The device and methods described herein relate to the isothermal heat transport through an intermittent liquid supply to an evaporator device, thereby enabling high evaporative heat transfer coefficients. A liquid and vapor mixture flows through miniature and micro-channels in an evaporator and addresses flow instabilities encountered in these channels as bubbles rapidly expand. Additionally, a high percentage of the fins are exposed to vapor and limit the required charge of refrigerant within the system due to effective condensate removal in the condenser.

BACKGROUND OF THE INVENTION

Passive heat transfer devices, such as heat pipes, are of much interest in applications such as electronics cooling. Heat pipes are a liquid and vapor device in which liquid is pumped through capillarity from the condenser to the evaporator. The pumping effect in this device requires a wick, which produces a high pressure loss and limits the maximum heat transport distance and or power that can be supported before dry-out occurs.

Another technology node that is useful is a thermosyphon as shown inFIG. 1. In operation, liquid104is vaporized in an evaporator101. The vapor then travels through a tube102to the condenser100. Heat is removed from the condenser100causing the liquid104to accumulate at the bottom. The accumulated liquid104in the condenser is driven by gravity through a liquid line103back to the evaporator101. The evaporators in these devices are typically pool boiling devices with an enhanced surface105that may consist of fins, a porous layer or even an etched surface. The maximum boiling heat transfer coefficient can be limited in this device because there are a finite amount of nucleation sites, and therefore a limited length of solid/liquid/vapor contact, where the heat transfer rate is the highest.

In conventional thermosyphon design, a flow pattern that enters one side of the evaporator and leaves the other side, through a series of channels is typically not used. While this general concept is widely used in most heat transfer products, the implementation in thermosyphon design for electronics is generally prohibited by the limited pressure head provided by gravity to drive the flow and flow instabilities encountered with vapor expansion in a confined channel as shown inFIG. 2. As a channel size201decreases to the same size of a vapor bubble202, the expansion of the vapor causes liquid203to flow outwards204, irrespective of the desired flow rate. This phenomena poses a few problems. One problem is that the pressure drop associated with high liquid velocities in a channel are quite high, especially relative to the small available pressure head in a thermosyphon device. A second problem that this phenomena can cause is that the middle of the channel is left dry and can increase in temperature, since the vapor has limited heat capacitance.

SUMMARY

This invention is directed toward thermosyphon technology. Certain embodiments are intended for use in electronics cooling applications, wherein a looped flow pattern through channels is formed by fins in the evaporator as well as in the condenser, while allowing for low pressure loss through these channels, thereby enabling this configuration to be applied in low profile systems where the gravitationally-induced liquid pressure head is limited.

The liquid supplied to the evaporator is intermittent, and passively regulated by the back flow of vapor bubbles. The passively regulated liquid supply enables enhanced solid/liquid/vapor contact, which yields high heat transfer rates on the channels within the evaporator. This characteristic is a solution to the limitations associated with pool boiling in an evaporator flooded with liquid.

Additionally, the problem of flow instabilities of expanding vapor bubbles in confined channels is addressed through a series of minor vapor and liquid distribution channels cutting across the major channels on the surface. These channels help enable the liquid and vapor to be stratified in a confined space, which provides a free path for vapor to escape the evaporator with minimum impedance of the liquid phase. Additionally, the liquid distribution allows for the bottom of the fins to maintain a wetted region, and maintain stable performance.

In various embodiments of the condenser, the vapor flow helps drag liquid along with it from the vapor intake orifices to the liquid exit orifice. The liquid exit orifice is located at the bottom of the fins, which helps minimize the required refrigerant charge as well as keeps the fins free from collected liquid, which can block the condensation process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an improved intermittent thermosyphon. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than an intermittent thermosyphon. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. In addition, the following terms shall have the associated meaning when used herein:

One embodiment of the present invention is presented inFIG. 3. It includes a condenser100, two evaporators101, a vapor tube102connecting the evaporator101to the condenser100primarily transferring vapor, a liquid tube103connecting the condenser100to the evaporator102primarily transferring liquid, and an access valve106, to pull a vacuum, charge and recapture working fluid at production as well as at end of life. The condenser100has fins107that allow for heat to be rejected to the air passing through. The bottom of the evaporators101will contact a heat generating electronics component, such as a central processing unit, through a thermal interface material. The contact surface will require force to be applied through an additional part, which is not detailed, so that adequate pressure may be obtained between the evaporator101and the heat generating component. This embodiment is described in detail, however, there may be variants, such as a system with a single evaporator101, and three or more evaporators101. In these scenarios, the implementation may require a separate vapor tube102and liquid tube103to each evaporator101in a parallel flow scheme or there is the possibility of using a serial flow scheme.

A cross-section of this embodiment through the vapor tube102is represented inFIG. 4. The evaporator101has fins201extending from the bottom surface to the top surface, creating a series of channels, and the fins201are partially submerged in liquid301. The evaporator fins201act to increase the heat transfer area as well as provide structural strength to withstand high internal pressures. Vapor300exits the evaporator101through an orifice210and enters the vapor tube102. Vapor300, travels through the tube102from the evaporator101to the condenser100in the direction represented by the arrows302. The axis of the vapor tube102generally parallels a horizontal axis. Vapor300enters the condenser100through two orifices206in the bottom of the condenser100. The condenser100also has fins200extending from the bottom surface to the top surface, creating a series of channels. The condenser fins200also act as a means to increase the heat transfer area as well as provide structural support. When height is limited, as is the case for the embodiment represented, the vapor entry orifices206in the condenser100may be located on the bottom side. In cases where there is additional space, these orifices206may also be located on the top side.

A cross-section of this embodiment through the liquid tube103is represented inFIG. 5. The center line of the liquid tube103parallels a horizontal axis. The liquid301primarily fills up the tube103. It leaves the condenser100through an orifice205located on the bottom of the condenser100. Since gravity forces the liquid301to stratify on the bottom half of the condenser100, allowing for liquid301to leave through the bottom of the condenser100limits the build-up of liquid301inside the condenser100, both reducing the required refrigerant charge as well as maximizing the exposure of the condenser fins200to vapor300. Liquid301travels along the liquid tube103and enters the evaporator101through an orifice209, and then distributes onto the floor of the evaporator101. The flow path of the liquid301is depicted by arrows303. Since the liquid301enters the evaporator101through an orifice209located at the top of the evaporator101, it competes to allow vapor bubbles304to escape the evaporator101through this same orifice209. The vapor bubbles304accumulate into larger plugs in the liquid tube103and flow back to the condenser100, and through the liquid orifice205in the condenser100, where the vapor300also competes to enter the condenser100, as liquid301exits. Since vapor300is accumulated in this tube103, it is necessary that any tube bends do not prevent significant vapor accumulation, where the vapor plugs may block liquid301from returning to the evaporator101entirely and cause a dry-out condition.

The flow pattern that is produced by the competing flow of the vapor300and liquid301in liquid tube103is intermittent, meaning that liquid301is supplied to the evaporator101as a series of slugs. This flow pattern is the same behavior that can be observed when turning over a soda bottle and observing the intermittent liquid flow leaving the bottle. Between liquid slugs supplied, there is a liquid starvation period, which must be overcome, which is discussed in a subsequent portion of this section. The liquid starvation period is the duration of time that no liquid is supplied to the evaporator101. The benefit of the unsteady liquid supply is that the evaporator fins201are only partially submerged in liquid301, allowing maximum solid/liquid/vapor contact and high evaporation heat transfer coefficients. A cross-sectional view showing the liquid301stratification in the evaporator101is depicted inFIG. 6. Liquid301primarily enters the evaporator101through an orifice209at one end and vapor300primarily leaves an orifice210at the other end after passing along channels created by fins201. The backflow of a vapor bubble304into the liquid tube103is represented as well, since vapor300is present on the top half of the evaporator101.

Since liquid301and vapor300both enter and exit an orifice209that is smaller than the width of the evaporator101, there is a need to allow for liquid301to distribute along the base and vapor300to collect along the top of the evaporator101. A close up of an evaporator fin201is represented inFIG. 7. This fin201has liquid channels202that allow liquid301to distribute across the fins201, so that every fin201is wet, to allow for evaporation. These channels202are repeated along the fins201, so that liquid301can easily distribute throughout the evaporator101, and help allow liquid301to easily flow to parts of the evaporator101experiencing a high heat flux. The evaporator fins201also have larger channels203near the top of the fin201to allow for vapor300to distribute along the fins201and easily flow to the orifice210. These vapor channels203allow for the fin density to increase, while reducing or eliminating the situation where a flow instability may occur due to the rapid expansion of a vapor bubble in a confined space (refer back toFIG. 2and the explanation in the background section). The combination of the liquid301and vapor300distribution allow for a steady supply of liquid301to the fins201as well as a steady removal of vapor300.

The evaporator may also have vertical ribs204imprinted into the fins201to form a corner in which liquid301may be pulled up by capillarity. As liquid301is pulled up, the length of the solid/liquid/vapor contact will increase and provide additional ability to vaporize liquid at low fin temperature elevation over the saturation temperature of the liquid301and vapor300mixture.

The aforementioned “steady” supply of liquid to the evaporator can be achieved if there is a large enough amount of liquid stored in the evaporator to overcome the unsteady delivery of liquid. The mass, mstorage, of the liquid stored in the evaporator should be greater than the mass of liquid that is vaporized during the starvation period, τstarvationas depicted in EQ 1, where the latent heat of vaporization is hfg. The higher the maximum heat load, Q, the greater the liquid reservoir that is required.

The concept of liquid storage in the evaporator is very important in many applications, including electronics applications, since the internal volume inside the evaporator is small and the power can be relatively high. There are situations where all the liquid in the evaporator can be vaporized in less than a single second. If the required liquid storage is not properly accounted for, the evaporator can dry-out and lose its functionality.

While evaporator performance is improved by balancing liquid delivery without flooding or starving the evaporator with liquid, condenser performance is improved by keeping as much of the fins exposed to vapor as possible. A cross-sectional view of the condenser100is presented inFIG. 8, in which vapor enters orifices206flows outward302along the fins200, cuts through openings211(not shown inFIG. 8, but described in detail below) created in the fins200and then flows inward305to the liquid exiting orifice205. The vapor helps to push liquid along with it, and prevent too much accumulation of liquid. The outward vapor flow302and inward vapor flow305are separated by a single fin207with openings only located at the far left and far right, as depicted inFIG. 8, forcing vapor to flow as depicted.

The vapor flow pattern within the condenser100may be varied, depending on vapor and tube routing requirements, allowable condenser depth and heat source location. For instance, vapor can simply flow from left to right, or even as a “Z” pattern.

The aforementioned openings211in the condenser fin200are depicted inFIG. 9. These openings211allow vapor to pass through while maintaining structural strength to withstand high internal pressures. At the inlet and outlet orifices, the fin200can have a cutout208allowing unobstructed vapor distribution (at the inlet) and liquid collection (at the outlet). Additionally, these fins200have dimples212which provide a means to reduce the thickness of the film of liquid created as vapor condenses on the surface and travels down the fin200. The dimple212creates a convex surface at its peak. The liquid's surface tension, in conjunction with the dimpled surface creates a relatively high capillary pressure. As the dimple212gradually merges into the flat surface of the fin200, the curvature continuously changes from a convex surface to a concave surface to a flat surface. In the regions where the curvature is changing, the capillary pressure changes, causing a pressure gradient in the liquid film. This pressure gradient drives the liquid from the relative high pressure to the relative low pressures and acts as a thinning agent. As the film thickness decreases, so does the temperature difference between the saturation temperature of the liquid and vapor mixture to the cooler fin temperature.

While determining sizing of the internal tube diameters, and maximum supported power, one can use the height difference from the bottom of the condenser to the top of the evaporator as the maximum pumping head potential of the system. The hydrodynamic losses along the tubes, condenser and evaporator may be estimated by determining the velocity of the fluids passing through. Since the flow pattern is transient, an experimental determination of the operating characteristics, such as maximum supported power before liquid cannot return to the evaporator is likely required. The details of the embodiment presented allow for the use of a higher pressure working refrigerant, such as R134a, R1234yf, R1234ze, R410a, or R290, at operating conditions of approximately −10 C to 85 C, which is the approximate range required for most electronics devices. The benefit of higher pressure refrigerants is that the vapor densities are greater, leading to lower vapor velocities and smaller tube diameters. Additionally, the volume of non-condensable gas within the system is compressed and takes up less volume, thereby limiting any adverse effects it may cause. Finally, leaks tend to go outward, and the use of valves may be considered, since the permeation of air through an elastomer O-ring is of minimal concern.

Another embodiment of the present invention is presented inFIG. 10. This embodiment has a condenser100, and two evaporators101on the same side of the condenser100. The evaporators100are fluidly coupled to the condenser with a vapor tube102and a liquid tube103. Integrated into each evaporator101are mounting hardware108, consisting of springs and screws, to couple the evaporator101to a heat generating device.

An isometric view of the evaporator with a transparent top lid214is presented inFIG. 11. The lid214has two orifices210near the center of the lid214which allow vapor to enter the vapor tube102. At the front and rear end of the lid214are two additional orifices209which allow liquid to enter the evaporator101from the liquid tube103. The use of multiple orifices (209&210) reduces pressure loss, which allows more power to be supported with limited liquid gravitational pressure head to drive the flow. In the evaporator101is a fin stack201, creating rectangular channels inside the evaporator with cross-cuts allowing vapor and liquid to flow freely between the channels.

One challenge to this embodiment, in which the two evaporators101are serially connected on a single side of the condenser100, is an increased sensitivity to vapor backflow through the liquid tube103. This vapor backflow, while in some situations is desired, can impede liquid from reaching the evaporator101, causing a dry-out situation. To limit the degree in which vapor is allowed to backflow through the liquid tube102, a vapor blocking fin213may be added to the fin stack. A view of the vapor-blocking fin213is presented inFIG. 12. Similar to the other evaporator fins201, the vapor blocking fin213has liquid cut-outs202, allowing liquid to freely pass through. The vapor blocking fin213removes the vapor cut-outs203, limiting or preventing vapor to freely flow past this fin213. In the space between the two vapor blocking fins213, the liquid and vapor will be stratified, as vapor tends to stay on the top. In order to better prevent vapor from crossing the vapor blocking fin213, the height of the liquid cut-outs202should be lower than the liquid height inside the evaporator101.

For a specific application, the design of the vapor blocking fin213may be tuned for a specific power range, by partially blocking the vapor cut-outs203. Another design consideration is the location of the liquid orifices209in the evaporator, relative to the vapor orifices210.

Yet another embodiment of the present invention is presented inFIG. 13, consisting of an evaporator101and a condenser100located above the evaporator101, a vapor channel102connecting the evaporator101to the condenser100and a liquid channel103connecting the condenser100to the evaporator101. In some embodiments, the liquid channel102and vapor channel103generally travel along a horizontal axis. However, in this embodiment, the liquid channel102and vapor channel103have vertical axes.

A cross section of the condenser100of the foregoing embodiment is presented inFIG. 14. This cross-section is located towards the bottom of the condenser fins200, exposing the cut-outs208adjacent to the liquid orifice205and vapor orifice206in the condenser100. The fluid flow306path inside the condenser100travels in a mirrored circular flow pattern. There is a dividing fin207that has no cut-outs through the center portion, separating flow that goes in opposite directions. Additionally, there is another added barrier215located between the liquid orifice205and vapor orifice206, preventing short-circuiting of the flow inside the condenser100.

A cross-sectional view of the evaporator101of the foregoing embodiment is presented inFIG. 15. In this embodiment, the liquid entry orifice209and vapor exit orifice210are located along the same channels formed by the evaporator fins201. The vapor backflow through the liquid orifice209is controlled by a solid barrier215. This barrier215blocks the top portion of the channels, but allows the bottom portion of the channels to be open. When the bottom portion of this barrier215is below the stratified liquid level inside the evaporator101, it can limit or prevent vapor backflow. The barrier215may extend across all of the channels, or just some of the channels, depending upon the permissible amount of vapor backflow.

Another embodiment of the thermosiphon of the present invention is presented inFIG. 16. In this embodiment, the evaporator and condenser are combined into a single evaporator/condenser109module. Fins107are attached to the evaporator/condenser109and allow air to pass through to remove heat. The core of the evaporator/condenser consists of a top piece, a bottom piece and internal fins216(not shown inFIG. 16, but described in detail below). The internal fins216are bonded to the top and bottom piece, and create internal channels. The internal fins216have several cross-cuts allowing liquid and vapor to flow across the channels. Heat is applied through the bottom piece, and removed through the top piece of this embodiment.

A cross-section of the evaporator/condenser109is presented inFIG. 17. This cross-section cuts through the internal fins216. The vapor and liquid flow in the same counter-rotating flow paths306. In this embodiment, heat is applied to the central region218of the bottom piece. The vapor flow306starts from this central region218, as liquid vaporizes as a result of the heat input. Since heat is removed from the entire region, condensation occurs along each and every flow channel. The flow pattern is driven by a flow control fin217. In the region adjacent to the central region218, liquid is allowed to flow307through the flow control fin217through liquid cut-outs202while vapor is not. The difference of liquid height on either side of this fin provides the gravitational pressure head needed to circulate the refrigerant flow306.

The flow control fin217may be divided up into several regions, which can be designed to dictate how the refrigerant will flow inside the evaporator/condenser109. A front view of this fin is presented inFIG. 18. The flow control fin217is made up in three distinct section types. The liquid cross section308, has liquid cut-outs202, but no vapor cut-outs203, thus only allowing liquid to pass through, since the vapor is stratified towards the top portion of the fin. The second portion is the flow separation region309. There are no vapor203nor liquid cut-outs202in this region. The flow separation region309allows isolation of countering flow currents. The third region is a flow crossing region310, which allows both vapor and liquid to pass through their respective cut-outs (202,203). This region may be utilized to allow the refrigerant flow to change directions.

It is possible to design an evaporator/condenser109without a flow control fin217, however the channel height typically needs to be higher, since liquid and vapor will flow counter to each other, which requires a larger gravitational pressure head to overcome the fluid flow losses.

While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions “in one embodiment” or “in another embodiment” are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms “a”, “an” and “the” mean “one or more” unless expressly specified otherwise. The term “connected” means “communicatively connected” unless otherwise defined.

When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device.

In light of the wide variety of methods for an intermittent thermosyphon known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto.

None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.