DOUBLE PIPE FOR HEAT EXCHANGE

Description

INCORPORATED BY REFERENCE

This application is a Continuation of International Application No. PCT/JP2024/009778 filed Mar. 13, 2024, which claims priority under 35 U.S.C. §§ 119 (a) and 365 of Japanese Patent Application No. 2023-053280 filed on Mar. 29, 2023, the disclosures of which are expressly incorporated herein by reference in their entireties.

BACKGROUND ART

1. Technical Field

The present disclosure relates to a double pipe for heat exchange used for heat exchanger, such as in automotive air conditioning systems and refrigerators.

2. Description of the Related Art

Conventionally, a double pipe for heat exchange in which an inner pipe is inserted into an outer pipe has been used for some of pipes of heat exchangers in automotive air conditioning systems (air conditioners) and in-vehicle refrigerators. A double pipe for heat exchange, as shown in U.S. Publication No. US 2023/003456 for example, enables heat exchange among a heat medium flowing inside an inner pipe and the heat medium flowing between the inner pipe and an outer pipe. By employing such a double pipe for heat exchange, efficiency of cooling/warming heat medium in refrigeration cycle can be improved by heat exchange among the heat medium in the double pipe for heat exchange.

SUMMARY

Meanwhile, in order to improve heat exchange efficiency among the heat medium flowing through the inner flow path inside the inner pipe and the outer flow path between the inner pipe and the outer pipe, US 2023/003456 employs a structure in which plural convex portions are provided circumferentially on the pipe wall of the inner pipe and these convex portions extend spirally. This increases the surface area of the pipe wall of the inner pipe compared to a pipe wall extending linearly with a constant circular cross-section, thereby improving heat exchange efficiency. In addition, the spiral shape of the pipe wall of the inner pipe prevents increase of flow resistance of the heat medium and reduces pressure loss, so there is no need for high pump performance, etc.

However, further research conducted by the inventors on double pipes for heat exchange with a spiral inner pipe wall such as in US 2023/003456 led to the discovery that there is room for further improvement. That is, in the structure of US 2023/003456, it was found that the heat medium flowing near the pipe wall of the inner flow path generates sufficient temperature change due to heat exchange with the heat medium flowing through the outer flow path, while the heat medium flowing through the central portion of the inner flow path, which is far from the pipe wall, does not sufficiently contribute to heat exchange with the outer flow path and result in small temperature change.

In particular, it was found that, the temperature difference of the heat medium between the region near the pipe wall of the spiral inner pipe and the central region of the inner pipe, which is far from the pipe wall, is larger in portions extending linearly in the pipe lengthwise direction than in portions curving or bending.

Meanwhile, there is a strong demand for further improvements in heat exchange efficiency of the entire double pipe for heat exchange, since changes of the heat medium, etc. are recently under consideration from the purpose of environmental conservation.

The present disclosure has been developed in view of the above-described matters as the background, and it is an object of the present disclosure to provide a double pipe for heat exchange with a novel structure in which pressure loss of a heat medium flowing through an inner flow path and an outer flow path is suppressed and heat exchange efficiency among the heat medium is further improved.

Hereinafter, preferred embodiments for grasping the present disclosure will be described. However, each preferred embodiment described below is exemplary and can be appropriately combined with each other. Besides, a plurality of elements described in each preferred embodiment can be recognized and adopted as independently as possible, or can also be appropriately combined with any element described in other preferred embodiments. By so doing, in the present disclosure, various other preferred embodiments can be realized without being limited to those described below.

A first preferred embodiment of the present disclosure provides

• • a double pipe for heat exchange in which an inner pipe is inserted into an outer pipe to enable heat exchange among a heat medium flowing through an inner flow path in the inner pipe and an outer flow path between the inner pipe and the outer pipe, comprising:
  • in a straight pipe portion extending linearly in a pipe lengthwise direction so as to constitute a heat exchange portion,
  • a pipe wall of the inner pipe having a plurality of convex portions in a circumferential direction so as to form a concave/convex cross-sectional shape, and
  • in the pipe wall of the inner pipe, an inclined pipe wall portion whose convex portion extends spirally in the pipe lengthwise direction and a turbulence-generating pipe wall portion whose convex portion has a lead angle different from that of the inclined pipe wall portion being formed in series in the pipe lengthwise direction.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, since the pipe wall of the inner pipe that separates the inner flow path and the outer flow path has the concave/convex cross-sectional shape with the plural convex portions in the circumferential direction, the surface area is increased. Therefore, heat exchange efficiency is improved among the heat medium flowing through the inner flow path and the outer flow path. Furthermore, since the pipe wall of the inner pipe wall has the prescribed concave/convex cross-sectional shape that extends spirally in a continuous manner in the pipe lengthwise direction, the heat medium flows smoothly in both the inner flow path and the outer flow path, so that the pressure loss due to the concave/convex shaped pipe wall of the inner pipe wall is reduced. Therefore, excellent heat exchange efficiency is achieved without any high-performance pump, etc.

Furthermore, the pipe wall of the inner pipe wall has the turbulence-generating pipe wall portions whose convex portion has a lead angle different from that of the inclined pipe wall portion, and the inclined pipe wall portions and the turbulence-generating pipe wall portions are arranged in series in the pipe lengthwise direction. Therefore, turbulence occurs in the heat medium flowing through the inner flow path and the outer flow path at the connecting portion between the inclined pipe wall portion and the straight pipe wall portion, so that the heat medium in the paths is agitated. As a result, the possibility that the heat medium which has finished transfer of heat remains near the pipe wall of the inner pipe is reduced or removed, thereby the temperature distribution in the inner flow path and outer flow path is uniformed. Therefore, temperature difference due to heat exchange is suppressed for the heat medium flowing through the inner flow path and the outer flow path near the pipe wall of the inner pipe, so that improvement in heat exchange efficiency is achieved.

A second preferred embodiment of the present disclosure provides the double pipe for heat exchange according to the first preferred embodiment wherein the turbulence-generating pipe wall portion is shorter than the inclined pipe wall portion in the pipe lengthwise direction.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, smooth flow of the heat medium due to the inclined pipe wall portion is realized in the longer region in the pipe lengthwise direction, and thus reduction in pressure loss is achieved. In addition, the turbulence-generating pipe wall portion exhibits agitation effect of the heat medium due to turbulence at the connecting portion with the inclined pipe wall portion. Therefore, even if the turbulence-generating pipe wall portion is shorter than the inclined pipe wall portion, the improvement of heat exchange efficiency is effectively achieved.

A third preferred embodiment of the present disclosure provides the double pipe for heat exchange according to the first or second preferred embodiment wherein the turbulence-generating pipe wall portion includes a straight pipe wall portion whose convex portion extends linearly in the pipe lengthwise direction.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, since the turbulence-generating pipe wall portion includes a straight pipe wall portion, the heat medium suffers a greater change in the flow direction at the connecting portion with the inclined pipe wall portion. Therefore, the improvement in heat exchange efficiency due to the generation of turbulence can be more effectively achieved.

A fourth preferred embodiment of the present disclosure provides the double pipe for heat exchange according to any of the first to third preferred embodiments wherein the inclined pipe wall portion and the turbulence-generating pipe wall portion in the pipe wall of the inner pipe have a constant cross-sectional shape across the entire length.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, pressure loss due to change in the cross-sectional shape of the pipe wall of the inner pipe is avoided, and the heat medium can flow more efficiently in the inner flow path and outer flow path.

A fifth preferred embodiment of the present disclosure provides the double pipe for heat exchange according to any of the first to fourth preferred embodiments wherein the convex portion of the inclined pipe wall portion extends spirally in the pipe lengthwise direction at a constant lead angle.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, a smoother flow of the heat medium is realized in the inner flow path and outer flow path at the position corresponding to the inclined pipe wall portion, and further reduction of pressure loss is achieved.

A sixth preferred embodiment of the present disclosure provides the double pipe for heat exchange according to any of the first to fifth preferred embodiments wherein the inclined pipe wall portion is located on each side of the turbulence-generating pipe wall portion in the pipe lengthwise direction.

According to the double pipe for heat exchange with the structure in accordance with the present preferred embodiment, the connecting portion with the inclined pipe wall portion is formed at each end of the turbulence-generating pipe wall portion, so that the heat medium in the inner flow path and outer flow path is agitated twice per turbulence-generating pipe wall portion, thereby improving the heat exchange efficiency among the heat medium with a small number of turbulence-generating pipe wall portions.

According to the present disclosure, it is possible to suppress pressure loss of the heat medium flowing through the inner flow path and the outer flow path and heat exchange efficiency among the heat medium is further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or other objects, features and advantages of the disclosure will become more apparent from the following description of a practical embodiment with reference to the accompanying drawings in which like reference numerals designate like elements and wherein:

FIG. 1 is a front view of a double pipe for heat exchange as the first practical embodiment of the present disclosure;

FIG. 2 is a plan view of the double pipe for heat exchange as shown in FIG. 1;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is an enlarged view of the cross section taken along line 4-4 of FIG. 3;

FIG. 5 is an enlarged view of the cross section taken along line 5-5 of FIG. 3;

FIG. 6 is a perspective view of an inner pipe constituting the double pipe for heat exchange shown in FIG. 1;

FIG. 7 is a perspective cross-sectional view of the inner pipe shown in FIG. 6;

FIG. 8 is a diagram showing results of simulation of a flow of a heat medium in an inner flow path of the double pipe for heat exchange; and

FIG. 9 is a diagram showing results of simulation of temperature distribution of the heat medium in the inner flow path of the double pipe for heat exchange.

DETAILED DESCRIPTION

Hereinafter, practical embodiments of the present disclosure will be described in reference to the drawings.

FIGS. 1-5 show the double pipe for heat exchange 10 as the first practical embodiment of the present disclosure. The double pipe for heat exchange is configured to be a straight pipe that extends linearly wherein an inner pipe 12 is inserted into an outer pipe 14 as shown in FIGS. 3-5. In the following description, as a general rule, the vertical direction refers to the vertical direction in FIG. 1, the left-right direction refers to the left-right direction in FIG. 1, which coincides with the pipe lengthwise direction, and the front-back direction refers to the vertical direction 10 in FIG. 2, respectively.

The inner pipe 12 extends linearly in the left-right direction and has an inner flow path 16 that penetrates in the pipe lengthwise direction. It is desirable that the material of the inner pipe 12 has a high thermal conductivity for the sake of increased heat exchange efficiency among the heat medium described below, and also has excellent corrosion resistance to the heat medium described below. Specifically, for example, aluminum alloys, iron (stainless steel), and copper alloys can be suitably employed as the material for the inner pipe 12. The inner pipe 12 is constituted of a pipe wall 20a of an intermediate region 18 as a straight pipe portion which forms the heat exchange portion in cooperation with an outer pipe 14 described below, and pipe walls 20b, 20b of end regions 22, 22 which forms the inflow and outflow portions of the heat medium at ends of the heat exchange portion.

As shown in FIGS. 4 and 5, the pipe wall 20a of the intermediate region 18 has a concave/convex cross-sectional shape with three convex portions 24, 24, 24 in the circumferential direction. Due to three convex portions 24, 24, 24 in the circumferential direction at equal intervals, the pipe wall 20 of the inner pipe 12 is an approximate triangle with rounded corners as a whole. The convex portions 24 are circular arcs with a curvature radius smaller than that of the inner surface of the outer pipe 14. The intervals between the convex portions 24, 24 adjacent in the circumferential direction have curvature radius larger than that of the inner surface of the outer pipe 14.

As shown also in FIGS. 6 and 7, the pipe wall 20a of the intermediate region 18 of the inner pipe 12 has inclined pipe wall portions 26 and straight pipe wall portions 28 as turbulence-generating pipe wall portions, both of which being formed continuously in the pipe lengthwise direction. In this practical embodiment, the plural inclined pipe wall portions 26 and plural straight pipe wall portions 28 are arranged alternately and continuously so as be aligned in series in the pipe lengthwise direction.

The inclined pipe wall portions 26 have a spiral form in which the convex portion 24 extends in the pipe lengthwise direction in an inclined manner in the circumferential direction. It is desirable that the convex portion 24 of each inclined pipe wall portion 26 extends spirally in the pipe lengthwise direction at a constant lead angle. The lead angles of the convex portions 24 that extend spirally between the plural inclined pipe wall portions 26 may differ from each other, however in this practical embodiment, they are the same. The lead angle θ of the convex portion 24 of the inclined pipe wall portion 26 is set within the range 0°<θ<90°.

The straight pipe wall portion 28 extends linearly in the pipe lengthwise direction without the convex portion 24 being inclined in the circumferential direction. The straight pipe wall portion 28 has a lead angle of 90°, which differs from that of the inclined pipe wall portion 26. The straight pipe wall portion 28 is shorter than the inclined pipe wall portion 26 in the pipe lengthwise direction.

As shown in FIGS. 4 and 5, the inclined pipe wall portion 26 and the straight pipe wall portion 28 have approximately the same cross-sectional shape as each other. Both the inclined pipe wall portions 26 and the straight pipe wall portions 28 have a roughly constant cross-sectional shape throughout the pipe lengthwise direction. Therefore, in the inner pipe 12, the pipe wall 20a of the intermediate region 18 has a constant cross-sectional shape for the entire length.

Each end of the pipe wall 20a of the intermediate region 18 of the inner pipe 12 is constituted of the inclined pipe wall portions 26, 26, and any of the plural straight pipe wall portions 28 are located in the middle, off the ends of the pipe wall 20a in the pipe lengthwise direction. Therefore, the inclined pipe wall portions 26 are located on both sides of each of the straight pipe wall portion 28 in the pipe lengthwise direction.

The pipe walls 20b of the end regions 22, 22, off the intermediate region 18 of the inner pipe 12 in the pipe lengthwise direction are cylindrical pipe wall portions 30. The cylindrical pipe wall portion 30 has a roughly constant circular cross-section and extends in the pipe lengthwise direction. The cylindrical pipe wall portion 30 has a cross-sectional shape different from that of the inclined pipe wall portion 26 or the straight pipe wall portion 28. The cylindrical pipe wall portions 30, 30 of the end regions 22, 22 are provided in a continuous manner integrally with the inclined pipe wall portions 26, 26 that constitute the ends of the intermediate region 18, and inner holes of the intermediate region 18 and the end regions 22, 22 interconnect so as to form the inner flow path 16 that extends in the axial direction. In this practical embodiment, the illustrated cylindrical pipe wall portion 30 extends linearly along the pipe, however, the cylindrical pipe wall portion 30 may bend as appropriate in the middle.

The manufacturing method for the inner pipe 12 is not particularly specified, however, for example, a prescribed shaped inner pipe 12 can be easily obtained by hydroforming. Namely, by applying large hydraulic pressure to the inner surface of a cylindrical blank pipe inserted into the cavity of a metal mold, the hydraulic pressure causes the blank pipe to expand in diameter, the blank pipe is pressed against the inner surface of the metal mold cavity, and concave/convex of the inner surface of the cavity is transferred to the blank pipe, so that the prescribed shaped inner pipe 12 can be formed. In this way, by forming the inner pipe 12 by hydroforming, it is possible to easily obtain the inner pipe 12 with a complex shape that has the plural inclined pipe wall portions 26 and straight pipe wall portions 28 arranged alternately. Moreover, it is also possible to form the inner pipe 12 with a large degree of shape freedom.

The outer pipe 14 extends linearly in the left-right direction and has an inner diameter that allows the inner pipe 12 to be inserted. It is desirable that the outer pipe 14 is formed from a material that has excellent corrosion resistance to the heat medium described below, for example, formed from aluminum alloys, iron (stainless steel), copper alloys. In the outer pipe 14, a pipe wall 32a of the intermediate region 18 is an intermediate pipe wall portion 34 extending along the pipe with a roughly constant circular cross-section. The pipe wall 32a of the outer pipe 14 has an inner diameter dimension roughly same as or slightly larger than the maximum outer diameter dimension of the inner pipe 12, so that it can be externally fitted about the inner pipe 12.

Each of pipe wall 32b of the end regions 22, 22 of the outer pipe 14 is a connecting pipe wall portion 36. The connecting pipe wall portion 36 has a diameter-enlarged portion 38 with a diameter larger than the intermediate pipe wall portion 34, and the portions on outer sides of the diameter-enlarged portions 38 in the pipe lengthwise direction are mating portions 40 with a diameter smaller than the intermediate pipe wall portion 34.

The diameter-enlarged portion 38 has a connecting hole 42 that penetrates a portion of the circumference portion of the outer pipe 14, and a connecting pipe path 44 that is a separate element from the outer pipe 14 is inserted into and attached to the connecting hole 42. As shown in FIG. 3, one end of the connecting pipe path 44 is connected to an inner hole of the outer pipe 14, so that it is connected to the outer pipe 14 in series. In this practical embodiment, the connecting pipe path 44 is shaped so that it extends radially outward from the diameter-enlarged portion 38 of the outer pipe 14 and then bends and extends left-rightly outward. However, the shape (piping route) of the connecting pipe path 44 is set as appropriate, taking into account factors such as interference with other components and the position of the equipment (e.g. condenser 54 and expansion valve 56 described below) to which the other end is connected.

The inner pipe 12 is inserted into the outer pipe 14. The mating portions 40, 40 of the connecting pipe wall portions 36, 36, which constitute the ends of the outer pipe 14, are fitted onto the outer surface of the cylindrical pipe wall portions 30, 30 of the inner pipe 12, thereby positioning the inner pipe 12 and outer pipe 14 relative to each other. The cylindrical pipe wall portions 30, 30, which constitute the pipe walls 20b, 20b of the end regions 22, 22 of the inner pipe 12, extend outward from the ends of the outer pipe 14 in the pipe lengthwise direction. Besides, the mating portion 40 may be formed by diameter reduction process on the ends of the outer pipe 14 after the inner pipe 12 has been inserted into the outer pipe 14, that is, it is not necessary to form the mating portion 40 on the outer pipe 14 in advance. In this case, the mating portion 40 is fitted onto the cylindrical pipe wall portion 30 when the diameter of the mating portion 40 is formed by diameter reduction, for example. Besides, the spaces between the ends of the outer pipe 14 and the inner pipe 12 are sealed fluid-tightly by means of gluing, brazing, or interposing an O-ring, etc. to prevent leakage of the heat medium described below.

The intermediate region 18 is a double pipe structure since the pipe wall 20a of the inner pipe 12 is inserted into the pipe wall 32a of the outer pipe 14. As shown in FIGS. 4 and 5, the convex portions 24, 24, 24 of the pipe wall 20a of the inner pipe 12 are close to or in contact with the pipe wall 32a (the intermediate pipe wall portion 34) of the outer pipe 14, and portions of the pipe wall 20a offset from the convex portions 24, 24, 24 are separated from the pipe wall 32a due to differences in curvature. As a result, an outer flow path 46 extending in the pipe lengthwise direction is formed between the pipe wall 20a of the inner pipe 12 and the pipe wall 32a of the outer pipe 14. Therefore, in the intermediate region 18, an inner flow path 16 formed by the inner hole of the inner pipe 12 and an outer flow path 46 formed between the inner pipe 12 and the outer pipe 14 around the inner flow path 16 are provided on the inner side and outer side of the pipe wall 20a of the inner pipe 12, respectively. Besides, the outer flow path 46 may be a single flow path continuous in the circumferential direction, or it may be constituted of substantially independent plural flow paths in which the outer surface of the inner pipe 12 (convex portion 24) and the inner surface of the outer pipe 14 contact each other partially in the circumferential direction.

As shown in FIG. 3, the inner flow path 16 is connected to an evaporator 50 and a compressor 52 that constitute the refrigeration cycle 48 of a heat exchanger (refrigerator or heat pump), for example, through the inner hole of the cylindrical pipe wall portions 30, 30 of the inner pipe 12. The outer flow path 46 is connected to an inner hole of the connecting pipe path 44 through the circumferential gap between the cylindrical pipe wall portion 30 of the inner pipe 12 and the diameter-enlarged portion 38 of the outer pipe 14, so that each end of the connecting pipe paths 44 are connected to the condenser 54 and the expansion valve 56, respectively. In the inner flow path 16, the heat medium is cooled by the expansion valve 56, passes through the evaporator 50, become low-pressure and low-temperature and flows towards the compressor 52. In the outer flow path 46, the heat medium is warmed by the compressor 52 and condenser 54, become high-pressure and high-temperature and flows towards the evaporator 50.

The compressor 52, condenser 54, expansion valve 56 and evaporator 50 are not components of the double pipe for heat exchange 10, however, they are components of the refrigeration cycle (heat exchange cycle) 48 equipped with the double pipe for heat exchange 10. Accordingly, they are explained briefly.

The compressor 52 is a device that pressurizes and warms the heat medium flowing in the refrigeration cycle 48 by compression. The compressor 52 is connected to the downstream side of the inner pipe 12 of the double pipe for heat exchange 10. The heat exchanged heat medium flowed in from the inner flow path 16 is pressurized and warmed.

The condenser 54 is a heat exchanger for warming the conditioned air, and is a device that condenses and liquefies the heat medium, which is in gaseous-phase due to heating by the compressor 52, by exchanging heat with the outside air. The condenser 54 is connected to the downstream side of the compressor 52 directly or via piping, and is connected to the upstream side of the outer flow path 46, so that the condensed heat medium flows into the outer flow path 46.

The expansion valve 56 is a device that cools the liquid-phase heat medium that has flowed from the condenser 54 to a prescribed temperature by depressurizing and expanding. The expansion valve 56 is connected to the downstream side of the outer flow path 46 of the double pipe for heat exchange 10. The heat exchanged heat medium flowed in from the outer flow path 46 is depressurized and cooled.

The evaporator 50 is a heat exchanger for cooling conditioned air, and is a device that evaporates the heat medium, which is in liquid-phase due to depressurizing and cooling by the expansion valve 56, so as to cool the conditioned air using the latent heat of vaporization. The evaporator 50 is provided on the downstream side of the expansion valve 56. The heat medium is depressurized and cooled by the expansion valve 56 and flow in. The evaporator 50 is provided on the upstream side of the inner flow path 16 of the double pipe for heat exchange 10. The warmed heat medium flows into the inner flow path 16 by cooling the conditioned air.

The heat medium circulating in the refrigeration cycle 48 is a fluid (gas or liquid), such as Freon, carbon dioxide, ammonia, etc., that is, a fluid (refrigerant) that is generally used in a refrigeration cycle 48 of air conditioning equipment and refrigerators (freezers).

In the refrigeration cycle 48 equipped with such various devices, in the double pipe for heat exchange 10, heat transfer (heat exchange) occurs among the low-temperature heat medium flowing through the inner flow path 16 and the high-temperature heat medium flowing through the outer flow path 46, based on the temperature difference among the heat medium. As a result, the heat medium flowing through the inner flow path 16 of the double pipe for heat exchange 10 into the compressor 52 is warmed by the heat from the heat medium flowing through the outer flow path 46 before being pressurized and warmed by the compressor 52. Also, the heat medium flowing through the outer flow path 46 of the double pipe for heat exchange 10 into the expansion valve 56 is cooled due to heat dissipation to the heat medium flowing through the inner flow path 16 before being depressurized and cooled by the expansion valve 56. In order to increase efficiency of heat exchange among the heat medium, it is desirable that the flow direction of the heat medium in the inner flow path 16 and the flow direction of the heat medium in the outer flow path 46 are mutually opposite. In FIG. 3, the direction of flow of the heat medium through the inner flow path 16 is indicated by a solid arrow, and the direction of flow of the heat medium through the outer flow path 46 is indicated by a dashed arrow.

In the inclined pipe wall portion 26, the heat medium flowing through the inner flow path 16 flows as a laminar flow without agitation of the heat medium through the outer circumference and the heat medium through the inner circumference. In this way, the pressure loss is suppressed by stabilizing the flow of the flowing heat medium without disturbing them in the inclined pipe wall portion 26, so that the heat medium flows efficiently in the refrigeration cycle 48.

The inclined pipe wall portion 26 and the straight pipe wall portion 28 are configured to have the same shape in the cross-section perpendicular to the pipe lengthwise direction. The pipe wall 20a of the intermediate region 18 is configured to have a roughly constant cross-sectional shape throughout the pipe lengthwise direction. Therefore, pressure loss caused by changes in the cross-sectional shape of the inner flow path 16 and the outer flow path 46 is avoided.

In this practical embodiment, the total length of the inclined pipe wall portions 26 is longer than the total length of the straight pipe wall portions 28 in the pipe wall 20a of the intermediate region 18 of the inner pipe 12. Accordingly, even if turbulence occurs at the connecting portion between the inclined pipe wall portion 26 and the straight pipe wall portion 28, the heat medium flows stably over a wide area of the inner flow path 16 and the outer flow path 46 so that the pressure loss is suppressed, as described below.

The heat medium flowing through the inner flow path 16 is subject to turbulence at the connecting portion of the inclined pipe wall portion 26 and the straight pipe wall portion 28 due to sudden change in the lead angle. As a result, the heat medium flowing through the inner circumference of the inner flow path 16 and the heat medium flowing through the outer circumference of the inner flow path 16 are agitated at the connecting portions between the inclined pipe wall portion 26 and the straight pipe wall portion 28. Therefore, the temperature of the heat medium in the inner flow path 16 is uniformed, thereby avoiding the case wherein the heat medium flowing through the outer circumference becomes hot by heat exchange whereas the heat medium flowing through the inner circumference remains cold with little contribution to heat exchange. As a result, the heat medium flowing near the pipe wall 20a in the inner flow path 16 is maintained at relatively low temperature, so that heat exchange efficiency with the heat medium flowing through the outer flow path 46 is improved.

Similarly, the heat medium flowing through the outer flow path 46 is subject to turbulence at the connecting portions of the inclined pipe wall portions 26 and the straight pipe wall portions 28 due to the sudden change in the lead angle, so that the heat medium in the outer flow path 46 is agitated, thereby the temperature is uniformed. As a result, a case wherein temperature of the heat medium flowing through the inner circumference of the outer flow path 46 becomes low can be avoided. Owing to increase of the temperature difference from the heat medium in the inner flow path 16, heat exchange efficiency is improved. Note that the term uniforming the temperature of the heat medium does not mean that the temperature of the entire heat medium is strictly constant, however, rather that the temperature difference within the heat medium is reduced.

As described above, in this practical embodiment, in the straight pipe portion of the inner pipe 12 that constitutes the heat exchange portion, the inclined pipe wall portion 26 is provided wherein the plural convex portions 24 are formed circumferentially and extend spirally in the pipe length direction. In the intermediate portion of the pipe lengthwise direction, the turbulence-generating pipe wall portion 28 with a different lead angle of the convex portions 24 is provided. In addition, since the plural inclined pipe wall portions 26 and the plural straight pipe wall portions 28 are alternately provided in the pipe lengthwise direction, turbulence occurs at the connecting portions of the inclined pipe wall portions 26 and the straight pipe wall portions 28 at multiple points in the pipe lengthwise direction. As a result, for the heat medium in the inner flow path 16 and the outer flow path 46, the temperature difference between inner and outer circumferences is more effectively reduced, so that further improvement in heat exchange efficiency is achieved.

Since the inclined pipe wall portions 26 are provided on both sides of each straight pipe wall portion 28 in the pipe length direction, agitation effect of the heat medium due to the generation of turbulence is exerted on both sides of each straight pipe wall portion 28. As a result, it is possible to achieve improvement in heat exchange efficiency with a relatively small number of straight pipe wall portions. In particular, in this practical embodiment, in the inner pipe 12, the pipe walls 20b, 20b of the end regions 22, 22, which are continuous with the pipe wall 20a of the intermediate region 18, are the straight cylindrical pipe wall portions 30, 30. Therefore, if the ends of the pipe wall 20a are constituted of the inclined pipe wall portions 26, agitating effect on the heat medium is expected due to the difference in the lead angle between the inclined pipe wall portion 26 and the cylindrical pipe wall portion 30.

FIG. 8 shows simulation results of the flow of the heat medium in the inner flow path 16. Comparative Example shown in the upper part of FIG. 8 is the simulation result of the conventional structure of a double pipe for heat exchange, in which there is no straight pipe wall portion (turbulence-generating pipe wall portion), and an inclined pipe wall portion 26 is provided with a convex portion 24 that extends spirally with a roughly constant lead angle throughout pipe wall 20a. Example shown in the lower part of FIG. 8 is simulation result of the double pipe for heat exchange 10 of the first practical embodiment with the straight pipe wall portions 28. Besides, in FIG. 8, multiple points and their movement trajectories represent the flow of the heat medium over a prescribed period. In particular, FIG. 8 observes the heat medium in each of the convex portions 24 of the inclined pipe wall portion 26 of the inner pipe 12, where there is a risk of stagnation.

In Comparative Example in FIG. 8, the heat medium flowing near the pipe wall of the inner pipe flows spirally along the pipe wall. There is almost no flow of the heat medium flowing spirally within each convex portion of the pipe wall flowing from the outer circumference to the inner circumference towards the center of the inner pipe, nor flowing from the inner circumference to the outer circumference. Meanwhile, in Example in FIG. 8, the heat medium flowing near the pipe wall 20a flows spirally along the pipe wall 20a in each convex portion 24 of the inclined pipe wall portion 26, whereas the flow is disturbed in the straight pipe wall portion 28, and there is a flow of heat medium that flows from the outer circumference to the inner circumference towards the center of the inner pipe 12, and flowing from the inner circumference to the outer circumference. In details, in the heat medium flowed spirally in each convex portion 24, multiple small vortex-like flows occur near the boundary from the inclined pipe wall portion 26 to the straight pipe wall portion 28. Since these vortex-like flows promotes the turbulent state, it is confirmed that the heat medium flowed spirally through the convex portions 24 and the heat medium flowed linearly through the central part of the inner pipe 12 with little effect from the convex portions 24 were mutually agitated. As shown above, the simulation result also confirm that the double pipe for heat exchange 10 of the present disclosure, which has the turbulence-generating pipe wall portions (straight pipe wall portion 28), causes the temperature to become more uniform due to the agitation of the heat medium in the inner flow path 16.

FIG. 9 shows the simulation results for the temperature distribution of the heat medium flowing through the inner flow path. According to FIG. 9, in Comparative Example without a turbulence-generating pipe wall portion, it is found that the outer circumferential high-temperature heat medium flowing along the pipe wall of the inner pipe and the inner circumferential low-temperature heat medium flowing through the central region of the inner pipe flow in a state of almost complete separation, with little mixing. Meanwhile, in Example where the straight pipe wall portion 28 is provided as a turbulence-generating pipe wall portion, FIG. 9 shows the outer circumferential high-temperature heat medium flowing along the pipe wall 20a of the inner pipe 12 in the inclined pipe wall portion 26 and the inner circumferential low-temperature heat medium flowing through the central region of the inner pipe 12 in the inclined pipe wall portion 26. FIG. 9 shows that change in the flows occurs where these flows mix each other in the connecting region between the inclined pipe wall portion 26 and the straight pipe wall portion 28. In other words, the simulation result of the temperature distribution in FIG. 9 also confirm that the straight pipe wall portion 28 causes the outer circumferential high-temperature heat medium flowing toward inner circumference of the inner pipe 12, and conversely the inner circumferential low-temperature heat medium flowing toward outer circumference.

Although the practical embodiment of the present disclosure has been described in detail above, the present disclosure is not limited by that specific description. For example, the double pipe for heat exchange do not have to be a straight pipe that extends linearly as a whole, however, it is sufficient if the inclined pipe wall portion and the turbulence-generating pipe wall portion are formed in a portion straight in the pipe lengthwise direction. Therefore, the double pipe for heat exchange can be shaped to bend as appropriate in the area offset from the inclined pipe wall portion and the turbulence-generating pipe wall portion.

The turbulence-generating pipe wall portion is sufficient if the lead angle of the convex portion differs from that of the inclined pipe wall portion, so that it does not have to be limited by a straight pipe wall portion whose convex portion extends linearly. For example, the turbulence-generating pipe wall portion may be a portion whose convex portion extends spirally with a lead angle larger or smaller than the lead angle of the convex portion of the inclined pipe wall portion. From the perspective of suppressing increases in pressure loss, etc., it is desirable that the lead angle of the convex portion in the turbulence generating pipe wall portion is set close to 90° and towards the flow direction towards the center axis of the pipe, and is desirable to be set in the same direction of inclination as the inclined pipe wall portion, but not in the opposite direction of inclination. From the perspective of suppressing increases in pressure loss, etc., it is also possible to provide the connecting pipe wall portion that gradually or stepwise changes the lead angle of the convex portions so as to transit from the inclined pipe wall portion to the turbulence generating pipe wall portion at the boundary portion connecting the inclined pipe wall portion and the turbulence-generating pipe wall portion.

In case plural inclined pipe wall portions are provided, the lead angles of the convex portions of these plural inclined pipe wall portions may be the same or different from each other. In addition, although it is desirable that the lead angles of a single inclined pipe wall portion are roughly constant, it may vary partially or entirely. Furthermore, the lengths of the plural inclined pipe wall portions may differ from each other in the pipe lengthwise direction.

In case plural turbulence-generating pipe wall portions are provided, the lead angle of the convex portions of these plural turbulence-generating pipe wall portions may be the same or differ from each other. Therefore, one or some of the plural turbulence-generating pipe wall portions may be straight pipe wall portions with a lead angle of 90°, while the other turbulence-generating pipe wall portions may be spiral shaped with a lead angle of less than 90°. Moreover, although it is desirable that a lead angle of a single turbulence-generating pipe wall portion is roughly constant, it may vary partially or entirely. Furthermore, the lengths of the plural turbulence-generating pipe wall portions may differ from each other in the pipe lengthwise direction.

The number of inclined pipe wall portions and turbulence generating pipe wall portions, their length in the pipe lengthwise direction, and their arrangement in the pipe lengthwise direction are not particularly limited. Furthermore, the number, shape, and arrangement in the circumferential direction of the convex portions in the inclined pipe wall portions and the turbulence-generating pipe wall portions are not particularly limited. Besides, it is possible to ensure a larger cross-sectional area of the outer flow path formed between the inner pipe and the outer pipe by providing a groove-shaped portion concave toward the outer circumference in a part of the inclined pipe wall portion and the turbulence-generating pipe wall portion offset from the convex portions in the circumferential direction. Although it is desirable that the length of the turbulence-generating pipe wall portion is shorter than the inclined pipe wall portion, it may be longer than the length of the inclined pipe wall portion.

The above-mentioned practical embodiment is an example of a case where the refrigeration cycle 48 equipped with the double pipe for heat exchange 10 is used for cooling/warming conditioned air so as to constitute an air conditioning system for automobile. However, the double pipe for heat exchange of the present disclosure can be used for any heat exchange purpose, and it may be suitably employed in refrigeration cycle of an in-vehicle refrigerator, etc.