Water cooling: heat transfer basics
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Heat transfer
Heat transfer is the basis for all computer cooling systems; in water cooled computers we make this more complicated by using multiple heat transfers:
CPU core to water block; water block to water; water to radiator; radiator to air
Heat moves driven by temp differential but what we want is to have the smallest posible temperature difference. We have a cold media and we would like the heat producer to be at this same temperature, so in an ideal system you would have the source at the cold media temperature. Basically you can think that the of heat transferred is equal to temp diff times heat conductivity:
Transferred Heat = Temp diff * Thermal Conductivity
We have a fixed temperature heat receiver, air, and a source producing a (rather) fixed amount of heat, The source will rise its temperature till all heat it produces is transferred, having then reached an equilibrium between added and outgoing heat. We have to find the way of moving out that heat using the smallest temp diff so source rises it temp the least possible. We have a cold media and ideally we would like the heat producer to be at this same temperature, so in an ideal case you would have your CPU, for instance, at ambient temperature.
You can imagine heat going from source to cool media through a series of steps one after another, each needing some temp diff to transfer the amount of heat being produced in the source, so then you add the temp diff that each step needs to transfer the heat, and you get the temp above the ambient your source is gonna be.
Three ways to transfer heat: conduction; radiation; mass flow (including convection) and one way to use it up: phase change
Heat Path
How the properties of the heat path affect heat transfer.
Length
The longer the heat path the higher temp diff you need to keep heat flowing,Surface
The bigger the cross area heat is travelling less temp diff you need,Heat conductivity
The bigger the better; silver and copper are the best readily available materials, aluminum is good also.Mass
The more material you have the greater amount of heat you have to use to rise its temp one degree; bigger flows allow heat transfer with less temp diff.Higher flow rates work better because the same amount of heat goes to heat more water so you need less temp diff; the same in radiator: more water goes by so each water mass unit transfers less heat so you need less temp diff; also higher velocity water changes the way water interacts with block or radiator (more about his in Type of Flow section)
Sources of confusion
There are some variables that have made this more confusing in practice though: Pump Heat Type of Flow: Turbulent or Laminar Component Flow Resistance Pump Type Fans
Pump Heat
An active cooling method means you are spending some energy moving heat. After you put this energy into your system it does not vanish, and it appears as added heat.
There is heat that your cooling device generates within itself because of its inefficiency, and there is the useful work listed in its specs as its potency. The heat due to pump inefficiency has to go somewhere: a submerged pump (inside of a reservoir) must add all of the heat it generates to the water; inline pumps are usually designed to use the pumped fluid as a coolant, most of the heat is laso going into water, (some amount is conducted to the outer surface of an inline pump, but this is a fairly small amount of the heat produced).
The useful work that pump does moves water; in a closed system, like a phase or water cooling system, it is used up overcoming frictions in the cooling loop and finally appears as heat inside your system; in an open system like an air cooling system, a small part of it goes into your system as friction produced heat and the rest goes out as free energy in cooling media that exits, (air).
Basically, all the energy a pump uses moving a fluid finally heats this fluid. If you use a small pump say 5W you add that heat production to your system, if you use a 50W pump you add 50W heat (just about what a CPU produces).
Now, in a simple water cooling system (cpu and water block only) we actually have two sources of heat: the cpu and the pump. Pumps with a higher Flow Rate will generate more heat than pumps with a lesser Flow Rate, and there lies our first bit of confusion: It is possible to add more heat from a larger pump than will be removed by the higher Flow Rate.
Flow Type
We can divide flow into two categories, laminar and turbulent.
Laminar Flow
Water moving through a vessel (tube, block or radiator) tends to move in ordered layers, following smooth paths: this is Laminar Flow. Each portion of water does not mix with surrounding portions, and heat has to travel orderly from vessel wall through each liquid layer needing temp diffs in its way. Due to friction, water against the vessel walls is much slower than the water in the center and flow rate at the heat exchange surface is near zero. However this ordered flow has less frictional loss (or pressure drop) than turbulent flow, so laminar flow is preferred inside tubes.
A couple of pointers to help pick tubing size... Use large diameter tubing to minimize the frictional loss along the length of the tubing, of course very large tubing is unwieldy, so we must settle on a reasonable size.
Turbulent Flow
Turbulence means that the fluid does not move in ordered layers, it has eddies so each portion is mixing with its surroundings. If fluid from the center of the vessel mixes with fluid near walls we end up with more efficient heat removal, cause each liquid portion may interact directly with vessel wall.
Turbulent flow occurs naturally in a pipe when the fluid velocity exceeds a certain point, which is dependent on a lot of factors. Turbulence isn't an on/off thing - you can have more or less of it. Moving faster will result in more turbulence. Some useful ways to add turbulence: Pump the fluid at a very high rate. Rather difficult. Add turbulators. This is a simple way to do it, with a minimal flow restriction. The tuning of the turbulators, to maximize the effect of the turbulence is tricky, hard to do. Jet impingement. This means tapering tubes to a small diameter to get a high velocity fluid jet that is directed against the heat transferring area. This maximizes the pumping power into a small concentrated area; it is by far the easiest way to do it.
So, in short moving water through a block or radiator faster improves heat transfer between the block and water, which reduces the temperature differential required to move an amount of heat.
Water Blocks and Radiators
Water Blocks are some of the most restrictive components in a water cooling system - in part because of the relatively massive amounts of turbulence caused by pins, dimples, changes of direction, fins and impingement jets. A good radiator is in the same situation : by design they maximize surface area and induce turbulence. These turbulence inducing components more than compensate for the restriction they impose by increasing heat transfer dramatically.
The best radiators for use in water cooled computers have been found to be automotive heater cores. Many of the radiator sold by online vendors are of the bent-tube-and-fin type. These add flow restriction without the benefit of effective surface area or sufficient turbulence.
Component Flow Resistance
Pumping a liquid through a tube creates resistance. The resistance is determined by the cross section of the tube, the length and all fittings in the line.
Static Head(or Lift)
The number of feet of elevation that the pump must lift the fluid regardless of flow rate.
Friction Head
A measure of resistance to flow (backpressure) provided by the pipe and its associated valves, elbows and other system elements: A smaller tube diameter will have greater resistance: even with identical fittings, pumps and water blocks, a system with larger diameter tubing will have a higher flow rate. A longer tube will also have greater resistance: even with identical fittings, pumps and water blocks, a system with shorter tubing lengths will have a higher flow rate. A straight length of tube will have less resistance to flow than one that is bent. A partially kinked tube easily proves this point. Any bend introduces some restriction to the flow: a sharper bend is much more restrictive than a gradual bend.
Head
The entire amount of flow resistance in a system.
Static Head + Friction Head = Head
This is what pump head capacity must overcome and is responsible for the reduction of flow rate in a system.
Pump Type
Positive displacement pumps will maintain constant flow rate but increase pressure as line restrictions interfere. Most pumps used in water cooling are centrifugal pumps and these are NOT positive displacement pumps, flow will diminish rapidly with flow resisitance. Getting the pump with the highest flow rating is NOT necessarily the best answer: centrifugal pumps tend to be extremely sensitive to flow restriction. A pump with a higher Head Capacity will be less sensitive to restriction and be more suitable for computer use. Which brings us back to the issue of pump heat: a pump with more head capacity and higher flow rate will add more heat to the system.
A Bit about Fans
Just as higher flow rates remove heat from the cpu faster, greater air flow rates through the radiator will improve performance at any given temperature. The actual equations differ since the fluid characteristics- water and air- differ, but the same principles apply. Similar to a water pump, the pressure needs to be taken into consideration and thin fans don't usually provide a lot of pressure; thicker fans generally will provide more air pressure. Example: a 120mm fan 38mm thick should be better for our purposes than a 120mm fan 25mm thick.
The advantage to more airflow is that it provides a good bang for the buck improvement in performance; the disadvantage to this is that a fan will create more noise than a water pump, and noise is often one of the areas we are trying to improve with water cooling
Some concluding remarks
A higher flow rate will give lower temperatures as long as other variables are not changed.Choosing an appropriate pump may be the hardest part: a high head capacity is best, high flow rating is important but less so; attention must be paid to the amount of heat generated by a pump as this heat will be added to the system.
Heat exchange is improved with turbulence- higher flow rate trhough the same diameter will give a bigger flow and be more turbulent. This will be beneficial in heat exchanging vessels (radiator or block).
Flow rate can be increased by using larger diameter tubing, the shortest total length of tube possible, fewest bends and fittings possible, lowest restriction radiator possible and by using a pump with a higher head capacity and flow rate.
Maintaining good airflow through the radiator is probably the easiest and noisiest way to improve performance.
