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different flow rates for different metals

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cookedcomp

Member
Joined
Mar 15, 2001
Location
Mt. Brudges, Ontario, CA
here is somthing that i found while testing my watercooled system:
[email protected] @2.5Vcore
1. water cooled superorb(fans removed)
high flow rate
idle: 51C
load:50C
low flow rate
idle:53C
load52C
2. homemade copper block
high flow rate
idle:49C
load:48C
low flow rate
idle:47C
load:46C

Conclution: copper requires lower flow rates due to its high heat spreading propoties and low radiation. While aluminum requires a high flow rate due to it's high radiation and low heat spreading proporties. My tests prove it!!
 

Spewn

Registered
Joined
May 1, 2001
Unfortunately, your tests don't actually prove anything. The differences in temperature between high and low flow-rate for the superorb are far too low to make any conclusions(slight fluxuations in ambient temperature could be to blame there). Also, since the copper block isn't radiating heat anywhere, it's literally in contact with the water and thus doesn't need to radiate the heat, the theory doesn't really make sense.
 

cjtune

Member
Joined
Dec 18, 2000
[pre]
Yes, I must agree with Spewn. Given more exposed surface area (to water),
aluminium blocks *will* beat copper blocks in transferring heat away from
your CPU. Here's why: Any heat transfer problem can be thought of as being
anologous to an electrical circuit. Between each contact interface there's a
certain amount of thermal resistance (just like electrical resistance, but here
the potential difference (voltage) is replaced by temperature difference).
First, there's contact resistance between your CPU core and the bottom of
your water block. Short of using Arctic Silver II instead of your regular
silicone goop and/or lapping both surfaces and/or having a bigger CPU
core die area, you can't change the thermal resistance there. Next, the heat
passes to the waterblock's metal from the CPU/block interface. Here, the
properties of the metal used will affect the temperature distribution across
the whole waterblock (but since copper is 27x (aluminium:16x) more
conductive than stainless steel and 600x-400x more conductive than water,
the thermal resistance here can be neglected compared to the previous
thermal resistance. This leaves us with the thermal resistance due to
convection (heat moved by fluid flow). Now, this CAN be changed by the
inclusion of fins or pins to increase the area of contact with water (just like an
air-cooled HSF combo) -as long as the extended surfaces do not severely
obstruct the fluid/coolant flow AND/OR increasing the flow rate. As you can
see now, your experiment has just only identified one factor out of many that
makes a good waterblock good. Here's a diagram of the thermal
resistances:


------------> Direction of heat transfer


T(core) Ra Rb Rc T(water)
-----------\NNNN\---------------\NNNN\-----------------------\NNNN\---------------
(core and (resistance (water flow and
waterblock of copper or internal area
contact) aluminium) of waterblock)


.....which is a series circuit. Heat transfer away from the waterblock by
radiation and natural air flow has been neglected as their contribution effects
are very small (otherwise then why bother with water cooling anyway?),
but they would appear as parallel resistances next to Rc.

[/pre]
 

inertia

Registered
Joined
May 24, 2001
He's not comparing alum and copper heat dissipation to a constant medium, he's comparing how the two metals react differently to a change in flow rate. I'm assuming that between the hi-flow and low-flow tests nothing about the interface to the heat source was changed which discounts a lot of what you're concerned about. It's unfortunate that we don't have a measurement of what ambient was doing, but you wouldn't necessarily have to go through a long process to change the flow rate (just pinch the tube or something more objective perhaps.) Now if we knew how long it was between the measurements per cooling schema we might could assume a fairly constant ambient.

So, assuming a constant ambient, we've got the different temps for alluminum are -2C if the flow rate is "high" vs. -2C for copper if the flow is low, Joe (couldn't help it.) Not hugely signifigant, but 2C is 2C.
 

cjtune

Member
Joined
Dec 18, 2000
inertia (Jun 15, 2001 02:34 p.m.):
He's not comparing alum and copper heat dissipation to a constant medium, he's comparing how the two metals react differently to a change in flow rate. I'm assuming that between the hi-flow and low-flow tests nothing about the interface to the heat source was changed which discounts a lot of what you're concerned about.

Not really, note that the geometry of the waterblocks used are different. It's not how a metal reacts to a different flow rate but rather how the geometry of the waterblock reacts to the different flow. Okay, for an example, in industrial applications, vanes are sometimes installed inside an elbow bend in order to help the fluid move more smoothly around the bend, otherwise head losses will be quite large. Head losses in joints and elbows of pipes follow the correlation of HL=K*V^2/(2g) -that means the head loss is proportional to the square(!) of the flow rate and to a constant 'K' WHICH DEPENDS ON THE GEOMETRY OF THE FITTING. If K is large enough due to a poor design of fitting (or in this case, a homemade waterblock), the head losses will be quite significant as to restrict the flow rate of the pump! Thus a flow may actually be slower than expected at a 'higher' flow rate (as perceived by cookedcomp) due to the turbulent head losses. Worse still, there will be zones of stagnation close to the hot inner surface of the waterblocks that increases in size with faster flow rates. Stagnation zones obstruct the transfer of heat from the waterblock's surface to the main flow. Again, this depends on geometry and not the kind of metal used.

Ah, heck. I'm no PhD holder in fluid mechanics; just an undergraduate...
 
OP
cookedcomp

cookedcomp

Member
Joined
Mar 15, 2001
Location
Mt. Brudges, Ontario, CA
YOU STUPID OVER ANALYSING IDIOTS!!!!!!!!!!!!
All i was trying for was to hopfully show you that copper works better with a lower flow rate and aluminum works better with a high flow rate!!!!!!
GAWD!!! HOW COULD YOU TWIST UP MY MEANING THAT BAD!!!
 

cjtune

Member
Joined
Dec 18, 2000
cookedcomp (Jun 16, 2001 02:34 p.m.):
YOU STUPID OVER ANALYSING IDIOTS!!!!!!!!!!!!
All i was trying for was to hopfully show you that copper works better with a lower flow rate and aluminum works better with a high flow rate!!!!!!
GAWD!!! HOW COULD YOU TWIST UP MY MEANING THAT BAD!!!

Geez, refer back to Spewn's posting and start again from there will you...

BTW, cookedcomp, do not presume that all the postings here are in response to your experiment; I was, in a way, disseminating some basic engineering knowledge. Hopefully, some other fellas have read and understood something about what you have done and about the subsequent replies to your undertaking.