| With the progress of technology and industrial manufacturing precision, the hydraulic machinery gained a significant development in the past few years. However, since they are becoming much faster and larger, the issue of cavitation cannot be ignored any more. The exsistence of cavitation would degrade the hydraulic efficiency, induce vibration and noise and even damage the solid wall, which severely affects the system operation safty. On the other hand, despite of such disadvantages caused by the cavitation, its special physical-chemical phase transformation property and high unsteady dynamic characteristic bring us a fantastic water treatment approach, with outstanding efficiency and low energy cost. In order to figure out the cavitation problems in typical hydraulic machinery, the investigation from simple hydraulic apparatus, such as venturi and orifice plate, to complex ones, like centrifugal pump and cavitation generator, were conducted via combined numerical and experimental method. The main research contents and creative achievements arrived are as follows:1. An automatic running program for cavitating flow simulation in hydraulic machinery was developed on the basis of windows Batch and ANSYS-CFX. Within the Batch language, the program can automaticly create a.bat file to run ANSYS-CFX and simultaneously adjust the boundary conditions. It enables the calculation move on to another step without any time consuming, successfully reducing the entire simulation duration.2. A combined experimental — numerical — image post-processing prediction approach for cavitation erosion was established. The relataionship between cavitation developing process and its induced damges was analyzed. A simultaneously testing method for cavitation structures and cavitation erosion was achieved by applying two high speed cameras aside a venturi section, and the aluminum foil was used as a damage measurement sample. The numerical simulation and image post-processing methods were employed to enrich the analysis. Considering the compressible property of cavitating flow the eddy viscosity over-prediction problem of the original turbulence models, an improved Compressible—Density corrected turbulence model (CDM) was presented. The results show that both shedding off and collapse processes of the cavitation cloud would cause damages, and the collapse comes at the first place for cavitation erosion. Furthermore, it is found that the increase of the damge pits number and area is not linear, but step-wise, which means the pits are only generated as cavitation cloud shedding or collapse and the gerneration is not transient.3. On the basis of the investigation on orifice plates, an image based post-processing cavitation erosion method was proposed. A commercial code MATLAB was employed to process the images under the frame of standard deviation method. It is proved the continuous cavitation evolution images can be used as a cavitation erosion prediction method as well. The erosion area is annular, located in the sites where bubbles collapse. The diameter of the area is identical with the prediction result. The cavitation shedding frequency increases with decreasing cavitation number. While the cavitation stays constant, higher flow velocity contributes higher cavitation intensity, and then leads to severe cavitation erosion.4. In the terms of the rotating movement characteristic of centrifugal pump and based on the CDM model, a modified Rotating—compressible—Density corrected turbulence model (RCD) was proposed, which is suitable of handling the cavitating flow in centrifugal pump. Comparison had been made with the original k-ε model and SST k-ω model, and aslo the experiment results. It is indicated that the RCD model has better performance on predicting the pump head and its cavitation chacteristic. Moreover, by applying both the visualization experiment and unsteady simulation, the cavtitation evolution mechanism was found. The RCD model can successfully reduce the over-prediction of eddy viscosity, and in the meanwhile accurately capture the unsteady cavitation shedding and collapse processes, as well as the variation of the fluid density.5. The ZGB cavitation model was investigated by analyzing the influences of the bubble radius, evaporation coefficient and condensation coefficient. The results show that the bubble radius and evaportation coefficient affect the cavity length and vapor volume fraction at the same time, while the condensation coefficient only impacts the cavity length. High vapor volume fraction expands the low pressure region on the leading edge of the blade, but hardly affects the head of the pump. The cavity length is the essential factor which would influence the predition result of the pump head.6. Considering the influence of rotating movement characteristic of centrifugal pump on bubbles, and the interaction between turbulent dynamic energy, a rotating based ZGB cavitation model (RZGB) was presented specially for cavitating flow in centrifugal pump. This model effectively connects the pump rotating speed, turbulent dynamic energy and bubble radius. In order to evaluate the RZGB model, three centrifugal pumps with different specific rotating speed were taken into accout as the research objects. The results show that, compared with original ZGB model, when the evaporation and condensation coefficients are 5000 and 0.001 respectively, the RZGB model obtains more accurate predictions on the pumps of both low and medium specific rotating speed pump. For the high specific ratating spedd pump, the RZGB model with coefficients of 50 and 0.01 is more suitable. Furthermore, compared with the visualization experiment, it demonstrates that the RZGB model can capture the cavitation structures more accurate than ZGB model. The simulated result matches the experiment very well:the triangle shape is obtained as in the visualization.7. By applying the previous research in the current dissertation, an image based post-processing cavitation erosion prediction method for rotating hydraulic machinery was established. Both the numersical simulation images and experimental visualization images can be used to predict the erosion area and its extent of the damage. The experimental test was also conducted to validate the method. The results show that the sites of the damages are identical with the cavitaion region, while the most potential erosion area is close to the trailing part of the attached cavity. It is implied that the unsteady trailing part of the attached cavity could also be a trigger inducing damages.8. Based on the visualization experiment and numerical simulation with the proposed RCD turbulence model and RZGB cavitation model, a rotor—stator hydraulic cavitation generator was studied detailedly. It is found that there are three kinds of cavitation generating mechanism in the generator:one is produced by the interaction movement of rotor and stator, which forms a nozzle tube at the moment. The others are caused by the high rotating rotor, producing the cavitation at the leading edge and the rear part of the blade. The pressure pulsation in the generator increases as the flow rate increases while keeping the rotating speed constant. When the flow rate remains the same, the pressure pulsation rises with increasing ratating speed. Increasing the distance between rotor and stator would slightly decrease the pressure. Furthermore, the oil ink method was employed to investigate the erosion area of the cavitation generator. It indicates that the rear part of the rotor blade, the leading edge and the rear part of the stator blade are the potential erosive area. According to the previous conclusion, the damages are generated by high intensity cavitation when the rotor and stator are crossing each other, leading to nozzle cavitating flow. And it reveals that the cavitation erosion in hydraulic machinery is primarily caused by tiny bubbles close to the solid wall, not big cavitation cloud. |
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