Experimental investigation of an adjustable guide vane system in a Francis turbine draft tube at part load operation

Abstract

Hydraulic turbines are increasingly used to regulate the electrical grid. They thus have to operate more often at off-design conditions where instabilities may develop leading to large pressure pulsations decreasing the life of the machines. In the present study, an adjustable guide vane system in the draft tube of a single regulated hydraulic turbine is developed. The aim is to mitigate the pressure pulsations when the turbine operates at part load operational regimes without significantly compromising the hydraulic efficiency of the turbine. The draft tube guide vane system, named DTGV, is composed of three guide vanes. Each guide vanes comprise two individual hydrofoils. The upper hydrofoil can rotate around an axis up to ±45°, and the lower is fixed. Therefore, the guide vanes are adjustable. The rotating vortex rope mitigation was investigated at two part-load operational regimes at a 12 m turbine head, a) Q/QBEP = 0.59, and b) Q/QBEP = 0.71. The results indicate that the DTGV system significantly mitigates the pressure pulsations, thereby improving the flexible operation of the turbine. The best mitigation outcome was for 0 GV configuration at both part-load operating conditions. The mitigation effects are better when the operating condition approaches BEP, and the rotating mode of the pressure pulsations tends to go towards the runner frequency (f/f0 = 1). The hydraulic efficiency is found to be marginally affected by the guide vane system at part load operations and is improved at the best efficiency point (Q/QBEP = 1) and high load Q/QBEP = 1.21.