Study on the experiment and numerical simulation of cavitation flow mechanisms at different flow rates in water-jet propulsion pumps

Cavitation is an important factor causing vibration and noise of water-jet pump. High-performance ships require water-jet pumps with high efficiency and low noise at high speeds. At present, the cavitation flow phenomenon and interference mechanism of the water-jet pump under different flow rates are not clear enough. This paper takes the mixed-flow water jet propulsion pump as the research object. The flow structure of the cavitation vortex of the water-jet pump under different flow rates was obtained by high-speed photography (HSP). Through comparison of numerical simulation and test results, the cavitation performance curves and cavitation flow structures of the water jet pump under five different flow rates were obtained. The influence of cavitation vortex structure on the performance of water-jet pump under different flow rates was obtained. The correlation between cavitation performance and cavitation vortex structure under different flow rates was established. This research work will help to understand the impact of the cavitation vortex structure of the water-jet pump under different flow rates and capture the evolution law of the cavitation vortex structure of the water-jet pump under different flow rates. And provide reference for improving the cavitation performance of water-jet pumps.


Introduction
Water-jet propulsion is a propulsion method that makes use of the response pressure of the water sprayed by using the propulsion pump to propel the ship ahead [1].This technology has been widely used in various fields due to its strong anti-cavitation performance, high propulsion efficiency, and low vibration and noise levels.Application and development [2][3][4].The flow rate and head of the mixed-flow water jet propulsion pump are moderate, and the advantages of axial flow pumps and centrifugal pumps are taken into account in terms of structure and performance [5].When cavitation happens in the water jet propulsion pump, it will now not solely limit the thrust, however additionally purpose a reduce in effectivity and extend in noise [6], which is no longer conducive to the protected and steady operation of the pump.The cavitation of the water jet propulsion gadget will lead to accelerated erosion of the blade surface, which will in addition lead to the degradation of the overall performance of the water jet propulsion pump.Serious surface erosion of hydraulic components requires suspension for repair or component replacement, which not only reduces the overall speed of the ship, but also causes high maintenance costs.Degraded overall performance of the water jet propulsion pump can additionally have an effect on the maneuverability of the ship.
Prior to this, the team built a water jet propulsion pump comprehensive performance test and test platform.Through the cavitation performance test, the model pumps under 1.000Q, 0.957Q, 0.913Q, 0.870Q and 0.804Q (0.46m 3 /s, 0.44m 3 /s, 0.42 m 3 /s, 0.40m 3 /s and 0.37 m 3 /s) head with the change curve of the NPSHr [7].In addition, combined with numerical simulation and experimental results, the cavitation flow structure, evolution law and influence on pump performance of the water jet propulsion pump under the critical NPSH at the design flow rate (0.46 m 3 /s) were described and analyzed [8].In order to set up the correlation between cavitation go with the flow shape and cavitation performance, the cavitation vortex glide traits of cavitation era and improvement are got to supply extra scientific aid for cavitation overall performance prediction [9][10].
By extracting the 24m/s water pace isosurface and examining the water floor pace on the isovalue surface, the group printed the float traits of the high-velocity fluid sector underneath one of a kind cavitation stages.Through the evaluation of the equal floor vortex structure, the foremost elements that have an effect on the improvement of the vortex shape in the high-velocity fluid place are summarized [11].Then the group simulated the cavitation traits of the impeller tip clearance leakage float and tip leakage vortex beneath distinct cavitation conditions.The glide mechanism of the impeller tip leakage waft and the separation vortex precipitated via the cavitation sector below special cavitation stipulations have been revealed.The predominant elements affecting the improvement of cavitation wake shape have been summarized [12].
Other groups have also done studies on the cavitation performance of waterjet propulsion pumps.Liu Chengjiang et al. [13] calculated and analyzed the cavitation performance of a mixed-flow water jet propulsion pump model by numerical simulation method, and numerically predicted the cavitation performance under different flow rates, and found that the trend was consistent with the test results.As the design point decreases, the error between the corresponding critical NPSH at each flow rate obtained by the test and numerical simulation shows a decreasing trend.Xu Shun et al. [14] took the axial-flow water jet propulsion pump as the object, and carried out numerical calculations on the head coefficient and efficiency under the design operating point.It is found that the average error between the numerically calculated lift coefficient and the experimentally measured value is 2%, and the average error of the efficiency between the two is 2.2%.The relative error between the test value and the cavitation coefficient corresponding to the numerical simulation is 8.89% at the critical cavitation condition point.
In order to further explore the flow structure of the cavitation vortex in the pump under different flow rates and the influence of the cavitation vortex on the performance of the water jet propulsion pump under different flow rates, this paper takes the high-performance water jet propulsion pump model developed by the team as the research object, and uses CFD simulation to reveal The cavitation flow structure and evolution mechanism of the jet pump under five different flow rates of 0.46m 3 /s, 0.44m 3 /s, 0.42 m 3 /s, 0.40m 3 /s, and 0.37 m 3 /s, to establish the cavitation flow structure and cavitation performance correlation between.Combined with high-speed photography (HSP) to obtain the flow structure of cavitation vortex under different flow rates, the influence of cavitation vortex structure on the performance of water jet propulsion pump is studied.

Model pump
The model pump studied is a mixed-flow water jet propulsion pump numbered WJP001 The threedimensional model of the impeller and rear guide vane mixed-flow hydraulic model is shown in figure 1, including the inlet suction pipe, deflector cap, impeller, guide vane body, perspective window, etc.Its physical map is shown in figure 2. See Reference 7 for the design parameters of the water-jet propulsion test pump.

Grid division
The number and quality of meshes have a great influence on numerical simulations.In order to ensure calculation accuracy and calculation efficiency, ICEM CFD is used to divide the calculation domain of the water jet pump inlet pipe and outlet pipe, BladeGen is used to construct the geometric model of the blade and flow channel, and Turbogrid is used to perform structural meshing on the impeller and guide vane divided.The specific division results are shown in figure 4.Under the premise of meeting the calculation requirements, the total number of grids is about 4.13 million.Mesh independence checks have been done in previous studies.

Numerical Method and boundary conditions
In order to obtain the cavitation flow structure inside the water jet propulsion pump under different flow rates, this study uses Ansys CFX commercial software to calculate the set model pump.The cavitation method used is consistent with the previous research, both using the SST k-ω turbulence model [12].When cavitation does not occur, the fluid is set to be liquid water at 25°C with a density of 997kg/m 3 .When cavitation occurs, add the cavitation model and set it to gas phase water vapor at 25°C with a density of 0.02308 kg/m 3 .The saturated vapor pressure of water at 25°C is 3169pa.Set the number of calculation steps to 4000 and monitor the head and efficiency of the water jet propulsion pump under different flow conditions.Monitor the head and efficiency of the jet pump under different working conditions.Under each flow condition, the inlet pressure setting value gradually decreases from 100000pa, and the NPSHa corresponding to the 3% head drop point is obtained in the post-processing.The setting of boundary conditions is the same as the previous research [12].

Cavitation performance curves at different flow rates
In order to comprehensively study the cavitation performance of the water jet propulsion pump, the CFX numerical simulation method was used to solve the problem, and the cavitation performance curve of the water jet propulsion pump under different flow rates was drawn.As shown in figure 5.The critical cavitation point corresponding to 3% head drop at different flow rates is obtained.At this point NPSHa is equal to the NPSHc of the pump.The red mark in the figure is the position of the critical cavitation condition point.In general, the head curves corresponding to different flow rates increase as the flow rate decreases, because the flow rate is inversely proportional to the head.The head decreases with the decrease of NPSHa, because as the inlet pressure decreases, the degree of cavitation continues to develop, and the head continues to decrease.The decrease trend of the head curves near the critical NPSH becomes gentler with the decrease of flow rate.This is due to the change of flow rate which causes the change of liquid flow angle, which leads to the change of angle of attack between the blades.This will further lead to deflow on the pressure surface or suction surface of the blade, thereby intensifying the development of cavitation.
Figure 6 is the cavitation performance curve of the water jet propulsion pump obtained according to the test data [7].Since the initial value of the initial inlet pressure drop in the numerical simulation and the test is different, there is a certain difference in the range of the abscissa.Comparing the cavitation performance curves of the two at different flow rates, it is not difficult to find from the figure that the overall change trends of the two are almost the same, and the change laws of the cavitation performance curves at different flow rates are also consistent.The downward trend of the head curve near the critical NPSH under the condition of large flow rate is steeper, which is the same rule as reflected by the numerical simulation.Comparing figure 5 and figure 6, it can be found that in the stage where no cavitation occurs, the error between the numerical simulation and the test is very small, within the allowable range of engineering.It shows that the error of numerical simulation prediction is relatively large under low flow rate.The error of numerical simulation prediction is small under large flow rate.In the cavitation development stage, the head drop process obtained by numerical simulation lags behind the head drop process obtained by the test, indicating that the cavitation model weakens the occurrence and development of cavitation, and it is necessary to study more accurate cavitation and select a more accurate cavitation.The model can accurately simulate the cavitation performance curve.8 that whether it is the NPSHc curve obtained by numerical calculation or experiment, as the flow rate decreases from 0.46m 3 /s to 0.37m 3 /s, the critical NPSH first decreases and then increases.And it reaches the minimum value near 0.4m 3 /s, the minimum value obtained by numerical simulation is about 6.31m, and the minimum value obtained by experiment is 7.106m.The error between the numerical calculation value and the experimental value under the flow rate of 0.46m 3 /s is very small, only 0.69%.As the flow rate decreases, the error between the two tends to increase.The NPSH value corresponding to the critical cavitation condition point obtained by the test is greater than the NPSH value corresponding to the critical cavitation condition point obtained by the numerical simulation.Comparing figure 5 and figure 6, it can also be seen that with the development of cavitation degree, the head drop process in numerical simulation lags behind that in experiment, indicating that the cavitation model weakens the occurrence and development degree of cavitation.As the flow rate decreases and deviates from the design flow condition, the cavitation model weakens the occurrence and development of cavitation more deeply.

Efficiency variation curves with NPSHa in different flow rates
Figure 8 is the performance curve of efficiency decreasing with NPSHa at different flow rates obtained through numerical simulation.Figure 9 is the performance curve obtained through the test.At different flow rates, the efficiency decreases with the decrease of NPSHa.From the two graphs, it can be seen that the overall curve law trend is consistent.From the stage where cavitation does not occur to the point where cavitation occurs, the overall flow rate of 0.44m 3 /s corresponds to the highest efficiency, followed by 0.46m 3 /s.Then as the flow rate decreases from 0.44m 3 /s, the efficiency performance curve of the water jet propulsion pump also gradually decreases.This is because when cavitation occurs in the pump, the energy exchange between the fluid and the impeller is disturbed and destroyed, and the efficiency is reduced in terms of external characteristics.As the flow rate decreases from 0.46m 3 /s to 0.37m 3 /s, the corresponding efficiency decay rate becomes slower after the critical cavitation point.This is because the friction loss in the hydraulic loss during pump operation is proportional to the square of the flow rate.Therefore the rate of efficiency loss is faster at high flows.The efficiency of the critical cavitation point at each flow presents a decreasing trend as the flow decreases.Numerical simulations show that as the flow rate decreases, the efficiency of the critical cavitation point at each flow rate also shows a decreasing trend.When the flow rate drops from 0.46m 3 /s to 0.44m 3 /s, the efficiency of the critical cavitation point will have a slight upward trend and reach the maximum at around 0.44m 3 /s.By comparing the efficiency changes at the critical cavitation point in figure 8 and figure 9, it is found that when the flow rate is 0.37m 3 /s, the efficiency of the critical cavitation point obtained by numerical simulation is 79.77%, and the efficiency obtained by experiment is about 78.13%.When the flow rate is 0.4m 3 /s, the efficiency of critical cavitation point obtained by numerical simulation is 81.95%, and the efficiency obtained by experiment is about 80.27%.When the flow rate is 0.42m 3 /s, the efficiency of critical cavitation point obtained by numerical simulation is 82.88%.The efficiency obtained by testing is about 80.63%.When the flow rate is 0.44 m3/s, the efficiency of critical cavitation point obtained by numerical simulation is 83.41%, and the efficiency obtained by experiment is about 81.98%.When the flow rate is 0.46m 3 /s, the efficiency of critical cavitation point obtained by numerical simulation is 83.5%, and the efficiency obtained by experiment is about 81.35%.The efficiency performance curve obtained by numerical simulation is slightly higher than the experimental value, and the error of numerical simulation also increases with the increase of flow rate.This is because the

The Cavitation volume distribution on impeller suction surface under critical cavitation condition at different flow rates
Through the numerical calculation and post-processing of the cavitation model, the cavitation area distribution diagram of the impeller blade suction surface corresponding to the critical cavitation state under five different flow rates is obtained, as shown in figure 10.It can be seen from figure 10 that as the flow rate decreases, the cavitation area on the suction surface of the impeller blade under the corresponding critical cavitation condition gradually expands.When NPSHc=7.44m;Q=0.46m 3 /s, the cavitation area on the suction surface of the impeller blade is the smallest, and when NPSHc=6.40m; Q=0.37m 3 /s, the cavitation area on the suction surface of the impeller blade is the largest.With the decrease of the flow rate, the location of the maximum cavitation volume fraction on the suction surface of the impeller blade under the corresponding critical cavitation condition is getting closer to the position of the hub and the leading edge of the blade.Previous studies have found that under the condition of 0.46m 3 /s, as the degree of cavitation increases, the cavitation area on the suction surface of the impeller blade gradually increases.When the critical cavitation condition point is exceeded, the cavitation volume fraction near the trailing edge of the suction side of the impeller blade is very high.Because as the degree of cavitation develops, a strong vortex will form at the trailing edge of the blade, causing the cavitation to fall off the suction surface [12].

Comparison of CFD and HSP on the flow structure of cavitation vortices in different flow rates
In order to clearly observe the cavitation flow state of the impeller part, the team set up a plexiglass visualization window on the impeller rim part so that excellent test images can be obtained.For the specific distribution of high-speed camera devices and the selected models of the devices, please refer to Reference [7].
The cavitation vortex structure captured by the high-speed camera test under the corresponding critical cavitation conditions under different flow rates was captured by high-speed camera technology, and compared with the cavitation vortex structure obtained by numerical calculation, as shown in figure 11   Comparing the numerical simulation and high-speed camera pictures of critical cavitation conditions under different flow rates, we found that the cavitation isosurface with a volume fraction of 0.03 and the vortex structure with a Lambda 2-Criterion with a level of 0.0073 can be well displayed Cavitation region and vortex center region.Numerical simulation results under critical cavitation conditions at different flow rates can be found that Blowing Vortex is formed at the tip of the blade, and the main flow direction forms a stagnant zone on pressure surface of the blade.A certain degree of sheet-like cavitation also appeared near the pressure surface of the blade leading edge.Angular vortex cavitation also occurs in the tip clearance.Sheet cavitation exists on the blade pressure surface.The tip clearance leakage vortex cavitation occurs on the suction surface of the blade tip.Shedding cloud-like cavitation occurs near the blade trailing edge and mid-flow surface [7].
By observing the high-speed camera photos of the cavitation vortex structure under different flow critical cavitation conditions.It can be found that there is a large degree of tip cavitation in the impeller tip area under all critical cavitation conditions under different flow rates.Large-scale violent tumbling cavitation can be observed in the entrained tip leakage vortex.The trailing edge of the triangular area extends toward the mainstream direction, and the size of the cavitation shedding at the trailing edge is large, and the entrainment and tumbling are very violent.The tail of the cloud-like cavitation begins to shed large pieces of cavitation, and the falling cavitation moves toward the adjacent blades.Its rotation direction is from the suction surface of the blade to the pressure surface of the adjacent blade.Affected by the entrainment of the blade tip leakage and the friction of the main flow, the shed cloud-like cavitation begins to be perpendicular to the pressure surface.The cloud-like cavitation that breaks up with the main cavitation appears in the triangular area and rolls vertically with the main flow cavitation, and the vortex enters.tip clearance [11].At this time, the tip cavitation and shedding cloud-like cavitation block the tip flow channel on a large scale, causing strong flow separation on the blade surface.This had a noticeable effect on pump performance, with a drop in head.
Comparing the cavitation vortex structures at critical cavitation conditions at five different flow rates, it is found that the area of the scraping vortex near the blade tip and the attached cavitation on the pressure surface of the blade leading edge is the largest at Q=0.46m 3 /s, The area is the smallest under Q=0.37m 3 /s.As the flow rate decreases, the attached cavitation area on the pressure surface of the leading edge of the scraping vortex blade at the blade tip tends to decrease.From Fig. 12 and Fig. 13, it can hardly be seen that there is much difference in the cavitation area of the angular vortex in the gap leakage flow at the blade tip gap.The cavitation area on the pressure surface of the blade increases with the decrease of the flow rate.The shedding vortex-like cavitation area near the trailing edge of the blade and the middle flow surface first decreases and then increases with the decrease of the flow rate, and the area is the smallest at Q=0.4m 3 /s.This is related to the previous study that the minimum value of critical NPSH corresponding to different flow rates is also obtained near Q=0.4m 3 /s.It can also be seen from Figure 11 that the cavitation area attached to the hub of the impeller increases as the flow rate decreases.
Although we have obtained the numerical simulation and high-speed camera pictures of the critical cavitation condition point of the water jet propulsion pump at different flow rates, and compared the two, we have obtained some different cavitation vortices in the numerical simulation and high-speed camera pictures.The distribution of structures and the evolution of different cavitation vortex structures with the change of flow rate.However, the general law of the correlation between the cavitation flow structure and the cavitation performance of the jet needs to be further explored.

Figure 2 .
Real hydraulic components of Water-jet Pump[7].The fluid domain of the hydraulic components of a waterjet propulsion pump, including inlet pipes, impellers, guide vanes and outlet pipes.The specific water body domain is shown in figure3.

4
(a) Assembly mesh (b) impeller (c) diffuser (d) The tip clearance Figure The mesh generation.

Figure 5 .
Figure 5. Waterjet pump cavitation performance curves in different flow rates by numerical calculation.

Figure 6 .
Figure 6.Waterjet pump cavitation performance curves in different flow rates by experimental data.The numerical calculation results and experimental comparison of the critical NPSH curves of the water jet propulsion pump at different flow rates are shown in figure 7. The position of the critical cavitation condition point is different under different flow rates.It can be seen from Fig.8that whether it is the NPSHc curve obtained by numerical calculation or experiment, as the flow rate decreases from 0.46m 3 /s to 0.37m 3 /s, the critical NPSH first decreases and then increases.And it reaches the minimum value near 0.4m 3 /s, the minimum value obtained by numerical simulation is about 6.31m, and the minimum value obtained by experiment is 7.106m.The error between the numerical calculation value and the experimental value under the flow rate of 0.46m 3 /s is very small, only 0.69%.As the flow rate decreases, the error between the two tends to increase.The NPSH value corresponding to the critical cavitation condition point obtained by the test is greater than the NPSH value corresponding to the critical cavitation condition point obtained by the numerical simulation.Comparing figure5and figure6, it can also be seen that with the development of cavitation degree, the head drop process in numerical

Figure 7 .
Figure 7. NPSHr curve of water-jet pump in different flow rates by numerical calculation and experimental comparison condition.

Figure 8
Figure 8 Curves of efficiency decreasing with NPSHa under different flow rates obtained through numerical simulation.

Figure 9 .
Figure 9. Curves of efficiency decreasing with NPSHa under different flow rates obtained through experiments.

Figure 10 .
Figure 10.The distribution of cavitation region of the impeller suction surface at critical cavitation condition in different flow rates.
and figure 12.