[AS_Aero]

In my turbine simulation, I am running Frozen rotor simulation with RANS and by changing the RPM I am trying to calculate the Power of the turbine. But unfortunately I am getting negative value for torque for some RPM

I tried with 50,100,300,500,1000 etc
Here for different rpm torque value is as shown below

50 -7e-2 J
100 -5.43e-2

300 -6.4e-4
350 1.25e-2
2000 3.4e-1

So based on these values my Power value is also changing and I am not getting a power curve out of it. I dont know whats going wrong here. This looks like a linear curve

I am calculating the power by taking absolute value of torque and the rpm in rad/sec

[ghorrocks]

This is as expected. Assuming the rotation direction is in the positive torque direction, then a positive torque means the rotor wants to accelerate from that speed, a negative torque means the rotor wants to run decelerate from that speed.

[AS_Aero]

Its rotating in the negative direction as shown in the image. Thats why I have given -300 rpm in the set up.

Secondly I have attached other images of the velocity and streamlines. I am wondering if these recirculation might be creating problem for the power value since it can affect the turbine ? What do you think ? But now I am trying to find the optimum rpm but my power curve is linear and its going on increasing than having a parabolic curve. I tried for many rpm and its just increasing.

[ghorrocks]

So isn't this as expected? For low speeds the torque is in the direction of rotation and it generates power. For high speeds the torque is against the direction of power and it looses power and shows that these speeds are too high, the steady operating speed is slower than this.

The separation certainly will be affecting the power. That separation is inherent in this style of turbine and will be a major cause of loss.

Another thing - don't calculate power as the abs(torque * speed). You need to know when it is positive or negative (as you can see from my description).