System and method of boiling heat transfer using self-induced coolant transport and impingements

According to one embodiment of the invention, a cooling system for a heat-generating structure comprises a chamber and structure disposed within the chamber. The chamber has an inlet and an outlet. The inlet receives fluid coolant into the chamber substantially in the form of a liquid. The outlet dispenses the fluid coolant out of the chamber at least partially in the form of a vapor. The structure disposed within the chamber receive thermal energy from the heat generating structure and transfers at least a portion of the thermal energy to the fluid coolant. The thermal energy from the heat-generating structure causes at least a portion of the fluid coolant substantially in the form of a liquid to boil and effuse vapor upon contact with a portion of the structure. The effusion of vapor creates a self-induced flow in the chamber. The self-induced flow distributes non-vaporized fluid coolant substantially in the form of a liquid to other portions of the structure.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of cooling systems and, more particularly, to a system and method of boiling heat transfer using self-induced coolant transport and impingements.

BACKGROUND OF THE INVENTION

A variety of different types of structures can generate heat or thermal energy in operation. To prevent such structures from over heating, a variety of different types of cooling systems may be utilized to dissipate the thermal energy. To facilitate the dissipation of such thermal energy in such cooling systems, a variety of different types of coolants may be utilized.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a cooling system for a heat-generating structure comprises a chamber and structure disposed within the chamber. The chamber has an inlet and an outlet. The inlet receives fluid coolant into the chamber substantially in the form of a liquid. The outlet dispenses the fluid coolant out of the chamber at least partially in the form of a vapor. The structure disposed within the chamber receive thermal energy from the heat generating structure and transfers at least a portion of the thermal energy to the fluid coolant. The thermal energy from the heat-generating structure causes at least a portion of the fluid coolant substantially in the form of a liquid to boil and effuse vapor upon contact with a portion of the structure. The effusion of vapor creates a self-induced flow in the chamber. The self-induced flow distributes non-vaporized fluid coolant substantially in the form of a liquid to other portions of the structure.

Certain embodiments of the invention may provide numerous technical advantages. For example, a technical advantage of one embodiment may include the capability to enhance heat transfer in a coolant stream. Other technical advantages of other embodiments may include the capability to utilize pin fin configurations to alter the heat transfer phenomenology and thereby enhance the transfer of thermal energy. Yet other technical advantages of other embodiments may include the capability to reduce a flow requirement into a chamber utilized for heat transfer.

Although specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.

It should be understood at the outset that although example embodiments of the present invention are illustrated below, the present invention may be implemented using any number of techniques, whether currently known or in existence. The present invention should in no way be limited to the example embodiments, drawings, and techniques illustrated below, including the embodiments and implementation illustrated and described herein. Additionally, the drawings are not necessarily drawn to scale.

FIG. 1is a block diagram of an embodiment of a cooling system10that may be utilized in conjunction with other embodiments disclosed herein, namely surface enhancement embodiments described with reference toFIGS. 3A-4DandFIGS. 6A-6C. Although the details of one cooling system will be described below, it should be expressly understood that other cooling systems may be used in conjunction with embodiments of the invention.

The cooling system10ofFIG. 1is shown cooling a structure12that is exposed to or generates thermal energy. The structure12may be any of a variety of structures, including, but not limited to, electronic components, modules, and circuits. Because the structure12can vary greatly, the details of structure12are not illustrated and described. The cooling system10ofFIG. 1includes channels23and24, pump46, inlet orifices47and48, a condenser heat exchanger41, an expansion reservoir42, and a pressure controller51.

The structure12may be arranged and designed to conduct heat or thermal energy to the channels23,24. To receive this thermal energy or heat, the channels23,24may be disposed on an edge of the structure12or may extend through portions of the structure12, for example, through a thermal plane of structure12. In particular embodiments, the channels23,24may extend up to the components of the structure12, directly receiving thermal energy from the components. Further, in particular embodiments the channels23,24may be cold walls. Although two channels23,24are shown in the cooling system10ofFIG. 1, one channel or more than two channels may be used to cool the structure12in other cooling systems.

In operation, a fluid coolant flows through each of the channels23,24. As discussed later, this fluid coolant may be a two-phase fluid coolant, which enters inlet conduits25of channels23,24in liquid form. Absorption of heat from the structure12causes part or all of the liquid coolant to boil and vaporize such that some or all of the fluid coolant leaves the exit conduits27of channels23,24in a vapor phase. To facilitate such absorption or transfer of thermal energy, the channels23,24may be lined with pin fins or other similar surface enhancement devices which, among other things, increase surface contact between the fluid coolant and walls of the channels23,24. Further details of the surface enhancement configurations, namely pin fin configurations, are described below with reference toFIGS. 3A-4DandFIGS. 6A-6C.

The fluid coolant departs the exit conduits27and flows through the condenser heat exchanger41, the expansion reservoir42, a pump46, and a respective one of two orifices47and48, in order to again to reach the inlet conduits25of the channels23,24. The pump46may cause the fluid coolant to circulate around the loop shown inFIG. 1. In particular embodiments, the pump46may use magnetic drives so there are no shaft seals that can wear or leak with time.

The orifices47and48in particular embodiments may facilitate proper partitioning of the fluid coolant among the respective channels23,24, and may also help to create a large pressure drop between the output of the pump46and the channels23,24in which the fluid coolant vaporizes. The orifices47and48may have the same size, or may have different sizes in order to partition the coolant in a proportional manner which facilitates a desired cooling profile.

A flow56of fluid (either gas or liquid) may be forced to flow through the condenser heat exchanger41, for example by a fan (not shown) or other suitable device. In particular embodiments, the flow56of fluid may be ambient fluid. The condenser heat exchanger41transfers heat from the fluid coolant to the flow56of ambient fluid, thereby causing any portion of the fluid coolant which is in the vapor phase to condense back into a liquid phase. In particular embodiments, a liquid bypass49may be provided for liquid fluid coolant that either may have exited the channels23,24or that may have condensed from vapor fluid coolant during travel to the condenser heat exchanger41.

The liquid fluid coolant exiting the condenser heat exchanger41may be supplied to the expansion reservoir42. Since fluids typically take up more volume in their vapor phase than in their liquid phase, the expansion reservoir42may be provided in order to take up the volume of liquid fluid coolant that is displaced when some or all of the coolant in the system changes from its liquid phase to its vapor phase. The amount of the fluid coolant which is in its vapor phase can vary over time, due in part to the fact that the amount of heat or thermal energy being produced by the structure12will vary over time, as the structure12system operates in various operational modes.

Turning now in more detail to the fluid coolant, one highly efficient technique for removing heat from a surface is to boil and vaporize a liquid which is in contact with a surface. As the liquid vaporizes in this process, it inherently absorbs heat to effectuate such vaporization. The amount of heat that can be absorbed per unit volume of a liquid is commonly known as the latent heat of vaporization of the liquid. The higher the latent heat of vaporization, the larger the amount of heat that can be absorbed per unit volume of liquid being vaporized.

The fluid coolant used in the embodiment ofFIG. 1may include, but is not limited to mixtures of antifreeze and water. In particular embodiments, the antifreeze may be ethylene glycol, propylene glycol, methanol, or other suitable antifreeze. In other embodiments, the mixture may also include fluoroinert. In particular embodiments, the fluid coolant may absorb a substantial amount of heat as it vaporizes, and thus may have a very high latent heat of vaporization.

Water boils at a temperature of approximately 100° C. at an atmospheric pressure of 14.7 pounds per square inch absolute (psia). In particular embodiments, the fluid coolant's boiling temperature may be reduced to between 55-65° C. by subjecting the fluid coolant to a subambient pressure of about 2-3 psia. Thus, in the cooling system10ofFIG. 1, the orifices47and48may permit the pressure of the fluid coolant downstream from them to be substantially less than the fluid coolant pressure between the pump46and the orifices47and48, which in this embodiment is shown as approximately 12 psia. The pressure controller51maintains the coolant at a pressure of approximately 2-3 psia along the portion of the loop which extends from the orifices47and48to the pump46, in particular through the channels23and24, the condenser heat exchanger41, and the expansion reservoir42. In particular embodiments, a metal bellows may be used in the expansion reservoir42, connected to the loop using brazed joints. In particular embodiments, the pressure controller51may control loop pressure by using a motor driven linear actuator that is part of the metal bellows of the expansion reservoir42or by using small gear pump to evacuate the loop to the desired pressure level. The fluid coolant removed may be stored in the metal bellows whose fluid connects are brazed. In other configurations, the pressure controller51may utilize other suitable devices capable of controlling pressure.

In particular embodiments, the fluid coolant flowing from the pump46to the orifices47and48may have a temperature of approximately 55° C. to 65° C. and a pressure of approximately 12 psia as referenced above. After passing through the orifices47and48, the fluid coolant may still have a temperature of approximately 55° C. to 65° C., but may also have a lower pressure in the range about 2 psia to 3 psia. Due to this reduced pressure, some or all of the fluid coolant will boil or vaporize as it passes through and absorbs heat from the channels23and24.

After exiting the exits ports27of the channels23,24, the subambient coolant vapor travels to the condenser heat exchanger41where heat or thermal energy can be transferred from the subambient fluid coolant to the flow56of fluid. The flow56of fluid in particular embodiments may have a temperature of less than 50° C. In other embodiments, the flow56may have a temperature of less than 40° C. As heat is removed from the fluid coolant, any portion of the fluid which is in its vapor phase will condense such that substantially all of the fluid coolant will be in liquid form when it exits the condenser heat exchanger41. At this point, the fluid coolant may have a temperature of approximately 55° C. to 65° C. and a subambient pressure of approximately 2 psia to 3 psia. The fluid coolant may then flow to pump46, which in particular embodiments46may increase the pressure of the fluid coolant to a value in the range of approximately 12 psia, as mentioned earlier. Prior to the pump46, there may be a fluid connection to an expansion reservoir42which, when used in conjunction with the pressure controller51, can control the pressure within the cooling loop.

It will be noted that the embodiment ofFIG. 1may operate without a refrigeration system. In the context of electronic circuitry, such as may be utilized in the structure12, the absence of a refrigeration system can result in a significant reduction in the size, weight, and power consumption of the structure provided to cool the circuit components of the structure12.

Although components of one embodiment of a cooling system10have been shown inFIG. 1, it should be understood that other embodiments of the cooling system10can include more, less, or different component parts. For example, although specific temperatures and pressures have been described for one embodiment of the cooling system, other embodiments of the cooling system10may operate at different pressures and temperatures. Additionally, in some embodiments a coolant fill port and/or a coolant bleed port may be utilized with metal-to-metal caps to seal them. Further, in some embodiments, all or a portion of the joints between various components may be brazed, soldered or welded using metal-to-metal seal caps.

In boiling heat transfer, there are four general ways in which a fluid interacts with a surface: (1) pool boiling, (2) flow boiling, (3) jet impingement cooling, and (4) spray cooling. With pool boiling, a volume of coolant comes in contact with a heated surface and the coolant is not forced to flow over the surface using a pump or other forcible method. Rather, gravity feeds coolant to the heated surface to replenish the coolant that has turned to vapor. Pool boiling reaches a limit when the effusing vapor does not allow for adequate coolant replenishment. When this happens, a film of vapor develops between the heated surface and the body of coolant as the boiling action transitions to film boiling. As a result, a higher temperature of the heated surface is required to drive the heat through the vapor film for boiling to happen at the vapor-liquid interface. When this happens the heat transfer coefficient drops significantly resulting in poor transfer of heat.

With flow boiling, coolant is forced to flow across the heated surface. As a result, vapor bubbles are washed away allowing for liquid coolant to replenish the coolant that has vaporized. This approach, in general, supports higher heat fluxes and yields higher heat transfer coefficients than pool boiling. Flow boiling has a limit at which there can be so much vapor produced that even a forced flow of coolant can not ensure adequate wetting of the heated surface. Flow boiling can also transition to film boiling, but generally at significantly higher heat fluxes than pool boiling.

With jet impingement cooling, coolant is forced to impinge the heated surface due to the momentum of the coolant coming out of a jet. The coolant forcibly passes through any vapor bubbles or a vapor film to wet the heated surface and pushes the vapor bubbles outward along the heated surface. Once the coolant flow is away from the area of impingement, the coolant flow behaves like flow boiling as there is no more impingement action to penetrate a volume of bubbles or a vapor film.

Spray cooling creates a mist or stream of coolant drops that impact the heated surface. In this case a thin liquid film of coolant is “painted” on the surface. As the coolant vaporizes, it is replenished by a continuing stream of mist from the spay head. Because spray nozzles for spray cooling are typically positioned at an appreciable distance from the heated surface, spray cooling is unsuitable for particular applications, namely densely packaged electronics.

FIGS. 2A-2Cillustrate conventional coldplate configurations.FIG. 2Ashows straight fin stock90A that can be used in a coldplate;FIG. 2Bshows wavy fin stock90A that can be used in a coldplate; andFIG. 2Cshows a cross sectional view of a coldplate80with straight fin stock90A. With reference toFIG. 2C, thermal energy or heat from a structure is transferred to the coldplate80, then to the straight fin stock90A, and then to a cross-flowing coolant stream (illustrated by arrow82) within the coldplate80. To facilitate this flow, the coldplate80may include features such as an inlet84, an inlet passage86, a porous foam header87, a chamber85, and an outlet88. In operation, coolant generally flows parallel to the coldplate surface through the chamber85and through channels created by the walls of the straight fin stock90A.

For conventional two-phase coldplates to be efficient, coolant typically flows in excess of that needed for energy balance to ensure wetting of the surfaces by forcing the removal of created vapor—thereby preventing surfaces from transitioning to film boiling. In an energy balance, a flow rate of “1×” is the amount of flow necessary to absorb thermal energy with 100% conversion—that is, one unit of liquid is put in that totally goes to vapor and takes out all the heat. For coldplates that require low temperature gradients, the required flow rate can be many times that needed for energy balance (1×)—that is, flow rates on the order of 3×, 4×, 5×, or 6×. This is due to the confining action of the walls of the fin stock, which create channels that allow the effusing vapor to hold the liquid off the walls thus preventing wetting. Thus, such conventional configurations need very high flow rates to allow the momentum of the liquid stream to overcome the ability of the effusing vapor to hold the liquid trapped in the vapor stream.

To overcome the action of the vapor that prevents wetting of the walls, the above-referenced jet impingement cooling or spray cooling may be utilized. Although jet impingement and spray cooling work well, they are more complex and may not be able to be used due to packaging limitations. Accordingly, teaching of some embodiments of the invention recognize that pin fins configurations can be utilized to alter the heat transfer phenomenology and thereby enhance heat transfer. In some of such embodiments, when a small amount of liquid enters a field of pin fins, a violent, exploding, and chaotic reaction occurs as liquid vaporizes. This reaction disperses the unvaporized liquid as globs amongst the pin fins in a Brownian motion-like or stochastic-like manner. This dispersion can create jet impingement-like and spray-like qualities with a low flow rate.

FIG. 3Ais an isolated perspective view of a pin fin configuration110A andFIG. 3Bis a side cross-sectional view of a pin fin configuration110B that may be utilized in embodiments of the invention. In particular embodiments, the pin fin configurations110A,110B may be disposed within the channels23,24described with reference toFIG. 1. In other embodiments, the pin fin configurations110A,110B may be disposed in other heat transfer structures. For purposes of illustration, the pin fin configurations110A,110B will be described as being disposed in a channel operable to receive fluid. The pin fin configurations110A,110B ofFIGS. 2A and 2Bare examples of surface enhancements that may be utilized to enhance the transfer of thermal energy from a heat generating structure to a fluid. Other types of surface enhancements that may be utilized with other embodiments of the invention include, but are not limited to, conductive foam and conductive fibers.

With reference toFIGS. 3A and 3B, a plurality of pin fins113,115protrude from channel walls125and are arranged in pin fin configurations110A,110B. Pin fin configuration110A shows a staggered arrangement and pin fin configuration110B shows an inline arrangement.FIG. 3Badditionally shows a channel120with a fluid flow towards the pin fin configuration110B, indicated by arrow132, and a fluid flow away from the pin fin configuration110B, indicated by arrow134. In operation, thermal energy is transferred to the pin fins113,115(e.g., from the channel wall125to the pin fins113,115) and to a fluid traveling through the channel, for example, channel120. In particular embodiments, the pin fin configurations110A,110B may be utilized to enhance boiling heat transfer. In such embodiments, liquid fluid coolant (e.g., traveling in direction of arrow132towards the pin fins113,115) comes in contact with the pin fins113,115and is boiled and vaporized. The vaporized fluid coolant (e.g., traveling away from the pin fins in direction of arrow134) inherently contains the thermal energy transferred from the pin fins113,115to the fluid coolant during vaporization. As described in further details below, the fluid flow in particular embodiments may be relatively slow, allowing a reduction in components that are used to create the fluid flow (e.g., pumps, conduits, etc.). Further, in particular embodiments, components may not be necessary to create a fluid flow, for example, in gravity-fed embodiments.

In particular embodiments, pin fins113,115, may be used inside a coldplate, allowing the coolant to move in multiple directions since there are no confining walls. At high heat fluxes, the effusing vapor creates a violent and near chaotic flow with liquid entrained in the vapor. Because the effusing vapor is confined to the core of the coldplate, its motion is internal to the core thus forcing it over the pin fins113,115. As the vapor effuses, it transports unvaporized liquid coolant in a near chaotic swirl, which results in the pin fins being impinged with liquid coolant. The vapor produced at all sites of the pin fins113,115collectively energizes the swirl of vapor with liquid embedded in it. Additionally, globs of liquid coolant (e.g., formed from the vaporization of other liquid coolant) are thrown against downstream pin fins113,115—creating a spray cooling-like quality. Accordingly, the pin fin configurations110A,110B allow a cross flowing coolant to be used while taking advantage of the attributes of jet impingement and spray cooling, which are provided by the chaotic cross flowing liquid impacting the pin fins113,115. Such a phenomenology in particular embodiments may result in a need for less coolant flow rate because the effusing vapor self induces the flow of coolant and creates a form of spray cooling or jet impingement cooling as the liquid impinges the pins. In other words, the spraying effect in particular embodiments is self-induced by the effusing vapor—that is, the energizing means for the spraying effect is the effusing vapor itself.

As referenced above, in particular embodiments relatively small flow rates may be used with pin fin configurations. In some of such embodiments, reduced pumping requirements, reduced sizes of flow lines, and the like may be achieved. Generally, a flow rate of “1×” is the amount of flow necessary to absorb thermal energy with 100% conversion—that is, one unit of liquid is put in that totally goes to vapor and takes out all the heat. To ensure proper wetting, for example in conventional configurations described above, flow rates must typically be on the order of 3×, 4×, 5×, or 6×. Such flow rates provide excessive amount of liquids. In particular embodiments, such conventional configurations can be contrasted with pin fin configurations in which the chaotic action reduces the flow rate towards 1×. For example, in particular embodiments, flow rate requirements for pin fin configurations can be 1.1×, 1.2×, or 1.3×. Thus, a dribble of liquid can be provided at the opening of the chamber housing the pin fins. Upon the liquid contacting the pin fins, the chaotic reaction occurs dispersing the liquid in a Brownian motion-like manner amongst all pin fins in the chamber.

A further benefit of the above referenced distribution in particular embodiments is small temperature differentials amongst the pin fins. Because the chaotic reaction creates Brownian motion-like distributions, the—result is a near-equal energy transfer distribution amongst pin fins—that is, each of the pins fins is allowed to transfer its thermal energy to the fluid in more equal-like manner. Thus, the chaos of the effusing action creates order in the thermal energy transfer. In particular embodiments, temperature differentials amongst pin fins can be on the order of one degrees Celsius.

A yet further benefit of the above referenced distribution in particular embodiments is prevention of excess fluid. Excess fluid is created by either excess flows or spray nozzles that spray outside of an effective thermal transfer area. In particular embodiments, fluid slightly above that needed for energy balance (1×) may be placed in the chamber of pin fins and the chaotic action will equally distribute the fluid amongst the pin fins.

In particular embodiments, no powering mechanism (e.g., a pump or the like) is needed to create a flow in the system. Rather, flow may be gravity fed to the pins fins (as is seen inFIG. 7) where the liquid is vaporized. The vaporized liquid travels away from the pin fins, condenses in a separate area of the system and then is gravity fed back to the pin fins. Systems similar to this are sometimes referred to as thermal siphons (as is seen inFIG. 8).

The pin fins113,115may be made of a variety of materials and may take on a variety of sizes and shapes. In this embodiment, the pin fins are made of a nickel plated copper and vary in size from 0.04 inches high to 0.1675 inches high. The pin fins113,115are shown with a columnar shape. In other embodiments, the pin fins113,115may be made of other materials, may have heights less than 0.04 inches, may have heights greater than 0.1675 inches, and may have shapes other than columnar shapes. Additionally, in other embodiments the pin fins113,115may be arranged in configurations other than inline or staggered configurations. Further, in particular embodiments, the pin fins may be made of a conductive foam or conductive fiber.

FIGS. 4A, 4B, 4C, and 4Dshow pin fin configurations110A,110B,110C, and110D, according to embodiments of the invention. Pin fin configuration110A ofFIG. 4Ais an inline configuration, pin fin configuration110B ofFIG. 4Bis an inline configuration, pin fin configuration110C ofFIG. 4Cis an inline configuration with square columns, and pin fin configuration110D ofFIG. 4Dis an inline configuration with long tubular columns. The pin fin configurations110A,110B,110C, and110D ofFIGS. 4A, 4B, 4C, and 4Dillustrate only some of the many configurations that may be utilized, according to embodiments of the invention.

FIGS. 5A-6Cshow a comparison of two heat load test articles.FIGS. 5A, 5B, and 5Cillustrate a conventional forced flow configuration130and it associated parameters.FIGS. 6A, 6B, and 6Cillustrate a pin fin configuration140and its associated parameters. The forced flow configuration130shown inFIGS. 5A and 5Buses a forced flow of cross-flowing coolant that is supplied to each of sixteen heated blocks135.FIG. 5Ashows one side of the test assembly andFIG. 5Bshows the opposite side of the test assembly.

The pin fin configuration140ofFIGS. 6A and 6Bshow pin fins on each of sixteen heated blocks145. The pin fins create the chaotic reaction referenced above. In comparing the parameters associated with each as shown inFIGS. 5C and 6C, the assembly ofFIGS. 6A and 6Bhas almost 82% less coolant (140 ml/min versus 800 ml/min) for the 800 Watt heat load. Further, the maximum temperature differentials of the transmit/receive (TR) sites is 2.8 degrees Celsius in theFIG. 5Cand 1.9 degrees Celsius in theFIG. 5Cfor the 800 Watt heat load. Thus, in particular embodiments, the assembly ofFIGS. 6A and 6Bwould be preferred over the assembly ofFIGS. 5A and 5B.

The embodiments of pin fin configurations may be utilized in a variety of configurations including but not limited to:1) High Power Phased Arrays—for high powered phase arrays to cool the transmit-receive (TR) functions and power supplies.2) Cooling of Electronic Chassis—for removing heat from the cold walls of electronic chassis.3) Micro Channel Cooling—Micro channel cooling may be used to cool single electronic chips, small circuit assemblies, and device die.4) Heat Exchangers—There are two-phase heat exchangers built using straight and wavy fin stock. Some of these may perform better and be less expensive if built using pin fins.

Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformation, and modifications as they fall within the scope of the appended claims.