Phase change heat storage device based on bidirectional staggered fins and application

Abstract

The invention belongs to the technical field of phase change energy storage, and particularly relates to a phase change heat storage device based on bidirectional staggered fins and application, the phase change heat storage device comprises a cylindrical shell, a finned tube and a phase change material, and the finned tube comprises a circular base tube and a plurality of fins welded to the surface of the base tube; each fin comprises a main fin connected to the outer surface of the base tube and extending in the length direction and a branch fin set connected to the top of the main fin, the first branch fin and the second branch fin are the same in height and width, and each first branch fin and the corresponding second branch fin are adjacent and opposite in bending direction. The branch fins I and the branch fins II at the top of each main fin are alternately arranged in sequence; the multiple fins are evenly distributed in the heat storage cavity on the periphery of the base tube in a radial shape, the heat storage cavity is filled with the phase change materials, the finned tube is wrapped with the phase change materials, and on the basis of conventional rectangular fins, the heat storage rate and the heat storage efficiency are remarkably improved through the simple machining procedures of segmentation and bending.

Description

Technical Field

[0001] The present invention belongs to the technical field of phase change energy storage, and in particular relates to a phase change heat storage device based on bidirectional staggered fins and its application. Background Art

[0002] As the global energy crisis intensifies, energy conservation has become an urgent need. As a major energy consumer, the construction industry accounts for about 40% of the world's total energy consumption. Therefore, building energy conservation has become a key area of ​​research. Phase change heat storage technology, as an efficient and environmentally friendly way of energy storage and utilization, has important application value in building energy conservation. Phase change heat storage technology can store thermal energy in phase change materials, absorb or release heat through the phase change of materials, and achieve long-term storage and regulation of energy. This technology can effectively solve the problem of mismatch between energy supply and demand, improve energy utilization efficiency and system stability, and achieve efficient utilization of building energy and energy conservation and emission reduction. Therefore, conducting research on phase change heat storage technology is of great significance to promoting the development of energy-saving technology and achieving sustainable development of energy.

[0003] The existing heat storage performance enhancement technologies are mainly divided into three types: active technology, passive technology and composite enhancement technology. Among them, passive technology is widely used in phase change thermal storage devices due to its advantages such as low cost, simple structure and convenient maintenance. Adding fins is a typical passive enhancement technology. Adding fins can effectively expand the heat transfer area of ​​the phase change thermal storage device, enhance heat conduction, and thus accelerate the heat storage rate.

[0004] During the phase change process, the heat transfer between the phase change energy storage material and the thermal fluid in the phase change energy storage device mainly relies on heat conduction and natural convection. Since the phase change energy storage material itself is not easy to conduct heat, the fin-type energy storage device in the prior art is mainly rectangular fins, and the heat conduction of the phase change energy storage material in the middle between the two fins is uneven, and the heat storage rate and efficiency are very low. The invention patent with application number CN202311041101.9 discloses a horizontal phase change heat storage device with open-hole tree-shaped bionic fins, and a plurality of open-hole tree-shaped bionic fins for heat exchange of phase change materials are evenly arranged on the circumference of the hot water pipe. Compared with the traditional rectangular fin tube phase change heat storage device, it has a greater advantage in heat exchange area. However, special-shaped fins such as tree-shaped bionic fins and fractal fins are difficult to process and have high costs, which are not convenient for engineering applications. Therefore, it is necessary to propose a new fin strengthening technology and a corresponding phase change heat storage device to solve the contradiction between enhanced heat storage performance and low cost. Summary of the invention

[0005] The purpose of the present invention is to provide a phase change heat storage device based on bidirectional staggered fins and its application. Considering the difficulty of processing, a simple processing procedure of splitting and bending is used on the basis of conventional rectangular fins to achieve a significant improvement in heat storage rate and efficiency.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A phase change heat storage device based on bidirectional staggered fins, comprising a sealed cylindrical shell, a fin tube coaxial with the shell arranged in the shell, and a phase change material arranged in a heat storage chamber between the shell and the fin tube, wherein the fin tube comprises a circular base tube located in the center and a plurality of fins welded to the surface of the base tube, each fin comprises a main fin connected to the outer surface of the base tube and extending along the length direction, and a branch fin group connected to the top of the main fin, wherein the branch fin group comprises a branch fin group one and a branch fin group two, the branch fin group one comprises a plurality of branch fins one, the branch fin group two comprises a plurality of branch fins two, the branch fins one and the branch fins two have the same height and width, each branch fin one and the branch fin two are adjacent and have opposite bending directions, and the branch fins one and the branch fins two at the top of each main fin appear alternately in sequence; the plurality of fins are uniformly arranged in the heat storage chamber in a radial shape on the periphery of the base tube, and the phase change material is filled in the heat storage chamber and wrapped on the fin tube.

[0007] Furthermore, the branch fins between adjacent fins on the periphery of the fin tube base tube are arranged symmetrically or in the same bending direction.

[0008] Furthermore, the number of fins on the outer circumference of the finned tube is not less than 4.

[0009] Furthermore, the ratio of the height of each branch fin in the fin tube along the radial direction of the base tube to the total height of the fin is 0.4-0.7, and the total height of the fin is the sum of the heights of the branch fin and the main fin.

[0010] Furthermore, the ratio of the width of each branch fin of the fin tube along the axial direction of the tube to the width of the main fin is 0.020-0.063.

[0011] Furthermore, in the fin tube, the ratio of the diameter of the circumscribed circle of the outer edge of the fin to the diameter of the inner wall of the shell is 0.80-0.95.

[0012] Furthermore, the ratio of the bending angle of the bifurcated fins in the fin tube to the maximum allowable bending angle is 0.45-0.75.

[0013] Furthermore, the material of the fin tubes in the heat storage device is steel, aluminum alloy or copper; and the phase change material is a hydrated salt, paraffin or fatty acid phase change material.

[0014] An application of a phase change heat storage device based on bidirectional staggered fins, which is used for wastewater waste heat storage, especially for the storage and utilization of cooling water in breweries. When the water temperature is 65°C, it is supplied intermittently, and the phase change material uses paraffin with a melting point of 52°C. Compared with the phase change heat storage device using traditional longitudinal straight fins, the phase change heat storage device of the present invention has a 21.7% shorter complete melting time of the phase change material. At the same time, the temperature field is more uniform and the temperature is higher. It has more effective heat transfer characteristics, and heat can penetrate quickly. The heat storage capacity has increased by 24.2%. It can be seen that the heat storage performance of the phase change heat storage device of the present invention has been significantly improved.

[0015] The present invention has the advantages that: 1. The bidirectional staggered fins of the present invention inhibit the growth of the flow boundary layer on the fin surface, strengthen the natural convection process of the phase change material in each discrete area, and improve the heat transfer efficiency and heat storage rate of the fins; 2. The bidirectional staggered fins of the present invention transfer heat more evenly to the entire phase change material in the heat storage chamber, making the melting more uniform; 3. The present invention can effectively enhance the heat storage performance of the fin-tube phase-change heat storage device. Compared with the traditional longitudinal straight-fin phase-change heat storage device, its heat storage efficiency can be increased by about 20%, and the heat storage capacity can be increased by more than 20% in the same time; 4. The present invention makes structural innovation to the original longitudinal straight fins, and realizes the synchronous rapid melting of phase change materials in various regions in the heat storage device through bidirectional staggered fins; the bidirectional staggered fin type phase change heat storage device described in the present invention can be quickly obtained through cutting and bending processes, is easy to realize automated production, easy to realize batch production, is closely related to industrial production, has extremely strong industrial application value, and further broadens its application in waste heat recovery of waste water in chemical, construction and other industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of a vertical phase change thermal storage device with bidirectional staggered fins provided in an embodiment of the present invention; Figure 2 It is a schematic diagram of the fin arrangement of Example 1 of the bidirectional staggered fin high-efficiency heat storage device of the present invention; Figure 3 It is a front view of the bidirectional staggered fins of Example 1 of the bidirectional staggered fin high-efficiency heat storage device of the present invention; the left picture is a front view of the bidirectional staggered fins; the middle picture is a top view cross-sectional view AA of the bidirectional staggered fins; the right picture is a top view cross-sectional view BB of the bidirectional staggered fins; Figure 4 It is a curve comparison diagram of the temperature change over time of the same test point of the bidirectional staggered fin and the longitudinal straight fin provided in Example 1 of the present invention; the left figure is the temperature change diagram of the longitudinal straight fin, and the right figure is the temperature change diagram of Example 1 of the present invention; Figure 5 is a graph showing the variation of the heat exchange rate of the bidirectional staggered fins and the longitudinal straight fins over time provided in Example 1 of the present invention; Figure 6 is a graph showing the change of the liquid fraction of the bidirectional staggered fins, the longitudinal straight fins and the phase change material over time provided by Example 1 of the present invention; Figure 7 It is a liquid fraction diagram of the bidirectional staggered fins and the longitudinal straight fins and the phase change material at the same time and the same position provided by Example 1 of the present invention; Figure 8 It is a schematic diagram of the fin arrangement of Example 2 of the bidirectional staggered fin high-efficiency heat storage device of the present invention; Fig. 9 It is a front view of the bidirectional staggered fins of Example 2 of the bidirectional staggered fin high-efficiency heat storage device of the present invention; the left picture is a front view of the bidirectional staggered fins; the middle picture is a top view cross-sectional view A of the bidirectional staggered fins; the right picture is a top view cross-sectional view B of the bidirectional staggered fins; Fig.10 This is a graph showing the change of the liquid fraction of the bidirectional staggered fins, the longitudinal straight fins and the phase change material over time provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0017] Example 1 A phase change heat storage device based on bidirectional staggered fins, comprising a sealed cylindrical shell 1, a fin tube 2 arranged in the shell and coaxial with the shell, and a phase change material 3 arranged in a heat storage chamber between the shell and the fin tube, wherein the fin tube comprises a circular base tube 4 located in the center and a plurality of fins welded to the surface of the base tube, each fin comprises a main fin 5 connected to the outer surface of the base tube and extending along the length direction, and a branch fin group connected to the top of the main fin, wherein the branch fin group comprises a branch fin group 1 and a branch fin group 2, the branch fin group 1 comprises a plurality of branch fins 1 6, the branch fin group 2 comprises a plurality of branch fins 2 7, the branch fins 1 and the branch fins 2 have the same height and width, each branch fin 1 and the branch fin 2 are adjacent and have opposite bending directions, and the branch fins 1 and the branch fins 2 at the top of each main fin appear alternately in sequence; the plurality of fins are evenly arranged in the heat storage chamber in a radial shape on the periphery of the base tube, and the phase change material is filled in the heat storage chamber and wrapped on the fin tube.

[0018] As a preferred embodiment, Figure 2 Schematic diagram of the fin arrangement of the bidirectional staggered fin high-efficiency heat storage device of Example 1 of the present invention, Figure 3 It is a front view of the bidirectional staggered fins of Example 1 of the bidirectional staggered fin high-efficiency heat storage device of the present invention, which describes the detailed structure of the bidirectional staggered fins. The adjacent fins of the bidirectional staggered fins are symmetrically arranged.

[0019] Through a large number of calculations on the structural parameters of the phase change heat storage device, the following optimal values ​​are obtained: the ratio of the height of the branch fins to the total height in the fin tube is 0.4-0.7, the total height of the fins is the sum of the heights of the branch fins and the main fins, the ratio of the width of each branch fin to the width of the main fin along the axial direction of the fin tube is 0.020-0.063, the ratio of the diameter of the circumscribed circle of the outer edge of the fin in the fin tube to the diameter of the inner wall of the shell is 0.80-0.95, and the ratio of the bending angle of the forked fin in the fin tube to the maximum allowable bending angle is 0.45-0.75.

[0020] The bidirectional staggered fins are preferably such that the ratio of the branch fin height to the total height is 0.43, the total fin height is the sum of the heights of the branch fins and the main fins, the ratio of the width of each branch fin to the width of the main fin along the axial direction of the tube is 0.025, the ratio of the diameter of the circumscribed circle of the outer edge of the fin in the fin tube to the diameter of the inner wall of the shell is 0.84, and the ratio of the bending angle of the forked fin in the fin tube to the maximum allowable bending angle is 0.75.

[0021] Comparative experiment 1 First, a traditional longitudinal straight-fin heat storage device and a heat storage device according to Example 1 of the present invention were manufactured and processed for experimental testing. The structural parameters of the longitudinal straight-fin heat storage device are: the outer diameter of the circular base tube is 24mm, the length is 350mm, the fin height is 35mm, the inner diameter of the cylindrical shell is 100mm, the length is 330mm, and the thickness is 2mm. The fin volume, base tube size, and shell size of the heat storage device of the present invention are consistent with the former. It should be noted that the total height of the fins of the heat storage device is still 35mm, of which the main fin height is 15mm, the branch fin height is 20mm, the bending angle of the forked fin is 54°, the bidirectional staggered fin length of a unit is 20mm, and there are 6 groups of fins along the circumference. These two heat storage devices are used for wastewater waste heat storage and for the storage and utilization of cooling water in breweries. When the room temperature is 28°C, the water temperature is 65°C, and the water flow rate is 1.1m / s, the two structural devices are experimentally operated and intermittently supplied. The phase change material uses paraffin with a melting point of 52°C. The entire waste heat storage system is mainly composed of a phase change heat accumulator, a constant temperature water tank, a high temperature water pump, a temperature recorder and a computer. The constant temperature water tank is mainly used to provide a fluid with a constant temperature and a constant flow rate. The temperature recorder connects the thermocouple wire to the temperature recorder through the data board and the terminal block. The temperature recorder is connected to the computer by a data line, and the data is recorded and saved through the TempPatrol software. The temperature of the corresponding time of the monitoring points arranged in the phase change heat accumulator is monitored through different channels. The circular base and fins of the phase change heat accumulator are made of carbon steel, and the outer tube is made of acrylic. In order to reduce heat loss and achieve the adiabatic boundary condition as much as possible, a layer of insulation cotton is wrapped around the outer wall of the shell. The high temperature water pump can control the flow rate of the fluid flowing out of the constant temperature water tank. In order to reduce the experimental error, the heat storage characteristics of the longitudinal straight fin tube and the structure of the present invention are tested at the same time. Two high temperature water pumps and two tees are used for pipeline diversion connection to ensure that the inlet temperature is exactly the same. At the same time, the flow rates flowing out of the two shunt pipes at the same time were tested, and the same flow rates were also guaranteed. The experiment was measured by inserting thermocouples in the phase change heat storage device. The thermocouples used in the experiment were K-type thermocouples with a temperature measurement range of 273K~1573K. From top to bottom, the thermocouples along the tube are b1, b2; b3, b4; b5, b6, and from inside to outside, the thermocouples along the radial direction are a1, a2, a3. The naming rule is "radial hot thermocouple number + axial thermocouple number". Taking the thermocouple at the bottom layer close to the shell as an example, it can be named "a3b1". Similarly, taking the thermocouple at the bottom layer close to the tube body as an example, it can be named "a1b1".

[0022] The overall distribution of thermocouples on the entire device is as follows: there are 6 thermocouples in each axial direction (each column), and their insertion depths are consistent. One column of thermocouples is inserted every 120° along the circumference (3 columns in total), and the insertion depths are consistent with the position parameters of a1, a2, and a3, respectively. There are a total of 18 thermocouples on each phase change thermal storage device. During the experimental preparation process, the annular space between the inner and outer tubes is filled with phase change material (paraffin with a phase change temperature of 52°C is used in this experiment) but not filled to prevent overflow due to the thermal expansion of the phase change material. The experimental section is arranged vertically, and the hot fluid flows in from the bottom end of the lower part and flows out from the top end of the upper part, and flows back to the constant temperature water tank through the pipeline. During the experiment, first add water to the constant temperature water tank for heating before the experiment begins, and turn on the magnetic rotor to heat the water tank evenly. During the heating period of the constant temperature water tank, connect the pipeline, wrap the insulation cotton, and temporarily open the valve to inhale and check the air tightness of the system. After the inspection, close the water pump valve. Then turn on the temperature recorder to observe whether the thermocouple reading is stable and ensure that the thermocouple is connected correctly. When the water temperature is heated to the preset temperature, the experiment officially begins. First, connect the temperature recorder to the computer, record several sets of data, and then turn on the high-temperature water pump to introduce the high-temperature fluid into the phase change thermal storage unit for melting heat storage experiment. During this period, observe the thermocouple readings. When the inlet and outlet temperature difference is 0°C (due to the measurement error of the thermocouple, the melting process is considered to be completed when the temperature difference is ≤0.1°C), it proves that the phase change device stops absorbing heat. At this time, first drain the high-temperature fluid in the pipe and then close the water pump valve. Then remove the insulation cotton to allow the paraffin in the phase change device to cool naturally, turn off the power supply of the water tank and drain the liquid in the water tank to facilitate subsequent experiments. Finally, process the experimental data and analyze the experimental results.

[0023] Compared with the longitudinal straight fin tube in the prior art, the temperature variation of the test point over time is as follows Figure 4 As shown. Figure 4 It can be seen that compared with the commonly used longitudinal straight fin heat storage device with the same fin volume, the temperature rise rate in the heat storage device is faster with the structure of the present invention. The temperature field of the structure of the present invention is always higher and more uniform than that of the longitudinal straight fin structure. This shows that the structure of the present invention has more effective heat transfer characteristics, allowing heat to quickly penetrate into the PCM in the radial direction. Figure 5 The heat transfer of two different structures changes with time. Figure 5 It can be seen that the heat storage capacity of the phase change heat storage device of the present invention is increased by 85.87% compared with the phase change heat storage device using traditional longitudinal straight fins. It can be seen that the heat storage performance of the phase change heat storage device of the present invention is significantly improved. At the same time, in order to further compare the melting of the phase change materials in the two devices and the time of complete melting. The distribution of the solid-liquid phase interface in the two heat storage devices was studied by numerical simulation method. Compared with the longitudinal straight fin tube in the prior art, the change of liquid fraction over time is shown in Figure 2. Figure 6 As shown. Figure 6It can be seen that compared with the commonly used longitudinal straight fin heat storage device with the same fin volume, the complete melting time is reduced by about 20.9% by adopting the structure of the present invention; Figure 7 This is a comparison of the liquid fractions at the same cross-sectional position of two different structures at the same time. Figure 7 It can be seen that at the same time, the structure of the present invention melts faster and more uniformly, which is consistent with the experimental results.

[0024] Example 2 The difference between Example 2 and Example 1 is that the branch fins between adjacent fins on the outer periphery of the finned tube base tube are arranged in the same bending direction, see Figure 8-9 As shown, Figure 8 This is a schematic diagram of the fin arrangement of Example 2 of the bidirectional staggered fin high-efficiency heat storage device of Example 2 of the present invention; Fig. 9 This is a front view of the bidirectional staggered fins of Example 2 of the bidirectional staggered fin high-efficiency heat storage device of the present invention, which describes the detailed structure of the bidirectional staggered fins. Through a large number of calculations on the structural parameters of the phase change heat storage device, the following optimal values ​​are obtained: the bidirectional staggered fins are preferably such that the ratio of the branch fin height to the total height is 0.7, the ratio of the width of each branch fin to the width of the main fin along the axial direction of the tube is 0.05, the ratio of the diameter of the circumscribed circle of the outer edge of the fin in the fin tube to the diameter of the inner wall of the shell is 0.8, and the ratio of the bending angle of the forked fin in the fin tube to the maximum allowable bending angle is 0.5.

[0025] Comparative experiment 2 Numerical simulations were performed on two types of finned tubes before and after optimization. The physical model of the longitudinal straight-fin heat storage device is as follows: the inner diameter of the inner tube is 20 mm, the inner diameter of the heat storage shell is 50 mm, the length of the heat storage is 120 mm, the longitudinal fin height is 10 mm, the fin thickness is 1 mm, and the number of fins along the circumference is 6. The physical model of the heat storage device of Example 2 of the present invention: the main fin height is 7 mm, the branch fin height is 3 mm, and the bending angle is 45°. The liquid PCM is regarded as a laminar, unsteady, incompressible fluid. The thermal properties of PCM are considered to be constant and independent of temperature. By considering the density change of the buoyancy term, the Boussinesq approximation is used to solve the natural convection problem. The inner tube is set to a constant wall temperature of 343K, and the outer shell wall and the upper and lower annular walls are set to adiabatic boundary conditions. The initial temperature is constant at 300K, and the heat storage chamber composed of the inner tube and the tube shell is filled with RT54HC paraffin. The phase change temperature of RT54HC paraffin is 327K. The heat storage characteristics of the longitudinal straight fin tube heat storage device and the heat storage device of Example 2 of the present invention are numerically calculated respectively, and the change of the liquid fraction over time is shown in the following figure: Fig.10 As shown. Fig.10 It can be seen that compared with the longitudinal straight fin heat storage device, the complete melting time is reduced by about 17.87% by adopting the structure of the present invention.

[0026] It can be seen from the above embodiments that phase change heat storage can be used in the fields of solar energy heat storage, waste heat utilization of industrial equipment, etc. The bidirectional staggered fins of the present invention can improve the efficiency of the heat accumulator on the basis of easy processing. Compared with the longitudinal straight fins, whether the fins are arranged in a rotating array in the circumferential direction of the fin tube or the adjacent fins are arranged symmetrically, it can be improved by about 20%, which can improve the advantages in market products and promote profits. Phase change energy storage technology has the advantages of high energy density and small space occupation, but there is a problem of low thermal conductivity. Most of the previous studies have focused on enhancing thermal conductivity. This patent innovatively uses bidirectional staggering of fins to enhance natural convection between adjacent fins, improve longitudinal fin heat accumulators, and improve the overall heat storage capacity of heat accumulators. Phase change heat storage has the advantages of constant temperature, high energy storage density, and strong reusability, but the low thermal conductivity of phase change materials hinders its practical application development. The present invention improves the problem of low thermal conductivity of phase change materials by strengthening convective heat exchange through bidirectional staggered fins, thereby enhancing the heat exchange performance of the heat accumulator.

Claims

1. A phase change heat storage device based on bidirectional staggered fins, characterized in that: The invention comprises a sealed cylindrical shell, a fin tube coaxial with the shell and arranged in the shell, and a phase change material arranged in a heat storage chamber between the shell and the fin tube, wherein the fin tube comprises a circular base tube located in the center and a plurality of fins welded to the surface of the base tube, each fin comprises a main fin connected to the outer surface of the base tube and extending along the length direction, and a branch fin group connected to the top of the main fin, wherein the branch fin group comprises a branch fin group one and a branch fin group two, wherein the branch fin group one comprises a plurality of branch fins one, and the branch fin group two comprises a plurality of branch fins two, the branch fins one and the branch fins two have the same height and width, each branch fin one and the branch fin two are adjacent and have opposite bending directions, and the branch fins one and the branch fins two at the top of each main fin appear alternately in sequence; the plurality of fins are uniformly arranged in the heat storage chamber in a radial shape on the periphery of the base tube, and the phase change material is filled in the heat storage chamber and wrapped on the fin tube.

2. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The branch fins between adjacent fins on the outer periphery of the fin tube base tube are arranged symmetrically or in the same direction.

3. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The number of fins on the outer circumference of the fin tube is not less than 4.

4. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The ratio of the height of each branch fin in the fin tube along the radial direction of the base tube to the total height of the fin is 0.4-0.7, and the total height of the fin is the sum of the heights of the branch fin and the main fin.

5. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The ratio of the width of each branch fin of the fin tube along the axial direction of the tube to the width of the main fin is 0.020-0.

063.

6. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The ratio of the diameter of the circumscribed circle of the outer edge of the fin in the fin tube to the diameter of the inner wall of the shell is 0.80-0.

95.

7. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The ratio of the bending angle of the bifurcated fins in the fin tube to the maximum allowable bending angle is 0.45-0.

75.

8. The phase change thermal storage device based on bidirectional staggered fins according to claim 1, characterized in that: The material of the finned tubes in the heat storage device is steel, aluminum alloy or copper; the phase change material is a hydrated salt, paraffin or fatty acid phase change material.

9. Application of the phase change thermal storage device based on bidirectional staggered fins according to any one of claims 1 to 8, characterized in that: Use it for waste water waste heat storage.

Images