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heat transfer fluid dynamics guys

post #1 of 14
Thread Starter 
Let's say I have a vertically mounted rad with two fans.
Potentially sequentially operated...
so they will be divided...
The top fan is moving air hotter than the bottom would I want less area or more area for the top fan to pull from?

Ill give a little more information and maybe images later on

Thanks
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post #2 of 14
you want more area..... but how is one fan getting air from a diffrent source?
post #3 of 14
Thread Starter 
Due to the two fans being sequential the radiator needs to be split into two shrouds.
The fans are identical, but the conditions they run at are not. The
fan on the top side of the radiator is going to be dealing with a
hotter water, so my question is should you increase the velocity of the air by
decreasing the area of the top shroud? Or should you slow the air
(larger area) to ensure maximum heat transfer?

Im with you on the second option. Just need some kind of proof to make an arguement

Dimensions and other information:

Core dimensions 145x240x35
13 fluid channels roughly 1.5mm X 32mm
Fan curve: http://www.spalusa.com/pdf/30103011_SPEC.PDF
Inlet temp can be 95c
Radiator should expel 35kW? in 35c air

i attached a file...the soda can shaped entity...is a soda can...for reference
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post #4 of 14
Maximizing both airflow and area would be ideal, but in your case (where you have to choose) maximize the area.

As for your proof:

This equation for heat transfer shows that the rate of heat transfer is directly proportional to the external surface area. (This equation was borrowed from wikipedia.)

The rate of heat transfer between the fluid inside the pipe and the pipe's external surface is:

Edit: In case you were wondering what all the variables here are...
q = heat transfer rate (W)
h = heat transfer coefficient (W/(m2·K))
t = wall thickness (m)
k = wall thermal conductivity (W/m·K)
A = area (m2)
ΔT = difference in temperature

Edited by fajita123 - 4/18/11 at 6:47am
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post #5 of 14
Keep in mind that the water temperature right after the water block is usually within 1*c of the temperature right after the radiator. There is so little delta T in water cooling that assumptions on ways to increase thermal efficiency almost always produce negligible results.
    
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post #6 of 14
Thread Starter 
thanks that exactly what i needed its generalized but it works.

its been so horribly long since fluid dynamics or thermo.
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post #7 of 14
Thread Starter 
except, now I'm in a bit of an argument about velocity...which does have some logic to it...

his argument smaller area=faster air=potentially more air=potentially more heat transfer capacity

I was always subscribed to the train of thought slower air travel through the rad itself meant the air could suck up more heat per unit volume or whatever

I cant think of any where it applies although I am sure there's something, but the air has an initial velocity varying between 0 and 40 meters per sec

Not too concerned with the second part...but my opponent seems to think a CFD (computational fluid dynamics) analysis will be the end all or this discussion...

basically now my arguement is that the efficiency of flow through the rad may not be the only concern if the air doesn't have a chance to pick up and remove any heat.


Sorry I'm writing books, and that they're a bit sporadic it is Monday.
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post #8 of 14
CFD analysis of Turbulent flow is poorly defined near surfaces according to my friend whose research focuses on that. Most of this work is done with heuristics and so called dimensionless analysis / groups. The numbers most often used and important in this are the Reynolds (for pure flow considerations) and the Nusselt... how they relate to the rest of this is related in a light manner here: http://en.wikipedia.org/wiki/Nusselt_number

Also, with an inlet of 95 C, you have the potential with fluctuations to have water vapor. That's not good. You'd also need certain tubing that could support that - since most PVC-like materials (including various formulations of Tygon) are only good at 150 F, anything above causing the plasticizer to escape and the plastic to degrade.

Is this a single pass or a double pass radiator (e.g. U-Tube)?
post #9 of 14
Thread Starter 
Quote:
Originally Posted by Dellus View Post
CFD analysis of Turbulent flow is poorly defined near surfaces according to my friend whose research focuses on that. Most of this work is done with heuristics and so called dimensionless analysis / groups. The numbers most often used and important in this are the Reynolds (for pure flow considerations) and the Nusselt... how they relate to the rest of this is related in a light manner here: http://en.wikipedia.org/wiki/Nusselt_number

Also, with an inlet of 95 C, you have the potential with fluctuations to have water vapor. That's not good. You'd also need certain tubing that could support that - since most PVC-like materials (including various formulations of Tygon) are only good at 150 F, anything above causing the plasticizer to escape and the plastic to degrade.

Is this a single pass or a double pass radiator (e.g. U-Tube)?
95 c is getting on worst case...

the rad is single pass, but with a 35mm core i guess depth.

the tubing will likely be an amalgam of bent aluminum and some silicon blend

and thanks for the info on reynolds and nusselt Ill certainly take a loook at that later
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post #10 of 14
Quote:
Originally Posted by jdobbs86 View Post
I was always subscribed to the train of thought slower air travel through the rad itself meant the air could suck up more heat per unit volume or whatever
One of the key factors in heat transfer is temperature difference.
Faster moving air through the radiator will pick up more heat per unit of time than slower moving air because there will be a greater average temperature difference between the water and air as it passes through the radiator.

Slower moving air will pick up more heat per unit of volume that passes through the radiator, but will pick up less total heat from the water per unit of time due to the lower average temperature difference between the water and air as it passes through the radiator.
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