Journal of Scientific SARAVANAN & Industrial et al: Research PRESSURE DISTRIBUTION OF SMALL WIND TURBINE BLADES WITH WINGLET Vol. 71, June 01, pp. 45-49 45 Pressure distribution of rotating small wind turbine blades with winglet using wind tunnel P Saravanan 1 * K M Parammasivam and Selvi Rajan 3 1 Department of Aeronautical Engineering, Tagore Engineering College, Chennai 600 048, India Department of Aerospace Engineering, MIT Campus, Anna University, Chennai 600 044, India 3 Wind Engineering Laboratory, CSIR-Structural Engineering Research Centre (SERC), Chennai 600 113, India Received 18 January 01; revised 16 April 01; accepted 17 April 01 This study presents pressure distribution over an envelope of blade with and without winglets for a small wind turbine in boundary layer wind tunnel. Different winglet configurations were tried based on winglet height and curvature radius. Pressure measurements were made with different chordwise and spanwise locations on the blade with and without winglets nearby tip region. Winglet improves overall pressure difference between pressure surface and suction surface. Presence of winglet seemed to have the pressure increased at 0.3c and maximum pressure difference was observed at a span of 0.95R for all winglet configurations. In suction, surface pressure decrease was 10% for all winglet configurations. Keywords: Blade surfaces, Pressure distribution, Wind turbine rotor, Winglets Introduction Wind power potential within India is estimated at 45,000 MW. Although, large wind turbines (capacity, 300 kw- MW) are being installed, harnessing wind power through small wind turbines (aero generators: capacity, 400W-15kW; wind speed. 3-10 m/s) for powering homes, hill areas and agricultural farms is still budding in India. Small wind turbine has low efficiency. Adding a winglet on blade tip can improve wind turbine performance. Van Bussel 1 augmented power by increasing tip speed ratio. Jianwen et al used wind turbine with various tip vanes and found increase in power output (-6%). Pressure distribution of wind turbine blade with tip vane improved pressure difference between pressure and suction surface 3. Aerodynamic study 4 on wind turbine rotor with winglets showed increase of power production (1.6%). Blade tip modification enhances aerodynamic performance 5. Winglet added wind turbine rotor blades were tested dynamically and increased power (6.4%) by 5 mph 6. This study presents pressure distribution along spanwise and chordwise of small wind turbine blade with winglets experimentally to improve performance. *Author for correspondence E-mail: saro.aero@gmail.com Experimental Section Boundary Layer Wind Tunnel (BLWT) BLWT (overall length, 5 m; size of test section, 18.0 m x.5 m x.15 m; wind speed, 0.5-55 m/s) at CSIR-SERC, Chennai simulates wind flow in a controlled manner to represent flow characteristics in the nature and aerodynamic forces; such a long test section is preferred to achieve natural development of equilibrium boundary layer. Ceiling of test section can be adjusted to obtain a zero pressure gradient. Test section has two turntables: i) upstream side, where studies on models for uniform flow conditions are being carried out; and ii) downstream to conduct experiments under simulated atmospheric wind. Wind Turbine Rotor Models Five rotor models (diam, 340 mm; hub diam, 0 mm; blade length, 140 mm) were fabricated using aluminium. NACA 441 profile was used from blade root to tip and the same profile maintained for different winglet configurations. Winglet curvature radius and winglet height effects are more dominant compared to other winglet parameters 4 (sweep angle, cant angle, toe angle and twist angle). Winglet models (W1, W, W3 and W4) were fabricated with different winglet height (% rotor radius) and curvature radius (% winglet height), respectively as follows: W1,, 1.5; W,, 5; W3, 4,
46 J SCI IND RES VOL 71 JUNE 01 1.5; and W4, 4%, 5%. Winglets bended towards suction side and airfoil shape were maintained same as the wind turbine blade. Winglets tapers were used same as blade tapers. This study focused on variation of winglet height and curvature radius of winglet configuration by keeping cant angle constant (75 ) for all models. Instrumentation Rotor model blades were attached to hub in horizontal shaft and it was mounted on two support arm with each holding a low friction roller bearing to ensure rotor shaft Fig. 1 Wind turbine model in test section of wind tunnel rotational freedom. Rotor shaft was mounted on a vertical tower (ht, 300 mm) and a digital laser tachometer (accuracy, 0.0%) was located inside tower to measure rotational speed of rotor models. A small wind turbine rotor was mounted inside wind tunnel (Fig. 1). By considering height of prototype wind turbine, a geometric scale (1:0) was chosen to ensure negligible blockage effects on pressure measurements. In present case, blockage based on exposed cross sectional area is < 1%. To account for geometric scale ratio, model was tested in upstream test section of wind tunnel, where turbulence level is negligible (1%) and also to eliminate ground effect and complications involved in dynamic pressure due to turbulence intensity. Rotor blade pressure taping arrangement (Fig. ) was connected via metal tubes to pressure transducers set inside the tower nacelle. Pressure measurements were made at three radial locations near to the blade tip to study the various winglet effects. At each radial location, 6 pressure tapings were used for measurements. Pressure transducers (self calibrated with no additional amplification; sampling frequency, 60 Hz) were connected to transmitter. Transmitted data was acquired using data receiver and data was stored in PC trough AD/DA card. Tachometer sensor (Fig. 3), mounted near rotating shaft positioned at vertical axis of tower and connected to AD/DA card, measured rotational speed of wind turbine rotor models. Experimental Studies and Validation of Measured Data Wind tunnel was operated at a constant wind speed (5 m/s) for all test cases and pressure measurement studies were conducted on all four rotor models at a turbulence level of 1% with winglets individually, in addition to rotor model without winglets. Pressure data Fig. Pressure locations on rotor blade
SARAVANAN et al: PRESSURE DISTRIBUTION OF SMALL WIND TURBINE BLADES WITH WINGLET 47 l s Fig. 3 Measurement system where U, wind speed (m/s); p, local pressure (N/m ); p, free stream pressure (N/m ); ρ, density of air (kg/m 3 ); ω, rotational angular velocity (rad/s); r, local radius for the spanwise location (m); x, distance between leading edge to chordwise location (m); c, chord of blade (m); and R, model rotor radius (m). were analyzed for mean pressure coefficient for comparison. Measured values of mean pressure coefficients were validated by comparing results with reported values 7. Experimental values (Fig. 4) were found very close to predicted values by Ronsten 7 for the case of rotor model without winglet. Static surface pressure data, both chordwise and spanwise for all winglets were measured. The data is further analyzed for pressure coefficients as C P Fig. 4 Comparison of pressure coefficient p p = 1 ρ [ U + ( ωr) ] (1) Results and Discussion Spanwise Pressure Distribution At x/c =0.3 (Fig. 5a), winglet rotor models (span 95%) had high pressure coefficients compared to rotor without winglet. Remarkable difference in pressure distribution between winglet and without winglet is increased in suction surface for all rotor models. Even though, pressure difference increases as winglet height increase more in suction surface and less in pressure surface. Higher curvature radius winglets pressure difference is less compared to the lesser curvature radius. At x/c=0.6, pressure difference between surfaces is more only in higher winglet height rotors and remaining rotors shown the same as without winglet rotor. At x/c=0.9, pressure coefficients is same at with and without winglet rotor. Pressure coefficients of pressure surface of winglet rotor model blades are almost same as without winglet rotor in all the chordwise locations. Increment by winglet on leading edge is larger than that on trailing edge. Pressure difference is observed maximum where
48 J SCI IND RES VOL 71 JUNE 01 a) b) c) Fig. 5 Comparison of pressure coefficient at radial: a) 95%: b) 90%; and c) 85% thickness of airfoil is maximum along the span. Thus, smaller curvature and higher height winglets can increase pressure difference more. Pressure coefficient of all winglet rotor models (span, 90%) are higher (Fig. 5b) than without winglet rotor model. On suction surface, the lesser curvature winglet rotor models pressure coefficients are higher than the higher curvature rotor models at x/c=0.3. Pressure coefficient of W rotor model is higher than other winglet rotor models at x/c=0.6. At trailing edge, all rotor model pressure coefficients are same. On pressure surface, winglet rotor models pressure difference slightly decreases than without winglet rotor model. With decrement of radial position, influence of winglet gradually decreases. On pressure coefficient of all rotor model blades in chordwise locations at 85% span (Fig. 5c), at x/c 0.6 and 0.9 locations, pressure coefficients for all rotor model blades are almost same, but pressure difference is seen only at x/c=0.3. Influence of winglet is very less compared to other (95% & 90%) radial locations. Pressure difference between pressure surface and suction surface decreases from root to tip and also pressure difference are found more at a location nearby leading edge of the blade section (Fig. 5). Effects of winglets are more dominant at blade tip. Chordwise Pressure Distribution Pressure difference is more (Fig. 6a) in r/r=0.95 for all rotor models (chord, 30%). Pressure difference increases as winglet height increases or curvature radius of winglet decreases. At r/r=0.9, pressure difference of all rotor models are identical with r/r=0.95. On the other hand, pressure surface values are all most identical, but only slight difference is seen. Pressure difference for all rotor model decreases (Fig. 6b) as chord increases (chord, 60%). Maximum pressure is seen in W1 and W in r/r=0.95 and remaining rotor model blade pressure coefficient are almost identical. At r/r=0.9, W1 and W experiences pressure decrease as chord increases. But W3 and W4 give same pressure throughout the chord length of blade. On suction surface, pressure coefficient for all rotor models are same and also seen very small deviations. At r/r =0.85, W and W4 give more pressure difference than other winglet rotor models. Thus, decrement in radial position corresponds to decrease in pressure from root to tip of the blade.
SARAVANAN et al: PRESSURE DISTRIBUTION OF SMALL WIND TURBINE BLADES WITH WINGLET 49 a) b) c) Fig. 6 Comparison of pressure coefficient at chord: a) 30%: b) 60%; and c) 90% For all rotor model blades (chord, 90%), pressure difference for all radial positions are identical (Fig. 6c) on both suction and pressure surfaces with small deviation is seen. Pressure difference is slightly more for W. Pressure difference decreases from leading edge to trailing edge (Fig. 6). Thus, tip effects of blades are more at x/c=0.3, near by the blade tip. Conclusions Effect of winglets at blade tip of wind turbine is dominant. Winglet increases pressure difference by increasing pressure on pressure surface and decreasing pressure on suction surface. At the location of maximum thickness of airfoil, pressure difference is maximum and decreases as chord length increases. Pressure difference increases as winglet height increases or curvature radius of winglet decreases. Winglet W gives more pressure difference at tip region of the blade. As pressure difference of blade increases, the momentum transfer increases, hence blade can absorb more energy from wind. W winglet configuration can improve power output of wind turbine. References 1 Van bussel G J W, A momentum theory for winglets on horizontal axis wind turbine rotors and some comparison with experiments, in 4 th IEA Symp on Aerodynamics of Wind Turbine, (IEA, Rome) 19-0 November 1990, 1-18. Jianwen W, Ruibo J & Keqi W, Numerical simulation of power augmentation of a horizontal axis wind turbine with tip vane, in Int Sci Conf in Power Industry and Market Econ, (IFOST, Ulaanbaatar) 4-7 May 005, 155-160. 3 Jianwen W, Ruibo J & Keqi W, Numerical simulation effects of pressure distribution of wind turbine blade with a tip vane, J Therm Sci, 16 (007) 03-07. 4 Johansen J & Sørensen N, Aerodynamic investigation of winglets on wind turbine blades using CFD, Tech Rep Risø-R-1543(EN) (Risø National Laboratory, Denmark), February 006. 5 Chattot J J, Effects of blade tip modification on wind turbine performance using vortex model, J Comp Fluid, 7 (009) 1405-1410. 6 Shane Merchant J, Mattthew Bondy J & Treuren V, Wind tunnel analysis of a wind turbine with winglets, in 010 ECTC Proc, ASME Early Career Tech Conf, (Oral Roberts university, Tulsa), 5-7 March 010, 1-4. 7 Ronsten G, Static pressure measurements on a rotating and a non-rotating.375 m wind turbine blade, Comparison with D calculations, J Wind Eng Ind Aerod, 39 (199 ) 105-118.