# Resonable Fluid Speeds for cooling

## Main Question or Discussion Point

Hi everyone,

I have a general question on fluid speed for air.

What would be a reasonable design limit velocity for air?

Assuming that it does not pass the speed of sound which can bring an whole set of other problems 1116 ft/s = 761 mph at sea level.

Would 900ft/s be a reasonable limit?

Here is a little background on the problem. I am designing an electric motor cooling system with different size ducting/channels. As we all know through fluid dynamics, the velocity is a function of area/mass flow. I can change the area/mass flow to increase/decrease velocity. The velocity will determine my heat coefficient, which I am trying to optimize for each section of the cooling system. I have velocities from 2 to 900 ft/s in the system. I am trying to find the upper design limit for velocity but there are very little information on the web.

Alvin

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That seems really really fast for cooling an electric engine. I would imagine at those speeds you wil have to worry about heating due to friction. How are you going to generate speeds like that?

russ_watters
Mentor
Don't forget about the energy required to move air that fast. You'll end up expending more energy than you needed to if instead you moved the same volume through a larger space at a lower velocity.

Gold Member
At those speeds you'd probably "waste" a lot of the thermal mass of the air because there is almost no way you will have a heat transfer rate high enough to keep up with that.

Also keep in mind that even in a base flow moving at Mach 0.7 you can have local supersonic region.

In short: way too fast. Match your flow conditions to your heat transfer rate.

I'd personally treat it as a simple heat transfer problem. Start with an ideal temperature value at the wall. This'll allow you to determine your ideal heat transfer rate and then the speed of air required to achieve this given the area and flow rate of air through your motor. The equations involved are pretty simple, but it'd give a nice approximation. I've used this method to give me an idea of what values to put into a simulation of the heat transfer occuring on the heat shield of a reentry vehicle.

I was trying to avoid saying how ridiculous those numbers you have are, but I couldn't help myself. Mach 2 speeds inside a cooling system? wtf. Are you trying to cool the sun? At those speeds you'd start creating shock waves, which significantly increase the local heat transfer. You'd be heating the system up much higher than what the system is that you're actually trying to cool. You'd need a cooling sysem for the cooling system. You get other effects too, which I won't go into. You'd have to heat shield the walls, just for a simple electric cooling system.

Thankyou for making work a little interesting.

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Thats just super sic, eerrrr i mean cool vader..................literally.

I figured anywhere near Mach 1 speed are a bit too high.....actually way to high. The best literature for air velocity speed is in the 80-100 ft/s (Compressed air pipes). I did find a few papers that have air velocity speeds in the 250-300 ft/s range but all in experimental applications.

Just some background on my analysis

Pipe model > Hand Calcs > FEA

-Pipe model to find velocities
-Hand Calcs to find h-values using velocities
-FEA to do the simulations using the h-values and mass flow

Currently, I believe 250-300 ft/s would be my upper limit based on the experiment papers that I read.

Then next questions is does anyone have info on reference material on air flow speeds? or can point to the right direction? such as IEEE, ASME, Universities paper etc.

Gold Member
Look into ASHRAE.

Chestermiller
Mentor

I figured anywhere near Mach 1 speed are a bit too high.....actually way to high. The best literature for air velocity speed is in the 80-100 ft/s (Compressed air pipes). I did find a few papers that have air velocity speeds in the 250-300 ft/s range but all in experimental applications.

Just some background on my analysis

Pipe model > Hand Calcs > FEA

-Pipe model to find velocities
-Hand Calcs to find h-values using velocities
-FEA to do the simulations using the h-values and mass flow

Currently, I believe 250-300 ft/s would be my upper limit based on the experiment papers that I read.

Then next questions is does anyone have info on reference material on air flow speeds? or can point to the right direction? such as IEEE, ASME, Universities paper etc.