Experimental Investigation of Wind Loading on a Ducted Wind Turbine

Abstract

Ducted wind turbines (DWT) are designed to operate in the low-speed wind conditions when compared to the conventional horizontal axis wind turbines. The duct/shroud boosts the power output by increasing the mass flow through the turbine. To enhance our understanding on the impacts of turbulence on the loading of a ducted wind turbine, an experimental analysis with changing upstream turbulence characteristics was undertaken. One fourth scaled model of an aerofoiled shaped duct (Donqi Urban Windmills) loaded with an actuator disc with 50% porosity was tested in the wind tunnel. Turbulence was generated using a novel multi-grid approach that utilises two passive grids placed at a fixed distance from each other. Testing was done for four different turbulence intensities in the range of 1% to 16%. Fluctuating drag measurements were made using a multi axis load cell to record time averaged and unsteady duct behaviour. Surface pressure taps connected to the multi-channel pressure acquisition system helped in recording the mean and fluctuating pressures on the surface of the duct. A multi hole cobra probe attached to the one-dimensional traverse system was used for characterising the flow at different upstream and downstream locations of the model. To establish the limits of drag on the ducted actuator disc, a detailed investigation on the open and close duct configurations revealed Reynolds number independency for the closed duct case, with the drag coefficients being in the same order for increasing turbulence intensities. Moreover, the drag on the open duct is linearly related to the turbulence intensity. From the experimental investigations on the ducted actuator disc, disc has considerable effect on the loading of the ducted wind turbines. For the present configuration with the high thrust loading, the drag on the disc contributes to more than 70% of the total drag in low turbulence case. A detailed study on the surface pressure measurements of the loaded duct reveals separation at the leading edge of the duct’s outer contour. The spread of the separation is incremental with increasing wind speeds. It is observed that the increase in the upstream turbulence supressed the separation for both high and low speed scenarios. However, the effect of the separation was seen to be dominant during the low wind speeds. The present study introduces a novel concept of utilising multi-grids to generate flow with superior turbulence characteristics in the wind tunnel. Moreover, the loading response of a ducted actuator disc at high turbulence intensities was also studied for the first time.