Double-layer stepped heat collecting tube with symmetrical inclined fins and trough type solar heat collector

Abstract

The invention discloses a double-layer step heat collecting tube with symmetrical inclined rib plates and a groove type solar heat collector. The double-layer step heat collecting tube comprises a transparent glass outer tube, a heat collecting outer tube, a heat collecting inner tube and the symmetrical inclined rib plates which are horizontally arranged. Two sets of inclined ribs used for enhancing heat exchange are arranged below the heat collection inner tube and on the heat collection outer tube, and after the inclined ribs are arranged according to specific arrangement positions, a specific symmetrical rotational flow field can be formed in the heat collection outer tube, so that the convective heat exchange effect of the heat collection tube is improved, the heat collection efficiency of the heat collection tube is further improved, and the service life of the heat collection tube is prolonged. And the maximum temperature of the heat collecting outer pipe is reduced, and the service life of the heat collecting pipe is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of solar collectors, and particularly relates to a double-layer stepped heat collection tube with symmetrical inclined fins and a trough-type solar collector. Background Technology

[0002] To achieve efficient, stepped solar energy conversion within the mid-temperature range, trough-type concentrating solar collectors, as a mature and reliable technology, rely on the photothermal conversion performance of the collector tubes. Under actual concentrating conditions, the reflector concentrates solar radiation onto the lower half of the collector tube, creating a localized high heat flux density, while the upper half primarily receives diffused energy that is not concentrated. This results in a significant asymmetry in the circumferential heat load on the tube wall. This asymmetric heat load not only triggers localized thermal stress and deformation risks in the tube material but, more importantly, causes significant temperature stratification of the heat-collecting medium in the circumferential direction. The heat carried by the medium in the high-temperature zone is more easily dissipated through the tube wall to the lower-temperature environment in the upper half, hindering the improvement of the overall system thermal efficiency. To improve this situation, the industry commonly employs passive enhanced heat transfer technology. Existing technological approaches mainly focus on two aspects: first, directly modifying the heat exchange surface morphology, such as processing internal ribs, pits, or axial corrugations; second, installing flow-turbulence components inside the flow channels, such as spiral ribbons, staggered baffles, or porous packing. While these methods can enhance turbulence and heat transfer, in the non-uniform heating scenario unique to trough systems, indiscriminate disturbances of the entire flow field often accelerate the transport of high-temperature fluid from high-heat-fluid regions to the pipe walls in low-heat-fluid regions, thereby increasing unnecessary heat loss and weakening the net benefits of enhanced heat transfer.

[0003] To make fuller use of solar energy at different energy levels, tiered solar collectors with a double-layered tube structure have emerged, such as the double-layered tiered solar collector (CN115111786A) previously invented by the inventors. This structure allows media with different boiling points to flow in the inner and outer channels, thereby achieving graded absorption and conversion of energy. However, in this structure, the outer annular channel space is usually relatively wide, and the working fluid flow velocity is relatively low, resulting in insufficient convective heat transfer capacity. The fins arranged on the collector tube need further improvement in their effect on heat flow disturbance and heat transfer enhancement in areas with high heat flux density. These factors all affect the effective extraction of heat from the high-temperature stage, especially in the lower half of the high heat flux region. How to specifically enhance heat transfer in this region without causing disordered mixing of the overall fluid becomes the key to further improving the performance of this type of solar collector. Summary of the Invention

[0004] To address the deficiencies and shortcomings of the existing technologies, this invention provides a double-layer stepped heat collection tube with symmetrical inclined fins and a trough-type solar collector. Through the rational layout of the fins, the technical problems of insufficient heat exchange intensity in the outer flow channel of the double-layer heat collection tube and difficulty in controlling heat loss under non-uniform heat flow are solved, further improving the heat exchange efficiency of the heat collection tube.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A double-layer stepped heat collection tube with symmetrical inclined fins includes a horizontally arranged transparent glass outer tube, a heat collection outer tube, and a heat collection inner tube. The heat collection outer tube is fitted inside the transparent glass outer tube, and the heat collection inner tube is fitted inside the heat collection outer tube, with the upper end of the heat collection inner tube closely fitting the upper end of the heat collection outer tube. Two wing-shaped outer edges protrude along the length of each side of the heat collection inner tube, with the upper ends of the two wing-shaped outer edges fitting against the inner wall of the upper end of the heat collection outer tube, and the central angle α corresponding to the two wing-shaped outer edges being less than 180°. Inside the heat collection outer tube and in the region below the heat collection inner tube, two sets of inclined fins for enhancing heat transfer are arranged circumferentially along the inner wall of the heat collection outer tube. The two sets of inclined fins are symmetrical, and each set contains several discrete inclined fins. The plane of symmetry of the discrete inclined fins along the radial direction of the heat collection outer tube forms an angle with the plane of symmetry of the heat collection inner tube. included angle The value range is 20°≤ ≤60°; the discrete inclined fins form an inclination angle with the axis of the heat collection tube along the radial direction of the outer tube. The tilt angle ranges from 15° to 15°. ≤70°.

[0006] Furthermore, the length, height, and width of the discrete inclined ribs are respectively , and The inner diameter of the outer heat collection tube is D, and the values ​​of the length, height, and width of the discrete inclined fins are respectively within the range of 0.1D≤ ≤0.2D, 0.005D≤ ≤0.1D and 0.005D≤ ≤0.1D.

[0007] Furthermore, in each group of discrete inclined ribs, the pitch of two adjacent discrete inclined ribs arranged at equal intervals along the axial direction is p, and the value of the pitch is in the range of p≥0.2D.

[0008] Furthermore, the boiling point of the heat transfer fluid inside the outer heat collector tube is greater than the boiling point of the heat transfer fluid inside the inner heat collector tube.

[0009] Furthermore, the thermal conductivity of the inner heat-collecting tube and the outer airfoil is... ,in ≤0.2W / (m·K).

[0010] Furthermore, the inner diameter of the outer heat collection tube is D, and the outer diameter of the inner heat collection tube is d, wherein D ≥ 2d.

[0011] Furthermore, the two ends of the annular space between the transparent glass cover tube and the outer heat collection tube are sealed, and the interior of the annular space is evacuated.

[0012] On the other hand, the present invention also provides a trough solar collector containing the above-mentioned double-layer stepped heat collection tube with symmetrical inclined fins.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) By setting symmetrical inclined fins in a specific double-layered heat collector tube and arranging them according to the relative positional relationship between the inclined fins and the inner heat collector tube, on the one hand, the inclined fins can be located in the peak region of relatively high heat flux density of the heat collector tube, thereby enhancing the heat transfer area and fluid disturbance intensity at this local location; on the other hand, through this specific arrangement of the inclined fins, a unique swirling flow field can be generated in the high heat flux density region of the double-layered heat collector tube. Through this special swirling flow field, the rapid mixing of the high-temperature and low-temperature regions of the heat collector fluid in the outer heat collector tube can be promoted; more importantly, the heat transfer performance of the region with the highest heat flux density (i.e., the lowest point of the outer heat collector tube) can be enhanced, and local high temperature can be suppressed.

[0014] (2) By combining the arrangement of the fins and the specific design of the size of the inclined fins, the heat transfer efficiency and flow performance of the high-temperature heat transfer fluid in the heat collection tube can be moderate. While ensuring the improvement of heat exchange efficiency, the fluid flow in the outer tube of the heat collection tube can also be guaranteed, reducing the energy consumption required to drive the fluid flow. Attached Figure Description

[0015] The disclosure of this invention will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are merely illustrative of the technical solutions of this invention and are not intended to limit the scope of protection of this invention.

[0016] Figure 1 This is a simplified structural diagram of the integrated symmetrical inclined fin double-layer stepped heat collection tube according to an embodiment of the present invention; Figure 2 This is a cross-sectional view of the integrated symmetrical inclined fin double-layer stepped heat collection tube described in an embodiment of the present invention; Figure 3 This is a simplified structural dimension diagram of the integrated symmetrical inclined fin double-layer stepped heat collection tube according to an embodiment of the present invention; Figure 4 This is an axial cross-sectional view of the inclined ribs inside the external heating tube; Figure 5 This is a cross-sectional view of the trough-type solar collector with double-layer stepped heat collection tubes according to an embodiment of the present invention; Figure 6The swirling flow field morphology along the mainstream direction of the heat collection outer tube obtained under different Lagrange multipliers and fluid inlet temperatures in this embodiment of the invention; Figure 7 for Figure 6 The vortex flow field morphology in a cross section inside the outer tube of the heat collector when the fluid inlet temperature is 500K. Figure 8 Temperature and velocity vector distribution diagrams on the cross-section of the embodiments of the present invention compared with Comparative Examples 1 and 2 under the same working conditions; Figure 9 Velocity vector distribution diagrams on the cross-section of the embodiments of the present invention and Comparative Example 3 under the same working conditions; Figure 10 The graph shows the variation of Nuscher number Nu with volumetric flow rate V for the embodiments of the present invention and Comparative Examples 1 and 2. Figure 11 This is a graph showing the variation of the friction coefficient f with the volumetric flow rate V in the embodiments of the present invention and in Comparative Examples 1 and 2. Figure 12 The graph shows the change of heat collection efficiency η as a function of volumetric flow rate V for the embodiments of the present invention and Comparative Examples 1 and 2. Figure 13 The graph shows the variation of Nuscher number Nu with solar radiation intensity DNI for the embodiments of the present invention and Comparative Examples 1 and 2. Figure 14 The graph shows the variation of the friction coefficient f with solar radiation intensity DNI for the embodiments of the present invention and Comparative Examples 1 and 2. Figure 15 The graph shows the change of heat collection efficiency η as a function of solar radiation intensity DNI for the embodiments of the present invention and Comparative Examples 1 and 2. In the figure: 1-double-layer stepped heat collection tube, 11-transparent glass cover tube, 12-outer heat collection tube, 121-discrete inclined fins, 13-inner heat collection tube, 131-wing-shaped outer edge, 2-slot-type concentrating reflector. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0018] The following detailed description of the heat collection tube provided by the present invention is based on the embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0019] Example 1 like Figure 1 and 2 As shown, this embodiment of the invention provides a double-layer stepped heat collection tube with symmetrical inclined fins, including a horizontally arranged transparent glass outer tube 11, a heat collection outer tube 12, and a heat collection inner tube 13. The heat collection outer tube 12 is sleeved inside the transparent glass outer tube 11, and the heat collection inner tube 13 is sleeved inside the heat collection outer tube 12. The upper end of the heat collection inner tube 13 is in contact with the upper end of the heat collection outer tube 12. Two wing-shaped outer edges 131 are protruding along the length of both sides of the heat collection inner tube 13. The upper ends of the two wing-shaped outer edges 131 are in contact with the upper inner wall of the heat collection outer tube 12. Inside the heat collection outer tube 12 and in the area below the heat collection inner tube 13, two sets of inclined fins for enhancing heat exchange are arranged circumferentially along the inner wall of the heat collection outer tube 12. The two sets of inclined fins are symmetrical from left to right, and each set of inclined fins contains a number of discrete inclined fins 121.

[0020] like Figure 3 and 4 As shown, the central angles corresponding to the two airfoil outer edges 131 <180°; the discrete inclined fins 121 form an angle with the plane of symmetry of the outer heat collection tube 12 and the plane of symmetry of the inner heat collection tube 13 along the radial direction. included angle The value range is 20°≤ ≤60°; the discrete inclined fins form an inclination angle with the axis of the collector tube along the radial direction of the outer collector tube 12. The tilt angle ranges from 15° to 15°. ≤70°. The inner diameter of the outer heat collection tube is D, and the outer diameter of the inner heat collection tube is d, where D ≥ 2d. The length, height, and width of the discrete inclined fins are respectively... , and The discrete inclined ribs have length, height, and width values ​​ranging from 0.1D to 0.1D. ≤0.2D, 0.005D≤ ≤0.1D and 0.005D≤ ≤0.1D. In each group of discrete inclined ribs, the pitch of two adjacent discrete inclined ribs arranged at equal intervals along the axial direction is p, and the value of the pitch is p≥0.2D. Each group of inclined ribs contains 20 discrete inclined ribs.

[0021] The trough solar collector using the double-layer stepped collector tubes with symmetrical inclined fins of Example 1, such as... Figure 5 As shown.

[0022] Compare with Example 1 It is largely the same as Embodiment 1 above, except that there is no inner heat collection tube 13 and wing-shaped outer edge 131 inside the outer heat collection tube, and there are no symmetrical inclined ribs 121 on the inner wall of the outer heat collection tube.

[0023] Compare with Example 2 Similar to Embodiment 1 above, except that symmetrical straight fins are provided on the inner wall of the outer heat collection tube. The arrangement of the symmetrical straight fins is described in CN115111786A.

[0024] Compare with Example 3 Similar to Embodiment 1 above, the difference lies in the included angle of the discrete inclined fins on the inner wall of the outer heat collection tube. =70°.

[0025] The comprehensive heat transfer performance of the examples and Comparative Examples 1, 2, and 3 was simulated and tested using the commercial CFD software Fluent. The specific parameters of the collector tubes and fins used in the simulation were as follows: the length of the double-layer stepped collector tube was 7800 mm; the inner diameter of the transparent glass cover tube was 109 mm, the wall thickness was 3 mm; the inner diameter of the outer collector tube was D = 66 mm; and the space between the transparent glass cover tube and the outer collector tube was a near-vacuum environment. The high-temperature heat transfer fluid was Syltherm-800 heat transfer oil, the low-temperature heat transfer fluid was water, the inner collector tube diameter was d = 20 mm, and the central angle α corresponding to the two airfoil outer edges was 120°.

[0026] In Example 1, the lower part of the inner wall of the heat collection tube is provided with inclined ribs, and the number of inclined ribs is 2 rows. In each row, the pitch p of the discrete inclined ribs is 20 mm, the length is l = 10 mm, and the height and width are respectively set as follows: =0.5mm and =0.3mm, discrete inclined rib angle =50°, rib tilt angle with axial direction =45°.

[0027] In Comparison Example 2, the height and width of the straight ribs are set to h=10mm and t=4mm, respectively.

[0028] Compared to Example 3, the included angle of the discrete inclined fins on the inner wall of the outer heat collector tube. =70°, and the other dimensions of the discrete inclined ribs are the same as in Example 1.

[0029] Figure 8 It reflects the temperature and velocity vector distribution on the cross-section. Figure 9 The velocity vector distribution diagrams of the cross sections of the embodiment and Comparative Example 3 are compared. Simulation results show that inside the outer tube of the heat collector in the embodiment, the fluid forms a pair of symmetrical longitudinal swirling flow field structures. Detailed flow field patterns under different parameter conditions are shown below. Figure 6-8As shown, where C0 is the Lagrange daily multiplier in the Lagrange functional, T in This is the inlet temperature of the heat collection fluid. From Figure 9 As can be seen, this embodiment, through a specific discrete inclined fin arrangement, can generate a special symmetrical longitudinal swirling flow field structure. This flow field structure can significantly improve the convective heat transfer performance of the fluid inside the outer tube of the heat collector. This symmetrical longitudinal swirling flow field can guide the relatively low-temperature fluid at the center of the tube to scour the tube wall in the high heat flux density region. At the same time, the swirling flow field can significantly increase fluid mixing, thereby effectively improving heat transfer performance, especially significantly improving the local heat transfer performance in the high heat flux density region, thereby effectively reducing the temperature of the heat collector tube wall and reducing heat loss. In addition, the flow resistance of this swirling flow field structure is small, and the power consumption required to drive the fluid flow is significantly lower than that of Comparative Example 2. In contrast, there is no obvious longitudinal swirling flow field inside the heat collector tube of Comparative Example 1, and inside the outer tubes of Comparative Examples 2 and 3. As a result, the temperature of the outer tube of the heat collector in Example 1 is significantly lower than that of the outer tubes of Comparative Examples 1-3. The heat loss caused by heat transfer to the external environment in Example 1 is also significantly reduced, and the heat collection efficiency is significantly improved compared to Comparative Examples 1-3.

[0030] Figure 10 This graph reflects the variation of Nuschl number (Nu) with volumetric flow rate (V). Figure 10 This shows that Nu gradually increases with increasing volumetric flow rate V. Within the volumetric flow rate V range illustrated, Nu in the embodiment is 1.34 to 2.09 times that of Comparative Example 1. The double-layer cascade collector tubes corresponding to the embodiment exhibit better heat transfer performance at lower volumetric flow rates. The overall heat transfer performance Nu of the embodiment is lower than that of Comparative Example 2. Figure 11 The graph reflects the variation of the friction coefficient f with the volumetric flow rate V. The friction coefficient f of the embodiment is higher than that of Control Example 1, but significantly lower than that of Control Example 2. The friction coefficient f of the embodiment is 3.47 to 3.95 times that of Control Example 1 and 0.75 to 0.80 times that of Control Example 2. This indicates that the embodiment has better flow performance than Control Example 2. Therefore, in a comprehensive comparison, the present invention has better overall heat transfer performance than Control Examples 1 and 2.

[0031] Figure 12 This graph reflects the change in heat collection efficiency η as a function of volumetric flow rate V. Figure 12 It can be seen that as the flow rate inside the outer heat collector tube increases, the heat collection efficiency of the double-layer stepped heat collector tube gradually increases. The heat collection efficiency of the embodiment is significantly improved compared with Comparative Example 1 and Comparative Example 2. Compared with Comparative Example 1, the heat collection efficiency of the embodiment is improved by 2.0%-4.2%. Compared with Comparative Example 2, the heat collection efficiency of the embodiment is improved by 0.65%-2.07%, which shows that the double-layer stepped heat collector tube with integrated symmetrical inclined fins of the present invention has excellent heat collection performance.

[0032] Figure 13 This reflects the variation of Nuschl number (Nu) with solar radiation intensity (DNI), from... Figure 13 It can be seen that the heat exchange performance Nu of the embodiment is 2.10-2.22 times that of the control example 1, and the heat exchange performance Nu of the embodiment is 0.80-0.96 times that of the control example 2. Moreover, the solar radiation intensity DNI has almost no effect on the heat exchange performance of the heat collector outer tube, indicating that the embodiment of the present invention has good heat exchange performance under any solar radiation intensity. Figure 14 This reflects the variation of the friction coefficient f with the solar radiation intensity DNI, from Figure 14 It can be seen that DNI has almost no effect on the frictional resistance of the outer collector tube, and the flow resistance f of the embodiment is 4.03-4.45 times that of Comparative Example 1, and the flow resistance f of the embodiment is 0.794-0.837 times that of Comparative Example 2. This indicates that the increase in flow resistance of the double-layer stepped collector tube corresponding to the embodiment of the present invention is moderate under any solar radiation intensity. Therefore, the embodiment has the best overall heat exchange performance.

[0033] Figure 15 The graph shows the change of heat collection efficiency η with solar radiation intensity DNI for the embodiments and comparative examples. As DNI increases, η gradually increases. Furthermore, the heat collection efficiency η of the double-layer stepped heat collection tube with symmetrical inclined fins corresponding to the embodiments is always higher than that of comparative examples 1 and 2. Compared with comparative example 1, the heat collection efficiency of the embodiments is increased by a maximum of 4.52%, and compared with comparative example 2, the heat collection efficiency of the embodiments is increased by a maximum of 2.14%.

[0034] In summary, this invention achieves tiered energy utilization by adding an inner heat-collecting tube with a wing-shaped outer edge inside the outer heat-collecting tube and by rationally arranging and placing the heat exchange fins. This further improves the heat exchange performance of the tiered heat-collecting tube. Compared with the original heat-collecting tube, the improved heat exchange fins further enhance the heat collection efficiency, with a maximum increase of up to 2.14%. In addition, the highest temperature of the outer heat-collecting tube wall of this invention is lower than that of the traditional heat-collecting tube and the double-layer tiered heat-collecting tube in Comparative Example 2, which also further improves the safety performance of the heat-collecting tube itself.

[0035] The above description, in conjunction with specific / preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the inventive concept, and all such substitutions or modifications should be considered within the scope of protection of the present invention.

Claims

1. A double-layer stepped heat collection tube with symmetrical inclined fins, comprising a horizontally arranged transparent glass outer tube, a heat collection outer tube, and a heat collection inner tube, characterized in that, The outer heat-collecting tube is fitted inside the transparent glass outer tube, and the inner heat-collecting tube is fitted inside the outer heat-collecting tube, with the upper end of the inner heat-collecting tube fitting against the upper end of the outer heat-collecting tube. Two wing-shaped outer edges protrude along the length of each side of the inner heat-collecting tube, with the upper ends of both wing-shaped outer edges fitting against the inner wall of the upper end of the outer heat-collecting tube. The central angles corresponding to the two wing-shaped outer edges are... Inside the outer heat collector tube and in the region below the inner heat collector tube, two sets of inclined fins are arranged circumferentially along the inner wall of the outer heat collector tube to enhance heat transfer. The two sets of inclined fins are symmetrical from left to right, and each set of inclined fins contains several discrete inclined fins. The plane of symmetry of the discrete inclined fins along the radial direction of the outer heat collector tube forms an angle with the plane of symmetry of the inner heat collector tube. included angle The range of values ​​is The discrete inclined fins form an angle of inclination with the axis of the heat collection tube along the radial direction of the outer tube. The range of values ​​for the tilt angle is: .

2. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1, characterized in that, The length, height, and width of the discrete inclined ribs are respectively , and The inner diameter of the outer heat collection tube is D, and the ranges of the length, height, and width of the discrete inclined fins are respectively... , and .

3. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1 or 2, characterized in that, In each group of discrete inclined ribs, the pitch of two adjacent discrete inclined ribs arranged at equal intervals along the axial direction is: The range of values ​​for the pitch is: .

4. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1 or 2, characterized in that, The boiling point of the heat transfer fluid in the outer heat collector tube is greater than the boiling point of the heat transfer fluid in the inner heat collector tube.

5. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1 or 2, characterized in that, The thermal conductivity of the inner heat-collecting tube and the outer airfoil is: ,in .

6. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1 or 2, characterized in that, The inner diameter of the outer heat collection tube is D, and the outer diameter of the inner heat collection tube is d, wherein D ≥ 2d.

7. A double-layer stepped heat collection tube with symmetrical inclined fins as described in claim 1 or 2, characterized in that, The two ends of the annular space between the transparent glass cover tube and the heat collection outer tube are sealed, and the inside of the annular space is evacuated.

8. A parabolic trough solar collector, characterized in that, Includes the double-layer cascade heat collection tube as described in any one of claims 1-7.

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