[ghost82]

Hi!
I'm testing a new real rotor stator machine in a lab.
1000 l/h of water enter a cilindrical chamber, which internally have a stator disc and a rotor disc.
Water enters centrally in the chamber and it exits at the top.

The rotor is driven by a 7,5 kW electric motor.
The amperometer says that the absorbed power is near 7,5 kW.

This machine is installed in a closed loop, in which 7 liters of water continually circulate at a flow rate of 1000 l/h.

Water heats itself by 5 ˚C/min (friction effect, same as a 'water brake'), this means:
- 7,5 kW of electric energy are converted to near 7,5 kW of mechanical energy (from the electric motor to the shaft)
- 7,5 kW of mechanical energy are converted to: 2,5 kW of thermal energy, 'x' kW (negligible) into chemical energy (due to OH radical formation) and the remaining to turbulent+pumping energy.

I want to simulate this machine, so I build a 3d model and I'm using sliding mesh (without energy equation).

Now the question is:
Since I'm not simulating the heat transferred to the water the result given by fluent about absorbed power should be:

a) 7,5 kW minus 2,5 kW minus 'x' kW
b) 7,5 kW
c) 7,5 kW plus 2,5 kW plus 'x' kW

What do you think?

[Bionico]

Ciao Daniele,
If you put 7,5 kW you'll have a complete mechanical energy transport to the fluid: I think it would overstimate the flow in your pipe.
Therefore I suggest setting 7,5-2,5=5 kW of power (I think chemical energy negligible, too).

Best Regards

[ghost82]

Grazie Flavio Bionico!
So, my bc are velocity inlet and pressure outlet (the flow rate is fixed and it is imposed by another pump).
So, reading your answer fluent should "understimate" the power I'm reading on the real machine, and it should return about 5 kW, right?

[Bionico]

Ciao Daniele,
I misunderstood your data: I thought you imposed the power to the machine, but this is the output (flow rate is the BC, from the pump in the circuit).
So your machine is a turbine, not a pump, right?

[ghost82]

Hi Flavio!
no, this is not a turbine, sorry if I'm not clear.
The power I need to calculate is absorbed power, so I need to give power to the rotor, to make it rotate.
It is a sort of inline mixer, which requires power to rotate, where a secondary pump fixes the flowrate to this machine.
I want to make the model of this "inline mixer" so to estimate the power number curve and evaluating future absorbed powers for different fluids.

Daniele

[Bionico]

Ok, now I understand!
Well, I think your machine would absorb less power without thermal losses: so you can expect 5 kW. Anyway you should take into account that usually Fluent underestimates pressure losses in internal flow (it's difficult to define wall roughness), above all if you choose the simplest turbulence models, like 2-equations ones (at least from my experience). So probably the power consumption will be even lower...

Regards

[ghost82]

Just a thought about my question: I think the correct answer is 7,5 kW (i.e. same power with thermal effects).
In my opinion this is because thermal energy does not enters the system directly, but it is a conversion of turbulence energy and wall shear.
Neglecting changes in fluid properties vs temperature power required to rotate the rotor should be the same.