| Mixed flow pumps are widely used in agricultural irrigation,municipal water supply and drainage,power plant circulating water and other national economic fields with high flow,high head,wide range of high efficiency. Transient cavitation of mixed flow pump is one of the difficult problems to be solved in submarine weapon launching device. It is the basis of solving those problems to study the transient flow field characteristic and cavitation shape of the accelerated flow condition. In the closed hydraulic system,the mixed flow pump quickly starts to produce high strength water pressure,drives the torpedo out to warehouse,then the weapon is launched. In this process starting process is related to which has great difference with steady-state process. Because the starting process involves many parameters and these parameters change over time uncertainly. When the liquid cabin pressure is insufficient,cavitation is produced in starting process and affects the emission process. While the vibration and noise which are generated by pump cavitation are more likely to be detected by radar. In this case the safety performance will be decreased. Therefore,the study of transient cavitation characteristics in the starting process is of great significance. It provides theoretical basis and technical support for the development of high speed and large scale mixed flow pump in the national defense equipment domain.This paper supported by the National Natural Science Foundation of China “Study on the transient cavitation patterns and induced hydraulic oscillation in a mixed-flow pump at accelerated flow conditions” focuses on the research of hydrofoil and high specific speed mixed flow pump. The accelerated flow conditions for hydrofoil are simulated,using NACA0015 model to verify the DES turbulence model for the accelerated flow conditions. Then use NACA66 model to simulate and verify the unsteady cavitation characteristics. Finally use NACA66 model to simulate the cavitation characteristics for 3 different accelerated flow conditions. The paper analyzes the development of the cavitation process, the velocity distribution regularity,airfoil lift and drag characteristic and internal vapor volume fraction under 3 different accelerations(a1=5m/s2、a2=2.5m/s2、a3=1m/s2). Then repeated experiments are done for the mixed flow pump characteristics,cavitation performance and starting process. And the transient regularity and mechanism of cavitation are analyzed for starting process. The main research work and achievements are as follows:(1)The Detached Eddy Simulation turbulence model and Zwart cavitation model based on Rayleigh-Plesset equation can better predict the hydrofoil cavitation under accelerated flow conditions. In the accelerating process,vacuole is first born in the leading edge of the hydrofoil,grows after a period of time and finally separates. Then the vacuole in the leading edge is reduced and the vacuole at the tail of the hydrofoil is increased and backward spread until rupture. Cavitation is first generated at around 0.6 times of total acceleration time and finished a cycle at about 1.12 times of total acceleration time. A small vortex in the airfoil leading edge is generated by the mainstream flow and the back jet flow. And the tail large clockwise vortex is caused by the strong back jet flow. At the same time,with the passage of time,the airfoil leading edge vortexes disappear and the tail vortexes enhance. The change trend of airfoil lift and drag coefficient under different acceleration is consistent. Coefficient starts from a larger value to a smaller value,then transits smoothly to the stable point,and at this point reaches a maximum value,then fluctuates with the periodic oscillation trend. The greater the acceleration is,the oscillation range of lift and drag is smaller and steadier.(2)Through the external characteristic experiment,the maximum efficiency point of the model pump is obtained. The flow rate is about 433m3/h,the head is 3.22 m,the efficiency is 75.948%. Through the cavitation experiment the NPSHr is 2.9m. When starting the experiment,the different starting accelerations are obtained by changing the starting time controlled by the frequency converter,and then the results of external characteristic experiments are obtained. Five groups of start-up experiments are done for 1.1Q、1.05Q、1.0Q、0.9Q and 0.8Q. In the five groups of different flow,three different starting acceleration experiments are done for each group. The characteristics of the starting process are obtained by shooting 30 seconds. Through the analysis,the shorter the starting time is,the longer the time needs which the flow rate,speed and head reach the stable point and the lag phenomenon is more obvious for about 2s. In the large flow rate conditions,the longer the starting time is,the more lag the time point of the head began to rise. Compared to the small flow rate conditions,the lag is more obvious for large flow rate conditions in the start-up phase. In the small flow rate conditions,the flow in different acceleration time lags approximately for about 2s and speed does not appear lag phenomenon.(3)Experiments for cavitation characteristics of mixed flow pump in start-up process are carried out. The cavitation phenomenon is studied for four different flow conditions and three different starting times. By high speed digital imaging technology,the cavitation phenomenon is shot for 25 seconds. In the starting process the cavitation experiment does not appear because the speed at starting stage is small and the pressure does not reach vaporization pressure. When the starting time reaches stable point,the vacuole at the top of the impeller appears. Then the vacuole in the impeller passage is produced. Under the same flow rate conditions,the lower the inlet pressure is,the more serious the cavitation occurs. When the inlet pressure is-20 kPa,the vacuole is assembled near the top of the impeller and there is no vacuole in the impeller passage. When the inlet pressure is reduced to-60 kPa,the vacuole is almost filled with the impeller passage and the cavitation is very serious. At the same time,it can be found that under the same inlet pressure,degree of cavitation under different flow rate conditions is basically the same. |
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