SELF-PURGING THERMOSIPHON

Description

PRIORITY STATEMENT UNDER 35 U.S.C. § 119 & 37 C.F.R. § 1.78

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.

BACKGROUND OF THE INVENTION

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 to FIG. 1A and FIG. 1B which illustrate one shortcoming of conventional thermosyphon systems known in the art. Specifically, if a relative low spot in the vapor tube 105 exists 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 108, like a central processing unit (CPU), is presented in FIG. 1A. Liquid 106, occupies a substantial portion of the liquid tube 104, the vapor tube 105 and the evaporator 101. The condenser 102 is filled with vapor 107, as well as an optional accumulator 103. The accumulator 103 is useful for liquid management, when the volume of liquid 106 in the system could flood the condenser 102. The liquid-vapor line 109 shows the height of the liquid 106 at an initial condition where no heat load is applied to the system. Below liquid-vapor line 109, liquid 106 fills both the liquid tube 104 and the vapor tube 105.

FIG. 1B illustrates the distribution of liquid 106 and vapor 107 in the thermosyphon system shortly after thermal energy is applied to the heat-generating component 108. As heat is transferred to the evaporator 101, the liquid 106 within the evaporator undergoes a phase change from liquid 106 to vapor 107, generating an expanding vapor front. This accumulation of vapor 107 at the upper region of the evaporator 101 increases local pressure and displaces liquid 106 upstream in the vapor tube 105. The advancing vapor-liquid interface 110 in the vapor tube 105 pushes the entrained liquid 106 through the vapor tube 105 which, due to the presence of a geometrical low point, imposes an adverse hydrostatic pressure head (H_v) that resists forward motion of the vapor 107.

Simultaneously, this expansion of vapor 107 also acts on the liquid-vapor interface 111 within the evaporator, displacing liquid 106 through the liquid return tube 104 toward the accumulator 103. This motion produces a favorable gravitational pressure head (H_L), aiding the return of liquid from the condenser 102. However, in configurations where the adverse head H_v (caused by liquid pooling at the vapor tube low point) exceeds the favorable head H_L, the pressure differential opposes intended vapor flow. Consequently, liquid 106 within the evaporator 102 is displaced below the level of the heat-generating device 108, 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 108.

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.

SUMMARY OF THE INVENTION

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.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic of a thermosyphon design in accordance with prior art, with a localized low spot in the vapor tube when zero heat is applied as an initial condition;

FIG. 1B is a schematic of a thermosyphon design in accordance with prior art, with a localized low spot in the vapor tube shortly after heat is applied;

FIG. 2A is a schematic of a first embodiment of the present invention in an initial condition;

FIG. 2B is a schematic of a first embodiment of the present invention shortly after heat is applied;

FIG. 2C is a schematic of a first embodiment of the present invention at steady state;

FIG. 3A is a schematic of a second embodiment of the present invention at an initial condition; and

FIG. 3B is a schematic of a second embodiment of the present invention at steady state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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.

FIG. 2A 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 evaporator 101 thermally coupled to a heat-generating device 108, such as a CPU, to facilitate direct thermal conduction. In a preferred configuration, heat-generating device 108 is positioned vertically as shown in FIG. 2A. The evaporator 101 is fluidly connected to a condenser 102 via a vapor conduit or vapor tube 105, which provides a path for vapor-phase working fluid 107 during operation. A separate liquid return path is established through a liquid tube 104 that fluidly couples the evaporator 101 to an accumulator 103, which in turn is connected to the base of the condenser 102 to receive condensed working fluid.

In the startup condition, the internal volumes of the evaporator 101, substantial portions of the vapor tube 105, and the liquid return tube 104 are occupied by liquid-phase working fluid 106. The remaining internal volumes of the system, including the accumulator 103, condenser 102, and the residual space in both the vapor tube 105 and liquid tube 106, are filled with vapor-phase working fluid 107, thus maintaining a saturated two-phase equilibrium.

Note that, as shown in FIG. 2A, the geometry of the fluid tubes incorporates one or more localized vertical minima, such that the vapor tube 105 includes a low-point elevation that permits pooling of liquid 106 in the vapor tube 105 under gravitational forces. A similar low-point configuration exists in the liquid tube 104. 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 tube 104 connects to the evaporator 101 at a port located on the top region of the evaporator 101 housing. 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 tube 104 may be submerged slightly into the liquid reservoir, however it should not be submerged past the top level of the heat-generating device 108.

FIG. 2B depicts the thermodynamic and fluid dynamic behavior of the first embodiment shortly after thermal energy is introduced to the heat-generating device 108. Upon activation, heat is conducted into the base of the evaporator 101, initiating phase change of the working fluid from liquid 106 to vapor 107. The resulting vapor 107 accumulates in the upper region of the evaporator 101 and exits through the vapor tube 105. This expanding vapor volume displaces liquid 106 along both the vapor and liquid paths, thereby advancing two liquid-vapor interfaces: interface 110 within the vapor tube 105 and interface 112 within the liquid tube 104.

The movement of liquid 106 in the vapor tube 105, displaced by the upwardly migrating vapor 107, establishes an adverse hydrostatic pressure head, designated as H_v. This pressure head resists vapor flow due to the gravitational potential energy required to overcome the local minimum in the vapor tube's 105 elevation profile. In contrast, the vapor pressure generated in the evaporator 101 also forces liquid 106 through the liquid return tube 104 and into the accumulator 103, generating a favorable hydrostatic pressure head H_L.

One feature of this embodiment is that the liquid tube 104 connects to the top of the evaporator 101. This routing effectively stabilizes the liquid 106 in the evaporator 101 during the critical startup phase by trapping a volume of liquid 106 above the heat-generating component 108, thereby preventing premature depletion.

Due to the elevation profile of the system, the liquid tube 104 incorporates a local minimum that amplifies H_L by increasing the vertical height difference relative to the accumulator 103. Consequently, H_L exceeds Hv, producing a net positive pressure differential that initiates the desired circulation pattern. As further illustrated in FIG. 2C, this results in the onset of steady-state flow conditions: vapor 114 flows from the evaporator 101 to the condenser 102 via the vapor tube 105, while condensed liquid 113 returns from the condenser 102 through the liquid tube 104 back to the evaporator 101. This balance of pressure heads ensures reliable startup without external priming or mechanical assistance.

During the startup phase, the recession of the liquid-vapor interface 111 within the evaporator 101 is primarily driven by the phase transition of the working fluid from liquid 106 to vapor 107 in response to thermal input from the heat-generating device 108. As heat is conducted into the liquid 106, latent heat of vaporization is absorbed, causing the liquid 106 to 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, vaporizing 1 milliliter of liquid can produce approximately 100 milliliters of vapor under standard thermosyphon operating conditions.

This rapid volumetric expansion of vapor 107 plays a role in system dynamics during startup. As vapor 107 displaces liquid 106 from the evaporator 101 toward the vapor tube 105 and liquid tube 104, it is essential that the liquid 106 above the heat-generating device 108 remain 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 evaporator 101 must be geometrically configured to retain a minimum volume of liquid 106 above the heat-generating device 108 throughout 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 evaporator 101 located above the heat-generating device should exceed approximately 1% of the total internal volume of the vapor tube 105. 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.

FIG. 3A 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 component 108 is thermally interfaced with the underside of the evaporator 101 to 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 tube 105 is filled with liquid-phase fluid 106. This condition is often a result of gravitational settling during charging or extended system shutdown.

Due to the geometric routing of the vapor tube 105, its upper portion connects to the condenser 102 at 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 evaporator 101 must displace the denser liquid 106 residing in the lower segment of the vapor tube 105 in order to reach the condenser 101. If unaddressed, this condition can obstruct vapor flow, delay startup, and destabilize system operation.

To mitigate this challenge, this embodiment incorporates a liquid-vapor separator 115 and an associated purge tube 116. The separator 115 is strategically located near the same elevation as the accumulator 103 and is fluidly coupled to both the lower and upper segments of the vapor tube 105. The purge tube 116 provides a low-resistance gravitational return path from the bottom of the separator 115 to the accumulator 103. In the initial condition, the lower vapor tube 105, the liquid return tube 104, and the purge tube 116 are predominantly filled with liquid 106, while the condenser 102 and upper portions of the system contain vapor 107. This configuration enables passive drainage of excess liquid 106 from the vapor tube 105 into the separator 115 and subsequently through the purge tube 116, preventing vapor entrapment and establishing favorable pressure conditions for startup.

In this second embodiment, the liquid-vapor separator 115 is strategically positioned at approximately the same vertical elevation as the accumulator 103 to facilitate gravitational fluid return and minimize hydrostatic pressure imbalances during transient phases. The lower section of the vapor tube 105 is fluidly connected at one end to the evaporator 101 and at the other end to the inlet at the bottom of the liquid-vapor separator 115. This routing enables liquid 106 and vapor 107 exiting the evaporator 101 to be directed into the separator 115 without imposing significant elevation-driven resistance to flow.

The liquid-vapor separator 115 functions as a passive phase stratification chamber, wherein the incoming two-phase mixture from the lower vapor tube 105 is allowed to decelerate and separate under the influence of gravity. Due to density differences, liquid 106 settles toward the base of the separator 115, while vapor 107 rises and exits through an outlet located at the top of the separator 115. This outlet is fluidly coupled to the upper portion of vapor tube 105, which directs the purified vapor stream to the condenser 102 for heat rejection.

To manage excess liquid that accumulates in the bottom of the separator 115 during startup, a purge tube 116 is provided. The purge tube 116 is connected to a drain outlet located at the bottom of the liquid-vapor separator 115 and leads downward toward the accumulator 103. This configuration establishes a low-resistance return path for excess liquid 106, allowing it to flow under gravity back to the accumulator 103. By continuously draining excess liquid 106 from the separator 115, the purge tube 116 prevents liquid 106 from backing up into the upper section of the vapor tube 105.

This design ensures that the upper portion of the vapor tube 105 remains 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 102, 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 tube 104 interfaces with the evaporator 101 at an inlet port located on the upper surface of the evaporator housing. However, rather than terminating at the point of entry, the liquid tube 104 is configured to extend downward into the internal cavity of the evaporator 101, 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 fluid 106 throughout the start-up period.

The submersion of the inlet end of the liquid tube 104 is relevant because it enables reverse liquid displacement during early thermal transients. As vapor 107 is generated in the evaporator 101 due to heat input from the heat-generating device 108, localized pressure increases force liquid 106 upward into the liquid tube 104. Because the inlet remains below the liquid surface, the entrained liquid 106 can be displaced in a controlled manner back toward the accumulator 103, facilitating pressure equalization without entraining vapor 107 into the return path.

Simultaneously, the system's liquid-vapor separator 115, positioned downstream of the lower vapor tube 105, serves to remove excess liquid 106 that may otherwise accumulate in the vapor tube 105. This phase separation mechanism ensures that the upper portion of the vapor tube 105 remains predominantly vapor-filled, reducing flow resistance and suppressing adverse hydrostatic pressure buildup.

As excess liquid 106 is purged from the vapor path via the separator 115 and corresponding purge tube 116, even a small volumetric quantity of vapor 107 generated at the evaporator 101 can establish a sufficient buoyancy-driven pressure gradient. This gradient initiates the desired directional flow: vapor 107 is directed upward through the now-cleared vapor tube 105 toward the condenser 102, while liquid condensate returns via the liquid tube 104 from the condenser 102 to the evaporator 101. This arrangement provides a passive and reliable mechanism for start-up flow establishment without requiring mechanical assistance or external priming.

FIG. 3B 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 tube 116 and its geometric positioning becomes relatively more significant.

The purge tube 116 is specifically designed with a localized vertical minimum (i.e., a low point) below both the liquid-vapor separator 115 and the condenser 102. This low point determines the purge tube's hydraulic threshold and directly governs the maximum purge pressure head (H_P). H_P is defined as the vertical distance between the free liquid surface in the accumulator 103 and the lowest elevation point within the purge tube 116. This value represents the maximum gravitational pressure that can drive liquid from the separator 115 into the accumulator 103 through the purge path.

In contrast, the system's favorable pressure head (H_L) is defined as the vertical distance from the free liquid surface in the accumulator 103 to the submerged terminal end of the liquid return tube 104 within the evaporator 101. H_L represents the gravitational pressure potential that drives liquid 106 circulation from the condenser 102, through the accumulator 103, and into the evaporator 101, thereby sustaining the thermosyphon's return flow.

For the purge tube 116 to remain hydraulically inactive during steady-state operation (thus preventing unintended liquid or vapor flow through it) it is essential that H_P exceed H_L. When this condition is met, the purge tube 116 becomes 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 evaporator 101 and condenser 102 via the primary vapor tube 105 and liquid tube 104.

Importantly, the deliberate placement of the purge tube's low point serves a dual function. First, it ensures the purge tube 116 ceases 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_L, which is critical for sustaining buoyancy-driven vapor flow through the upper vapor tube 105 and for returning liquid condensate through the condenser 102. 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 accumulator 103 as a distinct component for managing the liquid-phase working fluid 106, 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 condenser 102 itself 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 liquid 106 before it is gravitationally returned to the evaporator via the liquid tube.

Therefore, in some configurations, the accumulator 103 may 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.