Abstract
Marine energy still plays a marginal role in the current global energy scenario, despite the incessant effort by research for more than thirty years in the exploitation of the so-called blue energy. Nevertheless, thanks to a raising awareness of the gravity related to the climate changes, energy poverty and energy security, the Nations are expressing a common willingness to increase also the global installed capacity by ocean energy. Among the wide range of marine technologies, wave energy harvesting can play a significant role in view of its potential and Oscillating Water Column (OWC) systems, coupled with Wells turbines, can be considered among the most mature wave energy technology. Due to the oscillating nature of the flow rate in this kind of applications, Wells turbines are affected by dynamic stall, which has significant effects in terms of performance, vibration, noise and structural integrity of the turbine.
Actually, during dynamic stall, the Wells turbine experiences evident high frequency torque fluctuations which overlay on the typical hysteresis loop, mainly during flow deceleration. The amplitudes of these fluctuations are damped as the flow rate decreases toward reattachment. Often these fluctuations are not evident because hysteresis loops are usually provided with phase-averaged data, which can significantly smoothen or even conceal them. Indeed, it is difficult to find in the literature high frequency torque measurements able to show these fluctuations. With the aim to better investigate this phenomenon, this work proposes a new 3D-printed monoplane Wells turbine, which has been tested at the open wind tunnel of the Polytechnic University of Bari, Italy. The interest of the experimental campaign has been mainly focused on the effects of main parameters of the oscillating flow rate (mean frequency, frequency amplitude and period of the vector control drive of the squirrel cage blower) on the performance of the machine. The machine has been firstly investigated in steady state, then under dynamic stall working conditions. As a result, unsteady torque fluctuations occur during the flow deceleration till the flow reattachment; moreover, this phenomenon is intensified as the amplitude of the oscillating flow rate increases and the wave period decreases. Hence, detecting these oscillations can be relevant in the turbine design phase to enhance the structural strength of the turbine. Moreover, it has been found that this unsteady behavior is due neither to the mass flow rate crossing the turbine nor to the stagnation pressure drop, but only to the detachment of vortices generated from the leading edge and travelling along the suction side of the blade.