Design method and structure for reducing resistance of ventilation and air conditioning duct

Abstract

The application discloses a design method for reducing the resistance of a ventilation and air conditioning pipeline, which comprises the following steps: S01, determining the size and form of the air pipe to be optimized; S02, changing the value of theta by dividing the wall surface of the variable-diameter section with the equal length L, listing the position of point P, and establishing the model of different P points; S03, carrying out numerical simulation on the model, exporting the simulation result, calculating the value of the local resistance coefficient ζ, and finding the position of P' point when ζ' is the smallest; S04, carrying out grid search on the basis of P', adjusting the values of Δx and Δy, repeating the step S03, further optimizing, and finding P'' which makes ζ' smaller; and S05, the position of P point when the value of the local resistance coefficient ζ is the smallest can be obtained through the above steps, and the low-resistance ventilation pipeline structure is determined. The design method and structure for reducing the resistance of the ventilation and air conditioning pipeline are provided, the optimized variable-diameter form effectively reduces the local resistance at the variable-diameter position, and the method is convenient for processing and manufacturing in the actual field and low in cost.

Description

Technical Field

[0001] This invention relates to the field of ventilation and air conditioning duct resistance reduction technology, specifically a design method and structure for reducing the resistance of ventilation and air conditioning ducts. Background Technology

[0002] With the rapid urbanization in my country, the building area has increased significantly. According to national data reports, my country's total energy consumption in 2021 was 5.2 billion tons of standard coal equivalent, of which the construction industry accounted for 33%. Building energy consumption mainly occurs during the construction and operation phases, with operational energy consumption accounting for approximately 80% of total building energy consumption. Ventilation and air conditioning systems have become an indispensable part of modern building operation, accounting for about 30-50% of building energy consumption, and fan energy consumption within ventilation systems accounts for 30-50% of the total energy consumption of ventilation systems.

[0003] The energy consumption of fans in ventilation and air conditioning duct systems is mainly caused by frictional resistance and local resistance. Local resistance (energy loss caused by boundary layer separation and vortex generation due to changes in flow direction and velocity when fluid flows through local components) accounts for 40% to 60% of the total resistance of ventilation and air conditioning ducts. It can be seen that the energy consumption caused by local components of the duct is significant. Therefore, optimizing the form of local components of ventilation and air conditioning duct systems (including valves, branch tees, confluence tees, elbows, etc.) to reduce the resistance of the duct system is a solution.

[0004] For example, Chinese patent CN113137737B discloses a ventilation duct that can effectively reduce the boundary layer height at the point of maximum turbulence during diameter change, thereby reducing energy dissipation and resistance during diameter change.

[0005] Chinese patent CN113639276A discloses a three-way flue for preventing smoke leakage and its control method. By optimizing the shape and position of the guide vanes in the three-way flue, smoke leakage can be prevented while reducing the energy consumption of the fan.

[0006] Chinese patent CN110081185A discloses a wind valve in a ventilation and air conditioning system, which can achieve a significant drag reduction effect compared to a traditional single-leaf valve under different wind speeds with the same diameter.

[0007] Among the existing comparative patents mentioned above, few inventions address reducing local resistance at diameter changes while maintaining a simple and easy-to-manufacture structure. This invention aims to provide a design method and structure for reducing the resistance of ventilation and air conditioning ducts, which can reduce local resistance at diameter changes and facilitate on-site processing and manufacturing at low cost. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a design method and structure for reducing the resistance of ventilation and air conditioning ducts, with the aim of solving the aforementioned problems.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A design method for reducing the resistance of ventilation and air conditioning ducts includes the following steps:

[0011] S01. Determine the size and form of the duct to be optimized;

[0012] S02. By dividing the wall of the variable diameter section with an equal length of L, change the value of θ, list the positions of point Pi, and establish models of different points Pi.

[0013] S03. Perform numerical simulation on the model, export the simulation results and calculate the value of the local drag coefficient ζ, and find the location of point P' when ζ' is minimum.

[0014] S04. Based on P', perform a grid search. By adjusting the values ​​of Δx and Δy, repeat step S03 to further optimize and find a P'' that makes ζ' smaller.

[0015] S05. After the above steps, the location of point P under the condition of minimum local resistance coefficient ζ can be obtained, and the structure of low-resistance ventilation duct can be determined.

[0016] Preferably, in S01, the ventilation and air conditioning duct is configured to consist of an air inlet straight duct section 1, a reducing section 2, and an air outlet straight duct section 3, wherein:

[0017] The dimensions of the inlet straight duct section 1 are a1×b1 (width×height), and the dimensions of the outlet straight duct section 3 are a2×b2 (width×height). The hydraulic diameter of the inlet straight duct section 1 is... Hydraulic diameter of straight air outlet pipe section 3 The calculation formula is as follows.

[0018] (1)

[0019] (2)

[0020] The variable diameter section 2 is connected between the inlet straight pipe section 1 and the outlet straight pipe section 3, and is used as a transition section when the size of the straight pipe section changes. The variable diameter section 2 can be in the form of a bottom flat eccentric tapered pipe, a bottom flat eccentric tapered pipe, a top flat eccentric tapered pipe, a top flat eccentric tapered pipe, a double-sided tapered pipe, and a double-sided tapered pipe.

[0021] Preferably, in S02, the wall length of the variable diameter section 2 is defined as L, the intersection of the inlet straight pipe section 1 and the variable diameter section 2 is defined as O1, and the intersection of the variable diameter section 2 and the outlet straight pipe section 3 is defined as O2.

[0022] For a tapered pipe, the angle between the inclined wall of the reducing section 2 and the extension line of the straight outlet pipe section 3 is defined as θ.

[0023] For a gradually expanding pipe, the angle between the extension line of the straight inlet pipe section 1 and the inclined wall of the variable diameter section 2 is defined as θ;

[0024] The process of establishing different Pi point models includes:

[0025] Step 1: Divide the wall of length L into four equal parts at points A, B, and C, resulting in four lines of length L / 4.

[0026] Step 2: Next, with O2 as the center, draw three curves, curve 1, curve 2 and curve 3, with radii of O2A, O2B and O2C respectively, and assume point P on each of these curves;

[0027] Step 3: By adjusting the angle of the variable diameter section wall each time, θ is decreased or increased by 2 degrees. The intersection of the straight line extended by angle θ and curves 1, 2, and 3 is point P.

[0028] Step 4: Assume Pi on curves 1, 2 and 3. For each Pi, the wall of the variable diameter section 2 is divided into two new parts, forming a new shape of variable diameter section 2. Based on this, model the ventilation duct.

[0029] Preferably, in S03, after modeling is completed, a structural mesh is selected for mesh generation. The fluid flow of different models is simulated using CFD simulation software. The RSM turbulence model is selected, the near-wall function is the scalable wall function, the pipe inlet condition is set to velocity-inlet, the pipe outlet condition is set to pressure-outlet, the wall boundary condition of other duct sections is set to Wall, the wall adopts a no-slip boundary condition, and the pipe wall roughness is set to 0.15mm.

[0030] The pressure-velocity coupling method adopts the SIMPLE algorithm. The discretization of momentum, turbulent kinetic energy, turbulent dissipation rate and Reynolds stress adopts the second-order upwind scheme, and the pressure discretization adopts the Standard form. The steady-state method is used for calculation.

[0031] After the numerical simulation converges, the plane pressure and distance data are exported, and the local drag coefficient ζ is calculated using the following formula:

[0032] (3)

[0033] In formula (3)

[0034] (4)

[0035] (5)

[0036] in, This represents the total pressure difference between section P3 and section P4, in Pa. This represents the static pressure difference between the pressure test section P1 and the test surface P2, in Pa. and L1-3 and L4-2 represent the dynamic pressures at pressure measurement sections P1 and P2, respectively, in Pa; L1-3 and L4-2 represent the distances from section P3 to pressure test section P1 and from section P4 to pressure measurement section P2, respectively. and The friction resistances between pressure measurement sections P1 and P3 and between section P4 and P2 are respectively represented by equations (6) and (7), and are calculated in Pa.

[0037] (6)

[0038] (7)

[0039] In the formula, This represents the static pressure difference between pressure measurement sections P1 and P3, in Pa. This represents the static pressure difference between section P4 and pressure measurement section P2, in Pa.

[0040] Following the above steps, the local resistance coefficient ζ' of the duct modeled at different points Pi is calculated. The drag reduction rate rζ of the local resistance loss is then calculated using the following formula.

[0041] (8)

[0042] Where ζ and ζ' represent the local resistance coefficient of the traditional ventilation duct without changing its shape and the local resistance coefficient of the ventilation duct after changing its shape, respectively.

[0043] The optimal position P' corresponding to the model with the smallest local drag coefficient ζ' (i.e., the largest drag reduction ratio rζ) in the same batch of models is the P' of the model found in this step.

[0044] Preferably, in S04, the optimal point is found by continuing to search through a "grid search":

[0045] Point P' is moved laterally by Δx and vertically by Δy. The values ​​of Δx and Δy are determined according to the scale of the model, and N points can be determined (e.g., Figure 7 (The 20 points shown in the diagram)

[0046] Then, for the determined N points, repeat step S03 to perform duct modeling and numerical simulation, and continue to compare the local drag coefficient ζ values ​​to determine whether there is a P'' position that makes the local drag coefficient ζ' obtained in step S03 smaller (the drag reduction rate rζ larger).

[0047] A ventilation duct structure obtained by a design method to reduce the resistance of ventilation and air conditioning ducts.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] This invention provides a design method and structure for reducing the resistance of ventilation and air conditioning ducts. The optimized diameter change effectively destroys the large vortex generated at the end, dividing the large vortex into two smaller vortices, weakening the fluid deformation at the connection, making the velocity distribution at the end of the diameter change more symmetrical, and at the same time reducing the impact of the vortex on the velocity downstream of the pipe section. Compared with the original pipe, the optimized pipe section has a reduced velocity gradient change rate close to the wall in the lower straight section, thus achieving the effect of reducing resistance. It is also easy to process and manufacture in actual field and has low cost. Attached Figure Description

[0050] Figure 1 This is a specific set of steps in a design method to reduce the resistance of ventilation and air conditioning ducts;

[0051] Figure 2 This is a schematic diagram of the ventilation duct structure of the present invention;

[0052] Figure 3 This is a schematic diagram of different ventilation duct diameter variations according to the present invention;

[0053] Figure 4 This is a schematic diagram illustrating the principle of finding Pi in step S02 of a design method for reducing ventilation duct resistance.

[0054] Figure 5 This is a schematic diagram illustrating the location of point Pi under different conditions in step S02 of a design method for reducing ventilation duct resistance.

[0055] Figure 6 This is a schematic diagram of the calculation section of the local resistance coefficient in step S03 of the duct size requirements and design method when the fluid inside the ventilation duct reaches full development.

[0056] Figure 7 This is a schematic diagram illustrating the principle of further mesh search in step S04 of a design method for reducing ventilation duct resistance;

[0057] Figure 8 This is a schematic diagram of the duct dimensions in the embodiment;

[0058] Figure 9 This is a diagram showing the optimized duct resistance results in the embodiment. Figure 9 (a) is the result of optimization in steps S02 and S03 of the embodiment. Figure 9 (b) is the result of the grid search re-optimization in step S04 of the embodiment;

[0059] Figure 10This is the duct structure finally determined in step S05 of the embodiment;

[0060] Figure 11 This is a comparison chart of drag reduction rates under different wind speeds for the embodiments;

[0061] Figure 12 This is a comparison of velocity cloud maps from the embodiments.

[0062] In the diagram, 1 is the straight inlet duct section; 2 is the variable diameter section; and 3 is the straight outlet duct section. Detailed Implementation

[0063] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0064] A design method for reducing the resistance of ventilation and air conditioning ducts mainly consists of the following steps.

[0065] Step S01: Determine the size and form of the duct to be optimized.

[0066] Specifically, such as Figure 2 As shown, the ventilation and air conditioning duct consists of an inlet straight pipe section 1, a reducing section 2, and an outlet straight pipe section 3. The dimensions of the inlet straight pipe section 1 are a1×b1 (width×height), and the dimensions of the outlet straight pipe section 3 are a2×b2 (width×height). The hydraulic diameter of the inlet straight pipe section 1 is... Hydraulic diameter of straight air outlet pipe section 3 The calculation formula is as follows.

[0067] (1)

[0068] (2)

[0069] The variable diameter section 2 connects the inlet straight pipe section 1 and the outlet straight pipe section 3, serving as a transition section when the size of the straight pipe section changes. The form of the variable diameter section 2 includes, for example, Figure 3 The tubes shown are: bottom flat eccentric tapered tube, bottom flat eccentric expanding tube, top flat eccentric tapered tube, top flat eccentric expanding tube, double-sided expanding reducing tube, and double-sided tapered reducing tube.

[0070] In the second step S02, by dividing the wall of the variable diameter section with an equal length of L, the value of θ is changed, the position of point Pi is listed, and a model of different points Pi is established.

[0071] Specifically, firstly, such as Figure 3The wall length of the variable diameter section 2 is defined as L. The intersection point of the inlet straight pipe section 1 and the variable diameter section 2 is defined as O1, and the intersection point of the variable diameter section 2 and the outlet straight pipe section 3 is defined as O2. For a converging pipe, the angle between the inclined wall of the variable diameter section 2 and the extension line of the outlet straight pipe section 3 is defined as θ; for a diverging pipe, the angle between the extension line of the inlet straight pipe section 1 and the inclined wall of the variable diameter section 2 is defined as θ.

[0072] by Figure 3 (a) Taking the bottom-flat eccentric tapered tube as an example, the optimization method is explained. The upper wall surface of length L is divided into four equal parts by points A, B, and C, resulting in four lines of length L / 4. Then, with O2 as the center, three curves, curve 1, curve 2, and curve 3, with radii O2A, O2B, and O2C respectively are drawn, as shown below. Figure 4 As shown. Assume point P on each of these curves. By adjusting the angle of the wall surface on the variable diameter section, decreasing or increasing angle θ by 2 degrees each time, the intersection point of the straight line extended by angle θ with curves 1, 2, and 3 is point P. For example... Figure 5 As shown in (a), assume Pi on curve A; as Figure 5 (b) Assume Pi on curve B; as Figure 5 (c) Assume Pi on curve C. For each Pi, the upper wall of the variable diameter section 2 is divided into two new parts, forming a new shape for the variable diameter section 2. Based on this, the ventilation duct is modeled, and the length requirements of the upstream and downstream straight pipe sections are as follows: Figure 6 As shown.

[0073] In the above methods, to reduce the quality of modeling work or to make the results more accurate, the model can be divided into thirds, fifths, sixths, etc. The angle θ can also be adjusted more coarsely or finely, using 3 degrees, 1 degree, 0.5 degrees, etc. as variables. The optimization methods for other variable diameter forms are the same as above, and will not be elaborated here.

[0074] In the third step S03, numerical simulation is performed on the model, the simulation results are exported and the value of the local drag coefficient ζ is calculated, and the location of point P' when ζ' is minimum is found.

[0075] Specifically, after modeling, a structured mesh was selected for mesh generation. CFD simulation software was used to simulate fluid flow in different models. The RSM turbulence model was chosen, with the scalable wall function selected for the near-wall function. The pipe inlet condition was set to velocity-inlet, the pipe outlet condition to pressure-outlet, and the wall boundary condition for other duct sections to be Wall. No-slip boundary conditions were used on the walls, and the wall roughness was set to 0.15 mm. The pressure-velocity coupling method employed the SIMPLE algorithm. Momentum, turbulent kinetic energy, turbulent dissipation rate, and Reynolds stress were discretized using a second-order upwind scheme, while pressure discretization used the Standard form. Steady-state calculations were performed.

[0076] After the numerical simulation converges, according to Figure 6 The plane pressure and distance data are exported as shown, and the local drag coefficient ζ is calculated using the following formula:

[0077] (3)

[0078] In formula (3)

[0079] (4)

[0080] (5)

[0081] in, This represents the total pressure difference between section P3 and section P4, in Pa. This represents the static pressure difference between pressure measurement sections P1 and P2, in Pa. and L1-3 and L4-2 represent the dynamic pressures at pressure measurement sections P1 and P2, respectively, in Pa; L1-3 and L4-2 represent the distances from section P3 to pressure measurement section P1 and from section P4 to pressure measurement section P2, respectively. and Let P1 and P3 represent the friction resistance between pressure measurement sections and P4, respectively, calculated using equations (6) and (7), in Pa.

[0082] (6)

[0083] (7)

[0084] In the formula, This represents the static pressure difference between pressure measurement sections P1 and P3, in Pa. This represents the static pressure difference between section P4 and pressure measurement section P2, in Pa.

[0085] Following the above steps, the local resistance coefficient ζ' of the duct modeled at different points Pi is calculated. The drag reduction rate rζ of the local resistance loss is then calculated using the following formula.

[0086] (8)

[0087] Where ζ and ζ' represent the local resistance coefficient of a traditional ventilation duct without changing its shape and the local resistance coefficient of a ventilation duct after changing its shape, respectively.

[0088] The optimal position P' corresponding to the model with the smallest local drag coefficient ζ' (i.e., the largest drag reduction ratio rζ) in the same batch of models is the P' of the model found in this step.

[0089] In step S04, a grid search is performed based on P'. By adjusting the values ​​of Δx and Δy, step S03 is repeated to further optimize the search and find a P'' that makes ζ' smaller.

[0090] Specifically, such as Figure 7 As shown, within a small distance, there may be other locations around point P' where the local drag coefficient is smaller. We continue searching for the optimal point using a "grid search". Moving point P' laterally by Δx and longitudinally by Δy, where the values ​​of Δx and Δy are determined according to the model's scale, can identify N points (e.g., ...). Figure 7 (The 20 points shown in the diagram) are then used to determine N points. Step S03 is repeated to perform duct modeling and numerical simulation. The local drag coefficient ζ value is then compared to determine if there is a P'' position that makes the local drag coefficient ζ' obtained in step S03 smaller (the drag reduction rate rζ is larger).

[0091] Step 5, S05: After the above steps, the location of point P under the condition of minimum local resistance coefficient ζ can be obtained, and the structure of low-resistance ventilation duct can be determined.

[0092] Example 1

[0093] This explanation will be based on two duct sections with dimensions of 250mm x 250mm and 250mm x 160mm, respectively. Figure 8 As shown, the inlet straight pipe section 1 is 4500mm long, the diameter-reducing section is 155.88mm long, the outlet straight pipe section 3 is 3200mm long, the original angle θ is 30°, and the duct type is bottom flat eccentric tapering diameter-reducing.

[0094] Take it as Figure 5 The diagram shows a four-part division, generating points A, B, and C. Then, with O2 as the center, three curves (1, 2, and 3) with radii O2A, O2B, and O2C are drawn respectively. Next, the angles of the upper wall of the variable-diameter section are successively increased or decreased by 2° to determine different points Pi. Excluding the original 30° angle of the variable-diameter section, there are 16 possible cases: 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 32, 34, 36, 38, 40, and 42. Therefore, a total of 48 models need to be modeled and simulated for comparison of their local drag coefficient ζ.

[0095] The compiled simulation results are as follows Figure 9 As shown in (a), it can be found that the curve 1 at point A, combined with the new type of variable diameter formed by the 18° angle, has the best effect in 48 working conditions with a smaller local drag coefficient (drag reduction rate rζ=5.914%). This point is the P' found in step S03.

[0096] Next, based on P', move its position in the x and y plane with P' as the center point, and set Δx and Δy to 5mm, thus generating the following... Figure 9 (b) shows the locations of 25 points including P'. These models are modeled sequentially and their local drag coefficients ζ are simulated and compared.

[0097] The compiled simulation results are as follows Figure 9 As shown in (b), it can be found that when Δx=10mm and Δy=-5mm, the new type of variable diameter formed at this point has a smaller local drag coefficient (drag reduction rate rζ=6.219%) in 25 working conditions, and the effect is the best. This point is the P'' found in step S04, and the coordinates of P'' are (4623.083,168.906).

[0098] Thus, it is now confirmed. Figure 10 The low-resistance ventilation duct structure shown in Example 1.

[0099] Further analysis:

[0100] (1) The effect of wind speed on the drag reduction rate of the new variable diameter model

[0101] The above operating conditions were all conducted under a wind speed of 7 m / s. To verify the drag reduction effect at different wind speeds, simulations were performed at wind speeds of 3-13 m / s. The pressure difference between the test sections before and after the novel diameter reducing device and the traditional ventilation duct was derived. as well as The values ​​and calculated drag reduction ratios are shown in the table below:

[0102] Table 1 Pressure difference between traditional ventilation ducts and novel variable diameter test sections and ζ value

[0103]

[0104] like Figure 11 As shown, at wind speeds of 3-13 m / s, the drag reduction effect of the new variable diameter device is relatively significant compared to traditional ventilation ducts. The drag reduction rates are 5.875%~29.839%.

[0105] (2) Velocity cloud diagram analysis of new variable diameter duct and traditional ventilation duct

[0106] This study simulates the velocity field and extracts the x0y plane when Z equals 1 / 2 of the pipe width (i.e., Z = 125 mm), observing and comparing the velocity field within the diameter variation before and after optimization. Figure 12As can be seen from the comparison diagram, due to the structural change of dividing the original single wall into two connected walls, the optimized diameter variation effectively disrupts the large vortex generated at the end, dividing the large vortex into two smaller vortices. This weakens the fluid deformation at the connection point, making the velocity distribution at the end of the diameter variation more symmetrical. Simultaneously, the influence of the vortex on the downstream velocity of the pipe section is also reduced. Compared to the original pipe, the optimized pipe's lower straight section has a smaller rate of change of velocity gradient close to the wall, thus achieving the effect of reducing resistance.

[0107] The preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0108] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0109] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content invented by this disclosure.

Claims

1. A design method for reducing the resistance of ventilation and air conditioning ducts, characterized in that, Includes the following steps: S01. Determine the size and form of the duct to be optimized; S02. By dividing the wall of the variable diameter section with an equal length of L, changing the value of θ, listing the positions of point Pi, and establishing models of different points Pi; In S02, the wall length of the variable diameter section is defined as L, the intersection of the inlet straight pipe section and the variable diameter section is defined as O1, and the intersection of the variable diameter section and the outlet straight pipe section is defined as O2. For a tapered pipe, the angle between the inclined wall of the variable diameter section and the extension line of the straight outlet pipe section is defined as θ. For a gradually expanding pipe, the angle between the extension line of the straight inlet pipe section and the inclined wall of the variable diameter section is defined as θ. The process of establishing different Pi point models includes: Step 1: Divide the wall of length L into four equal parts at points A, B, and C, resulting in four lines of length L / 4. Step 2: Next, with O2 as the center, draw three curves with radii O2A, O2B and O2C respectively, and assume point P on each of these curves; Step 3: By adjusting the angle of the variable diameter section wall each time, θ is decreased or increased by 2 degrees. The intersection of the straight line extended by angle θ and the curve is point P. Step 4: Assume Pi on the curve. For each Pi, the wall of the variable diameter section is divided into two new parts, forming a variable diameter section with a new shape. Based on this, model the ventilation duct. S03. Perform numerical simulation on the model, export the simulation results and calculate the value of the local drag coefficient ζ, and find the location of point P' when ζ' is minimum. S04. Based on P', perform a grid search. By adjusting the values ​​of Δx and Δy, repeat step S03 to further optimize and find a P'' that makes ζ' smaller. S05. After the above steps, the location of point P is obtained when the local resistance coefficient ζ is minimized, and the low-resistance ventilation duct structure is determined.

2. The design method for reducing the resistance of ventilation and air conditioning ducts according to claim 1, characterized in that: In S01, the ventilation and air conditioning duct is configured to consist of an inlet straight duct section, a reducing section, and an outlet straight duct section, wherein: The dimensions of the inlet straight duct section are a1×b1, and the dimensions of the outlet straight duct section are a2×b2. The hydraulic diameter of the inlet straight duct section is... Hydraulic diameter of the straight exhaust pipe section The calculation formula is as follows: ① ② The variable diameter section connects the inlet straight pipe section and the outlet straight pipe section, and serves as a transition section when the size of the straight pipe section changes. The variable diameter section can take the form of a bottom-flat eccentric tapered pipe, a bottom-flat eccentric expander pipe, a top-flat eccentric tapered pipe, a top-flat eccentric expander pipe, a double-sided expander pipe, and a double-sided tapered pipe.

3. The design method for reducing the resistance of ventilation and air conditioning ducts according to claim 1, characterized in that: In S03, after modeling is completed, structural mesh is selected for mesh generation. CFD simulation software is used to simulate the fluid flow of different models. The RSM turbulence model is selected, the near-wall function is the scalable wall function, the pipe inlet condition is set to velocity-inlet, the pipe outlet condition is set to pressure-outlet, the wall boundary condition of other duct sections is set to Wall, the wall adopts no-slip boundary condition, and the pipe wall roughness is set to 0.15mm. The pressure-velocity coupling method adopts the SIMPLE algorithm. The discretization of momentum, turbulent kinetic energy, turbulent dissipation rate and Reynolds stress adopts the second-order upwind scheme, and the pressure discretization adopts the Standard form. The steady-state method is used for calculation. After the numerical simulation converges, the plane pressure and distance data are exported, and the local drag coefficient ζ is calculated using the following formula: ③ In formula ③ ④ ⑤ in, This represents the total pressure difference between section P3 and section P4, in Pa. This represents the static pressure difference between pressure measurement sections P1 and P2, in Pa. and L1-3 and L4-2 represent the dynamic pressures at pressure measurement sections P1 and P2, respectively, in Pa; L1-3 and L4-2 represent the distances from section P3 to pressure measurement section P1 and from section P4 to pressure measurement section P2, respectively. and The friction resistances between pressure measurement sections P1 and P3 and between P4 and P2 are respectively represented, calculated using equations ⑥ and ⑦, in Pa. ⑥ ⑦ In the formula, This represents the static pressure difference between pressure measurement sections P1 and P3, in Pa. This represents the static pressure difference between section P4 and pressure measurement section P2, in Pa. Following the above steps, the local resistance coefficient ζ' of the duct modeled at different points Pi is calculated. The drag reduction rate rζ of the local resistance loss is then calculated using the following formula. ⑧ Where ζ and ζ' represent the local resistance coefficient of the traditional ventilation duct without changing its shape and the local resistance coefficient of the ventilation duct after changing its shape, respectively. The optimal position P' corresponding to the model with the smallest local resistance coefficient ζ' among the same batch of models is the P' of the model found in this step.

4. The design method for reducing the resistance of ventilation and air conditioning ducts according to claim 1, characterized in that: In S04, the optimal point is found again through "grid search": Move point P' laterally by Δx and vertically by Δy. The values ​​of Δx and Δy are determined according to the scale of the model, and N points are determined. Then, for the determined N points, repeat step S03 to perform duct modeling and numerical simulation, and continue to compare the local resistance coefficient ζ values ​​to determine whether there is a P'' position that makes the local resistance coefficient ζ' obtained in step S03 smaller.

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