Computational analysis of part-load flow control for crossflow hydro-turbines

Abstract

Crossflow turbines are commonly used in low-head small hydro systems, typically less than 50 m, and produce power up to a few hundred kWs. A crossflow turbine is a unique type of hydro-turbine in that the flow passes twice through the runner. Therefore, the power is extracted in two stages. These turbines often employ a guide vane in the nozzle for controlling the flow entering the runner. However, a guide vane significantly reduces the quality of the entry flow by splitting it into two jets and producing non-uniform entry flow angles which can cause a serious loss in turbine efficiency. Adhikari and Wood (2017) and Adhikari and Wood (2018) showed that well designed crossflow turbines without a guide vane can achieve full-load efficiencies of over 90%. This leaves open the issue of maintaining high efficiency at part-load, defined as reduced flow at constant head. As for other hydro-turbine types, efficient part-load operation cannot be achieved solely by employing power electronics to reduce the shaft speed as the flow decreases. This paper analyzes a slider or “Cink” control device at the entry to the runner to reduce the entry arc, i.e. the angular extent of the runner entry, as the flow rate decreases, (Sinagra et al., 2014). We show that a properly operated slider will maintain the radial and azimuthal entry velocities at their full-load values and therefore will not change the optimum shaft speed as the flow rate changes. This simplifies the power electronics required for the turbine generator. Three-dimensional Reynolds-Averaged Navier-Stokes simulations of a 0.53 kW turbine with 88% full-load efficiency showed that the slider maintains high efficiency at part-flow conditions. We conclude that the slider shows significant advantages over a guide vane as the primary means of crossflow turbine control at part-load.