The present invention relates to a microchannel-type evaporator in which a path for a liquid to be evaporated is narrower than diameter of departing bubbles and to a system using the same.
In a fuel cell, fuel gas such as hydrogen and oxidant gas containing oxygen are electrochemically reacted through electrolyte, and electric energy is directly extracted from electrodes provided on both sides of the electrolyte. A polymer electrolyte fuel cell using a solid polymer electrolyte operates at low temperature and is easy to use. Accordingly, the polymer electrolyte fuel cell has attracted attention as a power supply for vehicles.
As a method of supplying hydrogen to the fuel cell, there are a method of directly supplying hydrogen from a hydrogen storage unit such as high-pressure hydrogen tank or a hydrogen storage alloy tank and a fuel reforming method of extracting hydrogen from fuel such as methanol or hydrocarbon and supplying the same. In the fuel reforming method, when the fuel is liquid, fuel or water is evaporated by an evaporator and then introduced to a fuel reformer, in which hydrogen is generated by a fuel reforming reaction.
As a small evaporator with high efficiency which is suitable for vehicles, an evaporator for air conditioning which evaporates refrigerant has been known (see Japanese Patent No. 2786728).
However, the conventional evaporator has a problem of reduction of heat exchange capability in a region of high heat flux. Hereinafter, such a problem is described.
(Definition of Microchannel)
In the present invention, the microchannel is defined as follows. A channel whose space is smaller than diameter of bubbles departing from heat transfer surface is defined as a microchannel. In other words, when bubbles which are yet smaller than departure diameter are crushed by walls of a channel to form microlayers between the bubbles and heating surfaces, such a channel is defined as a microchannel.
The diameter of bubbles having departed depends on a type of liquid to be evaporated, surface properties of heat transfer plates, and degree of superheat. Specifically, as shown in
(Need for Formation of Thin Liquid Film on Heat Transfer Surface)
As for a parallel plate-type evaporator,
As shown in
The mechanism of such characteristics is conceived to be reduction of the heat transfer coefficient due to dryout of the heat transfer surfaces.
In the heat transfer surface 101, the gas-liquid interface 104 as an interface between a wet region in the upstream of the dryout position and a dispersed flow region 103 moves toward the upstream as the heat flux increases. Accordingly, when the heat flux is increased, the wet region 102, where heat transfer is effectively performed, is reduced, so that the heat exchange efficiency of the entire heat transfer surfaces is reduced. In order to increase the heat exchange efficiency, it is necessary to increase the proportion of the wet region 102 having high heat transfer efficiency in the heat transfer surface.
The present invention was made in the light of the aforementioned problem, and an object of the invention is to increase the heat exchange efficiency of a microchannel-type evaporator and reduce the size thereof.
To achieve aforementioned object, a microchannel-type evaporator according to an aspect of the present invention includes a path provided substantially vertically, through which a liquid to be evaporated passes, and is characterized in that a space size of the path is smaller than diameters of bubbles departing from a heat transfer surface of the path, and the space size of the path in a gas-liquid two phase region is a minimum size satisfying that a heat flux is not more than a critical heat flux with respect to a quality.
With reference to the drawings, a description is given of embodiments of a microchannel-type evaporator according to the present invention in detail. The evaporator according to the present invention, which is not particularly limited, is suitable for an evaporator which evaporates water or hydrocarbon type fuel and supplies the same to a fuel reformer for a fuel cell.
FIGS. 1(a), 1(b), and 1(c) show a basic constituent unit of Embodiment 1 of the evaporator according to the present invention.
An evaporator 1 in this embodiment includes two heat transfer plates 2 opposite to each other. Between the heat transfer plates 2, a path 3 through which liquid to be evaporated passes is provided. The path 3 through which liquid to be evaporated passes is placed vertically, that is, along the direction G of gravity force. In the present invention, vertically setting the liquid path means setting the liquid path at such an angle that heat transfer properties in the right and left heat transfer surfaces which form a microchannel do not significantly lose symmetry because of the inclination thereof and, for example, includes setting the same at an angle of ± about 20 degrees from the vertical.
Furthermore, in the outside of the heat transfer plates 2, paths 4 through which heating gas passes, is provided. To form a real evaporator, it is preferable that the evaporator has a structure in which a plurality of the basic constituent units shown in the drawing are arranged in parallel. In this description, the path 3 through which the liquid to be evaporated passes is abbreviated as the liquid path 3, and the path 4 through which the heating gas passes is abbreviated as the gas path 4.
At the lower end of the liquid path 3, a liquid inlet 5, through which the liquid to be evaporated is supplied to the evaporator 1, is provided. At the upper end of the liquid path 3, a vapor outlet 6 is provided. The liquid to be evaporated evaporates as flowing from the bottom to the top of the evaporator 1. On the other hand, the heating gas is supplied from gas inlets 7, which are provided at the upper end of the evaporator, and discharged from gas outlets 8, which are provided at the lower end of evaporator. The evaporator of Embodiment 1 is therefore a countercurrent flow type evaporator in which the flow directions of the liquid to be evaporated and the heating gas are opposite to each other. During operation of the evaporator 1, the inside of the liquid path 3 is composed of a liquid phase region 10, a gas-liquid two phase region 11, and a gas phase region 12 sequentially from the bottom.
Size of space S of the liquid path 3 gradually increases from the bottom to the top in the gas-liquid two phase region 11.
Material of the heat transfer plates 2 can be a corrosion-resistant metal, for example, stainless steel, titanium, and titanium alloy.
Furthermore, the surface of each heat transfer plate 2 is coated with titanium oxide or the like for hydrophilic treatment. This can provide higher capillary pressure and more penetration into the heating surfaces, thus further increasing the critical heat flux.
In this embodiment, the mass flow rate of the heating gas is sufficiently higher than that of the liquid to be evaporated, and there is almost no change in temperature of the heating gas between the gas inlets 7 and the gas outlets 8.
The gas-liquid two phase region 11 of FIGS. 1(a) and 1(b) can be considered to be a region in which the quality is 0 and 1 at the lower and upper ends, respectively, and gradually increases therebetween.
When such quality and critical heat flux are represented by the horizontal and vertical axes, respectively, the correlation between the quality X and the critical heat flux qc in cases of spaces SA, SB, and SC (SA>SB>SC) of the path of the liquid to be evaporated in the parallel plate microchannel-type evaporator is shown as
When the entire evaporation tube has a same space, in part of smaller quality, the ratio of liquid is high, and a larger amount of heat is transferred from the heat transfer plates. Accordingly, the critical heat flux tends to be higher. When the space size of the evaporation tube is increased, a larger amount of the liquid to be evaporated is held in the space, and the critical heat flux tends to increase.
Herein, when the path space is increased with respect to the path space SB as a criterion, the critical heat flux increases, and the rate of the increase is higher when the quality is smaller. On the contrary, when the path space is reduced, the critical heat flux is reduced, and the rate of the reduction is higher when the quality is smaller.
In this embodiment, it is assumed that the mass flow rate of the heating gas is sufficiently higher than that of the liquid to be evaporated and there is almost no change in temperature of the heating gas between the gas inlets 7 and the gas outlets 8. The heat flux given from the heating gas through the heat transfer plates to the liquid to be evaporated is therefore constant regardless of the quality and is indicated by a dashed line of
In this embodiment, to avoid such reduction of heat flux, in the region with a quality X of not less than XB, the path space is increased to SB+1
In a similar way, in a region with a quality X of not more than XB, even if the path space is reduced to SB−1
As described above, in the microchannel-type evaporator of countercurrent flow type of the present invention, the space S of the liquid path is set to a minimum space size satisfying that heat flux is not more than the critical heat flux according to the quality X, so that thin liquid films of the liquid to be evaporated is formed on part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region of the liquid path. The part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region can be therefore always maintained to have a high transfer coefficient. Accordingly, the heat exchange efficiency of the microchannel-type evaporator can be increased, and moreover the microchannel-type evaporator can be reduced in size.
FIGS. 3(a), 3(b), and 3(c) show a basic constituent unit of Embodiment 2 of the evaporator according to the present invention.
The configuration of the evaporator 1 itself of Embodiment 2 is, similar to the configuration of the evaporator of Embodiment 1, of the countercurrent flow type in which the flow directions of the liquid to be evaporated and the heating gas are opposite to each other. This embodiment is an embodiment in the case where the mass flow rate of the heating gas is not negligible compared to that of the liquid to be evaporated and the temperature of the heating gas decreases from the gas inlets 7 to the gas outlets 8.
In
In this embodiment, to avoid such reduction of heat flux, in the region with a quality X of not less than XB, the path space is increased to SB+1
In a similar manner, in a region with a quality X of not more than XB, even if the path space is reduced to SB−1
As described above, in the microchannel-type evaporator of countercurrent flow-type of the present invention, the space S of the liquid path is set to a minimum space size satisfying that heat flux is not more than the critical heat flux according to the quality X, so that thin liquid films of the liquid to be evaporated are formed on part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region of the liquid path. The part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region can be therefore always maintained to have a high transfer coefficient. Accordingly, the heat exchange efficiency of the microchannel-type evaporator can be increased, and moreover the microchannel-type evaporator can be reduced in size.
FIGS. 5(a), 5(b), and 5(c) show a basic constituent unit of Embodiment 3 of the evaporator according to the present invention.
As shown in
At the lower end of the liquid path 3, a liquid inlet 5, through which the liquid to be evaporated is supplied to the evaporator 1, is provided. At the upper end of the liquid path, a vapor outlet 6 is provided. The liquid to be evaporated evaporates as flowing from the bottom to the top of the evaporator 1. On the other hand, the heating gas is supplied from gas inlets 7, which are provided at the lower end of the evaporator, and discharged from gas outlets 8, which are provided at the upper end of evaporator. Accordingly, the evaporator of this embodiment is a same flow direction type evaporator in which the flow directions of the liquid to be evaporated and the heating gas are substantially in a same direction. During operation of the evaporator 1, the inside of the liquid path 3 is composed of a liquid phase region 10, a gas-liquid two phase region 11, and a gas phase region 12 sequentially from the bottom.
Size of space S of the liquid path 3 gradually decreases from the bottom to the top in the gas-liquid two phase region 11.
Material of the heat transfer plates 2 can be a corrosion-resistant metal, for example, stainless steel, titanium, and titanium alloy.
Furthermore, the surface of each heat transfer plate 2 which comes into contact with the liquid to be evaporated is coated with titanium oxide or the like for hydrophilic treatment. This can provide higher capillary pressure and more penetration into the heating surfaces, thus further increasing the critical heat flux.
This embodiment is an embodiment in the case where the mass flow rate m (g/s) of the heating gas is not negligible compared to that of the liquid to be evaporated and the temperature of the heating gas decreases from the gas inlets 7 toward the gas outlets 8.
In
To avoid such reduction of heat flux, in the region with a quality X of not more than XB, the path space is increased to SB−1
In a similar manner, in the region with a quality X of not less than XB, the path space is reduced to SB+1
As described above, in the microchannel-type evaporator of the same flow direction type of the present invention, the space S of the liquid path is set to a minimum space size satisfying that heat flux is not more than the critical heat flux according to the quality X, so that thin liquid films of the liquid to be evaporated are formed on part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region of the liquid path. The part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region can be therefore always maintained to have a high transfer coefficient. Accordingly, the heat exchange efficiency of the microchannel-type evaporator can be increased, and moreover the microchannel-type evaporator can be reduced in size.
FIGS. 7(a), 7(b), and 7(c) show a basic constituent unit of Embodiment 4 of the evaporator according to the present invention.
As shown in
At the lower end of the liquid path 3, a liquid inlet 5, through which the liquid to be evaporated is supplied to the evaporator 1, is provided. At the upper end of the liquid path, a vapor outlet 6 is provided. The liquid to be evaporated evaporates as flowing from the bottom to the top of the evaporator 1. On the other hand, the heating gas is supplied from gas inlets 7, which are provided at the right end of the evaporator, and discharged from gas outlets 8, which are provided at the left end of the evaporator. The evaporator of this embodiment is therefore a cross flow type evaporator in which the flow directions of the liquid to be evaporated and the heating gas are orthogonal to each other. During operation of the evaporator 1, the inside of the liquid path 3 is composed of a liquid phase region 10, a gas-liquid two phase region 11, and a gas phase region 12 sequentially from the bottom.
Size of space S of the liquid path 3 changes three dimensionally so as to gradually decrease from the bottom to the top in the gas-liquid two phase region 11 and gradually decrease from the right to the left.
Material of the heat transfer plates 2 can be a corrosion-resistant metal, for example, stainless steel, titanium, and titanium alloy.
Furthermore, the surface of each heat transfer plate 2 which comes into contact with the liquid to be evaporated is coated with titanium oxide or the like for hydrophilic treatment. This can provide higher capillary pressure and more penetration into the heating surfaces, thus further increasing the critical heat flux.
In this embodiment, the heat flux transferred from the heating gas to the liquid to be evaporated in the evaporator is the highest at the position L-L in the upstream of the heating gas and decreases along with a decrease in temperature of the heating gas toward the position M-M in the middle of the stream and the position N-N in the downstream. In this embodiment, similar to Embodiment 1, it is assumed that the mass flow rate of the heating gas is sufficiently higher than that of the liquid to be evaporated and there is almost no change in temperature of the heating gas between the gas inlets 7 and the gas outlets 8. The heat flux given from the heating gas through the heat transfer plates to the liquid to be evaporated is therefore constant regardless of the quality and is indicated by a dashed line of
In
In this embodiment, to avoid such reduction of heat flux, in a region with a quality X of not less than XB at the position M-M, the path space is increased to SB+1, SB+2, and SB+3 as the quality increases to XB+1, XB+2, and XB+3. The heat flux can be therefore prevented from exceeding the critical heat flux even if the quality becomes larger than XB, thus providing a maximum heat transfer coefficient. In the region of a quality X of not more than XB, even if the path space is reduced to SB−1
In a similar manner, the dashed line indicating the heat flux given from the heating gas to the liquid to be evaporated at the position L-L of the evaporator 1 and a line indicating the critical heat flux of the path space SA intersect with each other at a quality XA. In other words, the graph shows that the critical heat flux is supplied at the position of the quality XA of the evaporator with the path space SA. The critical heat flux is supplied at the position of the quality XA of the path space SA, and in the region with a quality of not less than the quality XA, supplied heat flux exceeds the critical heat flux. The heat transfer surfaces therefore change from the wet state to the dry state, and the heat transfer coefficient is reduced. Accordingly, the heat flux is drastically reduced in a similar manner to Embodiment 2, which is not shown in the drawing.
To avoid such reduction of heat flux, in the region with a quality X of not less than XA at the position L-L, the path space is increased as the quality increases to XA+1, XA+2, and XA+3. This can prevent the heat flux from exceeding the critical heat flux even if the quality becomes larger than XA, thus providing a maximum heat transfer coefficient. In a region with a quality X of not more than XA, even if the path space is reduced as the quality decreases to XA−1 and XA−2, the heat transfer surfaces can be maintained to be wet. The heat flux can be therefore within the critical heat flux even if the quality becomes smaller than XA, thus providing a maximum heat transfer coefficient.
In this embodiment, the same process as the processes performed for the positions L-L and M-M is performed across the entire region of the evaporator 1. The space size can be thus set to a minimum space size that allows the heat flux to be not more than the critical heat flux according to the quality X.
As described above, in the microchannel-type evaporator of the cross flow type, the space S of the liquid path is set to the minimum space size satisfying that the heat flux is not more than the critical heat flux according to the quality X, so that thin liquid films of the liquid to be evaporated are formed on part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region of the liquid path. The part of the heat transfer surfaces which comes into contact with the gas-liquid two phase region can be therefore maintained to have a high transfer coefficient. Accordingly, the heat exchange efficiency of the microchannel-type evaporator can be increased, and moreover the microchannel-type evaporator can be reduced in size.
FIGS. 9(a), 9(b), and 9(c) show a basic constituent unit of Embodiment 5 as a modification of Embodiment 2.
A difference between the evaporator of this embodiment and the evaporator of Embodiment 2 is that the path space S is gradually reduced in the gas phase region 12 of the liquid path 3. The other configuration is the same as that of Embodiment 2. Same constituent components are given same numerals, and redundant description is omitted. This embodiment has an effect on preventing droplets of the liquid to be evaporated from being discharged from the vapor outlet 6.
Gradually decreasing the path space S in the gas-phase region 12 of the liquid path 3 like Embodiment 5 can be applied to not only Embodiment 2 but also other embodiments.
FIGS. 10(a), 10(b), and 10(c) show a basic constituent unit of Embodiment 6 of the evaporator according to the present invention.
An evaporator 1 of Embodiment 6 includes, in addition to the cross flow-type evaporator of Embodiment 4, a plurality of turning sections provided with partitions 9 in a heating gas path 4 to cause the heating gas to meander. The other configuration is the same as that of Embodiment 4. The same constituent components are given same numerals, and redundant description is omitted.
In Embodiment 4, the boundary line between the gas-phase region 12 and the gas-liquid two phase region 11 and the boundary line between the gas-liquid two phase region 11 and the liquid-phase region 10 are sloped. However, according to this embodiment, these boundary lines can be kept more parallel to the flow direction of the heating gas than those of Embodiment 4. Embodiment 6 therefore has an effect on further uniforming the superheat of vapor generated from the end of the vapor outlet 6.
FIGS. 11(a), 11(b), and 11(c) show a basic constituent unit of Embodiment 6 as a modification of Embodiment 2.
An evaporator 1 of Embodiment 7 includes, in addition to the cross flow-type evaporator of Embodiment 4, a plurality of turning sections provided with partitions 9 in a heating gas path 4 to cause the heating gas to meander. Furthermore, a difference between Embodiment 6 and Embodiment 7 is that the gas inlets 7 are in the vicinity of the liquid inlet 5 and the gas outlets 8 are in the vicinity of the vapor outlet 6. Accordingly, hottest part of the heating gas heats the liquid phase region 10, whose critical heat flux is highest, so that the space S of the liquid path 3 can be configured to be narrower than that of Embodiment 6. Embodiment 7 therefore has an effect that the evaporator 1 thereof can be smaller than that of Embodiment 6. The other configuration is the same as that of Embodiment 4. The same constituent components are given same numerals, and redundant description is omitted.
This embodiment is described using FIGS. 12 to 18. FIGS. 12(a), 12(b), and 12(c) show a basic constituent unit of Embodiment 8 of the evaporator according to the present invention.
As shown in
At the lower end of the liquid path 3, a liquid inlet 5, through which the liquid to be evaporated is supplied to the evaporator 1, is provided, and at the upper end of the liquid path 3, a vapor outlet 6 is provided. The liquid to be evaporated evaporates as flowing from the bottom to the top of the evaporator 1. On the other hand, the heating gas is supplied from gas inlets 7, which are provided at the lower end of the evaporator, and discharged from gas outlets 8, which are provided at the upper end of the evaporator. The evaporator of this embodiment is therefore a same flow direction type evaporator in which the flow directions of the liquid to be evaporated and the heating gas are substantially in a same direction. During operation of the evaporator 1, the inside of the liquid path 3 is composed of a liquid phase region 10, a gas-liquid two phase region 11, and a gas phase region 12 sequentially from the bottom. Size of space S of the liquid path 3 is substantially constant over the entire region of the evaporator 1.
Next, a description is given of the map of
In Embodiment 8, as shown in
For example, a description is given of the following five patterns of the relation between the quality and the heat flux when the space S is set to SB. First, when the mass flow rate and temperature at the gas inlet are mg2 and Tg2, respectively, the heat flux of the high temperature gas and the liquid to be evaporated varies as indicated by a dashed line along with changes in quality and is not more than the critical heat flux of the space SB in a quality of 0 to 1. In a similar manner, also in cases of (mg2, Tg1) and (mg1, Tg2), the heat flux is not more than the critical heat flux of the space SB. These show that a thin liquid film can be formed to enable an efficient heat exchange.
On the other hand, in cases of (mg2, Tg3) and (mg3, Tg2), as the quality increases, the heat flux of the high temperature gas and the liquid to be evaporated intersects with the critical heat flux characteristic and transits to the dryout region to be considerably reduced. This causes the heat exchange coefficient to be reduced.
The aforementioned results are put together as
Next, using FIGS. 18(a) to 18(c), a description is given of a configuration and an operation of systems using the evaporator of this embodiment.
Each system show in FIGS. 18(a) to 18(c) includes the evaporator 1, a superheater 21, a heating gas inlet 31 of the system, valves 32, 33, and 34 each composed of a three-way valve to switch paths of the heating gas, and a gas outlet 35 of the system. The superheater 21 further heats vapor from the evaporator of the present invention to generate superheated vapor.
The system of
The system of
The system of
Places indicated by thick lines in the drawing show flows of the gas, and a black portion of each three-way valve indicates the direction that the gas flow is stopped. The embodiment is composed of two heat exchangers: one is the above described microchannel-type evaporator 1 mainly used in the wet region; and the other is the superheater 21 to generate superheated vapor. Gas passing through these heat exchangers which are the evaporator 1 and superheater 21 is supplied to a hydrogen generation element such as a not-shown auto thermal reactor (ATR). In a normal state not described below (in the case of the wet region), the gas bypasses the superheater 21 to be supplied. FIGS. 18(a) to 18(c) correspond to FIGS. 15 to 17, respectively, and FIGS. 15 to 17 are simplified and schematically shown. The configuration diagrams shown in FIGS. 18(a) to 18(c) are just examples and do not limit the present invention.
Next, a description is given of a specific control using FIGS. 14 to 17.
In the system of
In the system of
In the system of
As described above, in this embodiment, when the mass flow rate of the heating gas is not less than the prescribed value, the heating gas is supplied to the superheater and the microchannel-type evaporator in parallel. When the temperature of the heating gas is not lower than the prescribed value, the heating gas is supplied to the superheater and then supplied to the microchannel-type evaporator. The heat transfer surfaces can be therefore maintained to be wet at narrow spaces with a constant cross section. Accordingly, the evaporator can be reduced in size, and the gas-liquid two phase region can be maintained to have a high heat transfer coefficient. It is therefore possible to realize a compact evaporator with high efficiency.
Note that the space S is configured to be constant in Embodiment 8 but may be changed like Embodiments 1 to 7.
The entire contents of Japanese Patent Applications No. 2004-162011 (Filing Date: May 31, 2004) and No. 2002-254611 (Filing Date: Sep. 1, 2004) are herein incorporated by reference.
Hereinabove, a description is given of the contents of the present invention along the embodiments and examples. However, it is obvious for those skilled in the art that the present invention is not limited to the description about these embodiment and examples and various modifications and improvements can be made.
In the evaporator of the present invention, a thin liquid film of a liquid to be evaporated is formed on part of heat transfer surfaces coming into contact with a gas-liquid two phase region. The part of the heat transfer surfaces coming into contact with the gas-liquid two phase region can be therefore always maintained to have a high transfer coefficient. Accordingly, the heat exchange efficiency of the evaporator can be increased, and the evaporator can be reduced in size.
Number | Date | Country | Kind |
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2004-162011 | May 2004 | JP | national |
2004-254611 | Sep 2004 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP05/07428 | 4/19/2005 | WO | 11/29/2006 |