I have a question about fans; and don’t remember much about fluid dynamics so please excuse the naivete. Assume this question is about a standard fan, in a very large empty room.
If we drive a fan with the same power (eg. current*voltage is constant); and we assume the fan runs at the same efficiency (heat losses are proportional to input power): What can we say about the volume flow rate of the air the fan is pushing?
As air density changes, would the volume flow rate remain the same? Or would mass flow rate remain the same (this makes more sense since the fan is converting the input energy to kinetic energy ~ mass)?
Or are there too many variables in the equation to even come to a conclusion?
We are designing a fan control law to dissipate heat; and want it to work at different air pressures and looking for what assumptions we can make about it…
Comments
An issue is whether the speed of rotation of the fan is measurably influenced by the air density. I suspect that in practice the effect of air density is not the governing factor in rotational velocity, with friction effects in the fan itself playing a larger part.
Well, think of it this way. You are moving more matter as density increases. More matter requires more energy to move. If you don’t put more energy into the system, it ought to slow down. But this is a wicked simplistic take. Frictional forces might make the air density changes negligible in comparison.