New Bulb Turbine with Counter-rotating Tandem-runner

With the increasing demand for the clean sustainable power, the turbine design urgently turns to increase the capability significantly toward higher head for generating larger power. Currently, there are many studies in the field of the bulb turbine with single-stage runner, though reports about counter-rotating tandem-runner are rare. However, the further high-head application with the single-stage runner is very difficult to achieve due to the limit of the specific speed. In this paper, a new bulb turbine with the tandem-runner is designed in order to substantially increase the applicable limit toward higher head with larger power. A half of the net head is absorbed by the frontal runner which can generate output power, while the remaining half is absorbed by the rear runner. To generate the Euler energy required for the rear runner, the frontal runner has the counter-rotation against the rear runner so that the counter-rotating tandem-runner can meet the purpose of double head and power under the same size as the conventional bulb turbine. Supply and demand of Euler energy between the two runners are thoroughly optimized through the detailed flow analysis, in order to secure the stable operation. As a result, the interference of Euler energies between the outflow from the frontal runner and the inflow to the rear runner is confirmed to be very small on the counter-rotating interface between the two runners. The prediction method of on-cam performance between the two adjustable runners is also developed numerically, which provides optimal flow between the two runners. This research provides a theoretical basis for the optimal design and operation of the counter-rotating tandem-runner bulb turbines.

Optimization of the co-closing law of guide vanes and blades for bulb turbines based on CFD

The load rejection transient process of bulb turbine units is critical to safety of hydropower stations,and determining appropriate closing laws of guide vanes(GVs)and runner blades(RBs)for this process is of significance.In this study,we proposed a procedure to optimize the co-closing law of GVs and RBs by using computational fluid dynamics(CFD),combined with the design of experiment(DOE)method,approximation model,and genetic optimization algorithm.The sensitivity of closing law parameters on the histories of head,speed,and thrust was analyzed,and a two-stage GVs’closing law associating with a linear RBs’closing law was proposed.The results show that GVs dominate the transient characteristics by controlling the change of discharge.Speeding GVs’first-stage closing speed while shortening first-stage closing time can not only significantly reduce the maximum rotational speed but also suppress the water hammer pressure;slowing GVs’second-stage closing speed is conducive to controlling the maximum reverse axial force.RBs directly affect the runner force.Slowing RBs’closing speed can further reduce the rotational speed and the maximum reverse axial force.The safety margin of each control parameter,flow patterns,and pressure pulsations of a practical hydropower station were all improved after optimization,demonstrating the effectiveness of this method.

Numerical simulation of transient characteristics in a bulb turbine during the load rejection process

To evaluate the safety of the bulb tubular turbine,the dynamic hydraulic characteristics of a hydropower station system during the load rejection process are studied through numerical simulations and a prototype test.In the developed model,a dynamic grid technology(DGT)controls the closure of the guide vane and the blade,whilst the moment balance equation and the user-defined function(UDF)provide the runner’s rotation speed.The 3-D transient simulation method can well predict the rotation speed and mass flow curves in the state of load rejection.The simulation outcomes of the system performance are basically consistent with the measurement data of the prototype.As observed,the runner is subjected to the reversely increased torque and axial force,the system is in a braking phase,and the maximum speed peaks at 144.6%of the rated speed.Moreover,the internal flow of the runner is greatly affected by the closure of the guide vane,and the draft tube forms an eccentric spiral vortex rope.It breaks downstream,aggravating the instability of the draft tube.Overall,the transient characteristics span for the first five seconds,demonstrating the importance of establishing an efficient governing controller.The obtained results are useful for designing the turbine’s flow channel with a double regulating function and comprehending the turbine’s transient characteristics.

Correlation analysis of cavitation-induced pressure pulsation and vibration in a bulb turbine

Cavitation is one of the main causes of deteriorating stability of bulb turbines.To enhance their stability,this study examines the effects of runner cavitation on draft tube pressure fluctuation and vibration in bulb turbine through experimental methods.With varying cavitation coefficients,a synchronous test system,including a high-speed camera,vibration acceleration sensors and pressure pulsation sensors,is applied to obtain cavitation images of the runner,vibration and internal fluid pressure pulsation data of the draft tube.The results show that the correlated component of pressure pulsation signals during the cavitation process is the synchronous pressure pulsation of 16f_(n)With the development of cavitation,the amplitude of synchronous pressure pulsation increases first and then decreases.Cavitation enhances the high-frequency vibration on the wall of runner chamber.The root mean square(rms)of the vertical vibration component IMF3,the horizontal vibration components IMF2,IMF4 are linearly negatively correlated with the cavitation coefficient.The associated component between cavitation-induced vibration and pressure pulsation signal is 16f_(n)and its harmonics.In the process of cavitation,pressure pulsation plays a leading role in vibration.

An optimization method on runner blades in bulb turbine based on CFD analysis

In this paper an optimization method of the runner blades in a bulb turbine based on CFD analysis is proposed.In the method the main scales of the turbine including guide vane,runner and draft are maintained.Only the runner blades are modified based on the present method.In the optimization method the runner blade is expressed by spline surface with a gather of coordinate points.The B-spline curve is used to keep the modified blades smooth.In order to make the blade optimization simple and ef- ficient,one of the coordinates is fixed and only the angles of the points are changed according to different modification purposes.Three main optimization principles based on flow diagnosis are presented here.These three principles are all based on the CFD analysis of the internal flow in bulb turbine.For the purpose of method verification,the optimization method is used in a model bulb turbine.A three dimensional steady turbulent computation is carried out through the whole passage including the bulb body,guide vanes,runner and draft tube of the bulb turbine under seven different work conditions.An SST k-ωturbulence model is used during the CFD analysis and the performance of the turbine can be achieved.The runner blade is optimized according to the three optimization principles based on flow diagnosis.The CFD analysis is conducted again on the optimized turbine and another modification is needed if the new turbine can’t satisfy the required performance.Comparison of the computational results between the original turbine and an optimized one indicates that the optimization method is practical and does improve the performance of the bulb turbine.