Does more water flow = better cooling?
I wrote this in an attempt to reduce a large amount of data (which was largely in one very informative thread which was 17 pages long) to one simple and fairly easy to read document.
The original thread is HERE for anyone who wishes to get more in-depth knowledge on this subject.
This post is
not intended to be the “final word” on Flow Rates, nor do I plan on getting heavily involved with the mathematics of fluid dynamics- the equations are far beyond my abilities.
What this IS:
1) A simplified explanation of Flow Rate tailored to our purposes. Components will be taken into account and briefly discussed.
2) Understand a little more about how the components work together- what impact to expect with a change in tube or barb size, etcetera.
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 transfer works best with the biggest temp differential: ideal would be cold water to cpu and hot water in the radiator. We cannot achieve this because a closed loop will achieve equilibrium at some point. The principle holds true however- the most efficient transfer happens at the greatest temperature differential, therefore higher flow rates will always help with all other variables remaining the same.
*It is the Heat Transfer that we want to maintain as efficiently as possible, and that is best done with a higher flow rate. Rather than thinking that there won't be enough time for heat to move towards the cool water, and therefore compromising heat loss, it is better to think that there is more fresh water moving onto the CPU and therefore, there is increased cooling.
The reason higher flow rates work better in computer water cooling is this:
There is more water with a larger temperature differential moving through the water block- this removes more heat.
There is more water with a larger temperature differential moving through the radiator- again removing more heat.
This is true even though a system with a lower flow rate will have more time to heat the water in the block and also more time in the radiator to cool the water: since the heat exchange works best with the greatest temperature differential, longer “stay time” is counter-productive.
Sources of Confusion:
There are some variables that have made this more confusing in practice though:
*Pump Heat
*Type of Flow: Turbulent or Laminar
*Friction
*Component Flow Resistance
*Pump Design
I will attempt to end the confusion on these points next.
Pump Heat:
Pumps generate heat; rather than explain why, let it suffice to say that if you put your hand on a running pump it WILL be warmer than one that is not running.
This heat 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, so most of the heat is going into the water. There will be some amount of heat being conducted to the outer surface of an inline pump, but this should be considered a fairly small amount of the heat produced.
Now, in a simple water cooling system (cpu 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:
For our purposes I am dividing flow type into two categories- there are more, but since I have no degree in Fluid Dynamics two is enough…
High velocity and the resulting turbulent flow is desirable in the components in which we want heat transfer, but low velocity is desirable in the rest of the system, where the goal is to reduce frictional (and local, or minor) losses.
*Laminar Flow*
Water moving through a vessel (tube, block or radiator) meets resistance at the walls; this causes the water at the walls to move more slowly than the water in the center of the tube: this is Laminar Flow.
Laminar flow is bad for heat exchange because the water against the vessel’s walls is slower than the water in the center. Flow rate at the heat exchange surface has diminished.; however: it is better for the tubing in our system because it has less frictional loss (or pressure drop) than turbulent flow.
A couple of pointers to help pick tubing size…..
1. 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.
2. 3/8' tubing will result in approximately 4x the frictional loss of 1/2" tubing.
*Turbulent Flow*
If we can get fluid from the center of the vessel to mix with fluid toward the walls, we end up with more efficient heat removal. Turbulence increases heat transfer significantly over slower Laminar flow.
Turbulent flow occurs naturally in a pipe when the fluid velocity exceeds a certain point, which is dependent on a lot of factors. Also, turbulence isn't an on/off thing - you can have more or less of it. Moving faster will result in more turbulence.
1. Pump the fluid at a very high rate. This is not achievable with a normal pump.
2. Add turbulators. This is the simplest way to do it, with a minimal flow restriction. It's also where we get into fluid dynamics, and tuning of the turbulators, to maximize the effect of the turbulence. Very tricky, very hard to do (ref: CFD, Computational Fluid Dynamics), and infinitely difficult to tune, if you're a designer. I'm pretty sure that heater core designs weren't just a fluke.
3. Jet impingement. This is by far the easiest way to do it. It maximizes the pumping power into a small concentrated area. It is fairly restrictive, but only because of the pre-jet tubing required. On a separate note, and mostly irrelevant to most, I found out that it's also referred to as an "irrecoverable pressure drop", because you can't recover the flow speed to the outlet.
So, in short moving water through the block faster improves heat transfer between the block and the water, which reduces the temperature differential between the block and water required to move an amount of heat.
It is NOT intended to reduce the temperature increase in the water as it travels through the block, but rather to allow more heat to be removed.
Faster flow means more turbulence, and that is a good thing- and brings us to the next topic…..
Water Blocks and Radiators
Water blocks are one of the most restrictive components in a water cooling system- this is due in part to the relatively massive amounts of turbulence in these components caused by turbulators such as 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.
Please note that I said a GOOD radiator: the best radiators for use in water cooled computers have been found to be automotive heater cores. Many of the “radiators” 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.
Friction:
I am not going to say much on friction here: it generates heat in the pump, and it also generates an amount of heat as water flows through system components (a system with greater Flow Rate or Head, either Friction or Static, will produce more heat.)
Perhaps the main area friction is involved in a water cooling system is in flow resistance- the next area to be covered.
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) - number of feet of elevation that the pump must lift the fluid regardless of flow rate.
.
Friction Head- 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 at all introduces some restriction to the flow: a sharper bend is 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 entirely responsible for the reduction of flow rate in a system.
As an example:
*3/8' tubing will result in approximately 4x the frictional loss of 1/2" tubing.
Pump Design:
Positive displacement pumps will maintain constant flowrate but increase pressure as line restrictions interfere.
Most pumps used in water cooling are centrifugal pumps and these are NOT positive displacement pumps.
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.
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
Conclusion:
A higher flow rate will give lower temperatures as long as NO other variable is changed.
Heat exchange is improved with turbulence- higher flow rate THROUGH A GIVEN DIAMETER TUBE will flow faster and be more turbulent.
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.
Choosing an appropriate pump may be the hardest part: a high head capacity is best, high flow rating important but less so; close attention MUST be paid to the amount of heat generated by a pump as this heat will be added to the system.
Formulas:
Q=M x c x Delta T.
There is an elementary equation from basic thermodynamics that states that the rate of heat transfer (Q) equals the mass flow rate (M) times a constant (the specific heat of water) times the delta T (fluid temp out minus fluid temp in).
Links to source information:
Old Flow Rate Sticky
www.pump.net
Google
Much of this information is from the original Flow Rate Sticky and credit is due the original authors.
Thank you for your contributions.
Any inaccuracies are mine
Please PM me with any errors you see and include source information.
