Numerical research on water hammer phenomenon of parallel pump-valve system by coupling FLUENT with RELAP5

Water hammer caused by system devices closing or opening are important factors that threat the safety of nuclear reactors. A coupling scheme is proposed to analyze water hammer in a designed parallel pump-valve system. The developed FLUENT/RELAP5 makes it possible to obtain three-dimensional (3-D) flow field of the key area with lower computer resource than traditional computational fluid dynamic (CFD) analysis. The coupled code can reflect many sophisticated phenomena which system code ignores. In the developed coupling scheme, RELAP5 is compiled as a Dynamic Link Library (DLL). The compiled library is invoked as a subroutine at every time step by FLUENT. A testing case has been proposed to verify the implementation of the coupling scheme. Testing results prove the implementation of the coupling scheme successful and coupled results reliable. During the period of valve close, the coupled code predicts considerably well over movement of the valve element by the moving grid method. During the period of pumps start, three shock waves occur, and the mechanical loading oscillates continuously after 12 s. Moreover, the coupled results are compared with the standalone RELAP5 results. Contrast analysis represents that the coupled FLUENT/RELAP5 has the potential to obtain credible transient numerical results. The predicted 3-D transient characteristics directly reflect the generated pressure surges and forces in the designed pump-valve system, which is a fundamental guide for mitigating the threat of water hammer.