Effect of Wind Tunnel Blockage on the Performance of a Horizontal Axis Wind Turbine with Different Blade Number

Abdelgalil Eltayesh,Magdy Bassily Hanna,Francesco Castellani,Matteo Becchetti,A.S Huzayyin,Massimiliano Burlando,Hesham M El-Batsh

doi:10.3390/en12101988

Abdelgalil Eltayesh, Magdy Bassily Hanna + Show 5 more

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https://doi.org/10.3390/en12101988

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Abstract

Blockage corrections for the experimental results obtained for a small-scale wind turbine in a wind tunnel are required in order to estimate how the same turbine would perform in real conditions. The tunnel blockage is defined as the ratio of the wind turbine swept area to the wind tunnel cross-section area. Experimental measurements of the power coefficient were performed on a horizontal-axis wind turbine with two rotors of diameter equal to 2 m and different numbers of blades, namely three and five. Measurements were carried out for different tip speed ratios in the closed circuit open test section wind tunnel of the University of Perugia (Italy). The obtained experimental results were compared with the numerical ones carried out in free conditions by using a CFD approach based on the steady-RANS method with the SST k-ω turbulence model, adopting the multiple reference frame (MRF) strategy to reduce the computational effort. The comparison showed that the maximum value of blockage, which is reached in the asymptotic limit at very large tip speed ratio (TSR) values, does not depend appreciably on the number of blades. A higher number of blades, however, makes the occurrence of the maximum blockage come earlier at lower TSRs.

Highlights

The most important factor affecting measurements of wind turbine experiments in bounded wind tunnels (WT) is the existence of the bounding walls, which cause the change in the actual flow velocity entering the rotor of the wind turbine and the generation of non-uniform flows in the span-wise and vertical direction due to the production of secondary recirculations inside the test chamber
As a wind turbine is placed in a bounded domain, such as a wind tunnel, the velocity and pressure fields confined within the stream-tube entering the rotor change compared to the far upstream: the stream-tube upstream of the rotor turns out to be contracted with respect to the free field case, which leads to increasing the working mass flow rate and a higher
This effect should be reduced in an open test chamber wind tunnel with respect to a closed one, but it was only reduced because some secondary recirculation vortices occurred at the corners of the test section that limited the stream tube widening

Summary

Introduction

The most important factor affecting measurements of wind turbine experiments in bounded wind tunnels (WT) is the existence of the bounding walls, which cause the change in the actual flow velocity entering the rotor of the wind turbine and the generation of non-uniform flows in the span-wise and vertical direction due to the production of secondary recirculations inside the test chamber. Many investigations dealing with the study of the performance and aerodynamics of wind turbines were performed with no blockage correction, as reported for instance in [5,6]. These authors measured the performance of wind turbines in a wind tunnel with a blockage ratio of about 10%, where the wind tunnel blockage was defined as the ratio of the wind turbine swept area with respect to the wind tunnel cross-section area

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