Effects of tip gap size and aspect ratio on the performance of a Wells turbine

The Wells turbine is one of the most suitable power take offs for oscillating water column systems, due to its simple and effective design combined to a reasonable efficiency. In recent years, a significant number of local geometry modifications have been suggested, with the promise to provide some important performance improvements. In order to achieve the full potential of this machine, these local modifications need to be applied to an optimal configuration of the machine’s global geometric parameters, such as tip clearance, blade aspect ratio, hub-to-tip ratio and solidity. The aim of this work is to provide a comprehensive experimental analysis of the effects of aspect ratio and tip clearance on the overall performance and operating range of a Wells turbine. These two effects have been accurately isolated, excluding the influence of the Reynolds’ number, thus providing a larger and more detailed set of data with respect to the ones available in the literature. An important conclusion is that the tip clearance size, which plays a dominant role, is best expressed in non-dimensional form with respect to the blade height rather than to the blade chord. With this approach, the turbine performance appears independent from the blade aspect ratio, within the ranges of parameters analyzed in this work. A lower aspect ratio, however, helps to achieve a larger operational Reynolds’ number, leading to better overall performance.