Static Torque Coefficient Research Articles

Novel Cyclic Blade Pitching Mechanism for Wind and Tidal Energy Turbine Applications

A vertical axis drag-based turbine is proposed that allows for an improved performance by feathering its blades during recovery strokes to eliminate adverse blade forces. The turbine blades resemble flat plates and pitch by 90 ∘ between the two turbine strokes using a novel dual-cam mechanism. This passive mechanism orients the blades vertically during the drive stroke for maximum effective area and horizontally for minimum effective area during the recovery stroke. This allows maximizing the positive drive stroke force and minimizing the recovery stroke losses, in turn maximizing the net energy capture and the turbine performance. It is called the cyclic pitch turbine, and a mathematical model is developed that predicts the turbine performance. It shows that the turbine is self-starting for all orientations and has a higher and more uniform static torque coefficient than the popular Savonius turbine. The dynamic analysis also indicates a higher performance, and the predicted values for torque and power coefficients match very closely with those from water channel and wind tunnel experiments on a prototype. Results of testing several blade shapes indicate that airfoil section blades with long and narrow continuous shapes that have less area towards the blade’s tip result in higher performance.

Optimization Research on Lift-Type Vertical Axis Wind Turbine Airfoil by CFD

The blade airfoil has an important effect on the performance of a lift-type vertical axis wind turbine (VAWT). In this paper, a research has been carried out based on CFD numerical technology to compare the effects of 4 kinds of S series airfoil (S1012, S1016, S1046, S1048) and traditional airfoil NACA0015 on lift-type VAWT. The turbulence model is used k-ω SST model. The simulation results indicate that: The airfoil NACA0015 has the highest torque coefficient and power coefficient. In terms of S series airfoils, the airfoil S1012 has the best performance and airfoil S1016 performs worst. The airfoil S1016 has the largest static torque coefficient. And the airfoil which has higher power coefficient would have a lower static torque coefficient for S series airfoil. The static torque coefficient of airfoil NACA0015 is lower than airfoil S1016 and S1046.

Optimization of the helical Savonius rotor through wind tunnel experiments

In this paper, the authors propose a novel vertical wind turbine named helical Bach rotor. This rotor combines the helical rotor and the Bach rotor. The Bach rotor had a high value of power coefficient Cp; but it had negative values of static torque coefficient. These negative values would prevent the rotor to start on their own at certain rotor angles. Similarly, the helical rotor had a lower value of the power coefficient than the Bach rotor but it could provide positive values of static torque coefficient. Therefore, the combination of these two rotors would produce a helical Bach rotor with a better performance. Hence, this study attempted a new design of the rotor made of resin and glass fiber and tested it in an open wind tunnel. Tests were conducted to evaluate the power coefficient, torque coefficient and static torque coefficient for each rotor. The experimental results indicate that the helical Bach rotor had the highest power coefficient and static torque coefficient compared to the helical rotor. The helical Bach rotor yielded 0.2 power coefficient whereas the helical rotor yielded 0.18 power coefficient.

Research on Aerodynamic Characteristics of Straight-Bladed Vertical Axis Wind Turbine with S Series Airfoils

Background. In order to investigate the effect of aerodynamic characteristics of S series airfoils on the straight-bladed vertical axis wind turbine (SB-VAWT), numerical simulations and wind tunnel experiments were carried out using a small SB-VAWT model with three kinds of blade airfoils, which are asymmetric airfoil S809, symmetric airfoil S1046, and NACA0018 used for performance comparison among S series. The aerodynamics characteristics researched in this study included static torque coefficient, out power coefficient, and rotational speed performance. The flow fields of these three kinds of blade under static and dynamic conditions were also simulated and analyzed to explain the mechanism effect of aerodynamic performance. According to the results, the SB-VAWT with airfoil S1046 has better dynamic aerodynamic characteristics than other two airfoils, while the SB-VAWT with airfoil S809 is better in terms of the static characteristics. As the most suitable airfoil for SB-VAWT, the S series airfoil is worth researching deeply.

Modeling of flow-induced stress on helical Savonius hydrokinetic turbine with the effect of augmentation technique at different operating conditions

Conventional Savonius rotor experiences negative and high static torque coefficient at several rotor angles. To prevail over this issue Helical Savonius hydrokinetic turbine (HSHKT) twisted with 90° is proposed, which is able to avoid excessive shaking due to hydrodynamic forces on blades. Usually, HSHKT have a low efficiency, Therefore to enhance the performance coefficient of the rotor some augmentation techniques are required to be used. In this study modeling of flow-induced stresses on HSHKT are performed through fluid-structure interaction analysis. The maximum hydraulic load and von-Mises stress on rotor are found as 0.79 Mpa and 145.45 Mpa respectively in case of duct with single deflector augmentation techniques. Computation results show that edge of shaft and joint of end plate and shaft rotor experiences zone of high von-Mises stress which leads to fatigue crack in this area. In consequence to that augmentation techniques creates adverse effect on stability, smooth operation and service life of rotor at the higher free stream velocity, which opposes the fact that the rotor performance is enhanced by employing augmentation techniques in certain conditions. In this study, it is found that the possibility of fatigue crack increases at higher free stream velocity while using augmentation techniques.

Performance Analysis of a Helical Savonius Wind Turbine with Modified Rotor

This study aims to investigate numerically the performance of a helical Savonius wind rotor with a twist angle of 45°. The twist is performed on a modified profile of the rotor blade. The power coefficient (Cp), the torque coefficient (CT), and the static torque coefficient (CTS) are computed under a constant aspect ratio of 1.0 and overlap ratio of 0.15. The numerical results presented in this paper are obtained using the three-dimensional incompressible unsteady Reynolds-Averaged Navier-Stokes equations along with the RNG k-e turbulence model. A comparison between the present helical Savonius wind turbine of modified rotor and the helical Savonius wind turbine of conventional rotor shows better performance for the present modification. The maximum power coefficient and the maximum torque coefficient for the present helical modified rotor are 0.22 and 0.41, respectively.

Performance Assessment of Darrieus Turbine with Modified Trailing Edge Airfoil for Low Wind Speeds

Darrieus wind turbines are simple lift based machines with exceptionally high efficiencies in terms of power coefficient compared to similar drag based vertical axis turbines. However, in low Reynolds numbers, a notable performance loss was reported. As a potential solution, truncated NACA 0018 airfoil (NACA 0018TC-39) has been introduced with baseline cavity modification to achieve better start-up characteristics and to enhance the low wind speed performance. The baseline cavity will provide an additional benefit of reverse drag at low TSR which is obligatory for low wind speed start-up. Numerical optimization has been carried out on the conceived airfoil NACA 0018TC-39 to find out the effective truncation percentage in terms of the chord. The numerical study has been extended to compare NACA 0018 and NACA 0018TC-39 airfoil for their aerodynamic performances in terms of lift, drag coefficients and separation characteristics. The NACA 0018TC-39 airfoil was incorporated within a non-swept straight bladed Darrieus turbine miniature to experimentally evaluate the performance in terms of dynamic power coefficient, dynamic torque coefficient and static torque coefficient and compared with conventional NACA 0018 airfoil at six different Reynolds numbers 178917, 193827, 208737, 223646, 238556 and 268376. The experimental contrast implied that NACA 0018TC-39 airfoil turbine yielded almost double power coefficients at low Reynolds number compared to conventional NACA 0018 airfoil without hampering its performance at higher Reynolds number.

Wind tunnel testing of scaled models of a newly developed Darrieus-style vertical axis wind turbine with auxiliary straight blades

Renewable sources of energy, needed because of the increasing price of fossil derivatives, global warming and energy market instabilities, have led to an increasing interest in wind energy. Among the different typologies, small scale Vertical Axis Wind Turbines (VAWT) present the greatest potential for off grid power generation at low wind speeds. In the present work, wind tunnel investigations about the performance of an innovative configuration of straight-blades Darrieus-style vertical axis micro wind turbine, specifically developed for small scale energy conversion at low wind speeds, has been made on scaled models. The micro turbine under investigation consists of three pairs of airfoils. Each pair consists of a main and auxiliary airfoil with different chord lengths. A standard Darrieus configuration, consisting of three single airfoils, was also tested for comparison. The experiments were conducted in a closed circuit open chamber wind tunnel facility available at the Laboratory of Industrial Measurements (LaMI) of the University of Cassino and Lazio Meridionale (UNICLAM). Measured data were reported in terms of dimensionless power and torque coefficients for dynamic performance analysis and static torque coefficient for static performance analysis. The adoption of auxiliary airfoils has demonstrated to give more dynamic torque at the lower wind speeds with respect to a standard Darrieus rotor, resulting in better performance for all the wind speeds considered. In terms of dynamic power coefficient, the standard Darrieus configuration presented slightly better performance for the highest wind speed analyzed. The proposed configuration showed also an higher value of static torque coefficient with respect to the standard Darrieus micro turbine, resulting in a better self-starting ability.

Influence analysis of blade chord length on the performance of a four-bladed Wollongong wind turbine

The Wollongong wind turbine is a new kind of vertical axis wind turbine (VAWT) with its blades rotated by only 180° for each full revolution of the main rotor. A computational study on the effect of blade chord length on the turbine output performance of a four-bladed Wollongong turbine has been conducted using the commercial computational fluid dynamics (CFD) code ANSYS 13.0. A validation study was performed using a Savonius turbine and good agreement was obtained with experimental data. Both rotating and steady CFD simulations were conducted to investigate the performance of the VAWT. Rotating two-dimensional CFD simulations demonstrated that a turbine with a blade length of 550 mm has the highest power curve with a maximum averaged power coefficient of 0.3639, which is almost twice as high as that of a non-modified Savonius turbine. Steady two-dimensional CFD simulations indicated that the Wollongong turbine has a good self-starting capability with an averaged static torque coefficient of 1.09, which is about six times as high as that of a Savonius turbine.

Wind tunnel investigation on the two- and three-blade Savonius rotor with central shaft at different gap ratio

The Savonius rotor seems to be a promising wind turbine as it not only has the simplest and cheapest design but also is capable of yielding a higher annual energy output at low wind speed than the Darrieus rotor. Moreover, the Savonius rotor can also be used in ventilation systems, for local electricity production, as the start-up device for the Darrieus rotor, and small hydrokinetic turbines operating at low velocity. As a two-blade Savonius rotor suffers from negative average static torque coefficient (ACTS) at some azimuth angles and large-amplitude variation of ACTS, several studies have been conducted in recent years to improve ACTS. The three-blade rotor seems to be a potential candidate for ACTS improvement. However, less research has been done on three-blade rotors with a 180° arc and central shaft at different gap ratios (GRs) for different wind speeds. Therefore, the focus of the present work is to compare the two- and three-blade rotor in terms of ACTS and power coefficient (CP) through a wind tunnel experiment. Results show that the wind speed had a small effect on ACTS. However, negative azimuth angle range is narrowed and the negative azimuth angle range is moved upward as GR increased. Hence, the Savonius rotor with three blades could not only eliminate the negative range of ACTS but also smooth ACTS curves. In terms of the CP, the maximum power coefficient of the two-blade configuration was approximately 1.5 times that of the three-blade configuration. The 1/6 GR test data exhibited the attainment of super performance for all wind speed and blade number.

The effect of the opening ratio and location on the performance of a novel vertical axis Darrieus turbine

A new type of Darrieus turbine has been proposed to improve the starting ability of the Darrieus turbines using an opening. We conducted a detailed CFD (computational fluid dynamics) investigation to find the influence of the opening location a ratio on the performance of the Darrieus rotor.Generally speaking, the simulation results revealed that the power coefficient (CP) of a rotor with an opening is smaller than that of a rotor without an opening. However, the opening does increase the static torque coefficient (CTs) of the rotor, especially when the opening is located at the outer surface of the airfoil. Moreover, the inner opening has a fairly large negative effect on the ICT in the upwind zone. And the outer opening has a fairly small negative effect on the ICT in the upwind zone and has a greater negative effect on the ICT in downwind zone. It appears that the desirable rotors for inner opening are the rotors with 0.48 and 0.60 opening ratios. And the desirable rotors with an outer opening are the rotors with 0.72 and 0.84 opening ratios. They can be regarded as promising devices to replace the Savonius-Darrieus rotor considering the CP loss, CTs increase and SOICTs increase.

Computational investigation of Savonius wind turbines with classical and novel blade configurations

In this study, a modified blade profile configuration has been suggested to improve the low-performance levels of the classical Savonius wind turbine. Numerical investigation of both the classical and modified Savonius rotor has been carried out using ANSYS FLUENT 14.0. The proposed design has been observed to prevent the negative static torque coefficient that has occurred in the classical rotor at certain rotor angles relative to the wind direction. The performance of both the classical and the modified designs have been examined and compared at three different Reynolds Numbers. Results indicate better performance of the modified rotor when compared with that of the classical rotor. The improvement was found to be due to the higher overlap flow velocity for the modified Savonius rotor than that of the classical Savonius rotor. Moreover, significant improvement was observed in the torque and power coefficients as the free-stream Reynolds number increases.

Investigation of self-starting and high rotor solidity on the performance of a three S1210 blade H-type Darrieus rotor

Energy is an essential ingredient for socio-economic development and economic growth of a country. Energy is available in two different forms, fast depleting or non-renewable (coal, fuel, natural gas) and renewable (solar, wind, hydro etc). Wind is one of the potential renewable energy sources due to its abundance in the atmosphere in different scales of high, medium and low ranges. Vertical axis wind turbine (VAWT) can be installed in low wind speed regime for performing various small-scale functions ranging from electrifying a built environment to pumping water especially in remote places where grid-connected electricity is a scarce. Amongst various VAWT rotors, H-type Darrieus rotor has become more popular in the built environment for their straight blade designs and simpler construction features. However, the major problem facing such VAWT rotor is their non-self-starting characteristics due to symmetrical blade designs. Replacing VAWT's conventional blades with unsymmetrical blades and increasing rotor solidity could make potential solution to the above problem. However, there is still hardly any quantitative measure of the self-starting, torque, power coefficient etc. with increased rotor solidity so as to obtain some performance insights of high solidity unsymmetrical blade H-Darrieus rotor in low wind speed condition. In this paper a three-bladed H-type Darrieus rotor equipped with unsymmetrical S1210 blades is investigated first for its self-starting characteristics with different rotor solidities (from 0.8 to 1.2) at various azimuthal positions. Then the power coefficients (Cp) are evaluated for these solidities at various wind speeds. It will be shown that high blade solidity is in fact desirable for overall better performance of the rotor. There is an optimum rotor solidity at which power coefficient is the highest. And the maximum Cp of 0.32 is obtained for rotor solidity 1.0 and wind speed 5.7 m/s. The results are compared with some other symmetrical/unsymmetrical blade H-Darrieus rotors. Though the operating range is reduced but, for higher static and dynamic torque and comparable power coefficient with respect to existing rotors, the present rotor could be used for various small-scale applications especially that require high torque like pumping, grinding etc.

Investigations on self-starting and performance characteristics of simple H and hybrid H-Savonius vertical axis wind rotors

With recent surge in fossil fuel prices and demands for renewable energy sources, vertical axis wind turbine (VAWT) technologies have emerged out as one of the prime growing sector for small-scale power generation in the built environment. In such an environment, self-starting and high performances are of utmost importance. Amongst all VAWT designs, H-rotor, being a lift-driven device, exhibits a high power coefficient. However, it suffers from poor starting behavior due to its conventional symmetrical NACA airfoil blades. The objective of the present study is to design a VAWT rotor that possesses both self-starting and high power coefficient simultaneously. For this, a three bladed H-rotor with unsymmetrical cambered S818 airfoil blades is investigated, which shows self-starting characteristics at many of the azimuthal angles. However to make the rotor completely self-starting, the same H-rotor is incorporated in a hybrid system with Savonius rotor as its starter. It is found that the hybrid design fully exhibits self-starting capability at all azimuthal positions, signified by the positive static torque coefficient values. For improving power performance of the hybrid rotor, the same is subjected to rigorous experimentations on the wind tunnel at different Reynolds numbers (Re) between 1.44×105 and 2.31×105 for five different overlap conditions in the Savonius rotor part. The performance coefficients of the hybrid rotor are compared with the simple H-rotor. Out of all the designs investigated, the maximum Cp of 0.34 is obtained for the hybrid rotor at tip speed ratio (TSR) of 2.29 and Re of 1.92×105 for the optimum 0.15 overlap, followed by a maximum Cp of 0.28 at TSR of 2.42 and at same Re for the simple H-rotor. Furthermore insights of the performances of the optimum VAWT rotor are also obtained under different wind velocity conditions as prevail in the built environment. The optimum hybrid H-Savonius rotor demonstrates better power performance as compared to many of the existing VAWT rotors.

PERFORMANCE OF A STATIONARY SAVONIUS ROTOR WITH CIRCULAR ARC BLADE PROFILE

Static torque and drag coefficients of a stationary Savonius rotor with circular arc blade profile havebeen investigated by measuring the pressure distribution on the blade surfaces for different rotor angles.Experiments have been performed at a Reynolds Number 1.8×105 with rotors having no overlap. Resultsindicate that static torque coefficients vary considerably with the rotor angle.DOI: http://dx.doi.org/10.3329/jme.v43i2.17830

Evaluation of Savonius rotor performance: Static and dynamic studies

An aerodynamic performance of stationary and rotating Savonius rotor for various overlap ratios is numerically analyzed using four turbulence models. The simulation is compared with the present and published experimental results to determine the suitable turbulence model. The computations are statically and dynamically performed at different rotor angles ranging from 0° to 180°. Five rotors, having two semicircular blades, but with different overlap ratios; reading 0, 0.15, 0.2, 0.3 and 0.5 are considered. The results indicate that the SST k–ω turbulence model gives more accurate results compared with the other studied turbulence models. In addition the static torque coefficient is improved by increasing the overlap ratio especially on the returning blade, which is due to the pressure recovery effect by the flow through the overlap. Furthermore the results of the rotating rotor show that the maximum device performance can be satisfied with overlap ratio value of 0.15.

Wind tunnel testing and numerical simulation on aerodynamic performance of a three-bladed Savonius wind turbine

Abstract The purpose of this research work is to investigate experimentally and computationally the feasibility of improving the performance of the vertical-axis Savonius wind turbine. The authors first performed a series of wind tunnel investigations on semi-cylindrical three-bladed Savonius rotor scale models with different overlap ratios and without overlap. These experiments were conducted in front of a low-speed subsonic wind tunnel at different Reynolds numbers. Pressures around the concave and convex surfaces of each blade, as well as the static torque for the rotor models, were measured. Using these experimental data, the authors calculated aerodynamic characteristics such as drag coefficients, static torque coefficients, and power coefficients. The authors then performed computational fluid dynamics (CFD) simulations using the commercial CFD software FLUENT and GAMBIT to analyze the static rotor aerodynamics of those models. The experimental and computational results were then compared for verification. Three different models with different overlap ratios were designed and fabricated for the current study to find the effect of overlap ratios. The results from the experimental part of the research show a significant effect of overlap ratio and Reynolds number on the improvement of aerodynamic performance of the Savonius wind turbine. At higher Reynolds number, the turbine model without overlap ratio gives better aerodynamic coefficients, and at lower Reynolds number, the model with moderate overlap ratio gives better results.

Computational Study to Assess the Influence of Overlap Ratio on Static Torque Characteristics of a Vertical Axis Wind Turbine

Abstract This paper presents an unsteady two-dimensional computational study in order to observe the effect of overlap ratios on static torque characteristics of a vertical axis wind turbine (VAWT). The study is performed with the help of a finite volume based computational fluid dynamics (CFD) software package Fluent 6.3. The computational model is a two-bladed conventional VAWT having overlap ratios of 0, 0.10, 0.15, 0.20, 0.25 and 0.30. Initially, a comparative analysis is made using various k turbulence models and then the results are compared with the experimental data available in literature. A realizable k turbulence model with enhanced wall treatment is found suitable for further computational analysis. The flow field around the turbine model is also studied with the help of static pressure contour analysis. Based on this computational study, it is realized that an overlap ratio of 0.20 eliminates the effects of negative static torque coefficient, provides a low static torque variation at different turbine angular positions and also gives a higher mean static torque coefficient as compared to the other overlap ratios.

Numerical Simulations of Vertical Axis Wind Turbine with Controllable Blades

Wind power gives us non-polluting alternative to reduce the exploration of conventional fossil fuels.A novel vertical axis wind turbine(VAWT) has been developed that consists of several controllable plat blades.A computational investigation of the output characteristics of the VAWT has been conducted using the commercial computational fluid dynamics(CFD) code ANSYS 13.0.A validation study was performed using a Savonius turbine and good agreement is obtained with experimental data.Both fixed and rotating two-dimensional CFD simulations are conducted to further investigate the performance of the VAWT.Steady simulations have demonstrated that the VAWT has good self-starting ability with an averaged static torque coefficient of 1.09.The predicted maximum averaged power coefficient is 0.449,which is nearly 2 times of the Savonius turbine.

Numerical simulation on the aerodynamic effects of blade icing on small scale Straight-bladed VAWT

To invest the effects of blade surface icing on the aerodynamics performance of the straight-bladed vertical-axis wind turbine (SB-VAWT), wind tunnel tests were carried out on a static straight blade using a simple icing wind tunnel. Firstly, the icing situations on blade surface at some kinds of typical attack angle were observed and recorded under different cold water flow fluxes. Then the iced blade airfoils were combined into a SB-VAWT model with two blades. Numerical simulations were carried out on this model, and the static and dynamic torque coefficients of the model with and without icing were computed. Both the static and dynamic torque coefficients were decreased for the icing effects.