WO2015132250A1

WO2015132250A1 - Wick structure for two-phase heat transfer apparatus

Info

Publication number: WO2015132250A1
Application number: PCT/EP2015/054401
Authority: WO
Inventors: Cosimo Buffone; Marco Marengo
Original assignee: Universite Libre De Bruxelles; Universita Degli Studi Di Bergamo
Priority date: 2014-03-05
Filing date: 2015-03-03
Publication date: 2015-09-11

Abstract

The present invention is related to a wick structure for use in a two-phase heat transfer apparatus, such as a loop heat pipe (LHP), capillary pumped loop (CPL) or a heat pipe. The wick structure of the invention is characterized by the presence of at least a portion of the wick structure consisting of graphene. Possible embodiments comprise elongate cylindrical wicks having vapour grooves formed between graphene ribs, or having porous ribs covered with a layer of graphene.

Description

WICK STRUCTURE FOR TWO-PHASE HEAT TRANSFER APPARATUS

Field of the invention

[0001] The present invention is related to two-phase heat transfer systems, in which a working fluid is transported from a condenser to an evaporator through capillary forces. The best known examples of such systems are heat pipes (HP), Loop Heat Pipes (LHP) and Capillary Pumped Loops (CPL). State of the art

[0002] Two-phase heat transfer systems referred to above are equipped with a porous portion referred to as a wick. The wick is configured to pump a working fluid to a higher pressure through the action of capillary forces in its porous structure. In loop heat pipes, the wick may be made from sintered metal, for example stainless steel, Cu, Ti, Ni or from plastics, for example polypropylene, or from sintered ceramic material, for example silicon carbide.

[0003] Loop Heat Pipes are able to transfer large quantities of heat

(>1 kW) over long distances (10-15m) with small temperature drops and employing small quantities of liquids. For a number of fields of the application, the wick pore size that can be produced by the existing wick materials is insufficient. For example, today propeller blades of aero engines are mainly made of either aluminium or composite. In icing conditions, the blades need to be heated in order to avoid that large pieces of ice form on the blades, which are at risk of being catapulted, creating hazards for people on the ground and people inside the fuselage. One of the limitations of LHP in aero engine applications (rotating blades) is the fact that high g-forces should be overcome by the liquid circulating inside the LHP. In fact, for thermal management of propeller blades (1 m long) rotating at over 1 ,000 rpm, the acceleration is higher than 1 ,000g. The wick pore size to overcome such acceleration is around 30nm when water is used as the working fluid. This cannot be reached by powder metallurgy usually employed in manufacturing of Nickel/Titanium wicks. [0004] In the article 'Modifying the heat transfer and capillary pressure of

Loop Heat Pipe wicks with carbon nanotubes', Terrado et al., J. Phys. Chem. C201 1 ,1 15,9312-9319, a bi-layer wick based on ceramic material with a thin layer of carbon nanotubes (CNT) on its outer surface is proposed. This approach succeeded in modifying and improving the thermal conductivity and capillary pressure of the wick. However, a CNT layer is not a structured layer, which raises concerns in terms of the predictability of its effect on the overall performance of the wick.

Summary of the invention

[0005] The invention is related to a wick structure for heat transfer systems of the types described above, and to such heat transfer systems as such. The invention is particularly related to wick structures and apparatuses as disclosed in the appended claims.

[0006] In short, the invention is related to a wick structure for use in a two- phase heat transfer apparatus, such as a loop heat pipe (LHP), capillary pumped loop (CPL) or a heat pipe. The wick structure of the invention is characterized by the presence of at least a portion of the wick structure consisting of graphene. Possible embodiments comprise elongate cylindrical wicks having vapour grooves formed between graphene ribs, or having porous ribs covered with a layer of graphene. The use of graphene is advantageous in that it provides an ordered structure with pore sizes smaller than about 30nm.

[0007] The invention is more particularly related to a wick structure for a two-phase heat transfer apparatus, configured to be in physical contact, on the one hand with a working fluid in liquid condition, and on the other hand with a vapour space, wherein at least a part of the wick structure configured to be in contact with a vapour space is formed of graphene.

[0008] According to one embodiment, the wick structure comprises a porous structure with graphene ribs (16) attached to said porous structure.

[0009] According to another embodiment, the entire wick structure is formed of graphene.

[0010] According to an embodiment, the wick structure comprises a porous structure covered at least partially with a layer of graphene.

[0011] According to an embodiment of the invention, the thickness of said layer is between 1 nanometer and 1 micron. According to further embodiments, the thickness of said layer is between 1 micron and 20 micron, or between 1 mircon and 10 micron. [0012] In a wick structure according to the invention, the porous structure may be an elongate flat or cylindrical body provided with longitudinal ribs, wherein the graphene layer covers the totality of the porous structure.

[0013] According to another embodiment, the wick structure comprises multiple radially placed wedge-shaped wick portions attachable to a central liquid reservoir, at least the vertical sidewalls of the wick portions being covered by a graphene layer.

[0014] The invention is equally related to an evaporator for use in a two- phase heat transfer apparatus, provided with a wick structure according to the invention. The invention is equally related to a Loop heat pipe or capillary pumped loop provided with an evaporator comprising a wick structure according the invention, and to a heat pipe provided with a wick structure according to the invention.

Brief description of the figures

[0015] Figure 1 a illustrates the basic components of a Loop Heat Pipe.

Figure 1 b illustrates the cross section of a wick for use in an LHP, as presently known in the art.

[0016] Figure 2a illustrates the cross section of an evaporator for use in an LHP provided with a wick structure according to a first embodiment of the invention, wherein the wick is made of a sintered metal core provided with ribs made of graphene.

[0017] Figure 2b illustrates the cross section of an evaporator for use in an LHP provided with a wick structure according to a second embodiment, wherein the wick is made of a sintered metal core provided with sintered metal ribs and provided with a layer of graphene.

[0018] Figure 2c illustrates the cross section of an evaporator for use in an LHP provided with a wick structure according to a third embodiment, wherein the wick is entirely formed of graphene.

[0019] Figure 3 shows a cross section of an evaporator for use in an LHP provided with a wick structure according to a fourth embodiment, wherein the evaporator has a flat structure and the wick is provided with a graphene layer.

[0020] Figure 4 shows a cross section of an evaporator for use in an LHP provided with a wick structure according to a fifth embodiment, wherein the evaporator has a circular structure and a plurality of wicks is provided, each provided with a graphene layer. Detailed description of the invention

[0021] This section mainly focuses on wicks for LHP and CPL evaporators. However, the invention is not limited to that particular domain. In LHP, the wick structure may consist of a primary wick and a secondary wick, the primary wick constantly being in contact with the liquid in a liquid reservoir, the secondary wick being placed around or in any case in contact with the primary wick. In CPL, a single wick is generally used. The working principle of CPL and LHP is, however, the same and in the context of the present invention no clear distinction between the two is necessary. The present invention is related to a wick structure that is either a single wick or a double wick structure comprising a primary/secondary wick, as well as to other wick structure designs comprising several wick portions, such as the radial wick portions of a circular evaporator (see further).

[0022] The working principle of CPL and LHP is briefly summarized here with reference to figure 1 a. A working fluid circulates in a loop comprising an evaporator 1 and a condenser 2. The evaporator comprises the wick 3 that is in constant contact with the working fluid in liquid state in a liquid reservoir 4. The liquid is pumped through the pores of the wick through capillary force and evaporates at the surface of the wick as it comes into contact with a heat source. The vapour flows through the vapour line 5 to the condenser, where it loses heat to the environment or an appropriately configured heat sink, thereby condensing and forming liquid again that flows back to the reservoir via the liquid line 6.

[0023] The wick 3 may have a tubular structure arranged in a cylindrical housing 7, as illustrated in Figure 1 b, which shows a cross section of the evaporator 1 . Liquid enters the evaporator through the central channel 10. Capillary force draws the liquid through the pores of the wick structure 3 towards the surface of the evaporator wick. Said surface is provided with longitudinal ribs 1 1 and grooves that form vapour channels 12 between the wick and the evaporator housing. The vapour channels form the so-called 'vapour space' of the evaporator, i.e. the area of the evaporator that fills up with vapour formed at the wick surface, said area being connected to the vapour line 5 of the LHP/CPL circuit. Other wick geometries are possible as will be described hereafter in the description, but the invention is firstly described for the case of the tubular wick shown in Figure 1 b.

[0024] The main innovative aspect of the invention when applied to

LHP/CPL is the use of graphene in the wick structure of the LHP/CPL. Graphene can be described as a one-atom thick sheet of the mineral graphite. It is a material that is known as such in the art. In a wick structure according to the invention, graphene is used at least on the exterior part of the wick, i.e. graphene is in physical contact with a vapour space of the evaporator. The term 'layer of graphene' in the context of this description must be understood as a layer consisting of one or more graphene sheets stacked on top of each other. The graphene layer preferably can comprise or consist of a plurality of graphene sheets in order to obtain a graphene thickness suitable for generating a significant capillary force. A graphene structure such as 'graphene ribs' described in relation to a number of embodiments are to be understood as 3- dimensional bodies formed by depositing a plurality of graphene sheets on top of each other, followed by the shaping of such a graphene stack into a 3-dimensional shape.

[0025] According to a first embodiment, illustrated in Figure 2a, the wick comprises a core portion 15 that may be made from a porous material known in the art for producing LHP or CPL wick structures, for example sintered metal (Ti or Ni e.g.). The ribs 16 of this wick are graphene portions attached to the core 15.

[0026] According to a second embodiment, a wick is provided having a core portion 15 and ribs 17 of a material known in the art, for example sintered metal. On top of this wick structure, i.e. covering the surface of the ribs 17 and of the valleys 18 in between the ribs, is a layer of graphene 19. Alternatively, a graphene layer covers only the ribs 17 (and not the valleys 18). The thickness of the graphene layer may be between a fraction of a nanometer up to several microns.

[0027] According to a third embodiment illustrated in Figure 2c, the complete wick 3 is made from graphene.

[0028] Other evaporator and wick geometries are possible. Figure 3 illustrates the cross-section of a flat evaporator. A flat elongated wick structure 3 is provided, again with longitudinal ribs 25 and grooves 26. The wick is in physical contact with a liquid reservoir located at one side of the evaporator, while the grooves lead to a vapour space located at the other side of the evaporator, where the vapour is collected and from where it is evacuated to the vapour line of the LHP/CPL circuit. In the embodiment according to the invention shown in the figure, a layer of graphene 27 covers the surface of the wick 3. Alternatively, the ribs 25 could be entirely formed of graphene and attached to a porous metal main body of the wick, or the entire wick 3 could be formed from graphene.

[0029] Figure 4 shows a circular evaporator structure. A liquid supply 33 is provided to a centrally placed liquid reservoir 34, to feed liquid to a plurality of wedge-shaped wick portions 35 that are radially placed within a flat and circular evaporator housing 36. At least on the vertical side surfaces of the wicks, a graphene layer 37 has been deposited. Vapour is collected in the vapour space consisting of the areas 38 between the wick portions 35 and of the vapour ring 39, from which the vapour is evacuated via the vapour outlet 40.

[0030] The invention is equally related to wicks for standard heat pipes, wherein at least a portion of the wick consists of graphene. The wick for a heat pipe could have the geometry of the longitudinal wicks described above and illustrated in Figures 2a to 2c and 3.

[0031] The wicks illustrated in Figures 2b, 3 and 4 are examples of the preferred embodiment wherein a layer of graphene is covering at least a portion of the outer surface of porous wick structure. A minimum layer thickness is required in order to ensure that a required capillary force is produced by the graphene layer. This minimum thickness may depend on the specific application (surface onto which the graphene is deposited, shape of the wick, etc.). It is preferred that the layer comprises more than one graphene sheet, so as to obtain a 3-dimensional pore structure in the layer. According to an embodiment, the graphene layer is between 1 nanometer and 1 micron. According to another embodiment, the thickness is between 1 and 20 microns. According to still another embodiment, the thickness is between 1 and 10 microns.

[0032] In all of the embodiments, in which graphene is attached to a porous structure, either in the form of ribs attached to a porous core portion (as in the embodiment of Figure 2a), or in the form of a graphene layer attached to a porous structure (as in the embodiments of Figures 2b, 3 and 4), the wick can be produced on a porous metal structure, for example on a porous Cu or Ni structure or a ceramic structure such as silicon carbide, by depositing graphene onto the structure by chemical vapour deposition (CVD) at a temperature of about 1000°C. Another possibility is the transfer of graphene from a copper foil onto the structure, after deposition of the film by CVD onto said foil. These techniques are known in the art, as illustrated for example in "Review of Chemical Vapor Deposition of Graphene and Related Applications", Y. Zhang, L. Zhang, C. Zhou (2013), Accounts of Chemical Research, 46(10), pp. 2329 - 2339.

[0033] Another way of producing graphene sheets is by liquid phase exfoliation of graphite, illustrated for example in "Liquid Exfoliation of Defect-Free

Graphene", J. Coleman (2012), Accounts of Chemical Research, 46(1 ), pp. 14 - 22.

This technique may be applied for depositing a layer of graphene on the outer surface of a porous structure to obtain the wick structure according to the invention.

[0034] Also the chemical synthesis of graphene is a technique that could be applied for producing a wick structure provided with a layer of graphene. For example, in "Gram-Scale Production of Graphene Based on Solovothermal Synthesis and Sonication", M. Choucair, P. Thordason, J.A. Stride (2009), Nature Nanotechnology, 4, pp. 30 - 33, a direct chemical synthesis of carbon nanosheets in gram-scale quantities was reported with a bottom-up approach based on reagents which are reacted to give an intermediate solid that is then pyrolized, yielding a fused array of graphene sheets that are dispersed by mild sonication.

[0035] The production of a wick structure according to Figure 2c fully consisting of graphene could be done by depositing a plurality of graphene sheets onto a cylindrical support that is afterwards removed. A full graphene wick having the flat geometry of Figure 3 could be produced by providing the evaporator housing with the top surface removed, depositing graphene sheets on the bottom of the evaporator, shaping the graphene to form the ribs 25, and then attaching the evaporator's top surface. Given the very low thickness of graphene sheets, the latter embodiments comprising full graphene wicks may be useful for the production of miniaturised cooling devices applicable for example in micro-electronic applications.

Claims

1. A wick structure (3) for a two-phase heat transfer apparatus, configured to be in physical contact, on the one hand with a working fluid in liquid condition, and on the other hand with a vapour space, wherein at least a part of the wick structure configured to be in contact with a vapour space is formed of graphene.

2. Wick structure according to claim 1 , wherein the wick structure comprises a porous structure (15) with graphene ribs (16) attached to said porous structure.

3. Wick structure according to claim 1 , wherein the entire wick structure is formed of graphene.

4. Wick structure according to claim 1 , wherein the wick structure comprises a porous structure covered at least partially with a layer of graphene.

5. Wick structure according to claim 4, wherein the thickness of said layer is between 1 nanometer and 1 micron.

6. Wick structure according to claim 4, wherein the thickness of said layer is between 1 micron and 20 micron.

7. Wick structure according to claim 5, wherein said thickness is between 1 micron and 10 micron.

8. Wick structure according to any one of claims 4 to 7, wherein the porous structure is an elongate flat or cylindrical body provided with longitudinal ribs (17,25), and wherein the graphene layer (19,27) covers the totality of the porous structure.

9. Wick structure according to any one of claims 4 to 7, wherein the wick structure comprises multiple radially placed wedge-shaped wick portions (35) attachable to a central liquid reservoir (34), at least the vertical sidewalls of the wick portions being covered by a graphene layer (37).

10. Evaporator for use in a two-phase heat transfer apparatus, provided with a wick structure according to any one of the preceding claims.

11. Loop heat pipe or capillary pumped loop provided with an evaporator comprising a wick structure according to any one of claims 1 to 9.

12. Heat pipe provided with a wick structure according to any one of claims 1 to 9.