Abstract
To obtain the flow mechanism of the transient characteristics of a Kaplan turbine, a three-dimensional (3-D) unsteady, incompressible flow simulation during load rejection was conducted using a computational fluid dynamics (CFD) method in this paper. The dynamic mesh and re-meshing methods were performed to simulate the closing process of the guide vanes and runner blades. The evolution of inner flow patterns and varying regularities of some parameters, such as the runner rotation speed, unit flow rate, unit torque, axial force, and static pressure of the monitored points were revealed, and the results were consistent with the experimental data. During the load rejection process, the guide vane closing behavior played a decisive role in changing the external characteristics and inner flow configurations. In this paper, the runner blades underwent a linear needle closure law and guide vanes operated according to a stage-closing law of “first fast, then slow,” where the inflection point was t = 2.3 s. At the segment point of the guide vane closing curve, a water hammer occurs between guide vanes and a large quantity of vortices emerged in the runner and the draft tube. The pressure at the measurement points changes dramatically and the axial thrust rises sharply, marking a unique time in the transient process. Thus, the quality of a transient process could be effectively improved by properly setting the location of segmented point. This study conducted a dynamic simulation of co-adjustment of the guide vanes and the blades, and the results could be used in fault diagnosis of transient operations at hydropower plants.
Highlights
With the dramatically worsened conflicts between energy supply and environmental protection around the world, due to the small investment, short construction period, and less negative impact on the environment, low-head hydraulic turbines have become the top priority in the development and utilization of renewable energy [1,2]
The discrepancies between the two methods may have been caused by the following reasons: some vibration signals may have interfered with the measured pressure results in the model experiment; the positions of the monitored points in the simulation were not completely identical to those in the actual dynamic test; the simulation and the experiment used two different sampling frequencies; the pressure tank was not established in the geometric model for the simulation, while the physical model contains a pipe system that had an impact on the curves of the turbine, that is, the boundary condition was not completely consistent
The numerical results could truly reflect the characteristics of the load rejection process only up to a certain degree
Summary
With the dramatically worsened conflicts between energy supply and environmental protection around the world, due to the small investment, short construction period, and less negative impact on the environment, low-head hydraulic turbines have become the top priority in the development and utilization of renewable energy [1,2]. Load rejection is a common transient process with large fluctuations of dynamic parameters such as rotational speed, mass flow rate, torque, axial thrust, and static pressure. During the load rejection transient, the rotational speed and system water pressure rise to the effect that the flow pattern in the flow channel becomes more complex, which makes the variation regularity of external characteristics change dramatically and induce additional inertial force. The dynamic load caused by the mass imbalance in the rotor and the hydraulic instability in the flow passage components can be greatly increased by these changes, which can result in strong hydraulic pulsation and vibration and pose great threat to the safe operation of the entire power station [10]. It is of great importance to ensure the operational safety of power stations with proper control schemes for regulating the turbine components
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