Phase change heat exchanger with trapezoidal ring fins for thermodynamic buoys

Abstract

This invention discloses a phase change heat exchanger with trapezoidal ring ribs for thermoelectric buoys, solving the problems of poor heat transfer performance and excessively long phase change time of phase change materials in phase change heat exchangers used in thermoelectric buoys. The phase change heat exchanger includes a heat exchanger shell and a rubber oil bladder disposed within the heat exchanger shell and coaxial with the central axis of the heat exchanger. Phase change material is filled between the rubber oil bladder and the heat exchanger shell. The inner wall of the heat exchanger shell has N annular ribs coaxial with the central axis of the heat exchanger. The N annular ribs are equally spaced, and the arc diameter of the inner edges of the N annular ribs decreases sequentially from top to bottom. The inner edges of the N annular ribs belong to the same frustum side. Each of the N annular ribs has a through-hole. This invention has a simple structure and low manufacturing difficulty. The addition of ring ribs can significantly improve the heat transfer performance of the heat exchanger, accelerate the phase change rate, and enhance the pressure resistance of the phase change heat exchanger. It can be applied to thermoelectric propulsion systems for underwater unmanned vehicles such as buoys.

Description

Technical Field

[0001] This invention mainly relates to the field of ocean thermal energy recovery and utilization, and is particularly applicable to phase change heat transfer applications such as the thermal energy power system of underwater unmanned vehicles, which use phase change materials to convert ocean thermal energy into pressure energy or kinetic energy. Background Technology

[0002] Buoys are devices permanently deployed in the ocean to acquire real-time underwater profile data and collect various underwater signals. They can freely float and dive between the surface and deep sea using their onboard buoyancy adjustment systems, making them an important type of marine information monitoring equipment. Buoys are non-recoverable; their energy source is an internal battery. However, the limited battery capacity directly restricts the number of times the buoy can float and dive in the seabed and deep sea, thus limiting its operational time and lifespan.

[0003] Ocean thermal energy (OTE) powered buoys can overcome this drawback, thus attracting widespread attention from researchers worldwide. Their basic principle is as follows: When the buoy is on the surface, the phase change heat exchanger absorbs heat, causing the internal phase change material to change from a solid to a liquid phase, expanding in volume and compressing the hydraulic oil in the rubber bladder, converting thermal energy into pressure energy. When the buoy is in the deep sea, the phase change heat exchanger releases heat, causing the internal phase change material to change from a liquid to a solid phase, contracting in volume and absorbing heat from the hydraulic oil in the rubber bladder. The buoy's reciprocating motion between the ocean surface and deep sea, with the phase change material undergoing cyclic solid-liquid phase changes, ultimately achieves the conversion of temperature difference energy into pressure energy. In an OTE buoy, the phase change heat exchanger is the direct medium for heat exchange between seawater and the phase change material, and is the main device for realizing the conversion of temperature difference energy into pressure energy. The heat exchange performance of the phase change heat exchanger directly determines the temperature difference energy-pressure energy conversion efficiency and the buoy's single-section period. However, current phase change heat exchangers have poor heat transfer performance, with phase change times exceeding 8 hours or even 24 hours, and there is a lack of research on heat exchange enhancement for thermoelectric buoy phase change heat exchangers. Summary of the Invention

[0004] To address the aforementioned issues in existing technologies, and to resolve the problems of poor heat transfer performance and excessively long phase change time in phase change materials for thermoelectric buoys, this invention provides a finned phase change heat exchanger for thermoelectric buoys, and proposes corresponding design procedures and guidelines. The annular finned phase change heat exchanger designed in this invention has a simple structure and is easy to manufacture. The addition of annular fins can significantly improve the heat transfer performance of the heat exchanger, accelerate the phase change rate, and enhance the pressure resistance of the phase change heat exchanger.

[0005] To address the aforementioned technical problems, this invention proposes a phase change heat exchanger with trapezoidal annular ribs for a thermoelectric buoy. The heat exchanger includes a heat exchanger shell and a rubber oil bladder disposed within the shell and coaxial with the central axis of the heat exchanger. The top of the rubber oil bladder has an oil inlet, and the bladder contains hydraulic oil. A phase change material is filled between the rubber oil bladder and the heat exchanger shell. The inner wall of the heat exchanger shell has N annular ribs coaxial with the central axis of the heat exchanger. These N annular ribs are equally spaced, and the arc diameter of their inner edges decreases sequentially from top to bottom. The inner edges of the N annular ribs belong to the same frustum side. Each of the N annular ribs has a through-hole.

[0006] The phase change heat exchanger with trapezoidal ring ribs described in this invention, wherein:

[0007] The number N of fins is calculated from the maximum allowable additional mass of the buoy fins; the fin height refers to the radial dimension of the annular fin from the inner wall of the heat exchanger to the inner edge of the fin; the height of the annular fin at the bottom of the phase change heat exchanger is H. max =0.9*L, the height of the annular fin at the very top of the phase change heat exchanger is H. min = (0.4~0.8)*H max Where L is the radius difference between the inner wall of the heat exchanger shell and the rubber oil bladder; the fin slope is (H max -H min) :((N-1)*A); The thickness of the rib is 1-5mm; The number of the guide holes n is 6-10, and the diameter of the guide holes d is 10-20mm.

[0008] The outer diameter D2 of the heat exchanger shell is 220mm, the inner diameter D1 of the heat exchanger shell is 200mm, the diameter of the rubber oil bladder is 70mm, the number of fins N is 6, L = 65mm, the thickness of the fins is 2mm, the fin spacing A is 110mm; the number of guide holes n on each fin is 8, and the diameter d of the guide holes is 10mm.

[0009] The phase change material is selected from materials with a phase change temperature in the range of 4 to 30°C and a phase change volume change rate greater than 8%. Preferably, it is n-hexadecane or n-pentadecanane.

[0010] Compared with the prior art, the beneficial effects of the present invention are:

[0011] (1) The trapezoidal fin phase change heat exchanger proposed in this invention is suitable for unmanned vehicles such as buoys. It has a simple structure and low cost, and provides a complete design process and design guidelines for trapezoidal fin design.

[0012] (2) The heat exchanger with the addition of annular fins has a significantly enhanced heat exchange capacity and a significantly shortened solidification and melting time of the phase change material. At the same time, the design of the annular fins acts as a reinforcing rib, which can enhance the mechanical strength of the outer wall of the phase change heat exchanger and increase the overall pressure resistance of the heat exchanger to a certain extent. Furthermore, by adding circular flow guide holes on the annular fins, the pressure difference between the two sides of the fins can be reduced, preventing fin deformation and reducing the inhibition of the fins on the internal convection of the PCM, thereby further enhancing heat exchange.

[0013] (3) Experimental studies have shown that the melting process of phase change materials in phase change heat exchangers with no fins or fins of equal height is a top-to-bottom phase change process. Premature melting of the upper phase change material will block the hydraulic oil flow channel, causing hydraulic oil pressure loss and affecting the energy conversion efficiency of the phase change heat exchanger. Therefore, this invention designs the annular fins to be shorter at the top and taller at the bottom (e.g., Figure 1 and Figure 3 As shown, the trapezoidal structure (with the radial dimensions of the annular ribs gradually increasing from high to low) helps optimize the phase change interface during the melting process, reduces the flow pressure loss of hydraulic oil, and improves the pressure energy conversion efficiency.

[0014] (4) Due to the low added mass (which can be controlled within 1 kg) of the fins added to the phase change heat exchanger of the present invention, it has the potential for large-scale application in thermoelectric underwater power devices. Attached Figure Description

[0015] Figure 1 This is a simplified cross-sectional view of the three-dimensional structure of the trapezoidal ring-ribbed phase change heat exchanger of the present invention;

[0016] Figure 2 This is a design flowchart of the trapezoidal ring-ribbed phase change heat exchanger fins of the present invention;

[0017] Figure 3 This is a schematic diagram of the 220mm, 6-fin phase change heat exchanger designed in this invention;

[0018] Figure 4 This is a comparison chart showing the change of solid fraction over time during the solidification process of n-hexadecane in a finless phase changer and a six-fin phase changer.

[0019] In the picture:

[0020] 1-First ring rib 2-Second ring rib 3-Third ring rib

[0021] 4-Fourth ring rib 5-Fifth ring rib 6-Sixth ring rib

[0022] 7-Heat exchanger shell; 8-Phase change material; 9-Rubber oil bladder

[0023] 10-Flow guide hole Detailed Implementation

[0024] To address the issues of poor heat transfer performance and excessively long phase change time in phase change materials used in thermoelectric buoys, this invention proposes a phase change heat exchanger with trapezoidal ring fins for thermoelectric buoys. The design concept is as follows: by adding ring fins within the phase change heat exchanger, the heat transfer performance can be significantly improved, the phase change rate accelerated, and the pressure resistance of the phase change heat exchanger enhanced. A simplified three-dimensional structural diagram of the trapezoidal ring fin phase change heat exchanger is shown below. Figure 1 As shown, the design principles and design strategies are as follows: Figure 2 As shown.

[0025] The structure of the phase change heat exchanger of this invention is as follows: Figure 1 As shown, the innermost part is a rubber oil bladder 9, with an oil port at its top. The rubber oil bladder 9 contains hydraulic oil. The outermost part of the heat exchanger is the inner wall of the metal heat exchanger shell 7. The phase change material 8 is located between the rubber oil bladder 9 and the inner wall. N annular ribs, coaxial with the central axis of the heat exchanger and equidistantly arranged, are installed in the hollow ring between the metal inner wall and the rubber oil bladder. These N annular ribs enhance heat transfer. The inner edges of the N annular ribs are all circular, with the diameter increasing with height. The inner edges of the N annular ribs belong to the same frustum side, resulting in a trapezoidal distribution. Furthermore, each annular rib has a through-hole 10 to enhance convection during the melting of the phase change material.

[0026] The phase change heat exchanger designed in this invention can be applied to the thermoelectric power system of underwater unmanned vehicles such as buoys. The addition of annular fins significantly improves the heat exchange performance of the phase change heat exchanger and shortens the phase change time of the phase change material. Furthermore, the trapezoidal structure of the annular fins, shorter at the top and longer at the bottom, slows down the melting rate at the top of the phase change material, and the change in the phase change interface is more conducive to hydraulic oil flow, reducing hydraulic oil flow losses and improving the energy conversion efficiency of seawater thermoelectric energy into hydraulic oil pressure energy. Designing guide holes on the fins reduces the pressure difference between the two fins, resulting in stronger convection than without guide holes, further shortening the phase change time. The phase change heat exchanger disclosed in this invention has a simple structure and low cost. The added mass due to the fins is low, limited to less than 1 kg, and has the potential for large-scale application in thermoelectric underwater power devices. The design principles and flowchart of the trapezoidal fins of the phase change heat exchanger are shown below. Figure 2 As shown, the specific design process for the phase change heat exchanger fin structure and dimensions will now be described:

[0027] (1) Calculation of fin height and slope: Fin height refers to the length from the base of the fin (inner wall of the heat exchanger) to the highest point of the fin. The trapezoidal fins suitable for thermoelectric phase change heat exchangers are generally distributed with shorter upper fins and longer lower fins. The bottom annular fins have the largest fin height, which mainly depends on the radius difference L between the inner wall of the heat exchanger shell 7 and the rubber oil bladder 9. Based on experience, the maximum fin height H... maxWe can take 0.9 times the radius difference, that is:

[0028] H max =0.9*L

[0029] The rib slope characterizes the height difference between two adjacent ribs, and the rib slope is (H max -H min) :((N-1)*A), where A is the fin spacing. The greater the height difference, the lower the slope, and vice versa. Within the same phase change heat exchanger, the height difference between adjacent fins is equal, meaning the fin slope is constant. Therefore, determining the height of the uppermost fin determines the fin slope. The design of the fin slope is mainly to optimize the phase change interface. Its magnitude depends primarily on the intensity of natural convection during the melting of the phase change material. The magnitude and intensity of natural convection heat transfer within a confined space can be characterized by the dimensionless Gr number:

[0030]

[0031] For phase change heat exchangers, when Gr < 2860°C, the heat transfer during the melting process of the phase change material mainly relies on thermal conduction. In this case, trapezoidal ring ribs with a large slope can be used, with the top rib height H. min :

[0032] H min =0.8*H max

[0033] When the Gr number exceeds 2860, natural convection begins to form within the phase change heat exchanger. At this point, the ratio of the top fin height to the bottom fin height can be between 0.4 and 0.8. Furthermore, the higher the Gr number, the stronger the natural convection, requiring the use of trapezoidal ring fins with a greater slope. When Gr > 290000, the minimum ratio of the top fin height to the bottom fin height of 0.4 can be used.

[0034] H min =0.4*H max

[0035] (2) Fin thickness calculation: Studies have shown that fin thickness has little impact on the heat exchange performance of phase change heat exchangers, and thicker fins will increase the mass of the float. Therefore, the fin thickness only needs to ensure that the fins do not undergo significant deformation under the working pressure and temperature of the heat exchanger. This value can be calculated using the fin material strength. Furthermore, since the two sides of the fin are connected and the pressure is equal, the fin thickness is usually small.

[0036] (3) Calculation of the number of ribs: Under the premise of ensuring the height and thickness of the ribs, the number of ribs can be calculated from the maximum allowable additional mass of the buoy ribs.

[0037] (4) Calculation of fin spacing: The fins are evenly distributed in the axial direction of the phase change heat exchanger, and their spacing can be calculated from the phase change heat exchanger structure and the number of fins.

[0038] (5) Fin-guided hole design: Phase change heat exchangers with annular fin structures may experience fin deformation due to excessive pressure difference between the top and bottom of the fins, and the annular fins may weaken the convective heat transfer effect inside the PCM. Therefore, this invention adds circular guide holes to the annular fins to reduce the pressure difference between the two sides of the fins, prevent fin deformation, reduce the inhibition of the fins on the internal convection of the PCM, and enhance heat transfer. The main function of the guide holes is to enhance the internal convection. Therefore, the number and size of the guide holes should be designed according to the strength of the convection of the PCM. The stronger the convection, the more guide holes there should be, and the larger the diameter of the guide holes should be. The diameter of the guide holes should preferably be set to 10-20 mm, and the number should preferably be 6-10.

[0039] (6) Strength test: The strength of the phase change heat exchanger and phase change heat exchanger fins after the design is completed must meet the design requirements. If it meets the design requirements, the design is completed; if it does not meet the requirements, the fin thickness is increased (or decreased) and the calculation is repeated according to the calculation results.

[0040] The following example uses a float-type thermoelectric phase change heat exchanger with n-hexadecane as the phase change material and an outer diameter of 220mm as an example to illustrate the trapezoidal ring rib design. However, the following embodiments are not intended to limit the invention in any way. In this embodiment, the designed phase change heat exchanger has a radius difference L = 65mm between the inner wall of the heat exchanger and the rubber oil bladder, as determined by the heat exchanger structure. Based on the strength of the convection heat exchanger, the maximum rib height is taken as 60mm, the minimum rib height as 40mm, and the remaining rib heights are distributed at equal arithmetic intervals. The rib thickness is selected as 2mm, and with a maximum rib added mass of 1kg, the number of ribs N is taken as 6. The rib arrangement and specific dimensions of the heat exchanger after the design are as follows: Figure 3 As shown, the outer diameter D2 of the heat exchanger shell 7 is 220mm, the inner diameter D1 is 200mm, the diameter D3 of the rubber oil bladder 9 is 70mm, and the six annular ribs are designated as first ring rib 1, second ring rib 2, third ring rib 3, fourth ring rib 4, fifth ring rib 5, and sixth ring rib 6 from bottom to top. The arc diameter of the inner edge of the six annular ribs decreases sequentially from top to bottom. The height of the sixth ring rib 6 at the top is H6 = 40mm, and the height of the first ring rib 1 at the bottom is H1 = 60mm. The thickness of the ribs is 2mm, and the spacing A between the ribs is 110mm. Each rib has eight guide holes 10 with a diameter d of 10mm. The heat exchange performance of the phase change heat exchanger after the completion of this embodiment is compared as follows. Figure 4As shown in the illustration, taking the 6-finned phase change heat exchanger of this invention as an example, compared to a finless heat exchanger of the same size, it can shorten the melting time by 54.8% and the solidification time by 39.8%, which is beneficial to reducing the single-section cycle of the buoy. Compared to annular fins of equal height, annular fins with trapezoidal height can reduce hydraulic oil flow loss by 0.5 MPa and increase pressure energy conversion by 2.5%.

[0041] The trapezoidal multi-ring fin structure and corresponding design process proposed in this invention are applicable to conventional cylindrical buoy phase change heat exchangers. Fin thickness has minimal impact on heat transfer performance; when designing the ring fin thickness, only the mechanical strength of the fins needs to be considered. The higher the thermal conductivity of the selected ring fin material, the more significant the heat transfer enhancement effect. Welding is the preferred method for connecting the ring fins to the inner wall of the heat exchanger. While ensuring sufficient thermal conductivity, it is recommended that the material of the ring fins be the same as that of the heat exchanger outer wall to reduce processing difficulty and increase fin strength. The trapezoidal arrangement of the ring fins aims primarily to improve the phase change interface of the phase change material during melting. The slope of all the trapezoidal ring fins mainly depends on the natural convection intensity during the melting of the phase change material; the higher the intensity, the smaller the slope.

[0042] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many modifications under the guidance of the present invention without departing from the spirit of the present invention, and these modifications are all within the protection scope of the present invention.

Claims

1. A phase change heat exchanger with trapezoidal ring ribs for a thermosiphon buoy, comprising a heat exchanger shell (7) and a rubber oil bag (9) arranged in the heat exchanger shell (7) coaxially with a heat exchanger center axis, a top of the rubber oil bag (9) being provided with an oil port, the rubber oil bag (9) containing hydraulic oil, the rubber oil bag (9) and the heat exchanger shell (7) being filled with a phase change material (8), an inner wall of the heat exchanger shell (7) being provided with N ring ribs coaxial with the heat exchanger center axis, the N ring ribs being arranged at equal intervals, circular arc diameters of inner edges of the N ring ribs decreasing in turn from top to bottom, the inner edges of the N ring ribs belonging to the same circular truncated cone side surface, the N ring ribs each being provided with a through flow guide hole (10), characterized in that, The determination of the number N of the annular fins, the fin height, the fin slope, the fin thickness, the fin spacing A, the diameter d and the number n of the flow guide holes: The number N of the fins is calculated by the maximum allowable added mass of the buoy fins; The fin height refers to the radial dimension of the annular fins from the inner wall of the heat exchanger to the inner edge of the fins; The height of the annular fin at the bottom of the phase change heat exchanger is H max =0.9*L, the height of the annular fin at the top of the phase change heat exchanger is H min =(0.4~0.8)*H max Wherein, L is the difference between the inner wall of the heat exchanger shell (7) and the radius of the rubber oil bag (9). The rib inclination is (H max -H min) :((N-1)*A) The fin thickness is 1-5 mm; The number n of the flow guide holes is 6-10, and the diameter d of the flow guide holes is 10-20 mm.

2. The phase change heat exchanger with trapezoidal ring ribs according to claim 1, characterized in that, The outer diameter D2 of the heat exchanger shell (7) is 220 mm, the inner diameter D1 of the heat exchanger shell (7) is 200 mm, the diameter of the rubber oil bag (9) is 70 mm, the number N of the fins is 6, L=65 mm, the fin thickness is 2 mm, and the fin spacing A is 110 mm; the number n of the flow guide holes (10) on each fin is 8, and the diameter d of the flow guide holes is 10 mm.

3. The phase change heat exchanger with trapezoidal ring ribs according to claim 1, characterized in that, The phase change material is selected from a material with a phase change temperature in the range of 4-30℃ and a phase change volume change rate greater than 8%.

4. The phase change heat exchanger with trapezoidal ring ribs according to claim 3, characterized in that, The phase change material is n-hexadecane or n-pentadecane.

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