Patent Publication Number: US-2020284519-A1

Title: Cyllindrical helical core geometry for heat exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 62/815,847 filed Mar. 8, 2019 for “CYLLINDRICAL HELICAL CORE GEOMETRY FOR HEAT EXCHANGER” by A. Becene, G. Ruiz, F. Feng, M. Maynard, M. Doe, M. Hu, and E. Joseph. 
    
    
     BACKGROUND 
     The present disclosure is related generally to heat exchangers and more particularly to heat exchanger core designs. 
     Heat exchangers can provide a compact, low-weight, and highly effective means of exchanging heat from a hot fluid to a cold fluid. Heat exchangers that operate at elevated temperatures, such as those used in modern aircraft engines, often have short service lifetimes due to thermal stresses, which can cause expansion and cracking of the fluid conduits. Thermal stresses can be caused by mismatched temperature distribution, component stiffness, geometry discontinuity, and material properties (e.g., thermal expansion coefficients and modulus), with regions of highest thermal stress generally located at the interface of the heat exchanger inlet/outlet and core. 
     A need exists for heat exchangers with increased heat transfer, reduced pressure loss and vibration excitation, and improved performance under thermal stresses. 
     SUMMARY 
     In one aspect, the present disclosure is directed toward a heat exchanger that includes a tubular inlet, a tubular outlet, and a core. The core fluidically connects the tubular inlet to the tubular outlet via a plurality of tubes. Each of the tubes has a helical shape and is circumferentially displaced from each of the others of the plurality of tubes. 
     In another aspect, the present disclosure is directed toward a heat exchanger that includes a first fluid manifold extending along a first fluid axis from a first fluid inlet to a first fluid outlet. The first fluid manifold includes first fluid inlet and outlet headers, and a helical core section. The inlet header is disposed to branch the first fluid inlet into a plurality of first fluid branches, and the outlet header is disposed to combine the plurality of first fluid branches into the first fluid outlet. The helical core section fluidly connects the inlet header to the outlet header via a plurality of helical tubes, such that each helical tube corresponds to one of the plurality of first fluid branches. 
     The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims, and accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematized side view of a heat exchanger with a helical core. 
         FIG. 2  is an exaggerated perspective side view of the heat exchanger of  FIG. 1 , in a compressed state. 
     
    
    
     While the above-identified figures set forth one or more embodiments of the present disclosure, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features and components not specifically shown in the drawings. 
     DETAILED DESCRIPTION 
     A heat exchanger with a rotationally symmetric helical core is presented herein. This helical core is made up of a plurality of structurally independent, circumferentially distributed and helical tubes. These tubes can be distributed in a cylindrically distributed spring arrangement. The helical geometry of the core increases heat exchanger functional length and surface area as a function of the total axial length of the core, and provides structural compliance that allows the core to serve as a spring to relieve thermal and other stresses from the heat exchanger and adjacent (connecting) flow elements. 
       FIG. 1  is a schematized side view of heat exchanger  10 , which includes first fluid manifold  12  and second fluid guide  14 . First fluid manifold  12  includes inlet header  16 , outlet header  18 , and core section  20 . Inlet header  16  forks from inlet passage  22  into a plurality of inlet header branches  24 , and outlet header  18  recombines outlet header branches  26  into outlet passage  28 . Core  20  is formed of a plurality of structurally independent helical tubes  30  that each extend from a separate inlet header branch  24  to a separate outlet header branch  26 . 
     During operation of heat exchanger  10 , hot fluid flow F 1  is provided to inlet header  16 , flows through core  20 , and exits through outlet header  18 . Thermal energy is transferred from hot fluid flow F 1  to cooling fluid flow F 2  as hot fluid flow F 1  passes through core  20 . It will be understood by one of ordinary skill in the art that the disclosed independent cold flow structure can be tailored for use with a wide variety of core geometries and is not limited to the embodiments shown. Furthermore, although the present disclosure refers to some flow as “cold” and other as “hot,” the present geometry can more generally be applied to any two fluid flows in a heat exchange relationship, e.g. wherein F 1  and F 2  are exchanged i.e. as cold and hot flows, respectively. 
     As illustrated in  FIG. 1 , heat exchanger  10  is oriented substantially symmetrically along a fluid axis A, which connects extends from inlet passage  22  to outlet passage  28 . In this embodiment, axis A is a straight line defining a primary flow direction of hot fluid flow F 1  through first fluid manifold  12 . In variations on the depicted embodiment, however, heat exchanger  10  can extend along a contoured (non-straight) axis, e.g. due to space constraints. 
     Headers  16 ,  18  distribute and receive fluid, respectively, substantially evenly across core  20 . Specifically, inlet header  16  splits into inlet header branches  24 , and outlet header  18  recombines from header branches  26 . In the illustrated embodiment, header  16  is a successively fractally branching manifold with multiple stages of branches, each narrowing in cross-sectional flow area with respect to the previous stage of less numerous branches, finally terminating in the full count of outlet header branches  22  as the narrowest and most axially distant from inlet passage  22 . More specifically, the present figures illustrate each stage of header  16  branching rotationally symmetrically about axis A into an even number of tubes evenly circumferentially distributed across a common plane transverse to axis A. More generally, however, header  16  can be of any shape capable of distributing fluid from a single source at inlet passage  22  across the multitude of separate helical tubes  30  of core  20 . The illustrated embodiment, however, advantageously reduces pressure drop and provides additional mechanical compliance along axis A, within header  16 . 
     As depicted in  FIG. 1 , header  18  substantially mirrors header  16 , across core  20 . In at least some embodiments, headers  16 ,  18  and core  20  are all formed monolithically. More generally, all components of heat exchanger first fluid manifold  12  can be formed partially or entirely by additive manufacturing. For metal components (e.g., Inconel, aluminum, titanium, etc.) exemplary additive manufacturing processes include but are not limited to powder bed fusion techniques such as direct metal laser sintering (DMLS), laser net shape manufacturing (LNSM), electron beam manufacturing (EBM). For polymer or plastic components, stereolithography (SLA) can be used. Additive manufacturing is particularly useful in obtaining unique geometries (e.g., varied core tube radii, arcuate core tubes, branched inlet and outlet headers) and for reducing the need for welds or other attachments (e.g., between headers  16 ,  18  and core  20 ). However, other suitable manufacturing process can be used. For example, header and core elements can in some embodiments be fabricated separately and joined via later manufacturing steps. 
     Second fluid guide  14  is illustrated schematically in  FIG. 1 . Second fluid guide  14  can be included in some embodiments to constrain cooling fluid flow F 2 . Second fluid guide  14  is illustrated as a baffle surrounding mechanically unconnected to first fluid manifold  12 . In other embodiments, second fluid guide  14  can have additional sub-layers or separations to further channel cooling fluid flow F 2  through and across first fluid manifold  12 . In still other embodiments, by contrast, second fluid guide  14  can be omitted altogether, and first fluid manifold  12  directly exposed to an unconstrained environment of cooling fluid flow F 2 . Second fluid guide  14  need not closely match the geometry of first fluid manifold  12 , but can in some embodiments parallel at least some aspects of the geometry of first fluid manifold, e.g. to more closely capture core section  20  as a whole. In the illustrated embodiment, second fluid guide  14  channels cooling fluid flow F 2  in a direction substantially antiparallel (i.e. parallel to but opposite) hot fluid flow F 1 . In other embodiments, second fluid guide  14  can instead direct cooling fluid flow F 2  in a direction transverse to F 1 , e.g. in a cross-flow direction. 
     The majority of heat transfer enabled by heat exchanger  10  is accomplished within core section  20 . Core section is formed by a plurality of separate, structurally independent helical tubes  30 . Each helical tube  30  has a helical or spring-like geometry, extending axially and turning in common about fluid axis A. Helical tubes  30  are distributed circumferentially about axis A, such that each helical tube  30  is substantially identical to all other tubes  30 , but shifted circumferentially relative to adjacent tubes. All tubes  30  are depicted as cross-sectionally distributed in a circular array across a plane orthogonal to fluid axis A. More generally, tubes  30  can be distributed in any array with rotational symmetry about fluid axis A, e.g. in elliptical or cloverleaf patterns. Circular symmetry in the distribution of tubes  30  permits all tubes  30  to have identical geometry, with correspondingly identical and therefore uniform fluid flow and heat transfer characteristics. Asymmetric arrangements of tubes  30 , however, may be advantageous in tight space constraints, or where cooling fluid flow F 2  is non-uniform. The spacing between adjacent helical tubes  30  is primarily circumferential, which provides a substantially uniform gap spacing between all adjacent tubes  30 , so as to promote even airflow F 2  therebetween. All helical tubes  30  can have a substantially identical and uniform inner diameter with a circular cross-section, resulting in equal cross-sectional areas. In the illustrated embodiment, the spacing between adjacent helical tubes  30  is greater than this inner diameter. In general, the rotational symmetry of helical tubes  30  within core  20  permits flow paths within headers  16 ,  18  to be substantially equal in length, for greater uniformity in the distribution of fluid flow F 1  across tubes  30  relative to geometries with no such symmetry, or other symmetry types. 
     The helical shape of tubes  30  of core  20  serves several functions. First, helical tubes  30  have no sharp corners or interfaces (e.g. with headers  16 ,  18 ), and consequently reduce unnecessary pressure losses. Second, helical tubes  30  are compliant along axis A, acting as a spring capable of deforming to accommodation expansion or axial translation of adjacent components. In particular, helical tubes  30  can be capable of compliantly deforming along axis A so as to accommodate thermal growth of headers  16 ,  18 , and/or translation of headers  16 ,  18  due to thermal growth of adjacent (upstream or downstream) components. This mechanical compliance provided by core  20  allows heat exchanger to better distribute and weather thermal and other mechanical stresses.  FIG. 2  is an exaggerated (not to scale) perspective side view of heat exchanger  10  illustrating a compressed state of first fluid manifold  12 .  FIG. 2  illustrates the performance of core section  20  under such compression. 
     In at least some embodiments, helical core  20  is significantly less compliant laterally, i.e. in dimensions transverse to fluid axis A. This increased lateral stiffness provides first fluid manifold  12  with resonant frequencies of oscillation transverse to the first fluid flow that are greater than the range of operating frequencies of a surrounding engine or other components for at least its three highest amplitude natural frequencies, for example, so as to avoid excitation within the expected environment of heat exchanger  10 . The generally circular cross-section of each tube  30  contributes to this increased lateral stiffness. The helical geometry of tubes  30  also provides greater fluid flow length within each tube  30 , and correspondingly greater surface area exposed to cooling fluid flow F 2 . The overall passage length of each tube  30  can, for example, be double the axial length of core  30 , or more. Helical tubes  30  can introduce additional turbulence to fluid flows F 1 , F 2 , for additional heat transfer. 
     In view of the above, in comparison to conventional straight-line connected channels, core  20  provides heat exchanger  10  with improved axial compliance to handle thermal stresses, increased lateral stiffness to avoid potentially harmful resonance conditions, and increased surface area exposed to cooling fluid flow F 2  for greater heat exchange, all with only modest pressure losses from inlet passage  22  to outlet passage  28 . Furthermore, the geometry of core  20  is symmetrical along two axes (axial and radial), and can consequently improve the uniformity of stress distribution across first fluid manifold  12 . 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A heat exchanger comprising: a tubular inlet; a tubular outlet; and a core fluidically connecting the tubular inlet to the tubular outlet via a plurality of tubes each having a helical shape and circumferentially displaced from each of the others of the plurality of tubes. 
     A heat exchanger comprising: a fluid manifold extending along a first fluid axis from a first fluid inlet to a first fluid outlet, the first fluid manifold comprising: an inlet header disposed to fork the first fluid inlet into a plurality of first fluid branches distributed circumferentially about the first fluid axis; an outlet header disposed to combine the plurality of first fluid branches into the first fluid outlet; and a helical core section fluidly connecting the inlet header to the outlet header via a plurality of cylindrically arranged helical tubes, each helical tube corresponding to one of the plurality of first fluid branches. 
     The heat exchanger of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing heat exchanger, wherein each of the plurality of helical tubes is structurally independent from all others of the plurality of helical tubes, such that the plurality of helical tubes are mechanically connected to each other only at the inlet header and the outlet header. 
     A further embodiment of the foregoing heat exchanger, wherein each of the plurality of helical tubes extends axially along and circumferentially about the first fluid axis. 
     A further embodiment of the foregoing heat exchanger, wherein the first fluid axis extends linearly from the first fluid inlet passage to the first fluid outlet passage, and wherein the first fluid inlet and the first fluid outlet are themselves oriented along the first fluid axis. 
     A further embodiment of the foregoing heat exchanger, wherein each of the plurality of helical tubes is mechanically separated from adjacent of the plurality of helical tubes by a circumferential and axial gap. 
     A further embodiment of the foregoing heat exchanger, wherein a structural rigidity of the first fluid manifold along the first fluid axis is less than along any radial dimension with respect to the first fluid axis. 
     A further embodiment of the foregoing heat exchanger, wherein the first fluid manifold is situated in an environment with a known range of operating frequencies, and wherein the first fluid manifold has at least a highest amplitude natural resonance frequency of oscillation transverse to the first fluid axis that is greater than the known range of operating frequencies. 
     A further embodiment of the foregoing heat exchanger, wherein each of plurality of helical tubes has a total passage length at least double its extent along the first fluid axis. 
     A further embodiment of the foregoing heat exchanger, wherein the helical core section forms a spring shape extending between the inlet header and the outlet header, wherein the spring shape is principally compliant along the first fluid axis. 
     A further embodiment of the foregoing heat exchanger, wherein the helical core section is capable of compliantly deforming to accommodate axial growth of the inlet header and outlet header. 
     A further embodiment of the foregoing heat exchanger, further comprising a second fluid flow structure disposed to direct a second fluid to impinge on the first fluid manifold, wherein the second fluid flow structure is configured to direct the second fluid generally along a direction from the first fluid outlet to the first fluid inlet. 
     A further embodiment of the foregoing heat exchanger, wherein the inlet header branches the first fluid inlet passage into a first number N of first fluid branches, and wherein the plurality of helical tubes comprises N helical tubes even distributed circumferentially to form a cylindrical arrangement with a circumferential angular separation of 360°/N. 
     A further embodiment of the foregoing heat exchanger, wherein the entirety of the first fluid manifold is formed monolithically as a single structure. 
     A further embodiment of the foregoing heat exchanger, wherein all of the plurality of helical tubes have identical flow area. 
     A further embodiment of the foregoing heat exchanger, wherein all of the plurality of helical tubes have a circular cross-section with a common diameter. 
     Summation 
     Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.