FVM-CFD coupled method for 3d hydraulic simulation on a long-corridor air cushion surge chamber in hydropower plant

Abstract A 1D-3D coupled model combining Finite Volume Method (FVM) and 3D Computational Fluid Dynamics (CFD) is firstly developed to investigate 3D dynamic flow regime of air chamber and the influence of different corridor lengths and widths on the 3D transverse unsteady flow in the chamber, which are rarely explored, in particular under the high driving pressure conditions of hydropower plants. The FVM and CFD are separately used for the pipeline section and the air cushion surge chamber, while real-time computation data is exchanged at the FVM-CFD coupled interface. Moreover, the effect of unsteady friction was considered in the water hammer simulations. Experimental results demonstrate that the FVM-CFD coupled method can accurately reproduce the experimental data. Based on the local 3D simulations for different size air chambers, a common feature is observed that spouting geyser arises near the inlet of air chamber during the load rejection initial stage. Results show that when the chamber corridor is longer and narrower under stable conditions and constant gas volume, the peak pressure would be higher and the water level fluctuation becomes more significant, while the wave shape is more regular and the surging period along the chamber long direction is longer. Meanwhile, the water level change exhibits a certain hysteresis in the “spouting geyser area” where the water level fluctuation is minimal and the surging phenomenon occurs less frequently. The reversed results can be observed when the chamber is shorter and wider, the water level fluctuation is more chaotic and the wave shape is more irregular, while the period of surging along the chamber direction is shorter. The water level change in the “spouting geyser area” is more violent, while the water level fluctuation at the chamber inlet is greater and the frequency of surging phenomenon is higher, and the water level fluctuation curves are more regular and smoother. The fluctuation law of the average temperature and the lowest temperature of the chamber water surface are consistent with the pressure fluctuation law. Importantly, the low temperatures (below zero) are possible if the air chamber corridor is too narrow or too short.