Study Of The Characteristics Of Pressure Fluctuation On Draft Tube Of A Medium Specific Speed Francis

With exploitation of water resources and development of technology in designingand manufacturing Francis hydro-turbine, the hydro-turbine tends to a trend of highspecific speed and high power output. Accordingly, the pressure fluctuation issueshave attracted many researchers’ attention, since it has crucial impact on the runningstability and safety of hydro-turbine. This thesis focuses on the problem of pressurefluctuation due to draft tube vortex by using a transient numerical simulation withfull-channel model. Meanwhile, part flow channel models were adopted toinvestigate the ingredients and sources of pressure pulsation.To achieve the above purpose, numerical computation of internal flow fields forvarious flow conditions based on four guide vanes’ opening were conducted. Steadysimulations were done in order to study the hydraulic characteristics and verify thecomputing accuracy of the flow field. Then transient simulation were done to obtainthe time-dependent pressure fluctuations, analyze the frequency distribution overfrequency fields, and elucidate the reasons of major frequency components inpressure oscillations. To be specific, the research includes the following aspects.Firstly, steady numerical simulations were done to study the inner flow fields andthe corresponding factors for flow instabilities. The flow conditions included onesmall guide vanes’ opening condition, one optimal opening condition, and two largeopening conditions. The purpose was to study the hydraulic characteristics andhydraulic efficiency for each condtion.Then, transient study on full passage model was done to get the pressure pulsationdata at each monitoring point inside the flow. To better understand the energydistribution in pressure pulsation, the fast Fourier transform (FFT) method wasadopted to handle the pressure pulsation variations. The pressure information in thetime field was then transformed into the variation in the frequency field. Thecommon features of pressure oscillations at each monitoring point were analyzed,also the reasons for the pressure variations in the draft tube was revealed, which wasa result of periodic motion of draft tube vortex rope.Finally, transient numerical simulations based on the part flow channel modelswere conducted in order to study the resource of pressre fluctuations in the drafttube. The models included the non-spiral casing channel model and non-stay vanechannel model. The information on the pressure fluctuation on the time field andfrequency field of each measuring point was obtained, by comparing with the resultsof full flow passage model, it was found that the upstream structure componentsdidn’t produce new pressure fluctuation in the draft tube.