power conversion efficiency improves to between 30 percent and 40 percent, depending on the recuperator's effectiveness. This increase in efficiency is essential to acceptance of microturbine technology in certain markets and to successful market competition with conventional gas turbines and reciprocating engines.

Capstone Turbine Corp., the assignee of the present invention, has employed annular recuperators in 30 kW microturbines. These 30 kW microturbine engines are described in Treece and McKeirnan, "Microturbine Recuperator Manufacturing and Operating Experience," ASME paper GT-2002-30404 (2002), the details of which are incorporated herein by reference. Capstone has also developed and marketed 60 kW microturbines having similar annular recuperators. Commercial operating experience with Capstone's 30 kW and 60 kW microturbines has shown that annular recuperators perform well in these microturbines. The annular recuperators are more resilient to thermal cycling and have less total pressure drop as compared to box-type recuperators.

FIG. 1 shows the schematic diagram of a prototypical Capstone Microturbine. The airflow enters and exits the recuperator in a radial direction and the gas flows in an axial direction of the engine. The construction of the individual recuperator core segments of the C30 and C60 microturbines previously sold by the assignee of the present invention have included a pair of sheets of fin fold stainless steel material assembled with a plurality of spacer bars located between the sheets of material and including external stiffener bars, all of which are welded together in a suitable arrangement and have assembled therewith corrugated air inlet and outlet manifold inserts and gas side manifold inserts.

U.S. Pat. Nos. 6,112,403; 6,158,121; and 6,308,409 disclose recuperator core segments similar to those previously used by Capstone.

Other general background information on the state of the art of recuperator design for gas microturbines is found in the following: (1) McDonald "Gas Turbine Recuperator Technology Advancements", presented at the Institute of Materials Conference on Materials Issues in Heat Exchangers and Boilers, Loughborough, UK, Oct. 17, 1995; (2) McDonald, "Recuperator Technology Evolution for Microturbines", present at the ASME Turbo Expo 2002, Amsterdam, the Netherlands, Jun. 3-6, 2002; (3) "Ward and Holman", "Primary Surface Recuperator for High Performance Prime Movers", SAE paper number 920150 (1992); and (4) Parsons, "Development, Fabrication and Application of a Primary Surface Gas Turbine Recuperator", SAE paper 851254 (1985).

As a part of the US Department of Energy's Advanced Microturbine System (AMTS) Project, the assignee of the present invention developed a 200 kW microturbine engine with annular recuperator. The goals of the AMTS Project were to achieve: (1) 40/45 percent fuel-to-electricity efficiencies; (2) capital cost of less than $500 per kW of rated output power; (3) reduction in NOx emissions to less than 9 parts per millions; (4) mean period of machine operation between overhaul of several years; and (5) greater flexibility in types of usable fuels.

There is a continuing need for improvements in recuperator technology for microturbines, and particularly for recuperators suitable for use with larger microturbines such as the 200 kW microturbine developed by the assignee of the present invention. In particular, improving the efficiency of the radial distribution of compressed air within the recuperator core segments will allow use of recuperator core segments having a greater radial width to axial length ratio while maintaining a high level of heat exchanger effectiveness.

SUMMARY OF THE INVENTION

The much larger physical size and much greater heat transfer demands required for a recuperator suitable for use with a 200 kW microturbine led the assignee of the present invention to develop a completely new design for an annular counter-flow primary surface recuperator.

The physical dimensions of the microturbine, combined with the surface area required to provide the necessary heat transfer, led to the construction of an annular recuperator having a relatively high ratio of radial width to axial length, which in turn led to the design of an internal recuperator core segment geometry which substantially improves compressed air flow to the radially outer portions of each recuperator core segment.

Additionally, new manufacturing techniques provide a recuperator core segment construction having a minimum number of parts and providing for efficient and economical assembly thereof.

In one embodiment of the present invention a method is provided for assembly of a recuperator core. A supply of first heat exchanger foils and a supply of second heat exchanger foils are provided, the first heat exchanger foils having a first fin fold orientation and the second heat exchanger foils having a different second fin fold orientation. An indexing indicator is formed on each of the first heat exchanger foils and each of the second heat exchanger foils, such that an improper assembly of two first heat exchanger foils or two second heat exchanger foils is visibly distinguishable from a proper assembly of one first heat exchanger foil and one second heat exchanger foil. The indexing indicator is preferably provided by forming each heat exchanger foil with two corners of different radius. In a proper assembly of one first heat exchanger foil and one second heat exchanger foil, the respective corners are aligned. When an improper assembly is made of two first heat exchanger foils or two second heat exchanger foils, a misalignment of corners results thereby visibly indicating an improper assembly.

In another aspect of the invention a heat exchanger foil includes a foil sheet having an overall generally trapezoidal outer profile defined by a longer side, a shorter side parallel to the longer side, and first and second sloped manifold sides of substantially equal length. First and second indexing corners are each defined in the generally trapezoidal outer profile at an intersection of the shorter side and a sloped manifold side, each first and second indexing corner having a generally curved outer profile defined by a first indexing radius and a second indexing radius, respectively. The first indexing radius and the second indexing radius are selected such that, for two such identical foils, mating a first indexing corner of one foil with a second indexing corner of the second foil creates a distortion in the profile of the mated assembly identifiable by the human eye or by automated inspection means.

In another aspect of the invention a recuperator for a gas turbine engine includes a plurality of cells, or recuperator core segments, disposed in juxtaposed relation to one another in an annular array. Each of the cells includes a first plate having spaced integral ribs thereon at least partially defined in a plurality of spaced high pressure air channels, and a second plate welded to the first plate and having a plurality of spaced integral ribs, which in combination with the first plate of an adjacent cell, define a plurality of low pressure exhaust gas channels. First and second extended spacer bars are mounted on the radially inner edges of the first and second plates, respectively, and extend beyond the cell. The first spacer bar has a height less than the ribs on the first plate. The second spacer bar has a height greater than the ribs on the second plate. Due to the lesser height of the first extended spacer bar and the greater height of the second extended spacer bar, the first and second extended spacer bars provide an offset indexing lip along the radially inner edge of the cell. This offset indexing lip provides a visual and tactile indication of the proper orientation of the recuperator core segments relative to each other so as to insure proper assembly thereof.

In still another aspect of the invention a method of assembly of the recuperator core includes providing a supply of recuperator core segments, each made from a first heat exchanger foil having a first fin fold orientation and a second heat exchanger foil having a different second fin fold orientation. Each recuperator core segment is also provided with an offset indexing lip on a radially inner edge thereof, the offset indexing lip being consistently oriented relative to the first and second heat exchanger foils of each of the recuperator core segments. A plurality of the recuperator core segments are assembled together with their offset indexing lips nested together so that the first heat exchanger foil of each recuperator core segment is adjacent the second heat exchanger foil of the adjacent recuperator core segment, so as to prevent nesting of the fin folds of adjacent recuperator core segments.

Accordingly, it is an object of the present invention to provide an improved recuperator core segment construction.

Another object of the present invention is the provision of improved methods of construction of recuperator core segments and of annular recuperators.

And another object of the present invention is the provision of a recuperator core segment and a method of assembly thereof which insures proper assembly of the recuperator core segment from one first heat exchanger foil and one second heat exchanger foil, wherein the first and second heat exchanger foils have different fin fold patterns to prevent nesting of the fin folds of adjacent heat exchanger foils.

And another object of the present invention is the provision of a recuperator core segment construction and assembly method wherein each recuperator core segment is provided with an offset indexing lip along its radially inner edge, so as to insure proper orientation of one recuperator core segment relative to another and to prevent nesting of fin folds between adjacent recuperator core segments.

Other and further objects features and advantages of the present invention will be readily apparent to those skilled in the art upon reading of the following disclosure when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a microturbine having an annular counter flow recuperator.

FIG. 2 is an exploded view of a recuperator core segment of one embodiment of the present invention.

FIG. 3 is profile view of an inner surface or air side of one of one heat exchanger foil or sheet of the recuperator core segment of FIG. 2.

FIG. 4 is an outer surface or gas side view of the heat exchanger foil of FIG. 3.

FIG. 5 is a partial cross-section view of the transition zone of the heat exchanger foil of FIG. 3 taken along reference line 154 of FIG. 3.

FIG. 6 is a cross-sectional view of fin fold material of the heat exchanger foils of FIG. 3.

FIG. 7 is a plan view of the gas channel inserts.

FIG. 8 is a plan view of the air channel inserts.

FIG. 9 is an end view of the gas channel insert of FIG. 7.

FIG. 10 is a plan view of a recuperator core segment.

FIG. 11 is a radially inner edge view of a plurality of recuperator core segments of FIG. 10 in a nested configuration.

FIG. 12 is a cross-sectional view of the recuperator core segments of FIG. 11 along a centerline reference line like 150 of FIG. 10.

FIG. 13 is a detail, somewhat schematic, view of the radially inner edge region of the recuperator core segments of FIG. 12.

FIG. 14 is a detail view of the radially inner edge region of one recuperator core segment of FIG. 12.

FIG. 15 is a cross-sectional view of the recuperator core segments of FIG. 11 along manifold reference line 152 of FIG. 10.

FIG. 16 is a detail view of the radially inner edge region of the recuperator core segments of FIG. 15.

FIG. 17 is a detail view of the radially inner edge region of one recuperator core segment of FIG. 15.

FIG. 18 is a profile view of an inner surface of one heat exchanger foil having indexing corners.

FIG. 19 is a partial oblique view of indexing corners of a properly assembled recuperator core segment having no profile distortion.

FIG. 20 is a partial oblique view of indexing corners of an improperly assembled recuperator core segment having a profile distortion.

FIG. 21 is an oblique view of a recuperator core segment having first and second indexed stiffener support spacer bars.

FIG. 22 is an oblique view of a recuperator core segment of FIG. 21 having mismatched indexed stiffener support spacer bars.

FIG. 23 is a detail cross-sectional view of a plurality of recuperator core segments in a nested configuration, each recuperator core segment having first and second indexed stiffener support spacer bars.

FIG. 24 is a detail cross-sectional view of a plurality of recuperator core segments of FIG. 23 having mismatched indexed stiffener support spacer bars.

FIG. 25 shows a cross-sectional view of the recuperator showing the attachment of the hot end extensions of the stiffener support spacer bars to a support ring.

FIG. 26 is a recuperator sector.

FIG. 27 shows a cross-sectional view of the recuperator core showing the inner case and interface rings welded to the interior surface of the recuperator and showing the outer case surrounding the exterior edges of the recuperator core segments.

FIG. 28 is a flow chart illustrating the process of manufacturing the annular recuperator of FIG. 23.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and in particular to FIG. 1, a microturbine is shown and generally designated by the numeral 10. The microturbine 10 and its major components are schematically illustrated in FIG. 1. The microturbine includes a turbine 12, a compressor 14 and a generator 16 all of which are located upon a common shaft 18. The microturbine further includes a combustor 20 and a recuperator 22 which is the particular object of the present invention.

Fresh combustion air enters the microturbine 10 as indicated at the microturbine inlet air passage 24. The combustion air typically passes through the generator 16 to provide some cooling to the components of the generator 16. The inlet air is then compressed by compressor 14 and high pressure air exits compressor 14 via the recuperator compressed air passage 26 which directs the compressed air through the recuperator 22 along C-shaped path 28. The compressed air is preheated in the recuperator 22, and the preheated compressed air exits the recuperator via preheated compressed air passage 30 which carries it to combustor 20. The preheated compressed air is combined with fuel in combustor 20 in a known manner and the heated products of combustion are directed via turbine inlet passage 31 to the turbine 12 to power the turbine 12 which in turns drives the compressor 14 and generator 16 via the common shaft 18. Hot exhaust gas from the turbine 12 is carried via turbine exhaust passage 32 back to the recuperator 22. The exhaust gas flows in an axial path through the gas side the recuperator along the recuperator exhaust gas passage 34. The spent low pressure exhaust gas is exhausted via the microturbine exhaust passage 36 after it passes through recuperator 22.

The recuperator 22 can be generally described as an annular counter flow recuperator or heat exchanger. The annular recuperator surrounds the compressor 14 and turbine 12 and is made up of a large number of individual recuperator core segments as further described below.

FIG. 2 shows an exploded view of one of the individual recuperator core segments of one embodiment of the recuperator 22. The individual recuperator core segment is generally designated by the numeral 38. The recuperator core segment 38 may also be referred to as a recuperator cell 38.

The components of the recuperator core segment 38 are shown in exploded view in FIG. 2 and include first and second heat exchanger foils 40 and 42, respectively. Heat exchanger foils 40 and 42 may also be referred to as heat exchanger sheets or plates.

Referring now to FIGS. 2 and 8, the recuperator core segment 38 of this embodiment further includes an air manifold inlet insert 44 and an air manifold outlet insert 46 which are inserted between the heat exchanger foils 40 and 42 in a manner further described below. Other embodiments, not shown, do not require air manifold inserts. Referring now to FIGS. 2 and 7, the recuperator core segment 38 of this embodiment further includes gas channel inserts 54 and 56 which are attached to one side of the recuperator core segment and provide spacing between adjacent recuperator core segments to aid in the flow of hot exhaust gases, as further described below. Other embodiments, not shown, do not require exhaust manifold inserts. Recuperator core segment 38 further includes first and second stiffener support spacers 48 and 50 which are sandwiched about the heat exchanger foils 40 and 42 along their axially extending radially inner edge in a manner further described below. The air inserts 44 and 46 and the gas channel inserts 54 and 56 are preferably constructed from corrugated stainless steel sheet material 57 having a cross-section generally as shown in FIG. 9. Recuperator core segment 38 further includes a weld cap 52 which will be received along the axially extending radially outer edge of the recuperator core segment.

Each of the heat exchanger foils 40 and 42 is preferably constructed from a sheet of fin folded material. The material typically is stainless steel or nickel alloy sheet having a thickness of approximately 0.0040 inches. One suitable geometry for the fin fold corrugations of the fin fold sheet is shown in FIG. 6. Such fin fold material is readily available from a number of sources including for example Robinson Fin of Kenton, Ohio.

FIG. 3 is a plan view of the air side of one of the heat exchanger foils 40 and 42, and FIG. 4 is a plan view of the as side of one of the heat exchanger foils 40 and 42. It will be understood that as used herein the air side of the heat exchanger foils refers to the interior surfaces 41 of heat exchanger foils 40 and 42 of an assembled recuperator core segment 38 through which the compressed air will flow. By gas side the following description refers to those exterior surfaces of the heat exchanger foils 40 and 42 of an assembled recuperator core segment 38, past which the hot exhaust gases will flow.

A preferred embodiment of the heat exchanger foil is shown in FIGS. 3 and 4. The heat exchanger foil shown is a sheet 40 or 42 of fin fold material having first and second manifold zones 70, 72 separated by a primary surface zone 74. The primary surface zone 74 includes a central portion 84 made of generally uniform foil corrugations 79 of a full height, and a first transition zone 86 is located between the central portion 84 and the first manifold zone 70. The first transition zone 86 is made of foil corrugations 79 of heights less than a full height. The foil corrugations 79 of the first transition zone 86 continuously increase in height from the first manifold zone 70 to the central portion 84.

Referring now to FIG. 5, which is generally a cross section taken through the first transition zone 86 along first transition zone reference line 154 of FIG. 3, the transition zone 86 has an axial extending width 100. In the manifold zone 70, the corrugations 79 have been crushed and have a sheet thickness 104. In the central portion 84 of primary surface zone 74, the corrugations 79 have their full height. Herein, full height refers to crest to centerline distance. As illustrated in FIG. 6, the gas side crests 81 have a full height of 107, and air side crests 83 have a full height of 109. The fin fold material has a crest-to-trough height 102 equal to the combined full heights 107 and 109 of the gas side crests 81 and the air side crests 83.

Referring again to FIGS. 3 and 4, the first transition zone 86 is relatively narrower and the foil corrugations 79 of the first transition zone 86 are more steeply sloped in areas proximal the inlet area 96 of the first manifold zone 70. The transition zone is relatively wider and the foil corrugations 79 of the first transition zone 86 are less steeply sloped in areas distal to the inlet area 96. In this embodiment, the primary surface zone 74 is rectangular in shape, and the first transition zone 86 of the primary surface zone 74 is triangular in shape. In other embodiments of the invention, the first transition zone 86 may have continuous variations in width. In yet other embodiments, the first transition zone 86 may have discontinuous variations in width

In this embodiment of the invention, each corrugation 79 of the first transition zone 86 has a generally constant aspect ratio, that is rise/run. Other embodiments of the invention have corrugations 79 with aspect ratios that vary along the length of the corrugation 79 within the first transition zone 86. In the embodiment shown in FIG. 3, the aspect ratios of the foil corrugations 79 of the first transition zone 86 vary from corrugation 79 to adjacent corrugation 79 and continuously decrease in a direction away from the inlet area 96. The aspect ratios of the foil corrugations 79 of the first transition zone 86 vary between 1:60 (closest to outer edge 64) and 1:0.5 (closest to inner edge 62).

In the embodiment shown in FIG. 3, a second transition zone 88 is located between the central portion 84 and the second manifold zone 72. The second transition zone 88 has foil corrugations 79 of heights less than full height. In this embodiment, the foil corrugations 79 of the second transition zone 88 have aspect ratios generally equal to a constant aspect ratio, that is they all have substantially the same slope. The constant aspect ratio is selected to be an aspect ratio of between 1:2 and 1:0.5. This produces a narrow second transition zone 88 between the central portion 84 and the second manifold zone 72. As further described below, this feature provides greater strength in the hot end of the recuperator core segment and reduces the likelihood of distortion of the heat exchanger foils 40 and 42 under operating conditions and, therefore is one factor in eliminating the need for an air manifold insert 46 between the heat exchanger foils in this region of the heat exchanger foils.

In the embodiment shown in FIG. 3, the heat exchanger foils 40 and 42 have an overall generally trapezoidal outer profile defined by a longer axially extending radially inner edge 62, a shorter axially extending radially outer edge 64 parallel to the longer edge, and first and second sloped manifold sides 66, 68 of substantially equal length. The first and second manifold zones 70, 72 are located adjacent the first and second sloped manifold sides 66, 68, respectively. The generally rectangular primary surface zone 74 is located centrally between the first and second manifold zones 70, 72. Raised corrugations 79 extend entirely across the generally rectangular primary surface zone 74 and protrude above and below the manifold zones 70 and 79. The primary surface zone 74 includes the transition zone 86 located adjacent the first manifold zone 70 and having a plurality of raised undulating corrugations 79 extending generally parallel to the longer and shorter sides 62, 64 and increasing in height in a direction away from the first manifold zone 70. The corrugations 79 are shown as crests 80 in the patch work portions of FIG. 3, and preferably are undulating corrugations when seen in planar view. The second transition zone 88 is located adjacent the second manifold zone 72, the second transition zone 88 having a plurality of raised corrugations 79 extending generally parallel to the longer and shorter sides 62, 64 and increasing in height in a direction away from the second manifold zone 72. The central portion 84 is located between the two transition zones, the central portion 84 having a plurality of raised corrugations 79 extending generally parallel to the longer and shorter sides 62, 64 and generally uniform in height. In the embodiment shown in FIGS. 3 and 4, each opposite planar surface 41, 43 of the heat exchanger foil 40 or 42 includes two manifold zones 70, 72 and one primary surface zone 74, including one central portion 84 and two transition zones 86, 88.

Another aspect of this invention is here described with reference to FIGS. 2, 3, 10 and 17. The recuperator core segment 38 includes first and second heat exchanger foils 40, 42 each having a primary surface zone 74. The primary surface zones 74 are disposed in opposition so as to define an interior axial air passage 170 (see FIG. 17) having an axial air passage inlet 172 (see FIG. 3) and an axial air passage outlet 174. The axial air passage inlet 172 and axial air passage outlet 174 each extend generally transversely away from the inner edge 62 defined by the heat exchanger foils 40, 42. At least one of the primary surface zones 74 includes a plurality of generally evenly spaced corrugations 79 extending from the axial air passage inlet 172 to the axial air passage outlet 174. The corrugations 79 define a corresponding plurality of air channels 176 of even width, as shown in FIG. 3, 6 and 17. FIG. 17 shows a cross-sectional view of the recuperator core segment 38 of FIG. 10 along the manifold reference line 152. Outlet manifold zones 72 partially obscure the corrugations 79 in the central portion 84 of the primary surface area 74. (For clarity, the outlet transition zone corrugations have been omitted.) The axial air passage 170 includes at least one such plurality of air channels 176.

It will be understood that FIG. 17 is somewhat schematic, in that the corrugations of adjacent heat exchanger foils 40 and 42 do not neatly align at their points of engagement as illustrated. Instead they criss-cross each other due to the different corrugation patterns, so as to prevent nesting of the corrugations or fin folds.

Referring again to FIGS. 2, 3, 5 and 10, selected corrugations 79 each have an aspect ratio (rise/run) defined along a first transition length 100 of the selected corrugation 79 along which the height of the selected corrugation 79 rises from a reduced height 103 at the axial air passage inlet 172 to a full height 107 or 109. In this embodiment, the aspect ratios of the selected corrugations 79 are selected such that resistance to air flow through the total length of an air channel 177 (see FIG. 3) for air channels distal to the radially inner edge 62 is generally less than resistance to air flow through the total length of an air channel 178 for air channels proximal to the radially inner edge 62.

At least one of the two primary surface zones 74 further includes the first transition zone 86 defined by a plurality of the first transition lengths 100 of the selected corrugations 79. In this embodiment of the invention, each first transition length 100 has a generally constant aspect ratio, that is, it has a straight slope rather than a curved slope. Other embodiments of invention, not shown, have aspect ratios that vary over at least one transition length 100. In the embodiment of the invention shown in FIG. 3, the aspect ratios of a plurality of the first transition lengths of the first transition zone 86 continuously decrease in a direction away from the radially inner edge 62. These aspect ratios of the plurality of the first transition lengths 100 of the first transition zone 86 may vary between 1:60 and 1:0.5, and are more preferably between 1:30 and 1:1.

The very narrow second transition zone 88 is best described with reference to FIGS. 3 and 4. In second transition zone 88 each corrugation 79 has an aspect ratio defined by a second transition length 101 of the additional selected corrugation 79 along which the height of the selected corrugation 79 rises from a reduced height at the axial air passage outlet 174 to a full height. In this embodiment a plurality of the second transition lengths 101 of the second transition zone 88 each have a generally constant aspect ratio. Other embodiments of invention, not shown, have aspect ratios that vary over at least one second transition length 101. In yet another embodiment of the invention, the first transition zone 86 and the second transition zone 88 are symmetric with respect to the center reference line 150, as illustrated in FIG. 2. In still yet another embodiment the first transition zone 86 and the second transition zone 88 are both triangular, again as illustrated in FIG. 2.

In the embodiment of the invention shown in FIGS. 3, 4 and 10, the aspect ratios of a plurality of the second transition lengths 101 of the second transition zone 88 are a generally constant aspect ratio. These aspect ratios of the plurality of the second transition lengths 100 of the second transition zone are an aspect ratio of between 1:2 and 1:0.5, and are more preferably an aspect ratio of 1:1.

The full height crests of a central zone of one heat exchanger foil 40 engage the full height crests of an opposing central zone of one heat exchanger foil 42, while the crests of opposing transition zones do not engage each other unless there is distortion in the heat exchanger foils. Excessive temperatures tend to cause material creep and may cause distortion of recuperator core segments 38 in the air outlet/gas inlet regions. The narrow second transition zone 88 provides for a larger central zone 86 having full height crests 80. This cell geometry provides for additional structural support for the opposing sheets necessary for the `hot` end of the recuperator core.

Referring now to FIG. 11, the recuperator core segment further includes an air inlet 114 and an air outlet 115, each defined in the radially inner edge 62. An interior air passage 180 (see FIGS. 16 and 17) is formed by a plurality of interior air passage channels 176 and provides fluid communication between the inlet 114 and outlet 115. The interior air passage 180 includes an inlet manifold passage 182 (see FIG. 2) extending radially outward from the inlet 114; an outlet manifold passage 184 extending radially inward to the outlet 115; and the axial air passage 170 (see FIG. 17) extending generally axially between the inlet manifold passage 182 and the outlet manifold passage 184. First and second air manifold inserts 44, 46 are received within the inlet manifold passage 182 and the outlet manifold passage 184, respectively. The first and second air manifold inserts 44, 46 have first and second air manifold corrugations 57, as best seen in FIG. 9, extending from the inlet 114 and outlet 115 toward the axial air passage inlet 91 and an axial air passage outlet 93, respectively. Referring to FIGS. 2 and 3, the first and second air manifold corrugations 57 have axially outer corrugations 186 in fluid communication with generally corresponding radially outer primary surface zone air channels 177 and further have axially inner corrugations 187 in fluid communication with generally corresponding radially inner primary surface zone air channels 178. Corresponding primary surface zone air channels 176 and manifold corrugations 57 form interior air passage channels 185 defining channels of flow through the interior air passage.

The aspect ratios of this embodiment are selected such that resistance to air flow through the total length of any interior air passage channel 185 is sufficiently equal to air flow through the total length of any other interior air passage channel 185 that substantially uniform air flow rates are achieved across as much as possible of the area of the primary surface zone. The transition zone 86 has allowed this to be achieved for the primary surface zone 74 having a radial width 58 to axial length 60 ratio in a range of from 0.9 to 1.1.

Greater balance in airflow through the primary surface zones provides greater heat exchanger effectiveness. This allows a greater radial width to axial length of the primary surface zone. This is advantageous in design situations where there is a limit on the axial length of the recuperator.

With reference to FIG. 8, it is noted that the air channel insert 46 has an irregular shaped portion 46A extending toward its associated transition zone 88 adjacent a distal end of the air channel insert. Air channel insert 44 is similarly shaped. This aids in distributing air flow to and from the radially outermost portions of primary surface zone 74.

FIGS. 2, 3, 12, 13, and 14, illustrate another aspect of the present invention. As noted, the first and second heat exchanger foils 40 and 42 each having an integrally formed peripheral mating flange 94. The peripheral mating flange 94 of the first and second heat exchanger foils 40 and 42 are mated with each other and joined together to provide a recuperator core segment 38 free of any separate internal spacer bars. Each integrally formed peripheral mating flange 94 extends all around the periphery of the sheet except for the inlet 114 and outlet 115. At least one of the integrally formed peripheral mating flanges 94 is an offset flange. The peripheral mating flanges 94 of the first and second heat exchanger foils 40 and 42 are joined together by a peripheral weld and the weld cap 52 is received over at least a portion of the peripheral weld. In this embodiment of the invention, each of the first and second heat exchanger foils 40 and 42 is comprised of fin fold sheet material and the mating flanges 94 are crushed areas of the fin folded sheet material.

It is a distinct advantage to eliminate the need for internal spacer bars through the use of offset peripheral flanges. The offset peripheral flanges are of the same thickness as the rest of the sheet material and have generally the same thermal transient characteristics. By eliminating the relatively thick internal spacer bars of the prior art a recuperator core segment's transient thermal stress due to thermal lag is greatly reduced.

As best seen in FIGS. 2 and 11, f