Abstract Pumpturbine operation along the Sshaped curve may encounter difficulties in loading rejection process or unusual increases of water pressure resulted in violent machine vibrations. The prevailing and effective solution to this problem is misaligned guide vanes (MGV) technique, that is, to control the stability of the machine operation in the S region by preopening several guide vanes (usually an even number) to specified angles. In order to explore the feasible and effective way of preopening guide vanes, firstly, based on the numbers and the locations of MGVs, four MGVs were designed, in the hope of finding the most optimal solution to eliminate S characteristics through fully threedimensional numerical simulation of flow analysis; secondly, model tests for the selected most optimal program were performed and the experimental results and simulation results were compared. The analysis of the results of the four preopening programs shows that more number of preopening guide vanes does not necessarily mean better improvement of S characteristics and equallydistributed guide vanes is better than centrallydistributed ones in improving S characteristics. It is concluded that the program of preopening 2#,7#，12#，17# twopair guide vanes can most effectively mitigate the S characteristics of pumpturbine; CFD calculations and the comparison with model test show that CFD simulation is well consistent with experimental data.
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