WO2021129261A1

WO2021129261A1 - Blade load distribution-based method for designing centrifugal-pump compound impeller

Info

Publication number: WO2021129261A1
Application number: PCT/CN2020/130308
Authority: WO
Inventors: 朱祖超; 李晓俊; 陈波; 宋宝林; 陈晓武
Original assignee: 浙江理工大学
Priority date: 2019-12-26
Filing date: 2020-11-20
Publication date: 2021-07-01
Also published as: CN111271317B; CN111271317A; JP2022515556A; JP7329220B2

Abstract

Provided is a blade load distribution-based method for designing a centrifugal-pump compound impeller: calculating an initial impeller to obtain the Euler head gradient coefficient, and determining whether long blades should be replaced by short blades for the impeller; while ensuring that the total load of the long and short blades is consistent, performing rear loading design for the long blades and performing front loading design for the short blades, such that the composite impeller can significantly improve the flow conditions inside the impeller. The invention improves the jet-wake phenomenon at the exit of the impeller, enhances the anti-cavitation performance of the centrifugal pump, and reduces the impact of pressure pulsation on the pump, making the flow more stable.

Description

A Design Method of Centrifugal Pump Compound Impeller Based on Blade Load Distribution

Cross reference related references

This application claims the priority of a Chinese patent application filed on December 26, 2019 with the application number 201911366855.5 and the invention title "A design method for a centrifugal pump compound impeller based on blade load distribution", the entire content of which is incorporated into this by reference Applying.

Technical field

The invention relates to a pump body impeller structure and a design method thereof, in particular to a design method of a centrifugal pump compound impeller based on blade load distribution, and belongs to the fields of fluid mechanical engineering and power engineering.

Background technique

As a general-purpose machinery, pumps have been widely used in various fields of the national economy, especially in the fields of national defense, water conservancy, aerospace, petrochemicals, etc., playing a very important role. However, there are also outstanding problems in the operation of centrifugal pumps, which are mainly manifested as the hazard of cavitation and outstanding pressure pulsation. The cavitation phenomenon is caused by the high fluid velocity near the blade inlet, which results in the existence of a local low pressure area. It can be solved by using short blades to reduce the blade inlet displacement, thereby reducing the inlet flow rate. The protruding pressure pulsation is caused by non-uniform flow structures such as flow separation and secondary flow inside the flow channel, which can be solved by increasing the number of blades. In today's large chemical, large petrochemical and other industries, the normal operation of centrifugal pumps effectively guarantees the normal operation of the entire production process. Once the centrifugal pump fails to operate normally, the consequences will be disastrous. Therefore, it is particularly important for centrifugal pumps to study how to avoid the hazards caused by cavitation and pressure pulsation at the same time.

Numerous studies at home and abroad have shown that the split short blade can improve the efficiency and anti-cavitation performance of the centrifugal pump, and prevent the flow stall. This is of great significance to the normal operation of the centrifugal pump.

At present, the diffusion factor DF is commonly used as the basis for judging the flow stall. The expression of DF is:

However, this formula is more complicated and difficult to judge the flow stall of the impeller. Therefore, a simple and effective new criterion is needed to determine whether a flow stall occurs, so as to determine whether it is necessary to install short blades to make the flow more stable.

Secondly, when it is determined that the impeller needs to be installed with short blades, the composite impeller needs to be designed. At present, the commonly used design method of compound impeller is the head coefficient design method based on Euler's equation, such as the study of the design and performance prediction method of semi-open compound impeller multi-stage centrifugal pump published by Xu Binjie of Zhejiang University in 2011. This paper uses the head coefficient design method to design the compound impeller, and the results show that the compound impeller can effectively prevent the backflow and outflow phenomenon in the impeller, and can significantly increase the head coefficient. However, the composite impeller designed by this method can not significantly improve the cavitation performance and pressure pulsation of the centrifugal pump.

Therefore, the prior art lacks a way to determine whether a flow stall occurs, and at the same time avoid the adverse effects of centrifugal pump cavitation and pressure pulsation.

Summary of the invention

The purpose of the present invention is to overcome the deficiencies of the above-mentioned background technology. It proposes a centrifugal pump compound impeller based on blade load distribution and a design method thereof, and judges whether to replace long blades with short blades according to Euler head gradient coefficient. The invention can determine whether a flow stall occurs, and at the same time avoid the adverse effects caused by the cavitation and pressure pulsation of the centrifugal pump. As shown in Figure 1, the present invention is implemented through the following steps:

The centrifugal pump includes an initial impeller with known parameters. The initial impeller is provided with a plurality of blades uniformly spaced along the circumference. Each blade extends from the edge of the impeller to a position close to the center of the impeller, that is, one end of the blade. Extend to the edge of the impeller and the other end towards the center of the impeller. The methods include:

1) Calculate the Euler head gradient coefficient of each sampling point on the blade of the initial impeller;

2) According to the range of Euler head gradient coefficient, the following method is used to judge whether the flow stall, that is, the blade profile is divided into multiple sections, and the Euler head gradient coefficient is judged by the following three sections:

When 0<L<0.1, k _i >37/ω, and 0.6<L<0.7, k _imax >87/ω, and 0.9<L<1.0, k _i <-10/ω. Among them, L is the dimensionless blade profile length, that is, the ratio of the length from the streamline node to the beginning of the streamline to the total streamline length, the relative streamline is the blade profile, and k _imax is the maximum Euler head gradient coefficient Value, ω is the angular velocity of the impeller; k _i represents the Euler head gradient coefficient, and i represents the ordinal number of the sampling point;

If the above is met, the flow of the impeller is stalled, and the next step is to adjust the initial impeller to form a compound impeller;

If the above is not met, the flow of the impeller is not stalled, and the initial impeller is not adjusted;

3) When the total profile length of the half-spaced blades in the impeller is shortened to form short blades, the original blades are used as long blades, and the long blades and short blades are alternately arranged in the circumferential direction, and the long blades are post-loaded to obtain long blades. Blade load curve, under the condition that the total loading load of the short blade is consistent with the total loading load of the long blade, the short blade load curve is obtained by pre-loading the short blade; by adjusting the inlet flow angle and the outlet flow angle of the short blade, Make the total loading load of the short blade consistent with the total loading load of the long blade, that is, the total area of the load distribution curve of the short blade is consistent with the total area of the load distribution curve of the long blade. The abscissa of the load distribution curve is the relative streamline length, and the vertical The coordinate is the load. In this case, the impeller with an even number of blades is designed to be distributed with long and short blades to form an optimized design for the compound impeller.

The load curve of the long blades loaded after the load and the load curve of the short blades loaded at the front are used as input conditions. According to the differential equation of the blade profile, the impeller with even number of blades is designed with long and short blade intervals to form the optimal design of the compound impeller.

The inlet liquid flow angle and the outlet liquid flow angle are specifically the angles between the circumferential speed and the relative speed of the blade at an end at the center of the impeller and an end at the edge of the impeller, respectively.

Therefore, the present invention completes the redesign and manufacture of the compound impeller of the centrifugal pump.

The specific steps of step 1) are:

1.1) For the initial impeller with known parameters, divide the blade into several equal sections along the profile line, establish a sampling point at each equal section, and increase the sequence numbers of all sampling points from the center of the initial impeller to the outside;

1.2) Obtain the circumferential component of the absolute velocity of each sampling point by calculating the streamline velocity change and looking up the table;

1.3) Then use the following formula to get the Euler head of each sampling point;

H _i ＝υursω/g

Among them, υu represents the circumferential component of the absolute velocity of the sampling point, rs represents the distance from the sampling point to the axis of the impeller, g represents the acceleration of gravity, and ω is the angular velocity of the impeller;

1.4) Obtain the Euler head gradient coefficient of each sampling point of the initial impeller according to the following formula, and then obtain the Euler head gradient range distribution of the initial impeller:

k _i =(H _i+1 -H _i )/ωΔ _x

Among them, k _i represents the Euler head gradient coefficient of _{the i-th sampling point, Hi} represents the Euler head of the i-th sampling point, and Δ _x represents the distance between two adjacent sampling points.

The compound impeller of the centrifugal pump completed by the present invention adopts the mentioned Euler head gradient as the inverse gradient distribution along the blade profile, and obtains the Euler head gradient coefficient range based on this gradient, thereby optimizing the blade structure.

In the step 3), the overall profile length of the blade is shortened to form a short blade, specifically: the end of the blade close to the edge of the impeller remains unchanged, and the end of the blade close to the center of the impeller is shortened by 30% of the total profile length, So that the total profile length of the short blade becomes 70% of the total profile length of the long blade.

Based on the load distribution curve of the initial impeller blade, the load distribution curve is obtained by the post-loading method for the long blade, and then the area enclosed by the load curve and the abscissa is calculated; for the short blade, the load curve and the abscissa are enclosed by the long and short blades. In the case of equal areas, the load distribution curve is obtained by the front loading method, which can improve the cavitation performance of the impeller and ensure the stability of the impeller pressure pulsation. In the specific implementation, for the long and short blades, the load distribution curve is obtained according to the load calculation formula.

The abscissa of the load distribution curve is the relative streamline length, the ordinate is the load, and the load is based on the formula:

Where p ⁺ and p ^- are the pressure on the pressure surface and suction surface of the blade respectively, B is the number of blades, w _m is the relative velocity of the blade surface, ρ is the density of water, rV _θ is the velocity circulation, and m is the relative axial surface Streamline length.

The total number of blades on the initial impeller is an even number.

In the design of the compound impeller, there is a design method based on the reverse pressure gradient. Since the liquid flow in the impeller flow channel is affected by the blade's functional force, the function force is strong near the pressure surface of the blade and the function force is weak near the suction surface. Under the pressure gradient, the exit of the impeller is prone to backflow and outflow. Therefore, it is necessary to install short blades to improve the flow inside the impeller. The Euler head gradient used in the present invention is a gradient distribution along the blade profile. Based on this gradient, the Euler head gradient coefficient range is obtained, and then it is determined whether to replace the long blade with the short blade.

In the specific implementation of the present invention, test verification is also performed, and the following processing is performed:

First, the load distribution curves of the long and short blades are obtained by processing:

Then, the long and short blade load distribution curves are used as input conditions to obtain the blade parameters from the blade profile differential equation to obtain the new blade shape.

Example

In specific implementation, after the blade load is determined, the basic basis for calculating the blade shape is the blade profile differential equation, and the geometric parameters of the blade are obtained from the following blade profile differential equation.

Among them, f is the blade wrap angle, ω is the angular velocity of the impeller, r is the radius of the node on the blade, V _θ is the circumferential velocity of the node, υ _m is the axial velocity, s is the axial streamline length, and d _f is the opposite The total differential of the blade wrap angle, ds is the total differential of the axial streamline length.

Finally, according to the obtained blade parameters, three-dimensional modeling of the initial impeller and compound impeller in SolidWorks software, meshing in ANSYS ICEM software, and numerical simulation with CFX software to obtain the cavitation of the pump where the initial impeller and compound impeller are located Performance curve and pressure pulsation characteristics, and then determine whether the performance of the composite impeller meets the design requirements.

The beneficial effects of the present invention are:

The present invention deals with impellers with an even number of blades, and can simply and effectively determine whether the impeller has a flow stall condition, and then replace the long blades with short blades, based on the blade load distribution to ensure that the total loading load of the long and short blades is consistent ( That is, under the condition that the total envelope area of the blade load curve is the same), the optimized design of the composite impeller can simultaneously improve the adverse effects of cavitation and pressure pulsation on the pump.

The implementation results show that the composite impeller designed by the present invention can effectively reduce the damage caused by cavitation and pressure pulsation, and make the flow inside the centrifugal pump more stable.

Description of the drawings

Figure 1 is the design flow chart of the compound impeller;

Figure 2 is a structural diagram of the initial impeller;

Figure 2 (a) is a plan view of the initial impeller;

Figure 2(b) is the axial structure diagram of the initial impeller;

Figure 3 A partial schematic diagram of the point adopted at the inlet of the initial impeller (Part A of Figure 2)

Figure 4 is a structural diagram of the compound impeller;

Figure 4 (a) is a plan view of the composite impeller;

Figure 4(b) is the axial structure diagram of the composite impeller;

Figure 5 is the load distribution curve of the long blade of the compound impeller;

Figure 6 is the load distribution curve of the long and short blades of the compound impeller;

Figure 7 shows the dimensionless cavitation performance curve of the centrifugal pump with the initial impeller and the compound impeller;

Figure 8 is a graph showing the pressure pulsation characteristics of the centrifugal pump with the initial impeller and the compound impeller.

Detailed ways

The present invention will be further described below in conjunction with the drawings and embodiments.

Taking the design of a compound impeller of a centrifugal pump as an example, the design process of the present invention will be specifically described in combination with the compound impeller design flow chart 1, including the following steps:

Step 1: Determine whether the initial impeller is equipped with short blades

The performance of a centrifugal pump is: flow Q=180m ³ /h, head H=45m, speed n=2950r/min. The centrifugal pump impeller blades are all long blades. The impeller of the centrifugal pump is used as the initial impeller, and its structure is shown in Figure 2.

For the initial impeller, the blade profile is equally divided into 46 parts, and 45 Euler head sampling points are obtained. The partial schematic diagram of the sampling point at the inlet of the impeller is shown in Figure 3. Calculate the circumferential component of the absolute velocity of each sampling point through the streamline velocity change rule checklist; bring the circumferential component of the absolute velocity into the Euler head calculation formula to obtain the Euler head of the sampling point, and calculate the initial impeller sampling point The Euler head gradient coefficient. In this embodiment, the angular velocity of the impeller is 308.7 rad/s, and the Euler head gradient coefficient range of the initial impeller is calculated as:

At _{0 <L <0.1,0.1322 <k i} <0.1735, at _{0.6 <L <0.7,0.2815 <k i} max <0.3724, and at 0.9 <L <1.0, -0.0482 < k i <-0.0367.

Judgment of Euler head gradient coefficient according to the present invention:

When 0<L<0.1, k _i >37/ω, and 0.6<L<0.7, k _imax >87/ω, and 0.9<L<1.0, k _i <-10/ω.

After determining that the above conditions are satisfied, the long blades are replaced with short blades to improve the flow inside the impeller.

Step 2: Determine the load distribution curve of the long and short blades

The invention improves the design of the compound impeller of the centrifugal pump.

The load characteristic curve of the initial impeller is calculated by the following formula.

According to the load characteristic curve of the initial impeller, a post-loading method is adopted for the long blade to determine the load distribution curve of the long blade. The post-loading point of the load curve is NC≈0.8. The shape of the load curve is shown in Figure 5.

Under the condition that the total loading load of the long and short blades is the same (that is, the total wrapping area of the blade load curve is the same), the short blade adopts a forward loading method to determine the load distribution curve of the short blade. The forward loading point of the load curve is NA≈ 0.3, the shape of the load curve is shown in Figure 6.

Step 3: New blade shape

The determined load distribution curve of the long and short blades is used as the input condition of the design to optimize the design of the impeller. After determining the blade load, the geometric parameters of the composite impeller are obtained according to the differential equation of the blade profile. After the blade load is determined, the geometric parameters of the composite impeller are obtained according to the differential equation of the blade profile: the number of long blades of the composite impeller is 3, the number of short blades is 3, and the two are distributed at intervals; the thickness of the long and short blades 3-5mm, long blade inlet radius R1 is 35-40mm, long blade outlet radius R2 is 120-125mm, long blade inlet width B1 is 20-25mm, long and short blade outlet width B2 are both 10-15mm, short blade The inlet radius R1sp is 35-40mm, the long blade inlet placement angle β ₁ is 17-19 degrees, the short blade inlet placement angle β ₂ is 25-27 degrees, the long blade outlet placement angle β ₃ is 26-28 degrees, and the short blade outlet The placement angle β ₄ is 26-28 degrees. The structure diagram of the compound impeller is shown in Figure 4.

Step 4: Performance calculation

According to the above steps, the initial impeller and composite impeller structure parameters are obtained. First, perform 3D modeling in SolidWorks software; secondly, perform meshing in ANSYS ICEM software. Finally, use CFX software for numerical simulation. The cavitation performance curve and pressure pulsation characteristics of the initial impeller and the compound impeller are obtained.

Step 5: Determine whether the performance of the composite impeller meets the design requirements

According to the numerical calculation, the cavitation performance curve and pressure pulsation characteristics of the initial impeller and the composite impeller are obtained to determine whether the performance of the composite impeller meets the design requirements.

It can be seen from Fig. 7 that under different working conditions, the NPSHr/NPSHrd coefficient of the compound impeller is lower than the NPSH coefficient of the initial impeller. The results show that the cavitation performance of the composite impeller is better than that of the initial impeller, and the cavitation performance of the composite impeller is significantly improved. Finally, the pressure pulsation data at the outlet of the centrifugal pump impeller is obtained. It can be seen from Figure 8 that the pressure pulsation amplitude of the composite impeller is significantly reduced compared to the initial impeller. This result shows that compared with the initial impeller, the composite impeller of the present invention has better pressure pulsation characteristics, and the design performance of the composite impeller meets the design requirements.

It can be seen that the present invention can improve the jet-wake phenomenon at the exit of the impeller, improve the anti-cavitation performance of the centrifugal pump, can obviously improve the flow condition inside the impeller, reduce the influence of pressure pulsation on the pump, and make the flow more stable.

Claims

A design method for a centrifugal pump compound impeller based on blade load distribution. The centrifugal pump includes an initial impeller. The initial impeller is provided with a plurality of identical blades evenly spaced along the circumference. Each blade extends from the edge of the impeller to close to the impeller in a circular arc shape. The central location is characterized in that the methods include:

1) Calculate the Euler head gradient coefficient of each sampling point on the blade of the initial impeller;

2) According to the range of Euler head gradient coefficient, the following methods are used to judge whether the flow is stalled:

When 0<L<0.1, k i >37/ω, and 0.6<L<0.7, k imax >87/ω, and 0.9<L<1.0, k i <-10/ω. Among them, L is the dimensionless blade profile length, that is, the ratio of the length from the streamline node to the beginning of the streamline to the total streamline length, the relative streamline is the blade profile, and k imax is the maximum Euler head gradient coefficient Value, ω is the angular velocity of the impeller; ki is the Euler head gradient coefficient, and i is the ordinal number of the sampling point;

If the above is met, the flow of the impeller is stalled, and the next step is to adjust the initial impeller to form a compound impeller;

If the above is not met, the flow of the impeller is not stalled, and the initial impeller is not adjusted;

3) When the total profile length of half of the blades in the impeller is shortened to form short blades, the original blades are used as long blades, and the long blades and short blades are alternately arranged in the circumferential direction, and the long blades are post-loaded to obtain the long blade load. Curve, when the total loading load of the short blade is the same as the total loading load of the long blade, the short blade load curve is obtained by pre-loading the short blade; by adjusting the inlet flow angle and the outlet flow angle of the short blade, the short blade The total loading load of the blade is the same as the total loading load of the long blade.
The design method of a centrifugal pump compound impeller based on blade load distribution according to claim 1, wherein the specific steps of the step 1) are:

1.1) For the initial impeller with known parameters, divide the blade into several equal sections along the profile line, establish a sampling point at each equal section, and increase the sequence numbers of all sampling points from the center of the initial impeller to the outside;

1.2) Obtain the circumferential component of the absolute velocity of each sampling point;

1.3) Then use the following formula to get the Euler head of each sampling point;

H i ＝υ u r s ω/g

Among them, υ u represents the circumferential component of the absolute velocity of the sampling point, r s represents the distance from the sampling point to the axis of the impeller, g represents the acceleration of gravity, and ω is the angular velocity of the impeller;

1.4) Obtain the Euler head gradient coefficient of each sampling point of the initial impeller according to the following formula, and then obtain the Euler head gradient range distribution of the initial impeller:

k i ＝(H i +1-H i )/ωΔx

Wherein, ki represents the Euler head gradient coefficient of the i-th sampling point, Hi represents the Euler head of the i-th sampling point, Δ x denotes the distance between two adjacent sample points.
The design method of a centrifugal pump compound impeller based on blade load distribution according to claim 1, characterized in that: in step 3), the overall profile length of the blade is shortened to form a short blade, specifically: One end of the edge of the impeller remains unchanged, and the end of the blade close to the center of the impeller is shortened by 30% of the total profile length, so that the total profile length of the short blade becomes 70% of the total profile length of the long blade.
The design method of a centrifugal pump compound impeller based on blade load distribution according to claim 1, characterized in that: the long blades are subjected to a post-loading method to process and fit the load distribution curve, and then the load curve and the abscissa enclosed Area: For short blades, when the load curve of the long and short blades is equal to the area enclosed by the abscissa, the load distribution curve is obtained by the front loading method.
The design method of a centrifugal pump compound impeller based on blade load distribution according to claim 1, wherein the total number of blades on the initial impeller is an even number.
A composite impeller of a centrifugal pump based on blade load distribution, which is characterized in that it is manufactured by any one of the methods of claims 1-5.