Design and testing of a static rig for tesla turbine flow visualization

The aim of this work is to describe the design and the use of an innovative test rig for investigating the expansion of subcooled fluids inside a converging nozzle and the evolution of two-phase flows in Tesla-type turbines. The flow exiting the nozzle enters tangentially into a thin flat circular chamber and it finally is discharged in the center through a duct perpendicular to it. The experimental test rig has two nozzles placed in diametric position. This peculiar shape reproduces the geometry of a single gap between two discs of a Tesla turbine, a machine that potentially could replace the throttling valve in chillers and heat pumps to increase their COP. The study of a simple and static geometry is necessary in order to calibrate the CFD modeling of the phase change in nozzle and rotor chamber. The rig was designed and assembled by TPG of the University of Genoa in the framework of the Pump-Heat H2020 project. Here it is used subcooled water and, in order to fully characterize the expansion conditions, the rig has been equipped with pressure sensors at the nozzle inlet and at the rig outlet. A Coriolis mass flow meter and a temperature sensor were also placed at nozzle inlet. High-resolution cameras provided and managed by Ansaldo Energia were used to look at the position and shape of the front of the fluid phase change along and around the nozzle as a function of varying pressure and temperature conditions. The tests were performed in the 2.1- 5.1barG pressure range and in the 132-155°C temperature range, feeding either one or both nozzles. Future work involves the use of different nozzle profiles, such as a convergent/divergent in order to test both subsonic and supersonic flows, and experimental analysis of pressures in the rotor chamber, aimed to optimize the geometry of nozzles and Tesla turbines in two-phase applications.