I am studying about wind turbine and turbine.
I have some questions, but first, I explain some cases as follow:
Low velocity air over an airfoil at a zero-incidence angle does not initiate flow separation. If the air velocity slowly increases, the flow separation starts from the blade trailing edge (TE). If the velocity or the incidence angle increases further, the separation point or stall point moves towards the blade leading edge (LE). If the stall point can be pushed towards the TE, the airfoil performance or the turbine blade performance increases as the attached flow transfers more energy to the blade.
Now, I want to know that how I can claim that efficiency of turbine increase up to given angle of attack?
The efficiency increase with increasing angle of attack while flow separation also increase with increasing angle of attack. I don't understand this case!
With regards to above explanations, if angle of attack increases; separation point or stall point moves towards the blade LE and the turbine blade performance decreases as there is low attached flow. I know that when angle of attack increases, tangential force also increases and for this reason, the efficiency of turbine increases but I want to know about increasing efficiency from the point of flow separation!
How can these two things (increasing efficiency and increasing separation) be linked together?
I am grateful to all who guide me about these cases and if it is possible, please place useful links and papers related to these cases.
In fact, I am studying more about the Ws turbine. In my investigations, the air velocity(inlet velocity or axial velocity) is constant and rotational velocity(r $\omega$) is changed and in other words, rotational speed is decreased in my calculations. this method is performed in the following paper. In this paper, There is the used equation for efficiency.
A comparison of computational...
$\eta = \frac{T \cdot \omega}{\Delta p_\mathrm{t}\cdot Q}$
$T =$ shaft torque
$\omega =$ angular speed
$U = r \cdot \omega =$ rotational speed
$\Delta p_\mathrm{t} =$ total pressure drop across the turbine
$Q = V_\mathrm{inlet} \cdot A$
$A = \pi\left(R_\mathrm{casing}^2-R_\mathrm{hub}^2\right) =$ through flow area
$\frac{V_\mathrm{inlet}}{U} =$ flow coefficient
Also, please see the following figure:
In fact, The various values of the flow coefficient were achieved by varying the rotational speed of the rotor at a constant value of the axial velocity, which are similar with those performed by Kim et al. [1,2], Takao et al. 3 and Zahari et al. [4].
1 Kim TH, Setoguchi T, Kinoue Y, Kaneko K. Effects of blade geometry on performance of Wells turbine for wave power conversion. Journal of Thermal Science 2001;10(4):293–300.
2 Kim TH, Setoguchi T, Kaneko K, Raghunathan S. Numerical investigation on the effect of blade sweep on the performance of Wells turbine. Renewable Energy 2002;25:235–48.
3 Takao M, Setoguchi T, Kinoue Y, Kaneko K. Wells turbine with end plates for wave energy conversion. Ocean Engineering 2007;34:1790–5.
[4] Zahari Taha, Sugiyono, Sawada T. A comparison of computational and experimental results of Wells turbine performance for wave energy conversion. Applied Ocean Research 2010;32:83–90.
Now, I am grateful that guide me about these cases and discuss together about these cases.