Visual experimental study on cavitation performance of double-suction centrifugal pump

In this paper, the cavitation test results of double-suction centrifugal pump are further “visualized” and verified by using high-speed photography technology to photograph the development process and flow field of impeller inlet cavitation. By optimizing the impeller inlet parameters, the key geometric factors affecting the cavitation performance of the double suction pump were determined. The experimental results show that by optimizing the inlet parameters of the double-suction pump and combining the visualization test verification of cavitation, on the basis of ensuring the wide and efficient efficiency of the double-suction centrifugal pump, the optimization scheme of the experimental design greatly improves its cavitation performance, and provides a new design idea and reference for guiding the development and design of products with high cavitation performance.


Introduction
Double suction pumps are widely used in buildings, waterworks, power plants, steel mills, air conditioning circulating water, water conservancy projects, irrigation area water supply and other industries [1][2][3][4][5][6][7][8].When the double suction pump is operated with the absolute pressure of the impeller inlet decreased, and the local pressure is lower than the local vaporization pressure, cavitation and cavitation erosion may occur.
The occurrence of cavitation will not only cause the performance of the double-suction pump to decrease, but also cause vibration and noise, and even cause a series of problems such as the pump to stop running, which is a difficult problem that has not been effectively solved at present.Compared with the study of efficiency, the study of cavitation performance is more difficult, and the research results are far from meeting the actual needs.

Design and research method of visual test scheme in the double suction centrifugal pump
The double suction pump will determine its cavitation allowance value through the specified cavitation performance test before leaving the factory, and logically it is believed that cavitation should not occur when the double suction pump works normally, but it will still be seriously cavitated and cavitated in actual operation.Due to the complexity of cavitation, the current understanding of cavitation phenomenon is not deep enough, and the study of the mechanism of cavitation is relatively weak.
The cavitation failure of the double suction pump directly affects the operation reliability and service life of the pump, and because of its double helix inlet suction chamber, there is an inhomogeneity of the flow state, therefore, in order to ensure the safe, stable and efficient operation of the double suction pump, by adding a visual test and observation window to observe the primary cavitation of the pump blade inlet in the inlet suction section, by observing the flow state distribution of the blade inlet during cavitation primary, critical cavitation, analysing the law of cavitation primary birth allowance and critical cavitation allowance, and improving the cavitation performance of the double suction pump.It provides a strong reference for the operation of the pump without cavitation.
The flow of liquid flow in the impeller of the double-suction centrifugal pump has a great influence on the cavitation performance, and reasonable design of the impeller inlet parameters can effectively improve the cavitation performance, so it is of great significance to study the impeller inlet structure in depth to improve the cavitation performance.
The cavitation performance of centrifugal pumps is improved by using cavitation-resistant materials or by rationally designing the impeller structure.In this paper, the pump body and suction section of the experimental research scheme remain unchanged, and 304 stainless steel is selected as the impeller material, which achieves the purpose of improving the cavitation performance of the double-suction pump through the optimized impeller inlet structure.

2.1． Experimental design
In this paper, the orthogonal design is applied the mode of L9 (3 4 ).That is taking nine experiments, there are total four factors, each factor has three levels.The performance index is NPSHr (This article is equivalent to NPSH3.Which will be explained later and will not be repeated), this experimental design can reduce the number of tests and achieve maximum optimization performance of parameters.Table 1 shows the target parameters of the study model.Table 2 shows a description of the cavitation criteria and associated parameters that may be relevant in this article [12][13][14][15][16][17][18].• NPSH3: The cavitation allowance tested by the double suction centrifugal pump when the head drops by 3%.
• P: Matching motor power; • C: Cavitation ratio revolutions; The number of cavitation ratio revolutions corresponding to the cavitation allowance tested by the double-suction centrifugal pump when the head drops by 3%.The data of C serves as a number of similar criteria for cavitation and marks the quality of cavitation performance.The higher the C value (the smaller the NPSHr), the better the cavitation resistance of the pump.The approximate range of C values is shown in Table 3.

NO.
C Remarks Pumps with high cavitation resistance.

800~1000
Pumps that combine efficiency and cavitation resistance.

600~800
Cavitation resistance does not make a requirement, mainly consider improving the efficiency of the pump.
The impeller inlet diameter, the curvature radius of the front cover inlet part, the thickness of the blade inlet side, the position and shape of the inlet side of the blade, and other 4 factors, according to the level of three groups, nine sets of test schemes were obtained, as shown in Table 4, through cavitation visualization test, the performance index: NPSHr, the primary and secondary order of the four factors affecting the cavitation performance of the pump was obtained by comparative analysis, and the optimal parameter combination was obtained to achieve the goal of improving the cavitation performance of the pump.
Considering the measures to improve the cavitation performance of centrifugal pumps, there are generally two kinds of measures, one is to improve the shape of the vane inlet; one is to choose high-performance anti-cavitation materials.In this paper, the choice of the material of the overflow components are the same, only the first way that changes the shape of the vane inlet in order to obtain more excellent cavitation performance of the double suction centrifugal pump.There are many parameters that affect the shape of the vane inlet, this paper has completed the preliminary selection of the vane inlet parameters through numerical calculations, due to space limitations are not repeated.In this paper, only the important factors Di, δ, DR and RA are taken as the key factors for the study of cavitation performance for experimental verification.The symbols in table 4 are explained as follows.
• Dj: Impeller inlet diameter • δ: Blade inlet edge thickness; • DR: The location of the inlet side • RA: Radius of curvature of the front cover.

Remarks：
• Through the obtained test results, the range and variance are analyzed, the larger the range, the more obvious the impact of this factor on the test.• Determine the primary and secondary order of the influence of the four factors (according to the range size R), and determine the combination of four factors at the optimal level.(*Additional testing may be required later.)The cavitation test of the above nine groups of optimization tests was carried out, and the best scheme for cavitation performance was optimized, and the visual test verification and performance analysis were completed.

Research methods
The optimal scheme is visualized and analysed through orthogonal experimental design, and the dimensionless coefficients are defined to analyse the changes of pump performance under cavitation under different flow coefficients.
According to the definition of cavitation: the pressure at the lowest point in the pump is equal to the vaporization pressure ( ), and the pump is in a state where cavitation occurs.The lowest pressure point in the pump usually occurs after the inlet of the back of the blade, because it is inconvenient to measure, it will be replaced by the average pressure near the suction port section of the pump, and it is represented by the suction port pressure value when the pump cavitates at the beginning.According to GB/T13006 "Centrifugal pump, mixed flow pump and axial flow pump NPSH margin", defined in "Cavitation allowance for centrifugal pumps, mixed flow pumps and axial flow pumps": cavitation allowance when the first stage head of the pump drops by 3%: NPSH3 as the standard for critical cavitation allowance, and represents the suction pressure value under the GB/T13006 standard.
NPSH3 is the calculation formula of critical NPSH3, and the cavitation test keeps the flow rate unchanged, and the system pressure is gradually reduced by vacuum in the closed cycle system, so that cavitation occurs in the pump.
Formulas Containing (1): is the absolute pressure of the inlet pressure measuring point of the test device, measured by the absolute pressure transmitter, Unit：Pa; P ：It is the saturated vapor pressure at the test water temperature, Unit： Pa; • (4) h :It is the height value of the absolute pressure transmitter higher than the intersection point between the rotation center line of the pump blade and the impeller housing of the test device, Unit：m; • (5)  :medium density, Unit：kg/m 3 ; • (6) g :acceleration due to gravity, Unit：m/s 2 ; • (7) j: 0.8,0.9,1.0,1.1,1.2, x, etc.
During the test, the flow rate remains constant and the head drop of 3% is determined as the critical NPSH.
Under different working conditions, the initial head, cavitation allowance and efficiency of the double-suction centrifugal pump at the beginning of cavitation are E and i F are introduced, and the detailed formulas are shown in ( 2), ( 3), ( 4).Through such dimensionless parameters, the changes and distribution rules of the external characteristics of the corresponding cavitation primary and critical cavitation pumps of double-suction centrifugal pumps under different working conditions are explored.
In the formula, j=0.8,0.9,1.0,1.1,1.2, x, etc. x is the ratio of the maximum flow rate that can be tested by the test device to the rated flow.

Experimental research equipment
In this paper, the experimental research was carried out on the high-precision test bench of Shanghai Kai-Quan, and the visual test and research device of the cavitation of the double suction pump was designed on both sides of the suction section of the double suction pump to facilitate the observation of the initial generation of blade cavitation.

Specific steps:
• (1-1)Four visualization windows on each side of the suction chamber of the double suction pump are designed to observe the impeller inlet side, 2 observation windows are used for the observation of the initial generation of blade inlet cavitation, and the other 2 observation windows on the same side are used to assist in observing the light source input at the beginning of blade cavitation.
• (2-2)A high-speed camera used to observe the primary generation and flow field state of cavitation at the inlet of the impeller by designing and arranging outside the observation window of the primary generation of cavitation at the inlet of the blade.
• (3-3)The observation angle and arrangement position of the high-speed camera need to be arranged through the field test device to ensure that the initial phenomenon of leaf cavitation can be observed.
• (4-4)Set four pressure monitoring points evenly at the inlet of the suction section, and four pressure monitoring points evenly set in the outlet section of the extrusion chamber, shoot through the high-speed camera screen, when the observation window transmits the double suction pump impeller cavitation primary, record the average pressure of the inlet of the suction section and the outlet of the pressure-out chamber and calculate the corresponding cavitation primary margin.In this article, the hydraulic components, volute and impeller are tested, as shown in Figure 3.In Figure 4, in order to use a high-speed camera to capture the development process of clear cavitation, four transparent observation windows were opened in the upper part of the suction section of the double-suction centrifugal pump, and four auxiliary lighting windows were opened in the lower part of the suction section for light source lighting.The surface of the impeller used in the text is painted to prevent the glass from being inconvenient to observe.
Figure 5 shows the actual cavitation performance test site, this paper involves cavitation performance test, all completed on the multi-functional high-precision hydraulic R&D test bench of Shanghai Kai-Quan Pump Group Co., Ltd.(shown in Figure 6), its test accuracy is 0.25%, and the possibility of test can be guaranteed.

Analysis of test results
In this paper, the test was completed on the high-precision multi-functional hydraulic performance research test bench in Shanghai Kai-Quan(Fig.5), and the flow point of the cavitation performance visualization test: 0.8Qn、0.9Qn、Qn、1.1Qn、1.2Qn.

External characteristics and cavitation performance test
Firstly, the external characteristics and cavitation performance tests of 0.8Qn, Qn and 1.2Qn were carried out for the nine impeller schemes designed in orthogonal experiments.The test data are shown in Table 5.
Then, through the comparative analysis of the R&D requirements of Table 1 and the external characteristics of Table 5 and the cavitation test results, the preferred principle: the head error is 3%, the higher the efficiency, the better, and the lower the cavitation allowance, the better.
Finally, according to the optimization rule, S4 is selected as the experimental research scheme for the final visualization of cavitation performance.In Table 5, the C value is up to 1468 (S7), and more than 1400 up to 6 groups of schemes, and the lowest C value is 1008, which also meets the requirements of high cavitation resistance.The minimum critical cavitation allowance is predicted to be 3.16m (S7) based on a 3% drop in head, and a minimum cavitation allowance of 3.9 m (S4) is predicted according to 1% decrease in efficiency.
The efficiency of this scheme is up to 88.11%, and the head also meets the target requirements.It shows that when the critical cavitation allowance is predicted by 3% head, the performance of the pump is still degraded, and when the efficiency is predicted by 1%, the performance is still in a better state.
Whether cavitation occurs or not needs to be further confirmed by cavitation primary observation tests.
Description of physical parameters in Table 5: • Hn：The rated point head tested by the double suction centrifugal pump at the rated flow.
• ηBEP：Optimum efficiency tested at full flow with double suction centrifugal pumps.
• NPSHC：The cavitation allowance tested by the double-suction centrifugal pump at a 1% decrease in efficiency.• NPSH3：The cavitation allowance tested by the double suction centrifugal pump when the head drops by 3%.
• C1：The number of cavitation ratio revolutions corresponding to the cavitation allowance tested by the double-suction centrifugal pump when the efficiency decreases by 1%.
• C2: The number of cavitation ratio revolutions corresponding to the cavitation allowance tested by the double-suction centrifugal pump when the head drops by 3%.
• S1 is the original initial protocol, and the other protocols are orthogonal experimental design preferred schemes.

Cavitation visualization experiments
Using a high-speed camera, the cavitation visualization test of the optimized scheme S4 was carried out, and the cavitation allowance near the occurrence of cavitation that could not be carried out in the cavitation performance test was quantitatively tested by observing the cavitation first occurrence and development of the transparent glass pipe section inlet in the water absorption section, filling the gap of the traditional cavitation performance before.6 can be found that in different cavitation processes, the number of cavitations and the gathering area of the impeller inlet are changing, when cavitation is born, a small number of tiny cavitations are attached to the surface of the impeller inlet, and with the development of cavitation, the cavitations continue to increase and grow and collapse with the rotation of the blades, until the entire inlet side is covered by bubbles to achieve complete cavitation.
In Figure 6, it can be seen that the cavitation is not clear through the observation window, and due to complex reasons, such as the water quality of the system and the coating material of the overflow parts, according to the problems and results of this test, we will continue to improve and further study this topic.
Table 6, the external characteristic parameters of optimal scheme 4 are shown, the efficiency is 82.93% under 0.8 times the rated flow, the efficiency is 84.98% under 1.2 times the rated flow, and under the condition of 0.8 times the rated flow rate to 1.2 times the rated flow, the high efficiency zone is wider and the optimal efficiency point is one point higher than the energy saving and high efficiency, and the cavitation performance is excellent.In Table7, the characterization values of cavitation performance and the number of cavitation-less revolutions in different cavitation states are shown to characterize the advantages and disadvantages of cavitation.In the fourth scheme of this paper, the head is reduced by 3% to calculate the critical cavitation ratio revolution, reaching 1468, and the experimental data prove that this scheme is a double-suction centrifugal pump with ideal cavitation performance.Figure 7 shows the changes and curve distribution of primary cavitation allowance and efficiency decrease of 1% and head decrease of 3% under different flow conditions.It can be seen in the figure that under the rated working conditions, the three cavitation allowance values are the lowest, the flow rate decreases and increases, the pump runs under unstable working conditions, and the cavitation performance is deteriorating.
Figure 8 and Figure 9 respectively show the external characteristic curves of the double-suction centrifugal pump under different operating conditions, and the Hi and ηi characterized in the figure are data that are determined synchronously through the observation of high-speed cameras through the visualization window.
The head curve and efficiency curve meet the performance trend of double-suction centrifugal pump, and with the increase of flow, the head decreases, the efficiency shows a parabolic trend and there is an optimal solution.Because the difference between head and efficiency under different cavitation standards exists in different cavitation states, the flow characteristics of the medium change, and the flow field loss and pressure drop are different, resulting in the difference of the final result curve.As shown in Figure 9, when the head drop of 3% is the critical cavitation allowance defined by the standard, the cavitation state of the pump is more serious, and the efficiency decline is more obvious.Compared with the primary cavitation, the maximum efficiency difference under the same flow condition is about 7%.The conformity between cavitation and the optimal efficiency curve is good, which can be used as experience in this scheme research for further verification and induction.
Figure 10.Relationship curve between flow rate of double-suction centrifugal pump and dimensionless parameters corresponding to different cavitation allowances.Figure 10 characterizes the analysis results of the dimensionless test data defined in Equation ( 2) ~ (4), and it can be found that there is an extreme point for the dimensionless number Ei of the cavitation allowance, that is, there is a minimum under the rated working conditions.The dimensionless Di and Fi of head and efficiency are flat and smooth as the working conditions change.
When approaching a large number of working conditions, the change from the previous law may be related to the performance parameters reflected by the flow characteristics of the pump itself under large flow conditions, and this discovery needs to be summarized and analyzed through more practical cases in order to better apply it in engineering practice.
Among them, the parameters in the figure are explained as follows: • NPSHi: cavitation allowance tested at the beginning of cavitation of the double-suction centrifugal pump.• NPSHC: cavitation allowance tested by the double-suction centrifugal pump when the efficiency decreases by 1%.
• NPSH3: cavitation allowance tested by the double-suction centrifugal pump when the head drops by 3%.
• Hn: The rated point head of the double-suction centrifugal pump tested at the rated flow.
• Hi: Head value tested at the beginning of cavitation of double-suction centrifugal pump.
• HC: Head value tested by double suction centrifugal pump when efficiency decreases by 1%.
• H3: The head value tested by the double-suction centrifugal pump when the head drops by 3%.
• ηi: pump efficiency value tested at the beginning of cavitation of double-suction centrifugal pump.
• ηC: The pump efficiency value tested by the double suction centrifugal pump when the efficiency decreases by 1%.
• η3: The pump efficiency value tested by the double suction centrifugal pump when the head drops by 3%.
• ηBEP: The optimal efficiency value tested by the double suction centrifugal pump at full flow.
• Di: Dimensionless parameters of double-suction centrifugal pump in cavitation primary and critical cavitation (head drop by 3%).
• Ei: No element parameters of cavitation allowance of double-suction centrifugal pump in cavitation primordial and critical cavitation (head reduction of 3%).
• Fi: No dimensional parameters for the efficiency of double-suction centrifugal pump in cavitation primary and critical cavitation (head drop by 3%).

Conclusion
1.Through experimental verification, optimizing the impeller inlet structure and shape parameters of the double-suction centrifugal pump is a feasible way to improve the cavitation performance of the double-suction centrifugal pump.2.High-speed photography technology was used to photograph the cavitation primary bubbles and flow field of the impeller inlet, and further "visual verification" of the pump critical cavitation test results.Through the experimental analysis, when cavitation allowance standard is characterized by cavitation primary, the external characteristic parameters such as efficiency and head are in good agreement with the optimal value of the pump in this state.3.Through the flow field and performance distribution law of cavitation primary, critical cavitation, the test conditions required for the complete cavitation-free operation of the double-suction pump are predicted.When the cavitation of the double-suction pump is initially born to critical cavitation, the flow state evolution characteristics of the blade inlet and the cavitation volume distribution are based on the initial generation mechanism of cavitation, which provides strong test data support for the design or optimization of high cavitation performance of the vane pump.4.By refining and in-depth study of the external characteristics of pumps in the initial birth and development of cavitation of double-suction centrifugal pumps, this test law is expected to further promote the theoretical research of pump cavitation, and at the same time provide a new design idea and reference for the cavitation design of centrifugal pumps.5.In this paper, the visualisation of the cavitation performance of double-suction centrifugal pumps has some limitations in view of the limitations of model selection, test conditions and test methods.In the next stage of research, the author will optimize and adjust the test device and test programme.This will be a long-term engineering practice and solve the pump user site problems, while there is a certain degree of difficulty in the research topic.
corresponding head, cavitation allowance, efficiency and efficiency values when critical cavitation is reached, respectively:

Figure 2 . 3 .
Figure 2. Schematic diagram of the pump set of the visualization test research device for the cavitation performance of the double-suction centrifugal pump.In Figure 2, the components of the cavitation performance visualization test research device built here are as follows.①Importedtransparent glass section (It is used to observe the inlet flow state of the pump of the test device and the gas content of the system.) ②Water absorption section pressure test section ③ Plexiglass observation window ④Outlet pressure test section ⑤Pressure pulsation measurement point (The analysis of pressure pulsation is not explained in this article, which is noted here.)⑥ Test pump unit

Figure 4 .
Figure 4. Test site for cavitation visualization of a double-suction centrifugal pump.

Figure 6 .
Figure 6.Initial birth and development process of cavitation near the impeller inlet of double-suction centrifugal pump.Figure6can be found that in different cavitation processes, the number of cavitations and the gathering area of the impeller inlet are changing, when cavitation is born, a small number of tiny cavitations are attached to the surface of the impeller inlet, and with the development of cavitation, the cavitations continue to increase and grow and collapse with the rotation of the blades, until the entire inlet side is covered by bubbles to achieve complete cavitation.In Figure6, it can be seen that the cavitation is not clear through the observation window, and due to complex reasons, such as the water quality of the system and the coating material of the overflow parts, according to the problems and results of this test, we will continue to improve and further study this topic.Table6, the external characteristic parameters of optimal scheme 4 are shown, the efficiency is 82.93% under 0.8 times the rated flow, the efficiency is 84.98% under 1.2 times the rated flow, and under the condition of 0.8 times the rated flow rate to 1.2 times the rated flow, the high efficiency zone is wider and the optimal efficiency point is one point higher than the energy saving and high efficiency, and the cavitation performance is excellent.

Figure
Figure 6.Initial birth and development process of cavitation near the impeller inlet of double-suction centrifugal pump.Figure6can be found that in different cavitation processes, the number of cavitations and the gathering area of the impeller inlet are changing, when cavitation is born, a small number of tiny cavitations are attached to the surface of the impeller inlet, and with the development of cavitation, the cavitations continue to increase and grow and collapse with the rotation of the blades, until the entire inlet side is covered by bubbles to achieve complete cavitation.In Figure6, it can be seen that the cavitation is not clear through the observation window, and due to complex reasons, such as the water quality of the system and the coating material of the overflow parts, according to the problems and results of this test, we will continue to improve and further study this topic.Table6, the external characteristic parameters of optimal scheme 4 are shown, the efficiency is 82.93% under 0.8 times the rated flow, the efficiency is 84.98% under 1.2 times the rated flow, and under the condition of 0.8 times the rated flow rate to 1.2 times the rated flow, the high efficiency zone is wider and the optimal efficiency point is one point higher than the energy saving and high efficiency, and the cavitation performance is excellent.

Figure 7 .
Figure 7. Relationship curve between flow rate and different cavitation allowances of double-suction centrifugal pump.

Figure 8 .
Figure 8. Relationship curve between flow rate of double-suction centrifugal pump and corresponding heads of different cavitation allowances.

Figure 9 .
Figure 9. Relationship curve between flow rate of double-suction centrifugal pump and corresponding efficiency of different cavitation allowances.

Table 1 .
Target parameters of the study model.

Table 5 .
External characteristics and cavitation performance test of double-suction centrifugal pump

Table 6 .
Preferred protocol test data for double-suction centrifugal pumps.

Table 7 .
Data analysis table of cavitation visualization test of preferred scheme for double-suction centrifugal pump.