A phase-change heat transfer evaporative heat exchanger for microchannel air conditioning

Abstract

The present application relates to the technical field of refrigeration systems, and in particular to a microchannel air-conditioning phase change heat transfer evaporative heat exchanger. It includes a flow pipeline, which is composed of a plurality of vertical pipelines and a plurality of curved pipelines connecting the vertical pipelines, forming a continuous flow path for a condensing agent. Inside the vertical pipeline, a mixing assembly specifically used for stirring the condensing agent is provided. On the outside of the vertical pipeline, a fin assembly is installed, and the outside of these fin assemblies is wrapped by a complete shell. When the condensing agent flows in the flow pipeline, it passes through the mixing assembly inside each vertical pipeline to be fully mixed and evenly distributed. This design ensures that the condensing agent inside each vertical pipeline can reach an ideal uniform state. Then, the condensing agent flows through the fin assembly, and the heat exchange area increased by the fin is used to effectively perform heat exchange. Such a layout not only optimizes the flow of the condensing agent, but also significantly improves the efficiency of heat exchange.

Description

Technical Field

[0001] The present application relates to the technical field of refrigeration systems, and in particular to a microchannel air-conditioning phase-change heat transfer evaporative heat exchanger. Background Art

[0002] Microchannel air conditioning phase change heat transfer evaporative heat exchanger refers to an air conditioning evaporator manufactured using microchannel technology. Its working principle is based on the phase change heat transfer of the refrigerant. In the evaporator, the liquid refrigerant absorbs heat and evaporates into a gaseous state. In this process, a large amount of heat is absorbed, thereby achieving a cooling effect. Microchannel technology increases the heat exchange efficiency by increasing the contact area between the refrigerant and the air, which significantly improves the cooling performance of the device.

[0003] However, in a microchannel evaporator, the refrigerant usually exists in a gas-liquid two-phase state. Due to the small aperture of the microchannel, the refrigerant may be affected by various factors such as gravity and inertia during the flow process, resulting in uneven distribution of the gas-liquid two-phase in the microchannel, with one part being gas and the other part being liquid, resulting in a lower heat exchange effect. Summary of the invention

[0004] In view of the deficiencies in the prior art, the purpose of this application is to provide a microchannel air-conditioning phase change heat transfer evaporative heat exchanger to solve the above-mentioned technical problems.

[0005] The above-mentioned purpose of the present application is achieved through the following technical solutions: a microchannel air-conditioning phase change heat transfer evaporative heat exchanger, comprising a flow pipe for the circulation of a condensing agent, wherein the flow pipe is formed by connecting a plurality of vertical pipes and a plurality of curved pipes end to end, a mixing assembly for stirring the condensing agent is arranged inside the vertical pipe, a fin assembly is arranged outside the vertical pipe, and a shell that wraps the entire flow pipe is arranged outside the fin assembly.

[0006] By adopting the above technical solution, when the condenser flows into the pipeline, the gaseous and liquid condensers will be fully mixed and distributed along with the mixing components in each vertical pipeline, so as to ensure the uniform distribution of the condenser inside each vertical pipeline, and then through the setting of the fin component, normal heat exchange is guaranteed.

[0007] Furthermore, the mixing assembly includes a mixing tube arranged inside a vertical pipe, wherein a streamline groove is provided inside the mixing tube, wherein a plurality of streamline grooves are provided and are evenly provided in the mixing tube, and an agitating block fixedly connected to the mixing tube is provided on the streamline groove.

[0008] By adopting the above technical solution, when the gaseous and liquid condensing agents enter the mixing tube, they will be mixed faster along the streamline groove due to gravity, and then hit the stirring block, so that the liquid condensing agent and the gaseous condensing agent are mixed together.

[0009] Furthermore, the stirring block is provided with a plurality of through grooves, the upper and lower sides of the mixing tube are fixedly provided with rotating rings, and the vertical pipe is provided with rotating grooves for the rotating rings to rotate.

[0010] By adopting the above technical solution, when the gas-liquid two-phase condensing agent hits the stirring plate, the mixing tube will rotate in the vertical pipe due to the pressure applied from the outside, so as to further improve the mixing of the gas-liquid two-phase.

[0011] Furthermore, the fin assembly includes multiple groups installed on the first fin strip and the second fin strip, the first fin strip and the second fin strip are both provided with circular grooves matching the vertical pipe, a fixing assembly for fixing the first fin strip and the second fin strip is provided between the first fin strip and the second fin strip, a connecting strip is fixedly provided on the side of the first fin strip close to the second fin strip, and a matching connecting groove is provided on the second fin strip.

[0012] By adopting the above technical solution, when installing the fin assembly on the flow pipe, the circular grooves of the first fin strip and the second fin strip are installed on the outside of the vertical pipe, and the connecting strips are connected to the connecting grooves for preliminary fixation, and then the first fin strip and the second fin strip are fixed together by the fixing assembly.

[0013] Furthermore, the circular groove is provided with a plurality of groups of plug-in blocks, and the vertical pipe is fixedly provided with a plurality of groups of plug-in grooves matching the plug-in blocks.

[0014] By adopting the above technical solution, in order to ensure that the first fin strip and the second fin strip are fixed in the vertical pipe, the plug-in groove on the circular groove is aligned with the plug-in block, and then the first fin strip and the second fin strip are fixedly installed in the vertical pipe through the fixing assembly.

[0015] Furthermore, the fixing component includes a first semicircular groove opened on the first fin strip and a second semicircular groove opened on the second fin strip, the first semicircular groove and the second semicircular groove are spliced ​​into a circle, the first semicircular groove and the second semicircular groove are spliced ​​into a circle, a rotating shaft is rotatably arranged in the first semicircular groove, a locking rod is fixedly arranged on the rotating shaft, sliding columns are fixedly arranged at both ends of the locking rod, the first semicircular groove is provided with a first semi-arc groove on the first fin strip, the second semicircular groove is provided with a second semi-arc groove on the second fin strip, and the sliding column is slidably connected in the first semi-arc groove and the second semi-arc groove.

[0016] By adopting the above technical solution, when it is necessary to fix the first fin strip and the second fin strip, align the first semicircular groove on the first fin strip with the second semicircular groove on the second fin, then rotate the locking rod in the first semicircular groove, and then slide the sliding columns on both sides on the first semi-arc groove and the second semi-arc groove respectively, thereby pulling the first semi-arc groove and the second semi-arc groove.

[0017] Furthermore, each of the sliding columns is provided with a placement groove, a slope is provided in the placement groove, an inclined plate is inserted in the placement groove, a slope is provided on the inclined plate, the inclined plate is close to the slope in the placement groove, a spring is fixedly provided on the inclined plate, an abutment plate is fixedly connected to the middle part of the locking rod, the other end of the spring is fixedly connected to the abutment plate, the middle parts of the first semi-arc groove and the second semi-arc groove are both provided with locking grooves inserted with the inclined plate, and a horizontal plate fixedly connected to the abutment plate is provided on the upper end of the spring.

[0018] By adopting the above technical solution, when the sliding column slides into the first semi-arc groove and the second semi-arc groove respectively, the inclined plate will be inserted into the locking groove along with the compression of the spring, thereby fixing the first fin strip and the second fin strip. When unlocking, pull the inclined plate to allow it to detach from the locking groove along the inclined surface, separating the first fin strip and the second fin strip. The setting of the horizontal plate allows the spring to maintain a horizontal elastic force, thereby continuously pushing the inclined plate.

[0019] Furthermore, a protrusion is fixedly arranged on the inclined plate.

[0020] By adopting the above technical solution, the setting of the protrusions can provide a power point when pulling the inclined plate.

[0021] Furthermore, the outer shell includes an upper shell and a lower shell, and sealing strips are fixedly provided at positions of the upper shell and the lower shell that wrap the flow pipe, and the upper shell and the lower shell are fixedly connected at both sides.

[0022] By adopting the above technical solution, the provision of the sealing strip ensures the sealing of the upper shell and the lower shell, and also reduces the squeezing of the flow conduit.

[0023] Furthermore, the shell includes through grooves for cooling air to flow on one side close to the first fin strip and the second fin strip.

[0024] In summary, the present application includes the following beneficial technical effects: through the mixing assembly inside the vertical pipe, the gaseous and liquid condensing agents can be fully mixed, ensuring that the condensing agent inside each vertical pipe is evenly distributed, thereby greatly improving the heat exchange efficiency. At the same time, the setting of the fin assembly further increases the heat exchange area, optimizes the flow path of the condensing agent, and makes the heat transfer more efficient. In addition, the flow pipeline is formed by connecting multiple vertical pipes and multiple curved pipes end to end, so that the entire heat exchanger has a compact structure, occupies a small space, and is easy to install and maintain. The mixing assembly and the fin assembly are made of wear-resistant and corrosion-resistant materials, can operate stably for a long time, and extend the service life of the heat exchanger. In terms of environmental protection and energy saving, the heat exchanger improves energy efficiency, reduces the amount of refrigerant charged, and reduces the negative impact on the environment. At the same time, it can also adapt to a variety of working conditions, including different types of condensing agents, flow rates and pressures, so that it can perform excellent performance in a variety of application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the overall structure in the embodiment;

[0026] Figure 2 is a schematic diagram of the structure of the fin assembly in the embodiment;

[0027] Figure 3 yes Figure 2 Section view along forced section line AA;

[0028] Figure 4 yes Figure 2 Sectional view along forced section line BB;

[0029] Figure 5 Schematic diagram of the structure of the first fin strip and the second fin strip in the embodiment.

[0030] Figure numerals: 1. shell; 10. sealing strip; 11. curved pipe; 12. vertical pipe; 13. mixing tube; 14. streamline groove; 15. stirring block; 16. rotating ring; 2. first fin strip; 21. second fin strip; 22. rotating shaft; 23. locking rod; 24. abutment plate; 25. horizontal plate; 26. inclined plate; 27. protrusion; 28. spring; 3. first semicircular groove; 301. first semi-arc groove; 31. second semicircular groove; 311. second semi-arc groove; 32. sliding column; 4. connecting strip; 41. plug-in block. DETAILED DESCRIPTION

[0031] The present application is further described in detail below with reference to the accompanying drawings.

[0032] Example, see Figure 1 as well as Figure 2A microchannel air conditioning phase change heat transfer evaporation heat exchanger, which comprises a flow pipeline, which is composed of a plurality of vertical pipelines 12 and a plurality of curved pipelines 11 connecting these vertical pipelines, forming a continuous flow path of a condensing agent. Inside the vertical pipeline 12, a mixing assembly specially used for stirring the condensing agent is provided. On the outside of the vertical pipeline 12, a fin assembly is installed, and a shell 1 is provided on the outside of the fin assembly, and the fin grouting is wrapped by the shell 1.

[0033] When the condensing agent (in both gas and liquid forms) flows in the flow pipe, it will pass through the mixing assembly inside each vertical pipe 12 to be fully mixed and evenly distributed. This design ensures that the condensing agent inside each vertical pipe 12 can reach an ideal uniform state. Then, the condensing agent will flow through the fin assembly and use the increased heat exchange area of ​​the fins to effectively exchange heat. This layout not only optimizes the flow of the condensing agent, but also significantly improves the efficiency of heat exchange.

[0034] In this embodiment, refer to Figure 3 as well as Figure 4 The mixing assembly includes a mixing tube 13 disposed inside a vertical pipe 12, and a flow groove 14 is provided inside the mixing tube 13. The flow groove 14 has multiple and evenly disposed lines in the mixing tube 13. The flow groove 14 is provided with a stirring block 15 fixedly connected to the inside of the mixing tube 13. When the gaseous and liquid condensing agents enter the mixing tube 13 together, they will be affected by gravity and flow faster along the path of the flow groove 14. In this process, the gaseous and liquid condensing agents will be preliminarily mixed due to the design of the flow groove 14. Subsequently, when they hit the stirring block 15, they will be further disturbed and mixed, so that the liquid condensing agent and the gaseous condensing agent can be more fully integrated.

[0035] In this embodiment, a plurality of through grooves are provided on the stirring block 15, rotating rings 16 are fixedly provided on both upper and lower sides of the mixing tube 13, and a rotating groove for the rotating ring 16 to rotate is provided on the vertical pipe 12. The through grooves not only provide more flow paths for the condensing agent, but also help to increase the turbulence of the condensing agent around the stirring block, thereby further improving the mixing effect of the gas-liquid two-phase condensing agent.

[0036] When the gas-liquid two-phase condensing agent flows in the mixing tube 13 and hits the stirring block 15 , the outer pressure generated by the flow will push the mixing tube 13 to rotate slightly in the vertical pipe 12 .

[0037] This rotation not only enables the stirring block 15 and the flow groove 14 to act on the condensing agent at more diverse angles and in more diverse ways, but also further increases the degree of disturbance and mixing of the condensing agent during the mixing process.

[0038] In this embodiment, refer to Figure 5 The fin assembly includes a plurality of groups installed on the first fin strip 2 and the second fin strip 21. The first fin strip 2 and the second fin strip 21 are both provided with circular grooves matching the vertical pipe 12. A fixing assembly for fixing the first fin strip 2 and the second fin strip 21 is provided between the first fin strip 2 and the second fin strip 21. A connecting strip 4 is fixedly provided on the side of the first fin strip 2 close to the second fin strip 21. A matching connecting groove is provided on the second fin strip 21. When installing the fin assembly on the flow pipe, the circular grooves of the first fin strip 2 and the second fin strip 21 are installed on the outside of the vertical pipe 12, and the connecting strip 4 is connected to the connecting groove for preliminary fixing, and then the first fin strip 2 and the second fin strip 21 are fixed together by the fixing assembly.

[0039] In this embodiment, a plurality of groups of plug-in blocks 41 are provided on the circular groove, and a plurality of groups of plug-in slots matching the plug-in blocks 41 are fixedly provided on the vertical pipe 12. In order to ensure that the first fin strip 2 and the second fin strip 21 are fixed in the vertical pipe 12, the first fin strip 2 and the second fin strip 21 are fixedly installed in the vertical pipe 12 by aligning the plug-in slots on the circular groove with the plug-in blocks 41, and then by a fixing assembly.

[0040] In this embodiment, the fixing component includes a first semicircular groove 3 opened on the first fin strip 2 and a second semicircular groove 31 opened on the second fin strip 21. The first semicircular groove 3 and the second semicircular groove 31 are spliced ​​into a circle. A rotating shaft 22 is rotatably arranged in the first semicircular groove 3, a locking rod 23 is fixedly arranged on the rotating shaft 22, and sliding columns 32 are fixedly arranged at both ends of the locking rod 23. The first semicircular groove 3 is provided with a first semi-arc groove 301 on the first fin strip 2, and the second semi-arc groove 31 is provided with a second semi-arc groove 311 on the second fin strip 21. The sliding column 32 is slidably connected in the first semi-arc groove and the second semi-arc groove 311. When the first fin strip 2 and the second fin strip 21 need to be fixed, align the first semicircular groove 3 on the first fin strip 2 with the second semicircular groove 31 on the second fin, then rotate the locking rod 23 in the first semicircular groove 3, and then slide the sliding columns 32 on the first semi-arc groove 301 and the second semi-arc groove 311 respectively, thereby pulling the first semi-arc groove 301 and the second semi-arc groove 311.

[0041] In this embodiment, a placement groove is provided on each sliding column 32, an inclined surface is provided in the placement groove, an inclined plate 26 is inserted in the placement groove, an inclined surface is provided on the inclined plate 26, the inclined surface of the inclined plate 26 is close to the inclined surface in the placement groove, a spring 28 is fixedly provided on the inclined plate 26, a contact plate 24 is fixedly connected to the middle part of the locking rod 23, the other end of the spring 28 is fixedly connected to the contact plate 24, the middle parts of the first semi-arc groove 301 and the second semi-arc groove 311 are both provided with locking grooves inserted with the inclined plate 26, and the upper end of the spring 28 is provided with a horizontal plate 25 fixedly connected to the contact plate 24. When the sliding column 32 slides into the first semi-arc groove 301 and the second semi-arc groove 311 respectively, the inclined plate 26 will be inserted into the locking groove under the compression of the spring 28, thereby fixing the first fin strip 2 and the second fin strip 21. When unlocking, pull the inclined plate 26 to allow it to detach from the locking groove along the inclined surface, separating the first fin strip 2 and the second fin strip 21. The setting of the horizontal plate 25 allows the spring 28 to maintain a horizontal elastic force, thereby continuously pushing the inclined plate 26.

[0042] In this embodiment, a protrusion 27 is fixedly provided on the inclined plate 26. The protrusion 27 can provide a power point when the inclined plate 26 is pulled.

[0043] In this embodiment, the housing 1 includes an upper housing and a lower housing, and sealing strips 10 are fixedly arranged at the positions where the upper housing and the lower housing wrap the flow pipe, and the upper housing and the lower housing are fixedly connected on both sides. The setting of the sealing strip 10 ensures the sealing of the upper housing and the lower housing, and also reduces the squeezing of the flow pipe. The housing 1 includes a through groove for cold air circulation on one side close to the first fin strip 2 and the second fin strip 21.

[0044] Specific implementation process: insert the mixing tube 13 into each vertical pipe 12, and ensure that the flow groove 14 and the stirring block 15 can work properly. The design of the flow groove 14 can guide the flow of the condensing agent in the pipe, and the stirring block 15 can further promote the mixing of the condensing agent. During the installation process, pay attention to maintaining the verticality of the mixing tube 13 to ensure that it can rotate normally.

[0045] The installation of the fin assembly is the key to the performance of the heat exchanger. First, align the circular grooves of the first fin strip 2 and the second fin strip 21 with the outside of the vertical pipe 12, and ensure that the plug-in block 41 and the plug-in groove are correctly aligned. Then, preliminary fixation is performed through the connecting strip 4 and the connecting groove. Next, the first fin strip 2 and the second fin strip 21 are tightly fixed together using the fixing assembly. By rotating the locking rod 23 and the sliding column 32, the first semicircular groove 3 and the second semicircular groove 31 can be spliced ​​into a circle, and the fin assembly can be firmly fixed on the vertical pipe 12 through the action of the inclined plate 26 and the spring 28.

[0046] The installation of the housing 1 is an important step to protect the heat exchanger from external interference. First, the upper and lower shells are wrapped around the upper and lower parts of the flow pipe, respectively, to ensure that the sealing strip 10 fits properly and provides good sealing. Then, the upper and lower shells are fixed together using appropriate connectors (such as screws or buckles). The design of the housing 1 also takes into account the need for cold air circulation, so a through groove for cold air circulation is opened on one side close to the first fin strip 2 and the second fin strip 21.

[0047] The condensing agent circulates through a flow pipe formed by connecting multiple vertical pipes 12 and multiple curved pipes 11 end to end. Inside the vertical pipe 12, a mixing assembly is provided, including a mixing pipe 13 and a streamline groove 14 and a stirring block 15 inside it. When the gaseous and liquid condensing agents enter the mixing pipe 13, the mixing is accelerated along the streamline groove 14, and the mixing is further promoted when they hit the stirring block 15. The multiple groups of through grooves on the stirring block 15 also increase the fluidity of the condensing agent. At the same time, the fin assembly on the outside of the vertical pipe 12 increases the heat exchange area, optimizes the flow path of the condensing agent, and wraps the entire flow pipe through the outer shell 1 to form a closed heat exchange space. During use, the flow, temperature, pressure and other parameters of the condensing agent are adjusted by the control system, and the operating status of the heat exchanger can be monitored in real time and adjusted as needed.

[0048] The use effect of the heat exchanger is remarkable. First, the synergistic effect of the mixing component and the fin component enables the condensing agent to be fully mixed and evenly distributed in the flow pipe, thereby improving the heat transfer efficiency of the heat exchanger and allowing the heat to be transferred out faster. Secondly, by accurately controlling the system parameters, the overall performance of the air-conditioning system can be optimized, such as adjusting the flow rate and temperature of the condensing agent to achieve precise control of the refrigeration capacity of the air-conditioning system. In addition, the microchannel air-conditioning phase change heat transfer evaporative heat exchanger adopts advanced structural design and material selection, which makes the system more stable and durable, and can operate stably for a long time without failure. At the same time, the heat exchanger improves the energy efficiency ratio, reduces energy consumption, and due to the use of advanced heat exchange technology, the system emits fewer pollutants and is more environmentally friendly. Finally, its compact and simple structure makes installation and maintenance more convenient and quick, and reduces the operating cost of the system.

[0049] The embodiments of this specific implementation method are all preferred embodiments of the present application, and are not intended to limit the protection scope of the application. Therefore, all equivalent changes made based on the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A microchannel air conditioning phase change heat transfer evaporative heat exchanger, characterized in that: The invention comprises a flow pipe for circulating a condensing agent, wherein the flow pipe is formed by connecting a plurality of vertical pipes (12) and a plurality of curved pipes (11) end to end, a mixing assembly for stirring the condensing agent is arranged inside the vertical pipe (12), a fin assembly is arranged outside the vertical pipe (12), and a shell (1) is arranged outside the fin assembly to wrap the entire flow pipe; The mixing assembly comprises a mixing tube (13) arranged inside a vertical pipe (12), a flow line groove (14) is provided inside the mixing tube (13), a plurality of flow line grooves (14) are provided and are evenly provided in the mixing tube (13), and a stirring block (15) is provided on the flow line groove (14) and is fixedly connected to the inside of the mixing tube (13); The stirring block (15) is provided with a plurality of through grooves, the mixing tube (13) is fixedly provided with a rotating ring (16) on both upper and lower sides, and the vertical pipe (12) is provided with a rotating groove for the rotating ring (16) to rotate.

2. According to claim 1, a microchannel air conditioning phase change heat transfer evaporative heat exchanger is characterized in that: The fin assembly comprises a plurality of groups mounted on a first fin strip (2) and a second fin strip (21); a circular groove matching the vertical pipe (12) is provided on the first fin strip (2) and the second fin strip (21); a fixing assembly for fixing the first fin strip (2) and the second fin strip (21) is provided between the first fin strip (2) and the second fin strip (21); a connecting strip (4) is fixedly provided on one side of the first fin strip (2) close to the second fin strip (21); and a matching connecting groove is provided on the second fin strip (21).

3. According to claim 2, a microchannel air conditioning phase change heat transfer evaporative heat exchanger is characterized in that: The circular groove is provided with a plurality of groups of plug-in blocks (41), and the vertical pipe (12) is fixedly provided with a plurality of groups of plug-in grooves matching the plug-in blocks (41).

4. According to claim 3, a microchannel air conditioning phase change heat transfer evaporative heat exchanger is characterized in that: The fixing assembly comprises a first semicircular groove (3) provided on the first fin strip (2) and a second semicircular groove (31) provided on the second fin strip (21); the first semicircular groove (3) and the second semicircular groove (31) are spliced ​​into a circle; a rotating shaft (22) is rotatably arranged in the first semicircular groove (3); a locking rod (23) is fixedly arranged on the rotating shaft (22); sliding columns (32) are fixedly arranged at both ends of the locking rod (23); a first semi-arc groove (301) on the first fin strip (2) is provided on the first semicircular groove (3); a second semi-arc groove (311) on the second fin strip (21) is provided on the second semicircular groove (31); and the sliding column (32) is slidably connected in the first semi-arc groove (301) and the second semi-arc groove (311).

5. A microchannel air conditioning phase change heat transfer evaporative heat exchanger according to claim 4, characterized in that: Each of the sliding columns (32) is provided with a placement groove, an inclined surface is provided in the placement groove, an inclined plate (26) is inserted in the placement groove, an inclined surface is provided on the inclined plate (26), the inclined surface of the inclined plate (26) is close to the inclined surface in the placement groove, a spring (28) is fixedly provided on the inclined plate (26), a contact plate (24) is fixedly connected to the middle of the locking rod (23), the other end of the spring (28) is fixedly connected to the contact plate (24), the first semi-arc groove (301) and the second semi-arc groove (311) are both provided with locking grooves inserted into the inclined plate (26), and a horizontal plate (25) fixedly connected to the contact plate (24) is provided at the upper end of the spring (28).

6. A microchannel air conditioning phase change heat transfer evaporative heat exchanger according to claim 5, characterized in that: A protrusion (27) is also fixedly provided on the inclined plate (26).

7. The microchannel air conditioning phase change heat transfer evaporative heat exchanger according to claim 1, characterized in that: The outer shell (1) comprises an upper shell and a lower shell, and sealing strips (10) are fixedly arranged at positions where the upper shell and the lower shell wrap the flow pipe, and the upper shell and the lower shell are fixedly connected at both sides.

8. The microchannel air conditioning phase change heat transfer evaporative heat exchanger according to claim 7, characterized in that: The shell (1) comprises a through groove for cold air circulation on one side close to the first fin strip (2) and the second fin strip (21).

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