US20130269918A1

US20130269918A1 - Heat exchanger

Info

Publication number: US20130269918A1
Application number: US13/727,803
Authority: US
Inventors: Takuya Hongo; Tomoyuki Suzuki; Tomonao Takamatsu; Mitsuaki Kato; Katsumi Hisano
Original assignee: Individual
Current assignee: Toshiba Corp
Priority date: 2012-04-12
Filing date: 2012-12-27
Publication date: 2013-10-17
Also published as: JP5743948B2; CN103376021A; JP2013221625A

Abstract

According to an embodiment, a heat exchanger is provided with a manifold, a heat exchange unit and a first porous body. The manifold has an inlet for a medium and an outlet for the medium. The heat exchange unit has a channel which communicates with the outlet. The first channel has a cross section of a typical length which is not more than a predetermined constant. The porous body is provided between the inlet and the outlet, and contains a plurality of pores with a mean diameter which is not more than the typical length.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-091230, filed on Apr. 12, 2012, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a heat exchanger.

BACKGROUND

In recent years, radiators are sought to be developed in order to address an increase in heat generation density with higher integration of semiconductor devices, or miniaturization of electric devices such as a mobile phone. A momentum for recovering energy efficiently even from a low-temperature heat source radiating heat is enhanced from the view point of preventing global warming. Such heat has been wasted as exhaust heat. Accordingly, development of a heat exchanging technology using a microchannel heat exchanger progresses. The microchannel heat exchanger employs channels having a diameter of about tens of micrometers to 1 mm to realize a small-sized and highly efficient heat exchanger.
However, in the microchannel heat exchanger, air bubbles can not be removed from a heat-exchange medium thoroughly, even if remaining air bubbles are removed from the medium as much as possible by deaerating the medium. As a result, air bubbles may enter into the channels with flow of the medium to clog up the channels. The air bubble clogging reduces heat-exchange performance of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a configuration of a heat exchanger in accordance with a first embodiment schematically.

FIG. 2 is a cross section showing an inside of the heat exchanger of FIG. 1.

FIG. 3 is a sectional view taken along a X-X plane of the heat exchanger of FIG. 1.

FIGS. 4A and 4B are views to explain a relation between a microchannel and an air bubble in the heat exchanger in accordance with the first embodiment.

FIGS. 5A and 5B are views to explain operation of the heat exchanger in accordance with the first embodiment.

FIG. 6 is a characteristic view to explain an experimental result of a performance of the heat exchanger in accordance with the first embodiment.

DETAILED DESCRIPTION

A heat exchanger in accordance with an embodiment is provided with a first manifold, a heat exchange unit, and a first porous body. The first manifold has a first inlet for a first medium, and a first outlet for the first medium. The heat exchange unit has a first channel which communicates with the first outlet. The first channel has a cross section of a typical length H₁which satisfies the following expression. The first porous body is provided between the first inlet and the first outlet, and contains a plurality of pores with a mean diameter which is not more than the typical length H₁. In the above expression, σ₁is a surface tension of the first medium at the first outlet, g is a gravitational acceleration, ρ₁is a liquid-phase density of the first medium at the first outlet, and ρ₂is a gas-phase density of the first medium at the first outlet.
$H_{1} \leq \sqrt{\frac{σ_{1}}{g (ρ_{1} - ρ_{2})}}$
Hereinafter, further embodiments will be described with reference to the drawings.
In the drawings, the same reference numerals denote the same or similar portions respectively.
A first embodiment will be described below.
FIG. 1 is a view showing a configuration of a heat exchanger in accordance with the first embodiment schematically. FIG. 2 is a cross section taken in parallel to the plane of the paper, and shows an inside of the heat exchanger of FIG. 1. FIG. 3 is a sectional view taken along the X-X plane of the heat exchanger of FIG. 1.
As shown in FIGS. 1 and 2, the heat exchanger of the embodiment is provided with a heat exchange unit 10, a first manifold 20, a second manifold 30, a heat receiving unit 40, flow tubes 50 and 60, and porous bodies 70 and 80. The heat exchange unit 10 exchanges heat between first and second mediums. The first manifold 20 divides a flow of the first medium. The second manifold 30 divides a flow of the second medium. The heat receiving unit 40 receives heat from a heat source (not shown). The flow tube 50 is connected to the heat receiving unit 40, and flows the first medium. The flow tube 60 flows the second medium. As shown in FIG. 2, the porous body 70 is provided inside the first manifold 20 so that the porous body 70 can block the flow of the first medium. The porous body 80 is provided inside the second manifold 30 so that the porous body 80 can block the flow of the second medium. The embodiment employs water as the first and second mediums. The first and second mediums are employed as a high-temperature medium and a low-temperature medium, respectively.
The heat exchanger can be used for a hot water dispenser, for example. Specifically, the heat receiving unit 40 is provided near a heat source. The heat receiving unit 40 receives heat from the heat source and provides the received heat to the first medium. The heat exchange unit 10 exchanges heat between the first medium and the second medium. As a result, the heat of the heat source is transported from the first medium to the second medium. Further, heat from the second medium is transported to a liquid in the hot water dispenser, which can rise the temperature of the liquid. In the embodiment, the first medium or the second medium is conveyed by natural convection.
As shown in FIG. 3, the heat exchange unit 10 is a laminar member having channels including first channels 11 and second channels 12 with small diameters. The channels are referred to as “microchannels” hereinafter. The microchannels penetrate the heat exchange unit 10 in a thickness direction of the heat exchange unit 10. The first channels 11 and the second channels 12 are provided to be in parallel with the thickness direction of the heat exchange unit 10. In the following description, the typical length of the cross sections of the microchannels is defined as being not more than a Laplace constant of water (about 2.5 mm) under the conditions of a room temperature and an atmospheric pressure.
For the heat exchange unit 10, high thermal conductive materials such as aluminum, copper or stainless steel may be used. As shown in FIG. 3, the microchannels of the heat exchange unit 10 are illustrated as having circular cross sections. The cross sections of the microchannels may be ellipsoidal, semicircular, rectangular or various form in addition to the circular form.
As shown in FIG. 3, the microchannels of the heat exchange unit 10 are arranged in a lattice. In the embodiment, the microchannels are arranged to have four columns in a x-axis direction and four rows in a y-axis direction so that the number of the microchannels is 16 totally. In the following description, the microchannels are treated as a plurality of microchannel groups which have four rows and one column respectively. The microchannel groups which consist of the first channels 11 are defined as “A”, and the microchannel groups which consist of the second channels 12 are defined as “B.”
As described below in detail, the heat exchange unit 10 exchanges heat between the first medium which passes through the groups A, and the second medium which passes through the groups B. Thus, the performance of the heat exchanger affects the flow of the first medium or the second medium inside the respective microchannels. For example, air bubbles may be produced during circulation of the first medium or the second medium. The air bubbles may enter into the respective microchannels for circulating the respective mediums, which prevents the flow of the first medium or the second medium in cases.
The influence of air bubble clogging in the microchannels is considered to be suppressed by setting the mass flow rate of the first medium or the second medium which flows in each microchannel to a high rate of 100-200 kg/(m²·s), for example. However, such a high mass flow rate may increase a pressure loss between the inlet and the outlet of each microchannel. Accordingly, the mass flow rate is desirably as low as possible. For example, a mass flow rate as high as 10 kg/(m²·s) is desirable in a case of transporting mediums by natural convection as the embodiment.
Further, the influence of air bubble clogging in each microchannel may be eliminated by enlarging the diameter of each microchannel. However, the number of microchannels per unit volume of the heat exchange unit 10 needs to be reduced when the size of the heat exchange unit 10 is maintained. As a result, a heat transfer area decreases. Thus, the diameter of each microchannel is desirably as small as possible in order to enhance heat conducting performance.
In the embodiment, the typical length of each microchannel that is the minimum length H of the cross section of each microchannel in a direction of gravitational force is defined as follows.
$\begin{matrix} H_{1} \leq \sqrt{\frac{σ}{g (ρ_{f} - ρ_{g})}} & (1) \end{matrix}$
In the expression, a denotes a surface tension of a medium, g denotes gravitational acceleration, ρ_fdenotes a liquid-phase density of the medium, and ρ_gdenotes a gas-phase density of the medium. In the embodiment, the typical length H of the section of each microchannel is a diameter of each microchannel. The right-hand side of the expression (1) represents a Laplace constant. As the surface tension a, a surface tension at the inlet of each microchannel is employed which is calculated in advance based on designed use conditions such as temperature and pressure for performing the heat exchanger. As the liquid-phase density ρ_rand the gas-phase density σ_g, a density at the inlet of each microchannel is employed.
A Laplace constant shows a scale under which the gravity force acting on a medium and the surface tension of the medium becomes equal to each other, generally. Thus, when a scale larger than the Laplace constant is used, the gravity force acting on a medium is larger than the surface tension of the medium. When a scale smaller than the Laplace constant is used, the surface tension of a medium is larger than the gravity force acting on the medium.
As shown in FIG. 4A, in a case where the diameter of each microchannel is larger than the Laplace constant, when an air bubble 90 goes into each microchannel 11 or 12, the gravity force acts on the medium more than the surface tension of the medium. Thus, the bubble 90 is easy to be eccentrically-located in the direction of the gravity force inside each microchannel. In contrast, as shown in FIG. 4B, in a case where the diameter of each microchannel 11 or 12 is less than the Laplace constant, when the air bubble 91 goes into each microchannel 11 or 12, the surface tension acts on the medium more than the gravity force acting on the medium. Accordingly, the air bubble 91 is easy to be located to prevent flow of the first medium or the second medium independently on the direction of the gravity force rather than being eccentrically-located in an opposite direction of the gravity force.
As a lower limit of the typical length H shown in the above expression, 20 μm may be used. When the typical length H is less than 20 μm, each microchannel is difficult to be manufactured and the pressure loss increases remarkably. The respective microchannels may have the same diameter, or have different diameters within a range of the above expression.
The heat exchange unit 10 may be manufactured by laminating laminar members which have a plurality of through-holes with microscopic diameters, or by laminating laminar members which have many grooves with microscopic diameters. The laminar members may be fixed to each other by diffusion bonding, brazing, etc. The heat exchange unit 10 may be manufactured using a single member which has through-holes. The through-holes or grooves may be made by chemical processing such as etching, electroforming, optical shaping or anode oxidation, or by mechanical processing such as drilling, press working, electrospark machining or laser machining.
As shown in FIG. 2, the first manifold 20 is a hollow member which has a first inlet 21 and a first outlet 22 and allows the first medium to flow through the member. The first manifold 20 divides the flow of the first medium which enters into a hollow interior of the member, and discharges the first medium into the heat exchange unit 10 via the first outlet 22.
The first manifold 20 has the first inlet 21, a chamber 23, a buffer 24, and the first outlet 22 which are provided along a flow direction of the first medium. The chamber 23 is filled with the first medium. The buffer 24 divides the flow of the first medium. The first inlet 21 communicates with the flow tube 50. The first outlet 22 communicates with respective ends of the microchannel groups A of the heat exchanger 10 shown in FIG. 3. In addition, the other ends of the microchannel groups A communicate with the flow tube 50.
The second manifold 30 is a hollow member which has a second inlet 31 and a second outlet 32 and allows the second medium to flow through the member. The second manifold 30 divides the flow of the second medium which enters into a hollow interior of the member, and discharges the second medium into the heat exchange unit 10 via the second outlet 32.
The second manifold 30 has the second inlet 31, a chamber 33, a buffer 34 and the second outlet 32 which are provided along a flow direction of the second medium. The chamber 33 is filled with the second medium. The buffer 34 divides the flow of the second medium. The second inlet 31 is connected to the flow tube 60. The second outlet 32 communicates with respective ends of the microchannel groups B of the heat exchanger 10. In addition, the other ends of the microchannel groups B communicate with the flow tube 60.
The first porous body 70 is a member containing a plurality of pores. The first porous body 70 is provided inside the first manifold 20 so as to perform blocking between the first inlet 21 and the first outlet 22. In more detail, the first porous body 70 is provided between the chamber 23 and the buffer 24.
As the first porous body 70, a member which is obtained by hardening sponge or fiber bundles made of a material such as polyvinyl alcohol with binder may be employed. Desirably, the first porous body 70 is a lyophilic member for the purposes of allowing the medium to go through the body 70 and preventing transit of air bubbles. The “lyophilic” is defined as having a contact angle of less than 90° between the porous body and the medium.
As described above, in a case where the diameter H of the first channels 11 of FIG. 3 is defined by the expression (1), the heat transfer area per unit volume can be increased but the bubbles may clog up the channels 11 when air bubbles having a diameter more than H enters into the first channels 11. This may decrease the flow rate of the medium passing through each microchannel.
In the embodiment, the mean diameter of the pores which are contained in the first porous body 70 is set below the diameter H of the first channels 11. The mean diameter is calculated from the following expression (2) by using the distribution of the volume V of the pores and the diameter D of the pores corresponding to the volume V. The diameter D corresponding to the volume V is obtained on the assumption that the pores are spherical. The volume V can be measured by a mercury press-fit technique using a porosimeter.
$\begin{matrix} Mean Diameter \overline{D} = \frac{\int D \partial V}{\int \partial V} & (2) \end{matrix}$
When the mean diameter of the pores is set below the diameter H of the first microchannels, air bubbles having diameters larger than the diameter H can be removed using the first porous body 70 so that air bubbles can be prevented from clogging up the interiors of the first channels 11. The embodiment can achieve increase in the heat transfer area by setting the diameter H of the first channels 11 to satisfy the expression (1). In addition, the embodiment can achieve ensuring the flow rate of the medium by preventing air bubbles from clogging up the first channels 11. As a result, even when the first and second mediums have a low flow rate, the heat exchange performance is enhanced.
The second porous body 80 is a member containing a plurality of pores. The second porous body 80 is provided inside the second manifold 30 so as to perform blocking between the second inlet 31 and the second outlet 32. In more detail, the second porous body 80 is provided between the chamber 33 and the buffer 34.
As the second porous body 80, a member which is obtained by hardening sponge or fiber bundles made of a material such as polyvinyl alcohol with binder may be employed. Desirably, the second porous body 80 is a lyophilic member for the purposes of allowing the medium to go through the body 80 and preventing transit of air bubbles.
In the embodiment, the mean diameter of the pores contained in the second porous body 80 is set below the diameter H of the second channels 12. The mean diameter is calculated from the expression (2).
FIGS. 5A and 5B are views for explaining function of the heat exchanger in accordance with the embodiment. FIG. 5A is a view corresponding to FIG. 2, and FIG. 5B is a view corresponding to FIG. 3.
In FIG. 5A, a flow C+D of the first medium goes into the heat receiving unit 40 which is under receipt of heat from a heat source, and the first medium receives the heat. The flow C+D of the first medium goes through the flow tube 50, and further goes into the chamber 23 of the first manifold 20 through the first inlet 21. When the flow C+D of the first medium goes through the first porous body 70, the first porous body 70 removes air bubbles having diameters not less than the diameter H of the first channels 11, from among air bubbles produced inside the flow tube 50 or the chamber 23. The flow C+D of the first medium which goes through the first porous body 70 is divided into a flow C of the first medium and a flow D of the first medium. The respective flows C and D go through the first outlets 22, as shown in FIG. 5B, and further go into the microchannel groups A containing the first channels 11 of the heat exchange unit 10.
On the other hand, a flow E+F of the second medium goes through the flow tube 60 and further goes into the chamber 33 of the second manifold 30 via the second inlet 31. When the flow E+F of the second medium goes through the second porous body 80, the second porous body 80 removes air bubbles having diameters not less than the diameter H of the second channels 12, from among air bubbles produced inside the flow tube 60 or the chamber 33. The flow E+F of the second medium which goes through the second porous body 80 is divided into a flow E of the second medium and a flow F of the second medium. The respective flows E and F go through the second outlets 32, and further go into the microchannel groups B containing the second channels 12 of the heat exchange unit 10.
At this time, in the heat exchange unit 10, heat is transferred, from the first medium passing through the microchannel groups A, to the second medium passing through the microchannels B by heat transfer.
The flows C and D of the first medium go out of the microchannel groups A, and go back to the heat receiving unit 40 via the flow tube 50 for performing circulation. The flows E and F of the second medium going out of the microchannel groups B circulate through the flow tube 60. As a result, the heat of the heat source is transferred from the first medium to the second medium, which can warm a space other than the heat source.
FIG. 6 is a view showing an experimental results obtained by using the heat exchanger in accordance with the first embodiment. FIG. 6 shows a relation between mass flow rate and thermal resistance. The experimental conditions are following. As the microchannels, channels of 250 μm in diameter were used. As the porous bodies, hydrophilic porous bodes of polyvinyl alcohol were used. The hydrophilic porous bodies contained pores with an mean diameter of 130 μm, and had a contact angle of 36° to water. As the first medium, water was used. The water indicated a temperature of 60° C., a liquid-phase density of 983 kg/m³, a gas-phase density of 0.13 kg/m³, and a surface tension of 0.066 N/m. As the second medium, water was used. The water indicated a temperature of 15° C., a liquid-phase density of 999 kg/m³, a gas-phase density of 0.014 kg/m³, and a surface tension of 0.073 N/m.
As shown in FIG. 6, the heat exchanger having the porous bodies 70 and 80 in accordance with the embodiment could present a thermal resistance smaller than that of a heat exchanger without porous bodies 70 and 80, though the mass flow rate was 10 kg/(m²·s) which is much smaller than 100-200 kg/(m²·s). The thermal resistance of the embodiment was much close to a design value.
The heat exchanger of the embodiment can prevent air bubble clogging in the microchannels even at a low mass flow rate of 100 kg/(m²·s) or less, for example. In a case where natural convection is utilized for circulation of the mediums, the lower the height of the heat source is, the lower the mass flow rate is for efficient heat exchange. Thus, the heat exchanger can prevent air bubble clogging in the microchannels even at a low mass flow rate without being restricted due to the height of the heat source i.e. the size of the heat exchanger etc. Further, when the mass flow rate is low, increase in pressure loss can be prevented so that it is possible to reduce burden on a pump in a case where the pump is used to generate a flow of the mediums.
The heat exchanger in accordance with the embodiment can ensure the performance of heat exchange using microchannels.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims

What is claimed is:

1. A heat exchanger, comprising:

a first manifold having a first inlet and a first outlet for a first medium;

a heat exchange unit having a first channel which communicates with the first outlet, the first channel having a cross section of a typical length H₁which satisfies the following expression; and

a first porous body provided between the first inlet and the first outlet and contains a plurality of pores having a mean diameter not more than the typical length H₁,

where, in the following expression, σ_fis a surface tension of the first medium at the first outlet, g is a gravitational acceleration, ρ₁is a liquid-phase density of the first medium at the first outlet, and ρ₂is gas-phase density of the first medium at the first outlet.

H_{1} \leq \sqrt{\frac{σ_{1}}{g (ρ_{1} - ρ_{2})}}

2. The exchanger according to claim 1, wherein the first porous body is lyophilic.

3. The exchanger according to claim 1, further comprising a heat receiving unit for receiving heat from a heat source, and a flow tube for connecting the heat receiving unit and the first inlet.

4. The exchanger according to claim 2, further comprising a heat receiving unit for receiving heat from a heat source, and a flow tube for connecting the heat receiving unit and the first inlet.

5. The exchanger according to claim 1, further comprising a second manifold and a second porous body, wherein

the second manifold includes a second inlet and a second outlet for a second medium,

the second porous body is provided between the second inlet and the second outlet and contains a plurality of pores,

the heat exchange unit including a second channel which has a cross section of a typical length H₂which satisfies the following expression, the second channel communicating with the second outlet and being provided in parallel with the first channel, and

the plurality of pores have a mean diameter not more than the typical length H₂,

where, in the following expression, σ₂is a surface tension of the second medium at the second outlet, ρ₃is a liquid-phase density of the second medium at the second outlet, and ρ₄is a gas-phase density of the second medium at the second outlet.

H_{2} \leq \sqrt{\frac{σ_{2}}{g (ρ_{3} - ρ_{4})}}

6. The exchanger according to claim 5, wherein the second porous body is lyophilic.

7. The exchanger according to claim 1, wherein the typical length H₁of the cross section of the first channel is 20 μm or more.

8. The exchanger according to claim 5, wherein the typical length H₁of the cross section of the first channel and the typical length H₂of the cross section of the second channel are 20 μm or more.

9. The exchanger according to claim 1, wherein the first medium is water.

10. The exchanger according to claim 5, wherein the first medium and the second medium are water.