WO2001033153A1

WO2001033153A1 - Starter for loop heat pipe

Info

Publication number: WO2001033153A1
Application number: PCT/US2000/028256
Authority: WO
Inventors: David A. Wolf, Sr.; Edward J. Kroliczek
Original assignee: Swales Aerospace
Priority date: 1999-11-04
Filing date: 2000-10-11
Publication date: 2001-05-10
Also published as: AU1199301A

Abstract

Heat energy is removed from the reservoir (10) of a Loop Heat Pipe (LHP) and is transferred directly into a concentrated area of the evaporator (20) of the LHP. A Peltier cell (100) cools the reservoir (10) and couples the heat energy via a heat pipe (120) to act as a heat load concentrated on a localized portion (160) of the evaporator wick. This promotes vaporization on the surface of the evaporator wick and creates a temperature differential across the evaporator wick. This provides a reliable way to initiate steady state heat transfer operation of the LHP.

Description

Title: STARTER FOR LOOP HEAT PIPE

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the art of heat transfer. More specifically, the present invention relates to a starter for two phase loop heat pipes. 2. Background Information Loop heat pipes (LHPs) are efficient, high capacity heat transfer devices used in advanced two-phase thermal management systems. LHPs offer significant improvements in weight, heat transport capacity, and reliability compared to alternative thermal control technologies. A simple loop heat pipe is a closed series connection of a fluid reservoir, an evaporator, and a condenser, the elements being chosen to promote capillary flow of a working fluid through the loop. Heat energy is acquired by the evaporator and is transported to the condenser where it is discharged. LHPs are characterized by the close proximity of the loop's reservoir and its heat acquisition interface (i.e., its evaporator). This proximity permits the reservoir to be hydraulically coupled to the evaporator with a secondary wick, thereby enhancing reliability. However, the proximity of the reservoir and the evaporator to one another also creates a thermal coupling between the two that can make the LHP difficult to start at low power. This startup difficulty can extend to higher powers when the evaporator is coupled to a large mass, as a considerable portion of the input power increases the sensible heat of the mass, thereby reducing the heat input to the evaporator. Successful LHP startup requires that the following two condition be satisfied: (1) that vaporization must occur on the outer surface of the evaporator wick, and (2) that sufficient pressure difference be established across the evaporator wick to initiate and sustain fluid flow. Both conditions require that a temperature difference (ΔT) be established across the evaporator wick. In the case of vaporization on the outer surface of the evaporator wick, ΔT is required in order to develop the superheat required for bubble nucleation. In the case of establishing a pressure difference across the evaporator wick, ΔT is required to satisfy the overall pressure balance associated with the saturated liquid/vapor equilibrium states that must be preserved on either side of the wick. If there is no phase change (i.e., bubble nucleation) in the evaporator, then capillary flow will not start. If there is no capillary flow, then no heat energy is transferred to promote a change in temperature difference ΔT between the reservoir and the evaporator. The result is a classic "chicken and egg" dilemma for which the prior art has provided no graceful solution. Often, raising the temperature of the evaporator by providing heat input starts the loop heat pipe. The problem with this approach is that the reservoir may simply heat up to the same temperature right along with the evaporator, thereby maintaining thermal equilibrium. The result is no temperature difference ΔT and, thus, no start up of the heat loop pipe. What is needed is a way to reliably start operation of a LHP that is in thermal equilibrium, regardless of how much power is to be transferred or the amount of mass the evaporator is coupled to. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for reliably starting operation of a LHP in low power situations. It is also an object of the present invention to provide a method for reliably starting operation of a LHP when the evaporator component of the LHP is coupled to a large mass heat source. It is an additional object of the present invention to provide an apparatus that reliably starts operation of a LHP in low power situations. It is a further object of the present invention to provide an apparatus that reliably starts operation of a LHP when the evaporator component of the LHP is coupled to a large mass heat source. It is yet another object of the present invention to provide a LHP including a structure that reliably starts operation of the LHP in low power situations. It is still another object of the present invention to provide a LHP including a structure that reliably starts operation of the LHP when the evaporator component of the LHP is coupled to a large mass heat source. It is another object of the present invention to provide a combination of a thermoelectric device and a heat pipe, configured so as to simultaneously cool the reservoir of a LHP and heat a concentrated portion of the evaporator of the LHP. The present invention is a starter for a LHP that removes heat energy from the reservoir of an LHP and transfers it directly into a concentrated area of the LHP's evaporator. A thermoelectric device (such as a Peltier cell) cools the reservoir and transfers the heat energy via a heat pipe to act as a concentrated heat load on the evaporator. This promotes vaporization on the surface of the evaporator wick and creates a temperature differential across the evaporator wick. This starts operation of the LHP to perform heat transfer. Some of the above objects are achieved with a loop heat pipe that uses a two-phase working fluid for heat transfer from a heat source to a heat sink. The loop heat pipe has a condenser, an evaporator, a reservoir, and a starter device. The condenser is coupled so as to provide heat to the heat sink. The reservoir is coupled to the condenser so as to receive working fluid from the condenser. The evaporator is coupled to the reservoir so as to receive working fluid from the reservoir, and is further coupled so as to receive thermal input from a heat source. The evaporator is also connected to the condenser so as to provide the condenser with working fluid. The starter device selectively causes transfer of heat energy directly from the reservoir to a localized portion of the evaporator, so that phase transition is initiated in at least the localized portion of the evaporator. Others of the above objects are achieved with a starter device that is intended for use with a loop heat pipe having a reservoir and an evaporator. The starter device has a thermoelectric cooler and a heat pipe. The thermoelectric cooler is in thermal communication with the reservoir. The heat pipe conducts heat energy from the thermoelectric cooler to the localized portion of the evaporator. The starter device selectively causes transfer of heat energy directly from the reservoir to a localized portion of the evaporator when the thermoelectric cooler is energized. Some of the above objects are also achieved with a starter device intended for use with a loop heat pipe having a reservoir and an evaporator. The starter device has a means, in thermal communication with the reservoir, for causing migration of heat energy, and a means for conducting heat energy from the means for causing migration of heat energy to the localized portion of the evaporator. The starter device selectively causes transfer of heat energy directly from the reservoir to a localized portion of the evaporator. Still others of the above objects are achieved with a process for starting operation of a loop heat pipe that has a reservoir and an evaporator having a wick. The process includes the act of developing a temperature gradient across the evaporator wick. The process also includes the act of causing nucleation of the working fluid at least at a localized portion of the surface of the evaporator wick. BRIEF DESCRIPTION OF THE DRAWING Additional objects and advantages of the present invention will be apparent in the following detailed description read in conjunction with the accompanying drawing figure. The Figure illustrates schematically a loop heat pipe embodying a starter structure according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION In order to start operation of a stagnant LHP, heat energy is transferred from the reservoir of the LHP to a concentrated portion of the evaporator of the LHP. This has the dual effect of promoting a temperature difference across the surface of the evaporator and promoting nucleation in the evaporator to initiate continuous phase change. According to a preferred embodiment of the present invention, a thermoelectric cooler (TEC) is incorporated into a LHP for the purpose of facilitating LHP startup. The TEC has a cold junction, which absorbs heat energy, and a hot junction, which exudes heat energy. The TEC is embodied, for example, as a Peltier cell. The cold junction of the TEC is mounted in thermal commumcation with the loop's reservoir and the hot junction of the TEC is thermally coupled to the loop's evaporator. The cold TEC junction cools the reservoir, decreasing its temperature below the evaporator, thereby increasing the temperature difference across the evaporator wick and inducing fluid circulation in the loop. The hot TEC junction provides localized heating to the loop's evaporator in order to nucleate the fluid at the wick's outer surface. By effectively using the waste heat, developed by the function of cooling the reservoir, to locally heat the evaporator, the LHP will be reliably started. Referring to the Figure, an LHP according to the preferred embodiment of the present invention is illustrated. The reservoir 10 is adjacent the evaporator 20. The proximity of the reservoir 10 to the evaporator 20 is not important to the invention; the illustrated juxtaposition of these system components is merely consistent with common engineering practice. The invention works equally well when the reservoir 10 and the evaporator 20 are remote from one another. The evaporator 20 is in thermal communication with a heat source (not shown) which provides heat energy to induce phase change of working fluid in the evaporator 20. Working fluid, vaporized in the evaporator 20, is conducted by a conduit 40 to a condenser 30. The condenser 30 is in thermal commumcation with a heat sink (not shown) that conducts heat away from the LHP. Vapor entering the condenser 30 undergoes a reverse phase change back into the liquid phase of the working fluid. This condensed working fluid flows from the condenser 30, through a conduit 50 back into the reservoir 10. A TEC 100 is mounted on the reservoir 10 such that the cold junction of the TEC 100 is in thermal communication with the reservoir 10. The hot junction of the TEC 100 is connected to a heat pipe 120. When the TEC 100 is energized, the reservoir 10 is cooled, thereby moving heat out the hot junction of the TEC 100 into the heat pipe 120. The heat pipe 120 conducts the heat energy to the evaporator 20 via a conduction member 140 having high thermal conductivity. The heat from the heat pipe 120 is input to the evaporator 20 at a localized portion 160 that is substantially smaller than the overall wick surface area of the evaporator 20. Conduction of heat from the TEC 100 to the evaporator 20 need not necessarily be done via a heat pipe 120. A member having high thermal conductivity (such as a metal bar) may be substituted for the heat pipe 120 to move heat directly from the TEC 100 to the evaporator 20. The heat pipe 120 is preferred over a bulky metal bar in those circumstances when minimizing mass is a priority, such as in spacecraft applications. According to a first alternate embodiment, the reservoir is cooled by a TEC without coupling the waste heat into the evaporator. Although this embodiment may be effective to start the LHP in some circumstances, it is not as reliable as the preferred embodiment described above, and thus, is not preferred. Cooling of the reservoir alone with the TEC may not develop the required heat flux and temperature gradient through the LHP's evaporator wick to start the loop, particularly when the pump's core contains both phases of the loop's working fluid. According to a second alternate embodiment, a localized portion of the evaporator of the LHP is heated without drawing heat from the reservoir. Although this alternate embodiment of the invention can be effective to start an LHP, it is not as reliable as the preferred embodiment described above, and thus, is not preferred. Spot heating of the evaporator alone may not develop the required temperature gradient through the pump's wick, particularly when the evaporator is coupled to a large thermal mass. The present invention is useful in a number of contexts. For example, an LHP starter is appropriate for use in a LHP for cooling heat generating components aboard spacecraft. An LHP starter is also useful for an LHP used for cooling electronics in confined spaces. Additionally, an LHP starter is also appropriate to make an LHP more useful in the context of cooling computer components that generate substantial heat. The present invention has been described in terms of various embodiments. However, numerous modifications and improvements may be made to the described embodiments without departing from the scope of the invention as described. The scope of the present invention is limited only by the appended claims.

Claims

WHAT IS CLAIMED IS: A loop heat pipe using a two-phase working fluid for heat transfer from a heat source to a heat sink, the loop heat pipe comprising: a condenser coupled so as to provide heat to the heat sink; a reservoir connected so as to receive working fluid from the condenser; an evaporator connected so as to receive working fluid from the reservoir and coupled so as to receive thermal input from the heat source, the evaporator being connected to the condenser so as to provide the condenser with working fluid; and a starter device selectively causing transfer of heat energy directly from the reservoir to a localized portion of the evaporator, wherein phase transition is initiated in at least the localized portion of the evaporator.

2. The loop heat pipe recited in claim 1, wherein the starter device comprises: a thermoelectric cooler in thermal commumcation with the reservoir, and a heat pipe thermally coupled so as to conduct heat energy from the thermoelectric cooler to the localized portion of the evaporator.

3. The loop heat pipe recited in claim 2, wherein the thermoelectric cooler has a cold junction and a hot junction, the cold junction being in thermal communication with the reservoir and the hot junction being in thermal communication with the heat pipe.

4. The loop heat pipe recited in claim 2, wherein the thermoelectric cooler is a Peltier cell.

5. The loop heat pipe recited in claim 2, wherein the starter device further comprises: a conduction member for coupling heat from the heat pipe to the localized portion of the evaporator.

6. A loop heat pipe using a two-phase working fluid for heat transfer from a heat source to a heat sink, the loop heat pipe comprising: a condenser coupled so as to provide heat to the heat sink; a reservoir connected so as to receive working fluid from the condenser; an evaporator connected so as to receive working fluid from the reservoir and coupled so as to receive thermal input from the heat source, the evaporator being connected to the condenser so as to provide the condenser with working fluid; and a starter device selectively causing transfer of heat energy directly from the reservoir to a localized portion of the evaporator, wherein phase transition is initiated in at least the localized portion of the evaporator, and wherein the starter device comprises: a Peltier cell having its cold junction in thermal communication with the reservoir, a heat pipe, in thermal communication with the hot junction of the Peltier cell, to conduct heat energy from the Peltier cell, and a conduction member for coupling heat from the heat pipe to the localized portion of the evaporator.

7. A starter device for use with a loop heat pipe comprising a reservoir and an evaporator, the starter device comprising: a thermoelectric cooler in thermal commumcation with the reservoir, and a heat pipe thermally coupled so as to conduct heat energy from the thermoelectric cooler to a localized portion of the evaporator, wherein the starter device selectively causes transfer of heat energy directly from the reservoir to the localized portion of the evaporator when the thermoelectric cooler is energized.

8. The starter device for use with a loop heat pipe recited in claim 7, wherein the thermoelectric cooler has a cold junction and a hot junction, the cold junction being in thermal communication with the reservoir and the hot junction being in thermal communication with the heat pipe.

9. The starter device for use with a loop heat pipe recited in claim 7, wherein the thermoelectric cooler is a Peltier cell.

10. A starter device for use with a loop heat pipe comprising a reservoir and an evaporator, the starter device comprising: means, in thermal communication with the reservoir, for causing migration of heat energy, and means for conducting heat energy from the means for causing migration of heat energy to a localized portion of the evaporator, wherein the starter device selectively causes transfer of heat energy directly from the reservoir to the localized portion of the evaporator.

11. A process for starting operation of a loop heat pipe comprising a reservoir and an evaporator having a wick, the loop heat pipe containing a working fluid, the process comprising: developing a temperature gradient across the evaporator wick, and causing nucleation of the working fluid at least at a localized portion of the surface of the evaporator wick.

12. The process for starting operation of a loop heat pipe recited in claim 11 , wherein the act of developing the temperature gradient is substantially concurrent with the act of causing nucleation of the working fluid.

13. The process for starting operation of a loop heat pipe recited in claim 11, wherein the act of developing the temperature gradient is achieved by transferring heat energy directly from the reservoir to the evaporator, causing the localized portion of the evaporator wick to be heated.

14. The process for starting operation of a loop heat pipe recited in claim 11 , wherein the act of causing nucleation of the working fluid is achieved by transferring heat energy directly from the reservoir to the evaporator, causing the localized portion of the evaporator wick to be heated.

15. A system comprising: a condenser providing heat to a heat sink; a reservoir receiving working fluid from the condenser; an evaporator receiving working fluid from the reservoir, receiving thermal input from a heat source, and providing the condenser with working fluid; and a starter device selectively causing transfer of heat energy directly from the reservoir to a localized portion of the evaporator, wherein phase transition of the working fluid is initiated in at least the localized portion of the evaporator.