| Transient processes such as frequent start and stop and accident runaway often occur during the operation of tubular turbines.During this process,the external characteristic parameters(speed,flow,torque,etc.)of the unit and the internal flow pattern change drastically over time,which seriously affects the safe and stable operation of the unit.At the same time,the operating head of the tubular turbine is low,and both the fluctuation of the free surface of the upstream and downstream reservoirs and the effect of gravity will affect the characteristics of the turbine during the transition process.Based on the submerged boundary-lattice Boltzmann method(IB-LBM),this paper studies the transient flow characteristics of tubular turbines.The related work is as follows:(1)A three-dimensional model of the full flow path of the tubular turbine including upstream and downstream reservoirs has been established.Firstly,the external characteristic parameters of the tubular turbine with different guide vane openings were simulated for the steady state process.The results obtained are in good agreement with the experimental values,and the error rate is less than that of the traditional CFX calculation.Secondly,the runaway transition process was numerically simulated,and the runaway process curve obtained is highly consistent with the test results,which proves the feasibility of the method applied to the transient simulation of the threedimensional flow of the hydraulic turbine.(2)The pressure distribution in the internal flow field of tubular turbine and the evolution law of vortex rope in draft tube during accident runaway are examined in detail.The simulation results with and without upstream and downstream reservoir models are compared and analyzed,and the periodic development law of negative pressure area in draft tube and vortex on runner blade surface was obtained.It is revealed that the free surface of reservoir has great influence on the pressure distribution of flow field inside the turbine unit and the pressure fluctuation behavior in the runaway process.(3)The transient numerical simulation of the guide vane closing process of the tubular turbine under load rejection is carried out,with a rigid body dynamics model being introduced.In the simulation,three different guide vane closing laws are set through a custom function,the main turbine parameters are analyzed and the dynamic qualities were compared.It is found that the guide vanes closed in steps can effectively suppress the increase of the rotating speed and reduce the amplitude of the pressure pulsation near the runner.The change of the flow field inside the turbine during the load rejection process is further studied,and it is found that there are periodic vortices in the bladeless area between the guide vane and the runner. |
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