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General Liquid/Water Cooling Discussion For discussion about Full Cooling System kits, or general cooling topics. Keep specific cooling items like pumps, radiators, etc... in their specific forums.

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Unread 07-19-2002, 04:38 PM   #1
JimS
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Default Good article on effect of radiator size in watercooling

Here


Maybe time to add another rad. to the system, I have at least 300W of pelts. and waterblocks.
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Unread 07-19-2002, 04:41 PM   #2
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Did some temp. measuring and that graph is very accurate.
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Unread 07-19-2002, 05:10 PM   #3
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Good find Jim!

Hum... ok, so for practical purposes, considering what we use here, what surface area should a rad have, where +/- 20% of the surface yields little difference?

Wouldn't increasing the flow rate cover for the lack of size of a rad?

How can we roughly calculate the surface area of a heatercore?

How does finned tubing fit into all this?

I've got more questions than answers!
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Unread 07-19-2002, 05:49 PM   #4
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The article mentions different sized radiators but the question I have is the thickness of the radiator. My radiator has a face of 36 sq. inches(6x6) and it is 2 inches deep(excluding fan).

I am assuming that the article is calculating the face as I have, simply by multiplying height x width. There are varying thicknesses in radiators though so I don't know how this is figured in.

Based on this article, there is obviously a minimum size our rads should be, and at a certain point more size gives a diminishing return.
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Unread 07-20-2002, 05:24 AM   #5
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Thickness does not improve efficiency as much as pure surface area. Count less than 50% and you're close. On an older thread i posted a link to a car maker who explained all this thoroughly.
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Unread 07-20-2002, 07:34 AM   #6
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Yes, but doesn't thickness contribute to the surface area of the fins and tubes?
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Unread 07-20-2002, 08:36 AM   #7
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JimS,

Thickness contributes directly to surface area of fins. Obviously it only contributes to tube surface area if you use the additional thickness to fit in larger tubes or multiple rows.

Thickness has a diminishing return regardless of radiator size simply for the same reason that the fins/pins on air cooled heat sinks show diminishing returns. The greatest contrast between fin and air temperature occurs where the fin meets the tubing. Because the conduction coefficient of metals is so much higher than the convection coefficient to air, you don't need much metal (thickness) to conduct the heat from the tubes. What you do need is lots of surface area and the closer that area is to the tubing the better.

Unfortunately, it's the weekend and my heat transfer book is at work. There is an equation for fin efficiency based upon conduction and convection variables. I can post that next week if anyone cares.
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Unread 07-20-2002, 02:00 PM   #8
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Quote:
Originally posted by myv65
Unfortunately, it's the weekend and my heat transfer book is at work. There is an equation for fin efficiency based upon conduction and convection variables. I can post that next week if anyone cares.
I'm definitely interested!
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Unread 07-22-2002, 08:35 AM   #9
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Just remember, you asked for this stuff BigBen2k,

O.K. Here's the quick run down on fin effectiveness and efficiency equations.

Individual fin effectiveness is defined as the amount of heat transferred off the fin versus the amount that would be transferred off the portion of the base connected to the fin if the fin didn't exist. If we call fin heat transfer "qf" and heat transfer potential from the base equals h * A * delta-T then fin effectiveness equals qf / (h * A * delta-T). "h" is the convection coefficient, A is the area at the fin base, and delta-T is the temperature differential from base to ambient air.

Fin efficiency is defined as the heat transfer off the fin versus the heat transfer that would leave the fin if the entire fin was the same temperature as the base material. This is like saying that the fin has a conduction coefficient of infinity.

There are different equations that apply to fins based upon how you wish to model them. These variations include convection from the fin tip, no heat transfer from the fin tip, a given specific temperature at the fin tip, and an infinitely long fin. The easiest equation is for the infinite fin where qf = (h * P * k * A)^(.5) * delta-T. Here the additional terms are "P" for fin perimeter (approximated as 2 * width for very thin, flat fins) and "k" is fin conduction coefficient. This approximation actually fits "real world" fins of finite length reasonably well.

A better approximation comes from assuming that the fin is of finite length with no heat transfer from the tip. The effective length of the pin (Leq) may be considered as actual fin length + 2 * fin thickness. The effective surface area (Ap) becomes Leq * fin thickness. In this case, fin efficiency boils down to:

Leq^(3/2) * (h / k * Ap)^(1/2)

Things begin getting really fun when you start to consider arrays of fins (ala radiators). You can define an overall efficiency to equal actual heat transfer divided by potential heat transfer if all the fins matched the temperature of the base material.

I'll skip ahead and spit out the equation for overall efficiency. It is:

1 - Af / At * (1 - fin efficiency)

Where:

"Af" is total fin area
"At" is total area of fins and exposed base material
"fin efficiency" is a single fin's efficiency based on the earlier equations.

Slapping all this stuff down in this form is only so much gibberish without all the supporting material that goes with it. If you really want to get into this sort of stuff, I'd suggest picking up a heat transfer book by Frank Incropera and David DeWitt. Along with a little calculus experience, you'd be well on your way.
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Unread 07-22-2002, 09:13 AM   #10
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I'll have to pick up that book.

I see exactly what you mean, but the other reason that I was interested in this, is because the waterblock designs posted on misc forums seem to get into some very unusual fin designs. I find a lot of them interesting, but fail to see anyone justify why a fin should be this or that height, or how thick.

I like #2 (assumption that the fin is of finite length, with no heat transfer at the tip), but I'm looking to optimize fins so that the block doesn't have an unnecessary mass, or restriction

The spiral design is interesting because it is essentially a fin, but where flow restriction has been taken into account.

So, flow restriction versus fin design/efficiency, an item to be added here

Cool stuff... Thank you!
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Unread 07-22-2002, 09:48 AM   #11
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Quote:
Originally posted by bigben2k
The spiral design is interesting because it is essentially a fin, but where flow restriction has been taken into account.
On a macro-scale, this is where it's at. Ya gotta optimize the use of the metal for conduction's sake while maximizing the surface area for convection's sake while minimizing the metal and surface area for flow's sake. Yeah.

On the micro-scale, you get into the stuff mentioned by many others here before. Namely surface treatments to minimize boundary layer thickness. IIRC, one of the big airplane manufacturers has been experimenting with tiny chevrons protruding from the airskin surface. They have been working to optimize chevron height, width, angle, and array layout to minimize skin drag. We're talking stuff that sticks up a mere few thousandths of an inch. Same sorta idea applies to our WB passages. Modeling something like this is outa my current league as my software resources aren't what they used to be.
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Unread 07-22-2002, 09:57 AM   #12
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Quote:
Originally posted by myv65


On a macro-scale, this is where it's at. Ya gotta optimize the use of the metal for conduction's sake while maximizing the surface area for convection's sake while minimizing the metal and surface area for flow's sake. Yeah.

On the micro-scale, you get into the stuff mentioned by many others here before. Namely surface treatments to minimize boundary layer thickness. IIRC, one of the big airplane manufacturers has been experimenting with tiny chevrons protruding from the airskin surface. They have been working to optimize chevron height, width, angle, and array layout to minimize skin drag. We're talking stuff that sticks up a mere few thousandths of an inch. Same sorta idea applies to our WB passages. Modeling something like this is outa my current league as my software resources aren't what they used to be.
Yeah.

I used to work for an airline, so I can relate to what you're saying. They're also going to have to take noise into account, and find a balance point. If you've been on an Airbus 330 or 340 (like me), then you know how amazingly quiet they are: you can actually hold a quiet conversation during take off! For an airline, it's a big selling point.
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Unread 07-22-2002, 12:19 PM   #13
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I'm hoping that this book would cover different fin shapes, like pins or blades (ex. 1) or really odd shapes (ex. 2, although clearly inefficient)?

Would it cover air as well as water?
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Unread 07-22-2002, 01:02 PM   #14
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Quote:
Originally posted by bigben2k
I'm hoping that this book would cover different fin shapes, like pins or blades (ex. 1) or really odd shapes (ex. 2, although clearly inefficient)?

Would it cover air as well as water?
Yes on both counts, though as soon as you begin to stray from common shapes you're kinda out on your own. A lot of people without engineering backgrounds seem to think we've got equations or tables for everything. Well yeah, but darn near all we have is based on experiments that someone took the time to do. So if you wish to know the performance of some odd-ball shape in convection, you'll often have to do the testing yourself. Where the theory helps in this regard is understanding what to expect, what to test, and how to test it.

As far as the text is concerned, air and water are interchangeable. Until you get to Mach numbers of ~0.3, assuming air to be incompressible is the norm. Yeah, it's density follows Boyle's law, but water's density also varies with temperature. In many respects, gases and liquids aren't so far apart.

Only ridden on the 340 once. I attributed the lower noise levels to engine advancements and sound deadening as much as anything. I hadn't really given consideration to external factors aside from reducing the occasional protrusion (from fasteners, mainly). I've ridden the 777 a few times and found it to be very quiet and comfortable, too. Those days are pretty well gone for me, however. I travelled ~100 days/year at my former job but more like 10 at this one.
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Unread 07-22-2002, 01:06 PM   #15
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You want online resources ? Go here:
http://www.coolingzone.com/Content/D...Flomerics.html
or here:
http://www.coolingzone.com/Content/D...iz/default.htm
or here:
http://www.coolingzone.com/Content/D.../baseframe.htm

This mainly concerns solid to air transfers, and there are some good resources on rad fin sizing.
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