mi9 Some experimental measurements of the Diffuser flow in a Ducted Wind Turbine assisted by two ejectors Kypros F. Milidonis Department of Mechanical Engineering & Aeronautics University of Patras

Introduction Ducted Wind Turbines are known to be capable of delivering a power coefficient greater than of the conventional, horizontal axis Wind Turbines. The power coefficient is enhanced if the diffuser exhausts into its wake, where the static pressure is reduced by an amount comparable to the wind dynamic pressure. The present study considers a second enhancement mechanism, that of an ejector that re-energizes the flow inside the diffuser of the ducted wind turbine. Experimental data were obtained from a small angle diffuser, equipped with two jet ejectors fitted parallel to the diffuser walls. The main objective focuses on the impulse improvement due to the large slot injection, appropriate for ducted wind turbines, were outer air is sucked into the duct.

The experimental facility The experimental wind tunnel begins with a 500mm straight rectangular duct of 200 x 200 mm cross sectional area. Duct is followed by the diffuser which slopes symmetrically down 3° and opens the diffuser up to 10 times the entrance height where the cross sectional area is 20 x 200mm. Diffuser is preceded by a 500mm inlet channel. Two linear slots are incorporated 500mm from the diffuser entrance in order to function as ejectors. Ejector slots are specially treated so the air enters the diffuser as smooth as possible and tangential to the main flow

Flow measurements Transverse slots were drilled on the top wall of the tunnel in order to allow a 5-hole Pitot tube to enter vertical into the diffuser. Pitot tube was guided by a two axis transverse system driven by computer software in order to measure the flow pressure field in the transverse plane. Cross sectional area of each station was scavenged with 2mm density grid in both Y and Z direction taken every 2 sec and data were collected by Data Acquisition System. Static pressure was measured at specific positions by 5 mm tap holes at the lower duct wall. Air flow was created using a 3hp centrifugal suction pump controlled by an inverter, able to create free stream velocities ranging from 2 m/s to 20 m/sec.

` Experimental results Velocity distribution before diffuser entrance : X/D=-6,5 Average plane velocity: U O =18m/sec Plane average total pressure: P O =-330Pa

v Experimental results Y-axis Velocity distribution at transverse planes 2,3,4,5 Y/D limit increases along the duct, as its cross section area increases in respect to the diffuser’s aspect ratio. At X/D=25 (ejector location) velocity is up to 2.3 times higher than the diffuser. Ejector slot sucks in air at a much larger velocity to that of the diffuser given the greater total pressure of the atmosphere. This supports the basic idea for the duct application, i.e. that the static pressure on the suction plane remains quite uniform.

v Experimental results Y and Z-axis CPt* distribution at transverse planes 2,3,4,5 Before the entrance of the diffuser the CPt* distribution is quite uniform and gradually increases at downstream cross-sectional planes as expected Mixing generates significant losses, something which implies either that a contact area should be employed for the mixing as a staged suction approach. Lateral CPt* distribution is quite uniform. This implies one-dimensional mixing process.`

v Experimental results Y and Z-axis CPs* distribution at transverse planes 2,3,4,5 Transverse distributions are uniform along both the Y and Z directions and static pressure discharges only along X-axis. Pressure increase may be deduced from the CPs* reduction rate. The air flux through the slot, exhaust into the static pressure of the diffuser.

v Experimental results Impulse function I Y and I Z along the diffuser Air suction raises the flow impulse function, which gradually drops as the two fluxes, (outer air and inner diffuser) mix out.

Conclusions Measurements in this study were taken in specific transverse planes (stations) in order to demonstrate the effectiveness of the ejector slots in diffuser flow. The two ejectors re-energizes the flow inside the diffuser. The ejectors functions because the static pressure inside the diffuser is below the atmospheric one, so additional atmospheric air is entrained into the diffuser as a result of this pressure difference. Furthermore, the velocity of the flow in the diffuser increased because of the flow that enters by the ejectors. As a result appears a thrust increase inside the diffuser and finally an increase in the overall efficiency. A relevant theoretical analysis predicts that the turbine loading in such a plant exceeds conventional ones with the same rotor diameter, especially when the diffuser exhausts into his wake.

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