Variable Pitch Inducer Wheels on Performance of High-speed Centrifugal Pumps
With the development of high-speed centrifugal pump units, the requirements for the cavitation performance of centrifugal pumps are funded by the National Natural Science Foundation of China (50979034). Natural science research projects in universities and colleges in Jiangsu Province are getting higher and higher, and induction wheels are installed in front of the main impeller. It has become the consensus of domestic scholars that it can effectively improve its anti-cavitation performance.
In order to further improve the performance of the induction wheel, many scholars have conducted in-depth studies on the cavitation performance and lift of the inducer in various aspects, the matching of the induction wheel and the pump impeller in terms of structure and energy, and induction of the flow field in the wheel CFD analysis 56 , Analysis of stress and strength of inducer and hydraulic design method of variable pitch inducer. The installation of the induction wheel has a significant effect on improving the cavitation performance of the high-speed centrifugal pump, but the conditions and disadvantages of the induction wheel should also be considered. The pressure pulsation and radial force in the pump are the leading causes of induced rotation of the centrifugal pump. The influence of the induced wheel on the pressure pulsation and radial force in the high-speed centrifugal pump improves the speed of the pump. The reliability is of great significance.
In this paper, the pressure pulsation and radial force distribution in two high-speed centrifugal pump models with variable-pitch inducer and non-inducer-free wheel are studied by numerical simulation, and the influence of the inducer on the flow field and performance of the high-speed centrifugal pump is analyzed. The study of the causes of these effects has guiding significance for the improvement of the reliability of high-speed centrifugal pumps.
1 Calculation model and numerical calculation method Li model pump performance and main structural parameters = 7600rpm, number of blades is 6 long and 6 short, blade outlet diameter A = 100mm, impeller outlet width = 4mm, impeller inlet diameter A = 32mm. 1.2 variable pitch The parameters of the inducer wheel variable pitch inducer design depends largely on experience, its design method is visible and must conform to the actual structure of the model pump. Table 1 shows the main performance parameters of variable pitch inducer of this model pump. It is a three-dimensional model of this inducer.
Table 1 Inducer Wheel Main Performance Parameters Tab. Parameters Numerical Parameters Number of Numbers Number of Blades Imported Hub Specific Axial Length/mm Outlet Hub Ratio Rim Diameter/mm Rim Clearance/mm Cascade Thickness Variable Pitch Induced Wheel 3D Model 1.3 Numerical Calculation Method The positioning of the monitoring points was solved using the finite volume method. The ANSYSCFX12.0 fully-coupled multi-grid linear solver was used. The SS model was used to solve the entire field for all variables simultaneously, and the continuous equations and the momentum equations were solved at the same time. The ICEM-CFD software was used to preprocess the model to obtain a tetrahedral hybrid grid. The unsteady calculation uses the total pressure inlet, the velocity outlet as the boundary condition, and the solid wall surface is the no-slip boundary condition. Given a solid wall surface roughness of 0.0125mm, the accuracy of the prediction can be improved.
The impeller rotates for 5 cycles, the total calculation time is 0.039 47s, and the impeller rotates 3°. The pressure pulsation in the centrifugal pump is studied in the centrifugal pump. It can be seen that the pressure pulsation in the centrifugal pump is mainly caused by the dynamic and static interference between the impeller and the volute, especially at intervals. The most noticeable at the tongue. In order to study the variation of pressure pulsation in the volute, a negative monitoring point is set at the volute casing and an outer point is set at the outlet of the volute. This way, the pressure pulsation in the high-speed centrifugal pump can be known by monitoring the pressure pulsation at these points. distributed. The monitoring point is set at the inlet of the impeller and used to monitor the pressure change at the inlet of the impeller. See the location of each monitoring point.
2 Results and Analysis 1 The performance prediction of the two model pumps is based on the simulation results under different operating conditions for the two situations of inducer and no inducer, and the external performance curves of the two high-speed centrifugal pump models are predicted as shown. It can be seen that after the monitoring point distribution in the volute casing with the variable pitch inducer is installed, the lift of the pump is higher than that of Fig2IndiCatrPinSS without the inducer, which shows that the variable pitch inducer does the lift of the power boost pump. By comparing the efficiency curves of the two model pumps, the efficiency of the model pump is reduced after the variable pitch induction wheel is installed. This phenomenon is basically consistent with the reduction of centrifugal pump efficiency caused by the addition of equal-pitch inducer found in the model, which indicates that the variable-pitch induction wheel and The equal-pitch inducer is an axial-flow impeller. Although the airfoil is different, it has the same characteristics as the axial-flow impeller. From the efficiency curve, we can see that with the increase of the flow rate, the pump efficiency of the induction wheel model and that of the non-inducing wheel model pump decrease, which indicates that the efficiency of the axial flow induction wheel decreases with the increase of the flow rate, resulting in the pump efficiency of the belt induction wheel model. reduce.
The external characteristics curve of the two models of the pump predicted by the rotation of the forehead lift 2.2 The comparison of the pressure pulsation at the inlet of the impeller is the impeller inlet and the high-speed centrifugal pump model without the induction wheel. Each inlet of the impeller is at the flow rate Q, 0.7Q, 1.4 Time domain graph under Q.
Unsteady calculations are performed on the basis of steady calculations. The abscissas of the time-domain graphs for various monitoring points under different flow rates are time, and the ordinates are the static pressure values ​​of the monitoring points under various flows.
It can be seen that regardless of the presence or absence of the inducer, the inlet pressure of the high-speed centrifugal pump decreases with the increase of the flow rate, which is in line with the decrease of the anti-cavitation performance of the centrifugal pump under the condition of large flow. At each flow rate, the inlet pressure of the centrifugal pump with the inducer is higher than that of the high-speed centrifugal pump without the inducer, and the pressure change curve is gentler.
This shows that the lift generated by the inducer makes the impeller inlet energy increase and the impeller inlet pressure increase, thereby increasing the anti-cavitation performance of the pump and achieving the design of the inducer. The pressure fluctuation at the inlet of the non-induced impeller pump is more pronounced than that at the inlet of the inducer impeller, indicating that the inducer can improve the flow conditions at the inlet of the impeller, making the flow field at the inlet of the impeller more uniform, reducing the energy consumption, improving the performance of the pump and resistance to steam. Erosion performance.
2.3 The pressure pulsation analysis in the volute is a pressure pulsation time domain diagram at the diaphragm of two model pumps. It can be seen that the pressure pulsation waveforms at the diaphragms of the two model pumps show obvious periodicity, and the average value at the small flow rate of the non-inducing wheel pump diaphragms is the largest. The mean value of the pressure in the diaphragms of the time domain diagram wheel pump with induced and non-inducing wheel impeller inlet pulsation is the smallest under large flow conditions and the largest under small flow rates, which is the same as that of the general non-induced wheel pump. . Through comparative analysis, it can be seen that after the induction wheel is installed in the high-speed centrifugal pump, the pressure in the pump is increased due to the induced wheel work to increase the impeller inlet energy under low flow conditions, while the work done by the induced wheel and the impeller is done at a large flow rate. Compared to only a small percentage, there is little effect on the pressure change in the pump.
The pressure pulsation time domain diagram at the diaphragm is the frequency spectrum at the flow rate of 0. 4Q. The spectrogram has two important meanings. One is to decompose the size of the vibration component contained in each frequency from the waveform of the time domain, from which the excellent vibration frequency of the waveform can be obtained; another important significance is to convert the waveform of the time domain to the frequency domain. Or return the frequency domain signal to the time domain. Therefore, for this article, the spectrum map can be analyzed from the time-domain diagram, and the frequency spectrum can be used to analyze the excellent frequency causing the pressure fluctuation in the pump to find the main influence factor of the pressure fluctuation. The superior frequency is the frequency corresponding to the maximum amplitude point. In the reference, the frequency domain diagram of the pressure pulsations at the tongues of the two models of the inducing wheel was used. The frequency of the pressure pulsation at the tongues was mainly distributed in the area below 3099 Hz, and the influence of the higher frequency area was smaller. Under all operating conditions, the predominant frequency of the isolated tongue monitoring points is around 7602 and 1520 Hz, which is exactly equal to the product of the shaft frequency and the number of long blades and total blades. At 760HZ, the amplitude of the non-induced wheel monitoring point is much smaller than that of the induction wheel tongue-shattering monitoring point, while in the design and large flow conditions, the non-induced wheel monitoring point is slightly larger than the individual tongue monitoring point of the induction wheel pump. The amplitude of the same model pump at this frequency increases with the flow rate. Under 15202, the amplitude of the pump of the same model is not much changed, but the amplitude of the monitoring point of the inducing wheel is smaller than that of the non-inducing wheel centrifugal pump.
The sum is the time-domain and frequency-domain characteristics of the pressure pulsation at the outlet of the two model pump volutes. It can be seen that at different flow rates, the pressure at the monitoring point of the volute outlet with and without the inducer wheel model pump is significantly increased over time. With the installation of the induction wheel, the outlet pressure is reduced under the design conditions and the pressure is increased under the small flow rate. From the fact that the major frequencies at the exit of the spiral casing are the long and short vanes and the total number of vanes passing frequencies, the pulsation amplitude at the outlet of the induction scroll casing of the high-speed pump is lower than that at the outlet of the no-induction casing. It shows that the installation of the induction wheel can effectively improve the pulsation at the outlet of the volute.
Pressure volute pulsation in the volute at the time domain. Fig. Volute characteristics at the outlet of the scroll. 4 Impeller radial force analysis is a time-domain distribution of radial forces acting on the impeller. It can be seen that the radial force experienced by the impeller and the pressure pulsation in the pump all show obvious periodic fluctuations, and the trend of the radial force of the wheel with and without the inducer is basically consistent with time.
The statistical analysis of the radial forces applied to the two model pump impellers is given in Table 2. (a> Time-domain diagram of the radial force of the high-speed centrifugal pump impeller with and without inducer wheel. Fig. 2 The induction wheel model with or without the inducer The pump acts on the radial force of the impeller Tab. The flow rate induces the impeller. The radial force of the impeller has no inducer. The radial force of the impeller means the maximum maximum mean value. From Table 2, we can see that the diameter of the impeller is affected by the design condition of the impeller with or without the inducer. The average force is minimum, and the radial force under large flow conditions is greater than the radial force under small flow conditions. This is similar to the radial force variation on the centrifugal impeller.
However, after the induction wheel was installed, the mean and maximum radial force of the impeller under design conditions and small flow rates was slightly larger than that of the non-inducing wheel model pump. In the design condition, the mean and maximum values ​​increased by 15.13%. And 18.4%, which shows that the high-speed centrifugal pump after adding the induction wheel, the radial force on the impeller will increase, indicating that the pressure inside the impeller is greater than the situation without the induction wheel and the induction wheel to produce lift to improve the impeller inlet energy .
It is the radial force vector distribution diagram of two model pump impellers. The positive axis of the axis is downward in order to conform to the direction of the three-dimensional modeling axis. The vector coordinate of a point in the figure represents the magnitude and direction of the radial force at a certain moment. It can be seen that the radial force acting on the impeller with and without the inducer under different operating conditions is changing to the star's periodic law at all times. The radial forces on the impeller with and without the inducer are symmetrically distributed along the axis. From the vector map, the direction, size, and rotation of the impeller can be known at any time during one revolution of the impeller.
3 Conclusions By analyzing the external characteristics of the pump with and without the induction wheel, the pressure characteristics of the impeller inlet pressure, the pressure distribution in the volute, and the radial force distribution acting on the impeller, the following conclusions can be drawn: through comparative analysis of the impeller inlet In the case of transient pressure changes, the average impeller inlet pressure of variable pitch pitch inducer is greater than the inlet pressure of the impeller without inducer under all operating conditions. This indicates that variable pitch inducer can effectively improve the high speed centrifugal pump. And cavitation performance of radial force vector distribution diagram of high-speed centrifugal pump without inducer. In addition, the lift generated by the variable pitch inducer increases the energy of the impeller to lift the lift of the high speed pump. However, as the flow rate increases, the efficiency of the axial flow inducer itself decreases, and the efficiency of the entire model pump also decreases.
By analyzing the frequency domain analysis of the pressure pulsation between the induction wheel and the non-induction wheel, it is shown that the pressure pulsation in the centrifugal pump is mainly caused by the dynamic and static interference between the impeller and the volute, and the main frequency is consistent with the leaf frequency. Increasing the inducer has little effect on the pressure pulsation frequency in the centrifugal pump, but the amplitude at the small flow increases while the amplitude at the design flow and large flow decreases.
The radial force of the impeller with and without the inducer wheel model impeller is the minimum under the design conditions. The radial force under the large flow condition is greater than the radial force under the small flow rate. After the induction wheel is installed, the mean and maximum radial force of the impeller under design condition and small flow rate is larger than that of the non-inducing wheel model pump. In the design condition, the average value and the maximum value increase by 15.13% respectively. 18.4%.
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