This paper reports on the study of comparative analysis of aerodynamic characteristics of a vertical axis vane type rotor. The aerodynamic characteristics were investigated experimentally by both static and dynamic approach. In static approach, the dynamic characteristic of the rotor was predicted by calculating the static torque coefficient produced in the rotor by measuring the pressure distribution over both the concave and convex surfaces of the blades. While in dynamic approach the power coefficient was calculated by measuring the rpm of the rotor at the different loading conditions and the difference in tensions between the two ends of the friction belt. A good correlation is observed between the predicted and calculated power coefficient of the blades.
Now a days, renewable energy becomes a burning issue, not only due to the crisis of traditional energy sources but environment is a great concern as well. Wind energy is a promising sector of green renewable energy. Over the past few decades, enormous increases in research works concerning laboratory simulations, full-scale measurements and more recently, numerical calculations and theoretical predictions of flows over a wide variety of VAWT and HAWT is observed. Bowden, G.J. and McAleese, S.A. [E] made some measurements on the Queensland optimum S-shaped rotor to examine the properties of isolated and coupled S-shaped rotor. GF Homicz [F] worked on blade fatigue life and simulate the random loads of VAWT Stochastic Aerodynamic Loads produced by Atmospheric turbulences. Both horizontal and vertical axis rotor of different types and shapes are developed throughout the world for harvesting wind energy. For the present study, vertical axis vane type rotor is chosen. It is a drag based, slow running wind machine having lower efficiency. But still popular in developing countries because of its easy and simple construction technology and good starting torque characteristics even at low wind speed, furthermore, it is independent on wind direction. Kamal,F.M [A], worked on stationary five bladed vertical axis vane type rotor and predict the dynamic power coefficient of the rotor The present paper tries to make a comparison of this predicted result with the experimental one.Construction Detail of the Rotor
The rotor was made up of five half cylinders (blade) of diameter d=65 mm and height, H= 340 mm. The rotor was made of PVC material. The center to center distance between the blades were 137.5 mm. Rotor diameter, D is 200 mm, optimum value of d/D is taken 3. The whole rotor was fixed on an iron frame by using two side shafts and two ball bearings.
An open circuit subsonic type wind tunnel was used to develop the required flow and the rotor was positioned at the exit section of the wind tunnel. The tunnel was 5.93 m long with a test section of (490mm × 490mm) cross-section. The central longitudinal axis of the wind tunnel was maintained at a constant height from the floor. The converging mouth entry was incorporated into the system for easy entry of air into the tunnel and maintains uniform flow into the duct free from outside disturbances. The induced flow through the wind tunnel was produced by two-stage rotating axial flow fan of capacity 18.16 m³/s at a head of 152.4 mm of water and 1475 rpm with each of the fans connected to a motor of 2.25kW capacity and 2900 rpm. A butterfly valve, actuated by a screw thread mechanism was placed behind the fan and was used to control the flow. A silencer was fitted at the end of the flow controlling section in order to reduce the noise of the system. The diverging and converging section of the wind tunnel was 460 mm long and made of 16 SWG black sheets. The angle of divergence and convergence was 7°, which was done with a view to minimizing expansion and contraction loss and to reduce the possibility of flow separation. Other three outlet square (610 mm each) sections were used to make the flow straight and uniform.
In static approach, the mean velocity of air was measured in a vertical plane 100 cm downstream from the outlet of the wind tunnel (without placing the rotor) by a pitot static tube connected to an inclined manometer with kerosene as the manometric fluid. The pressure distribution over the blade surfaces (concave and convex) was measured step by step by using the multi-manometer. At first, the vane rotor with the frame was placed 100 cm downstream in front of the exit section of the tunnel on a table. One blade of the rotor was fixed parallel to the free stream velocity i.e. parallel to the horizontal, which was the reference plane and from this plane angle of rotation was also measured. The pressure measurements were made at 8 pressure tappings on each blade. The tappings were made with copper tubes of 1.5 mm outer diameter and 10 mm length that were press fitted to the tapping holes. The tappings were located at the mid-plane of one side of each blade, so that pressure distribution at every 10° on the blade surface could be measured. The pressure tappings were connected to an inclined multi-manometer (manometric fluid was water and had an accuracy of ± 0.1 mm of water column) through 2 mm PVC tubes. The pressures were measured at every 10° interval of rotor angle. At a particular rotor angle, α the rotor blades experience forces (per unit span length) due to the pressure difference between the concave surface and convex surface and these forces can be resolved into two components Fn and Ft. Finally static torque coefficient was measured by calculating normal and tangential drag coefficient from the forces Fn and Ft. Based on the static torque the dynamic power coefficient was predicted.
Where as in dynamic approach, the velocity was measured without the model turbine at the sections which was placed in front of the rotor at different locations and average velocity was measured directly. The experimental set-up is shown in Figure 2. Non-contact electrical tachometer was used to measure the speed of the model wind turbine at different loading conditions. Wind speed behind the rotor was measured by a digital anemometer and the speed of the model rotor at different Reynolds number was determined using a non-contact digital tachometer at different loading conditions.
Result And Discussion
This paper reports the comparative analysis of the aerodynamic characteristics of vertical axis vane type five bladed rotor, both static and dynamic method. In static approach, the analysis was basically concern study of pressure distribution over the concave and convex surfaces of the blade at different angles of rotation. The forces experienced by the rotor blades were calculated from this pressure difference and finally the drag
coefficient and torque coefficient were calculated by using raw data. The dynamic characteristic, power coefficient was predicted by calculating the relative velocity of the rotor, free stream velocity and the static drag coefficient for different tip speed ratio. It is found that the nature of the torque coefficient is opposite to that of the drag coefficient which contributes significantly for producing torque.
In dynamic approach, the power coefficient and torque coefficient was directly calculated at different tip speed ratio for different Reynolds number.
It can be concluded that the nature of curve of Power coefficient vs Tip speed ratio matches qualitatively for both static and dynamic method, though a greater deviation is observed. Both the static and dynamic methods are important from their subject point of view, static method gives more detailed information regarding flow separation, torque analysis and corresponding drag analysis. On the other side, dynamic method is able to give the dynamic properties i.e. power coefficient, torque coefficient more accurately and in a straight forward manner.