I'd like to invite Graystar over for a while. You see, my wife wants this lily pond in the back yard and Graystar seems the perfect person to keep digging a hole deeper, and deeper, and deeper. . . . .
Speaking as someone with a BSME and ~15 years into an engineering career, I must say that your writings in this thread would not make sense no matter how much beer one had consumed. Please, please heed the advice of countless folks here and step back for a bit. Not only are your posts technically incorrect, but they're wandering about rather aimlessly, too. Sorry for such harsh words, but you have earned them.
Getting back on track
The bit about radiator effectiveness vs flow rate has puzzled me for a while. I profess that I can not offer an explanation for the information that Bill has measured. I would say there are a couple of things to consider.
One is that all of our wonderful theory boils down to empirical observation. The equations for determining what happens with "an infinite flow field over a flat plate" are cumbersome enough to confuse most people, yet don't come close to describing what may happen inside a radiator. The fact is that flow regimes near a radiator's inner tube walls are beyond what simple equations can model. I can only surmise that the interior geometry and surface finish make them particularly amenable to specific flow velocities.
The other thing has been mentioned here before, but is worth restating for clarity. This involves the energy from pumps and the true energy balance on the system. As 8-Ball has observed, the power from a CPU is practically a constant. What will differ, albeit minutely, is the percentage of CPU heat lost through secondary pathways. As the chip gets closer to ambient, less of its thermal energy will leave via secondary paths and a greater percentage will go into the fluid. I will not attempt to quantify this other than to say "it's minor".
The pump energy remains one of the most misunderstood (and denied). When you look at the fluid you must consider all sources of heat input and exit. Even with inline pumps, most of the juice flowing from the wall socket ends up in the fluid. Because radiators are fairly restrictive and because pump power tends to rise approximately vs flowrate^2, you could reach a point where power dumped in via the pump starts to be a major player in the game. The radiator must get rid of both the CPU's energy as well as the pump's. Sure, speeding water up will generally increase the convective coefficient, but the total energy input to the water also goes up.
This may not matter if you're using a pump that peaks at 12 watts, but start getting into 30, 50, or 200 watt pumps (yeah, I own a 200W pump, but it's for that lily pond I mentioned earlier) and the CPU power can become the insignificant factor.
Given all of this, I would expect there to be a unique flowrate for each radiator where the increase due to added fluid velocity no longer outweighed the added power to make that fluid move faster. If one was to graph it, I suppose it would be sort of a wide, upside-down parabola. Bill's data seems to hint at this, but doesn't continue tailing off with added flow. Ah heck, maybe his data is just playing tricks with my mind. Then again, maybe there's something missing somewhere in his measurements. I don't think the latter is true, but I can't completely dismiss it either.
One thing remains certain in my mind. When you're working with ambient air as your radiator cooling source, the added bang from really high flowrates simply is not worth the bother. Getting even a few °C requires higher air flow through the radiator and good fluid flow through the system. So you buy your loud 120mm fans and your Iwaki pump. Congrats. Now you have something darn near as loud as a really good air heatsink, but with nowhere near the portability or reliability. It may only be viewed as another hobby, destined to consume time and money for a payback that can only be measured in one's sense of satisfaction with doing something new/different.
|