Self-Purging Thermosiphon

This non-provisional application claims priority based upon prior U.S. Provisional Patent Application Ser. No. 63/654,467 filed May 31, 2024, in the names of Jeremy Rice and Katherine Carpenter entitled “SELF-PURGING THERMOSYPHON,” the disclosures of which are incorporated herein in their entirety by reference as if fully set forth herein.

Conventional passive heat transfer devices, such as heat pipes and thermosyphons, are widely used in numerous applications, such as the cooling of electronic devices. Heat pipes are two-phase heat transfer devices in which a working fluid circulates between an evaporator and a condenser. The liquid phase is transported from the condenser back to the evaporator through capillarity generated by a porous wick structure that tubes the internal walls of the heat pipe. This wick creates a capillary pressure gradient sufficient to overcome gravitational and flow resistance forces, enabling passive fluid movement without mechanical pumps. Upon reaching the evaporator, the liquid absorbs heat and vaporizes, and the resulting vapor flows to the condenser where it releases latent heat and condenses back into liquid, completing the cycle. However, the use of a wick in this device produces a high-pressure loss which limits the maximum heat transport distance and/or power that can be supported before dry-out occurs.

By contrast, a thermosyphon is a gravity-driven, two-phase heat transfer system that operates based on natural convection and phase change. In this system, a working fluid is contained within a sealed loop comprising an evaporator, a vapor conduit, a condenser, and a liquid return path. During operation, heat is applied to the evaporator, causing the working fluid to absorb thermal energy and vaporize. The generated vapor rises through a vapor conduit toward the condenser, which is typically elevated relative to the evaporator. At the condenser, the vapor releases latent heat through a cooling medium and condenses back into liquid form. The condensed liquid collects at the base of the condenser or optionally in a separate accumulator designed to manage fluid buffering. Under the influence of gravity, this accumulated liquid returns to the evaporator through a liquid tube, completing the thermodynamic cycle without the need for mechanical pumps. The evaporators in these devices are typically pool boiling devices with an enhanced surface that may consist of fins, a porous layer or, in some instances, 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.

Referring now toandwhich illustrate one shortcoming of conventional thermosyphon systems known in the art. Specifically, if a relative low spot in the vapor tubeexists due to system level design requirements, the system can be unstable and not function properly. The initial condition, where zero heat is dissipated in the heat-generating device, like a central processing unit (CPU), is presented in. Liquid, occupies a substantial portion of the liquid tube, the vapor tubeand the evaporator. The condenseris filled with vapor, as well as an optional accumulator. The accumulatoris useful for liquid management, when the volume of liquidin the system could flood the condenser. The liquid-vapor lineshows the height of the liquidat an initial condition where no heat load is applied to the system. Below liquid-vapor line, liquidfills both the liquid tubeand the vapor tube.

illustrates the distribution of liquidand vaporin the thermosyphon system shortly after thermal energy is applied to the heat-generating component. As heat is transferred to the evaporator, the liquidwithin the evaporator undergoes a phase change from liquidto vapor, generating an expanding vapor front. This accumulation of vaporat the upper region of the evaporatorincreases local pressure and displaces liquidupstream in the vapor tube. The advancing vapor-liquid interfacein the vapor tubepushes the entrained liquidthrough the vapor tubewhich, due to the presence of a geometrical low point, imposes an adverse hydrostatic pressure head (H) that resists forward motion of the vapor.

Simultaneously, this expansion of vaporalso acts on the liquid-vapor interfacewithin the evaporator, displacing liquidthrough the liquid return tubetoward the accumulator. This motion produces a favorable gravitational pressure head (H), aiding the return of liquid from the condenser. However, in configurations where the adverse head H(caused by liquid pooling at the vapor tube low point) exceeds the favorable head H, the pressure differential opposes intended vapor flow. Consequently, liquidwithin the evaporatoris displaced below the level of the heat-generating device, leading to partial or complete exposure of the device. Without full submersion, boiling cannot be sustained, significantly reducing heat transfer efficiency and causing a rapid rise in the temperature of the heat-generating device.

There is a need, therefore, for a method and system for initiating proper flow in thermosyphon systems, such as those used in electronics cooling, where flexible or geometrically constrained tubing introduces low points in vapor tubes, so that adverse pressure heads that prevent vapor movement and disrupt startup can be avoided.

Embodiments of this invention relate to thermosyphon systems designed for two-phase heat transfer applications, particularly those used in electronics cooling where flexible or geometrically constrained tubing may introduce low points in vapor tubes. Traditional thermosyphons can fail to initiate flow properly if liquid accumulates in these low points, generating adverse pressure heads that prevent vapor movement and disrupt startup. Various embodiments of the present invention address this challenge by integrating fluidic design features such as a submerged liquid inlet in the evaporator, a liquid-vapor separator, and a purge tube with a defined elevation profile. These features facilitate passive drainage of trapped liquid, allowing vapor to clear the tubes and establish directional flow without requiring mechanical pumps or external intervention.

The system operates by leveraging gravity and phase-change dynamics to balance pressure heads and manage fluid distribution throughout various phases of operation. During startup, vapor generation in the evaporator displaces liquid through both vapor and liquid tubes, with excess liquid diverted to an accumulator or returned through the purge system. The purge tube's elevation and hydraulic characteristics are designed to deactivate once steady-state flow is achieved, preventing backflow and conserving thermal efficiency. This configuration enhances reliability, supports a variety of layout constraints, and maintains optimal cooling performance, even in compact or flexible systems. The invention is adaptable to multiple embodiments, including those where the condenser itself provides the volume necessary for liquid management, eliminating the need for a separate accumulator.

The foregoing has outlined rather broadly certain aspects of the present invention in order that the detailed description of the invention that follows may better be understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

Various embodiments of the present invention are directed to improved methods and systems for, among other things, improving the operation of a 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 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.

illustrates the thermosyphon system according to a first embodiment of the present invention, shown under static, non-operational conditions (i.e., prior to application of heat to the heat source). The system comprises an evaporatorthermally coupled to a heat-generating device, such as a CPU, to facilitate direct thermal conduction. In a preferred configuration, heat-generating deviceis positioned vertically as shown in. The evaporatoris fluidly connected to a condenservia a vapor conduit or vapor tube, which provides a path for vapor-phase working fluidduring operation. A separate liquid return path is established through a liquid tubethat fluidly couples the evaporatorto an accumulator, which in turn is connected to the base of the condenserto receive condensed working fluid.

In the startup condition, the internal volumes of the evaporator, substantial portions of the vapor tube, and the liquid return tubeare occupied by liquid-phase working fluid. The remaining internal volumes of the system, including the accumulator, condenser, and the residual space in both the vapor tubeand liquid tube, are filled with vapor-phase working fluid, thus maintaining a saturated two-phase equilibrium.

Note that, as shown in, the geometry of the fluid tubes incorporates one or more localized vertical minima, such that the vapor tubeincludes a low-point elevation that permits pooling of liquidin the vapor tubeunder gravitational forces. A similar low-point configuration exists in the liquid tube. These features reflect practical realities of system installation, such as flexible tubing or space-constrained layouts, and introduce challenges to startup operation due to adverse hydrostatic pressure effects. Notably, the liquid tubeconnects to the evaporatorat a port located on the top region of the evaporatorhousing. This inlet design is relevant for maintaining liquid stability during early heat-up and for facilitating proper pressure balance across the system. Note that, although not required, the liquid tubemay be submerged slightly into the liquid reservoir, however it should not be submerged past the top level of the heat-generating device.

depicts the thermodynamic and fluid dynamic behavior of the first embodiment shortly after thermal energy is introduced to the heat-generating device. Upon activation, heat is conducted into the base of the evaporator, initiating phase change of the working fluid from liquidto vapor. The resulting vaporaccumulates in the upper region of the evaporatorand exits through the vapor tube. This expanding vapor volume displaces liquidalong both the vapor and liquid paths, thereby advancing two liquid-vapor interfaces: interfacewithin the vapor tubeand interfacewithin the liquid tube.

The movement of liquidin the vapor tube, displaced by the upwardly migrating vapor, establishes an adverse hydrostatic pressure head, designated as H. This pressure head resists vapor flow due to the gravitational potential energy required to overcome the local minimum in the vapor tube'selevation profile. In contrast, the vapor pressure generated in the evaporatoralso forces liquidthrough the liquid return tubeand into the accumulator, generating a favorable hydrostatic pressure head H.

One feature of this embodiment is that the liquid tubeconnects to the top of the evaporator. This routing effectively stabilizes the liquidin the evaporatorduring the critical startup phase by trapping a volume of liquidabove the heat-generating component, thereby preventing premature depletion.

Due to the elevation profile of the system, the liquid tubeincorporates a local minimum that amplifies Hby increasing the vertical height difference relative to the accumulator. Consequently, Hexceeds Hv, producing a net positive pressure differential that initiates the desired circulation pattern. As further illustrated in, this results in the onset of steady-state flow conditions: vaporflows from the evaporatorto the condenservia the vapor tube, while condensed liquidreturns from the condenserthrough the liquid tubeback to the evaporator. This balance of pressure heads ensures reliable startup without external priming or mechanical assistance.

During the startup phase, the recession of the liquid-vapor interfacewithin the evaporatoris primarily driven by the phase transition of the working fluid from liquidto vaporin response to thermal input from the heat-generating device. As heat is conducted into the liquid, latent heat of vaporization is absorbed, causing the liquidto vaporize and expand significantly in volume. Due to the large disparity in density between the liquid and vapor phases of typical working fluids, often on the order of 100:1, the vapor generated from a small volumetric quantity of liquid occupies a much greater space. For example, vaporizingmilliliter of liquid can produce approximately 100 milliliters of vapor under standard thermosyphon operating conditions.

This rapid volumetric expansion of vaporplays a role in system dynamics during startup. As vapordisplaces liquidfrom the evaporatortoward the vapor tubeand liquid tube, it is essential that the liquidabove the heat-generating deviceremain sufficient to maintain wetting and submersion of the heat-generating device's surface. Submersion ensures continued boiling, which is relevant for efficient thermal energy transfer during this transient state.

To prevent premature dry-out and potential thermal overshoot, the evaporatormust be geometrically configured to retain a minimum volume of liquidabove the heat-generating devicethroughout initial vapor generation. Based on empirical modeling and vapor production characteristics of common refrigerants, a practical design criterion is that the fluid volume contained in the portion of the evaporatorlocated above the heat-generating device should exceed approximately 1% of the total internal volume of the vapor tube. This ratio ensures that sufficient liquid buffering is maintained during the early moments of startup, allowing vapor pressure to build without compromising thermal contact at the evaporator surface.

illustrates a second embodiment of the present invention under static, pre-operational conditions, i.e., before any thermal input is applied to the system. In this configuration, the heat-generating componentis thermally interfaced with the underside of the evaporatorto promote efficient heat transfer into the working fluid housed within the evaporator. The system is initially charged with a two-phase working fluid such that a substantial portion of the lower segment of vapor tubeis filled with liquid-phase fluid. This condition is often a result of gravitational settling during charging or extended system shutdown.

Due to the geometric routing of the vapor tube, its upper portion connects to the condenserat a significantly higher elevation relative to its lower portion. This elevation difference introduces a risk of forming a large adverse hydrostatic pressure head (Hv) during startup. Specifically, any vapor generated in the evaporatormust displace the denser liquidresiding in the lower segment of the vapor tubein order to reach the condenser. If unaddressed, this condition can obstruct vapor flow, delay startup, and destabilize system operation.

To mitigate this challenge, this embodiment incorporates a liquid-vapor separatorand an associated purge tube. The separatoris strategically located near the same elevation as the accumulatorand is fluidly coupled to both the lower and upper segments of the vapor tube. The purge tubeprovides a low-resistance gravitational return path from the bottom of the separatorto the accumulator. In the initial condition, the lower vapor tube, the liquid return tube, and the purge tubeare predominantly filled with liquid, while the condenserand upper portions of the system contain vapor. This configuration enables passive drainage of excess liquidfrom the vapor tubeinto the separatorand subsequently through the purge tube, preventing vapor entrapment and establishing favorable pressure conditions for startup.

In this second embodiment, the liquid-vapor separatoris strategically positioned at approximately the same vertical elevation as the accumulatorto facilitate gravitational fluid return and minimize hydrostatic pressure imbalances during transient phases. The lower section of the vapor tubeis fluidly connected at one end to the evaporatorand at the other end to the inlet at the bottom of the liquid-vapor separator. This routing enables liquidand vaporexiting the evaporatorto be directed into the separatorwithout imposing significant elevation-driven resistance to flow.

The liquid-vapor separatorfunctions as a passive phase stratification chamber, wherein the incoming two-phase mixture from the lower vapor tubeis allowed to decelerate and separate under the influence of gravity. Due to density differences, liquidsettles toward the base of the separator, while vaporrises and exits through an outlet located at the top of the separator. This outlet is fluidly coupled to the upper portion of vapor tube, which directs the purified vapor stream to the condenserfor heat rejection.

To manage excess liquid that accumulates in the bottom of the separatorduring startup, a purge tubeis provided. The purge tubeis connected to a drain outlet located at the bottom of the liquid-vapor separatorand leads downward toward the accumulator. This configuration establishes a low-resistance return path for excess liquid, allowing it to flow under gravity back to the accumulator. By continuously draining excess liquidfrom the separator, the purge tubeprevents liquidfrom backing up into the upper section of the vapor tube.

This design ensures that the upper portion of the vapor tuberemains predominantly vapor-filled during the critical startup phase, thereby eliminating or substantially reducing the formation of an adverse hydrostatic pressure head (Hv). By maintaining a vapor-only pathway to the condenser, the system promotes rapid flow initiation and thermodynamic stability, enhancing startup reliability and preventing thermal overshoot of the heat-generating device.

In the second embodiment, the liquid return tubeinterfaces with the evaporatorat an inlet port located on the upper surface of the evaporator housing. However, rather than terminating at the point of entry, the liquid tubeis configured to extend downward into the internal cavity of the evaporator, such that its terminal end is positioned well below the upper fluid level during the system's initial static condition. This internal protrusion ensures that the inlet remains fully submerged in liquid-phase working fluidthroughout the start-up period.

The submersion of the inlet end of the liquid tubeis relevant because it enables reverse liquid displacement during early thermal transients. As vaporis generated in the evaporatordue to heat input from the heat-generating device, localized pressure increases force liquidupward into the liquid tube. Because the inlet remains below the liquid surface, the entrained liquidcan be displaced in a controlled manner back toward the accumulator, facilitating pressure equalization without entraining vaporinto the return path.

Simultaneously, the system's liquid-vapor separator, positioned downstream of the lower vapor tube, serves to remove excess liquidthat may otherwise accumulate in the vapor tube. This phase separation mechanism ensures that the upper portion of the vapor tuberemains predominantly vapor-filled, reducing flow resistance and suppressing adverse hydrostatic pressure buildup.

As excess liquidis purged from the vapor path via the separatorand corresponding purge tube, even a small volumetric quantity of vaporgenerated at the evaporatorcan establish a sufficient buoyancy-driven pressure gradient. This gradient initiates the desired directional flow: vaporis directed upward through the now-cleared vapor tubetoward the condenser, while liquid condensate returns via the liquid tubefrom the condenserto the evaporator. This arrangement provides a passive and reliable mechanism for start-up flow establishment without requiring mechanical assistance or external priming.

illustrates the second embodiment of the thermosyphon system under steady-state operating conditions, where continuous two-phase circulation has been established and thermal equilibrium is maintained. At this stage, the relative fluid levels and pressure distributions within the system stabilize, and the functional role of the purge tubeand its geometric positioning becomes relatively more significant.

The purge tubeis specifically designed with a localized vertical minimum (i.e., a low point) below both the liquid-vapor separatorand the condenser. This low point determines the purge tube's hydraulic threshold and directly governs the maximum purge pressure head (H). His defined as the vertical distance between the free liquid surface in the accumulatorand the lowest elevation point within the purge tube. This value represents the maximum gravitational pressure that can drive liquid from the separatorinto the accumulatorthrough the purge path.

In contrast, the system's favorable pressure head (H) is defined as the vertical distance from the free liquid surface in the accumulatorto the submerged terminal end of the liquid return tubewithin the evaporator. Hrepresents the gravitational pressure potential that drives liquidcirculation from the condenser, through the accumulator, and into the evaporator, thereby sustaining the thermosyphon's return flow.

For the purge tubeto remain hydraulically inactive during steady-state operation (thus preventing unintended liquid or vapor flow through it) it is essential that Hexceed H. When this condition is met, the purge tubebecomes effectively sealed by a stagnant liquid column and functions as a passive liquid-trap. This prevents short-circuiting of the normal flow loop and maintains directional flow integrity between the evaporatorand condenservia the primary vapor tubeand liquid tube.

Importantly, the deliberate placement of the purge tube's low point serves a dual function. First, it ensures the purge tubeceases to act as a flow path once the steady-state pressure gradient normalizes. Second, it contributes to the preservation of the favorable pressure head H, which is critical for sustaining buoyancy-driven vapor flow through the upper vapor tubeand for returning liquid condensate through the condenser. By isolating the purge path once its transient function is fulfilled, the system achieves a self-regulating, gravity-stabilized loop with minimized parasitic losses and enhanced thermal performance.

Numerous alternative embodiments and design variations of the present invention are contemplated, all of which fall within the scope and spirit of the disclosed thermosyphon system. While the described embodiments typically incorporate a dedicated accumulatoras a distinct component for managing the liquid-phase working fluid, such a structure is not strictly required in all implementations.

The primary function of the accumulator is to provide dynamic liquid volume buffering and to accommodate transient variations in fluid distribution during start-up, steady-state, and shut-down phases. It also helps maintain system priming by stabilizing the liquid return head and isolating liquid surges caused by vapor formation or system reorientation. However, this functionality can be integrated into other system elements without the need for a physically separate accumulator.

In particular, the condenseritself may be configured to perform the accumulator function, provided it has sufficient internal volume and geometric design to allow for stratified storage of condensed liquid without compromising vapor flow or heat rejection performance. A properly dimensioned condenser can act as a dual-function chamber, offering both heat exchange and fluid reservoir capabilities. For example, incorporating an extended base or a dedicated sump region within the condenser allows it to temporarily store condensed liquidbefore it is gravitationally returned to the evaporator via the liquid tube.

Therefore, in some configurations, the accumulatormay be implicitly integrated within the condenser housing, and need not be depicted or fabricated as a separate physical component. Such embodiments maintain all core advantages of the invention-including passive liquid-vapor management, startup stability, and hydrostatic pressure balancing-while potentially simplifying system architecture and reducing part count or spatial footprint.

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 thermosyphon operation 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.

To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, the applicant wishes to note that it does not intend any of the appended claims or claim elements to invoke 35 U.S.C. 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.