Investigating the flow effects of slots and perforations on a flat blade

Abstract

The following study investigates the flow effects caused by slots and perforations on a flat blade for use in a simple, undershot, water-wheel to better understand the flow mechanics of these turbines. This study has a primary aim of increasing the power generated by these blades without significantly increasing manufacturing costs. Experimental tests were conducted in a wind tunnel with 2.4% blockage ratio. These tests aimed to measure the power coefficient of the blade over a range of flow velocities and angles of attack. Reynolds Number matching was used to ensure that the use of a wind tunnel for a water-based design was adequate. Initial tests were undertaken on four designs with similar porosity ratios. After the initial results were found, these designs evolved to find an optimal solution. The results show that, under the test conditions, the square slotted design with porosity, Φ, of 0.1 was optimal. It provided a slight increase in overall power coefficient at high angle of attack while using 10% less manufacturing material. The final design also improved downstream flow effects, causing a more rapid decay in the wake velocity. This increases the power generation of blades further downstream when used in a turbine configuration. Collectively, these results provide an insight into the flow effects of perforated and slotted flat blades in a controlled experimental environment, which will be used in the future to improve the design of the simple water-wheel.