US20100012299A1

US20100012299A1 - Heat exchanger unit

Info

Publication number: US20100012299A1
Application number: US12/523,579
Authority: US
Inventors: Hitoshi Sakamoto; Kazuyuki Mikubo; Sakae Kitajo
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2007-01-24
Filing date: 2008-01-15
Publication date: 2010-01-21
Also published as: CN101589287A; JPWO2008090726A1; WO2008090726A1

Abstract

According to the present invention, there is provided a heat exchanger unit having, over a base, a surface-modified portion composed of a metal, the surface-modified portion being brought into contact with a flow path provided for a liquid refrigerant, wherein the liquid refrigerant is a liquid having a surface tension smaller than that of water, and the surface-modified portion has a porous structure, in which a plurality of recesses are provided on the flow path side thereof, each recess has an introduction path having a cross-section area gradually reduced from the inlet of the recess, and a cavity communicated with the introduction path while placing an inflection portion in between, and the shortest distance between the inflection portion and the flow path is larger than the shortest distance between the cavity and the flow path.

Description

TECHNICAL FIELD

The present invention relates to a semiconductor heat exchanger unit, and in particular to a semiconductor heat exchanger unit making use of phenomenon of boiling.

BACKGROUND ART

For the purpose of conducting a large energy of heat generated by semiconductor devices, there has been developed a method of obtaining a high level of cooling effect based on latent heat of vaporization of a refrigerant capable of boiling at a temperature not higher than the upper limit temperature of operation of the semiconductor devices. In recent years, investigations have been focused on influences of surface conditions of a boiling surface and thermo-physical properties of refrigerant, which govern the size and density of generated vapor bubbles, aiming at stabilizing and optimizing the heat conduction effect obtainable by boiling of the refrigerant.
It has been known that the heat conduction characteristics may be improved in pool boiling on a flat plate, by forming an irregularity of several micrometers or smaller on the surface of the flat plate. The first possible reason may be such that fine projections and notches contribute to increase the area of contact between the heat radiation surface and the refrigerant.
The second possible reason may be such that the micro-structure of the flat plate contributes to formation of nuclei of vapor, which is the initial stage of boiling bubbles.
Patent Document 1 describes formation of a surface irregularity of a nanometer scale. This document describes that any surface structure “larger than” several micrometers may be “less likely to ensure effective nuclei for bubbling”, with respect to a refrigerant having a small surface tension. It has, however, been known from theoretical examinations into simple micro-notches that physical properties of the refrigerant may he important parameters with respect to the minimum diameter of a point of bubbling, but the thickness of a liquid layer super-heated beyond the boiling point in the vicinity of a heat conduction surface may be predominant with respect to the maximum diameter of the point of bubbling, as taught by literatures and so forth. This is because, in the process of growth of boiling bubbles, supply of the vapor necessary for growing the bubbles may be suppressed, as soon as the vapor composing the bubbles is brought into contact with a vapor not super-heated yet.
More ideal geometry of the micro-structure may be such as allowing the nuclei to stay on the heat radiation surface even after the grown bubbles dissociate from the surface, so as to promote growth of the next bubbles, and may further be such as avoiding disappearance of the nuclei by condensation, even when they are brought into contact with a liquid, cooled to a temperature slightly lower than the boiling point, flowing thereinto as a result of dissociation of the bubbles.
As explained in Non-Patent Document 1, any conventional efforts for assimilating such ideal geometry have resulted in thickening over the entire surface geometry (100 μm to 1 mm or thicker), and any efforts have failed in obtaining the ideal geometry of the recesses.
The thickness and the recesses of several hundred micrometers to 1 mm or around may result in higher thermal resistance as compared with smaller structures, and may raise a problem in retention property of the vapor nuclei, when a refrigerant having a surface tension smaller than that of water is used.
Furthermore, while the geometry shown in Non-Patent Document 1 is not given with a scale, the size and so forth of the opening in an actual structure may vary, so that it may be very easy to presume that this raises negative effects, depending on physical properties of the refrigerant.
Although various micro-structures have been experimented, no invention has been successful to provide a surface geometry obtained by optimizing an ideal double-inlet structure with respect to an actual refrigerant. Also no invention has been made on a heat exchanger unit having a surface modification assimilating an ideal structure described in Non-Patent Document 1, provided in contact with the flow path for liquid refrigerant, so as to be optimized with respect to physical properties of the refrigerant.
It has widely been known that an effect of heat conduction is enhanced by boiling, but the degree of effect actually obtainable may be affected by density of bubbles generated in the process of boiling, frequency of dissociation, and size of bubbles in the process of dissociation, and may largely be affected also by surface conditions of the boiling surface, which may be supposed to be an important factor governing these parameters. Various trails have been made on the surface conditions, only to fail in achieving the ideal geometry, because thermal resistance may increase as the thickness of the surface increases.
The surface conditions ideal for boiling may be such as promoting generation of vapor bubbles and allowing growth thereof, wherein retention of “nuclei” which provide origins of growth of the bubbles may be indispensable. The nuclei are fine vapor bubbles, and remain on the surface even after the grown bubbles dissociate therefrom, so as to facilitate growth of next bubbles. The surface geometry may therefore be useful to have a structure capable of retaining the fine bubbles. For stable retention of the nuclei, it may firstly be necessary that the structure for retaining the nuclei allows a refrigerant, even if the surface tension of which being smaller than that of water, to flow therein only with difficulty. Secondly, it may be necessary that the structure is not causative of condensation of the vapor bubbles in the process of dissociation, even if the vapor bubbles are brought into contact with a liquid flown to the vicinity of the nuclei, while being cooled below the boiling point. Most of the surfaces at present are not structurally idealized, and is therefore far from being optimized for the nuclei, with some exceptions exhibiting the effect of heat conduction by virtue of the micro-structure.
FIG. 9 illustrates an exemplary case where a refrigerant having a small surface tension is adopted to a heat conduction surface having V-shape notches as described in Patent Document 1. It may theoretically be supposed that the gas-liquid interface having a profile concaved towards the liquid 22 side may have a large radius of curvature, thereby dissociation of the nuclei is promoted, and retention of the vapor bubbles may be destabilized. On the other hand, FIG. 10 illustrates an exemplary case where a refrigerant having a large surface tension such as water was used on the same heat conduction surface. In this case, the radius of curvature of the gas-liquid interface concaved towards the liquid side becomes smaller, so that the nuclei may be made more stable. It may therefore be understood from the above that the radius of curvature of the gas-liquid interface is determined by the angle of contact point of three phases, that are gas, liquid and solid phases ( reference numerals 20 and 21 in FIG. 9 and FIG. 10, representing the angles on the liquid side), and that use of the V-shape structure for a refrigerant having a small surface tension may result in only a poor effect of retention of vapor nuclei.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-228389 (p. 3-4, FIG. 2)
[Non-Patent Document 1] Liquid-Vapor Phase-Change Phenomena (p. 330, FIG. 8.14)

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a heat exchanger unit having, over a base, a surface-modified portion composed of a metal, the surface-modified portion being brought into contact with a flow path provided for a liquid refrigerant, wherein the liquid refrigerant is a liquid having a surface tension smaller than that of water, and the surface-modified portion has a porous structure, in which a plurality of recesses are provided on the flow path side thereof, each recess has an introduction path having a cross-section area gradually reduced from the inlet of the recess, and a cavity communicated with the introduction path while placing an inflection portion in between, and the shortest distance between the inflection portion and the flow path is larger than the shortest distance between the cavity and the flow path.
In view of raising an effect expected for the case where a refrigerant having a surface tension smaller than that of water is adopted, the recess may preferably have a pore size of 1 μm to 10 μm.
In addition, the liquid refrigerant may preferably be an organic refrigerant, and the organic refrigerant may preferably be a hydrofluoroether or a fluorine-containing inert liquid.
The heat exchanger unit may preferably be configured to have the surface-modified portion, along a flow path of micro-channel type as a forced-convection boiling refrigerant type cooling unit.
According to the present invention, generation of the boiling bubbles may he promoted by providing the surface-modified portion having a multiple-inlet structure to the flow path for a liquid refrigerant having a surface tension smaller than that of water.
According to the present invention, various problems ascribable to generation of bubbles may be solved in an unified manner, by adopting the surface-modified portion of the present invention to other types of boiling refrigerant type cooling units.
A surface condition having a structure with cavities will now be discussed. FIG. 1 illustrates a state of liquid 1 having a surface tension smaller than that of water, flowing through an introduction path 2, having a simple inlet structure, into a cavity 3. By virtue of the direction of the upper wall surface of the cavity 3, the radius of curvature of the gas-liquid interface 4 achieved herein is apparently more closer to that achievable under a larger surface tension, as compared with that achievable in a V-shape notch. Recess structures, into which even a refrigerant having a surface tension smaller than that of water is less likely to flow, may be those having a double-inlet structure.
FIG. 2 illustrates a simple double-inlet structure. The liquid 1 having a surface tension smaller than that of water, which is ready to go through the introduction path 2, creeps on the wall surface of the upper portion of the cavity 3 and enters the recess, wherein the cavity 3 expresses resistance against further intrusion of the liquid, while making the radius of curvature of the gas-liquid interface 5 recessed to the vapor side, depending on the structure of the wall surface of the recess. None of the surface geometries ever manufactured have such double-inlet structure.
It may not always be necessary that the actual micro-structure is completely identical to that illustrated in FIG. 2, wherein a state of allowing stable residence of bubbles by virtue of the double-inlet structure, and uniform provision of the structure over the entire heat conduction surface may contribute to improvement and optimization of the heat conduction effect, based on boiling as a macroscopic consequence. The vapor bubbles of a refrigerant having a small surface tension may further downsize themselves in the process of dissociation, so that micronization of the recess structure to as small as several micrometers or around may be effective in view of promoting the downsizing. As a consequence, the boiling may be expected to start at a temperature more closer to the boiling point.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A schematic drawing illustrating an exemplary case where a refrigerant having a surface tension smaller than that of water enters a cavity.

[FIG. 2] A recess having a double-inlet, and expressing a large resistance against intrusion even with respect to a refrigerant having a small surface tension.

[FIG. 3] A porous copper plating having a multiple-inlet structure.

[FIG. 4] Relations between the radius of bubbles and ambient liquid temperature in the process of boiling.

[FIG. 5] A boiling refrigerant type cooling unit as one example of the present invention.

[FIG. 6] A heat conduction surface for pool boiling as one example of the present invention, selectively modified on the surface at the center thereof.

[FIG. 7] A perspective view illustrating a parallel straight pipe micro-flow path heat exchanger as one example of the present invention.

[FIG. 8] A thermal siphon-type, boiling refrigerant heat exchanger making use of boiling on a vertical flat plate, as one example of the present invention.

[FIG. 9] A three-phase contact angle and a radius of curvature of the gas-liquid interface obtainable by a liquid having a small surface tension in a V-shape notch.

[FIG. 10] A three-phase contact angle and a radius of curvature of the gas-liquid interface obtainable by a liquid having a large surface tension in a V-shape notch.

BEST MODES FOR CARRYING OUT THE INVENTION

Next, exemplary embodiments of the present invention will be detailed referring to the drawings.
FIG. 3 illustrates an electron microphotograph obtained when the surface of a porous copper plating is observed. Materials of the plating may preferably be copper characterized by a large heat conductivity, or may be nickel.
It is found from the result of observation, from the top of the plated surface formed on a flat surface, that micro-recesses of a maximum of approximately 10 μm are formed. These recesses uniformly arranged are internally communicated with each other, and contribute to make boiling phenomenon uniform. On the ribs, there are formed innumerable micro-recesses with a variety of sizes. It has been confirmed that, by virtue of these micro-recesses arranged so as to surround the larger ones, the smaller recesses more earlier help growth of the nuclei, and allow the boiling to start at temperatures more closer to the boiling point.
The present invention improves reliability of boiling refrigerant type cooling unit, by manufacturing a heat conduction surface capable of retaining a large number of vapor nuclei which originate growth of boiling bubbles, and thereby making generation of bubbles uniform, both on the spatial basis and on the temporal basis. The heat exchanger of the present invention is characterized by providing a porous modified surface having a multiple inlet structure with a pore size of 1 μm or larger and 10 μm or smaller, to the wall surface which serves as the boiling surface of refrigerant.
The minimum pore size of 1 μm herein is determined by comparing a pore size of a point of bubbling possibly becoming active, obtained by a theoretical calculation based on a simple notch model, with a pore size obtained based on a growth model of vapor bubbles. The minimum pore size (r′) capable of actively generating the vapor bubbles may be obtained by the equation (1) below, described in Liquid-Vapor Phase-Change Phenomen, p. 183.
$\begin{matrix} r^{*} = \frac{2 σ T_{sat} v_{l v}}{h_{l v} [T_{l} - T_{sat}]} & (1) \end{matrix}$
In the equation, a represents surface tension, T_satrepresents saturation temperature, v_lvrepresents difference in specific volume between vapor and liquid, and h_lvrepresents latent heat of vaporization, all of which being physical property values of a refrigerant. T_lrepresents liquid temperature in the vicinity of bubbles, and indicates that a larger degree of super-heating (T_l-T_sat) activates notches of smaller pore size. It has, however, been predicted that a refrigerant having a smaller value of surface tension σ, such as organic refrigerant, may become active on a smaller surface structure, because of molecule-dependent nature of surface tension, contradictory to the discussion given in Patent Document 1. According to the equation (1), once the refrigerant is given, the pore size is automatically determined based on difference (ΔT) between ambient temperature (T_l) and saturation temperature (T_sat) of the refrigerant. FIG. 4 illustrates a relation between r′ and ΔT by a solid line, referring to an exemplary case of using an organic refrigerant having a small surface tension.
It is predicted that the bubbling may become active making use of a given difference, if the pore size R (μm) falls in the region above the solid line in the graph.
On the other hand, active growth of the boiling bubbles requires nuclei for originating the growth, so that the pore size which is supposed to be effective for generation of the nuclei, may be determined also from the viewpoint of growth process of bubbles. The process of growth of vapor bubbles may generally be divided into a first stage allowing formation of non-matured bubbles called embryos, and allowing the bubbles to grow immediately thereafter based on difference between the inner and outer pressures; and a succeeding second stage allowing the non-matured bubbles to grow at the gas-liquid interface while being promoted by heat conduction. In the second stage, the bubbles grow just corresponding to supplied energy, and the grown-up bubbles further grow by absorbing energy through their enlarged surface area. On the other hand, in the first stage, the growth of bubbles is necessarily preceded by accumulation of pressure necessary for the growth as enough as pushing aside the ambient liquid, so that accumulation of energy does not directly. result in increase in size of the nuclei. For this reason, an ideal size of the vapor nuclei which can be surrounded by the surface structure may be equivalent to a diameter (r_trans) transiently attainable between the first stage and the second stage. The transiently-attainable diameter may be determined by considering the energy balance, as indicated by a dotted line in FIG. 4. In the region where the pore size R (μm) falls above the dotted line, the bubbles can grow depending on heat energy conducting towards the vapor bubbles surrounded by the surface structure.
In other words, it may be said from comparison between the solid line and the dotted line in FIG. 4, that the region above the dotted line may correspond to a structure assisting the first stage of growth of bubbles, and the region above the solid line may correspond to a structure assisting the second stage and dissociation. Since both regions are necessary for the purpose of achieving heat conduction effect by boiling, so that the surface structure which belongs to the region above both lines may be ideal. Since the temperature of the ambient liquid may vary depending on the state of operation, and since it may be necessary to consider the pore size at the point where both lines intersect, physical properties of an organic refrigerant having a small surface tension, and the thickness of the modified surface which is causative of thermal resistance and is necessarily be suppressed, it may be optimum to adjust the pore size within the range from 1 to 10 μm. The organic refrigerant adoptable herein may be exemplified by a hydrofluoroether or fluorine-containing inert liquid.
A large amount of bubbles may be produced in the process of boiling, from the surface-modified portion of the present invention. It may therefore be necessary to keep the state of supply of the liquid to the heat conduction surface at a high level, so that a mode of boiling refrigerant type cooling, which can efficiently expel the bubbles from the heat conduction surface by forced convection, may be considered as optimum.

Examples

A heat exchanger unit provided with a surface-modified portion of the present invention will be explained.

Example 1

As illustrated in FIG. 5, a heat exchanger unit given with a flow path 12 for refrigerant, formed so as to allow the refrigerant to pass over a flat plate 7 provided with a surface-modified portion, is manufactured. The heat exchanger unit has a heating element 6, and a heat exchanger block 10 provided over the heating element 6. The heat exchanger block 10 has a heat exchanger upper holder 11 and a modified surface 7. The heat exchanger upper holder 11 is provided with an inlet 8 and an outlet 9, allowing therethrough introduction and discharge of a liquid refrigerant, respectively. Inside the heat exchanger block 10, there is provided the flow path 12 allowing therethrough circulation of a liquid refrigerant. Since the flow path 12 is provided so as to allow the refrigerant to pass over the modified surface 7, the flow rate of the refrigerant may be varied, making it possible to avoid adverse effects such as dry-out ascribable to generation of a large amount of bubbles.

Example 2

The heat conduction surface provided with the surface-modified portion may be used under pool boiling, while needing a mode capable of avoiding the dry-out. One possible method may be such as selectively providing a surface-modified portion 14, rather than providing it over the entire surface of the heat conduction surface 13. For example, as illustrated in FIG. 6, the surface-modified portion 14 may possibly be provided only at around the center of the heat conduction surface 13, so as to induce natural convection.
The number of convection cells on the flat plate is an issue of the Rayleigh-Benard Convection problem, and is determined by a function of the depth of refrigerant and the area of the bottom surface. In this Example, by selectively providing surface-modified portion 14 while making use of the convection cells, the boiling bubbles may effectively be taken apart from the heat conduction surface soon after dissociation, and thereby the liquid may be supplied to the heat conduction surface 13.

Example 3

The surface-modified portion of the present invention may be adoptable also to a heat conduction portion having a form generally called micro-channel, among forced-convection boiling refrigerant type cooling units, aimed at achieving a heat conduction effect by forming fine flow paths (FIG. 7). The micro-channel limits the width of the flow path to as small as the size of the boiling bubbles or below, in order to extremely expand the contact area between the heat conduction surface and a refrigerant. As a consequence, the bubbles may stagnate and the flow may accordingly be unbalanced among the flow paths, depending on refrigerant and conditions of flow.
By providing the surface modification of the present invention, the boiling bubbles may be allowed to generate uniformly in the flow paths, and may therefore be reduced in size when they dissociate from the heat conduction surface. Since small bubbles are unlikely to stagnate, so that the flow may more readily be balanced among the flow paths if the stagnation of bubbles may be avoided.

Example 4

By further providing the surface for receiving heat and allowing boiling to proceed thereon in the vertical direction, so as to allow the vapor bubbles 17 generated in a space having a micro-thickness to ascend making use of buoyancy, an upflow may successfully be produced where the surface-modified portion 14 is provided (FIG. 8). By providing a heat radiation structure such as a fin 15 to the exterior of the top of the heat exchanger so as to radiate heat to the ambient air, vapor produced inside the heat exchanger may be condensed. The condensed liquid refrigerant passes through a portion provided alongside of the main heater unit, having no surface-modified portion provided Thereto, and is supplied to the lower portion of the heat exchanger. In FIG. 8, reference numeral 16 represents the gas-liquid interface, reference numeral 18 represents flow of the vapor bubbles 17, and reference numeral 19 represents flow of the liquid.
This is a currently-available structure based on the principle of thermal siphon, but is capable of causing more faster, and nearly-forced natural convection, from which the heat conduction effect may more readily be obtained, by virtue of a marked difference in the generated vapor bubbles depending on presence or absence of the surface-modified portion 14.
Applications of the present invention may be exemplified by a cooling unit for semiconductor, such as CPU, in need of a heat conduction effect larger than that obtainable by natural air cooling.

Claims

1. A heat exchanger unit having, over a base, a surface-modified portion composed of a metal, said surface-modified portion being brought into contact with a flow path provided for a liquid refrigerant,

wherein said liquid refrigerant is a liquid having a surface tension smaller than that of water, and

said surface-modified portion has a porous structure, in which a plurality of recesses are provided on the flow path side thereof,

each recess has an introduction path having a cross-section area gradually reduced from the inlet of said recess, and a cavity communicated with said introduction path while placing an inflection portion in between, and

the shortest distance between said inflection portion and said flow path is larger than the shortest distance between said cavity and said flow path.

2. The heat exchanger unit as claimed in claim 1,

wherein said liquid refrigerant is an organic refrigerant, and said organic refrigerant is a hydrofluoroether or fluorine-containing inert liquid.

3. The heat exchanger unit as claimed in claim 1,

wherein said recess has a pore size of 1 μm to 10 μm.

4. The heat exchanger unit as claimed in claim 1,

further comprising a flow path of micro-channel type as a forced-convection boiling refrigerant type cooling unit, and said surface-modified portion is provided along said flow path.