Patent Publication Number: US-2017363375-A1

Title: Heat exchanger with variable density feature arrays

Description:
RELATED APPLICATION DATA 
     This application claims priority from U.S. Provisional Patent Application No. 62/186,796, which was filed on Jun. 30, 2015, which application is hereby incorporated by reference. 
    
    
     BACKGROUND 
     In various scientific or engineering applications, there is a need to maintain distinct heat sources at an approximate temperature. For example, in power electronics, various components can generate heat including inductors, rectifiers, and transistors. Each component that generates heat can be considered a heat source that if not regulated, can lead to malfunction. Other applications include a phased array, which has a plurality of elements that generate heat. In these applications, there are distinct heat sources that need to be regulated at an approximate temperature. 
     Typical systems used to cool such heat sources involve a heat exchanger. For example, a heat exchanger can be a cold plate or heat sink affixed to the heat sources, with fluid such as air or water flowing through the cold plate or heat sink. The fluid can flow in series past each of the heat sources or can flow in parallel over them. These systems, however, present disadvantages. For example, in a system where fluid flows in series over each heat source, the fluid will enter the heat exchanger at a particular temperature and will move past the first heat source, followed by the next heat source and so on. The temperature of the fluid increases as it passes each successive heat source. So the fluid as it passes the last heat source before exiting the cold plate or heat sink will be hotter than when it entered. As a result, the last heat source is not cooled as effectively as the first. Where the heat sources are electronic devices such as transistors, the disparity in the cooling of the heat sources is not optimal and can result in the last transistor or heat source degrading before the other transistors earlier in the series. In general, therefore, thermal management systems are designed around this specific temperature difference to ensure that the temperature of the last component does not exceed the prescribed limit. As a result, heat sources upstream of the last component are cooled to a temperature below the required standards. 
     In heat exchangers where the fluid flows in parallel over the heat sources, the fluid can enter at one or more inlets to the cold plate and divide such that fluid flows past each heat source at the same time. These systems can also suffer from unreliability due to either more points of leaking and/or increased components. 
     SUMMARY 
     According to various aspects, exemplary embodiments are provided of heat exchangers and applications. In an exemplary embodiment, a heat exchanger can include at least a first feature array and a second feature array, a channel in an interior of the heat exchanger, through which a fluid can flow, an inlet for the fluid to enter the channel, an outlet for the fluid to exit the channel. The channel may include at least one surface and said first feature array and said second feature array are positioned on the at least one surface of the channel, the fluid configured to flow from said inlet, through said channel to said outlet, and the first and second feature arrays have different densities. 
     In another exemplary embodiment, a multi-phase inverter can have a heat exchanger, where the heat exchanger can include at least a first feature array, a second feature array, and a third feature array, the first, second, and third feature arrays positioned along a surface of a channel defined in an interior of said heat exchanger, said first, second, and third feature arrays having different densities, an exterior surface, an inlet for a fluid to enter the channel, an outlet for a fluid to exit the channel. The fluid configured to flow from said inlet, through said channel to said outlet, and at least a first phase, a second phase, and a third phase, each of said first phase, second phase, and third phase having at least one switch, said first, second, and third phases positioned along said exterior surface, the first phase is positioned adjacent to said first feature array, the second phase is positioned adjacent to the second feature array, and the third phase is positioned adjacent to said third feature array. 
     In another exemplary embodiment, a heat exchanger can include at least a first feature array and a second feature array, a channel in an interior of the heat exchanger, through which a fluid can flow, an inlet for the fluid to enter the channel, an outlet for the fluid to exit the channel. The channel includes at least one surface and said first feature array and said second feature array are positioned on the at least one surface of the channel. The fluid configured to flow from said inlet, through said channel to said outlet, and the features of said first array and the features of said second feature arrays have different cross-sectional shapes. 
     Further aspects and features of the present disclosure will become apparent from the detailed description provided hereinafter. In addition, any one or more aspects of the present disclosure may be implemented individually or in any combination with any one or more of the other aspects of the present disclosure. It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the present disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a heat exchanger showing hidden elements according to an embodiment of the disclosure. 
         FIG. 1A  is a perspective view of a heat exchanger according to an embodiment of the disclosure. 
         FIG. 2  is a perspective view with a cutout illustrating features of a heat exchanger according to an embodiment of the disclosure. 
         FIG. 3  is a graphical illustration of temperature versus distance in a heat exchanger. 
         FIG. 4A  is a perspective view of a heat exchanger according to an embodiment of the disclosure. 
         FIG. 4B  is a perspective view of a heat exchanger showing hidden elements according to an embodiment of the disclosure. 
         FIG. 4C  is a perspective view with a cutout illustrating features of a heat exchanger according to an embodiment of the disclosure 
         FIG. 5A  illustrates a schematic setup utilizing a heat exchanger according to an embodiment of the disclosure. 
         FIG. 5B  illustrates a schematic setup utilizing a heat exchanger according to an embodiment of the disclosure. 
         FIG. 6  is a graphical illustration of temperature versus distance in a heat exchanger. 
         FIG. 7  is a perspective view of a heat exchanger according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1, 1A and 2  illustrate a heat exchanger  100  according to one embodiment of the disclosure.  FIG. 1  illustrates the heat exchanger  100  showing hidden detail,  FIG. 1A  shows the heat exchanger  100  without the hidden detail, and  FIG. 2  shows the heat exchanger  100  with a cutout so that features of the heat exchanger  100  can be viewed. The heat exchanger  100  includes a first panel  104  and a second panel  106 . The first panel  104  includes an interior side  108  and an exterior side  110 . The second panel  106  includes an interior side  112  and an exterior side  114 . The heat exchanger  100  includes a first array of features  116  positioned on the first interior side  108  and a second array of features  118  positioned on the interior side  108 , with the second array of features positioned apart from the first array of features  116 . The first array of features  116  has a first density and the second array of features has a second density. The densities of the first and second array of features are different. And in the example of  FIGS. 1-2 , the second array of features  118  is denser than the first array of features  116 . The density of features refers to the number of features per area. The first array of features  116  also includes features  116   a , which are deposed along an edge of the channel 
     Positioned on the exterior side  110  of the first panel  104  is a first heat source  120  and a second heat source  122 . The first heat source  120  is located adjacent to the first array of features  116  and the second heat source  122  is located adjacent to the second array of features  118 . The interior sides  108  and  112  form a channel  124  in which a fluid can enter, flow through, and exit. The channel  124  is positioned on an interior of the heat exchanger  100 . The fluid enters the channel  124  through an inlet  126  and exits the channel through an outlet  128 . In the embodiment of  FIGS. 1, 1A and 2 , the fluid flows past the two arrays of features with the second array of features  118  downstream of the first array of features  116 . The direction of the flow of fluid is indicated by the arrows in  FIG. 2 . The fluid passing through the heat exchanger  100  cools the first heat source  120  and the second heat source  122 . Additionally, in the embodiment shown in  FIGS. 1, 1A and 2 , the channel  124  includes an edge where features  116   a  of the first array of features  116  are deposed. Similarly features  118   a  of the second array of features  118  are deposed along the edge of the channel  124 . Deposing features along an edge of the channel reduces the ability of the fluid, when flowing in the channel, to bypass the feature arrays  116  and  118 . It should be understood that having features on an edge of the channel is optional. 
     Stated previously, the density of the second array of features  118  is higher than the first array of features  116 . The higher density of the second array of features  118  leads to a heat transfer rate that is higher than the first array of features  116 . Tuning the feature densities of the first and second arrays of features  116  and  118  allows the heat sources  120  and  122  to be maintained at particular temperatures. 
     For example, as the fluid passes through the heat exchanger  100  via the channel  124 , the temperature of the fluid itself changes. The temperature of the fluid as it enters the channel  124  is lower than the temperature when the fluid exits the channel  124 . Put differently, the fluid temperature increases as it flows through the channel. The fluid temperature increase affects the temperature of the heat sources. If, for example, there were no first or second arrays of features, then the second heat source  122  would be hotter than the first heat source  120 . In another example, if there were first and second arrays of features, but they had a constant density, then again the second heat source  122  would be hotter than the first heat source  120 . The higher temperature of the second heat source  122  can be disadvantageous, especially where the two heat sources produce the same magnitude of heat. For example, if the first and second heat sources  120  and  122  were transistors, then the second heat source would be hotter than the first heat source. As a result, maintaining the ideal operating temperature of the second heat source (via the fluid flowing in the channel  124 ) would cause the first heat source to be unnecessarily cooled below the ideal operating temperature. In the embodiment of  FIGS. 1-2  where the second array of features is denser than the first array of features, the higher heat transfer of the second array allows the two heat sources to be maintained at substantially the same temperature. This notwithstanding the temperature increase of the fluid as it flows through the channel  124 . It should be understood that while in  FIGS. 1-2 , there are two heat sources and two arrays of features of the heat exchanger  100 , it is understood that heat exchangers could have more than two arrays of features with more than two corresponding heat sources without departing from the scope of this disclosure. 
       FIG. 3  illustrates graphically the concepts described above.  FIG. 3  is a chart illustrating temperature versus distance along an exemplary heat exchanger having three heat sources, each of which produce substantially the same heat. Distance 0 mm indicates where fluid enters the heat exchanger. The lines charted on the graph in  FIG. 3  are fluid temperature, indicated by reference  302 ; heat exchanger temperature, where each of three heat sources is adjacent to an array of features having constant feature density, indicated by reference  304 ; and heat exchanger temperature where each of the three heat sources is adjacent to an array of features having variable feature density, with the density increasing along the flow of the fluid, indicated by reference  306 . By increasing density, heat source  2  has a corresponding feature array of a higher density than heat source  1 , and heat source  3  has a corresponding feature array of a higher density than heat source  2 . As shown in  FIG. 3 , the fluid temperature increases as it flows through the heat exchanger. Where the feature arrays are of constant density, the temperature of the heat exchanger also increases. However, where the feature densities are variable, the heat exchanger maintains the three heat sources at substantially the same temperature. 
       FIG. 4A-C  illustrates a heat exchanger  400  according to another embodiment of the disclosure. The heat exchanger  400  includes a first panel  404  and a second panel  406 . The first panel  404  includes an interior side  408  and an exterior side  410 . The second panel  406  includes an interior side  412  and an exterior side  414 . The heat exchanger  400  includes a first array of features  416 , a second array of features  418 , and a third array of features  420  each of which is positioned on the interior side  408 . The first, second and third array of features  416 ,  418  and  420  have different densities, with the second array  418  have a higher density than the first array  416  and the third array  420  having a higher density than the second array  418 . 
     Positioned on the exterior side  410  of the first panel  404  is a first heat source  422 , a second heat source  424 , and a third heat source  426 . The first heat source  422  is located adjacent to the first array of features  416 , the second heat source  424  is located adjacent to the second array of features  418 , and the third heat source  426  is located adjacent to the second array of features  420 . The interior sides  408  and  412  form a channel  428  in which a fluid can enter, flow through, and exit. Put another way, the channel  428  is on an interior of the heat exchanger  400 . The fluid enters the channel  428  through an inlet  430  and exits the channel through an outlet  432 . In the embodiment of  FIG. 4 , the fluid flows past the three arrays of features with the second array of features  418  downstream of the first array of features  416 , and the third array of features  420  downstream of the second array  418 . 
     The heat exchanger  100  and  400  illustrate two and three heat sources respectively. Heat exchangers with more than two or three heat sources can be utilized without departing from the scope of the invention. For example, two or more heat sources can be placed in series on a heat exchanger. In another example, heat sources can be disposed on both exterior sides of the panels of the heat exchanger. An example of this mirrored heat exchanger is shown in  FIG. 7 , showing heat exchanger  700 . In  FIG. 7 , fourth, fifth and sixth heat sources referenced as  434 ,  436 , and  438 , are disposed on the exterior side  414  of the second panel  406 . In this particular embodiment, there are fourth, fifth, and sixth feature arrays  440 ,  442 , and  444  located on the interior side  414  of the second panel  406  that are adjacent to respective fourth, fifth and sixth heat sources  434 ,  436 , and  438 . As with the feature arrays  416 ,  418 , and  420 , the densities of the feature arrays  440 ,  442  and  444  are different. In the embodiment of  FIG. 7 , this mirrored heat exchanger allows for six heat sources to be regulated. 
       FIG. 5A  illustrates a schematic setup utilizing a heat exchanger  500 . The heat exchanger  500  may be any heat exchanger according to this disclosure, including for example heat exchangers  100  and  400 . The heat exchanger includes an inlet where fluid can flow into the heat exchanger, and an outlet where fluid can exit the heat exchanger. The outlet is connected to a thermal reservoir  506 , with the thermal reservoir connected to a pump  508 . The pump  508  is connected to the inlet of the heat exchanger  500 . This forms a loop so that during operation fluid flows from the inlet, through the heat exchanger  500 , through the outlet to the thermal reservoir  506  and from the thermal reservoir the pump  508  pumps the fluid back to the inlet the start the loop again. The direction of the loop is shown by the arrows in  FIG. 5 . As discussed above, during operation, the temperature of the fluid will change from an original temperature at the inlet to a modified temperature at the outlet. The thermal reservoir  506  works to return the fluid from its modified temperature back to the original temperature before it enters the heat exchanger through the inlet. 
       FIG. 5B  illustrates an alternative schematic setup to the one shown in  FIG. 5A . In  FIG. 5B , there is an additional heat exchanger  510 . This heat exchanger can be a heat exchanger with variable density feature arrays as described herein, or maybe another type of heat exchanger. The heat exchanger  510  is thermally coupled to a thermal reservoir  512 . In  FIG. 5A , the fluid from the outlet of the heat exchanger  500  flows to the thermal reservoir. In  FIG. 5B , the fluid from the outlet of the heat exchanger  500  flows through the heat exchanger  510 . The heat exchanger  510  exchanges heat with the thermal reservoir  512  (shown by the arrows between 510 and 512). This is another way in which the modified temperature of the fluid coming out of the heat exchanger  500  can be returned to its original temperature before it enters the heat exchanger  500  via the inlet. 
     The arrays of features described in this disclosure including as shown in the corresponding figures constitute a plurality of projections (i.e., features) that can be arranged in any suitable configuration without departing from the scope of this disclosure. For example, the arrays could include features that are arranged in a rectangular fashion, but can be in other configurations such as trapezoidal, rectangular, circular, or triangular. The configurations could also be staggered or aligned. Additionally, the features can have any suitable cross-sectional shape without departing from the scope of this disclosure. Such cross-sectional shapes can include, without limitation, circles, rectangles, ovals, rhomboids, or the shape of a hydrofoil or an airfoil. Furthermore, the features may also be located on an edge of the channel of the heat exchanger. The feature sizes are typically in the range of 0.1 mm to 5 mm, but can greater or smaller depending on a particular application. 
       FIG. 6  illustrates graphically a heat exchanger on which three heat sources are disposed in a linear array with a spacing of 10 mm between sources. Each heat source has a size of 15×20 mm and generate  300 W, resulting in a heat flux of 100 W/cm 2 . The heat exchanger is made out of aluminum  6061 . Micro-hydrofoils with a characteristic length of 500 μm are embedded under each of the heat sources.  FIG. 6  shows a graph of temperature versus distance along the heat exchanger just described. Distance 0 mm indicates where fluid enters the heat exchanger.  FIG. 6  shows the temperature of fluid (in this setup the fluid was water flowing at 12.82 grams/sec), shown by reference  602 . Reference  604  shows the range of temperatures for the heat exchanger at the three heat sources. As shown in  FIG. 6 , the temperature of the water increases as it flows through the heat exchanger. But the temperature at each of the heat sources remains substantially the same due to the variable density feature arrays. While the fluid flowing in the heat exchanger that is charted in  FIG. 6  was water, other suitable fluids may be used for heat exchangers described herein including, without limitation ethylene glycol, hydrogen gas, nitrogen gas, liquid nitrogen, air, silicon oil, industrial coolants and engineered fluids. 
     The heat exchangers described herein can also be used for any suitable application where there are multiple heat sources that need to be regulated. For example, the heat exchangers described in this disclosure can be used in power electronics, to regulate the temperature of components such as transistors, inductors, and rectifiers. The components needing thermal regulation can be placed on an exemplary heat exchanger with an array of features adjacent to each component, with the feature density tuned to regulate the heat source. For example, the heat exchangers described herein can be used to create a multi-phase inverter, with each phase of the inverter having utilizing typically a pair of switches. Each switch can comprise one or more transistors and/or other circuit elements. During operation, each switch generates heat (i.e., is a heat source). Manufacturers sell pairs of switches as a type of module. So a three-phase inverter can be constructed with three such modules, with each module generating heat. A setup like this could employ, for example, the heat exchanger shown in  FIGS. 4A-C . Other applications of the heat exchangers described in this disclosure would also include phased array antennas. The heat exchangers described herein can be used in applications where there are multiple heat sources at which a particular temperature should be maintained. 
     In the example of the multi-phase inverter, the heat produced by each phase would be uniform. In this case, and as described in  FIGS. 1A-2 and 4A -C, the corresponding feature arrays would have densities that would increase downstream from the inlet of the heat exchanger. However, in situations where the heat sources do not produce uniform heat, the densities may not necessarily increase downstream. For example, if the first heat source in the flow path of fluid produces the most heat, that source would have a higher density then a heat source downstream that produced much less heat. Thus the feature array density is tuned not only based on where along the heat exchanger it is, but also can be based on magnitude of heat produced by the source. 
     In certain applications the heat exchanger can operate as a cold plate. In other applications, rather than a heat source, there can be sources that cool over time, such that the heat exchanger should function as a hot plate. The variable density feature arrays described herein may be used where the heat exchanger operates as a hot plate. 
     The feature density under a heat or cool source can be calculated in a two-step process. In the first step the temperature of the plate was approximated by solving the  1 D heat equation with a Finite Difference Approximation (FDA) in the direction of the flow path. The heat transfer coefficients in the regions where the heat or cool sources are located on the heat exchanger are selectively tuned, by adjusting the shape and/or size of the feature, and/or the density of the feature array, to yield the desired temperatures profile in the heat exchanger. The calculation process is iterated and the heat transfer coefficients are adapted until the regions where each of the heat or cool sources are located are at the desired operating temperature. In the second step the desired heat transfer coefficients are mapped back to geometric parameters such as longitudinal and transversal pitches of the feature array. Generally accepted correlations of flow across aligned and staggered projections such as ones proposed by Zukauskas can be used to determine the feature density. 
     It should be understood that the heat exchangers described herein can be formed from a unitary structure. For example, the first and second panels described herein may be discrete or integral structures. Similarly, the feature arrays described herein may be integral or discrete with the heat exchanger. It should be further understood that the heat exchanger could be made from any suitable thermal conductor including, without limitation, aluminum, copper, thermally conductive plastic, gold, platinum, or silver. In addition, the inlets and outlets described herein, can comprise one or more openings without departing from the scope of this disclosure. 
     While embodiments in this disclosure illustrate tuning the density of features to maintain a temperature of a heated or cooled surface, it should be understood, as described above, that adjusting the shape and/or size of features can also serve to maintain a temperature of a heated surface (e.g., a hot plate) or cooled surface (e.g., cold plate). Applications of maintaining a heated or cooled surface include applications in the food industry including hot food and beverage applications, wet laboratory applications including the industrial versions of the same such as oil refineries, and chemical engineering applications. 
     While embodiments have been illustrated and described herein, it is appreciated that various substitutions and changes in the described embodiments may be made by those skilled in the art without departing from the spirit of this disclosure. The embodiments described herein are for illustration and not intended to limit the scope of this disclosure.