Design of a high-speed cavitation hydrodynamics test system

Due to the high sediment content in rivers in China and the relatively high concentration of “gas core” in water, cavitation of hydraulic machinery is likely to occur at high speeds, leading to a series of problems such as reduced efficiency of hydraulic machinery, intensified vibration and material erosion. To carry out the study of cavitation erosion, a test system (hereinafter referred to as the cavitation water tunnel) suitable for the basic study of cavitation dynamics at high velocity is designed in this paper. The whole system includes a circulating loop structure, which mainly consists of a suction chamber, contraction rectifier, and test section. The test section is convenient for observing different types of cavitation flow fields inside and the highest test velocity in the test section can exceed 35 m/s. A water-gas separation chamber is designed to collect bubbles over 100 microns to ensure that the inlet of the test section is not affected by cavitation. The functions of the test system include basic experiments such as flow field characteristics, cavitation dynamics, acoustic characteristics, and cavitation erosion characteristics around hydrofoils/blades, which can provide test conditions for the study of high-speed hydrodynamic characteristics such as pumps/turbines, propellers and underwater vehicles.


Introduction
Hydraulic machinery has played an important role in water conservancy projects such as inter-basin water diversion, agricultural irrigation and drainage, urban water supply and drainage, and hydropower generation.At present, the hydraulic machinery are developing towards large scale, the maximum velocity of water pumps can reach 60m/s [1].The sediment and "gas core" content of rivers in China are relatively high [2][3].So the cavitation problem of hydraulic machinery is very serious.Cavitation and cavitation erosion is difficult to predict in the design stage, eradicate in the operation process, and repair after the damage occurs, which is also called "cancer" of hydraulic machinery.Therefore, it is necessary to do more research on the cavitation of the hydraulic machinery.
At present, the test study of cavitation erosion is mainly based on the universal hydraulic machinery model test bench and high-speed cavitation water tunnel.The universal hydraulic machinery model test bed is mainly used to observe the cavitation phenomenon in the internal flow channels of the model unit.However, the three-dimensional structure of the hydraulic machinery impeller is complex, so the experiment is very difficult.The cavitation water tunnel is mainly designed for the ship propeller, and the maximum velocity is generally lower than 30m/s.To deeply understand the influence of cavitation and cavitation erosion on the performance of hydraulic machinery, it is urgent to build a high-speed cavitation water tunnel that can meet the needs of fundamental cavitation research of hydraulic machinery.
The development of cavitation water tunnels abroad can be traced back to 1896 when C. A. Parsons made the first attempt to study cavitation in Britain.In 1971, the Department of Oceanography, School of Ship Architecture and Ocean Engineering, Berlin University of Technology developed an open cavitation water tunnel for measuring propeller thrust and torque [4].A circulating cavitation water tunnel for cavitation research was built at Tokyo University in 1981, with a total length of 5m, and the speed of the test section can reach 19.5m/s [5].In 1987, the Swiss Federal Institute of Technology in Lausanne built a cavitation water tunnel specially used for cavitation research of hydraulic machinery.The maximum velocity in the test section reached 50 m/s [6].More than 200 cavitation water tunnels have been built abroad.
The research on the cavitation water tunnel in China started late, and the earliest cavitation water tunnel was built in Shanghai in 1957 [7].There is a big gap between domestic research on cavitation water tunnels and foreign advanced countries, but many universities and scientific research units have built about 13 cavitation water tunnels [8].For example, Shanghai Jiaotong University built the earliest closed-loop cavitation water tunnel in China, and Nanjing University of Science and Technology and Xi 'an Jiaotong University built cavitation water tunnels with low turbulence [9].Beihang University has built a cavitation water tunnel used for flow display [10].
Table 1 shows some information on some cavitation water tunnels at home and abroad.Table 1.Information of some cavitation water tunnels at home and abroad a ， b .

Overall structure
The cavitation water tunnel constructed this time is located in the experimental hall of College of Water Resources and Civil Engineering in the east campus of China Agricultural University.The length of the hall is 32.8m in the north-south direction, 30.2m in the east-west direction and 10.8m in the height.Due to the existence of cranes, the maximum available height is only 8m, while the area used for the construction of the cavitation water tunnel is only 170m 2 , with a length of 21.4m and a width of 7.9m.
Based on the investigation of the cavitation water tunnel, it is found that the circulating structure is the preferred structure in the design of most cavitation water tunnels, which not only can realize continuous experiments many times but also has the advantages of high safety, low test cost and less time consumption.Therefore, to save the construction cost and effectively use the construction space, the cavitation water tunnel designed in this paper is of vertical circulation structure, and the overall pipeline length is controlled at about 50m.

Design requirements
To observe the detailed flow field related to cavitation in hydraulic machinery, it is necessary to design a special structure for observing the flow field, which is generally called the test section.In the test section, a hydrofoil model used in the study of the cavitation mechanism of hydraulic machinery can be placed, its chord length is from 50mm to 150mm and its attack angle can be adjusted.The flow velocity in the test section should cover the range of flow velocity in the prototype of common hydraulic machinery.In addition, to expand the function of the cavitation water tunnel in the future, the test section itself should be detachable and replaceable.
To ensure that the water flow reaches the test section with good uniformity and low turbulence intensity, and to dissolve the gas in the water by increasing the flow time, a water storage tank with a big volume should be designed before the test section to meet the requirements of improving the flow and increasing the flow time.
To reduce the turbulent intensity of water flow between the water storage tank and the test section, it is necessary to set up a suitable device between them.According to the wind tunnel or other cavitation water tunnels, the device is generally a contraction nozzle, called a contraction rectifier.
To reduce the hydraulic loss of the system, the dynamic pressure of the fluid should be converted into static pressure after the water flow passes through the test section, so it is necessary to design a diffuser section that is as long as possible and has a good diffuser effect.It is necessary to install some rectifier plates at corners and other places where the flow changes sharply.
Oldenziel's research shows that when the diameter of bubbles in water is greater than 100μm, they will no longer dissolve with time [15].Therefore, a unique device needs to be developed to collect and discharge bubbles with a diameter greater than 100μm.
The cavitation water tunnel should be equipped with a pump with enough power to meet the requirement that the water flow reaches 35m/s in the test section.And the maximum flow of the pump should not exceed 0.85 m 3 /s to meet the flow time of not less than 100s.In addition, the pump itself should have good cavitation resistance and high efficiency.
Do not make special requirement for the temperature control of the cavitation water tunnel.However, to minimize the temperature change, the working time of the system should be as long as possible.And appropriate instruments should be selected to measure the temperature and speed of the system.
The above design requirements and corresponding implementation schemes are listed in Table 2.The signal is converted between the pressure transmitter and the host computer Cycle time is not less than 100s Control of length and volume of the cavitation water tunnel and pump flow

Overall layout
As shown in Figure 1, the north-south length, east-west width and height of the final cavitation water tunnel construction area are 21.4m, 7.95m, and 7.766m respectively.To prevent noise, dust, etc., its periphery is separated from the outside by an acrylic transparent plate with a thickness of 5mm, and the ground contains drainage ditches and drainage pipes, which are convenient for discharging water.

Circular loop layout
As shown in Figure 2, the final length of the cavitation water tunnel is designed to be 17.57m and the height is 7.766m m.After flowing out of the water pump, the water reaches the third floor, accelerates through the suction chamber and the contraction rectifier, and finally reaches the highest speed in the test section.After passing through the diffuser section with a length of 10.6m, the speed is reduced to about 3% of the experimental speed, and after rectifying through two elbows, it enters the upper layer of the water gas separation chamber in a good flow state.

Contraction rectifier
As shown in Figure 3, the contraction rectifier is formed by hot rolling of 20mm thick stainless steel to ensure that it can withstand the pressure of 2MPa.In 1976, Hussain et al. made a comparative study on the influence of the curves of several contraction nozzles on the flow, indicating that the curves can reduce the boundary layer thickness and the turbulence intensity.The profile of the longitudinal section and the cross-section of the contraction rectifier is formed by fitting two cubic equation curves of Formula (1) and Formula (2), respectively [16]: where x is the flow direction, and y is the radial length.With this design, the cross-section of the flow is changed from a rectangle of 2.1m×0.5m to a square of 0.15m×0.15m,and the shrinkage ratio is 46: 1.The larger shrinkage ratio and shrinkage profile greatly improve the uniformity of the fluid entering the test section.The interior is filled with concrete and epoxy, which can reduce the use of steel materials prevent water vapor from directly contacting with steel, and play an antioxidant role.

Suction chamber
The contraction rectifier of the cavitation water tunnel adopts the form of gradual change of square section.Therefore, between the contraction rectifier and the water storage tank, an appropriate component should be designed.
As shown in Figure 4, the whole suction chamber is made of stainless steel with a thickness of 20mm by hot rolling.In order to optimize the flow pattern and avoid flow separation, a fillet structure with a radius of 150mm was designed at the corner connecting the water storage tank and the suction chamber.To ensure that the fluid entering the contraction rectifier is fully developed turbulence, a honeycomblike rectifying plate is designed at 150mm from the exit of the suction chamber, the hydraulic diameter of the regular hexagonal holes and the thickness of the rectifying plate is 4.5mm and 60 mm respectively.To ensure the uniformity of the flow, a screen with a thickness of 5mm is arranged at the joint of the suction chamber and the contraction rectifier, which is composed of a number of round holes with a diameter of 6mm and a spacing of 7.5mm.Due to the existence of the suction chamber, the cross-sectional size of the flow changed from a circle with a diameter of 2.1m to a rectangle with a size of 2.1m×0.5m,and the water flow contracted here for the first time, with a contraction ratio of 3.3: 1.

Test section
As shown in Figure 5, the test section is composed of a contraction section flange, a diffusion section flange, an organic glass window, a window frame and two L-shaped brackets.The four sides of the test section are covered with transparent plexiglass with a thickness of 60mm, and the glass on each side can be tightly fixed on two L-shaped brackets through the window frame.This design ensures the crosssection of the internal flow channel with a length of 750mm is 0.15m×0.15msquare and this section is the smallest part of the whole system, so the velocity of water can reach the highest velocity here.A circular hole is opened in one of the glasses, which is specially used to install hydrofoils or other test models.In the selection of materials, all the other parts except plexiglass are made of QAl7 aluminum bronze with a tensile strength of 635Mpa, so the test section can withstand the static pressure of 2MPa.

Diffuser diversion section
To reduce the hydraulic loss of the system, it is necessary to design a diffusion diversion section to convert dynamic pressure into static pressure.As shown in Figure 6, the flow section of the upstream section gradually changes symmetrically with a square, from a square of 0.15m×0.15mto a square of 0.3m×0.3m,with a length of 3.4m, and the design method is similar to that of the contraction rectifier.The flow section of the downstream section changes asymmetrically from a circle with a diameter of 482mm to a circle with a diameter of 1000mm, with a length of 7.155m, and its top is horizontally aligned with the end of the first section.It is formed by direct hot rolling of stainless steel plate.When the water flow reaches the outlet of the diffuser section, its velocity can be reduced to 3% of the test section, which greatly reduces the dynamic pressure of the water flow.

Water-gas separation chamber
The optimal position of the water-gas separation chamber is located in the second floor of the circulating structure, and the overall structure is a large-diameter pipeline arranged horizontally, and the internal water flow should be able to turn around and return.
The water flowing out of the diffusion diversion section needs to be directed into the water-gas separation chamber, so reasonable elbows must be designed between them.As shown in Figure 7, the first elbow is connected to the outlet of the diffuser diversion section.Seven rectifier plates are designed inside to optimize the flow.The design of the rectifier plate is based on the double-arc airfoil design at the corner of the cavitation water tunnel of Nanjing University of Science and Technology.The rectifier plates at the other corners of the cavitation water tunnel all adopt the same design.The expansion joint is installed at the entrance of the first elbow, which can compensate for the installation error.As shown in Figure 8, the second elbow is mainly composed of two vertical plates and four curved plates, it can direct water into a water-gas separation chamber.To reduce the turbulence intensity, a screen with the same standard as the outlet of the suction chamber is designed at the outlet of the second elbow.The design of the water-gas separation chamber is shown in Figure 9.Its interior is divided into two internal channels, the upper one is the incoming channel and the lower one is the outgoing channel.The upper laminar flow channel contains four identical bubble collection devices, each device is welded to the pipe wall from the top of the flow channel by 14 inverted V-shaped steel plates and the ends of these steel plates are connected with a pipe for discharging bubbles.When water and air bubbles flow to each inverted V-shaped steel plate, the air bubbles will rise to the top of the inverted V-shaped steel plate along the inclined plane due to buoyancy, and then move horizontally along the top to reach the pipe at the end and then be discharged.Through many model experiments, it is necessary to design an upturned structure and a stainless steel plate folded back at an angle of 20 degrees from the horizontal direction at the rear edge of the plate, where bubbles can suddenly accelerate and gain more kinetic energy to enter the bubble collection tube to improve the efficiency of bubble collection.An expansion joint was designed at the outlet of the water-gas separation chamber.

Pump selection
The most important task of pump selection is to determine the pipeline characteristic curve, that is, to calculate the head loss of the pipeline system through Formula (3) [17]: where hf(m) is hydraulic loss, k(s 2 /m 5 ) is hydraulic loss coefficient, Q(m 3 /s) is flow rate.Because the cavitation water tunnel designed in this paper has a high structural similarity with the high-speed cavitation water tunnel of the Federal Institute of Technology in Lausanne, Switzerland, the cavitation water tunnel designed in this paper refers to the data of the high-speed cavitation water tunnel of the Federal Institute of Technology in Lausanne, Switzerland [6]: the flow(Q0) of the pump at rated speed is 1.125m 3 /s, the head (H0) is 35m, and the head loss of the pipeline is estimated to be 27.4% of the dynamic pressure of the test section.Equations ( 4) and (5) give the calculation process of its head loss: 12 where V0 is the speed of the test section, g is the acceleration of gravity, A0 is the cross-sectional area of the test section, and its value is 0.0225m 2 The final head loss is equal to the head of the pump at this velocity.Therefore, it can be concluded that the value of K is 27.58586 s 2 /m 5 , and finally, the characteristic curve of the cavitation water tunnel pipeline designed in this paper can be obtained.
According to the pump type spectrogram provided by Shanghai Kaiquan Company and the pipeline characteristic curve of the system, a large double suction centrifugal pump with good water absorption performance is selected.At the rated speed of 740r/min, the velocity of the test section reaches 35 m/s.At this working point, the velocity of the pump is about 0.82m 3 /s, the head is about 20.5m, and the efficiency is about 86%.Its available net positive suction head(NPSH) is 5.5m, which is less than the water absorption height of the pump 7.2m, which ensures that the pump itself does not cavitation.

Design of water storage tank
The water storage tank is mainly used to control the overall temperature of the system and the stability of the flow, so it should have a large volume.According to the layout of the circulation structure, as shown in Figure 10, it is designed to be L-shaped, with a diameter of 2.1m, a height of 6.32m, and a length of 13.615m.A rectifier plate with the same design standard as the diffuser section is installed at the turning to optimize the flow state.An expansion joint is arranged at the inlet of the water storage tank to offset the horizontal displacement during the operation of the pump.Because the cross-sectional area suddenly expands at the entrance and the flow changes dramatically, a screen with the same standard as the outlet of the suction chamber is designed.

Operation monitoring system
To determine the working conditions of the experiment, it is necessary to monitor the flow velocity, pressure, temperature and cavitation number in real time.Pressure taps are designed at the inlet of the contraction rectifier and the inlet of the test section, and the signals are transmitted to two high-precision digital pressure sensors through the equalizing ring to measure the absolute pressure here, and then the pressure is transmitted to the host computer for subsequent processing.The temperature is measured by a high-precision temperature sensor at the top of the suction The measured pressure can be converted into velocity by formula (6), and then into cavitation number by formula(7): where P1 is the average pressure at the inlet of the contraction rectifier, P2 is the average pressure at the inlet of the test section, U is the velocity of the test section, ρ is the density, Pv is the saturated vapor pressure, K is the velocity coefficient, σ is the cavitation number, and T is the temperature.Before the experiment, the value of K was calibrated by ultrasonic flowmeter.

Pressure control system
The pressure control system as shown in Figure 11 is designed.When the pressure of the cavitation water tunnel is too high, it is necessary to charge high-pressure gas into the pressure regulating tank to supplement the leakage at the joint, and the pressure value of the gas to be charged into the pressure regulating tank can be judged according to the pressure measured at the contraction rectifier.A vacuum pump is set to reduce the pressure value at the suction chamber and ensure that the test section can achieve the ideal cavitation effect at low velocity.At the same time, in order to check the water level conveniently and ensure that the cavitation water tunnel is level with the pressure regulating tank, a pipeline connected to the cavitation water tunnel is led out from the bottom of the pressure regulating tank.Tap water is injected into the system at the top of the pressure-regulating tank.

Conclusion
To improve the scientific research strength of the unit, this paper designs a high-speed cavitation water tunnel test system for the study of cavitation and cavitation of hydraulic machinery.The hydraulic design of flow components in the cavitation water tunnel and the design of auxiliary facilities such as operation monitoring system are mainly carried out.The following conclusions are obtained: (1) After determining the circular structure layout of the cavitation water tunnel, the detailed design of each flow component is carried out, the functions of each component are defined, and the detailed size parameters of each component are determined.200kW circulating pump can make the speed of the test section reach 35 m/s; The test section with a square section of 150mm can be equipped with hydrofoils with a spanwise length of which are surrounded by high-strength plexiglass, so it is convenient to observe the flow field of hydrofoils and other turbulence experiments.The diffusion diversion section can reduce the flow velocity to 3% of the test section, and reduce the head loss; The suction chamber, contraction rectifier, and corner have a good rectification effect on water flow; The water-gas separation chamber can collect bubbles with a diameter greater than 100μm in water, thus avoiding interference to the experiment.
(2) The operation monitoring system can monitor the pressure, velocity, cavitation number, and temperature of the cavitation water tunnel in real-time, so as to ensure that the operation status can be mastered in the experiment.The pressure control system can adjust the pressure of the system according to the test needs, including pressurization and vacuum pumping, and can ensure the stability of the system pressure.
(3) The test section is designed to be easy to disassemble and replace.The functions of the test system include basic experiments such as the characteristics of the flow field around hydrofoils/blades, lift-drag characteristics, cavitation dynamics, structural dynamics, measurement of steam content in cavitation flow fluid-solid coupling characteristics, acoustic characteristics, cavitation erosion characteristics, etc., which can provide test conditions for the study of high-speed hydrodynamic characteristics such as pumps/turbines, propellers, and underwater vehicles, and assist their design and development.

Figure 1 .
Figure 1.Overall layout of the cavitation water tunnel.

Figure 4 .
Figure 4. Schematic diagram of the suction chamber, the fillet and the rectification structure (Unit: mm).

Figure 9 .
Figure 9. Schematic diagram of the water-gas separation chamber structure(Unit: mm).

Figure 11 .
Figure 11.The pressure control system of the cavitation water tunnel.

Table 2 .
The cavitation water tunnel design requirements table.

Table 3 .
Parameters table of the motor and the water pump.