프란시스 수차 모델의 흡입 헤드가 캐비테이션 초생 및 발달에 미치는 영향

Abstract

Francis turbine is one of the highly efficient turbines which converts hydraulic energy to mechanical energy. At lower flow rate operating conditions, the turbine’s stable operation is affected due to the downstream swirl instabilities. Cavitation is also a very common and severe problem in these turbines due to which turbine components may fail because of erosion and pressure fluctuations. In the turbine, the suction head affects the cavitation’s inception and development. A numerical investigation of the effect of suction head on cavitation at part load condition is attempted in the present study using CFX code. The suction head is evaluated based on creating different suction pressure at the draft tube outlet. The steady-state numerical calculations are performed with a structured and unstructured grid. For the acceptance of generated grid, the grid convergence index is also calculated. Reynolds averaged Navier-Stokes (RANS) equations are solved accompanying the Rayleigh-Plesset model to capture the turbulent flow with cavitation. An experiment is also performed for the verification of the numerical results. As the suction head increases, the cavitation is initiated and developed and both the efficiency and power decrease. The head loss in the draft tube is maximum for fully developed cavitation with a 2% deviation from without cavitation case for which the loss is minimum. The vortex rope strength for the higher values of suction heads is higher compared to lower values of suction heads. Similarly, the swirl intensity is also higher for higher suction heads. The deciding parameter of the suction head can be predicted for the stable operation of the turbine using the current investigation.