Swapped an Eheim 1048 for a Eheim 1250, Interesting results!

[pHaestus]

Am I not reading this graph right?

http://www.overclockers.com/articles481/dissvsflow.gif

Granted it is at a rather typical 120mm fan pressure and not something more typical of a serious fan, but for the application of these heat exchangers to water cooling the 0.05in H2O is pretty tpical. Doesn't this figure imply that for some radiators there is a "sweet spot" where dissipation is improved (Dangerden Cube) and for most others that lowering flow rates increases heat dissipation to some extent?

[myv65]

In the absence of additional information, the graph doesn't mean a whole heckuva lot. I know that BillA has done a lot of work in the area, but I do not know the specifics of how he has performed his tests.

All I'm telling you is that convection from a liquid to a solid will improve with increased velocity. Whether this results in an overall improvement depends on the change in energy input to the system versus the change in convection efficiency.

[pHaestus]

From this article: http://www.overclockers.com/articles481/

Bill controls inlet water temperature to 0.1C with a recirculating water chiller, so no effects of pump power or throttling here. Surely he'll pop in soon to elaborate

I think the difference in convection efficiency must be what is observed here.

[BillA]

the conditions were explicitly described in the article;
flows, temps, pressure drop and static pressures were all controlled and recorded (to generate the graphs);
and the tests were run a number of times


yes pHaestus, you are reading them correctly;
and interesting is that this 'anomalous behavior' occurs only with a particular type of radiator,
the round tube configuration having many short lengths with 180° bends
- it is not a testing 'artifact', something real is occurring

my speculation as to what is the cause revolves around the transition zone between laminar and turbulent flow regimes
-> I suspect that turbulent flow is attained at a lower than 'normal' velocity due to the bends in conjunction with the tube length;
then as the flow is slightly increased the regime reverts, causing the dip, and then reverts back to the 'typical' turbulent regime as the flow increases some more

for the specific rads shown, and ONLY for those, the bump is quite real
(I cannot count the hours passed incrementally shifting the flow rate up and down observing this phenomenon)

for any and all other rads; the sweet-spot is a fiction
(from a systems perspective however, the notion is not so farfetched - though technically not correct)

[BillA]

just noticed you were linking to the old article
(which sorely needs revision in view of my excellent and ever improving 20-20 hindsight)

the graphs have been corrected for a liquid flow rate calibration error and are posted here, the revised graphs

[pHaestus]

Speak of the devil

Thanks for clarifying that Bill. Seems reasonable I guess. Still a little confused as to how the flow regime would revert back to a more laminar type when flow was increased. Also the Serck is not the same type of rad as the others with this maximum; why do you think you see it with that one (but not with the other heater core types)?

[BillA]

am inclined to make an analogy with the tuned stacks on race engines
in a certain velocity range the pressure pulses are 'in phase' (wrt the manifold vacuum) and their throughput increases
- but will be worse above and below this range

with the fluid changing direction, there will be an entrance effect and the flow will not become uniform for some distance (depending also on the velocity and diameter)
the same factors that promote a turbulent flow regime can also serve to inhibit it
note that the 'bump' occurs at different velocities for the different sized round tube rads
(disregard the very small rad data, high variation due to the small temp/flow rates)

I am quite sure myv65 can describe this with the correct terminology
(I'll ask another fellow also, but this is not a good forum for the uninitiated)

I just found out yesterday that the Serck is an oil cooler with pleated turbulators in the flat tubes
(as are Mocal rads, also from the UK)
the BeCooling 'dual' rad is a very exceptional performer (not in the original article) which also has fluted turbulators in the straight sections

turbulators are excellent devices, but make the rad atypical

[pHaestus]

Tons of good info; thanks.

[myv65]

In chatting this over with Bill and looking at his revised graphs, I'll say the following.

As a general rule, you will see improved heat transfer with increased flow rates. There exists the possibility that due to a given radiator's internal configuration, there may be a performance bump at a specific, and typically low flow rate. At the high-flow end of this bump, performance should be better than at the low-flow end of this bump. Performance outside of this bump will improve with increasing flow.

Why some radiators exhibit this performance is open for debate. I don't personally believe it is related to laminar flow. I think it's likely a result of secondary flow, mainly vortices, and how their shape varies across a narrow flow range. As I told Bill, this is the la-la land of CFD. It's also possible, though unlikely, that this is a fictional artifact due to the way Bill is testing and measuring his results. You can easily dismiss such a result in a handful of trials. Dismissal is not so easy as the number of trials grows.

[bigben2k]

I lean towards the vortices myself, but I have no intention of cracking open a heatercore to prove it.

You know that it's an aspect that's relatively new. There was an article in Scientific American, last year I think, about how it is that dolphins can move so fast, while they don't have the muscle mass to do it (they use vortices, like from a boat's bow). There was also a bit about an artificial fish test in some university somewhere.

[#Rotor]

BillA , on your example of the pressure pulses in an engine, it has very little to do with the actual velocity of the charge in the Plenum. It does involve the pressure and vacuum pulses generated by the charge passing through either an expansion or contraction phase in the plenum, but the concept is based on sound pulses traveling through the fuel air mixture and arriving at the valves/port-entrance at various stages of the reciprocating cycle. This however takes for granted that the medium is able to be compressed to a certain degree..... two stroke engine exhaust design is a perfect example of this technology. It uses both the vacuum pulse and compression pulse generated by the exhaust pulse passing through the defuser section and baffle section, receptively. by changing the lengths and angle of these cones, one is able to generate a intensely impressive powerband, not too unlike that of an Turbo-charger on a 4-stroke..

[BillA]

yea, it was a poor analogy for unstable/transitional fluid flow

[#Rotor]

I do however agree fully, flow resistance in this place( water-cooling) is directly proportional to the mediums(water) velocity, there is no arguing over that one from me. It is in fact this annoying hurdle that steered me into trying to optimize block design for maximum turbulence, in stead of maximum flow. That way, the plumbing to and from the block will not be the bottle-neck, but rather the block.

Of course I am of the opinion that flow-rate not being used to generate turbulence, is flow-rate utterly wasted....

[JimS]

Laminar flow is not really an issue with watercooling computers. Laminar flow is more prevalent in very, very low flow rates. With the rates of flow we use in our systems, it is not an issue. All these blocks that claim better performance with turbulence are just marketing hype.

[pHaestus]

For the bulk solution perhaps, but there is always a boundary layer between the surface of wb or rad and the bulk water. At the surface (with proper resolution to see) the water should be fixed, in fact. Increased water velocity improves thermal transfer because it will decrease this layer. Increased water velocity also directly affects Reynolds number (the measure of turbulence). Hard to discern what block designs are improving performance due to increased surface area vs. actual changes in flow dynamics though.

#rotor your comments are as usual in line with mine. There are many block designs that would be extremely low flow and high resistance (Lytron?) that most likely will outperform these blocks that are "high flow" simply for the sake of bulk water movement.

On a somewhat related note, how costly is it to have you make waterblocks? I have the crude beginnings of a plan...

[BillA]

oooohh

the wb bug has bitten another

[JimS]

Back to the main topic. nikhsub, I have had a very similar experience to yours. I increased tubing at one point to 1/2" and also increased pump size. The overall difference was nil.

I've said it before, I'll say it again. In watercooling computers, once you get to a flow rate beyond 20-25 gph, the benefit of any higher flow is almost nothing.

You can show me all the charts and graphs you like. I speak from first hand experience as well as the experiences of many others who have made the mistake of adding more pumping power and larger tubing to their watercooled computers.

I will say that there are some applications that may benefit slightly from more flow, specifically those setups where flow was horrible to begin with. But once the minimum flow rate is achieved, no amount of pumping power or tubing enlargement is going to make much difference.

[#Rotor]

Cost is directly proportional to the amount of hoops you are going to make me jump through [I'm joking]

..... if it's a design I already made. then I'm cheaper than almost anyone else out there...(doesn't sound good, I know ) but when it comes to custom requests, I'm sure you will agree that cost can not stay as low as with a production model.

I however pride myself in being one of only a handful that actually has the ability to produce a waterblock on the fly, with information and design schematics, sent to me via email...... NASA can't, or won't even do that....

Jim's, you might have a good point there.... using the turbulence factor as an sales pitch might be a good idea, only problem, almost everyone in the world seem to think flow-rate is what makes a block work.... very much similar to the predicament AMD has with regards to CPU speed. GHZ is not what makes a PC fast, it's what you do with that cycle, that will make it fast. Same goes for flow-rate/turbulence .

[BillA]

Originally posted by JimS
Back to the main topic. nikhsub, I have had a very similar experience to yours. I increased tubing at one point to 1/2" and also increased pump size. The overall difference was nil.

I've said it before, I'll say it again. In watercooling computers, once you get to a flow rate beyond 20-25 gph, the benefit of any higher flow is almost nothing.

You can show me all the charts and graphs you like. I speak from first hand experience as well as the experiences of many others who have made the mistake of adding more pumping power and larger tubing to their watercooled computers.

I will say that there are some applications that may benefit slightly from more flow, specifically those setups where flow was horrible to begin with. But once the minimum flow rate is achieved, no amount of pumping power or tubing enlargement is going to make much difference.

Jim
I'm not contesting the validity of YOUR experience, I do accept that you saw what you saw

but your conclusion
"In watercooling computers, once you get to a flow rate beyond 20-25 gph, the benefit of any higher flow is almost nothing."
is simply wrong

you are swapping several components in a specific system, and then trying to generalize a universal conclusion
you are wrong

I have tested radiators, waterblocks, have 6 different pumps, and have assorted tubing from 1/4 to 3/4 in. ID

flow rates above your described 0.3 to 0.4gmp can provide a measurable benefit
but it is the ENTIRE system that must be evaluated to achieve higher flow rates
NOT just swapping the pump and tubing
-> as any single component can effectively limit the ACTUAL flow no matter what the pump or tubing size

you did not identify your wb, rad, or if any 90s were in the system;
but if you go from a flow rate of 0.4 to 1.4gpm, you will see a substantial difference (if your measurement capability is functional)

#Rotor
quite agree with the 'virtues' of turbulence (and am fabricating a 0.7gpm @ 40psi system right now)

but your comments are masking a point:

for a given wb, higher flow WILL result in greater turbulence, a reduced boundary layer thickness, a higher convection rate, resulting in a reduced thermal gradient across the wb bp - hence lower CPU temps

yes, yes - wbs can be designed for more turbulence at lower flow rates
but even so the above statement is still valid

(why do you promote turbulence (= drag) also on the 'top' side of your wbs ? - ain't no heat 'up' there)

[#Rotor]

True, most of the heat will already be suspended in the liquid, resulting in the top of the block, almost not seeing any heat at all, with the emphasis on ALMOST... and I am somewhat of an perfectionist.... so as you can imagine, I very much do not like doing things halfway, even if the gains is only fractions of a degree...

You are making the point there, "for a given block". What I'm saying is that so many people are thinking of flow-rate, as being the mechanism at work to produce better cooling, hence the so popular spiral designs everywhere, Now I'm not saying the spirals ain't good.... They are good, very good. . You see, these blocks have been designed to make it easy for the liquid to get through.... how do you make life easy for water to get through??? you remove any turbulence generating characters in the design.... there is my problem.... what I'm saying is, don't let the block worry about flow-rate, the block needs to worry about getting the heat into the water, nothing more. if flowrate, or lack there of, is a result of the blocks design, the wrong thing to do, is to go open up the block, and take a dremel to it.... the right thing is to go buy a stronger pump...
heehe I sound like an American.... "if the car ain't fast enough, add more CI. to the motor"....

[morphling1]

LOL, yes and we Europeans put in direct fuel injection, turbo charge 5valves per cilinder and twin spark, and take care of aerodynamics...

[Miaumarramiau]

Hello,

This is the first time I'm posting/replying in this forum (although I'm not exactly new), so HI all!

About this setup, I just think the problem as mentioned be4 is the high flowrate trough the radiator.
In the radiator, the longest the water can stay in into it, the more it will be cooled down, BUT it is also true the faster the water can go though the waterblock, the more heat will be dissipated.

I think with other radiator for higher flow, less passes and more tubes, your new pump would outperform your new one, but as it was mentioned be4, you've lost your "sweet point" of your setup. The heat exchange in your waterblock is higher, but your radiator now it's less efficient at this flowrate. Just try to get another one.

Francisco

PD: Sorry for my bad english, but I'm not english

[JimS]

BillA, as my results are limited to my own system, I certainly agree that different systems may yield different results when it comes to flow rate. Obviously your testing is far more advanced than mine, and covers a wider array of blocks, radiators, etc.

My system is a chevette heater core 6" x 6" x 2", two 120 mm fans, one push, one pull, inline chillerblock, 500 GPH pump. There are two loops in the system, the above describes the hot side. The cold side is CPU wb, NB wb, inline chillerblock and reservoir, 500 GPH pump.

The main point I am trying to make is that once you get beyond a minimum flow rate, any further increase results in minimal gains. I believe even your own graphs show this to some extent.

NOTE: The testing that I did was with the chillerblocks removed and the radiator and reservoir used in a normal configuration. It consisted of the pump, CPU wb, NB wb, radiator and reservoir.

[BillA]

our difference lies with the definition of "a minimum flow rate"
had you said 1.5gpm I'd not have even posted
but 0.3 to 0.4 is far below what can be useful

but looking at your system description it is very clear why you see no benefit from higher flow rates:
you cannot achieve them

your 'chiller' is killing your flow (potential), and also any cooling benefit as the chiller 'can't cope'
-> chillers ONLY 'work' (and I misuse the word here) at very low flow rates
because their capacity is so limited

just saw your note

NB wb in series or parallel ?
(I would suspect this as a contributing factor)
and either way confuses the issue: in parallel diverts some of the flow, in series severely limits it

[bigben2k]

Originally posted by unregistered
our difference lies with the definition of "a minimum flow rate"
had you said 1.5gpm I'd not have even posted
but 0.3 to 0.4 is far below what can be useful

but looking at your system description it is very clear why you see no benefit from higher flow rates:
you cannot achieve them

your 'chiller' is killing your flow (potential), and also any cooling benefit as the chiller 'can't cope'
-> chillers ONLY 'work' (and I misuse the word here) at very low flow rates
because their capacity is so limited

just saw your note

NB wb in series or parallel ?
(I would suspect this as a contributing factor)
and either way confuses the issue: in parallel diverts some of the flow, in series severely limits it

I agree 100% with that.

From all the data that BillA (and others) have shared, if I looked at a range of flow rate where the difference in the cooling ability (c/w) is less than 5%, it seems like 300 gph would be a good target flow rate, but that's still an off-hand observation. 300 gph effective flow rate is not easy (read cheap) to achieve, and that only takes into account the WB, not the rad.

It then became apparent that 300 gph is not only hard to achieve, but most blocks have a cross-sectional channel that is too large, which, although less restrictive, makes 300 gph a futile attempt.

300 gph is also quite useless to a rad. A heatercore would be too restrictive for that kind of flow rate anyways.

So I'm back to the old addage: either you use a high flow rate to achieve turbulent flow, or use a lower flow rate, but figure out the best way to induce turbulation in the water.

Then there's the fins...

Just check out the waterblock design thread I started.