Free runner concept downstream of Francis turbines

Abstract

Current requirements of the industrialized countries impose to use as much renewable energy as possible. One meaningful problem of the renewable energy is that the produced power is fluctuating. Currently the only available method for energy compensation in the shortest time is given by hydroelectric power plants. Instead, the hydroelectric power plants (especially the plants equipped with hydraulic turbines with fixed blades) are designed to operate in the vicinity of the optimum operating point with maximum ±10% deviation. The energy market require that hydraulic turbines to operate in an increasingly wide area between -35 - 20% from the optimum operating point, [1]. Operation of hydraulic turbines far from the optimum operating point involves the appearing downstream from the turbine of a decelerated swirling flow with hydraulic instabilities (known in the literature as the vortex rope), Iliescu et al. [2]. The hydraulic instabilities are formed at the boundary between the main flow zone (when is operating at partial flow is close to the cone wall) and the stalled area located in the centre of the conical diffuser [3], [4]. The main purpose of the paper is to investigate numerically a new concept by using a free runner downstream on the main hydraulic runner turbine. The free runner concept supposes that rotates at the runaway speed with vanishing mechanical torque. The main purpose is to redistribute between the shaft and the periphery the total pressure and the moment of momentum such that the flux of total pressure and the moment of momentum are not altered. Moreover the free runner does not modify the operating point of the main hydraulic turbine runner. The benefits of the free runner downstream the main runner turbine approach are: i) reducing the pressure pulsations (synchronous or asynchronous) from the draft tube cone, ii) optimal configuration for the swirling flow ingested by the draft tube cone, iii) minimum draft tube losses and maximum pressure recovery. Similar approaches have been presented by Susan - Resiga et al. [5] by using a Francis turbine with tandem runners, Gokhman [6] by using a stay apparatus downstream the Francis runner or RAPT concept for a radial axial pump turbine, [7]. The free runner concept downstream the main turbine will add the required flexibility for the Francis turbine regulation and avoid the deterioration of hydraulic turbine components.