Enhanced heat exchange structure and processing method

Abstract

The invention belongs to the technical field of enhanced heat exchange, particularly relates to an enhanced heat exchange structure and a processing method, and aims to solve the problems of poor heat exchange effect and insufficient cost control or process stability. The enhanced heat exchange structure comprises a base plate and rib bodies, and the rib bodies are fixedly arranged on the surface, capable of making contact with a heat exchange medium, of the base plate; micro grooves used for increasing the number of gasification cores and the density of nucleation sites are formed in the surfaces of the base plate and the rib bodies, and irregular particles used for increasing the heat exchange surface and improving heat exchange medium disturbance are arranged around the micro grooves. Different output powers of an electric discharge machine are set on the surfaces of the substrate and the rib body structure containing the flow channels or the rib body structure is processed for multiple times, so that the parameters of the micro-grooves and the spherical or filamentous particles are matched with the boiling heat exchange working condition, the heat exchange efficiency is improved to the maximum extent, and then the heat dissipation efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of enhanced heat exchange, and in particular relates to an enhanced heat exchange structure and a processing method. Background Art

[0002] Nowadays, electronic products continue to develop towards high power density, and the heat dissipation demand of power electronic equipment continues to increase. Therefore, it is urgent to develop more efficient, safe and stable heat dissipation technology. Improving boiling heat transfer performance through surface modification technology has been one of the research hotspots in recent years. Various heat transfer surface enhanced heat transfer structures have been widely studied, such as surface microstructures, porous structures, micro-nano composite structure layers, functional coatings and non-uniform wettability surfaces.

[0003] These methods either increase the effective heat transfer area of ​​the boiling heat transfer surface or provide more potential gasification cores for the heat transfer surface. However, each surface modification technology has its own technical characteristics. For example, although the micro-rib and sintering methods have good strengthening effects, they are slightly insufficient in cost control or process stability.

[0004] The heat generated by current power electronic components is constantly increasing. In order to ensure the long-term safe and stable operation of the devices, it is often necessary to use multiple enhanced heat transfer technologies to improve the phase change heating intensity in the surface-mounted heat sink, so as to further improve the cooling efficiency of the surface-mounted self-circulating evaporative cooling system. Summary of the invention

[0005] In order to solve the problems in the prior art of poor heat exchange effect and inadequacies in cost control or process stability, the present invention provides, in one aspect, an enhanced heat exchange structure, including a base plate and a rib body;

[0006] The ribs are fixedly arranged on the surface of the substrate that can contact the heat exchange medium. The surfaces of the substrate and the ribs are both provided with micro grooves for increasing the number of gasification cores and the density of nucleation sites. Irregular particles are arranged around the micro grooves for increasing the heat exchange surface and improving the disturbance of the heat exchange medium.

[0007] According to an enhanced heat exchange structure provided in some embodiments of the present application, a plurality of ribs are provided, the plurality of ribs are arranged at intervals, a flow channel is formed between two adjacent ribs, and the directions of the ribs and the flow channel are consistent with the flow direction of the heat exchange medium.

[0008] According to an enhanced heat exchange structure provided in some embodiments of the present application, the end surface width of the rib body is 1mm-5mm, the height is 7mm-9mm, and the spacing between adjacent rib bodies is 3mm-5mm.

[0009] According to an enhanced heat exchange structure provided in some embodiments of the present application, a plurality of micro grooves are provided on the surface of the rib body, and the plurality of micro grooves are arranged in an interlaced manner.

[0010] According to an enhanced heat exchange structure provided by some embodiments of the present application, the irregular particles include spherical particles and filamentary particles, and the spherical particles and filamentary particles are arranged around the micro grooves.

[0011] According to an enhanced heat exchange structure provided in some embodiments of the present application, the microgrooves are irregularly shaped microgrooves, the width of the microgrooves is 2um-8um, the length is 40um-90um, and the diameter range of the irregular particles is 1um-5um.

[0012] Some embodiments of the present application further provide a processing method, which is applied to the enhanced heat exchange structure, and the specific steps are as follows:

[0013] S1. Connect the substrate or rib to the negative output electrode of the EDM machine;

[0014] S2. Adjust the distance between the positive electrode of the EDM machine and the substrate or rib;

[0015] S3, adjust the output power of the EDM machine and power it on;

[0016] S4, start moving the positive electrode of the EDM machine at a constant speed until the surface of the substrate and the ribs are completely swept, so that micro grooves and irregular particles are formed on the surface of the substrate and the ribs;

[0017] S5. After completing one EDM process, the surface roughness of the substrate and / or rib body is tested. If the surface roughness does not reach the range of 60%-80%, steps S1-S4 are repeated until the surface roughness of the substrate and / or rib body reaches the optimal use range.

[0018] According to a processing method provided in some embodiments of the present application, the distance between the positive electrode of the electric spark machine and the substrate is 0.12 mm, and the output power is 3000 W.

[0019] According to a processing method provided in some embodiments of the present application, micro-grooves are formed by melting local metal of a substrate and / or a rib body through spark discharge. During the spark discharge process, the electrode moves rapidly to achieve lateral pulling of the molten metal, and irregular particles are formed around the micro-grooves, so that a multi-particle, multi-groove structure is formed on the surface of the rib body.

[0020] According to a processing method provided by some embodiments of the present application, bubbles are generated in the microgrooves which are closer to the heat source. The sharp parts of the irregular particles arranged at the edges of the microgrooves can destroy the bubble growth boundaries, causing the bubbles to break at the tail and accelerate their detachment from the rib surface.

[0021] Beneficial effects of the present invention:

[0022] By setting different output powers of the EDM machine or performing multiple treatments on the surface of the substrate and rib structure containing the flow channel, the parameters of the micro-grooves, spherical or filamentary particles are matched with the boiling heat transfer conditions, thereby maximizing the heat exchange efficiency and thus improving the heat dissipation efficiency.

[0023] Specifically, it is divided into three-level strengthening structures: ribs, microgrooves, and irregular spherical or filamentary particles. The rib structure can increase the heat exchange area and enhance the turbulence, while increasing the attachment area of ​​the microgrooves and irregular spherical or filamentary particle structures; the microgrooves on the rib surface are closer to the heat source, making it easier for bubbles to nucleate; the width and depth of the microgrooves can reduce the contact area between the bubbles and the ribs, thereby reducing the viscosity; the high-density microgrooves effectively increase the density of nucleation sites; when the bubbles in the microgrooves increase and contact the irregular spherical or filamentary particles arranged around the microgrooves, the sharp protrusions can pierce the bubble boundary layer, cut off the bubble tail, accelerate the bubble to detach from the rib surface, and increase the local heat dissipation area and the turbulence of the heat exchange medium in the microgrooves. The combination of the three structures can more effectively improve the boiling heat transfer intensity, thereby enhancing the phase change cooling capacity of the surface-mounted radiator under high heat flux density conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

[0025] Figure 1 is a schematic diagram of the overall structure of the enhanced heat exchange structure of some embodiments of the present application;

[0026] Figure 2 is a schematic diagram of the principle of electric spark blasting in some embodiments of the present application;

[0027] Figure 3 is a schematic diagram of metal surface microstructure parameters after electrospark treatment in some embodiments of the present application;

[0028] Figure 4 is a schematic diagram of the morphology of bubbles detaching from a flat substrate surface in some embodiments of the present application;

[0029] Figure 5 is a schematic diagram of the bubble detachment morphology after treatment in some embodiments of the present application;

[0030] Figure 6 is a schematic diagram of electric spark electrode machining in some embodiments of the present application;

[0031] Figure 7 is a schematic diagram of an electric spark columnar electrode processing method according to some embodiments of the present application;

[0032] Figure 8 is a schematic diagram of an electrospark plate-shaped electrode processing method according to some embodiments of the present application;

[0033] Fig. 9 It is a schematic diagram of the electric spark wire electrode processing method of some embodiments of the present application.

[0034] In the figure: 1, substrate; 2, ribs; 3, micro grooves; 4, spherical particles; 5, flow channels; 6, filamentary particles; 10, positive electrode of the EDM machine; 11, arc. DETAILED DESCRIPTION

[0035] The present application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It is also necessary to explain that, for ease of description, only the parts related to the relevant invention are shown in the accompanying drawings.

[0036] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0037] like Figure 1-9 As shown, on the one hand, a heat exchange enhancement structure is provided, including a base plate 1 and a rib body 2;

[0038] The rib body 2 is fixedly arranged on the surface of the substrate 1 which can contact the heat exchange medium. The surfaces of the substrate 1 and the rib body 2 are both provided with micro grooves 3 for increasing the number of gasification cores and the density of nucleation sites. Irregular particles are arranged around the micro grooves 3 for increasing the heat exchange surface and improving the disturbance of the heat exchange medium.

[0039] In a specific implementation, the substrate 1 and the rib body 2 are integrated and the material may be copper, aluminum or stainless steel, etc. The rib body 2 is arranged on the surface of the substrate 1 that can contact the heat exchange medium to increase the heat exchange area between the substrate 1 and the rib body 2 and the heat exchange medium and the attachment area of ​​the micro grooves 3 and the irregular particles. In some embodiments, the rib body 2 can be processed by a milling process, and the micro grooves 3 and the irregular particles can be formed by electric spark machining.

[0040] In some embodiments, a plurality of rib bodies 2 are provided, and the plurality of rib bodies 2 are arranged at intervals, and a flow channel 5 is formed between two adjacent rib bodies 2, and the directions of the rib bodies 2 and the flow channel 5 are consistent with the flow direction of the heat exchange medium.

[0041] In specific implementation, the orientation of each rib 2 and flow channel 5 is consistent with the flow direction of the heat exchange medium. Preferably, the intervals between two adjacent ribs 2 are equal to improve the uniformity of the distribution of the ribs 2 on the substrate 1.

[0042] In some embodiments, the width of the end surface of the rib 2 is 1 mm-5 mm, the height is 7 mm-9 mm, and the spacing between adjacent ribs 2 is 3 mm-5 mm.

[0043] In specific implementation, the rib body 2 can be of various structures, and its cross-sectional shape includes rectangular, triangular, diamond, etc. The end face shape of the rib body 2 can also be rectangular, triangular, etc. Preferably, the end face width range of the rib body 2 is 3mm-5mm, the height range is 7mm-9mm, and the flow channel width range is 3-5mm. The rib body 2 uses a structure above the millimeter level, which is convenient for the heat of the substrate 1 to be transferred to the top of the rib body 2, while increasing the heat exchange area. The larger flow channel allows more supercooled media to exist around the exchange surface, and increases the boiling bubble flow space, so that the gaseous medium can quickly flow out of the heat exchanger and reduce internal cavities.

[0044] In some embodiments, a plurality of micro grooves 3 are disposed on the surface of the rib 2 , and the plurality of micro grooves 3 are arranged in an interlaced manner.

[0045] In a specific implementation, a plurality of micro grooves 3 are disposed on the surfaces of the substrate 1 and the rib body 2 , and the arrangement of the micro grooves 3 includes a single micro groove 3 arrangement and a staggered arrangement of a plurality of micro grooves 3 .

[0046] In some embodiments, the irregular particles include spherical particles 4 and filamentary particles 6 , and the spherical particles 4 and filamentary particles 6 are disposed around the micro grooves 3 .

[0047] In specific implementation, the enhanced heat exchange structure can be applied to surface-mounted radiators. When in use, the rib body 2 is used to increase the heat exchange area with the heat exchange medium, and at the same time increase the attachment area of ​​the microgrooves 3 and the irregular particle structure. The high-density microgrooves 3 on the surface of the rib body 2 can increase the density of nucleation sites; when the bubbles in the microgrooves 3 increase and contact the spherical particles or filamentary particles 6 arranged around the microgrooves 3, the raised sharp parts can pierce the bubble boundary layer, cut off the bubble tail, accelerate the bubble to detach from the rib surface, and increase the local heat dissipation area and the turbulence of the heat exchange medium in the microgrooves. The combination of the three can more effectively improve the boiling heat exchange intensity, thereby enhancing the phase change cooling capacity of the surface-mounted radiator under high heat flux density conditions.

[0048] Compared with a single machined structure such as fins and fins, the multi-stage boiling enhanced heat exchange structure using ribs 2, micro grooves 3, spherical particles 4 and filamentary particles 6 can further improve the boiling heat transfer coefficient and critical heat flux density, thereby more effectively enhancing the boiling heat transfer in the surface mounted radiator.

[0049] In some embodiments, the micro groove 3 is an irregularly shaped micro groove, the width of the micro groove 3 is 2um-8um, the length is 40um-90um, and the diameter range of the irregular particles is 1um-5um.

[0050] When implementing it, Figure 2-Figure 3 As shown, each micro groove 3 is an arc-shaped groove with uneven depth; wherein the micro groove 3 can be defined by the structural quantities h, z, and l, the parameters h and z are used to represent the width and depth of the micro groove 3, respectively, and the parameter l (not shown in the figure) represents the length of the micro groove, wherein the parameters h and z can be adjusted by adjusting the output power of the EDM machine, preferably, the range of h is 2um-8um, and the range of l is above 50um.

[0051] Each irregular particle includes two forms, namely, filamentary particles 6 and spherical particles 4, both of which protrude from the surface of the rib body 2. The filamentary particles 6 are defined by the w and j structural quantities, the parameter w is used to represent the thickness of the filamentary particles 6, and the parameter j is used to represent the length of the filamentary particles 6; the spherical particles 4 are defined by the r structural quantity, and the parameter r is used to represent the diameter of the spherical particles 4. For example, the range of w is below 2um, the range of j is 4um-7um, and the range of r is below 4um. The parameter s is used to represent the particle spacing, for example, the point / filamentary particle density is between 0.6-0.8, and the density is the ratio of the diameter r of the spherical or filamentary particles to the particle spacing s.

[0052] Some embodiments of the present application further provide a processing method, which is applied to an enhanced heat exchange structure, and the specific steps are as follows:

[0053] S1, connect the substrate 1 or the rib 2 to the negative output electrode of the EDM machine;

[0054] S2, adjusting the distance between the positive electrode of the EDM machine and the substrate 1 or the rib 2;

[0055] S3, adjust the output power of the EDM machine and power it on;

[0056] S4, start to move the positive electrode of the EDM machine at a constant speed until the surfaces of the substrate 1 and the rib 2 are completely swept, so that micro grooves 3 and irregular particles are formed on the surfaces of the substrate 1 and the rib 2;

[0057] S5. After completing one EDM process, the surface roughness of the substrate 1 and / or the rib 2 is tested. If the surface roughness does not reach the range of 60%-80%, steps S1-S4 are repeated until the surface roughness of the substrate 1 and / or the rib 2 reaches the optimal use range.

[0058] In specific implementation, the surface roughness is in the range of 60%-80% for optimal use. Figure 2As shown, the positive electrode 10 of the EDM machine releases an arc 11 when in use, and the contact point between the arc 11 and the substrate 1 or the rib body 2 will generate high temperature. The micro groove 3 uses the high temperature generated by the spark discharge to cause the metal surfaces of the substrate 1 and the rib body 2 to produce a tiny melting and resolidification process to form the micro groove 3 structure. The spherical particles 4 use the high pressure generated by the spark discharge to cause part of the metal melt in the micro groove 3 to be sputtered to the metal surface of the substrate 1 or the rib body 2 and then condensed to form irregularly distributed spherical particles 4. The filamentary particles 6 use the opposite positive and negative charges of the two electrodes during the electric spark treatment to generate an attraction so that part of the metal melt will move to the electrode of the EDM machine. During the movement of the electrode, the molten metal condenses to form filamentary particles 6 around the micro groove 3.

[0059] In some embodiments, the distance between the positive electrode of the EDM machine and the substrate 1 is 0.12 mm, and the output power is 3000W.

[0060] In specific implementation, the distance between the positive electrode of the EDM machine and the substrate 1 is 0.12mm, and the processing effect is best when the output power is 3000W. For multiple metal substrates with the same structure, the EDM machine can be adjusted to the optimal output power during the first processing, so that subsequent metal substrates can be formed in one processing.

[0061] In some embodiments, the micro grooves 3 are formed by melting local metal of the substrate 1 and / or the rib body 2 by spark discharge. During the spark discharge process, the electrode moves rapidly to achieve lateral pulling of the molten metal, and irregular particles are formed around the micro grooves 3, so that a multi-particle, multi-groove structure is formed on the surface of the rib body 2.

[0062] In specific implementation, the micro grooves 3 and irregular particles can be made by one or more of the electrospark machining processes. For example, the main components of the columnar electrode grinding method include a vibrating electrode, an electrospark machine, etc., wherein the electrospark machine is the core equipment. The power of the electrospark machine can be adjusted within the range of 3000W. There is a reset spring inside the vibrating motor. When the extrusion force is applied to the electrode, the electrode segment will continue to jump under the action of the spring, forming a certain distance with the substrate 1 or the rib body 2. The depth of the spark-machined micro grooves 3 and the density of irregular particles can be controlled by adjusting different output power gears. By continuously moving the handheld device on the surface of the workpiece, the surface is covered with micro grooves and irregular particles.

[0063] In some embodiments, the main components of the EDM forming method include prefabricated forming electrodes, EDM machines, etc., wherein the forming electrodes include block electrodes and plate electrodes. The power of the EDM machine in this processing method needs to be adjusted within the range of 3000W-6000W. By accurately replicating the complex shape of the workpiece and reducing the complex movement process of the electrode, the workpiece surface can be processed and formed faster.

[0064] In some embodiments, an electric spark wire cutting processing method can also be used. The main components include fine metal electrode wire, electric spark machine, etc. The power of the electric spark machine in this processing method needs to be adjusted within the range of 2000W-5000W. The electrode wire is used to move evenly on the surface of the workpiece, which is suitable for workpieces with regular surface structures.

[0065] In some embodiments, the micro groove 3 is closer to the heat source, bubbles are generated in the micro groove 3, and the sharp parts of the irregular particles arranged at the edge of the micro groove 3 can destroy the bubble growth boundary, causing the bubble to break at the tail and accelerate its detachment from the surface of the rib body 2.

[0066] When implementing it, Figure 4-5 As shown, the irregular microgrooves are closer to the heat source, and bubbles are preferentially generated in the microgrooves 3. The width of the microgrooves 3 limits the diameter of the bubble tail, reduces the contact area between the bubble and the rib body 2, and can increase the number of gasification cores. When the bubbles in the microgrooves 3 increase and contact the spherical particles 4 and filamentous particles 6 arranged around the microgrooves 3, the sharp protrusions of the irregular particles can pierce the bubble boundary layer, cut off the bubble tail, and accelerate the bubble to detach from the rib surface, so that the gaseous medium can quickly flow out of the heat exchanger, reduce internal cavities, and increase the local heat dissipation area and the turbulence of the heat exchange medium in the microgrooves 3, which can more effectively improve the boiling heat transfer intensity, thereby enhancing the phase change cooling capacity of the surface-mounted radiator under high heat flux density conditions.

[0067] In specific implementation, the substrate 1 and the ribs 2 effectively increase the heat exchange area. The enhanced heat exchange structure is not limited to the plane and rib structure, but can also be anisotropic ribs. The processing method provided in this application can be applied to a variety of complex surfaces. The number of gasification cores and the density of nucleation sites are increased through micro-grooves and irregular particles, thereby enhancing the heat exchange efficiency. The processing process is mature.

[0068] In the description of the present invention, the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings, which are only for the convenience of description, and do not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0069] In addition, it should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0070] The term "comprise" or any other similar term is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus / device that includes a list of elements includes not only those elements but also other elements not expressly listed, or also includes elements inherent to such process, method, article, or apparatus / device.

[0071] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A heat exchange enhancement structure, characterized in that: It comprises a base plate (1) and a rib body (2); The rib body (2) is fixedly arranged on the surface of the substrate (1) that can contact the heat exchange medium, and the surfaces of the substrate (1) and the rib body (2) are both provided with micro grooves (3) for increasing the number of gasification cores and the density of nucleation sites, and irregular particles are arranged around the micro grooves (3) for increasing the heat exchange surface and increasing the disturbance of the heat exchange medium.

2. The enhanced heat exchange structure according to claim 1, characterized in that: A plurality of the rib bodies (2) are provided, and the plurality of the rib bodies (2) are arranged at intervals, and a flow channel (5) is formed between two adjacent rib bodies (2), and the directions of the rib bodies (2) and the flow channel (5) are consistent with the flow direction of the heat exchange medium.

3. The enhanced heat exchange structure according to claim 2, characterized in that: The end surface width of the rib body (2) is 1 mm-5 mm, the height is 7 mm-9 mm, and the spacing between adjacent rib bodies (2) is 3 mm-5 mm.

4. The enhanced heat exchange structure according to claim 1, characterized in that: A plurality of micro grooves (3) are arranged on the surface of the rib body (2), and the plurality of micro grooves (3) are arranged in a staggered manner.

5. The enhanced heat exchange structure according to claim 4, characterized in that: The irregular particles include spherical particles (4) and filamentous particles (6), and the spherical particles (4) and filamentous particles (6) are arranged around the micro grooves (3).

6. The enhanced heat exchange structure according to claim 5, characterized in that: The micro groove (3) is an irregularly shaped micro groove, the width of the micro groove (3) is 2um-8um, the length is 40um-90um, and the diameter range of the irregular particles is 1um-5um.

7. A processing method, characterized in that: The enhanced heat exchange structure according to any one of claims 1 to 6 is applied, and the specific steps are as follows: S1, connecting the substrate (1) or the rib (2) to the negative output electrode of the electric spark machine; S2, adjusting the distance between the positive electrode of the electric spark machine and the base plate (1) or the rib body (2); S3, adjust the output power of the EDM machine and power it on; S4, starting to move the positive electrode of the electric spark machine at a constant speed until the surfaces of the substrate (1) and the rib body (2) are completely swept, so that micro grooves (3) and irregular particles are formed on the surfaces of the substrate (1) and the rib body (2); S5. After completing one electrospark treatment, the surface roughness of the substrate (1) and / or the rib body (2) is tested. If the surface roughness does not reach the range of 60%-80%, steps S1-S4 are repeated until the surface roughness of the substrate (1) and / or the rib body (2) reaches the optimal use range.

8. A processing method according to claim 7, characterized in that: The distance between the positive electrode of the electric spark machine and the substrate (1) is 0.12 mm, and the output power is 3000W.

9. A processing method according to claim 8, characterized in that: The micro grooves (3) are formed by melting local metal of the substrate (1) and / or the rib body (2) through spark discharge. During the spark discharge process, the electrode moves rapidly to achieve lateral pulling of the molten metal, and irregular particles are formed around the micro grooves (3), so that a multi-particle, multi-groove structure is formed on the surface of the rib body (2).

10. A processing method according to claim 9, characterized in that: The microgrooves (3) are closer to the heat source, and bubbles are generated in the microgrooves (3). The sharp parts of the irregular particles arranged at the edges of the microgrooves (3) can destroy the bubble growth boundary, causing the bubbles to break at the tail and accelerate their detachment from the surface of the rib body (2).

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