Abstract
Labyrinth seals are not usually included in the numerical models of hydraulic machinery to simplify the geometric modeling, and thereby reduce the calculation burden. However, this simplification affects the numerical results, especially in the load rejection process, because disc friction losses, volume losses, and pressure fluctuations in the seal ring (SR) clearance passage are neglected. This paper addresses the issue by considering all of the geometrical details of labyrinth seals when conducting multiscale flow simulations of a high head Francis turbine under a transient load rejection condition using the commercial software code. A comparison of the numerical results that were obtained with the experimental testing data indicates that the calculated values of both torque and mass discharge rate are 8.65% and 5% slightly less than the corresponding values that were obtained from experimental model testing, respectively. The obtained pressure fluctuations of the Francis turbine in the vaneless zone and the draft tube appear to more closely match with the experimental test data when including SR clearance. Moreover, the flow rates through SR clearance passages were very small, but the pressure fluctuations among them were significantly enhanced under the minimal load condition. The numerical model with SR clearance can more accurately reflect the fact that the water thrust on the runner only fluctuates from 800 N to 575 N during the load rejection process, even though the water thrust on the blades varies from −220 N to 1200 N. Therefore, multiscale flow study is of great significance in understanding the effect of clearance flow on the load rejection process in the Francis turbine.
Highlights
Special attention should be paid to the dramatically worsened conflicts between energy supply and environmental protection around the world, so renewable energy is becoming of paramount importance in energy exploitation
The results indicate that the value of Q that was obtained for the upper seal ring clearance (USRC) is slightly greater than that obtained for the lower seal ring clearance (LSRC)
The experimental test results t [s] and the results showed that the multiscale flow simulation method can force on runner, Fz describe theAxial flow characteristic, in the clearance
Summary
Special attention should be paid to the dramatically worsened conflicts between energy supply and environmental protection around the world, so renewable energy is becoming of paramount importance in energy exploitation. Schiffer et al [16] performed a numerical simulation of a Francis turbine with runner seals on the crown and band side, and presented a new approach to determine the total efficiency, including all of the losses occurring in the hydraulic components of the turbine These past studies have mostly focused on the calculation of steady-state conditions in an effort to obtain more accurate or detailed parameters and inner flow characteristics. The clearance flow between the runner and its adjacent stationary parts has a significant influence on the internal flow and dynamic characteristics of hydraulic turbines [32,33] This paper addresses this issue by considering all of the geometrical details of labyrinth seals in a Francis turbine during a transient load rejection process using a multiscale 3D CFD numerical method that was provided by the commercial ANSYS Fluent 17.0 software (ANSYS Inc, Canonsburg, PA, USA). The calculated dynamic operational parameters, such as leakage flow rate, static pressure of monitoring points, runner torque, and the axial force on the runner, are compared with the experimental results
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