Field Of Invention

This invention relates generally to two-phase heat transfer apparatuses, including capillary pumped loop apparatuses and a loop heat pipe apparatuses.

Background

Two-phase heat transfer apparatuses such as capillary pumped loops (CPL) and loop heat pipes (LHP) use the latent heat of vaporization and the condensation of a refrigerant to transfer heat and mass, and a capillary or surface tension force provided by a wicking material to circulate the refrigerant. Applications for such apparatuses include but are not limited to thermal management of semiconductor and high power electronics, such laser, radar and insulated gate bipolar transistors (IGBT).

Both a CPL and a LHP apparatus comprise an evaporator, a condenser, and a vapor conduit connecting a refrigerant outlet of the evaporator to a refrigerant inlet of the condenser, a liquid conduit connecting a refrigerant outlet of the condenser to a refrigerant inlet of the evaporator, and a refrigerant accumulator. The accumulator is sometimes referred to in the art as a reservoir or a compensation chamber. A capillary wicking material is provided in the evaporator and accumulator.

CPL and LHP apparatuses operate on the same general principle. As a heat load is applied to the evaporator, the liquid-phase refrigerant in the evaporator is vaporized and a meniscus is formed at a liquid / vapor phase in the wicking material. The surface tension (capillary) force develops a pressure gradient that moves the vapor-phase refrigerant through the vapor conduit to the condenser where it discharges heat and condenses into a liquid phase. The liquid phase refrigerant then moves through the liquid conduit back to the evaporator. The refrigerant accumulator can contain both vapor-phase and liquid-phase refrigerant and serves to control device saturation temperature and maintain proper fluid inventory in the refrigerant loop. The main difference between a CPL and LHP device is the construction of the evaporator and liquid accumulator, and physical location of the liquid accumulator. In a LHP device, the compensation chamber is located directly in the path of the liquid flow, and is integrated into the same body as the evaporator and connected to the evaporator by a wicking material. The reservoir in a CPL device is typically located remotely from the

evaporator and is outside the refrigerant circulation path.

The compensation chamber in an LHP apparatus is located in the refrigerant flow path, and its temperature is determined by an energy balance between heat "leaked" from the evaporator and sub-cooling of the incoming liquid refrigerant. Heat leak directly affects the refrigerant loop operating temperature, especially at low power levels and low flow rates, and can also affect startup performance. Therefore, controlling heat leak and the temperature inside the compensation chamber is a significant operational challenge of present art LHP devices, and controlling the temperature of the refrigerant loop is a challenge for all two-phase heat transfer devices including LHP devices.

Summary

According to one aspect of the invention, there is provided an evaporator sub-assembly for a loop heat pipe apparatus. The evaporator sub-assembly includes: an evaporator base having an interior and an exterior surface; an evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet; a compensation chamber inside the evaporator body and in fluid communication with the refrigerant inlet; a capillary wicking material inside the evaporator body and fluidly communicable with the compensation chamber and the refrigerant outlet; and a heat shield inside the evaporator body positioned such that the wicking material is between the interior surface of the evaporator base and the heat shield, the heat shield having fluid openings for fluidly coupling the wicking material to the compensation chamber.

The evaporator base may have a selected thermal conductivity and the heat shield may have a selected thermal conductivity that is lower than the thermal conductivity of the evaporator base. Additionally, or alternately, the evaporator base may have a selected thermal conductivity and the evaporator body may have a selected thermal conductivity that is lower than the thermal conductivity of the evaporator base. The heat shield may be positioned such that a space between the heat shield and the wicking material is 1 mm or less. The heat shield can be coated with a heat reflective coating. The diameter or diagonal of the fluid openings of the heat shield may be 1mm or less.

The evaporator body may include at least one external heat transfer structure protruding outwards from an exterior surface of the evaporator body and/or at least one internal heat transfer structure protruding inwards from an interior surface of the evaporator body into the compensation chamber.

The evaporator sub-assembly may further include a thermal separator between the evaporator body and the evaporator base, the thermal separator comprising a low thermal conductivity material coated with a metallic surface layer.

The capillary wicking material may be multi-layered and include: a vaporization layer having a surface in contact with the evaporator base and a pore size selected to promote at least one of refrigerant evaporation, bubble creation and bubble departure; a control layer adjacent to the vaporization layer and having a pore size selected to achieve a selected pumping pressure; and a liquid absorbing layer adjacent to the control layer and in fluid communication with the compensation chamber, and having a pore size selected to promote drawing of liquid refrigerant into the wicking material. The pore size of the control layer may be between 1 and 30 microns and the pore size of the liquid absorbing layer and/or the vaporization layer may be at least 100 microns or above.

The evaporator sub-assembly may further include at least one brace inside the evaporator body and arranged to exert pressure on a portion of the capillary wicking material against the evaporator base.

The evaporator sub-assembly may further include a thermoelectric cooler mounted on the evaporator body in proximity to the compensation chamber and operable to cool liquid phase refrigerant before the liquid phase refrigerant enters the compensation chamber via the refrigerant inlet. According to another aspect of the invention, there is provided a loop heat pipe apparatus comprising: a condenser having a refrigerant inlet and a refrigerant outlet; an evaporator sub-assembly; a vapor conduit fluidly coupling an evaporator body refrigerant outlet to the condenser refrigerant inlet; and a liquid conduit fluidly coupling the condenser refrigerant outlet to an evaporator body refrigerant inlet. The evaporator sub-assembly includes: an evaporator base having an interior and an exterior surface; an evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet; a compensation chamber inside the evaporator body and in fluid communication with the evaporator body refrigerant inlet; a capillary wicking material inside the evaporator body and fluidly communicable with the compensation chamber and the evaporator body refrigerant outlet; and a heat shield inside the evaporator body positioned such that the wicking material is between the interior surface of the evaporator base and the heat shield, the heat shield having fluid openings for fluidly coupling the wicking material to the compensation chamber.

According to a further aspect of the invention, there is provided an evaporator for a two phase heat transfer apparatus. The evaporator includes: an evaporator base having an interior and an exterior surface; an evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet; a thermal separator between the evaporator body and the evaporator base, the thermal separator comprising a thermally insulating material coated with a metallic surface layer; and a capillary wicking material inside the evaporator body in contact with the interior surface of the evaporator base and fluidly communicable with a liquid refrigerant inside the evaporator body.

The evaporator body may include an annular rim that connects to the evaporator base and the thermal separator may be positioned between the annular rim of the evaporator body and the evaporator base. The evaporator base may have a selected thermal conductivity and the evaporator body may have a selected thermal conductivity that is lower than the thermal conductivity of the evaporator base.

The capillary wicking material of the evaporator may be multi-layered and include: a vaporization layer having a surface in contact with the evaporator base and a pore size selected to promote at least one of refrigerant evaporation, bubble creation and bubble departure; a control layer adjacent to the vaporization layer and having a pore size selected to achieve a selected pumping pressure; and a liquid absorbing layer adjacent to the control layer and in fiuidly communicable with the liquid refrigerant inside the evaporator body, and having a pore size selected to promote drawing of the liquid refrigerant into the capillary wicking material. The pore size of the control layer may be between 1 and 30 microns and the pore size of the liquid absorbing layer and/or the vaporization layer may be at least 100 microns or above.

The evaporator may further include at least one brace inside the evaporator body and arranged to exert pressure on a portion of the capillary wicking material against the evaporator base.

According to another aspect of the invention, there is provided a two-phase heat transfer apparatus comprising: a condenser having a refrigerant inlet and a refrigerant outlet; an evaporator; a vapor conduit fiuidly coupling an evaporator refrigerant outlet to the condenser refrigerant inlet; a liquid conduit fiuidly coupling the condenser refrigerant outlet to an evaporator refrigerant inlet; and a refrigerant accumulator fiuidly

communicable with and operable to store the refrigerant. The evaporator includes an evaporator base; an evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet; a thermal separator between the evaporator body and the evaporator base, the thermal separator comprising a thermally insulating material coated with a metallic surface layer; and a capillary wicking material fiuidly communicable with a refrigerant and for generating a pumping pressure.

According to another aspect of the invention, there is provided an insert for inserting inside a heat exchange tube. The insert includes: a longitudinally elongated body having an exterior surface and a transverse dimension less than an inner diameter of a selected heat exchange tube; and at least one spacer extending transversely outwards from the body a distance that contacts an inner wall of the tube when the body is inside the tube, such that an annular space is formed between the inner wall and the exterior surface of the body, the spacer comprising at least one opening to allow flow of a fluid there through. One spacer may be positioned at one end of the body and another spacer may be positioned at the other end of the body. The spacer may include a plurality of fins protruding radially outwards from the body, each fin contacting the inner wall of the tube when the body is inside the tube.

The insert may further include at least one locating feature protruding from an end of the body or the spacer.

According to another aspect of the invention, there is provided a two-phase heat transfer apparatus comprising: a condenser having a heat exchange tube between a refrigerant inlet and a refrigerant outlet and an insert inside at least part of the heat exchange tube; an evaporator having a refrigerant inlet and a refrigerant outlet, and a capillary wicking material fluidly communicable with the refrigerant and for generating a pumping pressure; a vapor conduit fluidly coupling the evaporator refrigerant outlet to the condenser refrigerant inlet; a liquid conduit fluidly coupling the condenser refrigerant outlet to the evaporator refrigerant inlet; and a refrigerant accumulator fluidly

communicable with and operable to store the refrigerant. The insert comprises a longitudinally elongated body having an exterior surface and a transverse dimension less than an inner diameter of the heat exchange tube; and at least one spacer extending transversely outwards from the body a distance that contacts an inner wall of the heat exchange tube when the body is inside the heat exchange tube, such that an annular space is formed between the inner wall and the exterior surface of the body, the spacer comprising at least one opening to allow flow of a refrigerant there through.

According to another aspect of the invention, there is provided a two-phase heat transfer apparatus comprising: a condenser having a refrigerant inlet and a refrigerant outlet; an evaporator having a refrigerant inlet and a refrigerant outlet, and a capillary wicking material fluidly communicable with a refrigerant and for generating a pumping pressure; a vapor conduit fluidly coupling the evaporator refrigerant outlet to the condenser refrigerant inlet; a liquid conduit fluidly coupling the condenser refrigerant outlet to the evaporator liquid inlet; a refrigerant accumulator fluidly communicable with and operable to store the refrigerant; and a thermoelectric cooler fluidly communicable with and operable to cool the liquid phase refrigerant. According to another aspect of the invention, there is provided an evaporator for a two- phase heat transfer apparatus, comprising: an evaporator base having an interior and an exterior surface; an evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet; a capillary wicking material inside the

evaporator body in fluid communication with the refrigerant outlet and secured around its periphery to the evaporator base such that the wicking material is in contact with the interior surface of the evaporator base; and a brace inside the evaporator body and arranged to exert pressure on a central portion of the wicking material against the evaporator base.

According to yet another aspect of the invention, there is provided an evaporator for a two phase heat transfer apparatus, comprising: an evaporator base having an interior and an exterior surface and a selected thermal conductivity; a evaporator body connected to the evaporator base and having a refrigerant inlet and a refrigerant outlet, and a selected thermal conductivity that is lower than the thermal conductivity of the evaporator base; and a capillary wicking material inside the evaporator body and secured around its periphery to the evaporator base such that the wicking material is in contact with the interior surface of the evaporator base and is fluidly communicable with a refrigerant inside the evaporator body.

According to yet another aspect of the invention, there is provided an evaporator subassembly for a loop heat pipe apparatus, comprising: an evaporator base having an interior and an exterior surface; and an evaporator body connected to the base and defining a compensation chamber therein. The evaporator body has a refrigerant inlet, a refrigerant outlet, at least one external heat transfer surface (such as a fin, pin, wave, or other surface area enhancing structure) protruding outwards from an exterior surface of the evaporator body, and at least one internal heat transfer fin protruding inwards from an interior surface of the evaporator body into the compensation chamber. The subassembly also has a capillary wicking material inside the evaporator body in contact with the interior surface of the base and fluidly communicable with a refrigerant inside the evaporator body. According to yet another aspect of the invention, there is provided an evaporator subassembly for a loop heat pipe apparatus, comprising: an evaporator base having an interior and an exterior surface and a selected thermal conductivity; a evaporator body enclosing the interior surface of the base and having a refrigerant inlet and a refrigerant outlet; a compensation chamber inside the evaporator body and in fluid communication with the refrigerant inlet; a capillary wicking material inside the evaporator body and fluidly communicable with the compensation chamber and the refrigerant outlet; and a heat shield inside the evaporator body and in contact with the wicking material such that the wicking material is between the base and the heat shield. The heat shield may be made out of (or plated) in silver or gold, which enhances its heat reflection properties further, and fluid openings for fluidly coupling the wicking material to the compensation chamber.

According to yet another aspect of the invention, there is provided an evaporator for a two phase heat transfer apparatus, comprising: an evaporator base having an interior and an exterior surface and a selected thermal conductivity; an evaporator body enclosing the interior surface of the base and having a refrigerant inlet and a refrigerant outlet; and a multi-layered capillary wicking material located inside the evaporator body in contact with the interior surface of the base. The wicking material has: a porous vaporization layer having a surface in contact with the base and a pore size selected to promote at least one of refrigerant evaporation, bubble creation and bubble departure; a control layer adjacent to the vaporization layer and having a pore size selected to achieve a selected pumping pressure; and a liquid absorbing layer adjacent to the control layer and fluidly communicable with liquid refrigerant inside the evaporator body, and having a pore size selected to promote drawing liquid refrigerant into the wicking material.

According to yet another aspect of the invention, there is provided a two-phase heat transfer apparatus comprising: a condenser, an evaporator, a vapour conduit, a liquid conduit, and a refrigerant accumulator. The condenser comprises an upper manifold with a refrigerant inlet, a lower manifold with a refrigerant outlet, and a substantially vertically oriented heat exchange tubes fluidly coupling the upper manifold to the lower manifold. The evaporator has a refrigerant inlet and a refrigerant outlet, and a capillary wicking material fluidly communicable with a refrigerant and for generating a pumping pressure. The vapor conduit fluidly couples the evaporator refrigerant outlet to the condenser refrigerant inlet, and the liquid conduit fluidly couples the condenser refrigerant outlet to the evaporator liquid inlet. The refrigerant accumulator is fluidly communicable with and operable to store the refrigerant.

Brief Description of Drawings

Figures 1 (a) and (b) are schematics of two-phase heat transfer apparatuses according to one embodiment of the invention, wherein Figure 1 (a) shows a loop heat pipe apparatus and Figure 1 (b) shows a capillary pumped loop apparatus.

Figure 2 is perspective view of a condenser heat exchanger tube of the heat transfer apparatus having a tube insert therein.

Figure 3 is a side elevation view of heat exchanger tube and tube insert. Figure 4 is an end view of the tube insert.

Figure 5 is a rear perspective view of a thermoelectric powered sub- cooler of the heat transfer apparatus.

Figure 6 is a rear elevation view of the thermoelectric cooler. Figure 7 is a side elevation view of the thermoelectric cooler.

Figure 8 is a front perspective view of an evaporator sub-assembly of the heat transfer apparatus.

Figure 9 is a rear perspective view of the evaporator sub-assembly.

Figure 10 is a sectioned perspective view of the evaporator sub-assembly.

Figure 1 1 is a bottom perspective view of an evaporator body of the evaporator subassembly.

Figure 12 is a top perspective view of a base plate of the evaporator sub-assembly. Figure 13 is a top perspective view of a heat shield of the evaporator sub-assembly.

Figure 14 is a top perspective view of a capillary wicking material of the evaporator subassembly.

Figure 15 is a sectioned perspective view of the evaporator body and base plate connected together.

Figure 16 is a sectioned elevation view of the evaporator body and base plate

connected together.

Figure 17 is a schematic view of a gravity feed through condenser according to another embodiment of the invention.

Detailed Description

The embodiments described herein relate generally to heat transfer apparatuses including two-phase heat transfer apparatuses such as capillary pumped loop (CPL) apparatuses and loop heat pipe (LHP) apparatuses. Figure 1 (a) schematically illustrates a LHP apparatus 10, and Figure 1 (b) schematically illustrates a CPL apparatus 12. While this description is directed primarily at the LHP embodiment, the principles described in that embodiment can be readily applied to the CPL embodiment, except where noted in this description.

Referring to Figure 1 (a), the LHP apparatus 10 generally comprises an evaporator subassembly 14, a condenser 16, a vapor conduit 18 fluidly coupling a refrigerant outlet of the evaporator sub-assembly 14 to a refrigerant inlet of the condenser 16, and a liquid conduit 20 fluidly coupling a refrigerant outlet of the condenser 16 to a refrigerant inlet of the evaporator sub-assembly 14. The evaporator sub-assembly 14 generally comprises a compensation chamber 22 and an evaporator 23 with a capillary wicking material 24. In contrast, the CPL apparatus 12 has its compensation chamber 22 remotely located from the evaporator 23. Together, these components form a refrigerant loop in which a refrigerant fluid flows there through. Suitable refrigerants may include but not limited to R134a (Tetrafluoroethane), R290 (Propane), and R717 (Ammonia). By utilizing the phase changes of the refrigerant from a liquid phase to a vapor phase and back to a liquid phase, heat can be absorbed by the evaporator sub-assembly 14 and discharged by the condenser 16. The capillary wicking material 24 provides a capillary pressure on the refrigerant to move the refrigerant around the loop.

The vapor and liquid conduits 8, 20 are made of flexible metal tubing such as convoluted or corrugated stainless steel tubing.

Condenser: Tube Inserts

Referring now to Figures 2 to 4 and according to a first embodiment, a tube insert 25 is provided for heat exchange tubing 26 of the condensers 16 of both the LHP and CPL apparatuses 10, 12, but can also be used in any heat exchanger having heat exchange tubing for flow of a heat transfer fluid such as a refrigerant there through, such as chillers and refrigeration units, that would benefit from locating a heat exchange heat transfer fluid closer to the heat exchange tubing walls.

The tube insert 25 comprises a longitudinally elongated generally cylindrical body 28 with a diameter that is less than the inner diameter of the heat exchange tube 26 inside which the insert is intended to be placed. The ends of body 28 are conical with a gentle taper. A pair of centering spacers 29 on the conical portion at each end of the body 28 comprise a plurality of fins that protrude radially outwards and are dimensioned to cause the tube insert 25 to fit snugly inside the heat exchange tube 26 wherein the body 28 and heat exchanger tube 26 axes are aligned (i.e. the insert 25 is centered inside the tube 26) and an annular volume is formed between the inner surface of the heat exchange tube 26 and the outer surface of the body 28.

While a spacer that fits over the conical ends of the body 28 is shown in the Figures, the spacer 29 may be a separate piece that attaches to the end of the body 28, provided the spacer extends transversely outwards from the body to contact the inner surface of the heat exchange tube. For example, the spacer 29 may comprise a spacer body with a plurality of fins that protrude radially outwards from the spacer body and an

attachment mechanism for attaching the spacer to an end of the insert body 28. The tube insert 25 also includes a pair of locating features 30 each protruding from an end of the body 28. If the spacer 29 is a separate piece attached to the end of the body 28, the locating feature can be attached to the spacer 29. The locating feature 30 is in the form of a cylinder and serves to properly seat the tube insert 25 in a tube 26 with a "hairpin" or a "return bend" of the heat exchanger tube 26 (not shown). This locating feature is particularly useful in heat exchangers having a serpentine refrigerant pathway such as the condenser 16 of this embodiment. In this embodiment, the condenser 16 features multiple heat exchanger tubes 26 in a parallel spaced arrangement, which are interconnected at each end by a U-shaped return bend interconnect.

The tube insert 25 can be made from Teflon™ , metal, or any material that is suitable to withstand the operating conditions inside the heat exchange tubing 26, e.g. operating temperature, fluid flow.

The tube inserts 25 are installed inside the heat exchanger during assembly of the heat exchanger. In this embodiment, the tube inserts 25 are inserted into each heat exchange tube 26 before the return bend interconnects at one end of the condenser are soldered (or attached in other ways as known in the art - glued, welded, brazed, etc ..) onto their respective heat exchange tubes 26. The tube inserts 25 are inserted into the open end of the respective tubes 26 until the locating feature 30 at the leading end of each tube insert 25 touches the return bend interconnect at the end of each tube 26. U- shaped interconnects are then soldered onto the open end of the heat exchange tubes 26 thereby completing the refrigerant pathway inside the condenser 16. The tube inserts are sized so that the locating feature 30 at the trailing end of each tube insert 25 should be protruding slightly from the heat exchange tube 26 such that this locating features 30 at both ends of the tube insert 25 will make contact with the return bend interconnects at both ends of the heat exchange tube 26.

In operation, the tube insert 25 when inserted inside the heat exchange tube 26 eliminates the "dead space" at the center of the tube 26 and forces the heat transfer fluid flowing through the tube into the annular space and thus close to the wall of the tube 26. In effect the tube insert 25 increases the heat transfer contact area per unit volume of heat transfer fluid thereby improving the heat transfer performance of the heat exchanger, or in the case of the present embodiments, of the condenser 16. More generally speaking, the tube inserts 25 serve to reduce the overall system volume inside the heat exchanger tubes 26, which is expected to be particularly beneficial in LHP and CPL configurations and in other applications that use small diameter refrigerant tubing. In the current state of the art, tube expanders cannot fit into tubing having smaller than a 5 mm diameters, and thus the tube inserts are particularly useful in reducing system volume in refrigerant tubing having this or a smaller diameter.

While a cylindrical body is shown in the Figures, the insert 25 can have other cross- sectional shapes; for example, the body 28 can have a polygonal or oval cross section in which case the transverse dimensions of the insert 25 should be smaller than the inner diameter of the heat exchange tube 26. Also, the spacers 29 can have different configurations so long as the spacers 29 serve to locate the body 28 inside the tube 26 to create an annular space and to allow the flow of refrigerant fluid through the tube 26; for example, the spacers 29 can be an annular disc that fits over the conical ends of the body 28 and has circumferentially located openings for flow of refrigerant there through. Furthermore, the spacer 29 may be positioned to fit around the central area of the body 28, in which case only one spacer 29 may be required. Alternatively, spacers 29 may be positioned at either end of the body 28 and around the central area of the body 28; this configuration may be useful for long heat exchange tubes 26 to ensure the insert 25 is correctly positioned along the length of the tube 26.

Condenser: Gravity Fed Through Design

The condenser shown schematically in Figures 1(a) and (b) are traditionally considered to be orientation independent. According to another embodiment, the condenser can feature a gravity feed through design that would be orientation dependent, but is particularly useful in clearing liquid from the heat exchange tubes in the condenser.

As shown in Figure 17, a gravity feed through condenser 116 can be used in the system. Such a condenser 116 comprises an upper manifold 117 with a vapor refrigerant inlet 118, a lower manifold 119 with a liquid refrigerant outlet 120, and an array of substantially vertically oriented heat exchange tubes 121 that fluidly couple the upper manifold to the lower manifold 119. The heat exchange tubes 21 are generally oriented vertically such that when vapor enters into the condenser 6 and is

condensed into a liquid, the liquid flows by gravity though the tubes 121 to the bottom manifold 119, thereby preventing any liquid pooling in other parts of the condenser tubing 121.

This design promotes heat exchange tubes that are clear of liquid such that the most if not all of the condenser volume can be used for heat exchange and condensation of vapor, regardless of vapor flow velocity and means other than gravity to clear the tubes.

Thermoelectric Cooler

According to another embodiment and Referring to Figure 5, the LHP 10 and CPL 12 apparatuses can further comprises a thermoelectric cooler ("TEC") 34 thermally coupled to the refrigerant loop and operable to further cool the liquid-phase refrigerant in the loop.

As is known in the art, a TEC uses the Peltier Effect to create a heat flux between the junction of two different types of materials. The TEC 34 is an electrically powered solid state active heat pump which transfers heat from the refrigerant to atmosphere. The TEC sub-cooler assembly has a copper base 37 having a refrigerant inlet 36 and a refrigerant outlet 38 fluidly coupled to the inlet 36; inside the base is pin structure (not shown) to promote the heat transfer in a manner that is known in the art. Electrical conductors 40 are electrically coupled to a power source (not shown) and to the TEC 34 and serve to power the TEC 34. A finned aluminum heat sink 42 serves to dissipate the transferred heat from the refrigerant into atmosphere.

The TEC 34 can be coupled to the liquid conduit 20 downstream of the condenser's refrigerant outlet. In operation, liquid phase refrigerant flows through the liquid conduit 20 and through the TEC 34 where the already-condensed refrigerant is further sub- cooled. In refrigeration, sub-cooling is the process by which a saturated liquid refrigerant is cooled below its saturation temperature, forcing it to change its phase completely. Sub-cooling the refrigerant by the TEC 34 is expected to ensure consistent and proper functioning of the heat exchanger apparatuses 10, 12. Evaporator Sub-Assembly (LHP Embodiment)

Several embodiments of the evaporator sub-assembly 14 of the LHP apparatus 10 are shown in Figures 8 to 16. At least some of principles in the LHP embodiments described here can also apply to the evaporator 23 of the CPL apparatus 10 (not shown) as will be explained further below.

According to one embodiment as shown in Figures 8-10 and 12-14, the evaporator subassembly 14 comprises the following major components: an evaporator base plate 44, a evaporator body 46 connected to the base plate 44 and defining the compensation chamber (reservoir) 22 therein, the capillary wicking material 24 inside the evaporator body 46 and in contact with the evaporator base plate 44, and a radiant heat shield 52 inside the evaporator body 46 and between the wicking material 24 and the

compensation chamber 22.

As visible in Figure 8, the evaporator body 46 is provided with a refrigerant inlet 54 and a refrigerant outlet 56. The refrigerant inlet 54 is coupled to the liquid conduit 20 and the refrigerant outlet 56 is coupled to the vapor conduit 18 as shown in Figure 10. The refrigerant inlet 54 opens into the compensation chamber 22 and receives liquid phase refrigerant from the liquid conduit 20. As shown in Figure 13, the heat shield 52 is provided with openings 58 spaced around the periphery of the heat shield 52 which provide a fluid passageway between the compensation chamber 22 and the wicking material 24. The wicking material 24 is porous and is in fluid communication with the refrigerant outlet 56. Collectively, the base plate 44, wicking material 24 and heat shield 52 comprise the evaporator 23 in the evaporator sub-assembly 14.

Referring to Figure 12, the base plate 44 is made of a high thermal conductivity material such as copper or aluminum and has an exterior surface and an interior surface. The interior surface has a plurality of parallel embossed ridges which define open faced vapor channels 60 there between. As can be seen in Figure 11 , the evaporator body 46 has an open bottom with an annular rim 59 that contacts and seats the base plate 44. The base plate 44 is aligned with the evaporator body 46 such that the vapor channels 60 are facing the refrigerant outlet 56. The annular rim 59 has a cavity around the refrigerant outlet 56 which serves as a flow manifold that provides a flow pathway from the vapor channels 60 to the refrigerant outlet 56.

In operation, the exterior surface of the base plate 44 is exposed to a heat source (such as electronic components on a circuit board), and heat is conducted efficiently through the base plate and into the evaporator. However, the heat shield 52 is made of a low thermal conductivity material such as Teflon™ to reduce the amount of "heat leak" through the wicking material inside the evaporator and into the compensation chamber 22. Additionally or alternatively, the heat shield 52 can be coated with a heat reflective coating such as silver, or gold plating to reduce heat leak by increasing heat reflectivity. Utilizing a silver coating can also be useful for anti-bacterial purposes, i.e. for hindering bacterial growth inside the refrigerant loop and reducing the need to chemical additives which tend to reduce performance of the working refrigerant.

Gasket ring 61 is provided to space the heat shield 52 from the surface of the wicking material 24. A secondary gasket ring 50 is located between the heat shield 52 and evaporator body 46.

In order to reduce heat back conduction it is desirable to have the entire back surface of the wicking material 24 coated with liquid refrigerant, as any dry and exposed areas conduct heat much more readily to the compensation chamber 22. Furthermore, in order to reduce fluid pressure drop through the wicking material 24, absorption through the entire surface is desirable. When the heat shield 52 is positioned close enough to the wicking material 24 (for example at a distance of 1 mm or less) and the liquid openings 58 in the heat shield 52 are small enough (for example having a diameter or diagonal of 1mm or less) it will naturally create a capillary effect between the heat shield and wicking material 24. As a result, regardless of the fill level of the compensation chamber 22, the entire surface of the wicking material 24 will be covered with liquid refrigerant, thereby reducing heat back conduction and liquid pressure drop through the wicking material 24.

According to another embodiment or optionally in this first embodiment, a thermal separator 51 is positioned in the join between the annular rim 59 of the evaporator body 46 and the base plate 44. The thermal separator 51 is a thin ring like structure made of a thermal insulting material coated with a metallic surface layer (for example nickel or the like). Similarly, a thermal separator can be positioned between the evaporator body and base plate of the evaporator 23 in the CPL apparatus 12. By thermal insulting material it is meant a material that has a low thermal conductivity to reduce the degree of heat that is thermally conducted from the base plate 44 through the annular rim 59 of the evaporator body 46 and into the compensation chamber 22, thereby reducing the amount of heat leak from the evaporator into the compensation chamber 22. For example, the thermal conductivity of the thermal insulting material may be less than 100 WmK, less than 75 WmK, less than 50 WmK, less than 30 WmK, less than 20 WmK, less than 10 WmK, less than 5 WmK, less than 4 WmK, less than 3 WmK, less than 2 WmK, less than 1 WmK, less than 0.5 WmK, or less than 0.25 WmK. The thermal insulting material may be plastic, for example engineered plastic polyetherimide (PEI), polyetheretherketone (PEEK) or the like. PEEK has a thermal conductivity of about 0.25 WmK compared to copper which has a thermal conductivity over 350 WmK. The metallic surface layer enables the separator 51 to be soldered directly to the metal evaporator body 46 and metal base plate 44, thereby providing a hermetic joint or seal between the evaporator body 46 and base plate 44 which is required for multiphase devices such as LHP and CPL heat transfer apparatuses. Instead of a ring like structure, the thermal separator may be a different shape depending on the shape of the evaporator body and base plate, for example, if the evaporator body and

corresponding base plate are cube shaped or rectangular shaped, the thermal separator will shaped to correspond to the periphery of the cube or rectangular evaporator body/base plate combination so as to snugly fit between the base plate and the evaporator body.

According to another embodiment or optionally in this first embodiment, the evaporator body 46 may be made of a material that has a lower thermal conductivity than the base plate 44. This also reduces the degree of heat that is thermally conducted from the base plate 44 through the annular rim 59 of the evaporator body 46 and into the

compensation chamber 22, thereby reducing the amount of heat leak from the evaporator into the compensation chamber 22. A suitable low thermal conductivity material for the evaporator body 46 is brass when the base plate 44 is made of copper. Similarly, the evaporator body of the evaporator 23 in the CPL apparatus 12 can be of a different thermal conductivity than the base plate of that evaporator 23.

Referring to Figures 10 and 11 and according to another embodiment or optionally in this first embodiment, the evaporator body 46 is provided with both external heat transfer fins 62 and internal heat transfer fins 64. These fins 62, 64 serve to conduct heat away from refrigerant in the compensation chamber 22 and into (cooler)

atmosphere, thereby reducing any deterioration in the degree of sub-cooling of the liquid refrigerant inside the compensation chamber 22. Instead of fins, other structures such as a pin, wave, or the like that increase surface area can be used to transfer heat away from the compensation chamber 22.

When the evaporator sub-assembly 14 is provided with the heat shield 52, low thermal conductivity evaporator body 46, thermal separator 51 , and internal and external fins 62, 64, the deleterious effects of heat leak into the compensation chamber 22 is expected to be reduced significantly.

Referring now to Figure 14 and according to another embodiment or optionally in this first embodiment, the wicking material 24 is provided with multiple layers each with different characteristics and designed to serve a specific function in the evaporator. In particular, the wicking material 24 has three layers, namely a top liquid absorption layer 70, a central control layer 72 and a bottom evaporation layer 74. Such a multiple layered wicking material 24 can also be used in the evaporator 23 of the CPL apparatus 12.

The liquid absorption layer 70 is the uppermost layer which contacts the heat shield 52 (if one is present) and is in fluid communication with liquid phase refrigerant in the compensation chamber 22. This layer 70 is made of a coarse material to facilitate liquid absorption into the wicking material 24.

The control layer 72 is sandwiched between the liquid absorption layer 70 and the evaporation layer 74 and serves to determine the actual head or pumping pressure generated by the wicking material 24. That is, the selection of the pore size of the control layer 72 has a direct correlation to the pumping pressure generated by the wicking material 24. The thickness of this layer 72 can be minimized to reduce pressure drop. In this embodiment, the largest pore size for the control layer 72 can be between 1 and 30 microns.

The evaporator layer 74 is the bottommost layer which contacts the interior surface of the base plate 44. Its pore size is selected to facilitate phase change (i.e. evaporation of the liquid phase refrigerant) with a low pressure drop, easy bubble creation and easy bubble departure. This pore size is usually finer than the pore size of the liquid absorption layer 70 but coarser than the pore size of the control layer 72. In this embodiment, the largest pore size for both the liquid absorption layer 70 and the evaporator layer 74 is at least 100 microns.

This three layered wicking material 24 is expected to improve thermal performance and operational characteristics of the evaporator sub-assembly 14 and reduce pressure drop through the wicking material structure than single layered wicking materials, thereby enabling higher head / pumping pressures.

The perimeter of the wicking material 24 is secured in place inside the evaporator body 46 by the clamping force of the body 46 against the base plate 44 (with the gaskets 61 and 50 and heat shield 52 if present also sandwiched there between); however, especially thin high performance wicking materials may deform during installation or operation, thus creating a small void between the base plate 44 and the wicking material 24. This can be detrimental to system performance as the void will tend to act as a thermal insulator thus reducing the performance of the evaporator.

Referring now to Figures 11 and 15 and according to another embodiment or optionally in this first embodiment, the evaporator sub-assembly 14 is provided with a brace 76 which protrudes from the top of the evaporator body housing and applies pressure to the wicking material 24 against the base plate 44. More particularly, the brace 76 is a post extending downwards from the top surface of the compensation chamber 22 and includes a gasket 78 at the bottom end of the post which makes contact with a central area of the heat shield 52 (or the wicking material itself in those embodiments that do not have a heat shield 52). In effect, the brace 76 impedes the formation of a void between the wicking material 24 and the base plate 44 (and also increases performance of the thermal joint by increasing pressure between wicking material and evaporator wall). While a single brace 76 is shown located in the middle of the wicking material 24, additional braces (not shown) can be provided to apply pressure onto other areas of the wicking material 24. Further, the brace 76 can have a different shape than a post, and can for example, have an "X"-shaped cross-section. Similarly, the evaporator 23 of the CPL apparatus 12 can feature a brace for the wicking material 24; such a brace extends not from the compensation chamber but from a wall inside the evaporator 23.

According to another embodiment or optionally in this first embodiment, the TEC 34 described above with reference to Figures 5-7 is mounted on top of the evaporator body 46 in close thermal proximity to the compensation chamber 22. As well as cooling the liquid phase refrigerant before it enters the compensation chamber 22, the mounted TEC 34 cools the evaporator body 46 to provide additional cooling to liquid refrigerant inside the compensation chamber 22 and to lower the temperature of the compensation chamber below ambient temperature conditions. This creates a heat load on the warmer base plate 44, thereby aiding low power start up conditions, as well as thermally de-coupling the compensation chamber 22 from the base plate 44.

Alternatively, one or more of the features in each of the evaporator sub-assembly embodiments described above can be included in yet another embodiment of the evaporator sub-assembly. For example, the evaporator sub-assembly 14 can include all, or some of the features in each of the described embodiments, namely, the multi- layered wicking material 24, the thermal separator 51 between the evaporator body 46 and base plate 44, the high thermal conductivity base plate 44 and low thermal conductivity evaporator body 46 combination, the radiant heat shield 52, external and internal fins 62, 64 on the evaporator body 46, the TEC 34 mounted on top of the evaporator body 46, and the brace 76 for applying pressure to the wicking material 24 against the base plate 44.

Evaporator Sub-Assembly (CPL Embodiment) As noted above a CPL apparatus 12 differs from a LHP apparatus 0 primarily in its absence of an evaporator sub-assembly 14. Instead, the compensation chamber (reservoir) is remotely located from the evaporator, unlike a LHP apparatus wherein the reservoir is in direct contact with the wicking material 24 in the evaporator subassembly. The CPL apparatus 12 also includes a heater (not shown) which is used to start the system.

The evaporator 23 of the CPL apparatus 12 still resembles the evaporator assembly 14 of the LHP apparatus 10 in the following manner:

• Both evaporators 23 have an evaporator body 46 and a base plate 44 with vapor channels, which in one embodiment can be of materials having different thermal conductivities, and which in another embodiment can include a thermal separator 51 between the evaporator body 46 and the base plate 44;

• Both evaporators 23 have a wicking material 24 inside the evaporator body 46, which in one embodiment can be braced by a brace like the brace 76 of the first embodiment, and which in another embodiment can be a three layered material like the wicking material 24 of the first embodiment.

Operation

In operation, the LHP apparatus 10 is started by clearing the evaporator vapor removal channels 60 and vapor transport lines from liquid and establishing a flow of condensed liquid refrigerant from the condenser 16 to the evaporator sub-assembly 14. By design, the LHP apparatus 10 will always have some liquid touching the wicking material 24, which enables startup without any prior startup sequences.

The CPL apparatus 12 on the other hand has a remotely located reservoir 22 and thus may face a situation of a dry wicking material 24 at start up. A heater on the reservoir (compensation chamber) 22 can be activated, which will cause liquid refrigerant to flow towards the wicking material 24 and pre-wet it for startup. Heat from a heat source, such as an electronic circuit that generates heat and requires cooling, conducts through the base plate 44 and into the wicking material 24. In the LHP apparatus 10, liquid phase refrigerant in the compensation chamber 22 will be drawn by capillary forces through the openings 58 in the heat shield 52 and into the wicking material 24.

For both the LHP apparatus 10 and the CPL apparatus 12, when the refrigerant is heated to its boiling temperature it changes phase from liquid to vapor. As there is a pressure gradient in the refrigerant between the two sides of the wicking material 24, the vapor phase refrigerant in the evaporator layer 74 of the wicking material 24 and vapor channels 60 of the base plate 44 move through the refrigerant outlet 56 and into the vapor conduit 8. The vapor phase refrigerant then flows through the vapor conduit 18 and into the condenser 16. More particularly, the refrigerant flows through heat exchange tubes 26 in the condenser 16 and is forced towards the inner wall of the tubes 26 by the tube inserts 25. Heat is transferred from the refrigerant through the tube walls and into the atmosphere, causing the refrigerant temperature to fall below its

condensation point and the refrigerant to condense into liquid phase. The liquid phase refrigerant then leaves the condenser 16 via the refrigerant outlet and flows through the liquid conduit 20 and through the TEC 34 which is coupled to the liquid conduit and may be mounted to the evaporator assembly 14 in the LHP apparatus 10. The condensed refrigerant is further cooled in the TEC 34 (the liquid phase refrigerant may already be a sub-cooled state at this time in which case cooling from the TEC 34 further contributes to maintaining the liquid refrigerant in its sub-cooled state). The sub-cooled liquid then enters the evaporator 23 for the CPL apparatus 12 or enters the evaporator subassembly 14 for the LHP apparatus 10 via the refrigerant inlet 54 wherein the cycle is repeated.

While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modifications of and adjustments to the foregoing embodiments, not shown, are possible.