Hydrodynamic Investigations in a Swirl Generator Using a Magneto-Rheological Brake

Currently there is a growing demand for renewable energy in the industrialized countries. However, these sources (e.g. solar, wind) introduce a fluctuating component within the electrical grid. Therefore, the hydraulic turbines have to operate far away from the best efficiency point (BEP) covering a wide range. As a result, the self-induced instability flow called vortex rope is developed downstream the runner, in the draft tube cone at part load conditions. The vortex rope induces pressure pulsations in all hydraulic circuit as well as additional hydraulic losses and unwanted events in the power plant. A swirl generator which mimics a hydraulic turbine operated at part load conditions (0.7 from the best efficiency discharge) was designed in order to explore several flow control techniques. The swirl generator is designed with two rows of blades (guide vanes and runner) to generate the flow configuration with self-induced instability. In this conditions, the runner spins freely at 1020 rpm corresponding to the runaway regime. A magneto-rheological brake (MRB) is designed and implemented on the test rig in order to provide several swirling flow configurations corresponding to the hydraulic turbine operation (from 0.7 to the best efficiency discharge). These swirling flow configurations are obtained slowing down the runner speed. The main challenge for MRB is associated with its operation underwater conditions. As a result, a special MR fluid is selected together with an appropriate sealing solution to avoid expelling the solid particles. In this case, no cooling system is needed due to the MRB is continuously cooled by water. The paper presents the hydrodynamic investigations performed with the swirl generator equipped with MRB working underwater conditions. The second section describes the mechanical design for the chosen type of MRB. Next section provides the experimental results and the MRB behaviour. The analysis concludes the fact that the MR brake slow down the speed with 35%. The pressure pulsation measurement reveals rotational component of pressure pulsation associated with the vortex rope at part load regime (1020–790 rpm) especially in first three levels of the draft tube cone. The pressure pulsations decrease according with the operation at best efficiency point (790–690 rpm), when the speed of the runner is slow down.