JP2005233477A - Evaporator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an evaporator improved in its wetting property by supplying the sufficient amount of evaporated liquid to an evaporation face by capillary pressure. <P>SOLUTION: In this evaporator having a path for the evaporated liquid, narrower than a diameter of released bubbles, a groove 3 is formed on an evaporated liquid-side surface of a heat transfer base 2 heating the evaporated liquid, and a skin 4 having a plurality of holes 5 for communicating the groove 3 and a heating surface is diffusion-joined to a surface of the heat transfer base at a joint face 6. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a microchannel evaporator in which the passage of a liquid to be evaporated is smaller than the diameter of a detached bubble.

  Conventional methods for improving the performance of an evaporator are roughly divided into two methods. The first method is to improve the performance by expanding the heat transfer area as seen in the fin tube, and the second method is a heat transfer mechanism viewed from a boiling heat transfer mechanism such as a sintered particle layer provided on the surface of the heating plate. This is an improvement in thermal performance.

  As a conventional technique of the second method, for example, there is a structure using a porous heat transfer surface as shown in FIG. 12 as a structure of a heating plate on an evaporation target side of an evaporator (Patent Document 1).

This porous heat transfer surface is constituted by a sintered particle layer 11 and a skin 12 having regular openings 13 on the surface of the sintered particle layer. A sintered particle layer 11 is provided on the heat transfer wall 10 and is further covered with a skin 12 provided with a number of regular openings 13. If this porous heat transfer surface is used in an evaporator, the opening 13 of the skin 12 becomes the resistance of bubbles and boiling liquid entering and exiting the sintered particle layer. For this reason, the pressure in the sintered particle layer 11 is applied as a governing factor to the behavior of the bubbles and the boiling liquid, and the growth and separation of the bubbles and the inflow of the liquid are linked through this pressure. Therefore, since the inflow and outflow of gas and liquid is self-controlled so that the inflow amount of the liquid is determined according to the amount of bubbles released, a thin liquid film is formed in the sintered particle layer 11 in a wide heat flux range, It is said that a high heat transfer coefficient can be obtained.
JP-B-5-059357 (page 3, Fig. 2)

  However, when the heat transfer surface having the structure described in Patent Document 1 is applied to the evaporation target liquid side of the evaporator, there are the following three problems.

(1) Since the cross-sectional area of the air gap is not uniform, the fluid resistance is large, and sufficient mass transport capability of the liquid to be evaporated due to capillary pressure cannot be obtained.

(2) A sufficient contact area between the skin and the sintered particles or between the heat transfer wall and the sintered particles is not ensured, and the thermal resistance is large.

(3) There is a portion where the sintered particles block the pores, and efficient heat and mass transfer through the pores, such as taking in droplets and evaporating from the voids of the sintered particles, are obstructed. Is done.

[Definition of microchannels]
In the present invention, the microchannel is defined as follows. The case where the diameter of the bubble that leaves from the heat transfer surface is larger than the channel gap is defined as a microchannel. In other words, a case where bubbles are crushed by a wall at a stage smaller than the separation diameter to form a microlayer between the bubbles and the heating surface is defined as a microchannel.

  The detached bubble diameter varies depending on the surface properties and the degree of superheat. Specifically, as shown in FIG. 7, for example, about 0.8 [mm] in the case of a highly hydrophilic titanium oxide film and about 2.5 [mm] in the case of a highly hydrophobic silicone film. The diameter is shown, that is, the value below that value is the dimension of the microchannel on the heated surface.

[Necessity of forming a thin liquid film on the heat transfer surface]
FIG. 8A is a graph showing the relationship between the degree of superheat and heat flux of the heat transfer surface when the gap between the heat transfer surfaces in the parallel plate evaporator is narrow (solid line) and wide (broken line). . As shown in FIG. 8 (a), when two solid lines intersect at a certain temperature and heat flux, and the gap between the heat transfer surfaces is narrow, the heat transfer characteristics are good in the low heat flux region. In the region where the heat flux is high, the heat transfer characteristic decreases, and the critical heat flux (Critical Heat Flux) decreases accordingly. On the other hand, when the gap between the heat transfer surfaces is wide, good heat transfer characteristics are exhibited in a region where the heat flux is high, but heat transfer characteristics are degraded in a region where the heat flux is low.

  As this mechanism, there is a decrease in heat transfer coefficient due to dryout of the heat transfer surface. FIG. 8B shows changes in the flow and heat transfer mode in the evaporator tube. The liquid in the evaporator tube is almost 100% liquid phase upstream (lower part in the figure), but gradually becomes a two-phase state in which bubbles increase in the liquid phase as it goes downstream (upper part in the figure). Downstream from the position (post-dryout), there is no liquid to be evaporated on the heat transfer surface, and the heat transfer surface is completely dry. In the post-dryout spray basin, there is no liquid that is always in contact with the heat transfer surface, indicating a significant decrease in heat transfer characteristics.

  FIG. 8 (c) shows the heating heat flux and heat transfer coefficient distribution in the evaporator tube. In the figure, the heat flux increases in the order of A → B → C → D. It is shown that the region of reduced heat transfer coefficient (spray flow) moves upstream as the heat flux increases. In order to improve the decrease in the heat transfer coefficient, it is necessary to supply the liquid to be vaporized to the spray flow (post dry out) region to keep the heat transfer surface wet.

  FIG. 9 is a schematic diagram showing the boiling aspect of the microchannel in the high heat flux region. The evaporation target liquid 106 is supplied from the lower side (upstream) of the heat transfer surface 101 in the figure, and the vapor 107 is discharged from the upper side (downstream). The liquid to be evaporated 106 is heated by the heat transfer surface 101, and the liquid to be evaporated becomes a spray flow 105 above the gas-liquid interface 104.

  The gas-liquid interface 104, which is an interface between the wet region (upstream from the dryout position) 102 of the heat transfer surface 101 and the spray flow region 103, moves downward (upstream) as the heat flux increases, and is efficient as heat transfer. The wet region 102 of the heat transfer surface that can be used for the heat transfer surface is limited, and the heat exchange efficiency of the entire heat transfer surface is reduced. In order to improve the heat exchange efficiency, it is necessary to increase the ratio of the wet region 102 having a high heat transfer coefficient in the heat transfer surface.

FIG. 10 is a schematic diagram of photographs of boiling states in each heat flux of (a) 9 [kW / m ² ], (b) 14 [kW / m ² ], and (c) 19 [kW / m ² ]. It has become. As shown in FIG. 10, the wetting area of the heat transfer surface is reduced as the heat flux increases. That is, in order to ensure the wettability of the heat transfer surface, the heat flux is controlled to be below the limit heat flux as shown in FIG. It is necessary to provide a structure and a configuration for improving the solid line from (b) to the broken line. The present invention improves the performance of the evaporator with the latter approach.

  In order to solve the above problems, the present invention provides an evaporator having a passage of the liquid to be evaporated which is narrower than the diameter of the separation bubble, and a groove is provided on the surface of the heat transfer substrate that heats the liquid to be evaporated. The gist is that a skin having a plurality of openings communicating the groove and the heating surface is joined to the surface of the heat transfer substrate.

  According to the present invention, in the gas-liquid two-phase region of the liquid to be evaporated, the liquid to be evaporated is supplied to the downstream of the heat transfer surface through the groove by capillary action, and the thin liquid is supplied from the groove to the surface of the heat transfer surface through the hole. Since the film is supplied, it is possible to maintain a high heat transfer coefficient, and there is an effect that the evaporator can be made compact and the evaporator core temperature can be made uniform by improving the heat exchange performance.

  Next, an embodiment of an evaporator according to the present invention will be described in detail with reference to the drawings. Although not particularly limited, the evaporator according to the present invention is suitable for an evaporator that evaporates water or hydrocarbon-based raw fuel and supplies it to a fuel reformer for a fuel cell.

  FIG. 1 is a (a) plan view and (b) AA sectional view of a heating plate 1 for explaining an embodiment 1 of an evaporator according to the present invention. As shown in FIG. 1, the heating plate 1 in this embodiment is configured by joining a heat transfer substrate 2 in which a groove 3 is formed and a skin 4 provided with a plurality of holes 5 at a joining surface 6. The heat transfer substrate 2 and the skin 4 are formed by diffusion bonding at the bonding surface 5.

  The width of the groove 3 of the heat transfer substrate 2 and the diameter of the hole 5 of the skin 4 are 0.5 [mm] depending on the temperature of the evaporator (in other words, the vapor pressure) when the liquid to be evaporated is water. In the following, it is considered that the effect of capillary action is expected to be preferably about 0.2 [mm] to 0.3 [mm]. Further, it is preferable that the length of the hole 5 is equal to or less than the width of the groove 3.

  Furthermore, the heat transfer substrate 2 and the skin 4 were made of corrosion-resistant metal materials such as stainless steel, titanium, titanium alloy, etc., and the grooves 3 and the holes 5 were formed by etching, respectively.

  The holes 5 are preferably provided corresponding to the positions where the vertical and horizontal grooves 3 of the heat transfer substrate 2 intersect, and penetrate the thin liquid film 7 that has risen through the grooves 3 by capillary pressure into the heating surface, and upstream of the heating surface. Trap droplets from. The thin liquid film 7 formed in the groove 3 increases in pressure due to surface tension, and its saturated vapor temperature is higher than that of the outside. As a result, even if the thin liquid film on the heating surface evaporates, the thin liquid film 7 in the groove 3 is maintained, and the liquid is supplied to the heating surface to enable the formation of the thin liquid film.

  Furthermore, by forming a film of titanium oxide or the like on the surface of the heating plate 1 that contacts the liquid to be evaporated and performing a hydrophilic treatment, it becomes possible to obtain a larger capillary pressure and a penetrating power to the heating surface, which are further limited. The heat flux can be improved.

  In the present invention, since the groove 3 is formed in the heat transfer substrate 2 and the surface 5 is provided with the hole 5, the groove 3 and the hole 5 are formed even in the high temperature and high pressure state at the time of diffusion bonding between the heat transfer substrate 2 and the skin 4. Is prevented from being deformed. When grooves and holes are formed in the skin 4, deformation occurs in a high-temperature and high-pressure state at the time of diffusion bonding, causing a problem that the shape of the grooves cannot be maintained.

  FIG. 2: is the (a) top view of the heating plate 1 explaining the Example 2 of the evaporator based on this invention, (b) BB sectional drawing, (c) AA sectional drawing. The present embodiment shows an example in which the groove 3 is provided along the flow direction of the liquid to be evaporated and the cross-sectional area of the groove 3 is sequentially reduced from the upstream toward the downstream. The materials of the heat transfer substrate 2 and the skin 4 constituting the heating plate 1, the method of forming the grooves 3 and the holes 5, the method of joining the heat transfer substrate 2 and the skin 4 and the like are the same as in the first embodiment.

  In the present embodiment, the size of the cross section of the groove 3 shown in (c) AA cross section on the upstream side of the liquid to be evaporated is larger than the cross section of the groove 3 shown in (b) BB cross section on the downstream side. It is. The size of the hole 5 is also made to correspond to the size of the groove 3, and the size of the hole 5 is made smaller from upstream to downstream.

  By adopting such a configuration, a larger amount of the liquid film is raised by the larger groove cross section on the upstream side of the liquid to be evaporated, and the capillary pressure is suppressed by suppressing the separation of the thin liquid film 7 from the surface of the groove 3 also downstream. Can be maintained.

  3A is a plan view of a heating plate 1 for explaining an embodiment 3 of the evaporator according to the present invention, FIG. 3B is a sectional view taken along the line AA (a <b), and FIG. a> b). The present embodiment shows the heating plate 1 in which the cross-sectional shape of the groove provided in the heat transfer substrate 2 is widened (FIG. 3B) or narrowed (FIG. 3C). The materials of the heat transfer substrate 2 and the skin 4 constituting the heating plate 1, the method of forming the grooves 3 and the holes 5, the method of joining the heat transfer substrate 2 and the skin 4 and the like are the same as in the first embodiment.

  As shown in FIG. 3B, when the width b of the groove bottom portion is made wider than the width a of the groove upper portion so that the cross-sectional shape of the groove provided in the heat transfer substrate 2 is divergent, the heat transfer substrate 2 and the skin It is possible to increase the cross-sectional area of the groove 3 while maintaining the contact area with 4, and to increase the amount of liquid film while maintaining a small thermal resistance from the heat transfer substrate 2 to the skin 4. it can.

  As shown in FIG. 3C, when the width b of the groove bottom is made narrower than the width a of the groove upper portion so that the cross-sectional shape of the groove provided in the heat transfer substrate 2 is narrowed, the heat transfer substrate 2 Etching for forming the groove 3 in the groove becomes easy, and the production cost can be reduced.

  FIG. 4: is the (a) top view of the heating plate 1 explaining the Example 4 of the evaporator based on this invention, (b) BB sectional drawing, (c) AA sectional drawing. This embodiment shows an embodiment in which the cross-sectional shape of the groove 3 formed in the heating plate 1 is circular. The materials of the heat transfer substrate 2 and the skin 4 constituting the heating plate 1, the method of forming the grooves 3 and the holes 5, the method of joining the heat transfer substrate 2 and the skin 4 and the like are the same as in the first embodiment.

  In the present embodiment, by making the cross-sectional shape perpendicular to the flow direction of the liquid to be heated in the groove 3 circular, the thickness uniformity of the evaporated liquid thin film held on the groove surface is improved, and a thinner liquid film It is possible to suppress the separation of the thin liquid film from the groove to the thickness and maintain the capillary pressure.

  FIG. 5: is the (a) top view and (b) AA sectional drawing of the heating plate 1 explaining Example 5 of the evaporator based on this invention. The present embodiment shows an example in which the vertical groove 3a, which is a groove provided along the flow direction of the liquid to be evaporated, is communicated with the horizontal groove 3b. The material of the heat transfer substrate 2 and the skin 4 constituting the heating plate 1, the method of forming the grooves 3 a, the grooves 3 b and the holes 5, the method of joining the heat transfer substrate 2 and the skin 4, and the like are the same as in the first embodiment. .

  In the example of FIG. 5, the example of the heating plate 1 in which the vertical grooves 3a and the horizontal grooves 3b are orthogonal to each other is shown. A groove communicating obliquely with respect to the longitudinal groove 3a may be used.

  According to the present embodiment, by providing the horizontal groove 3b that communicates with the vertical groove 3a that is a groove provided along the flow direction of the liquid to be evaporated, variations in the liquid film of the liquid to be evaporated in the vertical groove 3a are corrected. The critical heat flux can be improved.

  FIG. 6: is (a) top view of the heating plate 1 explaining the Example 6 of the evaporator based on this invention, (b) AA sectional drawing. In this example, the hole 5 connecting the groove and the heating surface is chamfered, and processing is performed so that the cross section of the hole 5 is configured with an obtuse angle. By adopting such a structure, the penetration of the thin liquid film 7 from the groove 3 to the heating surface is promoted, and the critical heat flux can be improved.

  In the heating plates of Examples 1 to 6 described above, grooves and holes caused by clogging of joints when brazing is performed by integrating the skin having holes connecting the grooves and the heating surface by diffusion bonding. It is possible to avoid the reduction and deformation of the cross-sectional area.

  Similarly, in the heating plates of Examples 1 to 6, the grooves and holes are processed by etching to form a fine shape of 0.5 [mm] or less and subjected to a hydrophilic treatment such as titanium oxide. By this, it becomes possible to obtain a larger capillary pressure and a penetrating force to the heating surface, and to further improve the critical heat flux.

It is (a) top view of the heating plate 1 explaining Example 1 of the evaporator based on this invention, (b) It is AA sectional drawing. It is (a) top view, (b) BB sectional drawing, (c) AA sectional drawing of the heating plate 1 explaining Example 2 of the evaporator which concerns on this invention. (A) Plan view, (b) AA sectional view (a <b), (c) AA sectional view (a> b) of the heating plate 1 for explaining an embodiment 3 of the evaporator according to the present invention. It is. It is (a) top view, (b) BB sectional drawing, (c) AA sectional drawing of the heating plate 1 explaining Example 4 of the evaporator based on this invention. It is (a) top view of the heating plate 1 explaining Example 5 of the evaporator based on this invention, (b) It is AA sectional drawing. It is (a) top view of the heating plate 1 explaining Example 6 of the evaporator based on this invention, (b) It is AA sectional drawing. It is a figure which shows the surface property of a heat-transfer surface, and the diameter of the isolation | separation bubble by superheat degree. (A) The figure explaining the relationship between the superheat degree and heat flux of a microchannel type evaporator, (b) The figure explaining the change of the flow and heat transfer mode in an evaporation pipe, (c) Heating heat flux and heat transfer distribution It is a figure explaining a relationship. It is a figure explaining the boiling aspect of the microchannel in a high heat flux area | region. (A) 9 [kW / m ² ], (b) 14 [kW / m ² ], (c) 19 [kW / m ² ] each of which is a photograph of the state of boiling in each heat flux. is there. (A) It is a figure which shows the heat transfer coefficient with respect to the quality of a gas-liquid two phase, (b) Critical heat flux (CHF) with respect to the quality of a gas-liquid two phase. It is a cross-sectional perspective view explaining the heating surface for evaporators of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Heating plate 2 ... Heat transfer board 3 ... Groove 4 ... Skin 5 ... Hole 6 ... Joining surface 7 ... Thin liquid film 8 ... Chamfer

Claims (9)

In an evaporator having a passage of the liquid to be evaporated which is narrower than the diameter of the separation bubble,
A groove is provided on the surface of the heat transfer substrate that heats the liquid to be evaporated, and a skin having a plurality of openings communicating the groove and the heating surface is joined to the surface of the heat transfer substrate. And the evaporator.   The evaporator according to claim 1, wherein the groove is provided along a flow of the liquid to be evaporated.   The evaporator according to claim 2, wherein the cross-sectional area of the groove is decreased from the upstream to the downstream of the flow of the liquid to be evaporated.   The evaporator according to claim 2 or 3, wherein a groove that communicates the grooves along the flow of the liquid to be evaporated is provided.   The evaporator according to any one of claims 1 to 4, wherein a cross-sectional shape orthogonal to the flow of the groove provided in the heat transfer substrate is divergent or divergent.   The evaporator according to any one of claims 1 to 5, wherein a cross-sectional shape of the groove is substantially circular.   The evaporator according to any one of claims 1 to 6, wherein the opening is processed so that a cross-section of the hole has an obtuse angle.   The evaporator according to any one of claims 1 to 7, wherein the heat transfer substrate and the skin are integrated by diffusion bonding.   The evaporator according to any one of claims 1 to 8, wherein a surface of the heat transfer substrate and the skin is subjected to a hydrophilic treatment.

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