Anti-cavitation water pump blade water inlet edge and design method thereof

Abstract

The invention relates to the technical field of water turbine blades, and discloses an anti-cavitation water pump blade water inlet edge, a blade inlet mounting angle is gradually increased from a front cover plate to a rear cover plate of an impeller, so that the flow speed of liquid on the blade water inlet edge is gradually reduced from front to back, and the blade wrap angle of the blade inlet close to the area of the front cover plate is larger than that of other areas. The invention discloses a design method for an anti-cavitation water pump blade water inlet edge. The design method comprises the following steps that 1, the progressive increase gradient of a blade inlet setting angle from a front cover plate to a rear cover plate is determined; 2, an increased blade wrap angle is set in the area, close to a front cover plate, of an impeller inlet; and 3, verifying the cavitation volume fraction through numerical simulation, and optimizing the blade inlet setting angle and wrap angle parameters. And the anti-cavitation operation range of the impeller is widened, and the anti-cavitation capacity is improved.

Description

Technical Field

[0001] This invention relates to the field of turbine blade technology, specifically to an anti-cavitation water pump blade inlet side and its design method. Background Technology

[0002] During pump operation, when the pressure at the impeller inlet is lower than the liquid's saturated vapor pressure, the liquid vaporizes, generating numerous bubbles. These bubbles, carried by the liquid into a high-pressure area, rapidly collapse, causing the surrounding liquid to surge towards the point of collapse at extremely high speed, creating a powerful water hammer effect. This phenomenon is called cavitation. Cavitation leads to decreased pump efficiency, vibration, and noise; in severe cases, it can cause impeller erosion damage and shorten the pump's lifespan.

[0003] Chinese patent document CN110657125B discloses a method for improving the anti-cavitation performance of an impeller. An inducer is installed before the first-stage impeller of the pump. A long blade with a slit is fixed around the impeller by an Archimedean spiral. A slit of 1mm to 2mm is opened at a distance of 1mm from the tip of the long blade, with the slit direction forming an acute angle with the flow direction. Based on the impeller inlet diameter, width, blade inlet flow angle, and inlet attack angle, the area of ​​the blade inlet throat and the cross-sectional area of ​​the impeller inlet are calculated, and the ratio of the blade inlet throat area to the impeller inlet cross-sectional area is set to be greater than 0.28. The working surface of the blade inlet is ground to make it nearly streamlined to reduce the impact of the liquid flow entering the blade. Simultaneously, the blade inlet attack angle is increased during grinding of the blade inlet working surface to increase the flow area of ​​the blade inlet and reduce blade displacement.

[0004] Currently, methods such as increasing the impeller inlet diameter and reducing the impeller speed are commonly used to reduce the impact of cavitation, which increases the structural size of the water pump and raises costs. Therefore, this patent addresses this issue by designing the impeller blades at the inlet side to reduce the liquid flow velocity, effectively suppressing cavitation and improving the pump's operating performance and reliability. Summary of the Invention

[0005] In order to solve the above-mentioned technical problems, the present invention provides an anti-cavitation water pump blade inlet side, characterized in that the blade inlet placement angle gradually increases from the front cover plate to the rear cover plate of the impeller, so that the flow velocity of the liquid at the blade inlet side gradually decreases from front to back.

[0006] Furthermore, the variation range of the blade inlet placement angle at the blade inlet side from the front cover plate to the rear cover plate is β1 to β1, respectively. n , where β n > … > β3> β2> β1.

[0007] Preferably, the blade inlet angle between the front and rear cover plates at the blade inlet side ranges from 16° to 25° and increases sequentially.

[0008] Liquid velocity at the inlet edge of the blade: v1 = Q × ηv / (A × ψ)

[0009] Where: v1 is the liquid flow velocity at the blade inlet (m / s);

[0010] Q is the water pump flow rate (m³ / s);

[0011] A is the blade inlet flow area (m²), A = π × D1 × b1 (D1 is the blade inlet diameter, b1 is the blade inlet width).

[0012] ηv is the volumetric efficiency.

[0013] ψ is the flow coefficient, which is positively correlated with the blade inlet angle. Increasing β results in a smoother blade profile and improved flow area utilization, thus increasing ψ.

[0014] To enhance cavitation resistance, the blade wrap angle near the front cover plate at the blade inlet is larger than in other areas. Optimizing the pressure distribution and velocity gradient in the impeller inlet region makes the fluid's turning and acceleration process after entering the impeller smoother.

[0015] A design method for the inlet side of an anti-cavitation water pump blade includes the following steps:

[0016] Step 1: Determine the increasing gradient of the blade inlet installation angle from the front cover plate to the rear cover plate;

[0017] Step 2: Set an increased blade wrap angle in the impeller inlet area near the front cover plate;

[0018] Step 3: Verify the cavitation volume fraction through numerical simulation and optimize the blade inlet placement angle and wrap angle parameters.

[0019] The present invention has the following beneficial effects:

[0020] The original design, with blades of uniform size and fixed inlet angles, resulted in minimal change in inlet velocity from the front to the rear impeller shroud. A new design, where the blade inlet angle gradually increases from the front to the rear shroud (i.e., the closer to the rear shroud, the larger the inlet angle), results in lower liquid velocity near the rear shroud, thus expanding the impeller's cavitation-resistant operating range.

[0021] By increasing the wrap angle at the inlet edge of the impeller blades, the fluid enters the impeller and the turning and acceleration process is smoother. The pressure distribution and velocity gradient changes in the inlet area are more uniform, which improves the flow stability. The pressure drop trend on the back of the blades is reduced, thereby improving the cavitation resistance. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the blade;

[0023] Figure 2 A schematic diagram showing the blade inlet placement angle;

[0024] Figure 3 A schematic diagram showing the increasing blade inlet angle;

[0025] Figure 4 This is a velocity triangle analysis diagram;

[0026] Figure 5 This is a schematic diagram showing the blade wrap angle before the change.

[0027] Figure 6 This is a schematic diagram showing the change in the blade wrap angle;

[0028] Figure 7 A schematic diagram showing the blade inlet placement angle;

[0029] Figure 8 The simulation analysis diagram is shown in Example 1;

[0030] Figure 9 The simulation analysis diagram is for Comparative Example 1;

[0031] Figure 10 The simulation analysis diagram is shown in Example 2;

[0032] Figure 11 The simulation analysis diagram is for Comparative Example 2. Detailed Implementation

[0033] The following detailed description illustrates the specific implementation method:

[0034] The reference numerals in the accompanying drawings include: blade 1, front cover plate 2, rear cover plate 3, blade inlet edge 4, impeller inlet 5, blade inlet placement angle 6, and blade wrap angle 7.

[0035] like Figure 1-7 As shown, in an anti-cavitation water pump, the blade inlet angle 6 gradually increases from the front cover plate 2 to the rear cover plate 3, so that the flow velocity of the liquid at the blade inlet edge 4 gradually decreases from front to back.

[0036] A design method for the inlet side of an anti-cavitation water pump blade includes the following steps:

[0037] Step 1: Determine the increasing gradient of the blade inlet placement angle 6 from the front cover plate 2 to the rear cover plate 3;

[0038] Step 2: Set an increased blade wrap angle 7 in the area of ​​impeller inlet 5 near the front cover plate 2;

[0039] Step 3: Verify the cavitation volume fraction through numerical simulation and optimize the blade inlet placement angle 6 and wrap angle parameters.

[0040] The blade inlet angle 6 is negatively correlated with the liquid inlet velocity. The design ensures that the liquid velocity at the blade inlet edge 4 gradually decreases from front to back, with the lowest velocity near the rear cover plate 3, thus expanding the anti-cavitation operating range. The blade inlet angle β determines the direction of the fluid's relative velocity. In the pump impeller, the absolute velocity v of the fluid can be decomposed into circumferential velocity u and relative velocity w, which form a velocity triangle. When the blade inlet angle 6 changes, the direction of the relative velocity w changes accordingly, thus affecting the fluid flow state at the inlet. Maintaining a constant flow rate within the flow channel, the design of gradually increasing the blade inlet angle 6 allows the flow area within the channel to match the velocity distribution, avoiding excessively high local velocities. A reasonable, gradual change in the blade inlet angle 6 reduces fluid separation on the blade surface 1, lowers energy loss within the boundary layer, and further suppresses cavitation.

[0041] Liquid velocity at the blade inlet: v1 = Q × ηv / (A × ψ)

[0042] Where: v1 is the liquid flow velocity at the blade inlet (m / s);

[0043] Q is the water pump flow rate (m³ / s);

[0044] A is the blade inlet flow area (m²), A = π × D1 × b1 (D1 is the blade inlet diameter, b1 is the blade inlet width).

[0045] ηv is the volumetric efficiency.

[0046] ψ is the flow coefficient, which is positively correlated with the blade inlet angle. As β increases, the blade profile becomes smoother, the flow area utilization rate improves, and ψ will increase.

[0047] By gradually increasing the blade inlet placement angle 6, the inlet flow velocity gradually decreases from front to back, with the lowest flow velocity near the rear cover plate 3, thereby expanding the anti-cavitation operation range.

[0048] At the impeller inlet 5 near the front impeller cover plate, the blade wrap angle 7 is increased (even if there is a certain angle difference between the wrap angle of the blade inlet edge 4 from the front cover plate 2 to the rear cover plate 3), which changes the pressure distribution, velocity gradient and flow stability in the impeller inlet 5 area and improves the cavitation resistance (the amount of wrap angle change is dynamically adjusted according to the operating conditions, rather than a fixed difference).

[0049] At the impeller inlet 5 near the impeller front cover plate 2, the blade wrap angle 7 is increased to optimize the pressure distribution and velocity gradient in the impeller inlet 5 area.

[0050] Increasing the wrap angle makes the fluid's turning and acceleration processes after entering the impeller smoother, reducing the pressure drop on the back of blade 1, improving flow stability, and further suppressing cavitation. The blade wrap angle 7 refers to the circumferential coverage angle of blade 1, and its size directly affects the flow path length and turning process of the fluid within the impeller. According to the angular momentum equation of fluid machinery, the energy gained by the fluid within the impeller is related to parameters such as the impeller speed and the blade wrap angle 7. Increasing the wrap angle prolongs the fluid's flow time within the impeller, making the turning and acceleration processes smoother and avoiding sudden local pressure drops caused by abrupt changes in flow direction. From the perspective of potential flow theory, a reasonable wrap angle design can make the potential flow distribution in the impeller inlet region 5 more uniform, reducing the negative pressure zone caused by potential flow distortion. A smooth flow state can reduce the rate of bubble generation and growth, thereby improving anti-cavitation capability.

[0051] The circumferential velocity component of a fluid can be expressed as:

[0052] If the change in fluid direction angle is Δβ and the flow path length is Δl, then the fluid flow time in this region is:

[0053] When a fluid changes direction, the change in the vector direction of the circumferential velocity component will result in a change in velocity:

[0054] The circumferential acceleration of the fluid:

[0055] Therefore: the relationship between the blade wrap angle 7° and the fluid diversion acceleration is:

[0056] In the formula: Let be the directional acceleration of the fluid in the circumferential direction (m / s²). Δβ is the circumferential velocity component of the fluid (m / s); r is the radius at the impeller inlet (m); Δβ is the change in the blade wrap angle (7); Δl is the flow path length of the fluid in the blade inlet region (m).

[0057] From the above equation, it can be seen that the circumferential velocity component of the fluid... With the impeller inlet radius r fixed, increasing the change in blade wrap angle Δβ will simultaneously increase Δl, but the rate of change of Δl will be greater than that of Δβ. Therefore, Δβ / Δl will decrease, resulting in a decrease in the fluid's directional acceleration. This will reduce the fluid's turning rate, making the process smoother.

[0058] Blade pressure calculation formula:

[0059] In the formula: P is the pressure at a point on the back of the blade (Pa); P0 is the inlet pressure (Pa); ρ is the liquid density (kg / m³); v is the fluid velocity at that point (m / s); hf is the head loss from that point to the inlet point (m).

[0060] Increasing the change in the wrap angle Δβ reduces the fluid turning acceleration and the rate of change of the flow velocity v. At the same time, the energy loss hf decreases. Therefore, the pressure drop of blade 1 is reduced, preventing the pressure from falling below the saturated vapor pressure and improving the cavitation resistance.

[0061] Cavitation margin (NPSH) is an indicator of cavitation resistance; to achieve cavitation-free operation, the following conditions must be met:

[0062] NPSHa ≥ NPSHr

[0063] in:

[0064] NPSHa: The available cavitation margin (m) of the system.

[0065] h f : Head loss in the inlet pipeline

[0066] NPSHr: Required cavitation margin (m)

[0067] : Minimum pressure at the blade inlet (Pa)

[0068] : saturated vapor pressure (Pa) of the liquid at the operating temperature.

[0069] Vin: Inlet flow velocity (m / s)

[0070] ρ: Liquid density (kg / m³) 3 )

[0071] g: is the acceleration due to gravity.

[0072] Because cavitation is more likely to occur at higher flow velocities and lower pressures, the system needs to provide greater pressure at the pump inlet to avoid cavitation. Reducing the liquid flow velocity at the blade inlet or increasing the pressure will improve the blade's resistance to cavitation. The method described here, using the change in the blade inlet angle 6 in conjunction with the wrap angle difference between the front and rear cover plates 3 of blade 1, is suitable for adjusting the cavitation performance of a pump under certain flow rate operating conditions, thus expanding the cavitation-free operating range.

[0073] Example 1

[0074] The blade inlet angle 6 at the water inlet edge 4 between the upper and lower cover plates of the impeller is increased by 17.7° to 20.1°. Under the same inlet pressure conditions, the simulation analysis results are as follows: Figure 8 As shown.

[0075] Example 2

[0076] With an inlet wrap angle difference Δβ of 9.1° between the upper and lower cover plates of the impeller blades at the inlet edge, and under the same inlet pressure conditions, the simulation analysis results are as follows: Figure 10 As shown.

[0077] Comparative Example 1

[0078] With the blade inlet angle 6 at the water inlet edge 4 between the upper and lower cover plates of the impeller set at 17.7°~17.7°, under the same inlet pressure conditions, the simulation analysis results are as follows: Figure 9 As shown.

[0079] Comparative Example 2

[0080] With an inlet wrap angle difference Δβ of 2.3° between the blade inlet edges of the impeller upper and lower cover plates and under the same inlet pressure, the simulation analysis results are as follows: Figure 11 As shown.

[0081] The simulation uses the volume fraction of air bubbles to determine cavitation; the smaller the volume fraction, the less cavitation. Figure 8 and Figure 9 In the comparison, the blade inlet placement angle 6 gradually increases from the front cover plate 2 to the rear cover plate 3 of the impeller, and the smaller the cavitation volume fraction.

[0082] The simulation uses the volume fraction of air bubbles to determine cavitation; the smaller the volume fraction, the less cavitation. Figure 10 and Figure 11 The comparison shows that a larger Δβ difference indicates a smaller Vapor Volume Fraction.

[0083] The above descriptions are merely embodiments of the present invention. Commonly known structures and characteristics are not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the application date or priority date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can, under the guidance of this application, improve and implement this solution in combination with their own capabilities. Some typical known structures or methods should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. An anti-cavitation water pump blade leading edge, characterized by, The blade inlet installation angle gradually increases from the front shroud to the rear shroud, so that the flow rate of the liquid at the blade water inlet edge gradually decreases from front to back.

2. The anti-cavitation water pump blade inlet edge of claim 1, wherein: The blade inlet setting angle of the blade water inlet edge varies in the range of β1 to β n wherein β n > … > β3> β2 > β1.

3. An anti-cavitation water pump blade inlet edge according to claim 2, wherein: The blade inlet installation angle of the blade water inlet edge between the front and rear shrouds ranges from 16° to 25° and gradually increases.

4. An anti-cavitation water pump blade inlet edge according to claim 3, wherein: The blade wrap angle of the blade inlet near the front shroud area is greater than that of other areas.

5. A method of designing the entry edge of a blade of a pump for pumping water, as claimed in any one of claims 1 to 4, characterized in that, The method comprises the following steps: Step 1, determining the increasing gradient of the blade inlet installation angle from the front shroud to the rear shroud; Step 2, setting an increased blade wrap angle in the area near the front shroud of the impeller inlet; Step 3, verifying the cavity volume fraction through numerical simulation and optimizing the blade inlet installation angle and wrap angle parameters.

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