Why blow when you can suck?

[Grub]

I was thinking the other day. Why do heatsink fans blow down on the heatsink. One of the first tenets of good heat exchange is to get the coolest part of your cooling medium to the hottest part of your item to be cooled first. That way, you have the biggest temperature differential across the heat exchanger. To put it into cpu terms...why doesn't the fan suck air into the fins at the bottom of the heatsink where the air is at it's coolest and the fins are at their hottest. You could put some sort of plastic shroud over the top portion of the fins to prevent air bypass. For you math guys: Q = cm(Th=Tc) where
Q= Heat transfer
c= thermal conductivity coefficient
m= mass flow rate
Th= hot temperature
Tc= cold temperature.

It the same mass flow rate, but the delta T goes way up, thus making your heat transfer go up. ANybody know why the manufacturers don't do this?


...ummm...sorry, I forgot what I was going to say...

 

[HammerBot]

If you suck air away from the heatsink, it will enter the fins from the side and be sucked out through the fan. I will expect that much air wont reach the center bottom part of the heatsink were cooling is required the most. However, if you blow, that wouldn't be a problem.
Considering the amount of research there is being put into good heatsinks and the amount of different types, I think the designers has considered the most optimal airflow. They all blow, so obviously thats better than sucking.
But go right ahead and turn around your fan, if you wan't your CPU to be hotter.

<i><b>Engineering is the fine art of making what you want from things you can get</b></i>
<A HREF="http://www.btvillarin.com/phpBB/viewtopic.php?t=655" target="_new">My systems</A>

 

[phsstpok]

There are several theories, first time I heard yours. Why do you think air is cooler near the bottom of the heatsink?

A differenent theory claims that the air furthest away from the heatsink and motherboard is coolest. On the motherboard you have other items generating heat. These include the chipset, memory, voltage regulators, etc. If you suck air though your heatsink you are pulling air over these other components warming the air before it enters the heatsink.

Regardless of theory, you are free to mount the fan either way and determine the outcome for yourself. When I tested it both ways I got a 4 degree cooler rating with the fan blowing than with the fan sucking.

<b>99% is great, unless you are talking about system stability</b><P ID="edit"><FONT SIZE=-1><EM>Edited by phsstpok on 11/29/02 09:32 PM.</EM></FONT></P>

 

[alc101ma]

If you want a sucking setup, you wanna make sure enough air flows over the hottest part of the heatsink (the bottom). The heatsink designs that do this have a shroud covering the top half of the fins. You can convert your blowing type heatsink to a sucking one by using simple masking tape around the top half of the heatsink.

 

[Grub]

Yeah...that's why in my original post I mentioned putting a shroud around the fins to prevent air bypass of the lower section. Guess I shoulda worded it better. What I was trying to say about he "coolest air" was this... Assuming that all air in the case is the same temp, then the air should enter the heat exchanger at the hottest part while it is still relatively cool. As the air travels through the fins, it will rise in temperature as it picks up heat from the fins and travels out of the heatsink. You want to maximize differential temperature throughout the cooler. That's why you want the coolest air at the hottest part of your heatsink. I agree that the hard part of this design would be getting all parts of the fins exposed to coolant. I think that this would be easy to accomplish. Just put a shroud that interlaces the fins and directs flow. When I get my spare system up I'm going to experiment with this.

...ummm...sorry, I forgot what I was going to say...

 

[knowan]

It's been tried. Empirically blowing works best. This arguement, suck vs blow, raged all over this forum about a year ago. Finally people tested it by putting their fans on upside down and found that blowing works best.

--------------
Knowan likes you. Knowan is your friend. Knowan thinks you're great.

 

[lhgpoobaa]

It really depends on the cooler in question.
Most work better with the fan blowing down. SOme, like the Alpha 8045 work better with the fan sucking.

Experiment for yourself.


<b>Just because someone's a member of an ethnic minority doesn't mean they're not a nasty small-minded little jerk. <i>Terry Pratchett</i></b>

 

[LtBlue14]

don't you want the air hitting the heatsink (particularly the bottom) hard instead of just sort of being sucked through it?
seems to me you wanna force the air onto the metal for better heat conduction
i'd like to see the results of your shroud experiment though, do post that whenever you get the chance

 

[Grub]

I just got all my components to cobble together a new system. I'm going to try this on an Athlon tbird 1.4... that should give definitive results since it puts out enough heat to measure easily. I'll use the stock aluminum cooler it came with. Should have some results next Monday. I know that the engineers at the heatsink manufacturing plnats have been through this problem...but you never know. Sometimes they go with one design because of other considerations besides performance. I just can't help myself...I've got to tweak it...

...ummm...sorry, I forgot what I was going to say...

 

[Crashman]

Alpha coolers suck, Swiftechs blow. But many users have found that REVERSING the fans on their sucking Alpha gives better results. For one thing, air is more DENSE when it's compressed, aka blown. Dense air has more molecules to exchange heat. And the other problem is getting the air to flow past the bottom of the sink, it doesn't work in the suck configuration. From automotive applications I also know that blowing is more efficient than sucking. Oh, and there's always the problem of the hot chipset, etc, being near the inlet in the sucky configuration.

<font color=blue>You're posting in a forum with class. It may be third class, but it's still class!</font color=blue>

 

[kimbahpnam]

i just gotta say nice subject title. lol