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Been playing around with some flow specific radiator testing and getting wierd results. After referring to other tests, I noticed the old coolingmasters review measured something similar. On their review they measured that with lower RPM fans, many of the rads begain showing worse performance with higher flow rates:
http://www.cooling-masters.com/image...mages/g3en.png
Any theories as to why?
:confused:
maybe because the fluid is moving too faster for the slower fans to keep up with cooling the radiators? Or I just completely missing it?
your completely missing it.. :ROTF:Quote:
Originally Posted by millertime359
im gonna blame the eternal badboy AMBIENTS martin.
I would imagine that the heat transfer between the water and the fins is quite high but the forced air the air cannot remove the heat from the fins fast enough. That's just my guess.
I don't know.
I know the net amount of time the water is in the radiator is the same regardless of flow rate when you consider the recirculation part of the loop. Double the flow rate, and it's only in there half the time, but twice as often, etc.
But the water itself and turbulence I figured would always be better with higher flow rates and increased velocity.
I can't make sense of it..?
How about liquid friction? :shrug:
That could amount to some, but at most I think it could only be a few watts. If the pump only consumes 15-18 watts at 1.5GPM and you're only loosing a very small portion of that (Pressure/Flow energy) due to pressure drop, I would think pressure drop produced heat could only be a few watts at most.Quote:
Originally Posted by Sadasius
They are showing that some of these radiators are loosing nearly 20 watts worth of dissipation between 1 LPM(.2GPM) and 11 LPM(2.2GPM).
That's more loss than the pump would consume. Also all the radiators seem to have a similar loss. Since the GTS is serveral times more restrictive, if the loss was all pressure drop induced, I would have expected the GTS to show this stronger.??
I'm sure that's some sort of contribution though. Energy has to go somewhere and pressure drop converts to heat.:up:
O, ok I understand this then, but if you have less air moving over the fins, less heat is dissipated, even though water spends the same time in the rad no mater what, the quality of the time it spends there is worse as it moves faster?Quote:
Originally Posted by Martinm210
So seeing that a slower fan should perform worse?
There is no mystery here. Liquid flow rate and air flow rate are conjoined twins. This has been well documented years ago by Bill Adams and his extensive radiator testing.
I am curious on this as well...the first couple radiator test runs I did I forgot to control the flow rate and saw some odd numbers. I don't have the thermal engineering background to give a concrete reason as to why.
so, on this one... :popcorn:
Quote:
Originally Posted by millertime359
I think millertime has part of it right, in our normal loops it might not be but these guys tested with a water chiller and not a closed recirculating loop. There is probably a non linear curve to how much heat the radiator dissipates depending on flow, and a non linear curve on how well the air absorbs heat depending on cfm. There is I'm guessing a turning point where curves go down when the flow is increased, and this means there is a perfect equilibrium point where there is a certain amount of flow that you can reach that will give you the best temps at that certain CFM.Quote:
Originally Posted by Martinm210
Do you recall any specifics about what's going on it detail?Quote:
Originally Posted by nikhsub1
I don't understand it,but it's very interesting especially when there's becoming an increasing number of users that prefer very very slow speed fans....
It does me there are circumstances where you may want to PURPOSELY run radiators in parallel or series. You might even consider some sort of bypass plumbing setup to tune the radiator where it peaks in heat dissipation.:shrug:
In their test how do they vary their flowrate?
needle valve, change in voltage
Excuse me Martin but how slow is slow? Are we talking about sub 1000rpm?
heat dump from fans? Unlikely though.
If the flow through the radiator is higher, the pressure drop must be higher too. It is possible that the pressure is altering the radiator physically, albeit very subtly? Maybe making the tubes flex outwards a little, which would make the tubes' inner surface/CSA ratio worse?
You couldn't have waited another week or two to post this, Martin? I'm halfway through building a cabinet to house three triple rads, three pumps, and I was planning on running sub-600rpm fans. This is throwing a monkey wrench into my plans, darn it. :mad:
The cooling masters guys and in my test, I'm pulling with the fans, so it can't be heat dump from the fans. Also in the coolingmasters test, they show this affect occurring to a very minor degree with the nexus fans at 12V, it's not really flat past 1GPM until they tested with the Panaflos at 7V.
And we're not talking huge differences, but something and perhaps more with certain radiators than others.
I just see this as another possible optimization. The radiator manufacturers could either build in their own bypasses to emphasize performance in one area, or the user could do it with plumbing/valves.
I'm certainly seeing this in my closed loop testing now and I'm repeating my tests, so it's checking out.
I think your on to something there.:up:Quote:
Originally Posted by MpG
I believe that does happen with some. I know in my 480GTX pressure drop test, that the curve was very linear compared to most pressure drop curves. Those tubes are very thin for good heat transfer, it probably doesn't take alot of pressure to start flexing them round..
If this is the case, you might consider running those rads in parallel...
Maybe this is the reason for the gap inside the TC rads?Quote:
Originally Posted by Martinm210
Now that I think about it some more, any physical deformation that interferes with low-speed fan cooling, should only be magnified with the high-speed fans. Which doesn't seem to be happening. :shrug:
Could be, although I would have expected a cutout hole in the dividing plate if it was intentional.Quote:
Originally Posted by MpG
What I find interesting is how low a flow rate we're talking about. 1LPM is about .2GPM, that's hardly any flow rate at all:eek:
I'm seeing up to 20% performance differences depending on flow rate so far and possibly more. That's pretty significant especially if you consider doing some flow bypassing also has the benefit of reducing overall system pressure drop and increasing flow rate for your blocks where more flow rate has been shown to make improvements...win win..:up:
It wouldn't be magnified with high speed fans because the supposed deformation is caused by the water and not the fans. The tubes expanding would create a smaller space for air, which makes the radiator more restrictive for air passing through. Therefore it is less efficient on lower rpm fans while the higher rpm fans can power through it (like a thicker vs thinner rad or a high fpi vs low fpi).Quote:
Originally Posted by MpG
You are closer to the truth than NaeKuh thinks! I wasted my breath on this one a long time ago, blocks may like as much flow as you can feed them but radiators certainly do not unless you ramp up the amount of air that you are horsing through the core.Quote:
Originally Posted by millertime359
@Martin, if you have any super finger choppers (220cfm+ delta's), try those on the same radiators and the cooling performance will improve. Please wear gloves and ear plugs though!
I think I figured out the problem with my particular setup when trying to see flow rate effects:
A larger delta across the inlet and outlet of the radiator will net a lower "Water Out" temperature with lower flow rates. This is simply a function of the water's specific heat value and how lower flow rates does indeed mean a lower "Water Out" temperature, but if you were measuring the average loop temperature, THAT would be a different story.
What this means to me is that using "Water Out" temperature only in radiator testing while letting flow rate vary is BAD!!!
More appropriately, we should be measuring "Water Out" and "Water in" and averaging those two temperatures. This way the Delta across the radiator which is VERY dependent on flow rate is canceled out in testing.
That or you always need to test at exactly the same flow rate. That's not necessarily taking any pressure drop differences or flow rate performance differences into account, but it's at least canceling out the In vs Out delta dependency on flow rate.
Bottom line, measuring radiator performance must either fix the flow rate, or preferably measure and average the temperature of the inlet and out.
This same issue goes for CPU testing. Using the water out only is again going to create some skewing of results if flow rates vary and you compare water to core. If ambients were held exactly the same, the blocks that are the most restrictive will produce water temperatures from the radiator that are slightly lower, this is not so much due to lower pump heat dump, it's mostly because of the delta across the radiator.
Anyhow, I think I finally understand and learned some important test procedure things:
1) You should not test radiators with "Water Out" only and allow flow rate to vary. This will make the more restrictive radiators appear to perform better, when it's really just a specific heat and flow rate variable that's making the water out temperatures lower.
2) Radiator testing or comparing of water temperature should compare the water loop temperature average "Water In + Water Out/2". This will cancel out this delta T problem that's extremely sensitive to flow rate.
3) Radiator testing using "Water Out" should ONLY be used if you fix the flow rate constant for all tests. This still does not test the real world where the flow rate is dependent on the loop, and affected by the radiator pressure drop, but it's still at least eliminating the radiator delta problem.
4) CPU block testing - Again this "Water Out" comparison is not good with varied flow rates. Ideally you need to fix ambient and compare core to ambient only. If you fix the ambient temperature and compare core temps to water out temps, you'll artificially be making the lower restriction blocks look better because by nature of the higher flow rate the radiator water out temperature is higher. You will indeed get lower water out temperatures with the more restrictive blocks simply because of the lower flow rate across the radiator and larger delta across the radiator. To me this just means using "Water/Core" as a means to compare blocks is not an apples to apples comparison. Apples to apples is a direct comparison to ambient, and that will mean two slightly different water temperatures running across the block.
Anyhow, my testing setup is not working for flow rate testing. I either need to fix flow rates at 1.5 GPM, or if I want to see flow rate effects...I need to measure both inlet AND outlet water temps.
Regardless, I think the Coolingmasters test was good, so there is still some interesting thing happening...just not to the extent that my first test was showing.
I thought something wasn't right.....:D
I also realize that my early CPU block testing and comparison to water out temperature was WRONG! The rad testing is still ok though as I think fixing the flow rate at 1.5 GPM saves that one.
heres comes back martin setting another higher standard for standarized testing.
:up:
Very interesting Martian. I learned something today for sure. The problem was it wasn't what I should have been learning in class :doh:
I am having a hard time figuring out what methodology coolingmasters is actually using to get their numbers. In their review they mention two different ways of doing it ... and then never say which way they are using.
I think they are using flow rate and delta across the radiator to come up with the heat dissipated. Then they probably use delta from the air in sensors to the average of the two water sensors.Quote:
Originally Posted by Erasmus354
I still struggle with the whole Q = M X CP X dT equation myself with units but I think I got it.
To simplify it further and convert it to flow rate:
Q in watts = 263.43 x (Flow rate in GPM) x (dT in C)
So you need 263 watts to raise the delta 1 degree at 1 GPM
Or if you want to solve for dT
dT in Celcius = (Q in watts)/(263.42 x (Flow rate in GPM)
So for example, I've been testing with a 590 watt heat load at 1.5 GPM, so my dT would be 1.49 degrees across the radiator.
I think I need a beer, this stuff hurts..:D
Thanks for this, Martin.
I'm going to have to change the calculations in my recent tests again, then. I'm quite curious if it will make a significant difference in my results.
Oh I understand the math and physics just fine, always have. I don't find it that complicated either. Protip: Stop using imperial measurements. The calculations are much simpler if you use liters per minute. What I don't understand is whether that is actually what they are doing. A google translation of their methodology page:Quote:
Originally Posted by Martinm210
Notice how they are pointing out the need for highly accurate sensors to measure the way you describe, and they go on to mention the other way of calculating it (which is using the known heating element). They never actually state, or at least I can't tell from the translation, which of the two methods they are actually using. If they are using the second method I can see how they might have made the same mistake you did.Quote:
This temperature difference between water input and output is only a few degrees at most at very low speed and a big power to dissipate. It is less than 1 ° C typically (flow> 2 L / min). With this method, it requires appropriate instruments to measure just 0.01 ° C or better. Indeed, every hundredth of a degree equivalent to several watts after calculation, especially where high-speed input-output gap is reduced, which reduces the sensitivity of the measure:
English version
The advantage of this method is that we know exactly the power dissipated by each radiator. There is no loss side or powers unknown variables introduced in the measuring circuit (eg a pump.). Here, everything happens only between the entry and exit of the radiator.
The method is to measure the temperature rise of water compared to air in a single loop, where we introduce a constant heating power, is another method. It is valid if we take some precautions because it introduces factors more or less unknown, but it does not require tools for high resolution. Each method has its advantages and disadvantages vis-à-vis what is sought and what was available. Either one works at constant power deltaT to find the water-air, or are working on water-air deltaT to find the constant power.
It depends on the heat load and flow rate. A smaller heat load and small difference in flow rate may not amount to much, but it made a huge difference in my test where I ran a 600 watt heat load over a range of flow rates from .5 to 3.4 GPM. My original test was showing a 22% improvement at .5GPM vs 3.4GPM, but once corrected the higher flow rate shows a 2% improvement over the low flow rate. My results were really messed up, but all better now. I think I'll switch out one of my water sensors over the inlet side though regardless to start recording the average water instead of water out.:up:Quote:
Originally Posted by HESmelaugh
I'm thinking three simple points will do if the results are flat. Maybe one at .5GPM, 1.5GPM, and Max flow. This will help average out the three tests and give a good indication if there is any trends regarding flow rates.
I see, yeah I'm not sure you can tell by the data presented, but I think they did it right.Quote:
Originally Posted by Erasmus354
My error was causing a very pronounced parabolic like curve starting high at low flows similar to what you see on a block C/W curve. Their data seems to be very different in that regard (more linear where it progresses downward) but there's not alot of number checking since they don't provide any of the data collection details...only the watts and deltas.:shrug:
First, sorry for quoting the whole post, but I think this needs to be carried over on page roll. Also, glad I am fixing flow rate on all the radiator tests I do...whew, bullet dodged there. :p:Quote:
Originally Posted by Martinm210
As for CPU blocks: When you're looking to find a constant ambient but cannot keep the ambient dead on a specific temperature (my test lab has a 2ºC natural variance even with a dedicated HVAC zone in my house) you have two options to calcute the average core temps to a set ambient. (<- holy run on sentence batman)
Air to Average 4 Core dT + Set Ambient
and
Water to Core dT + Water to Air dT + Set Ambient.
Since we can't rely on the Water to Air and Water to Core numbers unless we test all blocks at set flow rates, you have to use Air to Average 4 core. Or I could be misinterpriting your #4. Without using an environmental chamber, true control of ambient is near impossible. Other possibility...I am completely missing something obvious.
I guess the only way to be sure is use both calcs to check your data calculations and also use two sensors for Rad Water Out.
edit: 1 more thing...varied flow rate testing at 3 seperate flow rates is a good idea. would you still calc down to an average C/W though for overall performance?
Good to see you figured it out.
My first assumption was increasing heat dump from the pump.
Assuming you have a varying flow rate through the pump, not 100% sure how you vary the flow. Increased flow --> more power required by the pump, and this should give you a larger heat dump. From my experience with pumps they draw much less current with no/low flow then on max flow, as you your self noticed when you created an overly free flowing system and tripped the over current protection.
On a side note:
Nice to see you back up and testing, even if its on a smaller scale, you have teched me allot about water cooling, keep up the good work.
Yeah, I think it's still fine to do some correction on the data for varied ambient, but you might want to figure out the error per degree since processor heat generation varies with heat. This can be done by simply logging one long run over a large range of temperatures. I would just avoid using the water out number by itself for comparisons if flow rates vary. OR if you want to compare to water, it should be the average water temperature between inlet AND outlet.Quote:
Originally Posted by skinnee
I still kind of like thinking in terms of water out of the radiator temperature since that's whats running through our block if setup that way, but it's just not a good way to compare stuff since flow rate messes with it.
I think the key here is that any delta measurements from water outlet temperature are affected by flow rate. That problem is eliminated by referencing either the average of inlet/out, by fixing flow rate, or by referencing ambient instead.
For CPU testing I think referencing the ambient makes the most sense, this is also something more users understand than water deltas, although it does introduce more errors such as dust building up on a radiator and stuff like that you have to watch for also.
Heh, very interesting. A simple source of error, but definitely one that's capable of skewing results without anyone realizing it. I wonder if this issue is entirely responsible for the funny curves on the radiator performance charts?
Hi Martin,
I totally agree with your idea of taking into account flow rate. I mentioned this in one of HES’s posts.
http://www.xtremesystems.org/forums/...=220874&page=2 post #38
In HES’s test,he was plotting delta Ts by a constant wattage, so in that case you want to non-dimensionalize your delta T with some characteristic temperature (which takes into account flowrate). This doesn’t take into account all the flow rate dependence because the water side heat transfer coefficient can change with flow rate (boundary layer), but it will account for the m*Cp term. If you were to plot this dimensionless delta T for the same radiator at different flow rates any change you would see would solely be due to changes in the waterside heat transfer coefficient. You could also plot this for various radiators and/or radiator configurations as a function of flow rate and see which radiator scales the best with flow rate.
As for the average temperature difference, I see what you are trying to do. It is common practice to use a mean temperature when trying to determine the overall heat transfer coefficient of heat exchangers (usually a log mean temperature difference). If you were to plot your average temperature differences versus flow rate at some constant wattage one would see your average temperature difference decrease with increase flow rate (Ideally). You could do this for multiple heat exchangers and see which exchanger scales best with flow rate (which I think is the point of this exercise). But I guess I don’t see what is to gain with this method relative to a characteristic temperature. Looking at your method the only difference I see is measuring an extra temp.
As for the cool masters, their strange plots might have to do with error in their measurements. At low fan speeds the air is clearly saturated and no matter how much you increase the flow rate you will not reach a lower the water temp. But then why does the total wattage decrease? They have a calibration probe with an error of .0001C but they measure with probes of .02C. Because they are taking the difference they could have a max error of .04C. For every liter per minute there could be difference of 2.8W. So over the range of 10 L/min one could see a difference of 28W. When fan speed is increased the air is no longer saturated and we start to see increase dissipation vs. flow rate like we would expect. If there was no error in the probes this trend at higher fan speeds might be greater.
:shakes:Quote:
Originally Posted by hellcamino
Closed loops mate, i thought we settled this back yonder!
Nice to see the mind is still sharp Martin ;) Good work on picking up on that inconsistency.
:shakes::shakes: I'm guessing you didn't actually start at the beginning of this thread and read it?Quote:
Originally Posted by twwen2
Great stuff here.
Would it be fair to say that the majority of us running just one pump in a loop are not going to experience any major issues, since we are working within a relatively narrow range for flow rate?
If your pc doesn't have overheating problems already then you won't ever experience any major issues due to this test:PQuote:
Originally Posted by shazza
Thanks, hellcamino. Not too worried about my system, as I can run pretty consistent temps with varying fan speeds (CPU only on PA120.3). Really just looking to get confirmation that this is not a real issue most people have to deal with. I consistently see new folks believing their temp problems are caused by water flowing too quickly through the radiator.Quote:
Originally Posted by hellcamino
I dunno, it still doesn't seem right to me. From a pure thermodynamics and fluid mechanics standpoint it doesn't make sense. More flow means faster fluid velocity through the tubing. Faster fluid velocity directly correlates to a higher Reynolds number. This means less instance of laminar flow and therefore better heat transfer from the water to the tubing.
Now admittedly there are a lot of variables to consider here. Slower air flow means you need a higher fin/tube temperature to dissipate the heat load. The higher the temperature the less "eager" the metal is to absorb more heat from the water. And on and on, it is like a set of domino's one thing effecting another which effects another etc... Meh, all this thinking makes me remember a really good read about thermodynamics in respect to processor cooling.
BASIC PRINCIPLES INVOLVED IN HEAT TRANSFER AND COOLING RELATED TO PROCESSORS
This was referenced by Bill Adams in something I saw back in like 2004. Pretty interesting read for people who want to understand a bit more about the properties of how heat energy is moved through a system.
I initially think when reading these results that this is what is happening (alluded to by Erasmus)
Higher flow brings more heat to the radiator. This we know; but with the slow fans the heat is dissipated from the radiator less quickly. Thus the delta T between the Rad tube surface and the water is smaller and less heat is transferred away from the water.
mdot*Cp*(T2-T1) = h*A*(T2r-T1r)
There are principally 2 reasons to describe the small downturn at very low airflow and high water flow rate. Bond Number had some clues.
First, the whole measurement chain has a responsability. The bigger the water flow rate is, the bigger the error about heat dissipated will be. Even if the RTD probes for water are very sensitive (0.01 °C) and well calibrated against a quartz thermometer, the deltaT "water_out-water_in" become so small (~0.1-0.3 °C) at high flow rate that the smallest error on this deltaT leads to a bigger error when calculations are made. A 0.01 °C error or rounding seems to be very insignifiant, but at 10 L/min, it's not ! To compensate with the method used (deltaT water-air imposed), better probes and calibration are needed to achieve 0.001 °C resolution and precision. I let you imagine the price of such material and this requires specific environnement... Other parameters have also a very small inherent error : air is taken by multiple probes to 0.1 °C, water temp, etc. For instance, the chiller is very good but water inlet variation is about 0.03 °C, so we have to take a mean value between the min and max recorded by probes and that introduces an unavoidable error, very small but real. The same applies to outlet. All these tiny errors and roundings from measurement devices have less influence on the results at low flow rate especially (deltaT are greater, so relative error tend to be very small), but it's not possible to get the same confidence range for all possibilities, especially at high flow rate and low airflow. I won't show you the big excel sheet with all factors, averages and corrections parameters taken in account, but when we're dealing with global error bars, we can see how the deviation increases with flowrate. See below.
http://img189.imageshack.us/img189/998/range.png
For a clarity purpose, these errors bars weren't put on charts. As usual, curves are "mean" curves by a statistic way (tests are done several times), their uncertainty is not shown (there's always one !). Anyway, the very high flowrate zone is the less important one because it's quite rare (let's say >500 L/h) and not really useful because gains are undetectable on a typical machine.
The second point is about some sort of saturation because of the air weakness. Air passing through the radiator will be warmer at the outlet, no mystery. From a thermophysical point, this air can't be warmer than the hottest point on the radiator. We can approximate that maximum saying it's the water temperature on inlet, let's say 30 °C. So, at best, air at the outlet will be at 30 °C too. Problem with very low airflow is that this air (20 °C at inlet) passing through the radiator will be warmed very quickly and it will hit a thermal wall without the possibility to absorb more energy (impossible to go >30 °C). Here, more water flow rate with an imposed deltaT water-air of 10 °C is directly equal to give more thermal energy to the rad. So, we inject more and more energy, but the small air volume provided by fans can't absorb such amount of energy (due to its limited heat capacity), the radiator efficiency is then +/- leveled, even if the convection efficiency in flat tubes is a bit improved. With the flow rate increase, more and more area of the radiator become "useless" in a manner of speaking, the fins tend to be at the same temperature than air surrounding them (all at ~30 °C), the thermal transfer tends to be very weak on these parts.
Someone talk about heat generated by friction into the rad due to its pressure drop, it's a nice try because it's real, but the power generated between the I/O is very negligible here (~0.9 W @ 10 LPM for the CR-22T). Heat dumps from fans are also to be neglected, especially at very low airflow.
We can suppose that the dissipation curve at very low airflow should be a horizontal line in the bottom right of the graph, like a asymptotic platter we can't cross, but it's a bit difficult to be sure because of multiple factors involved. Need a different way of measure and a cross-comparison to investigate this particular zone more deeply. In a typical loop, this problem can't be seen directly. At low airflow, water temperature will go higher to compensate the inefficiency.
At the end, the 2 classical ways of measuring radiator efficiency have to lead to same global conclusions with a proper analysis and modus operandi. Don't be too obsessed with ultimate accuracy, because all setups are flawed in a way or another, even with top notch equipment.
Thanks guys, awesome discussion, I've learned alot.
This is what I was seeing with my error in using water out only. It simply doesn't work right because it's affected by both flow rate and heat load. This also explains why I had a hard time with multiple heat loads as well. Using water out also tend to give a higher heat dissipated number than is probably correct.
http://img15.imageshack.us/img15/6895/badi.png
I'm switching my testing over to measuring both inlet and outlet water temperatures, but I still plan to measure heat using deltaT because my probes aren't that accurate. Anyhow with this first test, I wasn't able to measure any downward trends once I figured out my error, only a very very small upward gain, but pretty small at only 2% difference between .5GPM and 3.4GPM.
Great post rosco, thanks for the input. However, riddle me this:
So we are assuming that the air reaches water temp by the time it leaves the radiator. On any curve on the chart you have a constant airflow and a variable flow rate. You should as you said reach a point where the curve plateau's and it shouldn't go back down. At this point you have reached the limit of what the air can absorb. Since the airflow is constant, air in is constant (20C for arguments sake), air out is constant (30C - water temp), you should have constant temperature dissipation. You have the same flow of air through the same area with the same change in temperature. Yet obviously your measurements show that the water temperature is not decreasing by the same amount. It is decreasing less, so how is the air increasing by the same number of watts but the water is decreasing by less and less as the flow rate increases? You are gaining energy somewhere, it just doesn't make sense.
Good to see you Rosco! You need to post more... Threads like these remind me of the old days :D (that is a good thing)
This can be answered by that balance I posted:Quote:
Originally Posted by Erasmus354
h*A*(T2-T1) this is the air side heat transfer. IF, the air flow rate is the same, then h is constant, A and (T2-T1) are both constant as you mentioned.
This has to be balanced by
mdot*Cp*(T2-T1), which is the water side heat transfer.
mdot is your mass flow rate. Cp is going to be pretty constant at these small temperature changes. So, as mdot slows (T2-T1) (larger temperature difference) has to go up to balance the equation..or the other way around...as mdot gets higher your (T2-T1) (smaller temp difference) gets smaller..
Which is exactly what I am getting at. The situation as described by rosco has the low speed fans saturated from a certain flowrate onwards. This means that the airflow half of the equation is constant. The problem here is that the chart shows the water half of the equation going down .... how?Quote:
Originally Posted by relttem
Air temp would be equal to water temp if you have full (100%) water to air heat transfer, meaning the radiators are 100% efficient. I am running into weird numbers on my second round of radiator reviews because I am seeing 100%+ effeciency of water to air heat transfer, and I am controlling flow.Quote:
Originally Posted by Erasmus354
So I am going back through and using the water in sensor in my water average numbers for calculated C/W. The water temps do rise (~.2-.4C, thus far), but this does change the calculated C/W. I do not have enough data sets converted yet to show a C/W chart for any of the 6 triples tested. I will share the differences as soon as I can get at least one radiator using the new formulas.
I think the consensus is that the "water half" is not actually going down. What you are seeing is error in the system.Quote:
Originally Posted by Erasmus354
There is all kinds of stuff going on here.. Skinnee nailed one by mentioning that the radiator would have to be 100% efficient, which isnt going to happen. Also mentioned above was using average temperatures, which actually changes the problem around. If you get super technical and were able to take temperature readings from one side of the tube to the other (like the diameter of a circle) you would find that you have a temperature distribution. The temperature of the fluid right next to the wall of the tube is going to be different from the temperature at the center line of the flow. Though the temperature difference might be small, when you start crunching out numbers it will make a difference. Also mentioned above was 'error'.. There is an actual system for calculating the error of experiments; Kline-McClintock. That system goes through all the partial derivatives, resolution error, deviation, and mean to come up with an error. It is really tedious, but will give you a range of where your final result lies. BUT, most modern stuff that we use tells you what the error is (+/- .05% of the reading or whatever). And, then there is the almighty calibration - which I didn't really see mentioned. Calibrating thermocouples is easy..stick it in boiling water and it should read 100C, then make an ice-bath, stick it in that and it should read 0C. When calibrating our thermocouples we just put them in boiling water and found out how much off each one was...which was usually within the error given in the thermocouples description. All equipment should be calibrated before every test..pumps wear out, thermocouples degrade, etc etc.
So then more flow rate is still always better (or at least not worse). However I haven't seen that consensus that the coolingmasters results were erroneous.Quote:
Originally Posted by Bond Number
They were not erroneous, the test was a great test. They just reach the limits of their equipment. Basically it would be difficult and expensive to perform a similar test with less error.Quote:
Originally Posted by Erasmus354
Interesting post.
I was first wonderin if I could get a better cooling performance (in terms of lower CPU temperature) by slowing down the water flow to the radiator using a res. Looks like I will get a disappointing result :D
That is not necessarily true. Again, it is going to be based on the balance equation (basically, your first law of thermodynamics)Quote:
Originally Posted by ci2kla
h*A*(T2-T1) = mdot*Cp*(T2-T1)
h is your convective heat transfer coefficient, which is a function of how fast you the fluid is moving (in the radiator's case the fluid is air). So, if you increase the fan speed your h will go up. A (surface area) will always be the same unless you change radiators or it gets clogged with a lot of dust etc.
mdot is the water mass flow rate, Cp is constant.
So, if you slow down mdot and increase h more than the balance requires the water temperature change has to increase.
The thing with more than 100% efficiency happened to me too. In my case, it is caused by two factors as far as I can tell: Sensor placement and calibration.Quote:
Originally Posted by skinnee
The radiator that I get the weird results with was, let's say a non-standard radiator where I had to change the placement of the sensors. All th sensors are calibrated to match each other in their usual placement, but when I change their positions, the values from the air out sensors get skewed. With the fans blowing horizontally, I can't seem to find a way to get accurate air out measurements... :(
This is all using average water-in and water-out temperatures, by the way.
It probably has no relevance to what you're seeing, Skinnee, but I thought I'd just throw it out there anyway. May be helpful to someone or maybe someone can enlighten me as to what's going on.
That's basically the point to understand and that's why I said that another method is necessary to investigate special zones if necessary, a cross-comparison (two good tests are better than one). The deltaT imposed method can't be very good for each possibility in a very large range (water and air flows), there's always some drawbacks we can't solve. Fortunately, the maximum error is kept in zones where almost no one goes, so it's not a problem in my opinion.Quote:
Originally Posted by Bond Number
Relttem gave another possible source of unavoidable error I didn't want to introduce when we're at high flow rate, but the water temperature in the tube section is not exactly the same everywhere, even if there's insulation and a lot of turbulence at the radiator outlet to mix water near the probe. Every 0.01 °C counts there and even 0.001 °C probes won't solve this kind of problem. Never forget that error is fully part of measurement.
You all should think about shrouding your radiator inlets and outlets (outlet being more important). When the air leaves the backside of the radiator it is going from a section of high velocity to almost no velocity - this will cause eddies and vorticies to occur which will suck in air temperature from the surroundings, which will cause the temperature reading to be inaccurate. A shroud will protect the temperatures from the surrounding air
That's a good idea with the shrouding, relttem. I'd never thought of that.
I think I'll try to build some type of shroud for measuring air out and see how it affects results.
you can build it with some poster-board and tape. I would use the 20 diameter rule - meaning the length of the shroud should be 20x the diameter of your radiator. You'll have to use the hydraulic diameter to get a measurement of the radiator diameter, since it is not a circle. Wikipedia will tell you how to measure that. It is easy. Then, from fluid dynamics, fully developed flow occurs 10 diameters away. So, that is where I would put my thermocouple (as close to the middle of the duct as possible). At that point all mixing will be done and the temperature there will be a good estimate of the average temperature coming out of the backside of the radiator.
Wow, that's very specific stuff. I would have just gone ahead, and built some type of box and stuck my thermal probes in there. ^^
I'll look into those measurements. One question, though: Is the shroud allowed to be longer than 20x diameter? Because ideally, I would make a shroud that's long enough for the radiator with the largest diameter and use it for all the radiaotors. At least, for all the ones of the same type (i.e. 360s or 480s or whatever).
Actually, now that I think about it: What diameter are we talking about exactly? The total diameter of all the internal tubes of the rad? Or the diameter of the whole radiator? And if so, along what axis?
I'm sorry if I'm asking stupid questions here, but this is all new to me.
I just pulled 20x out of the air. I would make it longer than 10x - 11x would work. It can be as long as you want it as long as it is longer that 10x. I would put the thermocouple at least 10 diameters away. The reason behind this, other than fully developed flow, is that you have different temperatures at different locations coming out the back of the radiator, so you want to get a good average. You can test that to by taking readings at different positions. If you want to get really specific, I would make the duct, get a small diameter (very small) metal rod or wood dowel, put a hole in the duct for the rod, attach my TC to that and position it in the middle of the duct. You can get a compression fitting at the rod's diameters that would let you lock it down in the desired position.
Ok. Well, I'll have to see what size shroud is practical/possible to make, too.
And the problem with different temperatures at different distances from the radiator is very crucial, I think. I'm pretty sure that that's whats messing up my results so far. Especially since, without a shroud, at low fan speeds the warm air will rise up and away from the thermal probes much sooner than with higher airflow.
The shroud will mean only air that has passed thru the radiator is being measured. You can try insulating your duct, but I don't think that will matter much unless your room/ambient temperature is really really cold.
The problem with delta T testing I see is how long it can take to stabilize the run at high deltas and high heat loads with large volumes. I'm seeing some tests taking over 90 minutes to stabilize.
So much for production testing..lol!..:eek:..:D
http://img30.imageshack.us/img30/4817/samplerun.png
I'm also seeing huge variability in the air out numbers. I think every fan draws and spits out air a little differently and any minor movement at all in the sensors gives you a different result, so while it is interesting information, I don't trust air out myself unless you had some sort of flow chamber to collect and mix the air before measuring it. This would probably be a good idea for the air in also, but I'm not changing now, at least my air in sensors are permanently fixed.
I agree there are alot of flaws in any test setup. There's really no way around it even when you try really hard to minimize it. Probably the best we can do is collect more data points to try and average some of it out and last but not least, don't worry about it and just acknowledge that it's there and take the data with a grain of salt..:D
Do the air out temperatures correlate at all to the flow of the water through the rad? Hotter ones being where the water comes in and colder ones where it comes out (slightly hotter water vs colder water and therefore better and worse heat transference).
It might, but I think with my setup and only having 4 air out sensors, that there is error because the sensors are all on one side. It probably depends on both which side I made the inlet on the barbs and it would also depend on the flow rate because in a 2 pass radiator, the inlet side/half of the radiator would be hotter than the outlet, and as flow rate increases, that difference between sides would get smaller.Quote:
Originally Posted by Erasmus354
You'd probably need at least 4 sensors per fan and they would need to be permanently fixed in an outlet shroud of sorts to get any worthwhile air out data.
I have 8 sensors on my air inlet and that's really not that much when you consider how much it changes over the radiator. Fortunately, I'm not really using the air out sensors for any of the c/w or heat dissipated numbers, it's more just for information. Air is just extremely touchy when it comes to measuring temperature, it's pretty amazing how sensitive it is. A small thing like someone walking into a room will throw the sensors for a ride...:eek:
Also any air movement in a room like a fan or ceiling fan, or any little thing like that will give you different results. This all adds up to more error and makes it hard to make consistent comparisons.
Getting towards standardised methods here...Quote:
You'd probably need at least 4 sensors per fan and they would need to be permanently fixed in an outlet shroud of sorts to get any worthwhile air out data.
That was exactly BillA's early testing methodology (iirc)... airflow and temperature measured at each quadrant (minimum) of the fan (if in a duct, using duct traverse method [Link 2] - which is what relttem's on about above - there's an industry standard set of guidelines and methods for doing such things - these should be referred to and adhered to wherever possible to ensure valid data. Bill tried to highlight the necessity to adhere to such guidelines for any airflow based testing, which also points out that you shouldn't test axial fans using a duct - they aren't designed for use in such scenarios - use an airchamber instead - you can see his 'hints' at this in the aftermath relating to Vapor's original fan review threads... comments were also made on folks doing noise testing of fans, which basically meant read this and this). BillA's data was used to produce the coolingmasters article if memory serves, before he got the windtunnel/envirochamber (which he acquired to counter the "small thing like someone walking into a room" effect)... about 6 months prior to doing the PA testing (by which point he DID Have the windtunnel / envirochamber).
Ultimately, if testing fans and airflow, either on a radiator or not on a radiator... reference this book
If working out resistance to airflow of a radiator (or anything else) have a read of this one for an idea of correct methods.
Info on temperature fields of radiators (uses domestic household radiators, but gives some good clues to what goes on within w/c radiators) here
And SUPERB info on radiator testing as used in aeronautics - old (as in 1920s) but still very relevant - HERE
I heartily recommend serious testers read all the above links and bookmark them for future reference... the closer folks get to using the standardised methods for everything, the more accurate the data you get... and if all testers follow the same standardised methods, using recommended standardised equipment, then data begins to become comparable across different testers.
Air is a challenging one, unfortunately there's just not incentive for hobby testers having fun to spend that sort of money on equipement. We're all here for fun, not to go broke for the fourth decimal point.Quote:
Originally Posted by Marci
Absolute accuracy we'll never have, but I wouldn't go as far as dismissing the value in relative comparison testing. There is always some value in that even if it's just entertainment which is probably the main reason most of us do water cooling and hang out here..
The only solution to absolute accuracy in testing would be for the manufacturers to adopt some standards for them to test and publish data for. They are the only ones with the incentive and the funding to support that sort of effort. Unfortunately we can't even get simple testing like pressure drop tests out of most manufacturers, so it's wishful thinking that there would be any sort of collaborative effort to create standards.
Expecting hobby testers with shoestring budgets to meet that sort of expectation is completely unrealistic.
As long as people are having fun...it's all good! After all we only have a DTS sensor resolution of 1C and many folks live in homes with 10-15C ambient fluctuations and there's also alot of hardware that's not all that affected by temperature anyway. That's how I look at it. I enjoy the science and strive for accuracy where I realistically can and want to, but you have to stop tripping over pine cones now and then and take a view of the forest or you're missing out...
Martin, from your scatterplot above showing equilibrium times, what heat load was being applied for that run (I'm doing the same on my data sets)?
That was a 590 watt run.Quote:
Originally Posted by skinnee
The wierd thing I'm noticing is the time to equilibrium is not really heat based as much as it is the Delta and the volume of water in the test system.
I also ran an 200 watt load that took just as long because the delta was high from running very low 400RPM fans.
My reservoir for the heaters is a 4" x 24" piece of PVC, so when I add up the volume from my heater reservoir AND the large radiator, it's well over a full gallon of water in my test loop.
I have found that you can speed up the heatup process by estimating the delta from previous runs and heating the loop without fans first....just don't forget about it:eek:
I think ideally for this delta T testing you want the bare minimum of water volume. That and simply alot of time and leave the room and come back in a couple of hours for a really good run. My data points are falling on the trendlines much more accurately with this run. Now instead of stopping the logging run, I simply copy the file over to another and open it, plot the run and check to see if it's good enough to stop before I stop it.
I just wish there was a way we could automatically log and plot the temps on the crystalfontz. I'm sure it could be done if someone had the programming expertise.
I think the only time that the wattage load should make a difference is when the radiator has peaked out. A radiator will only be able to dissipate so much heat, and when you have reached that limit you will start to see stuff like run-away systems, but until then it should be the relatively the same thing.
Again, with the mdot*Cp*(T2-T1)
knowing the wattage, T1 and the flowrate you can solve for T2. That will be neglecting natural convection and radiation losses, but without and fans those two will be small. You can graph that equation for a constant Q and see how T2 will vary with your mdot if T1 is constant.
It would be nice to see a comparison of the equation versus data if you are running a few tests without any fans. That will give you an idea of losses from the radiator itself.
Not entirely true. Higher heat load means that you need a higher delta to dissipate that heat (assuming constant fan speed). That means the water needs to get hotter, which means you need to wait longer for that volume of water to heat up. The biggest thing here though will still be the volume of water in the system.Quote:
Originally Posted by relttem
That is exactly true, but not all radiators can provide that necessary delta, therfore they are limited. All radiators have a specified heat load that they max out at..Quote:
Originally Posted by Erasmus354
it all goes back to
mdot*Cp*(T2r-T1r) = h*A*(T2-T1)
again, h, is related to the air speed, but it has a limit. So, when that has peaked out you only have one option left and that is to lower T1. T1 is going to be your ambient temp, so it can only go so low in practical cases. Sure you can run cooled air thru the radiator, or Nitrogen gas etc, but in practical systems it will be ambient. So, the equation part on the right hand side is maxed out and will equal X Watts.
Now, we move to the left hand side. Cp is constant. T1r, mdot and T2r can/will vary. Mdot will max out due to pressure drop thru the radiator and pump limitations. Now, if we keep pumping heat into the system, T1 goes up and T2 goes up, thus the run away system
Can you explain the "Max Out" part?Quote:
Originally Posted by relttem
I haven't seen that in any testing yet, they all seem to progress with more and more heat dissipation as the delta increases. Obviously if we approached boiling in the the fluid, that would be a limit, but I just havn't even began to see any sort of limit. I've tested and measured up to 20C deltas which is approaching my own personal comfort level and I know my temp probes also have their limits, but I just havn't really seen any sort of limit in testing.:shrug:
I can see a limit when you say something like what is the limit for a "10C water/air delta", but that's just a point of reference. I know there are some small physical changes to the specific heat/thermal conductivity values, but I just haven't seen any sort of limit yet other than my own self imposed one.
I tend to stay away from the thought of a radiator "Limit". I hear that alot in the forums with folks thinking that if they have a 1000 watt radiator, there's no performance loss to be had by adding another 100 watts to the loop, etc. That's obviously an incorrect way in thinking because every single watt of heat adds to the water temperature and your receiving core temperatures.
From what I gathered, you could potentially dissipate 1000 watts from a single 80mm radiator if you had a high enough water/air delta and enough air flow.
Honestly, when I was worked with Modine the radiators were sized way above anything that you would typically see. But, you can max out a radiator if, for example, you find a tiny radiator online that isn't really made for cooling a CPU, find one that is cooling something small. Or, make a simple radiator..just flow your liquid thru a short copper tube and blow air over it. Then, slowly add heat. The tube-radiator will peak out at a certain load. I have done it with refrigeration systems in that the refrigerant mass flow rate wasn't able to handle the amount of heat, then the temperatures start to run away until something breaks.
Again, the statement "if you had high enough water/air delta and enough air flow" has a limit. That is why a radiator will peak out. I think another thread on here talked about blowing thru a straw. If you get a martini straw and blow as hard as you can you will only get a little bit of air out..doesn't matter how much pressure you apply. Your air-side and fluid-side flow in the radiator will max out. You can get the largest fan and the largest pump, but it will still only pump so much liquid thru and so much air thru the fins..your lines will burst and the fan-blown air that can't get thru the fins due to the pressure drop will go around or not move at all..that means Mdot and h are maxed. So, we have A, mdot, Cp, h, and Tambient as constants and possibly T1r if the computer is at steady state
I don't need a lesson in the math. I was referring to your statement that the heat load doesn't matter. I was pointing out that it does have a factor in the time it takes for the system to reach equilibrium. Something you ignored and instead decided to launch into yet another pointless lecture.Quote:
Originally Posted by relttem
Doesn't the Crystalfontz unit update the .csv file in realtime? It should be possible pull the file with another program in progress, throw it into Excel, and throw the data into the graphing wizard to see how the curve is forming. If Excel won't accept a file that another program owns, you could use a program like Textpad to do that, and save it as a different file first.Quote:
Originally Posted by Martinm210
sorry, I don't mean them to be pointless. Its hard to determine what everyone is referring to on here, so I just try to generalize. But, if your heat load is too high it will never reach equilibrium..that is all I was trying to point out.Quote:
Originally Posted by Erasmus354
My res holds about a half gallon, I'm using 3" PVC.Quote:
Originally Posted by Martinm210
I've got the templates down for testing, just have to copy paste from 635_log.csv into the excel template. About as minimal as you can make it...I'll send you a workbook. :up:
Apologies for a bit of thread necromancy.
The reason why more flow doesn't necessarily mean better is because ine can only remove so much heat from one surface given so much fluid flow. This is why radiators designed for low airflow can only do so much when given higher flow rate fans.
Similarly, this goes the same for coolant flow, but with a twist, you would want maximum turbulence and maximum amount of dwell time for your coolant inside the tubes, however this doesn't bode very well for your water blocks.
There's always a sweet spot between pressure drop and heat dissipation radiator designers want to achieve. More often than not, its very counter intuitive and requires a lot of testing and validation.
You can increase your FPI only so much that it prevents you from using audibly acceptable fans, but you can only bring it down so much that your heat transfer potential, relative to the radiator footprint, is optimally utilized.
Sorry Martinm, did not see this until today as I only stop in here occasionally. The question you are asking is studied in chemical engineering under reactor well dynamics. Without the math to confuse, consider the simple steam locomotive. As I add in more coal, I can drive the speed up to perhaps 60 mph. Quickly though, there comes a point, where no matter how much more coal I add in, I cannot drive the locomotive any faster (in fact, due to thermal convection resonance, I will actually slow down slightly). Same effect here, increasing flow rate (under fixed fan speed) increases turbulence as flow changes from laminar to turbulent (up to Reynolds # 4000 or so) and dissipates more heat for any given radiator until you pass the turbulent boundary and reach chaotic flow (this is the result of the partial derivatives to zero in Navier Stokes), at which point cavitation sets in and less reward is seen for ever increasing pressure, until the flow is so chaotic that initial boundary conditions consume more energy in transition than in radiation. In laymen's terms, the fluid is now moving under such increased pressure that more energy is exchanged within the fluid than can be radiated thru the rad's surface. That is why the delta T drops slightly.
@ hwlabs "Apologies for a bit of thread necromancy."
Thanks, hadn't read this one.
Ya, that last one sounds really good:up: