EXTERNAL RECIRCULATION IN PRS TYPE TURBINE: EXPERIMENTAL AND NUMERICAL RESULTS.

Abstract

Cavitation is a relevant phenomenon for structural safety and noise level in hydraulic turbines, occurring when water pressure falls below the vapor value at a given temperature. In this case bubbles of vapor grow inside the liquid and move along with it. When the pressure returns above the vapor value the bubble collapses, and the pressure can locally achieve very high values, up to 7000 bars (Kumar & Saini, 2010). Moreover, if the bubble was confined also by the solid wall of a blade, the solid particles suspended in the fluid can be transported by the fluid ones and hit the solid wall at very high velocity, generating erosion. Cavitation is also the source of high frequency noise, very disturbing for humans. Cavitation has been extensively studied mainly in reaction turbines, especially Francis and Kaplan types (Luo &Tsujimoto, 2016, Turi et al., 2019; Alligne et al., 2014). Adhikari et al. (2016) carried on a numerical analysis occurring at the tip of the blades of a Crossflow turbine. The conclusion was that, at least in the case study, cavitation occurred only at rotational velocity greater than the design one. Power Recovery System (PRS) turbine is a reaction turbine with a rotor like the Crossflow one, that can be used instead of the Crossflow in the case of outlet pressure greater than zero. PRS has a very simple design and can provide hydraulic regulation with the use of a mobile flap that can reduce the rotor inlet area. Due to its simplicity and small size, PRS can be easily installed by replacing a short part of the pipeline where the hydraulic jump is available. In the following paper it is first shown, by means of CFD simulations, that low pressure and cavitation can easily show up in PRS turbines, especially when the downstream outlet pressure is very low, and that pressure attains the minimum value immediately after the tip of the nozzle, according to the rotational direction. The physical explanation is that the momentum Пout of the water volume trapped in the blade channel, immediately after the tip of the nozzle, cannot be balanced by the Пin, inlet one and this leads to strong negative pressure gradients. A possible countermeasure is the use of external recirculation by means of a small pipe connecting the outlet pipe with a rectangular opening of the case immediately after the tip of the nozzle. The connection leads to a small recirculation flow and to a pressure rise in the target area. The length of the arc between the end of the nozzle and the rectangular opening is a fraction of the arc of the blade channel, so that an inlet momentum is always provided to the blade channel up to the end of the rectangular opening. The proposed change has been numerically and experimentally tested on a 5kW PRS prototype, installed in the experimental loop of the hydraulic lab of the University of Palermo.