C HAPTER –6
6.1 Experiments on conventional SSWT
Through unsteady simulations, the effect of overlap ratio (δ) on the performance of SSWT is discussed in Chapter-3. Looking at the outcome of study, initially experiments are planned with conventional SSWT models. As already discussed in Chapter-2, the turbine aspect ratio, AR (H/D) plays an important role in achieving a satisfactory turbine performance. However, the available literature does not give a clear-cut indication on the selection of a suitable AR.
In view of this, experiments are executed to find the effect of AR on the performance of SSWTs.
6.1.1 Effect of aspect ratio
The conventional SSWT models in the range of AR = 0.7–1.2 are studied. This is accomplished by altering the overall diameter of the turbine and keeping the height constant at 230 mm. The effect of end plates (Do = 1.1D) is also considered. The performance indices such as torque and power coefficients are calculated corresponding to variable rotational rate and mechanical load applied to the turbine. Tests are carried out at different wind speeds ranging between V = 3.8–9.8 m/s. Figures 6.1 through 6.8 show the effect of AR on the performance indices of SSWT at 4.5 m/s, 6.2 m/s, 7.8 m/s and 9.8 m/s, respectively. It is observed that low aspect ratios (AR< 1.0) are giving better performance at low TSR.
However, to be precise, AR = 0.70 stands better only upto TSR < 0.30, AR = 0.80 to 0.90 for 0.3 > TSR < 0.50 and AR = 0.90 to 1.0 give a better power characteristics at TSR = 0.50 to 0.60. In contrast, with the increase of TSR beyond 0.6, AR = 1.1–1.2 give better performance characteristics. It is mainly due to fact that with the increase of both AR and TSR, the turbine power to weight ratio increases and the effect of inertia on the turbine rotational rate reduces.
Figure 6.1: Variation of CT for variable AR at V = 4.5 m/s
Figure 6.2: Variation of CP for variable AR at V = 4.5 m/s
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR= 1.1 AR = 1.2
V= 4.5 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR
AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR = 1.1 AR = 1.2
V= 4.5 m/s Blockage corrected data
Conventional SSWT
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79 Figure 6.3: Variation of CT for variable AR
at V = 6.2 m/s
Figure 6.4: Variation of CP for variable AR at V = 6.2 m/s
Figure 6.5: Variation of CT for variable AR at V = 7.8 m/s
Figure 6.6: Variation of CP for variable AR at V = 7.8 m/s
Figure 6.7: Variation of CT for variable AR at V = 9.8 m/s
Figure 6.8: Variation of CP for variable AR at V = 9.8 m/s
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR= 1.1 AR = 1.2
V= 6.2 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR
AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR = 1.1 AR = 1.2
V= 6.2 m/s Blockage corrected data
Conventional SSWT
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR= 1.1 AR = 1.2
V= 7.8 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR
AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR = 1.1 AR = 1.2
V= 7.8 m/s Blockage corrected data
Conventional SSWT
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR= 1.1 AR = 1.2
V= 9.8 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR
AR = 0.7 AR = 0.8 AR = 0.9 AR = 1.0 AR = 1.1 AR = 1.2
V= 9.8 m/s Blockage corrected data
Conventional SSWT
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80 6.1.2 Effect of overlap ratio
The SSWT model with an overlap ratio (δ = 0.20) is fabricated for carrying out wind tunnel tests. This overlap ratio is selected from the simulation studies, details of which are reported in Chapter-3. The turbine aspect ratio of AR = H/D = 1.1 is chosen on the basis of the above experimental studies (Section 6.1.1). The overlapped blades are tested and compared with non-overlapped blades at different wind speeds. The torque and power characteristics curves obtained are shown in Figures 6.9 to 6.16.
At V= 4.5 m/s, a better torque characteristics is observed with overlapped blades (Figure 6.9), which in turn while plotted as power characteristics, gives CPmax of 0.21 at TSR = 0.67 (Figure 6.10). In contrast, for non-overlapped SSWT, the CPmax is found to be 0.16 at TSR = 0.59. Thus, a performance gain of 31.25% is achieved with overlapped SSWT (δ = 0.20).
This power improvement is mainly due to acceleration of flow through the overlap region that inserts pressure drag on the concave side of the returning blade and thus, enhances the net torque characteristics in the rotational direction of the turbine.
Similarly, for other wind speeds, the semi-circular SSWTs are tested for overlapped and non- overlapped conditions. For overlapped SSWT, at V= 6.2 m/s, 7.8 m/s and 9.8 m/s, the CPmax is found to be 0.23 (TSR = 0.71), 0.23 (TSR = 0.73), and 0.22 (TSR = 0.69), respectively, whereas, for non-overlapped SSWT, the obtained CPmax is 0.16 (TSR = 0.62), 0.17 (TSR = 0.64), and 0.16 (TSR = 0.61), respectively. This analysis indicates that the value of maximum CP for conventional SSWT can reach up to 0.23 with a suitable AR = 1.1 and δ = 0.20.
Figure 6.9: Variation of CT for conventional SSWT with and without overlap at V = 4.5 m/s
Figure 6.10: Variation of CP for conventional SSWT with and without overlap at V = 4.5 m/s 0.00
0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR δ = 0.20
δ = 0.00
V= 4.5 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR δ = 0.20
δ = 0.00
V= 4.5 m/s
Blockage corrected data Conventional SSWT
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81 Figure 6.11: Variation of CT for conventional SSWT with and without overlap at V = 6.2 m/s
Figure 6.12: Variation of CP for conventional SSWT with and without overlap at V = 6.2 m/s
Figure 6.13: Variation of CT for conventional SSWT with and without overlap at V = 7.8 m/s
Figure 6.14: Variation of CP for conventional SSWT with and without overlap at V = 7.8 m/s
Figure 6.15: Variation of CT for conventional SSWT with and without overlap at V = 9.8 m/s
Figure 6.16: Variation of CP for conventional SSWT with and without overlap at V = 9.8 m/s 0.00
0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR δ = 0.20
δ = 0.00
V= 6.2 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR δ = 0.20
δ = 0.00
V= 6.2 m/s
Blockage corrected data Conventional SSWT
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR δ = 0.20
δ = 0.00
V= 7.8 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR δ = 0.20
δ = 0.00
Blockage corrected data Conventional SSWT V= 7.8 m/s
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
0 0.2 0.4 0.6 0.8 1 1.2
CT
TSR δ = 0.20
δ = 0.00
V= 9.8 m/s Blockage corrected data Conventional SSWT
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.2 0.4 0.6 0.8 1 1.2
CP
TSR δ = 0.20
δ = 0.00
V= 9.8 m/s
Blockage corrected data Conventional SSWT
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