A porous wing tip structure for suppressing leakage flow in axial flow machinery and its design method

Abstract

The present invention discloses a porous wingtip structure and a design method thereof for suppressing leakage flow in an axial flow machine. The present invention selects a reasonable porous medium material, processes a partial area of ​​the wingtip, and replaces the original wingtip with a porous medium wingtip. The porous medium has two effects on the leakage flow: damping and drainage. First, the leakage flow velocity is weakened by damping, so that an effective low-speed damping layer is formed in the porous wingtip. Second, the low-speed damping layer is discharged to the vicinity of the gap by drainage, thereby enhancing the dissipation degree of the vortex system and weakening the strong interference effect of the rigid blades on the flow at the same time, thereby weakening the wingtip separation vortex, and then weakening the tip gap vortex system and the tip gap cavitation intensity, reducing related negative effects.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluid machinery, and in particular relates to a porous wing tip structure for suppressing leakage flow in an axial flow machinery and a design method thereof. Background Art

[0002] Axial flow fluid machinery is widely used in my country's industrial and civil fields, such as fans, pumps, compressors, etc. In these devices, there is a certain gap between the blades and the shell to prevent interference, which causes high-pressure fluid to leak through the gap flow channel to the low-pressure area, forming a high-speed leakage flow, and then generating a series of complex vortex structures. These vortices will not only increase flow losses and reduce equipment efficiency, but also cause noise and vibration, and even induce strong cavitation in hydraulic machinery, causing cavitation damage to the blades and shells. Traditional suppression methods such as increasing blade thickness or installing tip gap closure devices can suppress vortices to a certain extent, but they are also prone to cause new problems, such as increased flow resistance, increased structural complexity, and a significant decrease in mechanical properties. Summary of the invention

[0003] In view of the deficiencies of the prior art, the present invention provides a porous wing tip structure for suppressing leakage flow in an axial flow machine and a design method thereof.

[0004] The design method provided by the present invention is:

[0005] Determine the original blade tip load distribution and collect the pressure distribution near the wing tip; determine the chord length range of the wing tip that needs to be processed based on the pressure difference variation trend along the chord direction on the blade surface;

[0006] The maximum pressure difference on the blade surface is used to calculate the height of the porous medium wing tip; the thickness of the porous medium wing tip is determined based on the original blade thickness; the pressure difference on the blade surface is integrated in the chord direction to calculate the average pressure difference on the blade surface under the original wing tip;

[0007] The pressure difference reduction is used to characterize the flow resistance and determine the resistance generated by the porous medium area. The Bernoulli principle is used to calculate the average velocity of the leakage flow in the gap.

[0008] Establishing the quantitative relationship between the characteristic parameters of porous media, obtaining the first quantitative expression between porosity and equivalent particle size; solving the quantitative relationship between the characteristic parameters of porous media, obtaining the second quantitative expression between porosity and specific surface area;

[0009] Screen porous media materials based on the second quantitative expression, media properties, and operating environment; establish a physical model, and perform effect verification and parameter tuning based on porous media flow simulation;

[0010] Determine the final characteristic parameters and process the porous media wing tip.

[0011] The porous wing tip structure for suppressing leakage flow in an axial flow machine of the present invention is obtained by adopting the above method.

[0012] The gain effect of the present invention:

[0013] First: By using a porous medium structure in some areas of the blade tip, it is possible to directly act on the tip gap vortex generation position under different gap scales and working conditions, and promote the weakening of the overall strength of the tip gap vortex through viscous dissipation, thereby alleviating problems such as flow channel blockage and efficiency reduction.

[0014] Second: The porous medium wing tip has little effect on the mechanical efficiency, and by steadily weakening the separation vortex and leakage vortex in the gap, it helps to enhance the spatiotemporal stability of the tip gap vortex system and reduce the flow-induced vibration, cavitation, noise and other problems caused by it. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the formation of tip gap vortex system;

[0016] Figure 2 Schematic diagram of the flow trend inside the porous medium wing tip under two types of gaps;

[0017] Figure 3 This is the NACA0009 hydrofoil model after installing the porous wing tip;

[0018] Figure 4 This is a schematic diagram of the original hydrofoil wing tip load flow distribution under a 2mm gap. C p is the pressure coefficient, and the abscissa is the ratio of the axial position x to the wingtip chord length c;

[0019] Figure 5 This is a schematic diagram of the spanwise distribution of the original hydrofoil wing tip load under a 2mm gap. C p is the pressure coefficient, and the abscissa is the ratio of the spanwise position z to the wingtip chord length c;

[0020] Figure 6 The porous wing tip structure is made of foamed iron-nickel alloy;

[0021] Figure 7 Comparison of cavitation distribution near the original wing tip and porous wing tip at two gap scales;

[0022] Figure 8 The influence of porous wing tips on the lift and drag coefficients of hydrofoils at two gap scales. The horizontal axis Time in the figure is the physical time of the flow. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0024] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other, and the embodiments can refer to and quote each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0025] like Figure 1 As shown, the fluid is affected by the pressure difference between the blade pressure surface 11 and the blade suction surface 10 of the blade body 1. The leakage flow passes through the gap (formed by the original wing tip surface 7 and the side wall surface 3) to form a tip gap vortex system composed of a blade tip leakage vortex, a separation vortex and an induced vortex, and induces cavitation in the low-pressure area at the vortex center. The arrows in the figure represent the average pressures of the hydrofoil pressure surface and the suction surface, respectively.

[0026] Based on the above-mentioned tip gap vortex formation mechanism, this application proposes a method for impeller wingtip design based on porous medium materials. By selecting reasonable porous medium materials, partial areas of the wingtip are processed, and the original wingtip is replaced with a porous medium wingtip. The porous medium has two effects on the leakage flow: damping and drainage. First, the leakage flow velocity is weakened by damping, so that an effective low-speed damping layer is formed in the porous wingtip. Second, the low-speed damping layer is discharged to the vicinity of the gap through drainage, thereby enhancing the dissipation degree of the vortex system and at the same time weakening the strong interference effect of the rigid blade on the flow, thereby weakening the wingtip separation vortex, and then weakening the tip gap vortex system and the tip gap cavitation intensity, reducing related negative effects.

[0027] Figure 2 and Figure 3 The present invention is a hydrofoil blade with a porous medium structure at the wing tip of an embodiment of the present invention, comprising a hydrofoil blade body 1, a porous medium wing tip 2, a blade wing tip guide edge 4, a blade trailing edge 5, and a blade fixed end 6. The porous wing tip part causes damping and dissipation to the flow, and the region forms a damping channel, and at the same time has a certain degree of diversion, providing a speed-increasing damping layer fluid to the gap region; the original gap flow channel is a diversion channel (the region between the wing tip and the side wall surface 3), and under the influence of the low-speed fluid in the damping channel, the leakage flow and flow velocity in the separation channel will decrease synchronously.

[0028] In one embodiment of the present application, a full coverage method of the wing tip area is adopted. Preferably, the porous wing tip can partially cover the blade wing end to reduce the possible additional leakage loss and increase the efficiency of the whole machine. Therefore, it is necessary to calculate the installation location area that matches the operating conditions and environment of the implemented fluid machinery, as well as the physical property parameters of the porous medium material.

[0029] The parameters involved in the calculation process include: density ρ of the fluid medium, dynamic viscosity ν, inlet velocity u0 of the flow component, blade load P(l), and medium characteristic parameters in the porous medium wing tip design include porous wing tip length L, wing tip width t, porosity .

[0030] In one embodiment, the design steps are as follows:

[0031] (1) Determine the original blade tip load distribution. According to the original rotor blade design, at a position 1 mm away from the tip surface, i.e., load collection position 8 near the wing tip, collect the pressure difference curve P(l) between the working surface and the back surface along the chord length of the wing tip, as shown in Figure 4 For simplicity, the load distribution representation method in the form of pressure coefficient is adopted in the figure.

[0032] (2) Determine the wingtip chord range to be treated. The coverage area should start from the leading edge of the blade wingtip and cover the wingtip area where the pressure difference ΔP(l) between the working surface and the back surface drops by 60%-70% along the wingtip chord direction.

[0033] (3) Calculate the wing tip height of the porous medium. At the maximum wing tip pressure difference position ΔP(l)_max, extract the blade load distribution P(r) along the blade span direction (radial direction in rotating machinery), that is, the span load collection position 9, as shown in Figure 5 The spanwise distance within the range of 10% pressure gradient change is taken as the porous medium wing tip height h.

[0034] (4) Determine the thickness of the porous medium wing tip. The thickness of the porous area is consistent with the thickness of the main blade tip: t = T.

[0035] (5) Calculate the average pressure difference on the blade surface under the original wing tip. Integrate the pressure difference on the wing tip surface within the original wing tip chord length and calculate the average pressure difference along the chord length:

[0036]

[0037] (6) Determine the resistance generated by the porous medium region. In this embodiment, the resistance is represented by the pressure difference reduction after the fluid passes through the porous region. The pressure difference reduction that should be obtained after applying the porous medium wing tip is:

[0038]

[0039] Where k is the resistance coefficient, which is between 0.2 and 0.3.

[0040] (7) Calculate the average velocity of the leakage flow in the gap. According to Bernoulli's principle, calculate the average velocity of the leakage flow near the gap u t :

[0041]

[0042] Where ΔCp is the pressure coefficient difference between the working surface and the back surface.

[0043] (8) Establish the quantitative relationship between the characteristic parameters of porous media. Use the Ergun equation to establish the porosity of porous media. Equivalent particle size d p The relationship between the two and the resistance pressure drop Δp, the average velocity of the leakage flow in the gap u t , the density ρ of the flowing medium and the dynamic viscosity μ should satisfy the following:

[0044]

[0045] (9) Solve the quantitative relationship between the characteristic parameters of porous media. The equivalent particle size and the average pore size of the porous media satisfy: Where A is the specific surface area of ​​the material, which represents the surface area per unit volume of the porous medium and is easier to measure than the equivalent particle size. Based on this, the porosity Equivalent particle size d p The relationship between the porosity of porous media is converted into The relationship between the specific surface area A.

[0046] (10) Screening of porous media materials. The quantitative relationship between the specific surface area A and the medium characteristics and operating environment is combined to select the appropriate type in the foam metal material. The selection criteria should give priority to the porosity above 80%. Figure 6 The porous wing tip structure made of foamed iron-nickel alloy is shown.

[0047] (11) Numerical simulation verification and optimization. The basic porous wing tip configuration designed in the above steps is a rough selection result and usually does not have the optimal characteristics. The relevant geometric and structural parameters should be further observed and improved in combination with numerical simulation methods. According to the geometric parameters of the porous medium area and the main blade shape of the blade, a porous wing tip geometric model is established for flow field simulation. The simulation process does not need to establish a strict physical model for the porous medium, but instead adds a damping source term to the control equation. The corresponding momentum equation is:

[0048]

[0049] in represents the partial differential operation, p represents the fluid pressure, x i and x j All represent spatial coordinates, subscript i and j represent different directions in space, such as x, y, z wait, u i For fluid i The velocity component in the direction, u j For fluid j The velocity component in the direction, S ij is the damping source term, which approximately simulates the blocking effect of porous media on flow.

[0050] Furthermore, the Darcy-Forchheimer equation is used to solve the damping source term, and the flow in the porous medium is considered to be isotropic:

[0051]

[0052] Where D and F are Darcy coefficient and Forchheimer coefficient, respectively representing the viscous loss and inertial loss of the fluid passing through the porous medium. The specific calculation method of D and F needs to be combined with the Eurgn equation:

[0053] ,

[0054] Through numerical simulation, the control effect of the porous metal foam wing tip on the tip gap flow and the overall efficiency are calculated under various porosities and corresponding specific surface areas, and the porous medium material with the best matching relationship is selected.

[0055] (12) Porous medium wing tip processing. After determining the final porous medium characteristic parameters, the porous medium wing tip structure is cut out according to the porous medium area geometric parameters and the main blade shape. During the cutting process, care should be taken to avoid damaging the surface morphology of the porous material. Wire cutting, laser cutting and other methods can be used for one-step forming. Do not stop, so as not to damage the pore structure on the surface of the porous medium.

[0056] In one embodiment of the present application, the main blade wing tip is completely covered with a porous medium structure, which has a good control effect on the tip gap vortex and tip gap cavitation. Numerical simulation and experimental research show that the porous medium wing tip has a good vortex suppression effect at different gap scales. Figure 7In addition, the porous medium wing tip helps to reduce the turbulence near the gap, enhance the temporal and spatial stability of the gap flow field, and reduce the fluctuation amplitude of the lift and drag coefficients accordingly, as shown in Figure 8 As shown, specifically:

[0057] like Figure 7 As shown in the figure, the cavitation area represented by the white area is significantly weakened after the porous wing tip is applied. The leakage vortex cavitation under the large-scale gap shows obvious discontinuity characteristics, and the attached cavitation area in the small-scale gap is weakened at the same time. Figure 8 It can be seen from the lift-drag curve of the airfoil shown (the lift-drag coefficients are defined as: , , L and D are the lift and drag of the hydrofoil, obtained by simulation monitoring, A ~ is the reference area of ​​the hydrofoil), the performance of the airfoil is not significantly affected after the porous wing tip is applied, that is, the lift coefficient decreases slightly and does not increase the drag coefficient. It can be seen that this design method is based on physical mechanism, has a simple structure, and has broad application prospects.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto, and any equivalent modifications or replacements should be covered within the protection scope of the present invention.

Claims

1. A method for designing a porous wing tip structure for suppressing internal leakage flow in an axial flow machine, characterized in that: Determine the original blade tip load distribution and collect the pressure distribution near the wing tip; determine the chord length range of the wing tip that needs to be processed based on the pressure difference variation trend along the chord direction on the blade surface; Calculate the wing tip height of the porous medium using the maximum pressure difference on the blade surface; Determine the wing tip thickness of the porous medium based on the original blade thickness; Integrate the pressure difference on the blade surface in the chord direction to calculate the average pressure difference on the blade surface under the original wing tip; The pressure difference reduction is used to characterize the flow resistance and determine the resistance generated by the porous medium area. The Bernoulli principle is used to calculate the average velocity of the leakage flow in the gap. Establishing the quantitative relationship between the characteristic parameters of porous media, obtaining the first quantitative expression between porosity and equivalent particle size; solving the quantitative relationship between the characteristic parameters of porous media, obtaining the second quantitative expression between porosity and specific surface area; Screen porous media materials based on the second quantitative expression, media properties, and operating environment; establish a physical model, and perform effect verification and parameter optimization based on porous media flow simulation; Determine the final characteristic parameters and process the porous media wing tip.

2. The method according to claim 1, characterized in that: The pressure distribution near the wing tip is characterized by the load distribution in the form of a pressure coefficient.

3. The method according to claim 1 or 2, characterized in that: The wingtip chord length range is the wingtip area where the pressure difference between the working surface and the back surface drops by 60%-70%.

4. The method according to claim 3, characterized in that: The calculation of the porous medium wing tip height is specifically as follows: at the position of maximum wing tip pressure difference, blade load distribution is extracted along the blade span direction, and the span distance within the range of 10% pressure gradient change is taken as the porous medium wing tip height.

5. The method according to claim 1, characterized in that: The first quantitative expression between porosity and equivalent particle size is established by the Ergun equation.

6. The method according to claim 1, characterized in that: A damping source term is added during the flow simulation.

7. The method according to claim 6, characterized in that: The damping source term is solved using the Darcy-Forchheimer equation, and the flow is considered to be isotropic in the porous medium.

8. The method according to claim 1, characterized in that: After determining the final porous medium characteristic parameters, the porous medium wing tip structure is cut out according to the porous medium area geometric parameters and the main blade shape of the blade, and is formed in one step by wire cutting and laser cutting.

9. A porous wing tip structure for suppressing internal leakage flow in an axial flow machine, obtained by the method according to any one of claims 1 to 8.

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