Please enable JavaScript.
Coggle requires JavaScript to display documents.
Venting on the Savonius turbine blade - Coggle Diagram
Venting on the Savonius turbine blade
Slatted-blade Savonius wind rotors (Reupke, AE 1991)
Research objective
To investigate the performance of rotors modified with 8 and 16 hinged flaps compared to unmodified rotor
Result
The unmodified S-rotor was not self-starting, even in a high wind with zero mechanical load imposed upon it.
the modified rotors were self-starting in wind speeds exceeding 4m/s, but would run at lower wind speed.
the flap operated as expected and as the machine gained speed, the flap swung less, eventually locking together due to the centrifugal forces they experience.
Once the flaps had locked, the rotor accelerated much more quickly, so suggesting that the turbine operated more effectively if the blades did not have flaps.
While the turbine is running with moving flaps, the torque coefficient is slightly greater than when the flaps are locked together, but the maximum power coefficient are much smaller.
the modified rotor was very noisy when the flaps were swinging.
The overall power coefficient of the basic Savonius rotor is far superior to that for the modified rotor with flaps.
the modified rotor with light, narrow flaps has better dynamic performance than the modified rotor with the heavier, wider flaps due partly to the difference in centrifugal force experienced.
The slatted rotors develop higher torques than the standard rotor at very low tip-to-wind speed ratios
Possible reasons for the relatively poor performance of the slatted rotor turbines
Savonius rotor is not solely a drag-operated machine, and lift effect occurs as well. the flow over the retreating blade is disrupted by the opening of the flaps in the advancing blade, and so a poorer performance ensues.
Loss is incurred in the movement of the flaps. Kinetic energy is lost due to movement of flaps (noise generation, friction etc).
(Alom, Energy 2018) Performance evaluation of vent-augmented elliptical-bladed savonius rotors by numerical simulation and wind tunnel experiments
Research objective
To numerically and experimentally investigate the performance of vent-augmented elliptical-bladed Savonius rotor
Turbine configuration tested
Conventional semicircular profile
vs
Elliptic profile at various cut angle, i.e. 45, 47.5, 50 & 55 deg
Convergent section is given on the vent to prevent the loss of wind in case of advancing blade. The vent on the blade is created 30 deg above and 30 deg below the blade central axis.
Overall rotor diameter, D = 257mm
blade thickness = 0.0066m
Overlap distance = 20% of chord length of the elliptic profile
Aspect ratio, AR = 0.7
Wind speed = 6.2 m/s
Venting slot = 0.03m
Result
Numerical result
non-vented profile with 47.5 deg elliptic cut angle shows improved performance coefficient compared to other non-vented elliptical profiles, hence is chosen for further study.
From total pressure contour, a reasonably higher total pressure is noticed on the advancing blade of the elliptical blade (47.5 deg) in comparison to other elliptical profile. Also, there exist an adverse pressure gradient behind the rotor blades of 45, 50 and 55 deg.
From velocity contours, at elliptical profile with cut angle of 47.5 degree, the maximum velocity magnitude observed near the flow separation point (rotor tip) is less compared to other elliptical profiles, hence the tip loss is less.
From turbulence intensity contours, turbulence intensity is much lower at elliptical profile with cut angle of 47.5 deg, which diminishes the formation of vortices at the downstream
Total pressure on the convex side of the returning blade is reduced in the vent-augmented elliptical profile. A comparatively higher total pressure is detected near the advancing vented profile, thereby improving its performance.
High velocity zone is noticed near the surface of advancing blade of vented elliptical profile compared to the non-vented one.
For vented profile, the convex side of the returning blade shows less velocity magnitude than the non-vented one, which indicates the reduction on negative drag
Experimental result
Vented elliptical rotor shows higher CPmax than non-vented elliptical and semicircular rotors at same TSR. (Improvement of 8.08% in vented elliptical profile compared to non-vented elliptical profile, improvement of 23.06% in vented elliptical profile than conventional semicircular profile)
(Rathod, 2019) Effect of capped vents on torque distribution of a semicircular-bladed savonius wind rotor
Research objective
To evaluate the performance evaluation of a conventional semicircular-bladed Savonius rotor with capped vents (CVs) or nozzle chamfered vents with difference sizes
Turbine configuration tested
Conventional blade
7% CV blade
14% CV blade
21% CV blade
(CV loacted at center of the advancing and returning blade)
Rotor height, H = 250mm
Rotor diameter, D = 250mm
Aspect ratio, AR = 1
Overlap length = 15% of rotor diameter
End plate diameter = 10% larger than the rotor diameter
Wind velocity:
Experimental:
Re = 1.12 x 10^5
Re = 0.96 x 10^5
Re = 0.77 x 10^5
Re = 0.61 x 10^5
Numerical:
7.4m/s (Re = 1.12 x 10^5)
Result
Experimental result
Conventional vlade attains the highest value of C_P and C_T followed by CV blades. The C_P and C_T values decreases with increment in vent ratio.
Working tip-speed ratio range is wider for conventional blades and it decreases with the increment of CV ratio of CV blades.
Performance decrement of roughly 20-35% with a shift of TSR_opt towards lower value side is noted for 21% CV blade compared to conventional blade.
Numerical result
C_P value of all CV blades are lower than the conventional blade, and their value decrease with increment in vent ratio.
C_L, C_D & C_R decreases as vent ratio increases, and remain lower than conventional blade
There is a flow leakage through CV of the advancing blade in all cases except for the conventional blade. The leakage flow interacts with the Coanda flow region downstream of the convex side, which lead to formation of vortices in the downstream side of the advancing blade.
The leakage at the returning blade influences the flow field around the concave side of the returning blade and introduces vortices in the wake of the returning CV blade.
There is an abrupt decrement of pressure coefficient, C_pr at the location of CV, on bothe the concave and convex sides. This C_pr decrement is higher on the concave side, indicating the change in pressure recovery effect on the concave side of the CV blade. The decrement of C_pr is due to leakage of high energy fluid through CV.
The leakage flow through CV increases C_pr over entire convex side of the advancing blade
the cumulative effect of CV is such that it decreases magnitude of positive C_LT (local torque coefficient) on the advancing blade and increases magnitude of negative C_LT on the returning blade
Summarized statement:
Implementation of CV with adapted design dimensions on conventional blade deteriorates performance due to altered pressure distribution in Coanda flow and pressure recovery region
(Borzuei, 2021) On the Performance Enhancement the three-Blade Savonius Wind turbine Implementing Opening Valve
Research objectives
To investigate the insertion of one-way opening valve on the three blade Savonius wind turbine to reduce negative torque on the convex side of the blades
To position the valve with specific size to avoid disturbing the required pressure gradient around the blade which generate positive torque
Turbine configuration (Numerical Simulation)
3 blades
OR (s/d) = 0.2
Height = 1m
A = 0.8888m^2
R = 0.4444m
t = 0.002m
shaft diameter, c = 0.03m
Velocity inlet = 7m/s with 5% turbulence intensity level
Result
large valve "III" with unlimited opening angle and counter-clockwise opening exhibits the best performance with 14.5% improvement
Negative torque decreases due to decrease in drag force at the convex surface (At theta = 48, 72 & 96 deg)
At theta = 0, 24 & 120 deg, opening the valve reduces the negative torque but this opening valve blocks the overlap section of the turbine which causes undesired effect on Ct
One of the blades limits the opening valve at some turbine angles which creates high pressure area and produces positive moment and helps turbine to rotate in the desired direction
Small valve with counter clockwise opening direction but limited opening angle (45 deg) at outer side of the blade (valve A) can increase torque coefficient by 5.5%
At theta = 0 & 96 deg, opening valve decreases the negative torque withoput adverse effect on positive torque which causes larger pressure gradient
At theta = 48 deg, the opening valve changes the concave surface area of the blade and decrease the produced positive torque which causes drop in torque coefficient
In overall, the performance is improved despite some undesired effect on torque coefficient at some angles.
In other configurations, performance is not improved (some configurations reduce the performance)
negative torques decreases, but drop in poritive torque is more dominant, causing and overall drop in torque coefficient