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Old   June 10, 2005, 11:29
Default Power calculation for mixing tank
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Hu
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I want to calculate the power input for the liquid in the mixing tank after using CFX. From the literature, I have found three different ways to calculate the power input.

1) CFX tutorial examples: Calculating the torques on the blades, then multiplied by the rotational speed. 2) Bird et al. Transfer Phenomena: Calculating the torques on the baffles and stationary walls, then multiplied by the rotational speed. 3) Some published papers: Integrating the turbulence eddy dissipation over the whole volume then multiplied by the liquid density.

Obviously I got different results from the three different methods. The first one the values are much higher than the other two. The second one is the lowest one, as the torque on one side of baffle is positive but on the other side is negative, the total torque on the baffles is much lower than that on the blades. I am quite confused which one I should go for.

Many thanks.
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Old   June 13, 2005, 18:28
Default Re: Power calculation for mixing tank
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Glenn Horrocks
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Hi,

Just from simple mechanics it tells you that providing the system is steady state the first two methods should give the same torque. If they don't maybe your simulation is not converged properly yet.

The final method relies on an accurate turbulence model. I suspect you will find this difficult to get similar to the previous two methods due to limitations in the turbulence model.

As for what is the correct power, that depends on what you are trying to do. Most likely you are looking for how much power the impeller will take to turn, so in that case use the torque the impeller sees by the rotational speed.

Glenn Horrocks
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Old   June 29, 2005, 09:25
Default Re: Power calculation for mixing tank
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Andrea
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