For a prototype turbine operating under part load conditions, the turbine output is fluctuating strongly, leading to the power station incapable of connecting to the grid. The field test of the prototype turbine shows that the main reason is the resonance between the draft tube vortex frequency and the generator natural vibration frequency. In order to reduce the fluctuation of power output, different measures including the air admission, water admission and adding flow deflectors in the draft tube are put forward. CFD method is adopted to simulate the three-dimensional unsteady flow in the Francis turbine, to calculate pressure fluctuations in draft tube under three schemes and to compare with the field test result of the prototype turbine. Calculation results show that all the three measures can reduce the pressure pulsation amplitude in the draft tube. The method of water supply and adding flow deflector both can effectively change the frequency and avoid resonance, thus solving the output fluctuation problem. However, the method of air admission could not change the pressure fluctuation frequency.
Three-dimensional flow simulation and blade tip high-response static pressure measurements were performed on an isolated subsonic compressor rotor to gain more insight into the stall inception mechanism of the compressors. The Navier—Stokes solver, EURANUS, was used for computation. The steady-state flow solution was achieved at the convergence of a four-stage explicit Runge—Kutta integration scheme. The time-dependent calculation was implemented in the implicit dual time stepping scheme, which allowed for the solution of a steady-state problem at each physical time step. The high-response static pressure was measured using five Kulite sensors installed on the casing. The data acquisition frequency was 100 kHz. The recorded data were later analysed using wavelet analysis method. Correlating the simulation result and the measurement result, it was shown that as the compressor was approaching the near stall (NS) condition, the tip leakage vortex dissipation and shedding became violent. The shedding or dissipated flow structures would consequently scatter around the blade tip passage, which formed the initial onset of stall disturbances. These scattered vortexes would finally lead to compressor stall as they hit and merge with each other into low-frequency disturbance of significant size and energy. A stall pre-alarming method based on the monitoring of the low-frequency spectrum power at the NS condition was also suggested by this study.
The electrode reaction and the change in composition and morphology of the rust layer on rebar surfaces after electrochemical realkalisation treatment are investigated by electrochemical measurement and scanning electron microscopy. The results show that the rust layer is gradually reduced following the sequence Fe 3z RFe 2z RFe 0 through mechanisms postulated to involve proton exchange and dissolution-deposition. The realkalisation process can restore the alkalinity of the pore solution in carbonated concrete adjacent to cathodically polarised rebar and also reduce rust layers on rebar in severely carbonated concrete structures. However, though the alkalinity of pore solution is restored and the corrosion potential of the rebar in deeply carbonated concrete becomes more noble after cathodic polarisation, the rebar cannot repassivate while polarised. Thus, to assist in repassivation, additional inhibitor injection is proposed. The results demonstrate that the adjunction of inhibitor can limit the corrosion of the rebar. Therefore, the combination of inhibitor plus electrochemical treatment is likely to prolong the durability of deeply carbonated reinforced concrete structures.
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