Patent Publication Number: US-2020300551-A1

Title: Heat exchanger temperature change rate control

Description:
BACKGROUND 
     Modern aircraft engines and associated systems operate at increasingly higher temperatures that place greater demands on several pneumatic components, including heat exchangers. Heat exchangers that operate at elevated temperatures often have short service lives and/or require increased maintenance as a result of high cyclic thermal stress. The stress is caused by multiple system and component factors including rapid flow and/or temperature transients, geometric discontinuities, stiffness discontinuities, mass discontinuities, and materials of construction. For example, inlet and exit manifolds are typically pressure vessels that are welded or bolted to a heat exchanger core or matrix. Pressure requirements dictate the thickness of these manifolds, sometimes resulting in a relatively thick header attached to a thinner core matrix. This mismatch in thickness and mass, while acceptable for pressure loads, conflicts with the goal of avoiding discontinuities to limit thermal stress. Because much of the fatigue damage occurs during start-up and shut-down transients, it would be beneficial to slow the magnitude of these thermal transients. 
     In particular, hot air entering a plate-fin heat exchanger core typically encounters closure bars of the cold circuit that it must flow around to enter the hot circuit fin passages. Because these cold closure bars are exposed to high velocity air on three sides, the cold closure bars heat up rapidly from an initial temperature, and accordingly, can tend to expand rapidly. The stiffer surrounding structure takes longer to heat up and opposes the thermal expansion of the cold closure bars, thereby creating high material stress. Although the combined structure of the heat exchanger core eventually reaches steady state temperatures, the latent damage that occurs during the initial few seconds of the heat-up transient is cumulative, and can limits the fatigue life of the heat exchanger core. Cracking of the cold closure bars and adjacent parting sheets can impact the service life of the heat exchanger core, and/or require more frequent inspection, testing, and/or repair during the service life. The cold closure bars that are near the hot circuit inlet are particularly vulnerable to these effects. 
     Methods of controlling the rate of flow introduction to the heat exchanger core during the start-up transient by using flow-modulating valves and associated control systems are known in the art. The additional components and control systems associated with those methods can be useful in some applications. However, it can be beneficial to have a means of controlling the rate of the temperature increase of the cold closure bars that is integral to those cold closure bars, thereby not requiring components and control systems that are external to the heat exchanger core. 
     SUMMARY 
     A closure bar adapted for use in a heat exchanger core includes a center void region configured to be partially filled with a phase-changing material and sealed, thereby containing the phase-changing material. 
     A method of producing a closure bar adapted for use in a heat exchanger core includes forming a closure bar that has a center void region, partially filling the center void region with a phase-changing material, and sealing the closure bar, thereby containing the phase-changing material within the center void region. 
     A method of operating a heat exchanger core to reduce a rate of change of temperature in at least one portion thereof, where the heat exchanger core includes a closure bar having a center void region partially filled with a phase-changing material and sealed, the phase-changing material has at least one phase-changing point that is between an initial temperature and a hot fluid operating temperature, and the closure bar is disposed in a region of the heat exchanger core that is configured to receive a hot fluid having a hot fluid operating temperature. The method includes initiating flow of the hot fluid to the heat exchanger core, slowing a rate of temperature increase by absorbing latent heat as the phase-changing material changes phase in a forward direction, ceasing the flow of the hot fluid to the heat exchanger core, slowing a rate of temperature decrease by liberating a latent heat as the phase-changing material changes phase in a reverse direction. The phase-changing material has at least one phase-changing point, melting or boiling, that is between an initial temperature and the hot fluid operating temperature. The phase-changing material is configured to change phase in the forward direction as the flow of hot fluid over the closure bar begins, thereby slowing a rate of temperature increase by absorbing a latent heat as the phase-changing material changes phase in the forward direction, and change phase in the reverse direction as the flow of hot fluid over the closure bar ceases, thereby slowing a rate of temperature decrease by liberating the latent heat as the phase-changing material changes phase in the reverse direction. The forward-reverse directions are boiling-condensing or melting-solidifying. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a plate-fin heat exchanger core with hollow closure bars. 
         FIG. 2A  is an end view of the plate-fin heat exchanger core of  FIG. 1  showing the hot fins. 
         FIG. 2B  is a side view of the plate-fin heat exchanger core of  FIG. 1  showing the cold fins. 
         FIG. 3A  is a perspective view of a temperature rate-control closure bar shown in  FIG. 2B . 
         FIG. 3B  is a cross-sectional side view of the temperature rate-control closure bar shown in  FIG. 3A . 
         FIG. 3C  is a cross-sectional end view of the temperature rate-control closure bar shown in  FIG. 3A . 
         FIG. 4A  is a graph of temperature versus time for a closure bar of the prior art. 
         FIG. 4B  is a graph of temperature versus time for the temperature rate-control closure bar shown in  FIGS. 3A-3C . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a plate-fin heat exchanger core with rate-control closure bars. Shown in  FIG. 1  are heat exchanger core  10 , bottom end sheet  12 , hot closure bars  14 , hot fins  16 , parting sheets  18 , cold inlet closure bars  20 , cold outlet closure bars  22 , cold fins  24 , and top end sheet  26 . Heat exchanger core  10 , together with inlet and outlet manifolds (not shown) on each of the hot flow and cold flow circuits, can function as a plate-fin heat exchanger for providing a compact, low-weight, and highly-effective means of exchanging heat from a hot fluid to a cold fluid. Because heat exchanger core  10  transfers heat from one fluid to another while maintaining a fluid separation between the two, heat will generally flow from the hot fluid to the cold fluid across the various components in heat exchanger core  10 , described hereafter. Therefore, as used in this disclosure, “hot” will be used to describe the first fluid circuit and “cold” will be used to describe the second fluid circuit. The terms “hot” and “cold” are relative one to the other. As used in different embodiments, heat exchanger core  10  can encounter temperatures ranging from near absolute zero (for example, in cryogenic distillation) to 1,300 deg. F (704 deg. C) or more (for example, in gas turbine engine systems and related components). Moreover, “hot” and “cold” are used in this disclosure as descriptive terms to refer to the various components that are associated with the respective first and second fluid circuits in the heat exchanger core, without implying that particular temperatures or a temperature relationship exists for those components during the manufacturing process of heat exchanger core  10 . 
     Alternating hot and cold layers are sandwiched between bottom end sheet  12  and top end sheet  26 . Hot fins  16  channel hot flow, with boundaries defined by hot closure bars  14  on either side of each hot layer, and parting sheets  18  on the top and bottom of each layer (with the exception of the bottom layer which is bounded on the bottom by bottom end sheet  12 , and the top layer which is bounded on the top by top end sheet  26 ). Similarly, cold fins  24  channel cold flow, with boundaries defined by cold inlet closure bars  20  and cold outlet closure bars  22  on either side of each cold layer, and parting sheets  18  on the top and bottom of each layer. It is to be appreciated that cold inlet closure bars  20  are so-named because they are in the vicinity of the hot flow inlet to heat exchanger core  10 . Similarly, cold outlet closure bars  22  are so-named because they are in the vicinity of the hot flow outlet from heat exchanger core  10 . 
     In the illustrated embodiment, hot fins  16  and cold fins  24  are corrugated. In other embodiments, hot fins  16  and/or cold fins  24  can have any configuration, with non-limiting examples being rectangular, triangular, perforated, serrated, ruffled, and herringbone. In the illustrated exemplary embodiment, five hot layers and four cold layers are used. In other embodiments, there can be practically any number of hot layers and cold layers, and the number of hot layers can be different from the number of cold layers. For example, in a particular embodiment, there can be more than 100 layers (i.e., hot layers and cold layers). In referring to heat exchanger core  10  shown in  FIG. 1 , height will refer to a dimension in the vertical direction as shown in  FIG. 1 , and width will refer to a dimension in an orthogonal direction along an appropriate axis that is defined by either hot flow or cold flow. In referring to the width of a particular hot or cold layer, reference can also be made to length, for example, when describing the length of the associated closure bars. Of course, after assembly into a heat exchanger (not shown), heat exchanger core  10  can have any physical orientation without regard to the labels of height, width, and/or length as are used herein. 
     As noted earlier, heat exchanger core  10  can operate at elevated temperatures such as those in modern aircraft engines, where a typical application can be to provide cooling of super-heated gas. When heat exchanger core  10  is not being used to exchange heat (i.e., the associated heat exchanger is idle), heat exchanger core  10  components are at an initial temperature (T Init ) which can often be much cooler than the operating temperature. The initial temperature (T Init ) can also be referred to as a local ambient temperature, or as an idle temperature that is representative of the temperature of heat exchanger core  10  when not in operation. Accordingly, the initial temperature (T Init ) can vary depending on the local environmental conditions. During a system start-up, when heat exchanger core  10  (and accordingly, the associated heat exchanger) is put into operation, cold flow is initiated through the cold layers and hot flow is initiated through the hot layers. Accordingly, cold inlet closure bars  20  can be subjected to a rapid heat-up as the hot fluid having hot fluid temperature (T H ) flows over (i.e., flows past, flows around) cold inlet closure bars  20  into hot fins  16 . The advantage of the present disclosure can be described by contrasting the start-up transient of heat exchanger core  10  to that of a plate-fin heat exchanger of the prior art (not shown), in which a hot flow is directed at the cold inlet closure bars, thereby quickly raising their temperature from ambient temperature to a steady-state operating temperature. This heat-up transient can result in transient stress-loading in heat exchanger core  10  particularly in and near cold inlet closure bars  20 , which can affect the service life and/or the maintenance requirements. As will be described in regard to the figures that follow, heat exchanger core  10  of the present disclosure provides temperature increase rate control during the start-up transient which can lower the cyclic stress loading on heat exchanger core  10 , thereby reducing maintenance requirements and/or extending the service life. 
       FIG. 2A  is an end view of the plate-fin heat exchanger core of  FIG. 1  in which hot fins  16  are visible.  FIG. 2B  is a side view of the plate-fin heat exchanger core of  FIG. 1  in which cold fins  24  are visible. Shown in  FIGS. 2A-2B  are bottom end sheet  12 , hot closure bars  14 , hot fins  16 , parting sheets  18 , cold inlet closure bars  20 , cold outlet closure bars  22 , cold fins  24 , and top end sheet  26 , having descriptions substantially similar to those provided above in regard to  FIG. 1 . Also shown in  FIG. 1  are rate-control cores  30 , shown in phantom, which are located within cold inlet closure bars  20 . As described above in regard to  FIG. 1 , cold inlet closure bars  20  (i.e., near the hot fluid inlet) are subject to an extreme temperature transient within heat exchanger core  10 . Therefore, rate-control cores  30  are used to mitigate the temperature transient that cold inlet closure bars  20  experience, prolonging the heat-up transient and thereby reducing the cyclic stress loading. As used in this disclosure, rate-control core  30  can also be referred to as a temperature rate-control core. 
     In the illustrated embodiment, rate-control cores  30  are located within cold inlet closure bars  20 , which are exposed to the hot fluid temperature (T H ) during the heat-up transient. In some embodiments, some or all cold outlet closure bars  22  can also include a rate-control core  30 . In these and/or other embodiments, some (i.e., at least one) cold inlet closure bars  20  can include rate-control cores  30 . Cold inlet closure bars  20 , which include rate-control cores  30 , can also be referred to as temperature rate-control closure bars, or simply, as rate-control closure bars. Accordingly, as used in the present disclosure, cold inlet closure bars  20  and temperature rate-control closure bars  20  can be used interchangeably. In the illustrated embodiment, heat exchanger core  10  and its components (including temperature rate-control closure bars  20 ) can be assembled and metallurgically joined using one of several exemplary processes including brazing and welding (e.g., electron beam welding). In some embodiments, heat exchanger core  10  and its components can be manufactured by additive manufacturing, hybrid additive subtractive manufacturing, subtractive manufacturing, and/or casting, for example. Embodiments of features described herein can leverage any additive or partial-additive manufacturing method is within the scope of the present disclosure. 
       FIG. 3A  is a perspective view of temperature rate-control closure bar  20  shown in  FIG. 2B .  FIG. 3B  is a cross-sectional side view of temperature rate-control closure bar  20 .  FIG. 3C  is a cross-sectional end view of temperature rate-control closure bar  20 . Shown in  FIGS. 3A-3C  are temperature rate-control closure bar  20 , rate-control core  30 , upper/lower wall  32 , sidewall  34 , end plug  36 , weld  38 , chamfer  40 , phase-changing material  50 , and void space  54 . Also labeled in  FIGS. 3B-3C  are bar length L, bar height H, bar width W, core diameter D, sidewall thickness A, and upper/lower wall thickness B. In the illustrated embodiment, temperature rate-control closure bar  20  is first fabricated having void space  54  on the interior, and then later filled or partially-filled with phase-changing material  50  by affixing end plug  36  at each end. Temperature rate-control closure bar  20  is made of metal or a metal alloy, with non-limiting examples of metallic materials including nickel, aluminum, titanium, copper, iron, cobalt, and/or all alloys that include these various metals. In an exemplary embodiment, temperature rate-control closure bar  20  can be a corrosion-resistant steel (i.e., CRES, stainless steel). Temperature rate-control closure bar  20  can be made by extrusion. In some embodiments, temperature rate-control closure bar  20  can be formed by additive manufacturing, hybrid additive subtractive manufacturing, subtractive manufacturing, casting, and/or forging, for example. In other embodiments that utilize additive manufacturing processes, temperature rate-control closure bar  20  can be made from any of the previously listed metals and/or their alloys. In some of these other embodiments, various alloys of INCONEL™ can be used, with Inconel 625 and Inconel 718 being two exemplary alloy formulations. In other embodiments, HAYNES™ 282 can be used. In the illustrated embodiment, chamfer  40  is located on each of the outward-facing corners (i.e., the corners that are incident to the hot flow entering heat exchanger core  10 ). Chamfers  40  can help reduce stress in temperature rate-control closure bar  20 . In some embodiments, chamfers  40  can also assist in the aerodynamic/hydrodynamic properties of heat exchanger core  10  (e.g., flow channeling, flow smoothing). In other embodiments, one or both chamfers  40  can be omitted from temperature rate-control closure bars  20 . 
     Referring to  FIGS. 3B-3C , temperature rate-control closure bar  20  has bar length L, bar height H, and bar width W. In the illustrated embodiment, bar length L is about 12 cm (5 inches), bar height H is about 4 mm (0.16 in), bar width W is about 4.5 mm (0.18 inch), core diameter D is about 2 mm (0.08 in), sidewall thickness A is about 1.25 mm (0.05 inch), and upper/lower wall thickness B is about 1 mm (0.04 inch). In some embodiments, bar length L can range from about 5 cm (2 inches) to about 1.8 m (6 feet). In other embodiments, bar height H can range from about 0.64 mm (0.025 inch) to about 25 mm (1 inch). It is to be appreciated that in an exemplary embodiment, the dimensions of bar height H, bar width W, core diameter D, sidewall thickness A, and upper/lower wall thickness B can roughly scale with each other, thereby maintaining a cross-sectional aspect that is roughly similar to that depicted in  FIG. 3C , however these dimensions can vary widely while keeping within the scope of the present embodiment. In the illustrated embodiment, the cross-sectional shape of rate-control core  30  is circular (i.e., having core diameter D). In some embodiments, the cross-sectional shape of rate-control core  30  can be elliptical, oval, oblong, or polygonal. As used in the present disclosure, rate-control core  30  refers to the hollow internal region of temperature rate-control closure bar  20  and any included fill material (i.e., phase-changing material  50  in solid, liquid, and/or gaseous phase, and any remaining void space). The hollow internal region can also be referred to as a center void region. 
     Referring again to  FIGS. 3B-3C , temperature rate-control closure bar  20  contains phase-changing material  50  which fills a portion of rate-control core  30 . In the illustrated embodiment, phase-changing material  50  is a liquid at ambient temperature and void space  54  occupies the remainder of the volume of rate-control core  30 . Void space  54  can also be referred to as a residual void volume. In the illustrated embodiment, void space  54  is filled with air. In some embodiments, void space  54  can be filled with an inert gas, with helium, nitrogen, and argon being non-limiting examples. In other embodiments, void space  54  can be evacuated, thereby containing a vacuum. In some of these other embodiments, a partial vacuum (i.e., rarefied air and/or gas) can exist. End plug  36  hermetically seals each end of temperature rate-control closure bar  20 , thereby containing phase-changing material  50 . End plug  36  can have a thickness (not labeled) that is on the order of magnitude of sidewall thickness A. In the illustrated embodiment, welds  38  metallurgically join end plugs  36  to temperature rate-control closure bar  20 . In some embodiments, the metallurgically joining can be by brazing. In other embodiments, end plugs  36  can be joined to temperature rate-control closure bar  20  by interference fit or by the use of a threaded fastener (e.g., cap screw). In the illustrated embodiment, wall coating  48  coats the interior of temperature rate-control closure bar  20  (i.e., the interior surfaces of upper/lower walls  32 , sidewalls  34 , and end plugs  36 ). Wall coating  48  is a material that reduces or prevents the reaction of phase-changing material  50  with the material of temperature rate-control closure bars  20 . In an exemplary embodiment, wall coating  48  is high-temperature paint. In other embodiments, wall coating can be a polymer coating, ceramic coating, or a plating. In some of these other embodiments, wall coating can be metallic plating. An exemplary metallic plating is electroless nickel. In some embodiments, wall coating  48  can be omitted from some or all surfaces of rate-control core  30  (i.e., interior surfaces of temperature rate-control closure bar  20 ). 
     In the illustrated embodiment, phase-changing material  50  occupies approximately 30% of the volume of rate-control core  30  at ambient temperature (T Amb ). This can also be referred to as a fill volume ratio. In some embodiments, phase-changing material  50  can occupy between 25-35% of the volume of rate-control core  30  at ambient temperature (T Amb ). In other embodiments, phase-changing material  50  can occupy between 20-95% of the volume of rate-control core  30  at ambient temperature (T Amb ). It is to be appreciated that as phase-changing material  50  boils, void space  54  begins to fill with a vapor of phase-changing material  50 . As used in this disclosure, ambient temperature (T Amb ) is taken to be 20 deg. C (68 deg. F), unless otherwise specified. 
       FIG. 4A  is a graph of temperature versus time for a closure bar of the prior art, obtained from mathematical modeling. Shown in  FIG. 4A  are hot fluid temperature plot  60 , hot fluid heat-up region  62 , cold closure bar bulk temperature plot  64 , cold closure bar heat-up region  66 , and cold closure bar steady state point  68 . The units of temperature are degrees Celsius (C) with initial temperature (T Init ) indicated at the horizontal axis, however specific temperature values are not necessary to the description. The units of time are seconds (s) with zero beginning at the vertical axis, however specific time values are not necessary to the description. As described above in regard to  FIG. 1 , initial temperature (T Init ) is the temperature of heat exchanger core  10  in an idle condition, which can be influenced by the local environmental condition. In the illustrated embodiment, initial temperature (T Init ) is about 50 deg. F (10 deg. C), which can be exemplary of the local environmental condition for an aircraft that is idle on the ground. At time zero (i.e., t=0), hot fluid is directed at the heat exchanger core of the prior art and the hot fluid temperature begins to rise during hot fluid heat-up region  62 . Eventually, hot fluid temperature achieves the steady-state value of hot fluid temperature (T H ). As the hot fluid flows past the cold inlet closure bars of the prior art, cold closure bar bulk temperature begins to rise in response to the incoming hot fluid flow, during cold closure bar heat-up region  66 . Eventually, cold closure bar bulk temperature achieves cold closure bar bulk steady state temperature (T SS ) at cold closure bar steady state point  68  corresponding to the time to steady state (t 1 ). Accordingly, the stress loading on the cold inlet closure bars is a result of the temperature excursion from initial temperature (T Init ) to cold closure bar bulk steady state temperature (T SS ) over time to steady state (t 1 ). It is to be appreciated that during steady state, cold closure bar bulk steady state temperature (T SS ) is less than hot fluid temperature (T H ) as a result of the heat flux across the cold inlet closure bar. 
       FIG. 4B  is a graph of temperature versus time for temperature rate-control closure bar  20 , obtained from mathematical modeling. The advantages of temperature rate-control closure bar  20  of the present disclosure are described in contrast to  FIG. 4A  showing the closure bar of the prior art, while using a common time scale. Shown in  FIG. 4B  are hot fluid temperature plot  70 , hot fluid heat-up region  72 , cold closure bar bulk temperature plot  74 , cold closure bar first heat-up region  76 , phase change begin point  78 , phase change region  80 , phase change end point  82 , cold closure bar second heat-up region  84 , and cold closure bar steady state point  86 . As noted above in regard to  FIG. 4A , the units of temperature are degrees Celsius (C), with initial temperature (T Init ) indicated at the horizontal axis, however specific temperature values are not necessary to the description. Similarly, the units of time are seconds (s), with zero beginning at the vertical axis, however specific time values are not necessary to the description. As described above in regard to  FIG. 1 , initial temperature (T Init ) is the temperature of heat exchanger core  10  in an idle condition. In the illustrated embodiment, initial temperature (T Init ) is about 50 deg. F (10 deg. C). Accordingly, At time zero (i.e., t=0), hot fluid is directed at heat exchanger core  10  and the entering hot fluid temperature begins to rise during hot fluid heat-up region  72 . Eventually, hot fluid temperature achieves the steady-state value of hot fluid temperature (T H ). As the hot fluid flows past cold inlet closure bars  20 , cold closure bar bulk temperature begins to rise in response to the incoming hot fluid flow, during cold closure bar first heat-up region  76 . Eventually, phase-changing material  50  begins to boil at phase change begin point  78 . Phase-changing material  50  absorbs heat energy through the latent heat of vaporization (LHV), while maintaining essentially a steady temperature during phase change region  80 . Eventually, phase-changing material  50  has been converted into vapor (i.e., all available LHV has been absorbed) at phase change end point  82 , and cold closure bar bulk temperature begins to rise again. Eventually, cold closure bar bulk temperature achieves cold closure bar bulk steady state temperature (T SS ) at cold closure bar steady state point  86 , corresponding to phase-changing time to steady state (t 2 ). As can be seen from  FIGS. 4A-4B , the phase-changing time to steady state (t 2 ) is greater than the time to steady state (t 1 ) for a heat exchanger core of the prior art, thereby prolonging the heat-up of cold inlet closure bars  20 . Accordingly, the stress loading on temperature rate-control closure bars  20  is less than that of the prior art because the temperature excursion from initial temperature (T Init ) to cold closure bar bulk steady state temperature (T SS ) occurs over a much longer phase-changing time to steady state (t 2 ). Stated alternatively, the aggregate heat-up rate (i.e., the change in temperature divided by the change in time) is lower. It is to be appreciated that during steady state, the cold closure bar bulk steady state temperature (T SS ) is less than the hot fluid temperature (T H ) as a result of the heat flux across temperature rate-control closure bars  20 . 
     In the illustrated embodiment, phase-changing time to steady state (t 2 ) is about 2.5 times the value of time to steady state (t 1 ) in the prior art. The ratio of t 2  to t 1  can be referred to as the heat-up prolongation factor. In some embodiments, the heat-up prolongation factor can range from about 1.5-10. In other embodiments, the heat-up prolongation factor can be greater than 10. It is to be appreciated that several factors can affect the heat-up prolongation factor in a particular embodiment, with non-limiting examples including the hot fluid temperature (T H ), the physical size of temperature rate-control closure bars  20 , the volume of rate control core  30 , the specific heat capacity and thermal conductivity of temperature rate-control closure bars  20 , the material used for phase-changing material  50 , and the fill volume percentage. 
     As described above in regard to  FIG. 4B , phase-changing material  50  is selected to have a boiling point that occurs during the temperature transient when hot flow is initiated into heat exchanger core  10  (i.e., during the heat-up phase). Accordingly, when phase-changing material  50  reaches the boiling point, its temperature will remain relatively constant during boiling, as heat is absorbed by the latent heat of vaporization (LHV). In the illustrated embodiment, phase-changing material  50  is water. In some embodiments, phase-changing material  50  can be any compound that has a boiling point that occurs during the temperature heat-up transient of heat exchanger core  10 , with non-limiting examples including acetone, methanol, and titanium tetrachloride. 
     In some embodiments, phase-changing material  50  can be a solid at ambient temperature, with the phase-change temperature representing a melting point. Accordingly, phase-changing material  50  is selected to have a melting temperature that occurs during the temperature transient when hot flow is initiated into heat exchanger core  10  (i.e., during the heat-up phase). In these embodiments, phase-changing material  50  absorbs heat energy through the latent heat of fusion (LHF), while maintaining essentially a steady temperature during phase change region  80 , with non-limiting examples including includes sodium, potassium, cesium, lithium, and their salts. In these embodiments, phase-changing material  50  can be inserted within temperature rate-control closure bars  20  during the manufacturing process in any form including a solid rod, pellets, and/or a powder. Moreover, in these embodiments, the fill volume ratio can be any value up to 100%, because it can be unnecessary to accommodate a vapor phase of phase-changing material  50 . In an exemplary embodiment, the fill volume ratio can be about 90%, thereby allowing a small void space  54  to accommodate the thermal expansion of phase-changing material  50  within temperature rate-control closure bar  20 . In other embodiments, phase-changing material  50  can undergo both melting and boiling during the heat-up of temperature rate-control closure bars  20 , thereby resulting in two regions of steady temperature (i.e., two temperature plateaus) in cold closure bar bulk temperature plot  74 . 
     It is to be appreciated that for most materials, phase-changing is a reversible process in which the heat energy (i.e., LHV) that is absorbed during vaporization (i.e., changing phase from liquid to gaseous) is later liberated during condensation (i.e., changing phase from gaseous to liquid). Similarly, the heat energy (i.e., LHF) that is absorbed during melting (i.e., changing phase from solid to liquid) is later liberated during freezing (i.e., changing phase from liquid to solid). Accordingly, the present disclosure also provides temperature decrease rate control by lengthening the period of time it takes for temperature rate-control closure bars  20  to cool down from an operating temperature to a final temperature. Phase-changing material  50  can therefore said to have a forward and a reverse phase-changing direction. In a particular embodiment, the forward-reverse phase-changing direction pair can be boiling-condensing. In another particular embodiment, the forward-reverse phase-changing direction pair can be melting-solidifying. In some embodiments, phase-changing material  50  can have two phase-changing temperatures (i.e., melting and boiling) that occur between the initial temperature (T Init ) and the hot fluid temperature (T H ). 
     Moreover, it is to be appreciated that initial temperature (T Init ) can vary widely under various embodiments. In an exemplary embodiment where heat exchanger core  10  is on an aircraft that is parked on the ground, initial temperature (T Init ) can range from about −70 deg F to 140 deg. F (−57 deg C. to 60 deg C). In another embodiment, for example, where heat exchanger core  10  is exposed to cold circuit fan air prior to initiating hot air flow by the opening of a shut-off valve, initial temperature (T Init ) can be about 300 deg. F (149 deg. C). In yet other embodiments, initial temperature (T Init ) can be an elevated temperature (e.g., an operating temperature), whereby temperature rate-control closure bars  20  provide cool-down rate control. Accordingly, initial temperature (T Init ) ranging from near absolute zero (for example, in cryogenic distillation) to 1,300 deg. F (704 deg. C) or more are within the scope of the present disclosure. Therefore, the selection of phase-changing material  50  can be influenced by the expected range of temperatures that can be encountered in any particular embodiment. 
     In the embodiment shown in  FIGS. 2A-2B and 3A-3C , phase-changing material  50  was used in cold inlet closure bar  20  of heat exchanger core  10 , depicted as being a plate-fin heat exchanger core. All bars having rate-control core  30  that includes phase-changing material  50  for use in a heat exchanger core are within the scope of the present disclosure, whereby their use can include slowing the rate of temperature increase by absorbing latent heat as the phase-changing material changes phase in the forward direction (e.g., melting or boiling), and/or slowing the rate of temperature decrease by liberating the latent heat as the phase-changing material changes phase in the reverse direction (e.g., solidifying or condensing). 
     Discussion of Possible Embodiments 
     A closure bar adapted for use in a heat exchanger core, the closure bar comprising a center void region configured to be partially filled with a phase-changing material and sealed, thereby containing the phase-changing material. 
     The closure bar 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 closure bar, wherein: the center void region defines a center void region volume; the center void region is filled with the phase-changing material; and the phase-changing material fills between 20-95% of the center void region volume at an ambient temperature, thereby defining a residual void volume. 
     A further embodiment of the foregoing closure bar, wherein the residual void volume contains a vacuum. 
     A further embodiment of the foregoing closure bar, wherein the residual void volume is filled with a gas that includes argon, helium, nitrogen, air, or mixtures thereof. 
     A further embodiment of the foregoing closure bar, wherein: the closure bar is configured to be subjected to a flow of a hot fluid having a hot fluid operating temperature; the phase-changing material has a boiling point that is between an initial temperature and the hot fluid operating temperature; and the phase-changing material is configured to: boil as the flow of hot fluid over the closure bar begins, thereby slowing a rate of temperature increase by absorbing a latent heat of vaporization as the phase-changing material boils; and condense as the flow of hot fluid over the closure bar ceases, thereby slowing a rate of temperature decrease by liberating the latent heat of vaporization as the phase-changing material condenses. 
     A further embodiment of the foregoing closure bar, wherein: the closure bar is configured to be subjected to a flow of a hot fluid having a hot fluid operating temperature; the phase-changing material has a melting point that is between an initial temperature and the hot fluid operating temperature; and the phase-changing material is configured to: melt as the flow of hot fluid over the closure bar begins, thereby slowing a rate of temperature increase by absorbing a latent heat of fusion as the phase-changing material melts; and solidify as the flow of hot fluid over the closure bar ceases, thereby slowing a rate of temperature decrease by liberating the latent heat of fusion as the phase-changing material solidifies. 
     A further embodiment of the foregoing closure bar, wherein the phase-changing material is selected from the group consisting of: sodium, potassium, cesium, lithium, and salts thereof. 
     A further embodiment of the foregoing closure bar, wherein: the phase-changing material additionally has a boiling point that is between the melting point and the hot fluid operating temperature; and the phase-changing material is configured to: boil as the flow of hot fluid over the closure bar continues, thereby further slowing the rate of temperature increase by absorbing a latent heat of vaporization as the phase-changing material boils; and condense as the flow of hot fluid over the closure bar ceases, thereby further slowing the rate of temperature decrease by liberating the latent heat of vaporization as the phase-changing material condenses. 
     A further embodiment of the foregoing closure bar, wherein the phase-changing material is selected from the group consisting of water, acetone, methanol, titanium tetrachloride, and mixtures thereof. 
     A further embodiment of the foregoing closure bar, further comprising one or more materials selected from the group consisting of nickel, aluminum, titanium, copper, iron, cobalt, and alloys thereof. 
     A further embodiment of the foregoing closure bar, wherein each center void region is configured to be sealed by a method selected from the group consisting of: brazing, welding, sealing with an interference-fitted plug, and sealing with a threaded fitting. 
     A further embodiment of the foregoing closure bar, wherein: the center void region defines an interior surface; and the interior surface is coated with a material that is configured to prevent the phase-changing material from reacting with the first cold closure bars. 
     A further embodiment of the foregoing closure bar, further comprising a heat exchanger core comprising: a bottom end sheet; a plurality of alternately stacked individual hot and cold layers, the cold layers defining a hot layer inlet region and a hot layer outlet region; and a top end sheet; wherein: each individual hot layer includes: a hot fin element forming a plurality of parallel open-ended hot channels adapted to pass a fluid therethrough; a parting sheet separating each individual hot layer from the adjacent individual cold layer; and two hot closure bars positioned on opposite sides of the fin element, parallel to the open-ended hot channels and extending the length of the open-ended hot channels; each individual cold layer includes: a cold fin element forming a plurality of parallel open-ended cold channels adapted to pass a fluid therethrough; a parting sheet separating each individual cold layer from the adjacent individual hot layer; a first cold closure bar, positioned on a first side of the cold fin element proximate the hot layer inlet region, parallel to the open-ended cold channels and extending the length of the open-ended cold channels; and a second cold closure bar, positioned on a second side of the cold fin element proximate to the hot layer outlet region and opposite the first cold closure bar, parallel to the open-ended cold channels and extending the length of the open-ended cold channels; wherein the second cold closure bar is the foregoing closure bar. 
     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 individual hot layer comprises two hot closure bars, each defining a hot closure height between 0.64-25 mm (0.025-1 inch); and each individual cold layer comprises two cold closure bars, each defining a cold closure height between 0.64-25 mm (0.025-1 inch). 
     A further embodiment of the foregoing heat exchanger, wherein the heat exchanger core is manufactured by one or more processes selected from the group consisting of: additive manufacturing, hybrid additive manufacturing, subtractive manufacturing, and hybrid additive subtractive manufacturing. 
     A method of producing a closure bar adapted for use in a heat exchanger core, the method comprising: forming a closure bar, the closure bar defining a center void region; partially filling the center void region with a phase-changing material; and sealing the closure bar, thereby containing the phase-changing material within the center void region. 
     The method 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 method, wherein: the center void region is configured to be sealed by a method selected from the group consisting of: brazing, welding, sealing with an interference-fitted plug, and sealing with a threaded fitting; and the closure bar comprises one or more materials selected from the group consisting of nickel, aluminum, titanium, copper, iron, cobalt, and alloys thereof. 
     A further embodiment of the foregoing method, wherein: the closure bar is configured to be subjected to a flow of a hot fluid having a hot fluid operating temperature; the phase-changing material has at least one phase-changing point that is between an initial temperature and the hot fluid operating temperature; wherein the at least one phase-changing point is selected from the group consisting of: melting and boiling; the phase-changing material is configured to: change phase in a forward direction as the flow of hot fluid over the closure bar begins, thereby slowing a rate of temperature increase by absorbing a latent heat as the phase-changing material changes phase in the forward direction; and change phase in a reverse direction as the flow of hot fluid over the closure bar ceases, thereby slowing a rate of temperature decrease by liberating the latent heat as the phase-changing material changes phase in the reverse direction; wherein the forward-reverse directions are selected from the group consisting of: boiling-condensing and melting-solidifying. 
     A method of operating a heat exchanger core to reduce a rate of change of temperature in at least one portion thereof, wherein the heat exchanger core includes a closure bar having a center void region partially filled with a phase-changing material and sealed, the phase-changing material has at least one phase-changing point that is between an initial temperature and a hot fluid operating temperature, and the closure bar is disposed in a region of the heat exchanger core that is configured to receive a hot fluid having a hot fluid operating temperature, the method comprising: initiating a flow of the hot fluid to the heat exchanger core; slowing a rate of temperature increase by absorbing a latent heat as the phase-changing material changes phase in a forward direction; ceasing the flow of the hot fluid to the heat exchanger core; and slowing a rate of temperature decrease by liberating a latent heat as the phase-changing material changes phase in a reverse direction; wherein: the phase-changing material has at least one phase-changing point that is between an initial temperature and the hot fluid operating temperature; wherein the at least one phase-changing point is selected from the group consisting of: melting and boiling; the phase-changing material is configured to: change phase in the forward direction as the flow of hot fluid over the closure bar begins, thereby slowing a rate of temperature increase by absorbing a latent heat as the phase-changing material changes phase in the forward direction; and change phase in the reverse direction as the flow of hot fluid over the closure bar ceases, thereby slowing a rate of temperature decrease by liberating the latent heat as the phase-changing material changes phase in the reverse direction; and the forward-reverse directions are selected from the group consisting of: boiling-condensing and melting-solidifying. 
     The method 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 method, wherein: the center void region is configured to be sealed by a method selected from the group consisting of: brazing, welding, sealing with an interference-fitted plug, and sealing with a threaded fitting; and the closure bar comprises one or more materials selected from the group consisting of nickel, aluminum, titanium, copper, iron, cobalt, and alloys thereof. 
     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.