| In order to meet the growing demand for load regulation of the power grid,pumped storage power stations are subjected to frequent condition conversions.The pump-turbine is one of the core components of the pumped storage power station with its steady-state flow phenomenon and mechanism already been widely studied,while the complex flow characteristics and instability in the transient process has not yet been explored in depth.Therefore,it is of great practical significance to study the flow characteristics of threedimensional transient process of the pump-turbine.In this study,a numerical simulation method that couples the one-dimensional hydro acoustic model(1D HA)and three-dimensional computational fluid dynamics(3D CFD)was established.The rotational speed of the rotor in the transient process was updated based on the momentum equation of the rotating system and the guide vane motion during the process was simulated using the overset mesh technique.Based on the Schnerr & Sauer cavitation model,the mass exchange between the water and the steam in the transient process was calculated,and the mass excitation source caused by the cavitation in the draft tube were extracted from the three-dimensional flow field and then loaded into the one-dimensional draft tube model,correcting the one-dimensional mass conservation equation under cavitation conditions.The applicability and accuracy of the model were verified by comparing the numerical simulation results with the experiment data from on-site transient measurements.The numerical simulation of the start-up and shutdown process of the pump turbine was carried out by the coupling algorithm.The characteristics of the unit's external characteristics,runner force characteristics,pressure fluctuation characteristics and internal transient flow evolution were analyzed.The details of the transient flow structure and their associations with the transient properties of the system were analyzed as well as the unstable characteristics mechanism of in the no-load operation condition.It was found that the rapid decline of flow rate and runner torque in the “S” characteristic zone came from the combined effect of the clogging effect of the high-speed water ring in the vaneless space and the complex vortices in the runner.Based on the coupling algorithm with cavitation model,the numerical simulation of the load rejection process of the pump water turbine was carried out.The instantaneous flow characteristics and cavitation development were analyzed as the pump-turbine operates through the turbine zone,the turbine braking zone and the reverse pump zone after the sudden rejection of load.It was found that under the influence of “S” characteristics,the unit ran deep into the reverse pump zone after passing through the turbine braking zone and the maximum negative torque reaches 0.94 of the rated torque in the reverse pump zone,exerting a large torque reversal on the main shaft in a short time.Meanwhile,it was found that serious cavitation takes place in the draft tube when the unit operated near maximum speed.The flow field near the maximum speed was simulated by conserdering the mass disturbance and wave velocity change caused by the cavitation phase transition.The influence of the cavitation excitation source on the external charateristics and pressure fluctuation characteristics was further analyzed.It was found that the cavitation excitation source played a role in amplifying the pressure fluctuation amplitude in the draft tube.This study reveals the complex flow characteristics and mechanism in the transient process of startup,shutdown and load rejection,which can provide scientific basis for guiding the safe and stable operation of the transient process and improving the hydraulic stability of the system. |
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