Specific Heat & Viscosity
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One of the most basic issues with water cooling is the amount of flow to use. While it is extremely easy to calculate a reasonable value, I’ve yet to see it discussed elsewhere. The specific heat of water is roughly 4.18 kJ/kg-°C. One watt equals one joule per second. This means that 4.18 kilowatts would raise the temperature of one kilogram of water at the rate of one degree Celcius per second.
More practically, a CPU produces on the order of 75 watts and one gallon of water has a mass of about 3.8 kilograms. A typical CPU would raise the temperature of a gallon of water at the rate of 0.00472°C per second or about 0.3°C per minute.
Here comes a very, very important point.
If you know the power of the CPU and you know the flow rate of fluid, the temperature rise of the fluid is also a known value.
Alternately, if you have a target for temperature rise in the fluid and you know your CPU’s power you can calculate the required flow rate.
Where flow is gallons per hour, CPUpower is watts, and DeltaT is °C. Most overclocked CPUs consume less than 100 watts. So even with a temperature rise of only one degree Celcius, the required flow is only ~23 gallons per hour.
Given this information, we can construct a graph showing required flow rate versus temperature rise for a fictional 100-watt heat source. Note that as the temperature rise drops below one degree Celcius, the required flow rate increases rapidly. This is a region of diminishing returns because it takes a lot more pump to produce the required flow.
OK, so we’ve concluded that a mere 23 gallons per hour would result in a temperature rise in the water of only 1°C. The obvious question is “Why would I want more flow than this?”
Once more I’m going to cheat and tell you that you’ll have to wait for the full details to come later. For now, the simple answer is because of convection. The fluid relies on convection to absorb heat from the block. Convection is dependent on many factors with velocity being among the most important. At very low flow rates, the fluid may only need to rise one degree, but because of the low velocity, the water may need to be 20°C cooler than the CPU. This would hardly make for good system performance. When we wrap up the series, we’ll cover the concerns of balancing flow for good performance combined with good efficiency.
In the simplest sense, viscosity is a measure of how easily a fluid’s shape may be changed. Fluids with high viscosity, like honey, tend to ooze and are not readily deformed. Fluids like water have relatively low viscosity and rapidly take the shape of their surroundings.
In water cooling systems, flow resistance is a function of the flow path (tubing, fittings, etc.) and the fluid viscosity. As viscosity increases, the pump must work harder to push fluid through the loop. Unless you’re going to work on a system capable of cooling below the freezing point of water, viscosity will be no concern. Only when we begin investigating extreme cooling and the fluids that remain liquid at very low temperatures does viscosity become a concern.
The other reason viscosity is so important is its effect on convection. Fluid directly in contact with a stationary surface does not move much. As you get farther away from the stationary surface, the velocity of the fluid increases. In this boundary layer of relatively slow speed fluid, convection is restrained as heat builds up in the layer. As viscosity decreases, the thickness of the boundary layer also decreases. This makes it easier for heat transferred into the boundary layer to find its way to the main flow path. Turbulence in the fluid and surface roughness also help keep the boundary layer thin. All other factors constant, a fluid with a lower viscosity will have better heat transfer performance than a higher viscosity fluid.
Aside from surface geometry and viscosity, the thickness of a boundary layer is also a function of fluid velocity. For a given pump and piping system, velocity is also determined by viscosity. This makes the viscosity doubly important as it directly affects the boundary layer thickness for a given velocity and indirectly affects it further by determining fluid velocity.
Plain water has a low viscosity. You may reduce it even further with the addition of water wetting products. The temperature obtained with water wetter addition versus plain water is minor, but real. Some water wetting products also contain anti-corrosion additives. Anti-corrosion additives are virtually required if you have dissimilar metals present in your circulation system.