Study Of Hydraulic Stability On Large-scale Francis Turbine

Along with the increase of the unit installed capacity, the unit size, the specific speed and the weaken of the relative rigidity of hydraulic power unit, the operating stability of large-scale Francis turbine received much more attention increasingly. Meanwhile, the rapid development of technology makes the automaticity of unit operation more and more higher and appears a large amount of hydropower stations with nobody or a few on duty, remote control, so the requirement of the unit operating stability becomes strict increasingly. On the other hand, there come forth vibration problems in many large-scale Francis turbines at home and abroad, which influence the normal operation not only, some of them endanger the units but also, therefore, the hydraulic stability has brought more attention in trades of electricity and manufacturing, the new research field has been brought about to the professionals and scholars also. This paper's research is based on the background.The concerned reasons about the hydraulic stability in large-scale hydropower unit are complicated and the phenomenon are varied, such as the low-frequency pressure pulsation in draft tube, the Karman vortex and the vortex between blades etc. In which of these, the draft tube vortex is the primary reason to bring about the unit vibration. The research methods of hydraulic stability are mainly model test, field test and CFD numerical simulation. Aiming at the vibration problem appeared in DongJiang hydropower unit, this paper comprehensively analyzed the reasons applying the combination of above three methods.The model test of hydraulic turbine is the very important way to study the prototype's hydraulic stability. Although there can be provided the means and simulation technologies now to give the relatively accurate numerical simulation and the performance prediction of hydraulic turbines, finally the model test is required to determine the energy performance, cavitation performance as well as hydraulic stability of model runner. As to the units in use, it can research the prototype's operating performance by means of model test to simulate the operating condition of units. The results of model test in many aspects indicate that the energy performance of the runner is relatively low; in small opening condition, there exist low-frequency pressure pulsation in draft tube; the draft cone with different configuration has great influence with the low-frequency pressure pulsation in draft tube; the application of different supplying air method can abate the vibration of the units.The field test is the direct way to research the prototype's hydraulic stability. The model test is very important but with limitations, for example, the assumption condition of the geometric similarity between model and prototype is relatively, meanwhile, the flow in passing components of hydraulic turbine is very complicated, the structural design difference among the hydraulic turbines is great and it's difficult to predict the prototype's operating stability from the vibration performance of the model, all of those make the research of field test received more attention for professionals. Many results of research indicate that the characteristics of hydraulic units'vibration have not only commonness but also individuality. Obviously, it is impossible to reveal the prototype's general performance by means of model test especially the transient performance related to the unit vibration. Thus, although the field test has many limitations comparing with model test, but it is necessary in order to research the specific problem especially to find out the influence to unit operating stability of the manufacturing and installation quality without in model test information. The paper carried out many field test research including the passing components measurement, the test of load variation and excitation variation with different head, the frequency characteristic analysis of unit vibration, the influence supplying air to prototype's stability and the analysis of the relationship between unit vibration and dam vibration. The results indicate that there exist important objections in manufacturing and installation quality of DongJiang units, the units have two vibration region in different head, which has the tendency to offset to lower load with the increase of head, under part-load conditions, it brings about eccentric vortex in draft tube, which causes the low-frequency pressure pulsation, it is available to abate the vibration of DongJian units by means of suitable method and quantity of supplying air.The CFD numerical simulation has great advantageous in cost, periods and information content comparing with the model test and field test, so this method becomes more and more popular and ingoing, it also makes the main flow performance in large-scale Francis turbines clear increasingly. The paper numerically simulated the prototype's stable flow field in different operating conditions by means of partly coupling method and generally analyzed the flow and energy performance in passing components.It is not enough to resolve all of the problems related to the hydraulic stability only to analyze the stable field in passing components. Finally, the paper numerically simulated the transient field in prototype's draft tube under different operating conditions by means of Reynolds stress turbulence model and analyzed the law of motion of vortex rope as well as the performance of low-frequency pressure pulsation and compared with field test results.