Quote:
Originally posted by gone_fishin
If the heat delivered to the rad is in very small quantities then the time needed to dissipate is also small in the rad. The amount of heat absorbed per volume of water before it reaches the rad is smaller in high flow but repeated many times over compared to low flow in the same time frame. Is my understanding in this flawed?
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I'll tell ya, I'm not sure if I should reply here or to another thread running around here that says basically the same thing. You are correct in all aspects, but you don't really state a conclusion. Barring that, I can't say if you are right or wrong.
I posted that last bit late at night and didn't really put down quite all I wanted. Well that and the fact it was already a really long post.
Just about everyone seems to get the fact that energy input to a cooling system is almost constant regardless of flow. It is not quite constant because processors don't put out truly constant heat and pump energy will change ever so slightly as system temperature (and fluid viscosity) change. So if we ignore these minor changes, we can say that flow rate multiplied by delta-T equals energy transferred. Not stating anything new here.
Somewhere else in another thread, JimS said something like what you said above. Namely, the percentage of time water spends in the radiator is independent of flow rate. His reasoning thus went along the lines of at flow = 1X, the water is in the radiator 5% of the time. At flow=2X, the water is still in the radiator 5% of the time. Cooling should therefore be roughly equivalent.
The fact is, energy transferred must be equivalent, which means that the delta-T at 1X is two times higher than the delta-T at 2X. This says absolutely nothing, however, about the actual fluid temperatures.
Also, consider a person that uses a reservoir. Given the same tubing and flows, maybe their fluid only stays in the radiator for 2% of the time. Assuming the same flows, does this mean that their radiator somehow performs poorly in comparison? Absolutely not. A radiator has no "brain" to know that is has the water for 5% versus 2% of the time. It knows only flowrate (fluid AND air) and temperatures.
There is no steadfast answer that applies to all situations regarding "what's the best flow". The general rule-of-thumb answer is "use as much flow as possible without dropping the residence time in the radiator too much". OK, so that doesn't really tell us anything. What does it mean?
It means that heat transfer in an air-cooled radiator by its very nature has its peak cooling at the entrance and its minimum cooling at the exit (as fluid and air temperature approach one another). When the residence time is too low or the air flow is too low or the surface area is too low, the delta-T between fluid and air
must increase to get the same quantity of heat transferred. When residence time is high and air flow is high and surface area is high, the fluid will exit at practically the same temperature as the air. As you begin to decrease any of the three variables, the fluid-air delta-T will begin to increase. It will increase gradually at first and more dramatically as you continue dropping residence time/air flow/surface area.
So long as you don't "go over the hump" where the delta-T curve gets very steep, the flow through the radiator is not yet too high. Simple to state, hard to put an absolute number on it (and it's a different number for every setup).