A double-walled bend structure

Abstract

This invention discloses a double-walled bend structure, comprising an inner pipe, which consists of a central inner pipe bend and inner pipe straight arms of equal diameter integrally connected to both ends of the inner pipe bend. The key feature is the inclusion of an outer pipe integrally spaced and sleeved around the outer side of the inner pipe. The outer pipe includes an outer pipe bend spaced around the inner pipe bend and an outer pipe straight arm spaced around the inner pipe straight arms. A connecting inlet is provided on the inner pipe straight arm within the outer pipe straight arm on the inflow side, and a connecting outlet is provided on the inner pipe straight arm within the outer pipe straight arm on the outflow side. This invention optimizes fluid flow through a double-walled structure, making it more stable, significantly reducing the flow resistance of the bend section, thereby improving the fluid transport capacity of the pipeline system, reducing operating energy consumption, and making its operation more economical and efficient.

Description

Technical Field

[0001] This invention relates to the field of fluid transport pipeline structure technology, specifically to a double-walled bend structure. Background Technology

[0002] As a key turning component in HVAC systems and industrial fluid distribution networks, the flow resistance characteristics of bends directly determine the system's energy consumption and operating efficiency. When fluid flows through the curvature change region within a bend, the coupling effect of centrifugal and viscous forces creates a complex secondary flow field, accompanied by phenomena such as vortex shedding, flow separation, and backflow, leading to a significant increase in local resistance losses. Existing research has confirmed that local resistance losses caused by bends and other local components in centralized heating networks can account for 50% to 90% of the total local resistance losses. This high resistance loss not only requires a large amount of additional electrical energy to drive circulating water pumps to maintain system flow and pressure, significantly increasing operating costs, but may also cause problems such as flow field instability, pipeline vibration, and noise pollution, seriously affecting the system's economic efficiency and reliability. Therefore, research on flow resistance reduction in bends is of significant practical importance for reducing energy consumption and improving the operating efficiency of fluid distribution systems.

[0003] Existing pipeline drag reduction technologies are mainly divided into two categories. One category achieves drag reduction by adding polymer drag-reducing agents to change the fluid's own properties, such as the polymethyl methacrylate compound-based pipeline drag-reducing agent disclosed in CN202510379130.9 and the one disclosed in CN202010585051.0. The information disclosed in CN202010308307.3 These are all examples of such technologies; another type is boundary layer control drag reduction technology, which uses wall coatings or microstructures to construct a disturbed flow field and optimize the distribution of wall shear stress. Among these, wall coatings include epoxy resin-based pipe drag reduction and high-temperature resistant coatings disclosed in CN202410864130.3 and phenolic epoxy resin-organic silicone resin-based thermal pipe drag reduction and high-temperature resistant coatings disclosed in CN202210837392.1; wall microstructures include biomimetic shark skin pipe drag reduction structure disclosed in CN202123077821.X and wide and narrow alternating semi-circular groove pipe drag reduction structure disclosed in CN202411539857.0, all of which belong to similar technical solutions.

[0004] However, the aforementioned commonly used methods for reducing drag in bends all have significant limitations: while polymer additives can modify fluid properties to reduce drag, they irreversibly contaminate the medium and are only suitable for pipelines transporting heating fluids, not for pipelines requiring fluid transport. Boundary layer control technology, on the other hand, is limited by the low durability of its microstructure or coating and is prone to failure under erosion and corrosion.

[0005] Therefore, how to improve the structure of the bend itself to better suppress secondary flow and boundary layer separation, reduce flow resistance, improve conveying efficiency, and at the same time take into account structural strength and manufacturability has become a technical problem that needs to be considered and solved by those skilled in the art. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a double-walled bend structure that can better achieve flow field control to reduce drag and improve conveying efficiency, so that it has the characteristics of high drag reduction efficiency and more stable flow state, and can be adapted to different pipe diameters and working conditions.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A double-walled bend structure includes an inner pipe, which consists of a central inner pipe bend and inner pipe straight arms of equal diameter integrally connected to both ends of the inner pipe bend. The structure is characterized by further including an outer pipe integrally spaced and sleeved around the outer side of the inner pipe. The outer pipe includes an outer pipe bend spaced around the inner pipe bend and an outer pipe straight arm spaced around the outer side of the inner pipe straight arms. The two ends of the outer pipe bend and the outer pipe straight arms are integrally connected with equal diameter. The outer end of the outer pipe straight arm on the inflow side is sealed to the corresponding inner pipe straight arm through an outwardly tapered inlet diffuser. The outer end of the outer pipe straight arm on the outflow side is sealed to the corresponding inner pipe straight arm through an outwardly tapered outlet diffuser. A connecting inlet is also provided on the inner pipe straight arm within the outer pipe straight arm on the inflow side, and a connecting outlet is also provided on the inner pipe straight arm within the outer pipe straight arm on the outflow side.

[0009] In this design, when the double-walled bend structure is in use, the fluid flows in from the straight arm of the inner pipe on the inlet side, and then is diverted at the connecting inlet, allowing some fluid to enter the space between the inner and outer pipes and pass through the bend. Finally, it flows out from the connecting outlet back into the straight arm of the inner pipe on the inlet side. This annular interlayer space created by the double-walled bend provides an effective technical approach for intervening in the flow field within the pipe and improving secondary flow issues in the bend. Compared to ordinary bends, the double-walled bend structure can effectively increase the cross-sectional area of ​​the flow path, thereby reducing the mainstream flow velocity and mitigating energy loss caused by turbulent pulsations at the source.

[0010] Furthermore, an inner tube guide vane that bends in the same direction is fixedly connected inside the inner tube elbow along the axis. The inner tube guide vane is located at the middle position between the inner and outer sides of the inner tube elbow cavity and is arranged in a straight line in the width direction.

[0011] In this way, by placing guide vanes at the middle position of the inner tube bend, the original large-scale, high-intensity double-vortex structure can be symmetrically divided into four low-energy small-scale vortices. This maximizes the reduction of the rotational kinetic energy of individual vortices, weakens the interaction between the vortex structure and the wall and the mainstream, and thus effectively reduces energy loss during the flow process. Furthermore, the straight arrangement in the width direction ensures uniform division of the flow channel, avoiding local impact and separation losses caused by irregular structures.

[0012] Furthermore, an outer pipe guide vane that bends in the same direction is fixedly connected between the outer pipe elbow and the inner pipe elbow. The outer pipe guide vane is located at the center between the inner and outer sides of the inner cavity of the outer pipe elbow and is arranged in a straight line in the width direction.

[0013] In this way, compared with the scheme of setting guide vanes only in the inner tube, setting guide vanes in the interlayer space between the inner tube and the outer sleeve can effectively cut off the circumferential flow path of the fluid in the interlayer, reduce its disturbance to the mainstream vortex in the central region, and further weaken the rotational intensity of the central vortex structure, thereby achieving a more significant drag reduction effect. Furthermore, the straight arrangement in the width direction ensures uniform channel segmentation, avoiding local impacts and separation losses caused by irregular structures.

[0014] Furthermore, the connecting inlet is a rectangular hole, and its length direction is located between the connection position of the outer straight arm pipe and the inlet diffuser pipe on the inflow side and the position of the inner pipe bend; the connecting outlet is a rectangular hole, and its length direction is located between the connection position of the outer straight arm pipe and the outlet diffuser pipe on the outflow side and the position of the inner pipe bend.

[0015] In this way, the fluid can flow smoothly from the inner tube into the outer tube through the gradually expanding structure, and then smoothly flow back from the outer tube to the inner tube through the gradually contracting structure; at the same time, it maximizes the effective flow area of ​​the connecting holes and achieves a smooth transition of the overall flow.

[0016] Furthermore, both the connecting inlet and the connecting outlet are positioned facing inwards, and the inner straight arm tube retains a section of connecting pipe on the outer side of the connecting inlet and the connecting outlet.

[0017] In this way, the direction of the upstream inlet opening of the elbow is consistent with the direction of the mainstream flow shifting inward under the action of centrifugal force. This can preferentially guide the fluid near the inside to enter the interlayer space in an orderly manner, which not only diverts and relieves pressure on the mainstream flow, but also avoids the high-speed fluid on the outside directly impacting the orifice and causing disturbance. Downstream of the elbow is a low-speed negative pressure zone. The rectangular outlet orifice opened here can allow the buffer fluid in the interlayer to be replenished to the mainstream area of ​​the inner pipe in the reverse direction, effectively alleviating the backflow phenomenon in the downstream low-speed stagnation zone, while balancing the pressure gradient inside and outside the elbow and suppressing the secondary flow enhancement caused by the pressure difference. Under the guiding effect of the inner arc side wall of the inner pipe elbow, the mainstream flow in the pipe becomes more gentle, which can avoid the formation of obvious low-speed stagnation zone and high-speed separation zone, and significantly reduce local flow resistance. Meanwhile, the inner tube adopts a non-disconnected design with the outer part retained and the opening only on the inner side, which can achieve multiple flow field control effects: the retained outer tube body forms a rigid guiding boundary, which not only constrains the mainstream to flow orderly along the inner side of the inner tube and avoids the formation of large-scale eddies by unconstrained diffusion, but also makes the inner tube and the straight tube arm form a smooth flow channel transition, reducing local disturbances and suppressing premature boundary layer separation; at the same time, this design allows the fluid exchange between the inner tube and the interlayer to be in an inner-side directional interconnection mode, avoiding direct impact between the interlayer fluid and the high-velocity mainstream on the outer side of the outer tube, reducing the velocity gradient and turbulent energy loss inside the tube.

[0018] Furthermore, the width of the connecting pipe body retained by the inner tube straight arm tube outside the connecting inlet is greater than that of the connecting pipe body retained by the inner tube straight arm tube outside the connecting outlet (i.e., the opening depth of the connecting inlet is shallower and the opening is smaller, while the opening depth of the connecting outlet is deeper and the opening is larger).

[0019] Thus, the width of the connected entrance This configuration ensures that the upstream pipe cross-section of the bend maintains a symmetrical and regular double-vortex structure, avoiding additional energy dissipation caused by asymmetrical vortices; connecting the outlet width The design effectively widens the mainstream flow cross section, which can transform the original two vortices with greater strength and smaller area into two vortices with less strength and larger area by widening the downstream flow area. This reduces drag, lowers the rotational kinetic energy and energy dissipation intensity of the vortex structure, weakens the continuous interference of large-scale vortices on the mainstream region, and achieves efficient control of the secondary flow in the bend.

[0020] Furthermore, the length of the connecting inlet is less than the length of the connecting outlet.

[0021] Therefore, a longer connecting outlet will result in better drag reduction.

[0022] Furthermore, the bending angle of the inner pipe elbow and the outer pipe elbow is 90 degrees.

[0023] Furthermore, the double-walled elbow structure is manufactured using the following method: first, the inner tube guide vanes inside the inner tube elbow are welded, and then the outer tube guide vanes on one side of the outer tube elbow are welded; then, the inner tube elbow is installed into the outer tube elbow, and the welding between the outer tube guide vanes already welded inside the outer tube elbow and the outer surface of the inner tube elbow is completed; then, the outer tube guide vanes on the other side are inserted into the installation position along the arc direction and the welding between them and the outer tube elbow and the inner tube elbow is completed; then, the inner tube straight arm pipe, the outer tube straight arm pipe, the inlet diffuser pipe, and the outlet diffuser pipe, which have been cut to connect the inlet and the outlet, are welded in sequence.

[0024] This allows for convenient and quick processing and manufacturing of the internal structures in the double-walled bend pipe structure.

[0025] Furthermore, the dimensional parameters of the double-walled bend structure are determined using the following methods and steps:

[0026] Step 1: Using a regular bend (without connecting inlet and outlet in appearance, and meeting the pipe requirements of the location to be used) with the same dimensions as the inner pipe in the double-wall bend structure as a comparison, establish a computer virtual model together with the double-wall bend structure.

[0027] Step 2: Perform structured mesh generation on the model of ordinary bend pipe; perform unstructured mesh generation on the area where the inner and outer pipes connect, as well as the area around the connecting inlet and outlet (and other geometrically complex areas) in the model of double-wall bend pipe structure; perform structured mesh generation on the remaining (relatively regular) main pipe area.

[0028] Step 3: Use Fluent fluid simulation software to perform numerical simulation of fluid flow in the two structures respectively, and obtain the turbulent flow field structure inside each structure.

[0029] Step 4: Calculate the drag reduction rate based on the comparison of the two structures. Based on the drag reduction rate, return to Step 1 to adjust the outer tube diameter, inlet diffuser length, outlet converging tube length, outlet straight arm length, inlet straight arm length, outlet depth, and inlet depth of the double-walled bend structure. Repeat Steps 1 to 4 until the calculated drag reduction rate is maximized. The corresponding dimensional parameters at this point are the final parameters of the double-walled bend structure.

[0030] Step 5: Normalize the structured parameters that affect the resistance coefficient of the double-walled bend to a dimensionless value relative to the inner pipe diameter d, so that they are applicable to all bend diameters.

[0031] Further, in step 1, the structural parameters are named as follows: inner diameter d of ordinary bend, curvature ratio r / d of ordinary bend, wall thickness s of ordinary bend; inner diameter d of inner tube in double-wall bend, curvature ratio r / d of inner tube in double-wall bend, wall thickness s of inner and outer tubes in double-wall bend, length of connecting inlet in double-wall bend a1, depth of connecting inlet in double-wall bend b1, length of connecting outlet in double-wall bend a2, depth of connecting outlet in double-wall bend b2, inner diameter D of outer tube in double-wall bend, curvature ratio R / D of outer tube in double-wall bend, length L1 of inlet expanding tube in double-wall bend, length L2 of outlet contracting tube in double-wall bend, length L3 of straight tube arm of outer tube on the inlet side of double-wall bend, and length L4 of straight tube arm of outer tube on the outlet side of double-wall bend.

[0032] Furthermore, in step 2, in order to comprehensively evaluate the overall flow loss and local flow field characteristics of the double-walled bend, the local resistance coefficient and the total pressure of the inner pipe bend and its upstream and downstream near-wall key nodes are selected as core evaluation indicators, and grid independence tests are performed on ordinary bends and double-walled bends respectively.

[0033] Furthermore, in step 3, during the numerical simulation, incompressible liquid water is selected as the fluid inside the pipe; The model is a turbulence model; the SIMPLEC algorithm is selected for solution, and the momentum equation, energy equation, and turbulent transport equation are all solved in second-order upwind form. The convergence criterion for the computational residuals of each equation is set to [value missing]. The scale; the boundary conditions are set as follows: the equivalent roughness of the pipe wall is set to 0.5 mm, the inlet is defined as a velocity inlet, and the outlet is defined as a free outflow.

[0034] Furthermore, in step 4, section 1 is selected 20D upstream of the bend and section 2 is selected downstream of the bend. The average static pressure of the two sections is obtained by area-weighted averaging. Thus, the total resistance loss of the bend section is obtained. Simultaneously, an equivalent straight pipe model without bends was established to obtain the frictional resistance loss under the same pipe length. Local resistance coefficient of bend The calculation formula is as follows:

[0035]

[0036] In the formula, The density of water, ; The average velocity at the pipe inlet is denoted as , in m / s.

[0037] Calculate the local resistance coefficients for ordinary pipe bends and double-walled pipe bends respectively. Calculate the drag reduction rate based on the local drag coefficient. :

[0038] .

[0039] Furthermore, in step 5, the final dimensionless representation of each parameter is as follows: L1=0.8d, L2=0.8d, L3=3d, L4=5d, h=0.2d, D=d+2s+2h; a1=L1+L3=3.8d, b1=0.5d, a1=L2+L4=5.8d, b1=0.85d.

[0040] Thus, using Fluent fluid simulation software for numerical simulation allows for in-depth analysis of the evolution of turbulent structures and flow mechanisms within pipelines, obtaining accurate flow data and providing a reliable basis for engineering optimization design. For the double-walled bend model, a hybrid mesh generation strategy is employed to more accurately capture fluid flow details, improving the accuracy and reliability of the simulation results. In this scheme, the turbulence model selected is... The model, by reconstructing the expression for the turbulent viscosity coefficient and introducing flow curl constraints, significantly improves the prediction accuracy of velocity distribution, pressure changes, and turbulent fluctuations within bends, providing theoretical support for the calculation of local drag coefficients. The numerical solution employs the SIMPLEC algorithm to handle pressure-velocity coupling, optimizes the pressure correction equation, effectively reduces numerical oscillations during iteration, and accelerates convergence. Simultaneously, a second-order upwind scheme is used, incorporating gradient information from adjacent grid points to reduce numerical dispersion errors and more accurately capture local changes in the flow field. To ensure computational accuracy, the residual convergence criterion for each equation is set to... The equivalent roughness of the pipe wall is set to 0.5 mm to align with actual engineering conditions; a velocity inlet is used, and the outlet is set to free outflow to avoid interference from boundary conditions on the downstream flow field. Therefore, this method effectively evaluates the improvement effect of structural parameters on flow resistance by calculating the drag reduction rate, providing a quantitative basis for optimizing pipeline configuration and thus improving engineering economy. By performing dimensionless optimization of structural parameters, the resulting structure can be directly applied to bend structures with different dimensional parameters, enhancing the adjustability and adaptability of double-wall bend design, enabling it to flexibly match diverse flow conditions and pipeline configuration requirements.

[0041] Therefore, this invention solves the problem of increased system energy consumption and low fluid transport efficiency caused by large local disturbances and high fluid resistance in ordinary elbows in fluid transmission and distribution pipeline projects. Under the same pipe diameter and flow velocity, the local resistance coefficient of this invention is reduced by 40% to 60% compared to ordinary elbows. It is evident that this invention optimizes the fluid flow pattern through a double-wall structure, making it more stable and significantly reducing the flow resistance of the elbow section, thereby improving the fluid transport capacity of the pipeline system, reducing operating energy consumption, and making its operation more economical and efficient. Attached Figure Description

[0042] Figure 1This is a schematic diagram of the double-walled bend pipe structure used in the implementation. To show the internal structure, the pipe body is shown transparently in the diagram.

[0043] Figure 2 for Figure 1 A schematic diagram of the structure of the separate inner tube.

[0044] Figure 3 for Figure 1 A schematic diagram of the structure of the separate outer tube.

[0045] Figure 4 This is a schematic diagram for the grid independence test.

[0046] Figure 5 This is a schematic diagram illustrating the method for calculating the local resistance coefficient of a bend. Detailed Implementation

[0047] The present invention will now be described in further detail with reference to specific embodiments.

[0048] For specific implementation: see [link / reference] Figures 1-5 A double-walled bend structure includes an inner pipe, which consists of an inner pipe bend 1 in the middle and inner pipe straight arm pipes 2 of the same diameter integrally connected to both ends of the inner pipe bend. The structure is characterized by further including an outer pipe integrally spaced and sleeved on the outside of the inner pipe. The outer pipe includes an outer pipe bend 5 spaced and sleeved on the outside of the inner pipe bend and an outer pipe straight arm pipe 6 spaced and sleeved on the outside of the inner pipe straight arm pipes. The two ends of the outer pipe bend and the outer pipe straight arm pipes are integrally connected with the same diameter. The outer end of the outer pipe straight arm pipe on the inflow side is sealed to the corresponding inner pipe straight arm pipe through an outwardly tapered inlet diffuser 7. The outer end of the outer pipe straight arm pipe on the outflow side is sealed to the corresponding inner pipe straight arm pipe through an outwardly tapered outlet diffuser 8. A connecting inlet 3 is also provided on the inner pipe straight arm pipe inside the outer pipe straight arm pipe on the inflow side, and a connecting outlet 4 is also provided on the inner pipe straight arm pipe inside the outer pipe straight arm pipe on the outflow side.

[0049] In this design, when the double-walled bend structure is in use, the fluid flows in from the straight arm of the inner pipe on the inlet side, and then is diverted at the connecting inlet, allowing some fluid to enter the space between the inner and outer pipes and pass through the bend. Finally, it flows out from the connecting outlet back into the straight arm of the inner pipe on the inlet side. This annular interlayer space created by the double-walled bend provides an effective technical approach for intervening in the flow field within the pipe and improving secondary flow issues in the bend. Compared to ordinary bends, the double-walled bend structure can effectively increase the cross-sectional area of ​​the flow path, thereby reducing the mainstream flow velocity and mitigating energy loss caused by turbulent pulsations at the source.

[0050] The inner tube elbow has a guide vane 9 that is bent in the same direction and is fixedly connected to the center line inside. The guide vane is located in the middle between the inner and outer sides of the inner tube elbow cavity and is set in a straight line in the width direction.

[0051] In this way, by placing guide vanes at the middle position of the inner tube bend, the original large-scale, high-intensity double-vortex structure can be symmetrically divided into four low-energy small-scale vortices. This maximizes the reduction of the rotational kinetic energy of individual vortices, weakens the interaction between the vortex structure and the wall and the mainstream, and thus effectively reduces energy loss during the flow process. Furthermore, the straight arrangement in the width direction ensures uniform division of the flow channel, avoiding local impact and separation losses caused by irregular structures.

[0052] Among them, an outer pipe guide vane 10 with the same direction of bending is fixedly connected between the outer pipe elbow and the inner pipe elbow. The outer pipe guide vane is located at the middle position between the inner and outer sides of the inner cavity of the outer pipe elbow and is arranged in a straight line in the width direction.

[0053] In this way, compared with the scheme of setting guide vanes only in the inner tube, setting guide vanes in the interlayer space between the inner tube and the outer sleeve can effectively cut off the circumferential flow path of the fluid in the interlayer, reduce its disturbance to the mainstream vortex in the central region, and further weaken the rotational intensity of the central vortex structure, thereby achieving a more significant drag reduction effect. Furthermore, the straight arrangement in the width direction ensures uniform channel segmentation, avoiding local impacts and separation losses caused by irregular structures.

[0054] The connecting inlet 3 is a rectangular hole, and its length is located between the connection position of the outer straight arm pipe and the inlet diffuser pipe on the inflow side and the position of the inner pipe bend; the connecting outlet 4 is a rectangular hole, and its length is located between the connection position of the outer straight arm pipe and the outlet diffuser pipe on the outflow side and the position of the inner pipe bend.

[0055] In this way, the fluid can flow smoothly from the inner tube into the outer tube through the gradually expanding structure, and then smoothly flow back from the outer tube to the inner tube through the gradually contracting structure; at the same time, it maximizes the effective flow area of ​​the connecting holes and achieves a smooth transition of the overall flow.

[0056] The connecting inlet 3 and the connecting outlet 4 are both located facing inwards, and the inner straight arm tube has a section of connecting pipe on the outside of the connecting inlet and the connecting outlet.

[0057] In this way, the direction of the upstream inlet opening of the elbow is consistent with the direction of the mainstream flow shifting inward under the action of centrifugal force. This can preferentially guide the fluid near the inside to enter the interlayer space in an orderly manner, which not only diverts and relieves pressure on the mainstream flow, but also avoids the high-speed fluid on the outside directly impacting the orifice and causing disturbance. Downstream of the elbow is a low-speed negative pressure zone. The rectangular outlet orifice opened here can allow the buffer fluid in the interlayer to be replenished to the mainstream area of ​​the inner pipe in the reverse direction, effectively alleviating the backflow phenomenon in the downstream low-speed stagnation zone, while balancing the pressure gradient inside and outside the elbow and suppressing the secondary flow enhancement caused by the pressure difference. Under the guiding effect of the inner arc side wall of the inner pipe elbow, the mainstream flow in the pipe becomes more gentle, which can avoid the formation of obvious low-speed stagnation zone and high-speed separation zone, and significantly reduce local flow resistance. Meanwhile, the inner tube adopts a non-disconnected design with the outer part retained and the opening only on the inner side, which can achieve multiple flow field control effects: the retained outer tube body forms a rigid guiding boundary, which not only constrains the mainstream to flow orderly along the inner side of the inner tube and avoids the formation of large-scale eddies by unconstrained diffusion, but also makes the inner tube and the straight tube arm form a smooth flow channel transition, reducing local disturbances and suppressing premature boundary layer separation; at the same time, this design allows the fluid exchange between the inner tube and the interlayer to be in an inner-side directional interconnection mode, avoiding direct impact between the interlayer fluid and the high-velocity mainstream on the outer side of the outer tube, reducing the velocity gradient and turbulent energy loss inside the tube.

[0058] Among them, the width of the connecting pipe body retained by the inner straight arm pipe on the outside of the connecting inlet is greater than that of the connecting pipe body retained by the inner straight arm pipe on the outside of the connecting outlet (that is, the opening depth of the connecting inlet is shallower and the opening is smaller, while the opening depth of the connecting outlet is deeper and the opening is larger).

[0059] Thus, setting the inlet width b1=0.5d allows the upstream pipe cross-section of the bend to maintain a symmetrical and regular double-vortex structure, avoiding additional energy dissipation caused by asymmetrical vortices of different sizes. The outlet width design b2=0.85d effectively widens the mainstream flow cross-section, which can transform the original two vortices with larger strength and smaller area into two vortices with smaller strength and larger area by widening the downstream flow area. This reduces drag, lowers the rotational kinetic energy and energy dissipation intensity of the vortex structure, weakens the continuous interference of large-scale vortices on the mainstream region, and achieves efficient control of the secondary flow in the bend.

[0060] The length of the connecting inlet is less than the length of the connecting outlet.

[0061] Therefore, a longer connecting outlet will result in better drag reduction.

[0062] The bending angle of the inner pipe elbow and the outer pipe elbow is 90 degrees.

[0063] The double-walled elbow structure is manufactured using the following method: First, the inner tube guide vanes inside the inner tube elbow are welded, and then the outer tube guide vanes on one side of the outer tube elbow are welded. Next, the inner tube elbow is installed into the outer tube elbow. First, the welding between the outer tube guide vanes already welded inside the outer tube elbow and the outer surface of the inner tube elbow is completed. Then, the outer tube guide vanes on the other side are inserted into the installation position along the arc direction and welded to the outer tube elbow and the inner tube elbow. Then, the inner tube straight arm pipe, the outer tube straight arm pipe, the inlet diffuser pipe, and the outlet diffuser pipe, which have been cut to connect the inlet and outlet, are welded in sequence.

[0064] This allows for convenient and quick processing and manufacturing of the internal structures in the double-walled bend pipe structure.

[0065] The dimensional parameters of the double-walled bend structure are determined using the following methods and steps:

[0066] Step 1: Using a regular bend (without connecting inlet and outlet in appearance, and meeting the pipe requirements of the location to be used) with the same dimensions as the inner pipe in the double-wall bend structure as a comparison, establish a computer virtual model together with the double-wall bend structure.

[0067] Step 2: Perform structured mesh generation on the model of ordinary bend pipe; perform unstructured mesh generation on the area where the inner and outer pipes connect, as well as the area around the connecting inlet and outlet (and other geometrically complex areas) in the model of double-wall bend pipe structure; perform structured mesh generation on the remaining (relatively regular) main pipe area.

[0068] Step 3: Use Fluent fluid simulation software to perform numerical simulation of fluid flow in the two structures respectively, and obtain the turbulent flow field structure inside each structure.

[0069] Step 4: Calculate the drag reduction rate based on the comparison of the two structures. Based on the drag reduction rate, return to Step 1 to adjust the outer tube diameter, inlet diffuser length, outlet converging tube length, outlet straight arm length, inlet straight arm length, outlet depth, and inlet depth of the double-walled bend structure. Repeat Steps 1 to 4 until the calculated drag reduction rate is maximized. The corresponding dimensional parameters at this point are the final parameters of the double-walled bend structure.

[0070] Step 5: Normalize the structured parameters that affect the resistance coefficient of the double-walled bend to a dimensionless value relative to the inner pipe diameter d, so that they are applicable to all bend diameters.

[0071] In step 1, the structural parameters are named as follows: inner diameter d of ordinary bend, curvature ratio r / d of ordinary bend, wall thickness s of ordinary bend; inner diameter d of inner pipe in double-wall bend, curvature ratio r / d of inner pipe in double-wall bend, wall thickness s of inner and outer pipes in double-wall bend, length of connecting inlet a1 of connecting inlet in double-wall bend, depth of connecting inlet b1 of connecting inlet in double-wall bend, length of connecting outlet a2 of connecting outlet in double-wall bend, depth of connecting outlet b2 of connecting outlet in double-wall bend, inner diameter D of outer pipe in double-wall bend, curvature ratio R / D of outer pipe in double-wall bend, length L1 of inlet expanding pipe in double-wall bend, length L2 of outlet contracting pipe in double-wall bend, length L3 of straight pipe arm of outer pipe on the inlet side of double-wall bend, and length L4 of straight pipe arm of outer pipe on the outlet side of double-wall bend.

[0072] In step 2, to comprehensively evaluate the overall flow loss and local flow field characteristics of the double-walled bend, the local resistance coefficient and the total pressure at key nodes near the wall upstream and downstream of the inner pipe bend are selected as core evaluation indicators. Mesh independence tests are then performed on both ordinary bends and double-walled bends. More specifically, the implementation can be as follows: Figure 4 As shown, six sets of gradient-refined meshes were designed (445,500, 1,045,000, 1,430,100, 1,793,800, 2,455,300, and 3,030,900). When the mesh count reached 2,455,300, the fluctuation of the local drag coefficient was less than 1.2%, and the fluctuation of the nodal pressure was less than 5%, both meeting the accuracy requirements for engineering calculations. Therefore, a mesh count of 2,455,300 was selected to study the drag reduction characteristics of double-walled bends. The traditional bend mesh generation also followed the same mesh independence verification principle.

[0073] In step 3, during the numerical simulation, incompressible liquid water is selected as the fluid inside the pipe; The model is a turbulence model; the SIMPLEC algorithm is selected for solution, and the momentum equation, energy equation, and turbulent transport equation are all solved in second-order upwind form. The convergence criterion for the computational residuals of each equation is set to [value missing]. The scale; the boundary conditions are set as follows: the equivalent roughness of the pipe wall is set to 0.5 mm, the inlet is defined as a velocity inlet, and the outlet is defined as a free outflow.

[0074] In step 4, during implementation, such as Figure 5 As shown, section 1 is located 20D upstream of the bend and section 2 is located 20D downstream of the bend. The average static pressure of the two sections is obtained by area-weighted averaging. Thus, the total resistance loss of the bend section is obtained. Simultaneously, an equivalent straight pipe model without bends was established to obtain the frictional resistance loss under the same pipe length. Local resistance coefficient of bend The calculation formula is as follows:

[0075]

[0076] In the formula, The density of water, ; The average velocity at the pipe inlet is denoted as , in m / s.

[0077] Calculate the local resistance coefficients for ordinary pipe bends and double-walled pipe bends respectively. Calculate the drag reduction rate based on the local drag coefficient. :

[0078] .

[0079] In step 5, the final dimensionless representation of each parameter is as follows: L1=0.8d, L2=0.8d, L3=3d, L4=5d, h=0.2d, D=d+2s+2h; a1=L1+L3=3.8d, b1=0.5d, a1=L2+L4=5.8d, b1=0.85d.

[0080] Thus, numerical simulations using Fluent fluid simulation software enable in-depth analysis of the evolution of turbulent structures and flow mechanisms within pipelines, obtaining accurate flow data and providing a reliable basis for engineering optimization design. A hybrid mesh generation strategy is employed for the double-walled bend model, which more accurately captures fluid flow details, improving simulation accuracy and the reliability of results. In this scheme, the Realizable k-ε turbulence model is selected. By reconstructing the turbulent viscosity coefficient expression and introducing flow curl constraints, the prediction accuracy of velocity distribution, pressure changes, and turbulent fluctuations within the bend is significantly improved, providing theoretical support for the calculation of local drag coefficients. The SIMPLEC algorithm is used for numerical solution to handle pressure-velocity coupling, optimizing the pressure correction equation, effectively reducing numerical oscillations during iteration, and accelerating convergence. Simultaneously, a second-order upwind scheme is adopted, incorporating gradient information from adjacent mesh points to reduce numerical dispersion errors and more accurately capture local changes in the flow field. To ensure computational accuracy, the residual convergence criterion for each equation is set to 10⁻. 6 The equivalent roughness of the pipe wall is set to 0.5 mm to align with actual engineering conditions; a velocity inlet is used, and the outlet is set to free outflow to avoid interference from boundary conditions on the downstream flow field. Therefore, this method effectively evaluates the improvement effect of structural parameters on flow resistance by calculating the drag reduction rate, providing a quantitative basis for optimizing pipeline configuration and thus improving engineering economy. By performing dimensionless optimization of structural parameters, the resulting structure can be directly applied to bend structures with different dimensional parameters, enhancing the adjustability and adaptability of double-wall bend design, enabling it to flexibly match diverse flow conditions and pipeline configuration requirements.

Claims

1. A double-wall elbow structure comprising an inner pipe composed of an intermediate inner pipe elbow and inner pipe straight arm pipes of equal diameter integrally connected at both ends of the inner pipe elbow, characterized in that, The outer tube is arranged outside the inner tube, and the outer tube comprises an outer tube elbow arranged outside the inner tube elbow and an outer tube straight arm tube arranged outside the inner tube straight arm tube.

2. The double-wall elbow structure according to claim 1, wherein The inner tube elbow is internally and fixedly connected with an inner tube guide vane which is curved in the same direction.

3. The double wall elbow structure of claim 1, wherein, The outer tube elbow and the inner tube elbow are fixedly connected with an outer tube guide vane which is curved in the same direction.

4. The double wall elbow structure of claim 1, wherein, The communication inlet is a rectangular hole, and the length direction is located between the connection position of the outer tube straight arm tube on the inflow side and the inlet expansion tube and the inner tube elbow.

5. The double wall elbow structure of claim 1, wherein, The communication inlet and the communication outlet are arranged opposite to the inner side, and the inner tube straight arm tube is reserved with a connecting pipe body outside the communication inlet and the communication outlet.

6. The double wall elbow structure of claim 5, wherein, The width of the connecting pipe body reserved outside the inner tube straight arm tube at the communication inlet is greater than that at the communication outlet.

7. The double wall elbow structure of claim 5, wherein The length of the communication inlet is less than that of the communication outlet.

8. The double-wall elbow structure of claim 1, wherein The double-wall elbow pipe structure is obtained by the following method: welding the inner tube guide vane inside the inner tube elbow, welding the outer tube guide vane on one side inside the outer tube elbow, installing the inner tube elbow into the outer tube elbow, welding the outer tube guide vane inside the outer tube elbow and the outer surface of the inner tube elbow, inserting the outer tube guide vane on the other side into the installation position along the curvature direction and welding the outer tube guide vane on the other side and the outer tube elbow and the inner tube elbow, and welding the inner tube straight arm tube, the outer tube straight arm tube, the inlet expansion tube and the outlet expansion tube which are cut with the communication inlet and the communication outlet.

9. The double-wall elbow structure of claim 1, wherein The size parameters of the double-wall elbow pipe structure are determined by the following method steps: Step 1: an ordinary elbow pipe with the same size as the inner tube in the double-wall elbow pipe structure is taken as a comparative example, and a computer virtual model is established for each of the two structures. Step 2: the model of the ordinary elbow pipe is structured and meshed, the model of the double-wall elbow pipe structure is meshed with unstructured meshes at the connection between the inner tube and the outer tube and the peripheral area of the communication inlet and the communication outlet, and the remaining pipe body area is structured and meshed. Step 3: the Fluent fluid simulation software is used to simulate the fluid flow of the two structures respectively, and the turbulent flow field structure inside each structure is obtained. Step 4, calculate the drag reduction rate according to the comparison of two structures, according to the drag reduction rate, return to step 1 to adjust the outer tube diameter, inlet gradual expansion tube length, outlet gradual contraction tube length, outlet side of the outer tube straight arm tube length, inlet side of the outer tube straight arm tube length, the depth of the communication outlet and the depth of the communication inlet in the double-wall elbow structure; repeat steps 1 to 4 again until the calculated drag reduction rate is maximum, at this time the corresponding size parameter is the final parameter of the double-wall elbow structure; Step 5, normalize the structural parameters affecting the drag coefficient of the double-wall elbow relative to the inner tube diameter d in multiples to make it applicable to all elbow diameters.

10. The double-wall elbow structure of claim 9, wherein In step 1, the structural parameters are named as: ordinary elbow inner diameter d, ordinary elbow curvature ratio r / d, ordinary elbow wall thickness s; double-wall elbow inner diameter d, double-wall elbow inner tube curvature ratio r / d, double-wall elbow inner tube and outer tube wall thickness s, double-wall elbow communication inlet length a1, double-wall elbow communication inlet depth b1, double-wall elbow communication outlet length a2, double-wall elbow communication outlet depth b2, double-wall elbow outer tube diameter D, double-wall elbow outer tube curvature ratio R / D, double-wall elbow inlet gradual expansion tube length L1, double-wall elbow outlet gradual contraction tube length L2, double-wall elbow inlet side of the outer tube straight tube arm length L3, double-wall elbow outlet side of the outer tube straight tube arm length L4; In step 2, to comprehensively evaluate the overall flow loss and local flow field characteristics of the double-wall elbow, the local resistance coefficient and the total pressure of the inner elbow and its upstream and downstream near-wall key nodes are selected as the core evaluation indexes, and the grid independence test is carried out for the ordinary elbow and the double-wall elbow respectively; In step 3, during the numerical simulation, incompressible liquid water was selected as the fluid inside the pipe; The model is a turbulence model; the SIMPLEC algorithm is selected for solution, and the momentum equation, energy equation, and turbulent transport equation are all solved in second-order upwind form. The convergence criterion for the computational residuals of each equation is set to [value missing]. The magnitude; the boundary conditions are set as follows: the equivalent roughness of the pipe wall is set to 0.5 mm, the inlet is defined as a velocity inlet, and the outlet is defined as a free outflow; In step 4, the upstream 20D of the elbow pipe is selected as section 1, and the downstream 20D is selected as section 2, and the area weighted average is used to obtain the average static pressure of the two sections , thereby obtaining the total resistance loss of the elbow pipe section ; at the same time, an equivalent straight pipe model without elbow is established to obtain the friction resistance loss under the same pipe length ; and the calculation formula of the local resistance coefficient of the elbow pipe is as follows ​ ； wherein is the density of water, ; is the average velocity at the pipe inlet, m / s; The local resistance coefficients of the common elbow and the double-wall elbow are calculated respectively as The drag reduction rate is calculated according to the local resistance coefficients : ； In step 5, the final dimensionless representation of each parameter is: L1=0.8d, L2=0.8d, L3=3d, L4=5d, h=0.2d, D=d+2s+2h; a1=L1+L3=3.8d, b1=0.5d, a1=L2+L4=5.8d, b1=0.85d.

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