CN111306076A - Device and method for testing backflow vortex cavitation of vane pump - Google Patents
Device and method for testing backflow vortex cavitation of vane pump Download PDFInfo
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Abstract
The invention provides a device and a method for testing backflow vortex cavitation of a vane pump, which are used for testing the performance and the dynamic characteristic of the backflow vortex cavitation of the vane pump under the working condition of low flow. In the device, a vane pump is fixed on a workbench at a distance of 1X D from an inlet of an impellersThe organic glass inlet pipe is provided with 1 pressure pulsation sensor, a plurality of pressure pulsation sensors are respectively arranged at intervals of 45 degrees along the circumferential direction of the volute flow channel, and the distance from the pump outlet pipe is 2X DsA pressure pulsation sensor is arranged, and the sensor and the acquisition card are connected in series and are sequentially connected to a computer system provided with signal acquisition and data processing software. The testing method is based on the fact that backflow occurs at the inlet of the pump impeller, mainstream flow and hydraulic performance of the inlet of the pump are affected, pressure pulsation information inside the pump is collected in real time through the pressure pulsation sensor, and identification and judgment of backflow vortex cavitation of the vane pump are accurately achieved by combining the flowing condition inside the pump when the backflow vortex cavitation occurs.
Description
Technical Field
The invention belongs to the technical field of hydromechanics, relates to an experimental test method for centrifugal pump cavitation, and particularly relates to a device and a method for realizing backflow vortex cavitation test of a vane pump.
Background
The centrifugal pump can be widely used for conveying slurry containing solid particles in the industries of electric power, metallurgy, coal, building materials and the like, is widely applied to the important national economy fields of aerospace, petroleum, chemical engineering, water conservancy and the like, and has the advantages of simple structure, reliable performance, convenience in maintenance and the like. When the pump runs under the deviated design working condition, particularly the small-flow working condition, the inlet of the centrifugal pump is easy to generate backflow, the pressure of the center of a backflow vortex is extremely low, steam bubbles and sudden rupture phenomena are generated in liquid water flow, a low-pressure area is easy to appear in a water suction pipeline, cavitation can be caused in the low-pressure area, low-frequency pressure pulsation of the centrifugal pump and a pipeline system of the centrifugal pump can be induced seriously, strong mechanical vibration is generated, energy is consumed, and the running reliability and the working performance of the pump are reduced.
Disclosure of Invention
The invention aims to provide a device and a method for testing backflow vortex cavitation, which can more simply and conveniently judge whether backflow vortex cavitation occurs when a vane pump runs by observing the dynamic characteristic of cavitation change during pump internal cavitation and combining the cavitation development condition of cavitation phenomenon, pulsation signal and axial or radial characteristics of the pulsation signal.
The present invention achieves the above-described object by the following technical means.
A testing device for realizing backflow vortex cavitation of a vane pump is characterized by comprising a water tank, the vane pump, a visual testing system for realizing the reappearance of a pump inlet flow field, a pressure pulsation signal acquisition system and a signal processing device;
the visual testing system consists of a transparent organic glass inlet pipe, a high-speed camera and a digital camera, wherein the transparent organic glass inlet pipe is used for connecting the vane pump and the water tank, and the high-speed camera and the digital camera are respectively arranged on the front surface and the side surface of the inlet section of the transparent organic glass inlet pipe; the outlet of the vane pump is connected with the water tank through a pipeline to form a loop, and a flowmeter and a valve are arranged on the outlet of the vane pump and the pipeline of the water tank; the water tank is also connected with a vacuum pump, and an exhaust valve is also arranged on a pipeline between the vacuum pump and the water tank;
the pressure pulsation signal acquisition system comprises a pump inlet and a pump outlet which are arranged on the vane pump and a plurality of pressure pulsation sensors which are arranged along the circumferential direction of a volute flow passage of the vane pump; the pressure pulsation sensor at the pump inlet and the pump outlet pressureThe force pulsation sensors are respectively arranged at a distance of 1 × D from the inlet of the impellersRight above and at a distance 2 x D from the transparent plexiglass inlet tubesAt least one of (1) and (b);
the signal processing device is composed of a first acquisition card, a second acquisition card, a partial pressure plate, a power box and a computer provided with signal acquisition and data processing software, wherein the first acquisition card is connected with the pump inlet pressure sensor and the pump outlet pressure sensor, the second acquisition card is connected with the pressure pulsation sensors, the first acquisition card and the second acquisition card are connected to the computer provided with the signal acquisition and data processing software, and the first acquisition card and the second acquisition card are connected with the power box through the partial pressure plate.
Further, a plurality of pressure pulsation sensors are uniformly distributed in the circumferential direction inside the vane pump.
Further, the number of the pressure pulsation sensors is 8.
The testing method for realizing the return vortex cavitation testing device of the vane pump is characterized by comprising the following steps of:
1) completing the installation of the backflow vortex cavitation test device;
2) starting a power supply, starting the vane pump and enabling the rotating speed of the vane pump to reach the required testing rotating speed; starting a computer, and carrying out sensor offset acquisition before starting the computer;
3) setting the flow value for carrying out the pump cavitation performance test as a low-flow working condition by adjusting a valve in an outlet pipeline of the pump; in each flow working condition test, the valve in the outlet pipeline is continuously adjusted so as to keep the flow value constant in the whole backflow vortex cavitation experiment;
4) regulating the pressure at the inlet of the water tank by starting the vacuum pump to change the pressure at the inlet of the vane pump, so that the inside of the vacuum pump is not cavitated; collecting corresponding cavity flow field distribution and pressure pulsation information thereof when the cavity in the vane pump occurs at the moment, and detecting the state that the cavity in the vane pump does not occur at the moment;
5) gradually reducing the pump inlet pressure of the vane pump through the vacuum pump, and setting the inlet pressure of the vane pump so as to gradually generate backflow vortex cavitation in the vane pump;
6) under the condition of constant inlet pressure or constant cavitation coefficient, acquiring cavitation test information of the vane pump synchronously acquired by a pressure pulsation sensor, a high-speed photography camera and a digital camera when the vane pump is cavitated in real time, and processing the following data:
a) calculating cavitation performance data of the vane pump:
by stepwise reduction of the inlet pressure p of the centrifugal pump1Thereby reducing the effective cavitation margin NPSHA until cavitation occurs; the cavitation starts from the initial state, and the initial stage of cavitation is referred to as NPSHA (NPSHR)3Indirectly calculating the cavitation allowance of the pump based on a pump cavitation characteristic equation to determine the initial cavitation performance point of the pump, and evaluating the pump cavitation performance; different NPSHA values correspond to different lift H values, a series of lift H are finally obtained and used for calculating a lift drop value caused by cavitation development, a curve of the lift changing along with cavitation allowance is further drawn, a cavitation performance curve of the pump can be obtained, the cavitation performance of the pump under various working conditions is compared, and a cavitation initial point is judged;
b) visual data of the cavitation flow field of the vane pump:
acquiring the process that the backflow vortex cavitation at the inlet of the pump occurs, develops and disappears;
c) vane pump pressure pulsation test data:
synchronously collecting pressure pulsation signals flowing inside the vane pump, introducing the pressure pulsation signals of pressure pulsation sensors at monitoring points of an inlet pipe, an impeller and an outlet pipe of the vane pump into data processing software for pressure pulsation frequency domain analysis by extracting the pressure pulsation information of the pressure pulsation sensors at the monitoring points, obtaining a pressure pulsation frequency domain diagram of each monitoring point by performing fast Fourier transform on the pressure pulsation signals of each monitoring point so as to obtain a frequency spectrum analysis diagram of cavitation flow inside the pump, and extracting pressure pulsation frequency domains of the inlet pipe, the outlet pipe and the volute, namely extracting peak signals in the pressure pulsation frequency domain diagrams of the inlet pipe, the outlet pipe and the volute; meanwhile, performing cross correlation analysis on pressure pulsation signals acquired by 2 pressure pulsation sensors which are the same in axial position but different in installation angle to acquire cross correlation phases of the pressure pulsation signals, determining the axial and radial characteristics of various cavitation phenomena of the pump, namely the number and the rotation direction of the rotating unit bodies, and judging whether the cavitation phenomena are axial instability phenomena or rotation instability phenomena so as to acquire the flow instability phenomena and the characteristics of the interior of the vane pump;
7) adjusting the working conditions, and repeating the steps 3) -6) until all the working conditions are finished.
Further, the specific method of c) of step 7) is as follows:
firstly, dividing pressure pulsation signals flowing inside the vane pump according to different time periods: each series of data having a total length of TrDivided into N time intervals of TdWhere d is 1,2, …, nd(ii) a Windowing the pressure pulsation signal of each interval by adopting a Hanning window function;
for data n in each time intervaldThe fourier transform is performed using equation (2),
wherein i is 1,2, …, nd(ii) a k is 0, 1, …, (N-1); Δ t is the sampling time in units of s; x is the number ofinAny collected data point; n isdThe number of data in equal interval is nd；Xi(fk) Is a frequency domain signal, Hz.
Taking the influence of a Hanning window function into consideration, and carrying out scale conversion on the data subjected to Fourier transform; obtaining a frequency domain diagram of each monitoring point, carrying out pressure pulsation frequency domain analysis, obtaining a frequency spectrum analysis diagram of the pump internal cavitation phenomenon, thus obtaining a frequency spectrum analysis diagram of the pump internal cavitation flow, and extracting pressure pulsation frequency domains of a pump inlet pipe, a pump outlet pipe and a volute;
the cross correlation analysis is carried out on a frequency spectrum analysis chart of the pump internal cavitation phenomenon obtained by carrying out Fourier transform on signals collected by 2 pressure sensors at the same axial position but different installation angles based on a formula (3),
in the formula, Sxy(fk) Analyzing a function for signal correlation; xi(fk)，Yi(fk) Is the pressure pulsation frequency domain signal at 2 different locations, Hz.
If it is cross-correlated in phase
The value is 0 degrees, and the unstable cavitation phenomenon of axial flow occurs in the vane pump;
if it is cross-correlated in phase
When the pump is in operation, the phenomenon of unstable cavitation appears in the vane pump, and the number of the rotating unit bodies
Where Δ θ is the angular interval at which the 2 adjacent pressure sensors are actually mounted.
Further, acquiring initial and developed flow field information of cavitation in the vane pump under small flow is to respectively shoot the front and the side of the transparent organic glass inlet section of the vane pump by adopting a high-speed photographic camera and a digital camera.
Further, in step 7), c), the data processing software is Origin or Matlab software.
The present invention has advantageous effects
According to the invention, the inlet pressure of the vane pump is gradually reduced to enable the interior of the pump to generate backflow vortex cavitation, and meanwhile, the dynamic process of the backflow vortex cavitation generated in the interior of the vane pump is described by means of visualization means of flow in the interior of the pump, so that the development process of cavitation bubbles is visually shown, the dynamic characteristics of the change of the cavitation bubbles when the cavitation of the front edge of the vane of the impeller is generated are better observed and mastered, and the observation of the onset and the development of the vane pump and the internal cavitation are facilitated, so that the optimal visualization effect is achieved.
According to the invention, the performance data of the pump when cavitation occurs is synchronously acquired, the cavitation allowance of the pump is calculated based on the pump cavitation characteristic equation to evaluate the pump cavitation performance, the cavitation performance of the pump under various working conditions is compared, and the cavitation onset point of the pump is judged, so that the cavitation occurrence conditions under different cavitation coefficients are obtained.
The invention can collect pressure pulsation signals flowing in the pump in real time under the non-cavitation and cavitation states based on signal collection program software, and obtains a spectrogram of the unstable phenomenon in the pump to analyze the occurrence and the characteristics of various flow unstable phenomena by extracting pressure pulsation information of a pressure pulsation sensor at monitoring points of an inlet pipe, an impeller and an outlet pipe of a vane pump.
By comprehensively analyzing the cavitation generation and development of the backflow vortex cavitation and the pressure pulsation characteristics of the backflow vortex cavitation, the correlation among the initiation and development of the pump internal cavitation, the correlation between the pump cavitation performance data and the change of the pump internal pressure pulsation signal are established, and finally, according to the experimental result, the correlation between the initiation and development of the pump internal cavitation, the correlation between the pump cavitation performance data and the change of the pump internal pressure pulsation signal are established and displayed by adopting a graphical method. The method provides a simple test method for detecting whether the vane pump generates backflow cavitation, provides reference for researching problems such as unstable work caused by flow instability such as backflow and cavitation generated by the vane pump under an off-design working condition, and provides basis for guiding the actual operation of the pump in engineering, so that the performance and fault diagnosis of the vane pump are deeply analyzed, whether the pump stably operates is detected to a great extent, the use efficiency of the pump is improved, and the pump is well protected.
Drawings
Fig. 1 is a schematic view of the backflow of fluid at the impeller.
FIG. 2 is a schematic structural diagram of the testing device for realizing the backflow vortex cavitation.
Fig. 3 is a schematic diagram of the connection of the electronic instrument device.
FIG. 4 is a schematic diagram of the electrical connection of the sensor to the acquisition card.
Fig. 5 is an installation schematic and a cross-sectional view of an inlet pressure pulsation sensor. Wherein, the length L of the transparent organic glass pipeline is 10Ds。
FIG. 6 is a block diagram of a visual test system for inlet flow field testing.
Fig. 7 is an installation schematic diagram and a sectional view of 8 pressure pulsation sensors at the periphery of the volute.
FIG. 8 is a graph of pump cavitation performance at low flow conditions.
Fig. 9 shows the change of the internal cavity volume of the small-flow pump during cavitation, wherein the cavitation coefficients (a1) and (a2) are 2.8; (b1) (b2) indicating that the cavitation coefficient is 1.7; (c1) (c2) the cavitation coefficient is 1.1; (d1) and (d2) the cavitation coefficient σ was 0.9.
FIG. 10 is a pressure pulsation diagram of monitoring points in a cavitation pump under a low flow condition.
In the figure:
1-a water tank, 2-a vane pump, 3-a visual test system, 4-a pressure pulsation signal acquisition system, 5-a signal processing device, 6-a base, 7-a transparent organic glass inlet pipe, 8-a digital camera, 9-a high-speed camera, 10-a first pressure pulsation sensor, 11-a second pressure pulsation sensor, 12-a third pressure pulsation sensor, 13-a torque meter, 14-a motor, 15-a flange, 16-a flow meter, 17-a valve, 18-an exhaust valve, 19-a vacuum pump, 20-a first acquisition card, 21-a second acquisition card, 22-a pressure divider, 23-a power supply box, 24-a computer and 25-a writing desk.
Detailed Description
The invention will be further described with reference to the following figures and specific examples, but the scope of the invention is not limited thereto.
FIG. 1 is a schematic representation of the backflow of fluid at an impeller; the device for realizing the backflow vortex cavitation test of the vane pump is shown in figure 2. The device for realizing the vortex cavitation of the backflow of the vane pump comprises a water tank 1, a vane pump 2, a visual test system 3 for realizing the reappearance of a pump inlet flow field, a pressure pulsation signal acquisition system 4 and a signal processing device 5.
The water tank 1 is fixedly arranged on the ground, and the vane pump 2 is fixedly arranged on the base 6 and connected with the motor 14. The vane pump 2 and the water tank 1 are connected by a transparent organic glass inlet pipe 7 to form a visual testing system of an inlet flow field, and then the outlet of the vane pump 2 and the water tank 1 are connected by other pipelines, so that a fluid flow loop from the inlet to the outlet of the pump is formed.
The visual testing system 3 comprises transparent organic glass import pipe 7, a high-speed camera 8 and a digital camera 9, and transparent organic glass import pipe 7 is used for connecting vane pump 2 and water tank 1, and high-speed camera 8 and digital camera 9 are installed respectively in the front and the side of transparent organic glass import pipe 7 import section, as shown in fig. 6. The outlet of the vane pump 2 is connected with the water tank 1 through a pipeline to form a loop, and the outlet pipelines are connected through a flange 15. A flow meter 16 and a valve 17 are arranged on the outlet of the vane pump 2 and the pipeline of the water tank 1; the water tank 1 is also connected with a vacuum pump 19, and an exhaust valve 18 is also arranged on a pipeline between the vacuum pump 19 and the water tank 1.
As shown in fig. 4, the pressure pulsation signal collecting system 4 includes a first pressure pulsation sensor 10 installed at the inlet of the pump, a second pressure pulsation sensor 11 installed at the outlet of the pump, and 8 pressure pulsation sensors 12 installed along the circumferential direction of the volute flow channel of the vane pump 2; the 8 third pressure pulsation sensors 12 are uniformly distributed in the circumferential direction inside the vane pump as shown in fig. 7. The inlet first pressure pulsation sensor 10 and the outlet second pressure pulsation sensor 11 are respectively arranged at a distance of 1 × D from the inlet of the impellersRight above and at a distance from the transparent plexiglass inlet tube 7 to the outlet tube 2 x DsAs shown in fig. 5.
As shown in fig. 3, the signal processing device 5 is composed of a first acquisition card 20, a second acquisition card 21, a voltage divider 22, a power box 23 and a computer 24 equipped with signal acquisition and data processing software, wherein the first acquisition card 20 and the second acquisition card 21 are connected with the power box 23 through wires and voltage divider to acquire power. The first acquisition card 20 is connected with a first pressure pulsation sensor 10 at the inlet of the pump and a second pressure pulsation sensor 11 at the outlet of the pump, the second acquisition card 21 is connected with 8 third pressure pulsation sensors 12 which are uniformly distributed in the circumferential direction in the vane pump, and the first acquisition card 20 and the second acquisition card 21 are connected to a computer 24 which is provided with signal acquisition and data processing software; inputting the collected pressure pulsation information into a computer provided with signal collection and data processing software, acquiring pressure pulsation data flowing inside the pump by using a signal collection program, simultaneously leading the pressure pulsation data into data processing software such as Origin or Matlab software, carrying out fast Fourier transform on the pressure pulsation signal of each monitoring point, acquiring a frequency domain diagram of each monitoring point, and carrying out pressure pulsation frequency domain analysis. The dynamic characteristics of backflow and backflow vortex cavitation of the vane pump are obtained by combining the visual inlet pipe of the pump and the occurrence and development changes of internal cavitation bubbles of the inlet pipe and the internal cavitation bubbles of the inlet pipe and combining the analysis results of pressure pulsation in the pump, the corresponding cavitation margin NPSHA value or cavitation coefficient sigma when the backflow vortex cavitation occurs is obtained through calculation, meanwhile, phase intersection and correlation analysis is carried out on pressure pulsation signals of monitoring points at different positions, the critical working conditions of the backflow and backflow vortex cavitation are judged by combining the analysis results, and the axial and radial characteristics of the backflow and backflow vortex cavitation and the number of units of unstable flow and the like are obtained. Therefore, a basis is provided for judging whether the interior of the pump generates backflow vortex cavitation in the future and preventing the pump from generating backflow vortex cavitation under the low-flow working condition.
The principle of the testing method of the invention is as follows: taking clear water as an example, when the vane pump operates under a small flow design working condition, water is pumped out of the water tank, flows through the transparent organic glass tube, enters the suction chamber, and then generates inlet pressure at an inlet and enters the impeller; meanwhile, part of clear water flows back to the pump inlet pipe from the impeller outlet to generate backflow, so that the backflow condition of fluid in the pipe can be preliminarily observed; the pressure in the inlet pipe can generate deviation to block the flow of the inlet main flow, and when the pressure of the reflux center is reduced to the saturated vaporization pressure of the fluid medium at the temperature, the cavitation condition is reached, namely the cavitation coefficient sigma is up to sigma ≈ sigmahead-dropAt this time, back-flow vortex cavitation occurs in the impeller.
p1Is the static pressure at the impeller inlet, pvIs the saturated vapor pressure of the clear water; u is the peripheral speed of the impeller inlet.
The experimental test process for realizing the backflow vortex cavitation comprises the following steps:
the testing of the back flow vortex cavitation performance of the vane pump is realized by an unsteady state testing method for continuously reducing the inlet pressure of the vane pump, and the experimental results obtained in the testing time are divided averagely. In the process of carrying out the backflow vortex cavitation experiment of the vane pump each time, the flow in the whole experiment device loop is kept constant. Cavitation is initiated and develops continuously as the inlet pressure decreases gradually and to a certain value. The front and the side of the transparent organic glass inlet section of the vane pump are shot by adopting a method of combining a high-speed photography camera and a digital camera respectively, the high-speed photography can be synchronous with a stroboscopic light source, finally, a series of flow field information of cavitation generation, development and the like can be obtained, and based on a signal acquisition program, cavitation performance data of the vane pump when no backflow vortex cavitation occurs and backflow vortex cavitation occurs is acquired, so that data of a series of lifts when the backflow vortex of the vane pump is reduced can be obtained. The specific experimental steps are as follows:
1) completing the installation of the backflow vortex cavitation test device;
2) starting a power supply of a pump cavitation experiment testing system, and starting a motor to enable the impeller of the vane pump to gradually reach the rotating speed required by the experiment; starting a computer provided with signal acquisition and data processing software; and carrying out sensor offset acquisition before starting.
3) Setting the flow value for carrying out the pump cavitation performance test as a low-flow working condition by adjusting a valve 17 in a pump outlet pipeline; in each flow condition test, the valve 17 in the outlet pipeline is continuously adjusted so as to keep the flow value constant in the whole backflow vortex cavitation experiment.
4) The inlet pressure information of the water tank is adjusted by starting the vacuum pump, so that the inlet pressure of the vane pump is changed, and the inside of the vacuum pump is not cavitated; and collecting corresponding cavitation flow field distribution and pressure pulsation information thereof when cavitation in the pump occurs at the moment, wherein the distribution is used for detecting the state that cavitation does not occur in the pump.
5) And the inlet of the pump is gradually reduced by adopting a vacuum pump, and the inlet pressure of the vane pump is set, so that the backflow vortex cavitation is gradually generated inside the vane pump.
6) Acquiring cavitation test information of the vane pump synchronously acquired by a pressure pulsation sensor, a high-speed photographic camera and a digital camera when the vane pump is cavitated in real time under the condition of constant inlet pressure or constant cavitation coefficient, and calculating inlet and outlet pressure values of the synchronously acquired pump when the pump is cavitated by a formula to acquire pump cavitation performance data, such as corresponding cavitation allowance NPSHA or cavitation coefficient sigma values when a series of pump lifts are reduced, based on acquired pressure pulsation signal data of the vane pump when the vane pump is cavitated; and the frequency characteristics of various cavitation phenomena in the vane pump and the axial and radial characteristics of various cavitation phenomena.
Specifically, the method comprises the following steps:
a) calculating cavitation performance data of the vane pump:
by stepwise reduction of the inlet pressure p of the centrifugal pump1Thereby reducing the effective cavitation margin NPSHA until cavitation occurs; the cavitation starts from the initial state, and the initial stage of cavitation is referred to as NPSHA (NPSHR)3Indirectly calculating the cavitation allowance of the pump based on a pump cavitation characteristic equation to determine the initial cavitation performance point of the pump, and evaluating the pump cavitation performance; and different NPSHA values correspond to different lift H values, a series of lift H are finally obtained and used for calculating a lift drop value caused by cavitation development, then a curve of the lift changing along with cavitation allowance is drawn, a cavitation performance curve of the pump can be obtained, the cavitation performance of the pump under various working conditions is compared, and a cavitation onset point of the pump is judged.
b) Visual data of the cavitation flow field of the vane pump:
the front and the side of the transparent organic glass inlet section of the vane pump are shot by adopting a high-speed photographic camera and a digital camera respectively, and the process that the backflow vortex cavitation at the inner inlet of the pump can occur and develop to disappear is obtained.
c) Vane pump pressure pulsation test data:
synchronously collecting pressure pulsation signals flowing inside the vane pump, introducing the pressure pulsation signals of pressure pulsation sensors at monitoring points of an inlet pipe, an impeller and an outlet pipe of the vane pump into data processing software for pressure pulsation frequency domain analysis by extracting the pressure pulsation information of the pressure pulsation sensors at the monitoring points, obtaining a pressure pulsation frequency domain diagram of each monitoring point by performing fast Fourier transform on the pressure pulsation signals of each monitoring point so as to obtain a frequency spectrum analysis diagram of cavitation flow inside the pump, and extracting pressure pulsation frequency domains of the inlet pipe, the outlet pipe and the volute, namely extracting peak signals in the pressure pulsation frequency domain diagrams of the inlet pipe, the outlet pipe and the volute; meanwhile, the pressure pulsation signals collected by the pressure pulsation sensors with the same axial position and different installation angles are subjected to cross correlation analysis to obtain cross correlation phases, the cross correlation phases are used for determining the axial and radial characteristics of various cavitation phenomena of the pump, namely the number and the rotation direction of the rotating unit bodies, and the cavitation phenomena are judged to be axial instability phenomena or rotation instability phenomena, so that the flow instability phenomena and the characteristics of the interior of the vane pump are obtained.
Firstly, dividing pressure pulsation signals flowing inside the vane pump according to different time periods: each series of data having a total length of TrDivided into N time intervals of TdWhere d is 1,2, …, nd(ii) a Windowing the pressure pulsation signal of each interval by adopting a Hanning window function;
for data n in each time intervaldThe fourier transform is performed using equation (2),
wherein i is 1,2, …, nd(ii) a k is 0, 1, …, (N-1); Δ t is the sampling time in units of s; x is the number ofinAny collected data point; n isdThe number of data in equal interval is nd；Xi(fk) Is a frequency domain signal, Hz.
Taking the influence of a Hanning window function into consideration, and carrying out scale conversion on the data subjected to Fourier transform; obtaining a frequency domain diagram of each monitoring point, carrying out pressure pulsation frequency domain analysis, obtaining a frequency spectrum analysis diagram of the pump internal cavitation phenomenon, thus obtaining a frequency spectrum analysis diagram of the pump internal cavitation flow, and extracting pressure pulsation frequency domains of a pump inlet pipe, a pump outlet pipe and a volute;
the cross correlation analysis is carried out on a frequency spectrum analysis chart of the pump internal cavitation phenomenon obtained by Fourier transform of signals collected by 2 adjacent pressure pulsation sensors based on a formula (3),
in the formula, Sxy(fk) Analyzing a function for signal correlation; xi(fk)，Yi(fk) Is the pressure pulsation frequency domain signal at 2 different locations, Hz.
If it is cross-correlated in phase
The value is 0 degrees, and the unstable cavitation phenomenon of axial flow occurs in the vane pump;
if it is cross-correlated in phase
When the pump is in operation, the phenomenon of unstable cavitation appears in the vane pump, and the number of the rotating unit bodies
Where Δ θ is the angular interval at which the 2 adjacent pressure sensors are actually mounted.
Adjusting the working conditions, and repeating the steps 2) to 6) until the experiment of all the working conditions is completed.
During the unsteady state test process of the whole backflow vortex cavitation performance of the vane pump, the pressure reduction amount of the inlet needs to be kept constant. The collected experimental data were equally divided by the quasi-steady state time period to keep the inlet pressure nearly constant. The time period of the average interval must be long enough to ensure that the collected data volume is not dispersed, and experimental data in the time interval are superposed, so that the test result of the whole continuous reflux vortex cavitation experiment is better.
In cavitation experiments, the inlet pressure p of the centrifugal pump is gradually reduced1Thereby reducing the effective cavitation margin NPSHA until cavitation occurs; cavitation is developed continuously from the initial state by using the initial state of cavitationTime of birth NPSHA ═ (NPSHR)3Indirectly calculating the cavitation margin (NPSHR) of the pump3And measuring the initial cavitation performance point of the pump. Different NPSHA values correspond to different lift H values, a series of lifts H can be finally obtained and used for calculating the lift drop value caused by cavitation development, and then a curve of the lift change along with the cavitation allowance is drawn, so that a cavitation performance curve of the pump can be obtained, as shown in FIG. 9. Table 1 shows the change of the volume of the internal cavity of the small-flow pump when the small-flow pump cavitates and the corresponding cavitation coefficient.
TABLE 1 internal flow field of cavitation pump under small flow and cavitation coefficient reference table
The internal flow field of the cavitation pump corresponding to the cavitation coefficient shown in table 1 is shown in fig. 10 of the figure, which is the change of the internal cavitation volume of the small-flow pump when the small-flow pump is subjected to cavitation, wherein (a1) and (a2) indicate that the cavitation coefficient is 2.8; (b1) (b2) indicating that the cavitation coefficient is 1.7; (c1) (c2) the cavitation coefficient is 1.1; (d1) and (d2) the cavitation coefficient σ was 0.9.
As can be seen from table 1, as cavitation in the pump progresses to an increasingly severe state, inlet return vortex cavitation occurs and progresses to a disappearance state. The cavitation coefficient is gradually reduced, the cavitation bubbles in the impeller are completely developed, the volume of the cavitation bubbles almost occupies a single blade flow channel, the fluid returning to the inlet pipe from the impeller is reduced, the inlet backflow is gradually weakened, the inlet backflow phenomenon is not obvious or even disappears, the inlet backflow vortex cavitation phenomenon disappears, and the cavitation bubbles do not appear in the inlet pipe. The pump cavitation performance is evaluated by combining with a numerical calculation or test result of the visualization of the internal flow field of the pump and calculating the cavitation allowance or cavitation coefficient of the pump based on a pump cavitation characteristic equation, the cavitation performance of the pump under various working conditions is compared, the cavitation initiation point of the pump is judged, the correlation between the initiation of cavitation in the pump and the cavitation volume distribution in the development and the pump performance data is established, and a foundation is provided for the next cavitation pressure pulsation test.
For obtaining cavitation variations produced by back-flow vortex cavitation of vane pumpsDynamic characteristics, combining the generation and development process of inlet reflux vortex cavitation, and keeping the inlet at a distance of 1 × DsAn inlet pressure sensor is installed at the position of the pipeline, wherein the pipeline is a transparent organic glass pipeline and is shown in figure 5; simultaneously, the front and the side of the organic glass inlet section of the vane pump are respectively shot by adopting a method of combining a high-speed photographic camera and a digital camera, so that the inlet backflow state can be better observed as shown in figure 6; and 8 pressure pulsation sensors are installed in the circumferential direction of the surface of the volute, and one pressure pulsation sensor is installed at every 45 degrees, as shown in fig. 7. Pressure pulsation signals flowing inside the pump are synchronously collected, and a frequency spectrum analysis chart of cavitation flow inside the pump is obtained by extracting pressure pulsation information of pressure pulsation sensors at monitoring points of an inlet pipe, an impeller and an outlet pipe of the vane pump. Meanwhile, the signals of the pressure pulsation sensors at two adjacent positions, namely the main frequency, are subjected to phase crossing analysis, so that the phenomenon of flow instability and the characteristics of the flow instability in the pump under the working condition of small flow can be analyzed.
FIGS. 10(a1) and (a2) are frequency domain graphs of pressure pulsation at monitoring points inside the pump under the condition of low flow rate. The pressure pulsation main frequency of the monitoring point inside the pump is f11.465Hz and f2Similarly, the main frequency of the signals at each of 2 pressure pulsation monitoring points was subjected to phase cross-correlation analysis at 125Hz, and the results are shown in fig. 10(b1) and (b2), from which it is apparent that the frequency f is1Cross phase of
The values are all 0 degrees, and the axial flow phenomenon corresponding to the frequency can be considered to occur in the pump; frequency f2Cross phase
The values almost all exhibit a linear relationship, and it can be considered that a rotationally unstable flow phenomenon corresponding to the frequency of the range occurs inside the pump. The cross phase obtained by cross correlation analysis is obtained according to the installation phase difference delta theta between the two sensors being 45 degrees and the frequency spectrums of the pressure pulsation signals collected by the two sensors
Frequency of occurrence in pump f2In the case of the rotational unsteady flow phenomenon corresponding to 125Hz, there are 3 rotating units, and the real frequency is 1/3 of the pressure pulsation monitoring point frequency and is corresponding to the frequency f1The corresponding axial flow unstable flow phenomenon of 1.465Hz has low frequency characteristics, which also indicates that the axial unstable flow phenomenon inside the pump has low frequency characteristics under the condition of low flow. The frequency characteristic is the frequency characteristic when the back-flow vortex cavitation occurs inside the pump.
The present invention is not limited to the above-described embodiments, and any obvious improvements, substitutions or modifications can be made by those skilled in the art without departing from the spirit of the present invention.
Claims (7)
1. A device for realizing vortex cavitation of backflow of a vane pump is characterized by comprising a water tank (1), the vane pump (2), a visual test system (3) for realizing the reappearance of a pump inlet flow field, a pressure pulsation signal acquisition system (4) and a signal processing device (5);
the visual testing system (3) consists of a transparent organic glass inlet pipe (7), a high-speed camera (8) and a digital camera (9), the transparent organic glass inlet pipe (7) is used for connecting the vane pump (2) and the water tank (1), and the high-speed camera (8) and the digital camera (9) are respectively arranged on the front side and the side surface of an inlet section of the transparent organic glass inlet pipe (7); the outlet of the vane pump (2) is connected with the water tank (1) through a pipeline to form a loop, and a flowmeter (16) and a valve (17) are arranged on the outlet of the vane pump (2) and the pipeline of the water tank (1); the water tank (1) is also connected with a vacuum pump (19), and an exhaust valve (18) is also arranged on a pipeline between the vacuum pump (19) and the water tank (1);
the pressure pulsation signal acquisition system (4) comprises a pump inlet and a pump outlet which are arranged on the vane pump (2), and a plurality of pressure pulsation sensors (10, 11, 12) which are arranged along the circumferential direction of a volute flow channel of the vane pump (2); the pressure pulsation sensor (10) at the pump inlet and the pressure pulsation sensor (11) at the pump outlet are respectively arrangedMounted at a distance 1 x D from the impeller inletsRight above and at a distance 2 x D from the transparent plexiglass inlet tube (7) of the devicesAt least one of (1) and (b);
the signal processing device (5) is composed of a first acquisition card (20), a second acquisition card (21), a partial pressure plate (22), a power box (23) and a computer (24) filled with signal acquisition and data processing software, wherein the first acquisition card (20) is connected with a pump inlet pressure sensor (10) and an outlet pressure sensor (11), the second acquisition card (21) is connected with a plurality of pressure pulsation sensors (12), the first acquisition card (20) and the second acquisition card (21) are connected to the computer (24) filled with the signal acquisition and data processing software, and the first acquisition card (20) and the second acquisition card (21) are connected with the power box (23) through the partial pressure plate (22).
2. The device for testing vortex cavitation for realizing backflow of the vane pump according to claim 1, is characterized in that: the plurality of pressure pulsation sensors (12) are uniformly distributed in the circumferential direction inside the vane pump.
3. The device for testing vortex cavitation for realizing backflow of the vane pump according to claim 1, is characterized in that: the number of the pressure pulsation sensors (12) is 8.
4. The testing method for realizing the return flow vortex cavitation testing device of the vane pump as claimed in claim 1, is characterized by comprising the following steps:
1) completing the installation of the backflow vortex cavitation test device;
2) starting a power supply, starting the vane pump and enabling the rotating speed of the vane pump to reach the required testing rotating speed; starting a computer, and carrying out sensor offset acquisition before starting the computer;
3) setting the flow value for carrying out pump cavitation performance test as a low-flow working condition by adjusting a valve (17) in a pump outlet pipeline; in each flow working condition test, a valve (17) in the outlet pipeline is continuously adjusted so as to keep the flow value constant in the whole backflow vortex cavitation experiment;
4) regulating the pressure at the inlet of the water tank by starting the vacuum pump to change the pressure at the inlet of the vane pump, so that the inside of the vacuum pump is not cavitated; collecting corresponding cavity flow field distribution and pressure pulsation information thereof when the cavity in the vane pump occurs at the moment, and detecting the state that the cavity in the vane pump does not occur at the moment;
5) gradually reducing the pump inlet pressure of the vane pump through the vacuum pump, and setting the inlet pressure of the vane pump so as to gradually generate backflow vortex cavitation in the vane pump;
6) under the condition of constant inlet pressure or constant cavitation coefficient, acquiring cavitation test information of the vane pump synchronously acquired by a pressure pulsation sensor, a high-speed photography camera and a digital camera when the vane pump is cavitated in real time, and processing the following data:
a) calculating cavitation performance data of the vane pump:
by stepwise reduction of the inlet pressure p of the centrifugal pump1Thereby reducing the effective cavitation margin NPSHA until cavitation occurs; the cavitation starts from the initial state, and the initial stage of cavitation is referred to as NPSHA (NPSHR)3Indirectly calculating the cavitation allowance of the pump based on a pump cavitation characteristic equation to determine the initial cavitation performance point of the pump, and evaluating the pump cavitation performance; different NPSHA values correspond to different lift H values, a series of lift H are finally obtained and used for calculating a lift drop value caused by cavitation development, a curve of the lift changing along with cavitation allowance is further drawn, a cavitation performance curve of the pump can be obtained, the cavitation performance of the pump under various working conditions is compared, and a cavitation initial point is judged;
b) visual data of the cavitation flow field of the vane pump:
acquiring the process that the backflow vortex cavitation at the inlet of the pump occurs, develops and disappears;
c) vane pump pressure pulsation test data:
synchronously collecting pressure pulsation signals flowing inside the vane pump, introducing the pressure pulsation signals of pressure pulsation sensors at monitoring points of an inlet pipe, an impeller and an outlet pipe of the vane pump into data processing software for pressure pulsation frequency domain analysis by extracting the pressure pulsation information of the pressure pulsation sensors at the monitoring points, obtaining a pressure pulsation frequency domain diagram of each monitoring point by performing fast Fourier transform on the pressure pulsation signals of each monitoring point so as to obtain a frequency spectrum analysis diagram of cavitation flow inside the pump, and extracting pressure pulsation frequency domains of the inlet pipe, the outlet pipe and the volute, namely extracting peak signals in the pressure pulsation frequency domain diagrams of the inlet pipe, the outlet pipe and the volute; meanwhile, performing cross correlation analysis on pressure pulsation signals acquired by 2 pressure pulsation sensors which are the same in axial position but different in installation angle to acquire cross correlation phases of the pressure pulsation signals, determining the axial and radial characteristics of various cavitation phenomena of the pump, namely the number and the rotation direction of the rotating unit bodies, and judging whether the cavitation phenomena are axial instability phenomena or rotation instability phenomena so as to acquire the flow instability phenomena and the characteristics of the interior of the vane pump;
7) adjusting the working conditions, and repeating the steps 3) -6) until all the working conditions are finished.
5. The test method as claimed in claim 4, wherein the specific method of c) of step 7) is:
firstly, dividing pressure pulsation signals flowing inside the vane pump according to different time periods: each series of data having a total length of TrDivided into N time intervals of TdWhere d is 1,2, …, nd(ii) a Windowing the pressure pulsation signal of each interval by adopting a Hanning window function;
for data n in each time intervaldThe fourier transform is performed using equation (2),
wherein i is 1,2, …, nd(ii) a k is 0, 1, …, (N-1); Δ t is the sampling time in units of s; x is the number ofinAny collected data point; n isdThe number of data in equal interval is nd；Xi(fk) Is a frequency domain signal, Hz.
Taking the influence of a Hanning window function into consideration, and carrying out scale conversion on the data subjected to Fourier transform; obtaining a frequency domain diagram of each monitoring point, carrying out pressure pulsation frequency domain analysis, thus obtaining a frequency spectrum analysis diagram of the cavitation flow in the pump, and extracting pressure pulsation frequency domains of the inlet pipe, the outlet pipe and the volute of the pump;
the cross correlation analysis is carried out on a frequency spectrum analysis chart of the pump internal cavitation phenomenon obtained by carrying out Fourier transform on signals collected by 2 pressure sensors at the same axial position but different installation angles based on a formula (3),
in the formula, Sxy(fk) Analyzing a function for signal correlation; xi(fk)，Yi(fk) Is the pressure pulsation frequency domain signal at 2 different locations, Hz.
If it is cross-correlated in phase
The value is 0 degrees, and the unstable cavitation phenomenon of axial flow occurs in the vane pump;
if it is cross-correlated in phase
When the pump is in operation, the phenomenon of unstable cavitation appears in the vane pump, and the number of the rotating unit bodies
Where Δ θ is the angular interval at which the 2 adjacent pressure sensors are actually mounted.
6. The testing method of claim 4, wherein the obtaining of the initial and developed flow field information of cavitation in the vane pump at low flow rate is performed by photographing the front and side surfaces of the transparent organic glass inlet section of the vane pump with a high-speed camera and a digital camera, respectively.
7. The test method of claim 4, wherein in step 7) c), the data processing software is Origin or Matlab software.
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