Dear all,
Basically, I am trying to draw the Cp ( coefficient of performance) over TRS ( tip speed ratio) in 2D analysis, and I am using the sliding mesh technique to evaluate the performance with different tip speed ratios.
The main problem is that I am getting high moment values so that when I put in the equation of performance, I get very high values that exceed Betz's limit.
My simulation was designed carefully, so I tested my mesh by Grid Independence Study, my time steps, the boundary conditions and the models. It was found that these factors do not affect my results.
In your opinion, why do I get such values? and how can I overcome this problem?
This is the equation I am using:
Cp= (torque * angular velocity of the turbine)/(0.5* water density* diameter of the turbine * height of the turbine * the stream velocity^3)
Thank you
Betz's Limit is used as a maximal energy rate to transfer kinetic energy of air flows (wind) into mechanical work or power based on the definition. It is useful in the wind power plant. You are writing about water turbine.
2D analysis is a good choice when you deal with straight blade. It also require shorter computational time.
Please check on this parameter on your setting:
1. Blade mesh size
- you can applied fine mesh. Make sure to have more than 10 points (20 points maybe) on the blade surface
2. y+ on the blade surface
- y+ depend on your turbulence model. You have your LR on this, please used that as guide.
- In your employed CP equation? maybe units? RPM or rad/s? area considered? (easy to get human errors here) ... How exactly did you get the torque?
- Blockage effect? How large is your domain? How close/far is the rotor from the boundaries?
- Time discretization? Courant number? Large time-steps can result in overpredicted power
Good luck!
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