Which direction to point a venting fan?

[DaveC426913]

I have a tiny fan installed in the lid of my aquarium (it's actually a CPU fan). It moves air from the air space over the water surface below the light. It's purpose is several-fold:
1] supply oxygen to the air-water interface, remove CO2
2] remove excess moisture
3] remove waste heat due to halogen light fixture (deprecated, as I have switched to LED)

The fan is over a hole in the back of the lid. Tghere is only one hole for air flow - the one in front of which the fan is mounted; it is assumed that the gap around the lid is enough to act as the other end of the air flow.

So, my question is: does it make a difference in fan efficiency which way the fan points (inward vs. outward)?

 

[DaveC426913]

Now with pix!

 

[Q_Goest]

I remember the same question coming up in one of my "cooling of electronics equipment" course in college. The answer they gave was that the fan should blow into the cavity but the reasoning wasn't particularly strong. The primary reason given was that the air flow downstream of the fan is generally turbulent so that the convective heat transfer coefficient would be higher than for laminar flow. That would be true also of entrainment of CO2 and water vapor since a more turbulent flow would aid in mixing those gasses with the incoming air when compared to laminar flow.

 

[RandomGuy88]

Would the turbulent flow on the water disturb the fish?

 

[DaveC426913]

Would the turbulent flow on the water disturb the fish?

Well, every proper tank must have good water circulation at the surface; that's where all the exchange occurs. So air circulation shouldn't be an issue.

 

[russ_watters]

The risk in having it suck is that it won't pull air across the surface of the water, whereas when it blows, a relatively coherent stream of air will go where-ever it is pointed.

 

[DaveC426913]

The risk in having it suck is that it won't pull air across the surface of the water, whereas when it blows, a relatively coherent stream of air will go where-ever it is pointed.

Ah! I see. Coherence. The outflowing stream will most definitely preturb a predictable location of volume of air. There's no such guarantee that the inflowing air will perturb a volume of air in any particular location.

For all I know, if pointed outward, it might suck all its air from a crack right next to the fan, leaving the rest of the airspace completely unperturbed, and thus stale.

No so if the fan points inward. I can force it to move a volume of air in a specific location, such as that volume right above the water's surface.

Thank you. That is an answer I can sink my teeth into.

(I always thought fans should be time-reversible, that you should be able to film it and run it forward and backward and not be able to tell whether it was blowing or sucking. But it's not true. Fans are very one-way.)

 

[AlephZero]

(I always thought fans should be time-reversible, that you should be able to film it and run it forward and backward and not be able to tell whether it was blowing or sucking. But it's not true. Fans are very one-way.)

They would be time-reversible in a fluid with zero viscosity.

But if there was no viscosity, they would neither blow nor suck.