EP3775747B1

EP3775747B1 - Two-phase thermodynamic system having compensational wick geometry to enhance fluid flow

Info

Publication number: EP3775747B1
Application number: EP19724698.6A
Authority: EP
Inventors: Shahar Ben-Menahem; Igor Markovsky; Tzu-Yuan Lin; Michael Nikkhoo
Original assignee: Microsoft Technology Licensing LLC
Current assignee: Microsoft Technology Licensing LLC
Priority date: 2018-05-18
Filing date: 2019-05-06
Publication date: 2023-03-15
Anticipated expiration: 2039-05-06
Also published as: WO2019221947A1; EP3775747A1; US20190353431A1

Description

BACKGROUND

Heat pipes and vapor chambers absorb heat that is emitted from external sources located adjacent to designated evaporator regions. The absorbed heat causes a working fluid to evaporate from a liquid into a vapor phase, which stored the heat in latent form at a slightly lower temperature. The vapor then flows to condenser regions, where it condenses back into liquid phase, causing the stored latent heat to be released and dissipated into an ambient environment. The condensed liquid is returned back to the evaporator regions, for instance via capillary forces and pressure gradients, so as to continue this cycle. Due to the thermodynamically spontaneous phase-change processes of evaporation and condensation respectively absorbing and releasing latent heat, heat pipe and vapor chamber devices behave as highly efficient, passive thermal conductors, so long as evaporator regions remain wet and flooding of condenser regions is avoided. Therefore, preventing designated evaporator regions from drying out and condenser regions from over-wetting (flooding) are two key considerations in the design of vapor chambers and heat pipes. Absent sufficient liquid "wetting" of evaporator regions, these regions may partially or entirely dry out, which moves the evaporation to device regions more distant from the external heat source. And if condensed liquid accumulates too rapidly at the condenser regions, the liquid return mechanisms may not be able to keep up, resulting in condenser flooding, which moves the effective condensation regions further from the least-thermal-resistance exit paths. When either drying or flooding occurs (or both), the phase-change processes cannot be exploited to maximize thermal conductivity. Both evaporator drying and condenser flooding, can be mitigated by designing the device so as to improve liquid flow in its return path
Various wicking structures have been designed that exploit capillary action to continuously absorb liquid into various solid structures as it condenses at the condenser regions, and to then draw the absorbed liquid back to the evaporator regions. In some wicking structures, the cavities that induce the capillary action are intentionally smaller at the evaporator regions as compared to the condenser regions. For example, some wicking structures are made of sintered metal powers that are smaller (e.g., in terms of granule size) in the evaporator regions than in the condenser regions. In this way, the capillary action may be stronger in the evaporator regions than in the condenser regions, increasing the tendency of the liquid to be drawn from the condenser regions and recycled back into the evaporator regions. An additional advantage of such schemes is that more thin-liquid-film evaporation area is available, reducing the overall thermal resistance across all fluid-vapor interfaces (menisci) at evaporation regions of the device. Yet unfortunately, reduced cavity size over part of the device may actually exacerbate the problem of evaporator regions drying, out due to an unintended increase in resistance to liquid return flow. For example, conventional techniques for reducing cavity size to improve capillary forces also lessen the total (e.g., aggregate) cross-sectional area of a path through which the liquid can flow back to the evaporator regions.
It is with respect to these and other considerations that the disclosure made herein is presented.
FR 2 280 872 A1 discloses a differential pressure heat transfer unit.
US 2007/0246194 A1 discloses a heat pipe which includes a metal casing and a composite capillary wick received in the casing.
US 4,989,319 A discloses the preamble of independent claim 1 and in particular etching technique which provides longitudinally extending capillary grooves of variable cross-sectional dimension on the interior surface of a heat pipe.
CN 101 162 133 A discloses a heat pipe.
EP 1 544 563 A1 discloses an internally enhanced heat pipe with a groove opening size that is smaller than the size of the groove bottom.

SUMMARY

The present invention provides a thermodynamic system according to claim 1. Certain more specific aspects of the invention are set out in the dependent claims.
Technologies described herein provide a two-phase thermodynamic system that includes a compensational wick geometry to enhance fluid round-trip flow between a condenser region or regions, and an evaporator region or regions. Generally described, systems disclosed herein include one or more geometric features that vary between a condenser region and an evaporator region to increase capillary forces within wicking structures without excessively increasing resistance to a fluid flowing through these wicking structures. Unlike conventional wicking structures (e.g., such as incorporate reduced cavity sizes that bias capillary forces toward evaporator regions but also drastically increase resistance to fluid flow), the novel wicking structures described herein include various wicking geometries that compensate for decreased cavity sizes at evaporator regions to mitigate the effects of increased resistances to fluid flow. In this way, capillary forces toward evaporator regions remain sufficiently high, while resistance to fluid flow remains sufficiently low so as to prevent evaporator regions from drying out even at thermal absorption rates that would easily dry out evaporator regions in conventional systems. Thus, the technologies described herein enable two-phase thermodynamic systems (e.g., heat pipes, vapor chambers, etc.) to replenish their evaporator regions with liquid and, therefore, to continue to exploit phase-change processes for maximizing thermal conductivity at higher throughput heat power rates than conventional systems.
An example of a thermodynamic system ("device") includes walls enclosing a cavity that contains a working fluid in two phases. During operation, the liquid phase of the working fluid absorbs heat that is conducted or otherwise transferred by a heat source external to the device (e.g., touching a portion of the walls of the thermodynamic system). The absorbed heat continually converts liquid mass to vapor phase, which then transfers this heat in latent form, away from the heat source. More specifically, inside the cavity is a designated evaporator region or regions for absorbing heat to convert a liquid fraction of the working fluid into a vapor fraction of the working fluid. For example, the working fluid may be a bi-phase fluid that evaporates from a liquid state into a gaseous (vapor) state upon absorbing latent heat. The working fluid may then flow through the cavity, in the vapor state, to carry the latent absorbed heat away from the evaporator region. Inside the cavity there is also a spatially disjoint condenser region or regions, designated for releasing the latent absorbed heat out of the working fluid, and expelling this heat into the external environment of the device. This release of latent heat occurs via spontaneous condensation, wherein the vapor fraction is continually converted back into the liquid fraction. It will be appreciated by those skilled in the art that the specific amount (e.g., mass) of the vapor fraction that condenses back into the liquid fraction in any given time interval of device operation, depends on the specific amount of latent heat that is dissipated from the vapor fraction. It will also be appreciated that in steady state operation of the device, this mass amount is equal to the mass amount of liquid fraction that is evaporated (converted to vapor) during an equal duration of time, provided all evaporator regions are summed in the second calculation, and provided also that all condenser regions are summed in the second calculation. It will further be appreciated that in steady-state operation of such a device as just described, the total heat power flowing into all evaporator regions from the external source, equals the total heat power flowing out into the device external environment via all condenser regions. Exemplary working fluids include, but are not limited to, water, refrigerant substances (e.g., R134), ammonia-based liquids, or any other fluid suitable for efficient absorption and release of heat to effect phase changes (evaporation and condensation respectively) change a liquid and a gaseous (vapor) state.
The thermodynamic system (device) may include a vapor flow path extending from the evaporator region to the condenser region, to enable the vapor fraction of the working fluid to convectively carry the heat that is absorbed at the evaporator region or regions away from the heat source. The vapor flow path may be any path suitable for the vapor fraction of the working fluid to freely flow through. The thermodynamic system may also include a liquid flow path that includes a plurality of channels extending from the condenser region or regions, back to the evaporator region or regions. The channels fill up (at least partially) with the liquid fraction as it condenses at the condenser region or regions. The channels then draw this liquid fraction back to the evaporator region or regions, e.g. via capillary action. In case capillary action plays an important or dominant role in liquid fraction return, it can be appreciated that the channels are a form of a wicking structure designed to ensure the evaporator region remains sufficiently wetted by continually drawing the liquid fraction back into the evaporator region, at sufficient rates to keep up with evaporation. Ideally, the liquid fraction is drawn into the evaporator or evaporators via the wicking structure (e.g., the channels in this example) at substantially the same rate it is being evaporated by virtue of absorbing the latent heat, i.e., to achieve a steady state of operation.
The liquid flow path or paths may include various segments at which geometric features of the wicking structure are modulated (i.e., controllably changed between adjacent segments) to induce capillary action toward the evaporator region or regions. For example, the liquid flow path may include a first segment that is located within a condenser region and a second segment that is located within an evaporator region. One or both of these segments may extend outside of the condenser region and/or evaporator region into an intervening adiabatic region, through which the working fluid liquid fraction passes without significantly absorbing or releasing heat.
Pursuant to the example wherein the wicking structure comprises channels, within the first segment one or more individual ones of the channels may have a first width and within the second segment one or more individual ones of the channels may have a second width that is different than the first width. The first width within a condenser region may be greater than the second width within an evaporator region, so as to move the liquid fraction toward the evaporator region with as little pressure drop as feasible, while also maximizing capillary forces where they are most needed. More specifically, by virtue of the individual channels having a narrower width within the second segment than within the first segment, the capillary action exerted on the working fluid's liquid fraction may be greater within the second segment than within the first segment, while keeping the total condenser-to-evaporator liquid pressure drop as small as feasible. Thus, in this exemplary embodiment of the thermodynamic system ("device"), wick design promotes continual wetting of the evaporator region, while also optimally leveraging capillary forces in the return paths of the liquid fraction of the working fluid.
As described above, conventional two-phase thermodynamic systems that induce return flow of the liquid fraction toward specific evaporator regions with reduced wick pore sizes, nevertheless remain highly susceptible to dry-out of these evaporator regions, because the reduced wick pore sizes excessively constrict liquid flow. For example, it will be appreciated by those skilled in the art, that given sufficient time to reach a steady state, the working fluid's condensed liquid fraction will close the thermodynamic cycle by flowing through the restricted cavities, paths or channels of the liquid return path of the conventional two-phase thermodynamic system. For high enough throughput heat power levels, however, the combined phase-change mass conversion rate at all the evaporator regions, exceeds the rate at which capillary action can pull liquid mass back to the evaporator region or regions through the restricted cavities. When this happens, the result is partial or total dry-out of one or more designated evaporator regions.
In contrast to such conventional designs, the novel wicking structures herein described, combine cavity or channel size reductions that achieve enhanced capillary action, with various other geometric features to mitigate or even offset the effects of increased resistances to liquid flow near evaporator regions. In this way, capillary forces pulling liquid toward evaporator regions remain at sufficiently high levels, while overall resistance to return-path liquid flow remains sufficiently low as to prevent any evaporator region from drying out, even at throughput heat power rates that would easily dry out evaporator regions in conventional two-phase thermodynamic systems. While it will be appreciated that evaporator dry-outs (and potentially also condenser floodings) will still occur at sufficiently high heat power rates, the onset of these deleterious phenomena can be pushed to significantly higher critical throughput heat power levels by implementing the structural designs herein described. Thus, the technologies described herein enable two-phase thermodynamic systems to replenish their evaporator regions with liquid while avoiding condenser flooding, throughout an expanded range of heat power levels. The technologies described herein thus enable the designer of a two-phase thermodynamic system as described, to push the exploitation of phase-change processes for maximizing thermal conductivity, up to significantly higher through-put heat power rates than conventionally achievable.
With respect to the example wherein the wicking structure comprises channels that are narrower within their second segments than in the first segments, the channels may also be deeper and/or more numerous within the second segment than in the first segment - both characteristics that decrease resistance to fluid flow, while either not affecting or even improving evaporator performance For example, deeper channels increase the cross-sectional areas through which the working fluid flows in the second segment of each channel- thereby decreasing the aggregated fluid flow hydraulic resistance (pressure drop over mass flow) offered to the return-path liquid fraction of the working fluid. In this sense of minimizing and/or eliminating increases to fluid flow resistance, the geometric feature of the channels being deeper in the second segment than in the first segment compensates for the geometric feature of the channels being narrower in the second segment than in the first segment. In some embodiments, the cross-sectional fluid path area of the second segment is equal to or even greater than the cross-sectional fluid path area of the first segment. As used herein, the term "cross-sectional fluid path area" may refer to the total area of a cross-section taken perpendicular to the mean direction of fluid travel.
The liquid return-path channels that extend between the condenser region or regions and the designated evaporator region or regions, may be formed by ribs having an undercut angle that varies between the various different segments of the liquid flow path. For example, the ribs may be formed with a greater undercut angle within the evaporator regions than in the condenser regions. As used herein, the term "undercut angle" refers generally to an angle measured between a rib wall and a bottom surface of a channel. Additionally, or alternatively, the ribs may include rounded shoulders that vary in radius between the various different segments of the liquid flow path. For example, the ribs may be formed with a smaller shoulder radius within the evaporator region than in the condenser region, which hardly affects flow resistance but optimizes capillary forces at the designated evaporation sites.
These and various other features will be apparent from a reading of the following Detailed Description and a review of the associated drawings.

DRAWINGS

The Detailed Description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same reference numbers in different figures indicate similar or identical items. References made to individual items of a plurality of items can use a reference number with another number included within a parenthetical (and/or a letter without a parenthetical) to refer to each individual item. Generic references to the items may use the specific reference number without the sequence of letters.

FIG. 1 is a perspective cut-away view of an exemplary two-phase thermodynamic system that includes a compensational wick geometry to enhance fluid flow between a condenser region and an evaporator region.
FIG. 2A is a top view of an exemplary wicking structure that is formed by channels that are deeper and narrower in an evaporator region than in a condenser region.
FIG. 2B is a cross-section view, taken along the line A-A (shown in FIG. 2A), of a segment of the exemplary wicking structure of FIG. 2A.
FIG. 2C is a cross-section view, taken along the line B-B (shown in FIG. 2A), of another segment of the exemplary wicking structure of FIG. 2A.
FIG. 3A is an exemplary condenser region profile of a wicking structure wherein ribs that form channels have undercut angles that vary between the condenser region and the evaporator region.
FIG. 3B is an exemplary evaporator region profile of the wicking structure of FIG. 3A.
FIG. 4A is an exemplary evaporator region profile of an exemplary two-phase thermodynamic system that includes a compensational pore geometry to enhance fluid flow between a condenser region and an evaporator region.
FIG. 4B is an exemplary condenser region profile of the two-phase thermodynamic system of FIG. 4A.
FIG. 4C is a side cut-away view of the two-phase thermodynamic system wherein a first cut-away section corresponds to the line C-C of FIG. 4A and a second cut-away section corresponds to the line D-D of FIG. 4B.

DETAILED DESCRIPTION

The following Detailed Description describes technologies for providing a two-phase thermodynamic system that includes a compensational wick geometry to enhance fluid flow between a condenser region and an evaporator region. Embodiments disclosed herein may include geometric features that vary between a condenser region and an evaporator region to increase capillary forces within wicking structures without excessively increasing resistance to a fluid flowing through these wicking structures. Unlike conventional wicking structures that bias capillary forces toward evaporator regions but fail to efficiently manage fluid flow resistances, the novel wicking structures geometries described herein compensate for decreased cavity sizes at evaporator regions to prevent (or at least minimize) increases to fluid flow resistances. In this way, capillary forces toward evaporator regions remain sufficiently high while resistance to fluid flow remains sufficiently low such that these evaporator regions remain "wetted" even at thermal absorption rates that would easily dry out evaporator regions in conventional systems. Thus, the technologies described herein enable two-phase thermodynamic systems (e.g., heat pipes, vapor chambers, etc.) to replenish their evaporator regions with a liquid fraction of a working fluid to exploit phase-change processes at higher rates than conventional systems.
The present technologies are believed to be applicable to a variety of two-phase thermodynamic systems and approaches involving drawing a liquid fraction of a working fluid into an evaporator region to exploit a phase-change process for rapidly transferring heat away from one or more heat sources. Examples disclosed below are predominantly described in the context of novel channel-based wicking structures. While the present disclosure is not necessarily limited to such channel-based wicking structures, an appreciation of various aspects is best gained through a discussion of examples in this context.
FIG. 1 is a perspective cut-away view of an exemplary two-phase thermodynamic system 100 (also referred to herein as "thermodynamic system" or "device") that includes a compensational wick geometry to enhance fluid flow between a condenser region 110 and an evaporator region 108. As illustrated, the thermodynamic system 100 includes walls 102 that contain a working fluid (omitted from illustration to expose wicking structure 104). During operation, a heat source 106 emits heat that is absorbed into a working fluid at a designated evaporator region 108 (as shown by the Q _IN symbol) of the thermodynamic system 100 - thereby converting a liquid fraction of the working fluid into a vapor fraction of the working fluid. As additional mass (as quantified e.g. in gram units) of the liquid fraction is continually evaporated into the vapor fraction, some mass of the vapor fraction is displaced towards a condenser region 110. Thus, as additional heat is absorbed into the working fluid converting the liquid fraction into the vapor fraction, some latent heat is carried by the vapor fraction through a vapor flow path 109 (shown as a white arrow disposed above the wicking structure 104) to the condenser region 110. Then, when the vapor fraction reaches the condenser region 110, the latent heat is transferred out of the working fluid (as shown by the Q _OUT symbol) via a combination of condensation phase change (converting the vapor fraction of the working fluid back into its liquid fraction) and conductive heat transfer out of the device and into the external environment adjacent to the outer device wall in the vicinity of the condenser region.
The thermodynamic system 100 may further include the wicking structure 104, designed to continually wick or draw the liquid fraction back into the evaporator region 108 to keep the evaporator region 108 wetted - enabling continual (cyclical) exploitation of the back-and-forth phase-change process to efficiently remove heat from the heat source 106. The wicking structure 104 defines a liquid flow path 203 (labeled in FIG. 2A) and may include various segments 105 at which one or more geometric features are modulated (i.e., controllably changed between adjacent segments) to optimize capillary action toward, and evaporation action at, the designated evaporator region 108 while minimizing overall hydraulic resistance to liquid flow in 203. In the illustrated example, the wicking structure 104 includes a first segment 105(1) that is disposed within the condenser region 110 and a second segment 105(2) that is disposed within the evaporator region 108.
In the illustrated embodiment, the wicking structure is made up of channels 112 that extend from the condenser region 110 to the evaporator region 108. As the vapor fraction of the working fluid releases latent heat and condenses back into the liquid fraction at the condenser region 110, this newly condensed liquid fraction enters the channels 112 which then draw this liquid fraction back to the evaporator region 108. It should be appreciated that the channels 112 are not illustrated to scale and that, in practice, these channels 112 may be micro-structures that are on the order of tens of microns in width and/or depth. In this way, the cavities (i.e., the area which the working fluid may enter and/or flow through) which are formed by the channels 112 are sufficiently small so as to induce capillary action from the condenser region 110 to the evaporator region 108.
In this example, the first segment 105(1) of the channels 112 is formed by first ribs 114(1) that are spaced apart a first width and that extend down a first depth. Thus, the first segment 105(1) of the channels 112 can aptly be described as having the first width and the first depth. In contrast, the second segment 105(2) of the channels is formed by second ribs 114(2) that are spaced apart a second width and that extend down a second depth. Thus, the second segment 105(2) of the channels 112 can aptly be described as having the second width and the second depth. As illustrated, the second width is less than the first width and, therefore, when saturated the second segment 105(2) will have a greater tendency to induce capillary action than the first segment.
However, all other things being equal, the narrower width of the channels 112 within the second segment 105(2) will tend to restrict fluid flow to a greater extent than the wider width of the channels 112 within the first segment. In this example, this issue is compensated for by modulating the depth of the channels 112 so that they are deeper within the second segment 105(2) than in the first segment. As opposed to leaving the depth of the channels 112 the same, deepening the channels increases the cross-sectional area through which the liquid is able to flow - thereby decreasing hydraulic resistance to liquid flow. Moreover, the issue of liquid flow restriction is further compensated for in this example by modulating the number of channels 112 through which the liquid fraction is able to flow. It can be appreciated that both of these geometric features (i.e., deepening and increasing the number of channels 112) effectively reduce the overall hydraulic resistance to fluid flow toward the evaporator region 108.
Thus, in contrast to conventional designs, the novel wicking structures described herein compensate for geometric features that achieve increased capillary action (e.g., channel width reductions) with various other geometric features to mitigate the effects of increased resistances to fluid flow (e.g., channel depth increases, rib undercut, channel count increases, etc.). In this way, capillary forces pulling liquid toward evaporator regions remain sufficiently strong, while hydraulic resistance to fluid flow remains sufficiently low so as to prevent evaporator regions from drying out even at heat transfer power rates that would easily dry out evaporator regions in conventional two-phase thermodynamic heat-transfer systems.
It should be appreciated that the illustrated embodiment is shown for illustrative purposes only and is not intended to be limiting. Other embodiments are also contemplated and within the scope of the present disclosure. For example, some embodiments may include geometrically varying segments of channels around the entire inner diameter of a substantially round heat pipe.
Turning now to FIG. 2A, illustrated is a top-view of an exemplary wicking structure 104 that is formed by channels 112 that are deeper and narrower in an evaporator region 108 than in a condenser region 110. As illustrated, a liquid fraction of the working fluid is being drawn from the first segment 105(1) into the second segment 105(2) due to the increased capillary action that is exerted on the interface(s) ("menisci") between the liquid fraction and the vapor fraction within the second segment 105(2) as compared to the first segment. As illustrated in FIG. 2A, this increased capillary action may be induced due to a greater amount of surface tension forces being exerted at this interface per unit-area of liquid within the second region than in the first region. For example, assuming that each of the illustrated forces (shown as black arrows) are substantially equal (which may or may not be the case in various applications), the six different forces that are pulling the second menisci 202(2) will generate greater net pull towards the evaporator region than do the two forces that are pulling the first meniscus 202(1) in the opposite direction (back toward the condenser region). This means that during initial start-up of the device or its ramp up to higher throughput heat power rate, and also during steady-state operation, the net capillary force helps to push the condensing liquid fraction back to the designated evaporator regions and sites. Thus, the liquid return mechanism in this design is less dependent on high (and thus dry-out inducing) liquid pressure drops. This is consistent with the fact that our wick design approach, disclosed herein, indeed minimized the condenser-to-evaporator liquid pressure drop.
FIG. 2B is a cross-section view, taken along the line A-A (shown in FIG. 2A), of the second segment 105(2) of the wicking structure 104 of FIG. 2A. As illustrated, within the second segment 105(2), a plurality of second channels 112(2) are formed by a plurality of second ribs (a.k.a. "fins") 114(2). FIG. 2C is a cross-section view, taken along the line B-B (shown in FIG. 2A), of the first segment 105(1) of the wicking structure 104 of FIG. 2A. As illustrated, within the first segment, a plurality of first channels 112(1) are formed by a plurality of first ribs 114(1). For purposes of the discussion, presume that a side-by-side comparison of FIGS. 2B and 2C is informative of relative dimensions between these two cross-sections. For example, it can be graphically appreciated that the second channels 112(2) are deeper than the first channel 112(1) and, furthermore, that the second channels 112(2) are narrower than the first channel 112(1).
As described above, in addition to modulating geometrical features of the individual channels 112 between segments, in some embodiments, the number of channels 112 may also be modulated. For example, each of the cross-sections illustrated in FIGS. 2B and 2C are shown to span the same distance (e.g., 100 microns or any other suitable distance). As shown in FIG. 2B, in the second segment 105(2) there are 3 channels per this distance. As shown in FIG. 2C, in the first segment 105(1) there is only 1 channel per this distance. Thus, in an embodiment of the thermodynamic heat-transfer system (a.k.a. "device") disclosed herein that is consistent with this pattern, there will be three times as many channels for the fluid's liquid fraction to flow through within the second segment than there will be in the first segment.
As described above, the individual cavities (e.g., channels and/or pores) of the wicking structures 104 described herein may be on the order of tens of microns. As a specific but non-limiting embodiment of the wicking structure 104 shown in FIGS. 1 through 2C, the depth and width of the first channels 112(1) may be 30 microns and 90 microns, respectively. In the illustrated example, in the condenser region 110, the total cross-sectional area of the liquid flow path (per the distance shown in FIG. 2B and 2C) is equal to the cross-sectional area of a single channel. In this example, the cross-sectional area of the 30 micron by 90 micron channel is equal to 2.7 × 10^-9 m ². Turning now to the evaporator side, the depth and width of the second channels 112(2) may be 60 microns and 16 microns, respectively. As illustrated, in the evaporator region 108, the total cross-sectional area of the liquid flow path (per the distance shown in FIG. 2B and 2C) is equal to the aggregate cross-sectional area of three channels. In the illustrated example, this is equal to 2.88 × 10^-9 m ². Moreover, each individual rib 114 may have a width of roughly 15 microns.
Based on this specific example, it can be appreciated that in some embodiments the total-cross sectional area of the liquid flow path within the evaporator region 108 may be even greater than the total-cross sectional area of the liquid flow path in the condenser region 110. For example, the aggregated cross-sectional area of three of the second channels 112(2) is greater than the area of a single first channel 112(1). Thus, in contrast to conventional wicking structures which are designed to maximize capillary action within the evaporator region (but which also induce greater fluid flow restriction in the evaporator region), the techniques described herein similarly enable the capillary action to be maximized within the evaporator region but without causing drastic increases to fluid flow hydraulic resistance within the evaporator region. Those skilled in the art will appreciate that liquid channel hydraulic resistance in each wick segment depends not only on cross-sectional area but also upon channel length. It will also be appreciated that liquid channel hydraulic resistance in each wick segment also depends on cross sectional shape. It will furthermore be appreciated that the capillary forces depend not only upon meniscus surface tension and channel widths, but also upon meniscus contact angle (material and finish dependent) as well as channel "shoulder" radius of curvature. All these wick attributes should be factored in when optimizing wick segments design.
To illustrate the advantage of the design approach disclosed herein, consider a conventional wicking structure made of sintered metal powders that are smaller (e.g. in terms of granule size) in the evaporator region than in the condenser region. It can be appreciated that these smaller metal powders will pack more tightly together relative to larger metal powders and, therefore, will induce relatively more capillary action and easier evaporation. It can further be appreciated, however, that because these finer metal powders pack more tightly together, the available area through which the liquid fraction may flow is also less relative to the larger metal powders - which tends to increase the wick structure's hydraulic resistance to liquid flow. According to the techniques herein disclosed, to offset the restriction to liquid flow caused by decreased cavity sizes, various compensating geometric features may be incorporated to ensure that the working fluid's liquid fraction still encounters an acceptably low hydraulic resistance to its flow through the wicking structure towards the evaporator region or regions (region 108 in the illustrated embodiment).
Turning now to FIGS. 3A and 3B (collectively referred to as FIG. 3), illustrated is an embodiment of the wicking structure 104 wherein the ribs 114 that form the channels 112 have undercut angles that vary between the condenser region 110 and the evaporator region 108. With specific reference to FIG. 3A, an exemplary condenser region profile is shown wherein the first ribs 114(1) that form the first channels 112(1) have a first undercut angle Θ¹ . With specific reference to FIG. 3B, an exemplary evaporator region profile is shown wherein the second ribs 114(2) that form the second channels 112(2) have a second undercut angle Θ² that is different than the first undercut angle Θ¹ of the condenser region 110. As used herein, the term "undercut angle" refers generally to an angle measured between a rib wall 304 and a bottom surface 306 of a channel 112. Although the profiles shown in FIGS. 3A and 3B are illustrated as "dry" in the sense that no liquid fraction of the working fluid is shown as filling the channels 112, it can be appreciated that when these channels 112 are adequately charged with liquid, an interface (meniscus) between the liquid fraction and the vapor fraction may touch the rounded shoulders 302.
In the illustrated embodiment, the first undercut angle Θ¹ is greater than the second undercut angle Θ² . As a specific but non-limiting example, the first undercut angle Θ¹ that is within the condenser region 110 is 30 degrees and the second undercut angle Θ² that is within the evaporator region 108 is 10 degrees. In some embodiments, the first undercut angle Θ¹ is within the range of 25 degrees to 35 degrees and the second undercut angle Θ² is within the range of 5 degrees to 15 degrees. In some embodiments, the first undercut angle Θ¹ is within the range of 30 degrees to 40 degrees and the second undercut angle Θ² is within the range of 0 degrees to 10 degrees. In some embodiments, the first undercut angle Θ¹ is within the range of 15 degrees to 40 degrees and the second undercut angle Θ² is substantially 0 degrees.
In some embodiments, the individual channels 112 may be formed by ribs 114 that include rounded shoulders 302. For example, as shown in FIG. 3A, the first ribs 114(1) that form the first channels 112(1) include a first rounded shoulder of a first radius R¹. In some embodiments, the radius of the rounded shoulders 302 is substantially the same in the evaporator region 108 as in the condenser region 110. As a specific but non-limiting example, in an exemplary embodiment each of the first ribs 114(1) and the second ribs 114(2) may include rounded shoulders 302 of radius 800 nanometers. In some embodiments, the top portion of each rib (fin) may in fact have a rounded-vertex rectangular shape rather than be a sector of a circular disc, as depicted in Figs. 3.
In some embodiments, the radius of the rounded shoulders 302 may be different within the condenser region 110 than within the evaporator region 108. As a specific but non-limiting example, the first radius R¹ of the first rounded shoulders 302(1) that are in the condenser region 110 is 4000 nanometers and the second radius R² of the second rounded shoulders 302(2) that are in the evaporator region 108 is 700 nanometers. In some embodiments, the first radius R¹ is within the range of 3500 nanometers to 4500 nanometers and second radius R² is within the range of 650 nanometers to 900 nanometers. In some embodiments, the first radius R¹ is more than 4500 nanometers and second radius R² is less than 650 nanometers.
Turning now to FIGS. 4A through 4C (collectively referred to as FIG. 4), illustrated is an exemplary two-phase thermodynamic system 400 that includes a compensational pore geometry to enhance fluid flow between a condenser region 110 and an evaporator region 108. With specific reference to FIG. 4A, illustrated is an exemplary profile for the evaporator region 108 of the two-phase thermodynamic system 400. In the illustrated example, the evaporator region 108 includes thirty-six ("36") pores 402 through which a working fluid is able to flow. In FIG. 4, the liquid fraction of the working fluid is illustrated as a pattern fill of black dots. Thus, individual pores 402 which are filled with this fill pattern are illustrated as being filled with the liquid fraction whereas other pores 402 (e.g., which have no fill) are illustrated as being filled with the vapor fraction. It will be appreciated that for this wick design to work, the pores in each segment of the wick must be connected by suitable channels (not shown), to enable evaporation and condensation.
Turning to FIG. 4B, illustrated is an exemplary profile for the condenser region 110 of the two-phase thermodynamic system 400. In the illustrated example, the condenser region 110 includes four ("4") pores 402 through which a working fluid is able to flow. In some embodiments, the aggregate cross-sectional area of the "small" pores within the evaporator region 108 is substantially equal to or even greater than the aggregate cross-sectional area of the "large" pores within the condenser region 110. As a specific but non-limiting example, individual ones of the "small" pores within the evaporator region 108 may have a diameter of twenty ("20") microns and, therefore, an individual cross-sectional area of three-hundred and fourteen ("314") square microns. Thus, the aggregate cross-sectional area of the pores within the evaporator region 108 is very close to eleven-thousand and three-hundred ("11,300") square microns. In this specific example, individual ones of the "large" pores within the condenser region 110 may have a diameter of sixty ("60") microns and, therefore, an individual cross-sectional area of two-thousand eight-hundred and twenty-seven ("2827") square microns. Thus, the aggregate cross-sectional area of the pores within the condenser region 110 is also very close to eleven-thousand and three-hundred ("11,300") square microns.
Turning now to FIG. 4C, illustrated is a side cut-away view of the two-phase thermodynamic heat-transfer system 400 wherein a first cut-away section corresponds to the line C-C of FIG. 4A and a second cut-away section corresponds to the line D-D of FIG. 4B. The direction that a vapor fraction is flowing through the pores of the two-phase thermodynamic system 400 are illustrated as white arrows within the top half of the two-phase thermodynamic system 400. The direction that a liquid fraction is flowing through the pores of the two-phase thermodynamic system 400 are illustrated as gray arrows within the lower half of the two-phase thermodynamic system 400. It can be appreciated that the two-phase thermodynamic system 400 of FIG. 4 is at least partially orientation independent
Individual ones of the "large" pores shown in FIG. 4B may be interconnected with one or more individual ones of the "small" pores shown in FIG. 4A. For example, the two-phase thermodynamic system 400 may include one or more segment boundaries at which individual pores from one segment branch out into multiple pores in the adjacent segment.
In the specific but non-limiting example illustrated in FIG. 4, the two-phase thermodynamic system 400 includes three discrete segments as separated by the two discrete segment boundaries shown in FIG. 4C. At each of these segment boundaries, the number of pores increases by a factor of three. Thus, in the condenser region 110 there are four individual pores which are each interconnected with three individual pores of a smaller size within an adiabatic region (e.g., a region through which the working fluid passes without significantly absorbing or releasing heat). Then, at a segment boundary where the adiabatic region meets the evaporator region 108, the twelve individual pores may each interconnect with three individual pores of an even smaller size within the evaporator region 108.
The pores 402 may be arranged in accordance with a substantially ordered arrangement. For example, in the illustrated example, the thirty-six ("36") pores 402 shown in the evaporator region 108 are arranged as an ordered matrix. Embodiments wherein the arrangement of the pores 402 within one or more segments of the two-phase thermodynamic system 400 is substantially ordered may enhance the overall predictability of the system performance over some conventional heat pipes and vapor chambers. For example, many conventional heat pipes and vapor chambers include largely random wicking structures (e.g., sintered metal powders, wire mesh wicks, etc.) such that the performance varies largely between even the same manufacturing lot of these systems. In contrast, because the illustrated embodiment is a substantially ordered arrangement of pores having specific diameters that are controllably modulated from segment to segment, the operational behavior of the presently described system should be easier to predict using model simulations.
Although illustrated as highly ordered structure (with regular inter-pore channels - not shown in Figures), one skilled in the art will recognize that the term "pore" may refer to random interconnected structures such as, for example, openings in a wire mesh, or to high-aspect-ratio liquid regions between solid fibers in a fibrous-bundle wick component. Those skilled in the art will further recognize that term "pore" may also refer to void volumes between grains in a sintered-metal-grains wick component that are partially filled with liquid.

Claims

A thermodynamic system (100) that contains a bi-phase fluid, the thermodynamic system comprising:
an evaporator region (108) for absorbing heat to convert a liquid fraction of the bi-phase fluid into a vapor fraction of the bi-phase fluid;

a condenser region (110) for dissipating the heat out of the bi-phase fluid to convert the vapor fraction of the bi-phase fluid into the liquid fraction of the bi-phase fluid;

a vapor flow path (109) extending from the evaporator region (108) to the condenser region (110); and

a liquid flow path (203) that includes a plurality of channels (112) extending from the condenser region (110) to the evaporator region (108),
wherein the liquid flow path (203) includes a first segment (105(1)), that is disposed at least partially within the condenser region (110), at which the plurality of channels (112) have a first width and a first depth, and

wherein the liquid flow path (203) further includes a second segment (105(2)), that is disposed at least partially within the evaporator region (108), at which the plurality of channels (112) have a second width that is less than the first width and characterized in that the plurality of channels (112) have a second depth that is greater than the first depth
The thermodynamic system of claim 1, wherein the first segment (105(1)) includes a first number of channels (112) spanning across a particular distance that is substantially perpendicular to the liquid flow path (203), and wherein the second segment (105(2)) includes a second number of channels (112) spanning across the particular distance, the second number of channels (112) greater than the first number of channels (112).
The thermodynamic system of claim 1 or claim 2, wherein individual channels of the plurality of channels are formed by ribs (114) having at least one first undercut angle (Θ¹ ) within the condenser region (110) and at least one second undercut angle (Θ² ) within the evaporator region (108), and wherein the at least one first undercut angle (Θ¹ ) is less than the at least one second undercut angle (Θ² ).
The thermodynamic system of claim 3, wherein the at least one first undercut angle (Θ¹ ) within the condenser region (110) is within a range of 25 degrees to 35 degrees, and wherein the at least one second undercut angle (Θ² ) within the evaporator region (108) is within a range of 5 degrees to 15 degrees.
The thermodynamic system of any one of claims 1 to 4, wherein individual channels (112) of the plurality of channels are formed by ribs (114) that include rounded shoulders (302) having a first radius within the condenser region (110) and a second radius, that is different than the first radius, within the evaporator region (108).
The thermodynamic system of claim 5, wherein the first radius that the rounded shoulders (302) have within the condenser region (110) is greater than the second radius that the rounded shoulders (302) have within the evaporator region (108).
The thermodynamic system of any one of claims 1 to 6, wherein a first cross-sectional area of the liquid flow path within the first segment (105(1)) is equal to or greater than a second cross-sectional area of the liquid flow path within the second segment (105(2)).
The thermodynamic system of any one of claims 1 to 7, wherein the second depth is at least two times greater than the first depth, and wherein the first width is at least three times greater than the second width.