Patent Publication Number: US-2023141836-A1

Title: Tube Bank Heat Exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. Pat. Application No. 17/078,720, filed Oct. 23, 2020, and entitled “Tube Bank Heat Exchanger”. 
    
    
     BACKGROUND 
     The disclosure relates to gas turbine engines. More particularly, the disclosure relates to gas turbine engine heat exchangers. 
     Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) include a variety of heat exchangers. 
     Examples of gas turbine engine heat exchangers are found in: U.S. Pat. Application Publication 20190170445A1 (the ‘445 publication), McCaffrey, Jun. 6, 2019, “HIGH TEMPERATURE PLATE FIN HEAT EXCHANGER”; U.S. Pat. Application Publication 20190170455A1 (the ‘455 publication), McCaffrey, Jun. 6, 2019, “HEAT EXCHANGER BELL MOUTH INLET”; and U.S. Pat. Application Publication 20190212074A1 (the ‘074 publication), Lockwood et al., Jul. 11, 2019, “METHOD FOR MANUFACTURING A CURVED HEAT EXCHANGER USING WEDGE SHAPED SEGMENTS”, the disclosures of which three publications are incorporated by reference in their entireties herein as if set forth at length. 
     An exemplary positioning of such a heat exchanger provides for the transfer heat from a flow (heat donor flow) diverted from an engine core flow to a bypass flow (heat recipient flow). For example, air is often diverted from the compressor for purposes such as cooling. However, the act of compression heats the air and reduces its cooling effectiveness. Accordingly, the diverted air may be cooled in the heat exchanger to render it more suitable for cooling or other purposes. One particular example draws the heat donor airflow from a diffuser case downstream of the last compressor stage upstream of the combustor. This donor flow transfers heat to a recipient flow which is a portion of the bypass flow. To this end, the heat exchanger may be positioned within a fan duct or other bypass duct. The cooled donor flow is then returned to the engine core (e.g., radially inward through struts) to pass radially inward of the gas path and then be passed rearward for turbine section cooling including the cooling of turbine blades and vanes. The heat exchanger may conform to the bypass duct. The bypass duct is generally annular. Thus, the heat exchanger may occupy a sector of the annulus up to the full annulus. 
     Other heat exchangers may carry different fluids and be in different locations. For example, instead of rejecting heat to an air flow in a bypass duct, other heat exchangers may absorb heat from a core flow (e.g., as in recuperator use). 
     Among recently proposed annular heat exchangers are those in U.S. Pat. Application Publication 20150101334A1 (the ‘334 publication), Bond et al., Apr. 16, 2015, “HEAT EXCHANGERS” and U.S. Pat. 10,184,400 (the ‘400 patent), Cerny et al., Jan. 22, 2019, “Methods of cooling a fluid using an annular heat exchanger”. 
     SUMMARY 
     One aspect of the disclosure involves a heat exchanger comprising: a first manifold assembly comprising a stack of plates; a second manifold assembly comprising a stack of plates; and a plurality of tubes extending from the first manifold assembly to the second manifold assembly. The plurality of tubes comprises a plurality groups of tubes. For each of the groups of the tubes: the tubes of the group have first ends mounted between plates of the first manifold assembly; and the tubes of the group have second ends mounted between plates of the second manifold assembly. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the plurality of groups form a respective plurality of stages of tubes, with the tubes of each group being fluidically in parallel with each other and the tubes of the different groups being fluidically in series. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, adjacent plates of the first manifold assembly combine to form associated plenums common to the tubes of the associated group of tubes and adjacent plates of the second manifold assembly combine to form associated plenums common to the tubes of the associated group of tubes. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the first manifold assembly is an inner manifold assembly and the second manifold assembly is an outer manifold assembly at least partially surrounding the inner manifold assembly so that the tubes of each group diverge from each other from the first manifold assembly to the second manifold assembly. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the groups form respective rows of tubes and the heat exchanger has at least 3 said rows of tubes. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the groups form respective rows of tubes and the heat exchanger has at least 20 tubes in each of the rows. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the tubes each have a plurality of bends. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the tubes’ plurality of bends each have a respective bend axis transverse to a stacking direction of the first manifold assembly and second manifold assembly. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the tubes’ at least one bend is, for each tube, a first bend and a second bend; and between the first bend and the second bend, each tube has a continuous arc of at least 50% of a length of said tube. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, in at least one of the first manifold assembly and the second manifold assembly: the stack of plates comprises a first end plate, a second end plate, and a plurality of intermediate plates; and the plurality of intermediate plates are a plurality of first intermediate plates identical to each other and one or more second intermediate plates identical to each other if a plurality but different from the first intermediate plates and alternating with the first intermediate plates. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, in the first manifold assembly: the stack of plates comprises a first end plate, a second end plate, and a plurality of intermediate plates; and the plurality of intermediate plates are an alternating plurality of first intermediate plates identical to each other and second intermediate plates identical to each other but different from the first intermediate plates; and in the second manifold assembly: the stack of plates comprises a first end plate, a second end plate, and a plurality of intermediate plates; and the plurality of intermediate plates are an alternating plurality of first intermediate plates identical to each other and second intermediate plates identical to each other but different from the first intermediate plates. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, in the first manifold assembly: the first intermediate plates have a first face with a first channel, a second face with a second channel, and no through-holes between the first channel and second channel; and the second intermediate plates have a first face with a first channel, a second face with a second channel, and a plurality of through-holes between the first channel and second channel. In the second manifold assembly: the first intermediate plates have a first face with a first channel, a second face with a second channel, and a plurality of through-holes between the first channel and second channel; and the second intermediate plates have a first face with a first channel, a second face with a second channel, and no through-holes between the first channel and second channel. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the manifold plates and tubes each comprise Ni-based superalloy or stainless steel. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, a turbine engine includes the heat exchanger and further comprises a gas path (e.g., a core flowpath or a bypass flowpath) passing gas across exteriors of the plurality of tubes. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively: the tubes each have a first bend and a second bend; and between the first bend and the second bend, each tube has a portion of at least 50% of a length of said tube. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the first bend and second bend shift said portion downstream along the gas path. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the turbine engine further comprising a recuperator comprising: a turbine coupled to the at least one outlet of the outlet manifold; and a compressor having an outlet coupled to the at least one inlet of the inlet manifold. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the recuperator comprising a generator driven by the turbine. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the recuperator comprising a supercritical carbon dioxide or other cryogenic working fluid. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, a method for using the heat exchanger comprises: passing a first fluid flow along a first flowpath across exteriors of the tubes; and passing a second fluid flow through interiors of the tubes. The tubes each have at least one bend. The at least one bend shifts a portion of the tube downstream along the first flowpath. The passing of the first flow and the second flow thermally expands the length of the tubes to further shift the portion downstream along the first flowpath. 
     Another aspect of the disclosure involves a method for manufacturing a heat exchanger. The heat exchanger comprises: a first manifold assembly comprising a stack of plates; a second manifold assembly comprising a stack of plates; and a plurality of tubes extending from the first manifold assembly to the second manifold assembly. The plurality of tubes comprises a plurality groups of tubes. For each of the groups of the tubes: the tubes of the group have first ends mounted between plates of the first manifold assembly; and the tubes of the group have second ends mounted between plates of the second manifold assembly. The method comprises: stacking the respective plates of the first manifold assembly and the second manifold assembly with the respective first and second ends of the tubes between the associated plates; applying compression across the first manifold assembly’s plates while applying an electrical current across the first manifold assembly’s plates so as to bond the first manifold assembly’s plates to each other; and applying compression across the second manifold assembly’s plates while applying an electrical current across the second manifold assembly’s plates so as to bond the second manifold assembly’s plates to each other. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the method further comprises thermally heating during the applying compression across the first manifold assembly’s plates and the second manifold assembly’s plates. 
     In a further embodiment of any of the foregoing embodiments, additionally and/or alternatively, the first manifold assembly’s plates and the second manifold assembly’s plates are alloy plates. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a view of a circumferentially segmented annular heat exchanger. 
         FIG.  2    is an end view of a segment of the heat exchanger. 
         FIG.  3    is a partial axial/radial sectional view of the heat exchanger. 
         FIG.  4    is an enlarged cutaway axial/radial sectional view of the heat exchanger. 
         FIG.  5    is an inboard end view of a first end plate of an inner diameter (ID) manifold of the heat exchanger. 
         FIG.  6    is an off-center inner diameter (ID) view of the plate of  FIG.  5   . 
         FIG.  7    is an inboard end view of a second end plate of the ID manifold. 
         FIG.  8    is an off-center ID view of the plate of  FIG.  7   . 
         FIG.  9    is an outboard end view of a first penultimate plate in the ID manifold. 
         FIG.  10    is an inboard end view of the plate of  FIG.  9   . 
         FIG.  11    is an off-center outer diameter (OD) view of the plate of  FIG.  9   . 
         FIG.  12    is outboard end view of a second penultimate plate in the ID manifold. 
         FIG.  13    is an inboard end view of the plate of  FIG.  12   . 
         FIG.  14    is an off-center OD view of the plate of  FIG.  12   . 
         FIG.  15    is a first end view of a baffle plate in the ID manifold. 
         FIG.  16    is a second end view of the plate of  FIG.  15   . 
         FIG.  17    is an off-center OD view of the plate of  FIG.  17   . 
         FIG.  18    is a first end view of a boundary plate in the ID manifold. 
         FIG.  19    is a second end view of the plate of  FIG.  18   . 
         FIG.  20    is an off-center OD view of the plate of  FIG.  18   . 
         FIG.  21    is an inboard end view of a first end plate in an outer diameter (OD) manifold of the heat exchanger. 
         FIG.  22    is an inboard end view of a second end plate in the OD manifold. 
         FIG.  23    is a first end view of a baffle plate in the OD manifold. 
         FIG.  24    is a second end view of the plate of  FIG.  23   . 
         FIG.  25    is an off-center ID view of the plate of  FIG.  23   . 
         FIG.  26    is a first end view of a boundary plate in the OD manifold. 
         FIG.  27    is a second end view of the plate of  FIG.  26   . 
         FIG.  28    is an off-center ID view of the plate of  FIG.  26   . 
         FIG.  29    is a side view of a tube in the heat exchanger. 
         FIG.  30    is a schematic view of a gas turbine engine having the annular heat exchanger in a recuperating supercritical CO 2  bottoming cycle. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG.  1    shows a heat exchanger  20  providing heat exchange between a first flowpath  900  and a second flowpath  902  and thus between their respective first and second fluid flows  910  and  912 . In the exemplary embodiment, the flowpaths  900 ,  902  are gas flowpaths passing respective gas flows  910 ,  912 . In the illustrated example, the first flow  910  enters and exits the heat exchanger  20  as a single piped flow and the flow  912  is an axial annular flow surrounding a central longitudinal axis  500  of the heat exchanger.  FIG.  1    also shows an axial direction  502  as a generally downstream direction along the first flowpath  100 . In a coaxial duct within a gas turbine engine, the axis  500  may be coincident with a centerline of the engine and an axis of rotation of its spools, the direction  502  is an aftward/rearward direction, and a radial direction is shown as  504 . 
     The heat exchanger  20  has a first flow inlet  22  ( FIG.  3   ) and a first flow outlet  24 . The exemplary inlet and outlet are, respectively, ports of an inlet manifold  26  and an outlet manifold  28 . Exemplary manifolds are metallic (e.g., nickel-based superalloy). The inlet manifold and outlet manifold may each have a respective fitting  30 A,  30 B providing the associated port  22 ,  24 . Each manifold  26 ,  28  further has a body  32 A,  32 B extending circumferentially about the axis  500  from the associated fitting  30 A,  30 B, and port  22 ,  24 . The exemplary manifolds have continuously curving arcuate form. 
     The exemplary heat exchanger  20  is circumferentially segmented into a plurality of segments  40  (four segments shown in  FIG.  1   ). Each segment  40  may, itself, be identified as a heat exchanger. Depending upon situation, the segments  40  may be plumbed to have respective first flow segments in parallel, in series, or two totally different first flows. In the illustrated example, they are plumbed in parallel with each other along both flowpaths  900  and  902 . As is discussed further below, each segment  40 , itself, has a first manifold  42 , a second manifold  44 , and a plurality of heat transfer tubes  46  extending between the first manifold and the second manifold. Interiors of the tubes fall along the associated branch of the first flowpath to pass an associated portion of the first flow. Central exposed exterior surfaces of the tubes are along the second flowpath in heat exchange relation with the second flow. 
     The manifolds  42  and  44  of the individual segments may be mated to the overall heat exchanger manifolds  26  and  28  (if separately-formed). In the exemplary implementation, the segment first manifold  42  is a combined inlet and outlet manifold separately in fluid communication with the overall heat exchanger manifolds  26 ,  28 . They may be mated/sealed by welding, brazing, or gasketed bolting. The segment second manifold  44  is thus a turn manifold lacking external fluid communications. 
     In alternative embodiments (not shown), one of the segment manifolds  42 ,  44  may be an inlet manifold mated to (or otherwise integrated with) the heat exchanger manifold  26  while the other is an outlet manifold mated to (or otherwise integrated with) the heat exchanger manifold  28 . 
     As is discussed further below, each of the manifolds  42 ,  44  is formed as an assembly of a stack of plates ( FIGS.  3 &amp; 4   ) extending between a first axial end  50 ,  52  and a second axial end  54 ,  56 , respectively. Each of the plates has a pair of opposite faces (axially-facing or radially/circumferentially extending), a pair of circumferential ends, an inner diameter (ID) surface, and an outer diameter (OD) surface. In the exemplary embodiment, the plates of each manifold  42 ,  44  are stacked with the aft/downstream face of one plate contacting and secured to the forward/upstream face of the next. From upstream-to-downstream along the second flowpath  902  or fore-to-aft in the axial direction  502 , the end sections or portions of groups of the tubes  46  are mounted in pockets  58  ( FIG.  4   ) formed between the mating plates. As is discussed further below, the tubes of each group are staggered to relative to the adjacent row(s) provide out-of-phase registry with the tubes of the adjacent groups fore or aft (e.g., each tube of a given row is circumferentially directly between two adjacent tubes of each of the two adjacent rows - except at circumferential or axial ends). 
       FIG.  4    shows pockets  58  at the plate junctions accommodating the tube end sections. With exemplary circular-section tubing, the pockets are essentially right circular cylindrical pockets split evenly between the two plates and provided by respective semicylindrical grooves  90  ( FIG.  5   ) in the two faces. The grooves (or pocket segments/sections)  90  have surfaces  91  and extend between the associated OD surface of an ID plate or ID surface of an OD plate on the one hand and a plenum discussed below on the other hand. 
     With the exemplary arcuate manifold segment configuration, the first manifold assembly  42  is an inner (inner diameter (ID)) manifold and the second manifold assembly  44  is an outer (outer diameter (OD)) manifold assembly at least partially surrounding the first manifold assembly so that tubes  46  in each group circumferentially diverge from each other in the radial direction from the first manifold assembly to the second manifold assembly. Despite this radial fanning arrangement, each group may be identified as a “row” as is common practice with tube-bank heat exchangers. 
     The plates of the first manifold assembly  42  include a first end plate  60 , a second end plate  62 , and one or more intermediate plates. Depending on implementation, the intermediate plates may be the same as each other or different from each other. In the illustrated example, the first manifold intermediate plates are: first and second penultimate plates  64  and  66  respectively adjacent the first and second plates  60  and  62 ; and alternating first intermediate plate(s)  68  and second intermediate plate(s)  70 . 
     In the illustrated example: the various plates are symmetrical from one of their circumferential ends to the other (e.g., across a central axial/radial plane); and the rows thus alternate in number of tubes by a single tube with even rows having one number of tubes and odd rows differing by one. An exemplary number of rows is at least two or at least three. Upper limits may be influenced by diminishing return on heat transfer and by increasing fluidic losses along both flowpaths. Thus, an exemplary upper limit on rows is ten with a likely sweet spot of three to six rows. For full annular heat exchangers there may be a thousand or more tubes per row. Even for a smaller segment, there may be hundreds of tubes per row or more. There may be at least an exemplary twenty in a segment (whether stand-alone or assembled with other segments such as sectors discussed above) or a range of twenty to one thousand or twenty to two hundred). 
     Alternatively: the end plates  60  and  62  may be identical to each other but oppositely facing; and the penultimate plates  64  and  66  may be identical to each other. Circumferential end-to-end asymmetry of the alternative plates may allow the first circumferential ends of the plates to align with each other and the second circumferential ends of the plates to align with each other. This would allow staggered groups of the same number of tubes (in distinction to the illustrated difference of one tube between the two alternating sets of rows). In other implementations, this alignment may be lacking (e.g., the plate ends at a given circumferential end of the ID manifold stagger). This stagger potentially allows mechanical interfitting between circumferential ends of adjacent manifolds. This, in some implementations, may allow full annulus manifolds to be assembled from circumferential segments. In other full annulus manifold situations, the plates are full annulus. 
     In yet further variations, the overall heat exchanger manifolds  26  and  28  may be more extensively integrated with the segment manifolds  42  and  44 . For example, an end plate and a penultimate plate may combine to also form the associated overall heat exchanger manifold  22  or  28  or a segment thereof. In yet other variations, one or both of the overall manifolds may be integrated with the second manifold assembly  44 . 
     Similarly, the plates of the second manifold assembly  44  include a first end plate  80 , a second end plate  82 , and one or more intermediate plates. Depending on implementation, the intermediate plates may be the same as each other or different from each other. In the illustrated example, the second manifold intermediate plates are alternating first intermediate plates  86  and second intermediate plates  88 . The possibilities for end-to-end circumferential symmetry or asymmetry and the implications thereof are the same as those discussed for the first manifold assembly  42 . 
     The ends of the tubes of each row are in respective communication with plenums formed between adjacent plates.  FIG.  4    shows, from fore to aft, plenums  110 ,  112 ,  114 ,  116 ,  118 , and  120  in the ID manifold  42  and  130 ,  132 ,  134 ,  136 ,  138 , and  140  in the OD manifold  44 . The various plenums are formed by channels in the mating axial faces of adjacent plates. In the exemplary embodiment, all but the axially outboard faces of the end plates have such channels. In the exemplary embodiment, with separate inlet manifold  26  and outlet manifold  28 , the ID manifold aft plenum  120  and forward plenum  110  open radially inward (along the ID face of the ID manifold  42 ) so as to form an aft inlet slot  126  ( FIG.  4   ) and a forward outlet slot  128 . To form the forward plenum  110  and aft plenum  120  of the ID manifold, the associated plate faces have radially inwardly open circumferentially elongate channels  150  ( FIG.  5   ) each having an open inboard radial end  151  and circumferential ends at circumferential end walls  152 . The open inboard radial ends  151  of the mating pairs of plates form the associated ports  126  and  128 . The remaining plate faces of the ID manifold have radially inwardly closed circumferentially elongate channels  155  ( FIG.  10   ) having closed radially inboard ends along an intact ID wall  156  and closed circumferential ends at intact end walls  157 . A web  158  of material is left between the channels of a given such plate. In the baffle plates, the web  158  is pierced by the ports  122  ( FIG.  15   ). 
     In similar fashion to the respective mating pairs of channels  155  forming the  FIG.  4    plenums  112 ,  114 ,  116 ,  118  of the ID manifold  42 , the various axial faces of the OD manifold plates (except the front face of the first end plate  80  and aft face of the second end plate  82 ) have outwardly closed circumferentially elongate channels  165  ( FIG.  21   ) forming the  FIG.  4    plenums  130 ,  132 ,  134 ,  136 ,  138 ,  140 . These channels  165  extend between circumferential ends at circumferential end walls  67  and have an outboard end at an intact outboard or outer diameter (OD) end wall portion  166 . 
     The exemplary flow  910  (or a branch thereof) passes into the aft-most plenum  120  from the inlet  22 . This flow branches into respective branches (or sub-branches) passing through the tubes of the last (aft-most) row of tubes to enter the aft-most OD plenum  140 . The flow then passes forward through ports  142  in the plate  86  to enter the next forward OD plenum  138 . The flow then re-branches to pass through the associated row of tubes to pass to the ID plenum  118 . The flow then passes forward through ports  122  to the ID plenum  116  and, again re-branches to pass radially outward through the associated row of tubes, repeating the process until it passes to the forward-most ID plenum  110  and exits the outlet  24 . This particular arrangement provides counterflow (more particularly multi-pass, cross-counter flow) heat exchange between the two flows  910  and  920 . Thus, the tube row that is upstreammost along the flowpath  900  is downstreammost along the flowpath  902  and so forth. 
     Each tube  46  ( FIG.  29   ) extends between an opening at a first end or rim  200 A and an opening at a second end or rim  200 B. Proximate each end, the tube has a respective straight section or portion  202 A,  202 B of which a portion is received in the associated manifold socket  58 . 
     The exemplary tube  46  has an arcuate center section  204  joining the sections  202 A,  202 B at respective bends  206 A,  206 B. Thus, in this example, there are three bends with the arcuate portion  204  forming an intermediate bend of opposite direction to the bends  206 A,  206 B. For each individual tube,  FIG.  29    shows an overall end-to-end length, spacing, or span S 1 , a span or spacing S B  between the bends  206 A,  206 B, and a radius of curvature Rc of the arcuate section  204 . The arc of the arcuate section  204  offsets the centerline of the tube at the center of the arc by a spacing So relative to the centerline at the ends  200 A,  200 B. Each tube has an inner or interior surface  220  ( FIG.  4   ) and an outer or exterior surface  222  and has a centerline shown as  520 . The arcuate section may represent a majority of the overall length along the tube centerline and a majority of the end-to-end direct length or span S 1  and a majority of the radial span S F  ( FIG.  3   ) between ID manifold OD surface and OD manifold ID surface. 
       FIG.  4    shows an on-center row-to-row axial spacing S L  and  FIG.  2    shows a circumferential on-center spacing within rows as S T .  FIG.  19    shows a tube outer diameter as D. With the radial fanning, S T  may increase from the ID manifold to the OD manifold. Relaxed So (e.g., all components isothermal at  21 C) may be greater than the row-to-row axial spacing S L , for example it may be 1.0 to 5.0 times the row-to-row axial spacing S L  (more narrowly 1.5 to 4.0 or 1.5 to 2.5). This provides an advantageous combination of mechanical stability while accommodating flexing. Too low an So may cause excessive compressive stresses. Too high an So may allow the first flow  910  to resonate the tubes and produce flutter. The flutter may produce hysteresis or, if out of phase, may cause tube collisions. 
     In an exemplary typical general use situation, the first fluid flow  910  is a relatively cool heat recipient flow and the second flow  912  is a relatively warm heat donor flow. In an initial transient startup situation, the relatively high thermal mass of the manifold structures (typically associated with greater material thickness) will typically mean that the second flow  912  heats the tubes (more particularly the exposed portions of the tubes) faster than it heats the manifolds. This will cause a differential thermal expansion of the tubes relative to the manifolds. Differential thermal expansion expands the tubes. The arcuate nature of the tubes allows the tubes to expand limiting stress. In the illustrated example, this will cause So to increase (whereas a straight tube might suffer very high compressive stresses or might buckle unpredictably). Aerodynamic stability considerations suggest that the tubes be oriented to bend downstream along the first flowpath  900 . 
     There may also be cool-down or other transient in the opposite direction. If the temperature of the second flow decreases (or the second flow is stopped), the exposed portions of the tubes may cool more rapidly than the manifolds and differentially thermally contract reducing So potentially even beyond its initial pre-use value. 
     Alternatively, the first flow  910  may be the heat donor flow and the second flow  912  be the heat recipient flow. Unless, however, the second flow is cooled below ambient, the initial transient may still be in the same direction discussed above if the first flow heats the exposed portions of the tubes faster than the manifolds heat. 
     An exemplary specific use situation is in a recuperator or waste heat recovery wherein the first flow  910  is of the recuperator working fluid (e.g., carbon dioxide). The heat exchanger  20  may be used as a heat absorption heat exchanger in the hot section of the engine (e.g., absorbing heat from combustion gases (as the second flow  912 ) in an exhaust duct downstream of the turbine). Alternatively, the heat exchanger may be used as a heat rejection heat exchanger (e.g., rejecting heat to air (as the second flow  912 ) in a fan duct or other bypass). 
       FIG.  30    schematically illustrates a gas turbine engine  800 , including the heat exchanger  20  in a waste heat recovery system (recuperator)  801 . The exemplary engine is an aircraft propulsion engine, namely a turbofan. The engine has a fan section  805 , one or more compressor sections  810 , a combustor section  820  and one or more turbine sections  830 , sequentially along a primary fluid flowpath (core flowpath). The fan also drives air along an outboard bypass flowpath. The exemplary engine is a two-spool engine with the low spool directly or indirectly (e.g., via reduction gearbox) driving the fan. Exemplary combustors are annular combustors and can-type combustor arrays. 
     A downstream section of the core flowpath provides the second flowpath  902 . Downstream of the turbine section  830  is an exhaust casing  840  which exhausts combustion gas (as the fluid flow  912 ) into an ambient atmosphere downstream of the turbine. 
     In order to recapture the waste heat from the combustion gas flow  912  and convert the waste heat to work, the heat exchanger  20  is positioned within the exhaust casing  840 . The first flowpath  900  is a leg of a supercritical CO 2  (sCO 2 ) bottoming Brayton cycle (referred to herein as the waste heat recovery system  801 ). The heat exchanger  20  is connected to transfer heat from the turbine exhaust to the waste heat recovery system  801 , and the waste heat recovery system  801  converts the heat into rotational work (which may be used for various purposes such as driving an electrical generator (not shown) to power aircraft systems). The waste heat recovery system  801  may additionally recuperate waste heat within the recovery system  801  and is referred to as a recuperating bottoming cycle. 
     The waste heat recovery system  801  has a turbine  870  with an inlet  872  connected to an output of the heat exchanger  20 . The turbine  870  expands the heated working fluid (CO 2  or other cryogenic fluid  910 ) and expels the heated working fluid through a turbine outlet  874 . The expelled working fluid is passed through a relatively hot passage of a recuperating heat exchanger  880 , and is passed to a relatively hot passage of a heat rejection heat exchanger  882 . The heat exchanger  882  may be positioned to reject thermal energy from the working fluid to ambient air (e.g., fan bypass air). After passing through the heat rejection heat exchanger  882 , the working fluid is passed to an inlet  892  of a compressor  890 . The compressor  890  (driven by the turbine  870  (e.g., co-spooled)) compresses the working fluid, and passes the compressed working fluid from a compressor outlet  894  to a cold passage of the recuperating heat exchanger  880 . 
     During operation of the waste heat recovery system  801 , the compressor  890  compresses the working fluid, and passes the compressed working fluid through the recuperating heat exchanger  880  and the heat exchanger  20 , causing the compressed working fluid to be heated in each of the heat exchangers  20 ,  880 . The heated working fluid is provided to the inlet  872  of the turbine  870  and expanded through the turbine  870 , driving the turbine  870  to rotate. The rotation of the turbine  870  drives rotation of the compressor  890  and of an output shaft  802 . The output shaft  802  may be mechanically connected to one, or more, additional turbine engine systems and provides work to those systems using any conventional means for transmitting rotational work. Additionally or alternatively, the rotational work can be converted into electricity and used to power one or more engine or aircraft systems using a conventional electrical generator system coupled to the output shaft. 
     Other variations on stacked manifolds are possible. Variation areas may have interdependencies. One area for variation is the manifold footprint. Several variations on manifold footprint are discussed above. For example, whereas the  FIG.  1    embodiment forms the heat exchanger  40  as an annular sector for ease of manufacture and/or installation/service, full annulus variations are possible. 
     A related area for variation is the relative positioning of tubes in a row. For example, in contrast to the radial tubes between concentric inner and outer manifolds, there may be parallel tubes extending between two facing/parallel manifolds (e.g., a left and right manifold, an upper and lower manifold, or other absolute orientation or orientation relative to the overall engine or aircraft). 
     Other variations involve tube shape (e.g., bending). A variation on the continuously arcing central portion is to have a straight central portion offset from the end portions by short legs. Respective bends/turns separate the ends of the short legs from the straight central portion and the associated end portion. Differential thermal expansion can thus mostly lengthwise extend or contract the central portion and flex the bends. This has an advantage of relatively low upstream or downstream movement and prevents tube collision when there is a small axial spacing S L  (particularly when the thermal expansion differs from row to row). 
     Other variations involve tube cross-section. The tube or a portion thereof along the external flowpath may be flattened (e.g., ovalized, ellipsoidalized or obroundized) such as by pressing or rolling to narrow transverse to the external flowpath and lengthen along the external flowpath. Such flattened tubes may provide aerodynamic stability and enhanced heat transfer. 
     Other variations involve differing internal flow arrangements. Whereas, in the first embodiment, each row of tubes represents a single fluidic stage, other configurations may differ. For example, alternating tubes in a given row may represent fluidically different stages along the internal flowpath. For example, the internal flowpath may enter a given tube in the row in one manifold, pass to an adjacent tube in that row in the other manifold, and then pass to a tube in the next row, and so forth. One advantage of such a system is that it facilitates use of completely identical intermediate plates rather than two alternating groups. Two groups of identical plates may be rotated by an increment about the axis relative to each other for annular plates or may be flipped front-to-back. In one group of examples, rather than full annular plenums being formed between the mating plates, one of the manifolds may have a circumferential array of two-port plenums. 
     Exemplary manufacture of manifold plates may be via casting and machining, via machining from billet or plate stock, via powder metallurgical consolidation, via additive manufacture, or the like. Exemplary tubes may be formed by extrusion or rolling followed by bending/cutting. Exemplary plates and tubes are metallic (e.g., nickel-based superalloy or stainless steel). In an exemplary manufacture technique, the plates are successively stacked starting with corresponding end plates of the two manifolds. In the stacking orientation, the pocket segments face upward and the tubes of the associated row/group are put in place. With the exemplary bent tubes, it is, thus, easiest to start with the downstreammost plates  62 ,  82  along the first flowpath  900  because gravity will keep the arcuate center portions of the tubes in their proper orientations. The next plate is then placed over the prior plate and the next row of tubes put in place until the stacks are complete. In some manufacture techniques, the pocket segments may be dimensioned to have a slight interference with the tubes to assure tight fit and sealing. In other manufacture techniques, the pocket segments may be dimensioned to have a slight clearance with the tubes for ease of assembly. 
     The stacking process may be used with any of a number of securing processes. One securing process (not shown) is to use through-bolts (e.g., nutted) extending through each stack and tightened down. In such a situation, there may be optional gaskets between plates in the stacks. 
     An alternative securing technique is brazing. In such a situation, braze foils or braze paste beads may be included in the stacks between the plates and at the tube-to plate junctions. After assembly, pressure may be applied across the stacks (e.g., via clamps) and the assembly heated (e.g., in a furnace) to braze the plates of each stack together. 
     An alternative securing process somewhat similar to the brazing is a welding process (sintering if the plates are formed powder metallurgically) in which pressure and electric current are applied across the stacks (optionally accompanied by additional heating beyond that provided by the pressure and current) to bond adjacent plates to each other and, optionally, to the tubes. A similar technology in sintering of powder-formed bodies is field assisted sintering technology (FAST), also known as spark plasma sintering. 
     The brazing and welding or sintering are particularly suited for the initial tube-to-plate (pocket) clearance fit mentioned above. When subject to the furnace heating, the low thermal mass of the tubes and conduction from the exposed portions of the tubes to the end portions differentially thermally expands the tubes from a clearance fit to an interference fit. Once interfering, the braze or welding/sintering process bonds and seals the tubes to the plates. 
     The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description. 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baseline configuration, details of such baseline may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.