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CPU Convective Water Cooling
biso writes "The possibility of cooling a CPU with gravitational convective flow of water is here analyzed and experimented with positive results.
Many liquid cooling systems have been experimented by overclockers to better dissipate the heat from CPUs. The major part of these coolers is characterized by a relatively complex system requiring pumps or other active devices. Sometimes even liquid nitrogen is used. I built a simpler cooler, able to dissipate the same heat flux of a normal heatsink."
Abstract
The possibility of cooling a CPU with gravitational convective flow of water is analyzed and experimented with positive results.
Introduction
Many liquid cooling systems have been experimented by overclockers to better dissipate the heat from CPUs. The major part of these coolers is characterized by a relatively complex system requiring pumps or other active devices. Sometimes even liquid nitrogen is used.
My intent was instead to build a cooler able to dissipate the same heat flux of a normal heatsink, but without the annoying noise of the fan.
A first prototype was built out of a regular heatsink. Holes were drilled in the aluminium finning, and copper tubes passed through them. An aquarium pump provided the necessary pressure for circulation.
Figure 1. First Prototype--Front View
(picture)
Figure 2. First Prototype--Side View
(picture)
The system was silent and reliable. But with bigger pipes and a lower pressure drop would it have been possible to take away the pump? Simple calculations showed that it would have been perhaps feasible and a prototype was built.
Temperature on Heatsink Surface
Roughly:
Power to be dissipated: powd = 80 W
If the heatsink is a little copper box to put over the CPU, a reasonable value for the surface available at copper-water interface can be: surfc = 0.01 m2
The heat transfer coefficient on the water-copper boundary layer can vary from a few watt per square meter per kelvin if the flow is slow and laminar to more than 1 kW K-1m-2 when the flow is very fast and turbulent. If the coefficient is supposed to be: texc = 100 W K-1m -2
The difference of temperature on surface will be: delt = powd / (texc surfc) = 80 K
It appears that the water should boil on the surface of such a little heatsink, but radiation wasn't taken into account and the geometry of the box is complex, so it's not clear if there could be turbulence and with which effect. If necessary the surface could be enhanced with fins or by increasing the dimension of the equipment.
Convection
Supposing that the heatsink could be able to exchange the heat between the CPU and the water, would it flow through the pipes?
Power to convey: powd = 80 W
Length of the circuit branches between the CPU and the radiator on the top of the computer case: heigh = 0.8 m
Equivalent length of the circuit (we take into account the bends too): len = 2 m
Radius of the pipe: rdp = 9 10-3 m
Rate of change of water density against temperature: dct = 0.55 kg m-3 K -1
Water density: rho = 103 kg m-3
Water viscosity: eta = 10-3 decapoise
Specific heat of water: wsh = 4180 J kg-1 K -1
Gravitational acceleration: grav = 9.8 m s-2
Pi: pi = 3.14
Difference of temperature between ascending and descending branch: deltat
Difference of density of the water in the two branches: deltarho = deltat dct
Difference of pressure due to the difference of density: deltap = deltarho grav heigh
Volume of water conveyed per unit time: vot
Pressure drop in the pipe: deltap = vot 8 eta len / (pi rdp 4)
Power conveyed: powd = wsh rho deltat vot
Putting it all together: deltat2 = 8 powd eta len / (wsh rho pi rdp 4 dct grav heigh) = 3.4 K2
Everything should work with a temperature difference of less than 2 kelvin. Consequently the radiator isn't required to be very efficient.
SIRPAL-1 Prototype
The SIRPAL-1 prototype was made using a 5 mm thick copper sheet for the base, and 2 mm thick copper sheets for the walls. The edge of the square base is 55 mm long. Inside there are two plates 25 mm wide. One is vertically aligned, soldered to the base, to increase the exchange surface near the CPU, the other is horizontal, soldered between the input-output pipe fittings, to guide the fluid in the right direction.
A test was performed on a K6-2 450MHz which dissipates a power of about 25 watt. The ambient temperature was 18 celsius degrees. After a few hours the CPU temperature, measured by the PC board sensor, was at least 1 kelvin lower than when the fan is used. External surface temperatures: 19 celsius degrees on the pipes; 24 celsius degrees on the copper box.
A drop of ink in the water revealed a slow flow as expected. It worked so well that I think a more powerful CPU would be efficiently cooled too.
Figure 3. SIRPAL-1
(picture)
Figure 4. SIRPAL-1--Testing
(picture)
Sure it is, he says right out he didn't eliminate the pump because it was the most expensive part, he wanted to eliminate it because it was the loudest part.
Yeah, and other times they go a little nutz and use Fluorinert
Nah... I'd use transformer oil, and I don't think a Lipton Cup-a-Soup would taste quite the same.
Transformer oil, however, is probably quite suitable for use in a CPU cooling system.
It has a higher breakdown voltage than air and is almost infinitely less conductive than real-world (ie. impure) water. Transformer oils are specifically designed for use as an insulating material in large power distribution transformers. Electric utility transformers at power substations, operating in the range of hundreds of thousands of volts, would arc between windings if the oil leaked out of them and air - with its lower breakdown voltage - seeped in. (Air breaks down at about 3kV per millimeter.) You can feel pretty confident that leaked oil won't short out IC pins on your motherboard. Hell, you could also ditch your power supply fan and fill that full of oil, too - just beware of relays and other mechanical components.
Heat transfer is a big reason for oil, too. In a car engine, much of the heat is generated by friction in the bearings, and motor oil pumped through the bearings takes that heat away. Transformer oil doesn't have to lubricate, nor does it have to carry away huge amounts of impurities or combustion by-products as in a car engine - the biggest requirements are heat carrying capability and high breakdown voltage. Large pole pigs (pole-mounted power transformers) are usually oil-filled and often have pipes coming from the bottom and going to the top - they serve as radiators. Oil flow is not by pump, the reliability would be too low - they're convective, too.
Finally, viscosity. Yes, this might be difficult, but transformer oils are available in a variety of thicknesses. You want a viscosity corresponding to SAE 0, which is the same as water. Even less might be available, though I've personally never seen it.
Density changes with temperature rise will have to be considered, since the lower density of hot liquids causes them to rise in the system (and is also the physics behind lava lamps). The system that guy designed is based on the density changes of water. Transformer oil won't behave the same way; accordingly, you'll have to whip out the old slide-rule and do some math. Calculus is your friend. Fortunately, the data on transformer oil should be readily available, it's an important design criteria.
Voltesso and Diala are good trade names which I've personally used in transformers loaded to hundreds of kilowatts at over 250,000V, at RF frequencies. (FAA obstruction lights on large VLF radio transmitting towers.) They're ALL PCB-free, and while you don't want to drink it, they're no more toxic than motor oil. And it takes a hell of a lot of work to make them catch fire.
In short, transformer oils are available in a variety of viscosities, are specifically engineered for their thermal transfer capabilities, are not electrically conductive, not dangerous, and are suitable for almost all of your electronic cooling needs.
The only problem I forsee is that you're gonna have a hard time buying them in quantities less than 45-gallon drums... though the drum would make a great passive radiator. Seriously, talk to a couple of linesmen with your local power utility, maybe you'll be able to talk your way into a couple of gallons of it.
And once that's done across all the machines in your compile farm, you can get to work tackling the big problems of why Linux isn't ready for the desktop yet.
Fire and Meat. Yummy.