Method and system for stabilizing loop heat pipe operation with a controllable condenser bypass

Abstract

A loop heat pipe includes a reservoir, an evaporator adjacent to the reservoir, and a condenser including a condenser inlet and a condenser outlet. The loop heat pipe further includes a vapor transport line connecting the evaporator to the condenser inlet, a liquid transport line connecting the condenser outlet to the evaporator, and a vapor bypass joining the vapor transport line near the condenser inlet and joining the liquid transport line near the condenser outlet. The vapor bypass includes a vapor bypass housing. The vapor bypass housing includes a temperature. The loop heat pipe also includes a thermally-controlled connection between the vapor bypass housing and the condenser, and a thermal controller connected to the thermally-controlled connection and regulating the temperature of the vapor bypass housing via the thermally-controlled connection.

Claims

1. A loop heat pipe (LHP) comprising: a reservoir; an evaporator adjacent to the reservoir; a condenser comprising a condenser inlet and a condenser outlet; a vapor transport line connecting the evaporator to the condenser inlet; a liquid transport line connecting the condenser outlet to the evaporator; a vapor bypass joining the vapor transport line near the condenser inlet and joining the liquid transport line near the condenser outlet, the vapor bypass comprising a vapor bypass housing, the vapor bypass housing comprising a temperature; a thermally-controlled connection between the vapor bypass housing and the condenser; and a thermal controller connected to the thermally-controlled connection and regulating the temperature of the vapor bypass housing via the thermally-controlled connection.

2. The LHP of claim 1, wherein the vapor bypass comprises a tubing comprising a vapor bypass inlet at the vapor transport line near the condenser inlet and comprising a vapor bypass outlet at the liquid return line near the condenser outlet, the tubing comprising a tubing diameter, the vapor transport line comprising a vapor transport line diameter, the tubing diameter being smaller than the vapor transport line diameter.

3. The LHP of claim 2, wherein the vapor bypass tubing, comprises thermal insulation.

4. The LHP of claim 1, wherein the thermally-controlled connection comprises thermal straps.

5. The LHP of claim 1, wherein the thermal controller comprises at least one of: a heater; and a thermal electrical cooler.

6. The LHP of claim 1, wherein the vapor bypass tubing comprises corrugated tubing.

7. The LHP of claim 1, further comprising: a least one heater connected to the liquid transport line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic of an exemplary loop heat pipe with a full bypass of the condenser.

(2) FIG. 2 shows another schematic of an exemplary loop heat pipe with a partial bypass of the condenser.

(3) FIG. 3 shows another schematic of an exemplary loop heat pipe with a partial bypass of the condenser.

(4) FIG. 4 shows another schematic of an exemplary loop heat pipe with a gas-loaded heat pipe used as a thermal strap.

DETAILED DESCRIPTION

(5) This invented method to stabilize a Loop Heat Pipe operation and to eliminate or at least significantly reduce instabilities such as thermal-fluid oscillations, partial evaporator dryouts, quick fluid dynamics events, etc., includes a controllable fluid path with a small-inner-diameter in the vicinity of the LHP condenser and bypassing the LHP condenser, which allows a fraction of the vapor flow in the LHP vapor transport line to flow directly into the liquid return line. The housing of the bypass path extending between the inlet at the vapor line and the outlet at the liquid transport line can be cooled or heated in a controllable manner in order to achieve appropriate warming of the liquid flowing in the liquid return line by releasing the latent heat of vaporization into the liquid return flow, where part of the warming effect can be supplemented by the heaters or thermal electric coolers on the liquid transport line, with overall objective to prevent, control or eliminate various operational thermal-fluid instabilities in the LHP.

(6) An aspect of the invention is directed to a two-phase heat transfer system, such as a Loop Heat Pipe, comprising: at least one two-phase loop heat pipe capillary evaporator; at least one condenser; a vapor bypass joining the vapor transport line near the inlet of the condenser with the liquid transport line at the outlet of the condenser; and a thermally-controlled thermal connection, for example thermal straps, of such bypass housing to the condenser plate for cold biasing, as well as heaters or thermal electric coolers to regulate the bypass housing temperature and the liquid return line housing temperature.

(7) External heating of a LHP evaporator 1, shown in FIG. 1, evaporates working fluid (liquid) inside the evaporator primary porous wick. Capillary pressure, developed by the primary wick, pushes the vapor flow into the vapor transport line 2 and further into condenser 3, which is attached to the condenser plate 4 cooled externally by a heat sink. Vapor is normally fully condensed inside condenser 3 and cold liquid coming out of the condenser 3 into the liquid return line 8 is usually colder than the saturation temperature of the reservoir 10. Liquid entering the reservoir 10 through the liquid transport line 8 cools the reservoir to some extent, compensating for the reservoir heating due to the internal heat leak from the evaporator 1 to the reservoir 10, which allows the reservoir to reach a steady state operational temperature. Additionally, excessively cold liquid entering the LHP reservoir might cause thermal fluid oscillations, destabilizing the LHP operation.

(8) Exemplary embodiments for stabilizing LHP operation utilize several additional small and simple components (without moving parts) added to the basic LHP schematic;

(9) a) Condenser bypass small diameter tubing 5, which potentially allows−2% of the vapor flow to bypass the condenser,

(10) b) Thermal Straps 6 between the condenser bypass tubing 5 and the cold condenser plate 4, providing some low-level cooling of the bypass tubing 5,

(11) c) Thermal Electric Coolers 7 integrated with the thermal straps, which can provide either heating or cooling, when turned on, to the bypass tubing by switching the polarity (or single electrical heaters instead of TECs),

(12) d) Small heaters 9 on the liquid return line to increase temperature of the cold liquid flowing to the reservoir (for the system fine-tuning and/or for redundancy purposes).

(13) Functions and thermal-fluid operation of these added components are explained below in more detail.

(14) The Condenser Bypass 5, shown in FIG. 1, can be adiabatic (covered with a thermal insulation) and can operate without any thermal straps or TEC, still preventing TFOs for a wide range of the heat loads on the LHP evaporator, provided it is sized correctly for that power range and environmental conditions. The Condenser Bypass 5 allows a small fraction (−2%) of the vapor flow in the vapor transport line to bypass the condenser, due to the existing pressure drop across the condenser at a particular operational time. It is important to note that vapor flow rate through the bypass instantly increases proportionally to the pressure drop across the condenser exactly at the moment when there is a cold liquid surge coming out of the condenser. It can be stated therefore that the bypass vapor flow rate is synchronized with and proportional to the condenser fluid mass flow rate. The bypass vapor mass flow rate of m.sub.v enters the liquid return line where the liquid flow rate is m.sub.L and elevates temperature of the cold liquid coming out of the condenser by condensing in the cold liquid and releasing its latent heat of vaporization so that the cold liquid temperature would increase by ΔT, which value is approximated for steady flow conditions as
ΔT=(m.sub.v h.sub.fg)/(m.sub.L c.sub.pL)  (1)

(15) Warming the cold liquid returning to the condenser during a potential cold liquid surge reduces or completely eliminates TFOs by stabilizing the reservoir temperature (essentially preventing its temperature variations in time).

(16) Predictions generated by a numerical model but without any condenser bypass (or heaters) show significant temperature oscillations exhibited on the reservoir and liquid return line. Such TFOs were in fact observed during LHP testing, as well.

(17) If the condenser bypass tubing is being cooled by TECs, or due to heat losses either to the environment or to the cold condenser plate, the vapor inside the capillary tubing bypass would be partially or fully condensed, depending on the level of such heat losses, with mainly liquid flowing through the condenser bypass. If there is no vapor flowing through the bypass tubing into the liquid return line, than the liquid return flow warming due to the latent heat of vaporization expressed by equation (1) does not exist and the LHP can experience TFOs, almost as if there is no condenser bypass. Numerical predictions for the case where the 1.5 mm ID bypass is cooled by the condenser plate (with the effective heat transfer coefficient of 310 W/mA-K on the entire outer surface of the bypass tubing) were also made. TFOs in this simulation developed from a complete initial steady state and are only slightly different from the case without any bypass mentioned above. Therefore a low-level cooling of the condenser bypass provides some control over the vapor flow in it and hence over its stabilizing function, without using any moving parts.

(18) Summarizing, the invented method includes adding a controllable small inner diameter tubing bypassing LHP condenser. Such condenser bypass stabilizes LHP operation, which otherwise can exhibit undesirable thermal-fluid oscillations under some conditions, especially with a mass attached to LHP evaporator (payload). Controllability of such condenser bypass function, positioned in close proximity to the cold LHP condenser plate, can be achieved by applying a very low auxiliary power (heating or cooling) to the bypass tubing, without utilizing any moving parts in the LHP system.

(19) Another part of exemplary LHP stabilization methods are electrical heaters or TECs 9 installed on the LHP liquid return line shown in FIG. 1. Such heaters or TECs can be used as an additional means to stabilize the LHP operation, separately from or jointly with the already existing condenser bypass, expanding range of the LHP operational parameters, such as the condenser sink temperature. In other words, the liquid return line heaters (or TECs) may be included into exemplary LHP stabilization system for fine-tuning, LHP enhanced adjustability, and redundancy of the stabilizing system. Moreover, such heaters on the liquid return line can serve an important role by themselves in terms of eliminating LHP TFOs (thus stabilizing the LHP operation), as explained below.

(20) Experiments have been conducted on the LHP presented above with only liquid line heaters (without any condenser bypass). Test data provides clear evidence that elevating the heat load level on the liquid line heaters to 9 W (3% of 300 W) completely eliminated TFOs observed on the LHP heat loaded at 300 W (without the attached mass) prior to the test time of 874,000 seconds. Additional test data for the power level of 175 W indicates that only 2 W (1.2% of 175 W) of the liquid transport line heating was sufficient to eliminate thermal-fluid oscillations. Turning off the liquid line heating was followed by re-appearance of the thermal fluid oscillations.

(21) Such methods of heating the liquid return line to suppress TFOs (and generally to stabilize the LHP operation) may be more effective than applying heat load to the LHP Reservoir. The method can be simplified to keeping the liquid return line heaters at about 2% or 3% of the evaporator electrical heat load level at all times, when instabilities are anticipated.

(22) For comparison, reservoir heating stabilizing approach to eliminate TFOs by utilizing heating of the reservoir to prevent its temperature from dropping below a certain level has been demonstrated. Such demonstration was only successful with a proportional-control approach (much more difficult to implement than a continuous heating), where the time-averaged control power consumption was about 2 W (for the 75 W heat load case). Subsequent attempts to continuously apply 2 W to the LHP reservoir (2.7% of 75 W) did not succeed in that the operating LHP temperature started to increase rather significantly, which is undesirable. This indicates that the reservoir heating method is less preferable than the liquid return line heating.

(23) Referring now to FIG. 2, shown is another exemplary embodiment LHP 200, with a partial bypass 210. In this case, the bypass only bypasses the upstream portion of the condenser by entering the condenser upstream from the condenser outlet 211.

(24) Referring now to FIG. 3, shown is another exemplary embodiment LHP 300, with a partial bypass 310. In this case, the bypass only bypasses the downstream portion of the condenser by exiting the condenser upstream from the condenser outlet 211.

(25) Referring now to FIG. 4, a gas-loaded heat pipe 412 can be used as a variable conductance thermal strap between the bypass tubing and the condenser plate so that such system can operate in a fully autonomous manner and without using any heaters or TECs. Such autonomous operation is possible due to compression of the non-condensable gas (due to increased saturation pressure of the working fluid) inside the gas-loaded heat pipe when the condenser plate temperature is elevated, at which time the heat pipe starts to progressively cool the bypass tubing more effectively and thus partially or fully preventing the vapor flow through the bypass.

(26) Additionally, a small electrical heater 413 can be installed on the heat pipe body 412. Turning on heater 413 can disrupt the heat pipe operation and keep the warm vapor flowing inside the bypass line when appropriate.

(27) One advantage of the exemplary methods (versus heating the LHP reservoir) is that it intends to prevent or at least reduce surges of the cold liquid into the LHP reservoir in an intrinsically synchronized manner, where the vapor flow in the bypass is synchronized with the pressure drop across the condenser and is proportional to the surges of the cold liquid out of the condenser. Heating the reservoir with an electrical heater can be only synchronized with the already developing variation of the reservoir temperature, which is already lagging behind the liquid flow rate variation. Therefore, the invented method is much faster and effective (versus the reservoir heating) in terms of stabilizing a LHP operation at very early stages of cold liquid surges in development.

(28) A second advantage of exemplary methods (versus heating the LHP reservoir) is that it significantly reduces the control power consumption by simply utilizing the latent heat of vaporization of the bypass vapor flow to warm the cold liquid returning to the reservoir to a needed extent.

(29) A third advantage of exemplary methods are that they allow adjustment of operation of the already-manufactured expensive LHP to changes in the requirements by simply altering or replacing the inexpensive thermal straps between the condenser plate and bypass tubing. In other words the additional benefit is that the condenser bypass function can be adjusted after the expensive LHP system is manufactured and delivered to the user as needed due to mission changes (heat load levels re-defined, orbital environments changed, etc.). Depending on the situation, the low-conductance thermal straps can be added, removed, or replaced rather readily as they are external add-on components and do not effect sensitive internal components of the expensive Loop Heat Pipe system itself during such addition or removal (as soon as the bypass was pre-installed during the LHP fabrication).

(30) A fourth advantage of exemplary methods, versus using a mechanical valve with moving internal parts, is that there are not any moving parts involved in the condenser bypass method, which is highly beneficial and desirable for the reliability of LHP operation, especially for long-term deep space missions.

(31) A fifth advantage of the invented method with a gas-loaded heat pipe used as a variable conductance thermal strap between the bypass tubing and the condenser plate is that such system can operate in a fully autonomous manner and without using any heaters or TECs. Such autonomous operation is possible due to expansion of the non-condensable gas inside the gas-loaded heat pipe when the condenser plate temperature is elevated, at which time the heat pipe starts to cool the bypass tubing effectively and thus prevents the vapor flow through the bypass.

(32) Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.